To what extent was the United States a world power in 1914 Essay

To what extent was the United States a world power in 1914 - Essay Example The main reasons as to why the foreign policy of the US government was isolationist in nature, is because the US still did not have enough resources and capability of worrying about the foreign affairs and problems of other countries (Hook and Scott, 2012). On this basis, the immediate concern of the United States was its internal environment. This was a period in which the US had just emerged from a costly civil war, and it was the intention of General Washington to expand the territories of United States (Hook and Scott, 2012). However, during the year 1890, the government of the United States began engaging with overseas territories (Black, 2003). The government decided to look for the markets of its products to foreign markets. This was not only for an economic benefit, but also for their territorial expansion (Hook and Scott, 2012). On this basis, the expansionist policy of the United States took an international approach. For instance, the American government sent an invasion force to Haiti, in 1915, with the intention of protecting American economic interests, as well as replacing the Haitian constitution which advocated against the foreign ownership of land (Kagan, 2003). On this basis, its invasion of Haiti was purely motivated by economic and territorial gains. It is important to denote that the expansionist policy practiced by the United States was under the context of the isolationist policy. The First World War played a significant role in shaping the global position of the United States. This is because the First World War negatively affected the global stability that was witnessed during the last 100 years (Kagan, 2003). On this basis, political isolation no longer served the interests of the United States., hence the country had to engage in a series of foreign affairs, that was never witnessed in the United States. It is important to denote that the First World War began in 1914 (Kagan,

Motivational Strategy and Action Plan Essay Example for Free

Motivational Strategy and Action Plan Essay Determine the motivational strategy or strategies that would likely be most appropriate for each of your three employees on basis of their individual characteristics. Indicate how you would leverage their employee evaluations to motivate each of the three employees. Describe one or more of the motivational theories and explain how the theories connect to each of your selected motivational strategies. Team Member Name Summary of Individual Characteristics Motivational Strategy and Action Plan Relevant Theory Tina Engaged Passionate Persistent Inspirational Leader Happy Emotional stable This employee would not really need much motivation, since already in engaged, passionate and is a leader at work. I would suggest that the employee uses an action plan to continue with positive attitude. Also since the employee has passed reviews, to advance in progress with promotions in the company. McClelland theory is relevant to this situation. It relates to this situation because for the most part the employee is happy at the job. Brittany Competitive Good communicator Influential Independent Emotional Quick thinker This employee shouldn’t much motivation since she is so competitive. However we could motivate her to be more engaged and happy at job since so emotional. An action plan for this strategy would be to assign her to do task she is interested and likes to do that are easy and take little thought since she is a quick thinker. This situation relates to the self-efficiency theory because employee is independent. She likes to do things on her own and feels she can. Denise Timid Considerate Neutral decision maker Eager Happy with job Encouraging This employee needs motivation in company involvement in communication since timid and shy. She can demonstrate these skills in an action plan by being more involved in conferences, presentations, and meetings. This situation could be also related to the McClelland theory because for the employee is happy with job, but there is room for opportunity.

Pain In Cardiothoracic Surgery Numerical Rating For Pain Nursing Essay

Pain In Cardiothoracic Surgery Numerical Rating For Pain Nursing Essay Pain is frequently experienced post-operatively, after cardiothoracic surgery, and is thus a core component of nursing practice (Kalso, Perttunen, and Kaasinen, 2002). This assignment introduces the concept of pain and highlights the importance of the accurate assessment of pain in terms of the Nursing and Midwifery (NMC, 2008) Code of Practice and recommended guidelines. This is followed by an evaluation of pain assessment outcome measures, with particular focus on the Numerical Rating Scale (NRS) for pain. The NRS, when used as a self-report outcome measure, is the gold standard for pain assessment. It is a psychometrically and operationally robust pain assessment measure, as supported by the evidence presented within this assignment. Not only is the NRS associated with a number of beneficial patient outcomes, but it has also been found to facilitate communication between patients, healthcare professionals, and multidisciplinary teams (de Rond et al., 2001). Pain in Cardiothoracic Surgery: The Numerical Rating Scale for Pain Assessment This assignment introduces the concept of pain and highlights the importance of the accurate assessment of pain within the cardiothoracic surgery setting. This is followed by an evaluation of pain assessment outcome measures, with particular focus on the gold standard self-report outcome measure, the Numerical Rating Scale for pain. Background In the UK, over 10,000 cases of thoracic surgery are carried out each year, with pain being frequently reported post-surgery (Perttunen, Tasmuth, and Kalso, 1999; Maguire et al., 2006). One study found that persistent pain lasting more than 6-months was reported by 44% of patients after a thoracotomy (Kalso, Perttunen, and Kaasinen, 2002). The prevalence of chronic pain after thoracic surgery has been reported as a significant problem that is consistently rated by patients as being one of the most difficult problems following surgery; it can impact a patients life for several years, severely depleting their quality of life (Maguire et al., 2006). Despite the prevalence and burden of pain, the literature highlights many cases of poor clinical practice in the assessment and management of post-operative pain (Dihle et al., 2006; Schoenwald and Clark 2006). This is regardless of past quality improvement initiatives and changes to practice, which comprised the establishment of clinical nurse specialists, multidisciplinary pain teams, and standardised pain assessment tools (The Royal College of Surgeons of England and College of Anaesthetists, 1990). Inadequate assessment and management of post-operative pain poses a number of implications for the patient and the NHS. For example, pain can result in increased levels of anxiety, sleep disturbance, restlessness, irritability, and aggression, as well as limitations in mobility (Macintyre and Ready, 2001; Carr et al., 2005). More importantly, post-operative pain is an unnecessary ordeal that causes heightened distress (Macintyre and Ready, 2001; Carr et al., 2005). It can also have physiological effects on patients, which may lead to complications and delayed discharge from hospital, including increases in heart rate and blood pressure, delayed gastric emptying, nausea, vomiting, and paralytic ileus (paralysis of the intestine). Difficulties coughing, resulting from increased pain on exertion, can result in chest infections and additional problems, such as deep vein thrombosis and pulmonary embolus (Sjostrom et al 2000; Macintyre and Ready, 2001). At worst, unrelieved pain can be lif e-threatening, especially in older people with comorbidities (Hamil, 1994). Pain is the fifth vital sign in the physiological assessment of patients, making it a core component of nursing practice (Chronic Pain Policy Coalition, 2008). The Joint Commission on Accreditation of Healthcare Organisations has made it mandatory for hospitals to assess pain in patients (Krebs, Carey, and Weinberger, 2007). Nurses are morally and ethically responsible for the accurate assessment of post-operative pain (Dimond, 2002), since this is vital for identifying the nature and severity of pain as well as for administering pain relief interventions and ascertaining the effectiveness of such interventions (Mackintosh, 2007). As an example, pain scores can be used alongside the WHO (1990) three-step analgesic ladder in the administration of pain relief. The assessment of pain is complex and decisions are required as to the most accurate method of assessment within different clinical environments and with different patients. One such complex decision is whether to measure pain observationally or via self-reports and this decision is most likely to be led by conceptions of pain. If defined as a subjective experience, or as described by McCaffery and Beebe (1968, p. 95) as, whatever the experiencing person says it is, existing whenever the experiencing person says it does, then choice of assessment is most likely to be self-report. Self-report is the gold standard for measuring pain since subjective experiences can only be measured from the perspective of the patient (Wood, 2004). The importance of self-report pain assessment is highlighted in a study by Whipple et al. (1995) whereby, out of 17 trauma patients admitted to an intensive care unit, 95% of doctors and 81% of nurses felt that the patients had adequate pain relief; in contrast, 74% of patients rated their pain as moderate or severe. Many other studies confirm this inconsistency between the subjective pain reported by patients and the objective pain reported by healthcare professionals (Sjostrom et al., 2000; Marquie et al., 2003; Sloman et al., 2005). There are a vast array of patient-reported outcome measures for assessing pain, including uni-dimensional scales that measure one element of pain (such as intensity) and multidimensional scales that measure more characteristics of pain and its impact (Macintyre et al., 2010). Whilst multidimensional tools might be better for chronic long-term conditions, uni-dimensional scales have been reported to be effective for acute pain, which can be experienced in the cardiothoracic surgery setting (Wood, 2008). Multi-dimensional measures of pain are rarely used post-surgery as they are more complex and time-consuming (Coll et al., 2004). Therefore, this assignment evaluates selected literature on uni-dimensional outcome measures within this context, with particular focus on the measure recommended by the Australian and New Zealand College of Anaesthetists and Faculty of Pain Medicine in their guidance on Acute Pain Management: Scientific Evidence (Macintyre et al., 2010): The Numerical Rating Scale for Pain. Methodology Literature pertaining to the Numerical Rating Scale (NRS) for pain was searched in order to identify articles on the NRS and comparative pain assessment tools. The following keywords were used within the search strategy: pain AND numerical rating scale OR NRS AND surgery. The search was limited to articles comprising adult participants. The search was also limited to records no earlier than 2005 in an effort to obtain the most recent evidence examining the NRS. Nevertheless, where these records have cited earlier research assessed as being applicable to this assignment, these records have also been obtained and used as evidence. Results A total of 88 records were retrieved from the pre-defined search criteria, 28 of which were not relevant to this assignment. A total of 60 records were evaluated for evidence to be included within this assignment. In synthesising the evidence within these records and the utilised pain assessment tools, a framework created by Fitzpatrick et al. (1998) was adopted. This framework, designed to facilitate the selection of the most appropriate patient-reported outcome measures, promotes a consideration of the following psychometric properties and operational characteristics: appropriateness of the instrument to the purpose and setting; reliability; validity; responsiveness; precision; interpretability; acceptability; and feasibility. Examples of three of the most frequently utilised uni-dimensional pain assessment measures, as discussed within the following appraisal of the literature, can be found in appendix 1. Literature Appraisal The reviewed evidence suggests that the four most commonly utilised uni-dimensional pain assessment tools are the verbal rating scales (VRS), numerical rating scale (NRS), visual analogue scale (VAS), and pictorial rating scale (PRS). Such tools were developed due to the lack of feasibility associated with using multidimensional outcome measures within the clinical environment (Wood, 2004). Uni-dimensional tools such as the VRS, NRS, VAS and PRS enable health professionals to quantify pain intensity from a subjective perspective without being too time-consuming and without creating a burden for patients. Verbal rating scales are descriptive in nature, allowing patients to rate their pain intensity on a scale of no pain, mild pain, moderate pain, or severe pain (Wood 2004; Williamson and Hoggart, 2005). They have been reported as being one of the easiest tools to understand and use, whilst also offering the option of being completed verbally or in written format. The VRS has been adopted and integrated into acute settings, with numbers to rate pain being used in observation charts (e.g. 1=mild pain; 2=moderate pain, etc.). Such integration into standard practice increases the feasibility of the scale, whilst using numbers to document pain provides ease of interpretability. Of caution, however, is that although verbal rating scales are easy to use, the adjectives do not necessarily represent equal intervals of pain. Indeed, patients may wish to express their pain in via a word not appearing within the list of adjectives they have been presented with (Schofield, 1995). Due to the use of words to describe pain, the VRS is dependent on both the respondents interpretation and understanding of the terms, as well as the health professionals interpretation. It has, therefore, been suggested that this scale lacks the sensitivity and accuracy of other pain rating scales (Baillie 1993). Jensen et al. (1994) suggested that the lack of sensitivity of the VRS could lead to an over or underestimation of changes in pain being experienced and, as such, could make it difficult to manage pain appropriately and effectively. In contrast to the VRS, the visual analogue scale (VAS) uses a 100mm horizontal or vertical line with extremes of pain placed at either end of the line so that the patient marks their pain intensity along the continuum. The distance to the line can then be measured and documented. A VAS rating of greater than 70mm is usually the threshold indicative of severe pain (Aubrun et al., 2008). However, the VAS poses a number of limitations within the clinical setting. It requires a greater degree of cognitive functioning, physical dexterity, and concentration than other measures of pain, and thus it is not suitable for some patients, including older patients and those with visual difficulties (Krulewitch et al., 2000). Indeed, Chapman and Syrjala (1990) estimated that 7-11% of adults would have difficulty using the VAS, whilst Wood (2004) went on to find that about 20% of patients are either unable to complete the VAS or find it confusing. Also, because it is administered verbally, it might be difficult to use after general anaesthesia or administration of some analgesics. In addition, the VAS has been found to be highly sensitive to changes in levels of pain, which can make it difficult to use (Bird and Dickson, 2001). Overall, the VAS has been found to be the least suitable uni-dimensional pain assessment measure, especially if administered after cardiac surgery (Pesonen et al., 2008). Numerical rating scales (NRS) offer an alternative to descriptive measures of pain by assessing pain intensity numerically, on a scale of 0 (no pain) to 10 (worst pain imaginable). A value of four or more is most often used as a threshold to guide clinical intervention (Mularski, 2006). On the other hand, the most recent guidance from the World Union of Wound Healing Societies (WUWHS, 2007) makes no reference to pain score thresholds, merely offering that change in pain level may indicate a need to reassess the patient. The NRS has been found to be highly acceptable to patients when compared to other pain scales (WUWHS, 2007) and, like the VRS, it has the advantage of being validated for verbal or written administration, which makes it feasible for use with patients who have differing levels of ability to complete such assessments (Paice and Cohen, 1997). It has been shown that older patients, post-operative patients, and patients with poor motor coordination are able to use the NRS (Rodriguez, 2001; Aubrun et al., 2003). It is not recommended, however, for patients with post-operative confusion (Ferrell et al., 1995). The NRS is more sensitive than the VRS, although some patients might find it difficult to describe their pain numerically (Carpenter and Brockopp, 1995). The NRS for pain is recommended by the The Australian and New Zealand College of Anaesthetists and Faculty of Pain Medicine developed guidance on Acute Pain Management: Scientific Evidence (Macintyre et al., 2010), which is endorsed by the Faculty of Pain Medicine, Royal College of Anaesthetists in the UK as well the International Association for the Study of Pain. The guidance aims to combine a review of the best available evidence for acute pain management with current clinical practice and was designed to provide information based on best evidence. The support cited within the document shows a good correlation between the VAS and NRS, indicating good levels if convergent validity with a measure purporting to assess the same construct. However, this correlation is not as strong in cardiothoracic patients compared to non-cardiothoracic patients (Ahlers et al., 2008). The document also highlights that the NRS is usually preferable, most certainly among patients (Herr et al., 2004). This is likely due to its feasibility in terms of burden to patients and staff since it only takes 30 seconds to complete (Downie et al., 1978). Importantly, the NRS has been found to be responsive to interventions such as patient-controlled analgesia (Li, Liu and Herr 2009), making it an effective instrument for monitoring pain management. The scale is also highly Downie et al. (1978) also found the NRS to have superior accuracy when compared to the VAS and simple descriptive measures of pain such as the VRS. It could be argued that the NRS provides a compromise between the VRS, which offers only a few descriptors for patients to choose from, and the VAS, which has been reported to offer too much choice and to be confusing. In terms of comparability against observer-based measures of pain, the NRS has been found to be more sensitive to detecting pain than the Behavioural Pain Scale (BPS) in both cardiothoracic and non-cardiothoracic patients (Ahlers et al., 2008). The authors rationalise that when using the NRS, health professionals tend to gather more background information on the patient, taking into consideration pain over time. In contrast, the BPS measures pain at one point in time, is objective, and lacks a contextual basis for interpretation. Therefore, the authors recommend that the BPS is only used alongside the NRS (Ahlers et al., 2008). This again supports self-reported pain as being the gold-standard for pain assessment and management. Importantly, studies have demonstrated that improvements in pain assessment and documentation frequently lead to more effective pain management (Erdek and Pronovost, 2004). In terms of the NRS, the evidence suggests that using this scale frequently results in favourable clinical outcomes such as decreased incidence of pain and agitation, as well as a decrease in the duration of mechanical ventilation (Chanques et al., 2006). It also enhances the nurse-patient relationship by providing acknowledgment of pain (Briggs, 2003). These positive outcomes are likely related to the fact that healthcare professionals are less liable to underestimate a patients level of pain when using the NRS than when compared to not using it. One study showed that of patients experiencing pain, where a discrepancy was reported between patient and nurse ratings, the NRS had not been used in 45% of such cases (Lorenz et al., 2009). This problem is especially apparent when patients rate their pain as being unacc eptable; nurses tend to underestimate the level of pain if not collecting pain ratings from patients using the NRS (Ahlers et al., 2008). In this sense, the NRS and, indeed, other pain measures are invaluable for facilitating patient communication of pain and expression of pain (Wood, 2004). The NRS also offers a number of practical advantages in that it is easy to teach to all staff and patients, as well as being easy to score and document. The documentation of all measures of pain is fundamental for the delivery of effective care, and it also facilitates communication between multidisciplinary team members (American Pain Society, 1995). The NRS is a valid and reliable measure of pain, but does still need to be used with caution and professional judgment as some studies have found a lack of consistency between ratings of pain. For example, one study found that whilst a patient might express a reduction in pain after an intervention, their score on the NRS remains the same (Mackintosh, 2005). Furthermore, a rating of, for example, seven by one patient might have a different meaning to another patient (Sloman et al., 2000). However, provided that such limitations are taken into consideration and efforts made to supplement the information gathered from the NRS, the instrument can be a highly effective tool for the assessment and management of pain (Mackintosh, 2005). Such supplementary enquiry might include observation and history taking, as recommended by McCaffery and Pasero (1999). Implications for Practice Despite the many benefits to uni-dimensional outcome measures of pain, such tools need to be used with caution as they only focus on limited aspects of the pain experience, arguable oversimplifying the complexity of the experience (Wood, 2004). They also pose the risk of being misinterpreted, a risk that cannot be rectified via descriptive tools since interpretation difficulties are also present when using the VRS. For example, what constitutes moderate pain might vary across patients and health professionals, as well as be dependent on factors such as personality, culture, and experience (Closs et al., 2004). The validity and reliability of all pain assessment tools, including the NRS, can be enhanced by familiarising the patient with the assessment tool and explaining the reasons for its use. Indeed, Giordano, Abramson and Boswell (2010) have emphasised the importance of listening to the patients subjective descriptions of pain and being consistent in the documentation of any pain assessment. At the same time, it is imperative to acknowledge that a pain assessment tool is only one aspect of the overall assessment of the patients pain (Duke, 2006). In the cardiothoracic setting, pain assessment should include static (rest) and dynamic (sitting, coughing, etc.) pain assessment and management (Macintyre et al., 2010). Although the assessment process should not be rushed, it does need to commence soon after surgery since studies have shown that high levels of pain immediately after surgery are associated with increased risk of developing chronic pain (Katz et al., 1996). In the clinical environment, it is not always feasible to carry out extensive assessments of pain, but the benefit of utilising a tool such as the NRS is that it provides an initial brief assessment of pain intensity. This, in turn, provides vital information on whether pain relief is required or whether a previously administered intervention has been effective. It is understood, however, that awareness of other pain measures is essential for the purpose of providing equal care to patients who might not be able to complete the NRS. For example, patients with cognitive impairments might find the Abbey Pain Scale easier to complete (Abbey et al., 2004), whilst patients with learning disabilities might prefer Zwakhalen et al.s (2004) scale of non-verbal indicators. Conclusions Effective pain management needs to commence with effective pain assessment, as well as the identification of factors requiring urgent intervention (Fear, 2010). The Numerical Rating Scale for pain provides a psychometrically robust method of assessing pain intensity and monitoring pain reduction interventions. As well as being psychometrically robust, the scale is acceptable to patients and feasible within busy clinical environments such as the cardiothoracic surgery setting. The best available evidence suggests the Numerical Rating Scale for pain is a suitable tool for the assessment and management of post-surgery pain and using this tool thus adheres to the Nursing and Midwifery (NMC, 2008) Code of Practice for providing a high standard of evidence-based practice at all times. Not only does this scale provide improved patient outcomes, but it also promotes communication between the patient, nurse, and multidisciplinary team (de Rond et al., 2001). More research is needed on the acc uracy and effectiveness of the NRS, as well as exploration as to any potential improvements to the instrument (Krebs, Carey, and Weinberger, 2007); however, until then, the evidence suggests that the NRS is an acceptable and efficacious screening tool for measuring pain in patients. Appendix 1: Pain Rating Scales Visual Analogue Scale (VAS) Numerical Rating Scale (NRS) Faces Rating Scale (FRS)

Comparing the Dream Deferred in Raisin in the Sun and Death of a Salesman :: comparison compare contrast essays

A Dream Deferred in A Raisin in the Sun and Death of a Salesman Most everyone in America would like to achieve financial success. Sometimes living in a capitalistic society entices many to become too materialistic. Greed is the characteristic that many Americans then attain. This is all in pursuit of the American dream. For most Americans, this high status is very difficult to achieve. In Arthur Miller's, Death of a Salesman, we see how difficult it was for Willy Loman and his sons to achieve this so called American dream, and these people were proud white Americans. In Lorraine Hansberry's ,A Raisin in the Son, she examines an African-American's family's struggle to break out of the poverty that is preventing them from achieving some sort of financial stability, or the American Dream. It focuses on Walter's attempt in "making it," or "being somebody." She also analyzes how race prejudice and economic insecurity affect a black mans role in his own family, his ability to provide, and his identity. What Hansberry is trying to illustrate is how West ern civilization has conditioned society to have materialistic aspirations and how these ideals corrupt the black man's identity and his family. Many black men have to deal with a systematic racism that effects their role in society. The frustrations that a black man has to deal with can affect the family a great deal. For example, if Walter gets upset at work or has a bad day, he can't get irate with his boss and risk loosing his job; instead he takes it out on his wife Ruth. Also, the job that he holds can only provide so much to the family. He's not even capable of providing his son Travis with some pocket change without becoming broke himself. What type of "breadwinner" can a black man be in America? Walter Younger is thirty-five years old and all he is, is a limousine driver. He is unhappy with his job and he desperately seeks for an opportunity to improve his family standing. He tells his mother how he feels about his job when she wouldn't give him the ten thousand dollars; I open and close car doors all day long. I drive a man around in his limousine and I say,"Yes sir;no sir,very good sir; shall I take the drive, sir? " Mama, that ain't no kind of job... that ain't nothing at all. Comparing the Dream Deferred in Raisin in the Sun and Death of a Salesman :: comparison compare contrast essays A Dream Deferred in A Raisin in the Sun and Death of a Salesman Most everyone in America would like to achieve financial success. Sometimes living in a capitalistic society entices many to become too materialistic. Greed is the characteristic that many Americans then attain. This is all in pursuit of the American dream. For most Americans, this high status is very difficult to achieve. In Arthur Miller's, Death of a Salesman, we see how difficult it was for Willy Loman and his sons to achieve this so called American dream, and these people were proud white Americans. In Lorraine Hansberry's ,A Raisin in the Son, she examines an African-American's family's struggle to break out of the poverty that is preventing them from achieving some sort of financial stability, or the American Dream. It focuses on Walter's attempt in "making it," or "being somebody." She also analyzes how race prejudice and economic insecurity affect a black mans role in his own family, his ability to provide, and his identity. What Hansberry is trying to illustrate is how West ern civilization has conditioned society to have materialistic aspirations and how these ideals corrupt the black man's identity and his family. Many black men have to deal with a systematic racism that effects their role in society. The frustrations that a black man has to deal with can affect the family a great deal. For example, if Walter gets upset at work or has a bad day, he can't get irate with his boss and risk loosing his job; instead he takes it out on his wife Ruth. Also, the job that he holds can only provide so much to the family. He's not even capable of providing his son Travis with some pocket change without becoming broke himself. What type of "breadwinner" can a black man be in America? Walter Younger is thirty-five years old and all he is, is a limousine driver. He is unhappy with his job and he desperately seeks for an opportunity to improve his family standing. He tells his mother how he feels about his job when she wouldn't give him the ten thousand dollars; I open and close car doors all day long. I drive a man around in his limousine and I say,"Yes sir;no sir,very good sir; shall I take the drive, sir? " Mama, that ain't no kind of job... that ain't nothing at all.

An event in your life that changed you Essay

A new house, new school, and a completely new atmosphere were ahead of my nine-year-old life. I had lived in the same house for what it seemed like forever and had attended the same school since first grade. I knew everyone. From January to December, my calendars were filled with birthday parties I had to attend. Performing in talent shows with my best friends Dezerey, Jasmine, and Nykchasia were a yearly activity at Garden Valley Elementary School. The Robinson Center’s summer camp was mandatory for my big sister, Breonna, and me. I was used to familiarities like the Chihuahua that ran to the front of its gate and barked every time I rode past it on my sleek, black rollerblades. Those rollerblades made me feel untouchable, at ease and confident until I had to ride them in a new area, surrounded by new, unfamiliar people. Close to the end of my fourth grade year, my mother told my sister and me that we were moving to Terricina Gold Apartments in Natomas. I did not think it was going to change anything. I was probably a bit excited for what was to come. I can clearly remember my first day at Two Rivers Elementary school being terrified. I believe that was when it first â€Å"hit me† that I was not at â€Å"home† anymore. My comfort zone left was tarnished. I remember having butterflies and feeling as if I was going to faint before I stepped foot onto school grounds. When I got to the blacktop all I could do was stand there in silence. The other kids were standing around in there cliques and it was obvious that the fifth graders controlled the far left gate on the black top. I remember one girl that stood out. She looked larger than life in my eyes. She was bright skinned, tall and everyone seemed to flock to her. She made me feel small without even knowing her. I was having trouble finding my classroom number on the blacktop where my class was supposed to  line up. I completely gave up after a few minutes mainly because I felt so out of place I just wanted to hide. Not to mention I am legally blind and even with my glasses I still cannot see all that well so finding my class line was a far reach for my nine-year old mind. I felt out of place and inferior so I â€Å"I took it upon myself† to run away and hide in the nearest bathroom. I stayed in that bathroom for a while, even after the bell rang. I eventually decided to take a step out of the bathroom and walk to my class. In fourth grade I absolutely did not have any social skills. Many children lived in my new apartments. I envied their large groups of friends and seeing them made me miss my old ones. I could never gather up the courage to talk to anyone. Thankfully, I had my older sister. She is the most outgoing between us and she managed to know everyone in nearly a week of our residency. I eventually met everyone that lived in the apartments but I never felt at ease with them as I did with my old friends. I rode my rollerblades all around those apartments and they made me happy like a piece of home was with me. I never realized that I was in fact an introvert. I suppose I had always been a timid person my mother brings up her memories of my behavior as a child often. I never realized my childhood behavior until now. I remember my mom changing my teacher’s and being too afraid to walk into the classroom even though I knew the teacher and all the students well. Moving away from my familiarities showed how quiet and unsociable I was. I did not have friends at school until I was well into my seventh grade year. Today, I am on the verge of twenty-years-of-age, and I still have to work on speaking up and being more sociable. Before I had many friends and after I moved, I did not. I believe if I stayed where I lived before I would have been given a sense of security. Looking back, I believe I would not have gained a great number of wonderful friends. My experiences with people in my apartments are always the topic of conversation with my family and friends and they bring on tons of laughs and feelings of embarrassment. I love revisiting my old Natomas neighborhoods and seeing my old friends. We always joke saying TG (Terricina Gold) for life. I guess you can call it a family.

Data and Computer Communications

DATA AND COMPUTER COMMUNICATIONS Eighth Edition William Stallings Upper Saddle River, New Jersey 07458 Library of Congress Cataloging-in-Publication Data on File Vice President and Editorial Director, ECS: Marcia J. Horton Executive Editor: Tracy Dunkelberger Assistant Editor: Carole Snyder Editorial Assistant: Christianna Lee Executive Managing Editor: Vince O’Brien Managing Editor: Camille Trentacoste Production Editor: Rose Kernan Director of Creative Services: Paul Belfanti Creative Director: Juan Lopez Cover Designer: Bruce Kenselaar Managing Editor,AV Management and Production: Patricia Burns  ©2007 Pearson Education, Inc.Pearson Prentice Hall Pearson Education, Inc. Upper Saddle River, NJ 07458 Art Editor: Gregory Dulles Director, Image Resource Center: Melinda Reo Manager, Rights and Permissions: Zina Arabia Manager,Visual Research: Beth Brenzel Manager, Cover Visual Research and Permissions: Karen Sanatar Manufacturing Manager, ESM: Alexis Heydt-Long Manufacturing B uyer: Lisa McDowell Executive Marketing Manager: Robin O’Brien Marketing Assistant: Mack Patterson All rights reserved. No part of this book may be reproduced in any form or by any means, without permission in writing from the publisher.Pearson Prentice Hallâ„ ¢ is a trademark of Pearson Education, Inc. All other tradmarks or product names are the property of their respective owners. The author and publisher of this book have used their best efforts in preparing this book. These efforts include the development, research, and testing of the theories and programs to determine their effectiveness. The author and publisher make no warranty of any kind, expressed or implied, with regard to these programs or the documentation contained in this book. The author and publisher shall not be liable in any event for incidental or consequential amages in connection with, or arising out of, the furnishing, performance, or use of these programs. Printed in the United States of America 10 9 8 7 6 5 4 3 2 1 ISBN: 0-13-243310-9 Pearson Education Ltd. , London Pearson Education Australia Pty. Ltd. , Sydney Pearson Education Singapore, Pte. Ltd. Pearson Education North Asia Ltd. , Hong Kong Pearson Education Canada, Inc. , Toronto Pearson Educaci n de Mexico, S. A. de C. V. Pearson Education Japan, Tokyo Pearson Education Malaysia, Pte. Ltd. Pearson Education, Inc. , Upper Saddle River, New Jersey For my scintillating wife ATSWEB SITE FOR DATA AND COMPUTER COMMUNICATIONS, EIGHTH EDITION The Web site at WilliamStallings. com/DCC/DCC8e. html provides support for instructors and students using the book. It includes the following elements. Course Support Materials The course support materials include †¢ Copies of figures from the book in PDF format †¢ A detailed set of course notes in PDF format suitable for student handout or for use as viewgraphs †¢ A set of PowerPoint slides for use as lecture aids †¢ Computer Science Student Support Site: contains a number of links and documents that the student may find useful in his/her ongoing computer science education.The site includes a review of basic, relevant mathematics; advice on research, writing, and doing homework problems; links to computer science research resources, such as report repositories and bibliographies; and other useful links. †¢ An errata sheet for the book, updated at most monthly T DCC Courses The DCC8e Web site includes links to Web sites for courses taught using the book. These sites can provide useful ideas about scheduling and topic ordering, as well as a number of useful handouts and other materials. Useful Web Sites The DCC8e Web site includes links to relevant Web sites, organized by chapter.The links cover a broad spectrum of topics and will enable students to explore timely issues in greater depth. iv WEB SITE FOR DATA AND COMPUTER COMMUNICATIONS, EIGHTH EDITION v Supplemental Documents The DCC8e Web site includes a number of documents that expand on the treatment in the book. Topics include standards organizations, Sockets, TCP/IP checksum, ASCII, and the sampling theorem. Internet Mailing List An Internet mailing list is maintained so that instructors using this book can exchange information, suggestions, and questions with each other and the author.Subscription information is provided at the book’s Web site. Simulation and Modeling Tools The Web site includes links to the cnet Web site and the modeling tools Web site. These packages can be used to analyze and experiment with protocol and network design issues. Each site includes downloadable software and background information. The instructor’s manual includes more information on loading and using the software and suggested student projects. This page intentionally left blank CONTENTS Web Site for Data and Computer Communications Preface xv 1 iv Chapter 0 Reader’s and Instructor’s Guide 0. Outline of the Book 2 0. 2 Roadmap 3 0. 3 Internet and Web Resources 5 0. 4 Standards 6 PART ONE OVERVIEW 9 Chapter 1 Data Communications, Data Networking, and the Internet 10 1. 1 Data Communications and Networking for Today’s Enterprise 12 1. 2 A Communications Model 16 1. 3 Data Communications 19 1. 4 Networks 22 1. 5 The Internet 25 1. 6 An Example Configuration 29 Chapter 2 Protocol Architecture, TCP/IP, and Internet-Based Applications 2. 1 The Need for a Protocol Architecture 33 2. 2 The TCP/IP Protocol Architecture 34 2. 3 The OSI Model 42 2. 4 Standardization within a Protocol Architecture 44 2. Traditional Internet-Based Applications 48 2. 6 Multimedia 48 2. 7 Recommended Reading and Web Sites 53 2. 8 Key Terms, Review Questions, and Problems 54 Appendix 2A The Trivial File Transfer Protocol 57 PART TWO DATA COMMUNICATIONS 62 Chapter 3 Data Transmission 65 3. 1 Concepts and Terminology 67 3. 2 Analog and Digital Data Transmission 78 3. 3 Transmission Impairments 86 3. 4 Channel Capacity 91 3. 5 Recommended Reading and Web Si te 96 3. 6 Key Terms, Review Questions, and Problems Appendix 3A Decibels and Signal Strength 99 Chapter 4 Transmission Media 102 4. Guided Transmission Media 104 4. 2 Wireless Transmission 117 4. 3 Wireless Propagation 125 32 96 vii viii CONTENTS 4. 4 4. 5 4. 6 Line-of-Sight Transmission 129 Recommended Reading and Web Sites 133 Key Terms, Review Questions, and Problems 134 Chapter 5 Signal Encoding Techniques 138 5. 1 Digital Data, Digital Signals 141 5. 2 Digital Data, Analog Signals 151 5. 3 Analog Data, Digital Signals 162 5. 4 Analog Data, Analog Signals 168 5. 5 Recommended Reading 175 5. 6 Key Terms, Review Questions, and Problems 175 Chapter 6 Digital Data Communication Techniques 180 6. Asynchronous and Synchronous Transmission 182 6. 2 Types of Errors 186 6. 3 Error Detection 186 6. 4 Error Correction 196 6. 5 Line Configurations 201 6. 6 Recommended Reading 203 6. 7 Key Terms, Review Questions, and Problems 204 Chapter 7 Data Link Control Protocols 207 7. 1 Flow Control 209 7. 2 Error Control 216 7. 3 High-Level Data Link Control (HDLC) 222 7. 4 Recommended Reading 228 7. 5 Key Terms, Review Questions, and Problems 229 Appendix 7A Performance Issues 232 Chapter 8 Multiplexing 239 8. 1 Frequency-Division Multiplexing 242 8. 2 Synchronous Time-Division Multiplexing 248 8. Statistical Time-Division Multiplexing 258 8. 4 Asymmetric Digital Subscriber Line 265 8. 5 xDSL 268 8. 6 Recommended Reading and Web Sites 269 8. 7 Key Terms, Review Questions, and Problems 270 Chapter 9 Spread Spectrum 274 9. 1 The Concept of Spread Spectrum 276 9. 2 Frequency Hopping Spread Spectrum 277 9. 3 Direct Sequence Spread Spectrum 282 9. 4 Code-Division Multiple Access 287 9. 5 Recommended Reading and Web Site 290 9. 6 Key Terms, Review Questions, and Problems 291 CONTENTS ix PART THREE WIDE AREA NETWORKS 295 Chapter 10 Circuit Switching and Packet Switching 297 10. Switched Communications Networks 299 10. 2 Circuit Switching Networks 301 10. 3 Circuit Switching Concepts 304 10. 4 Softswitch Architecture 307 10. 5 Packet-Switching Principles 309 10. 6 X. 25 317 10. 7 Frame Relay 319 10. 8 Recommended Reading and Web Sites 324 10. 9 Key Terms, Review Questions, and Problems 325 Chapter 11 Asynchronous Transfer Mode 328 11. 1 Protocol Architecture 329 11. 2 ATM Logical Connections 331 11. 3 ATM Cells 335 11. 4 Transmission of ATM Cells 340 11. 5 ATM Service Categories 345 11. 6 Recommended Reading and Web Sites 348 11. Key Terms, Review Questions, and Problems 349 Chapter 12 Routing in Switched Networks 351 12. 1 Routing in Packet-Switching Networks 352 12. 2 Examples: Routing in ARPANET 362 12. 3 Least-Cost Algorithms 367 12. 4 Recommended Reading 372 12. 5 Key Terms, Review Questions, and Problems 373 Chapter 13 Congestion Control in Data Networks 377 13. 1 Effects of Congestion 379 13. 2 Congestion Control 383 13. 3 Traffic Management 386 13. 4 Congestion Control in Packet-Switching Networks 13. 5 Frame Relay Congestion Control 388 13. 6 ATM Traff ic Management 394 13. 7 ATM-GFR Traffic Management 406 13. Recommended Reading 409 13. 9 Key Terms, Review Questions, and Problems 410 Chapter 14 Cellular Wireless Networks 413 14. 1 Principles of Cellular Networks 415 14. 2 First Generation Analog 427 14. 3 Second Generation CDMA 429 14. 4 Third Generation Systems 437 14. 5 Recommended Reading and Web Sites 440 14. 6 Key Terms, Review Questions, and Problems 441 387 x CONTENTS PART FOUR LOCAL AREA NETWORKS 444 Chapter 15 Local Area Network Overview 446 15. 1 Background 448 15. 2 Topologies and Transmission Media 451 15. 3 LAN Protocol Architecture 457 15. 4 Bridges 465 15. 5 Layer 2 and Layer 3 Switches 473 15. Recommended Reading and Web Site 478 15. 7 Key Terms, Review Questions, and Problems 479 Chapter 16 High-Speed LANs 482 16. 1 The Emergence of High-Speed LANs 483 16. 2 Ethernet 485 16. 3 Fibre Channel 500 16. 4 Recommended Reading and Web Sites 504 16. 5 Key Terms, Review Questions, and Problems 506 Appendix 16A Digital Sig nal Encoding for LANs 508 Appendix 16B Performance Issues 514 Appendix 16C Scrambling 518 Chapter 17 Wireless LANs 522 17. 1 Overview 523 17. 2 Wireless LAN Technology 528 17. 3 IEEE 802. 11 Architecture and Services 531 17. 4 IEEE 802. 11 Medium Access Control 535 17. 5 IEEE 802. 1Physical Layer 543 17. 6 IEEE 802. 11 Security Considerations 549 Recommended Reading and Web Sites 550 17. 7 17. 8 Key Terms, Review Questions, and Problems 551 PART FIVE INTERNET AND TRANSPORT PROTOCOLS Chapter 18 Internetwork Protocols 556 18. 1 Basic Protocol Functions 558 18. 2 Principles of Internetworking 566 18. 3 Internet Protocol Operation 569 18. 4 Internet Protocol 576 18. 5 IPv6 586 18. 6 Virtual Private Networks and IP Security 596 18. 7 Recommended Reading and Web Sites 599 18. 8 Key Terms, Review Questions, and Problems 600 Chapter 19 Internetwork Operation 603 19. 1 Multicasting 605 19. Routing Protocols 614 19. 3 Integrated Services Architecture 625 19. 4 Differentiated Services 636 554 CONTENTS xi 19. 5 19. 6 19. 7 19. 8 Service Level Agreements 645 IP Performance Metrics 646 Recommended Reading and Web Sites 649 Key Terms, Review Questions, and Problems 651 657 Chapter 20 Transport Protocols 655 20. 1 Connection-Oriented Transport Protocol Mechanisms 20. 2 TCP 674 20. 3 TCP Congestion Control 683 20. 4 UDP 693 20. 5 Recommended Reading and Web Sites 695 Key Terms, Review Questions, and Problems 695 20. 6 PART SIX Chapter 21 21. 1 21. 2 21. 3 21. 4 21. 5 21. 6 21. 7 21. 8 21. INTERNET APPLICATIONS 699 Network Security 701 Security Requirements and Attacks 703 Confidentiality with Conventional Encryption 705 Message Authentication and Hash Functions 713 Public-Key Encryption and Digital Signatures 720 Secure Socket Layer and Transport Layer Security 727 IPv4 and IPv6 Security 732 Wi-Fi Protected Access 737 Recommended Reading and Web Sites 739 Key Terms, Review Questions, and Problems 740 Chapter 22 Internet Applications—Electronic Mail and Network Managemen t 22. 1 Electronic Mail: SMTP and MIME 745 22. 2 Network Management: SNMP 760 22. 3 Recommended Reading and Web Sites 770 22. Key Terms, Review Questions, and Problems 771 743 Chapter 23 Internet Applications—Internet Directory Service and World Wide Web 23. 1 Internet Directory Service: DNS 774 23. 2 Web Access: HTTP 784 23. 3 Recommended Reading and Web Sites 795 23. 4 Key Terms, Review Questions, and Problems 796 Chapter 24 Internet Applications—Multimedia 799 24. 1 Audio and Video Compression 800 24. 2 Real-Time Traffic 808 24. 3 Voice Over IP and Multimedia Support—SIP 811 24. 4 Real-Time Transport Protocol (RTP) 820 24. 5 Recommended Reading and Web Sites 831 24. 6 Key Terms, Review Questions, and Problems 832 773 ii CONTENTS APPENDICES 835 Appendix A Fourier Analysis 835 A. 1 Fourier Series Representation of Periodic Signals 836 A. 2 Fourier Transform Representation of Aperiodic Signals 837 A. 3 Recommended Reading 840 Appendix B Projects for Teaching Dat a and Computer Communications B. 1 Practical Exercises 842 B. 2 Sockets Projects 843 B. 3 Ethereal Projects 843 B. 4 Simulation and Modeling Projects 844 B. 5 Performance Modeling 844 B. 6 Research Projects 845 B. 7 Reading/Report Assignments 845 B. 8 Writing Assignments 845 B. 9 Discussion Topics 846 References Index 858 ONLINE APPENDICES WilliamStallings. om/DCC Appendix C Sockets: A Programmer’s Introduction C. 1 Versions of Sockets C. 2 Sockets, Socket Descriptors, Ports, and Connections The Client/Server Model of Communication C. 3 C. 4 Sockets Elements C. 5 Stream and Datagram Sockets C. 6 Run-Time Program Control C. 7 Remote Execution of a Windows Console Application Appendix D Standards Organizations D. 1 The Importance of Standards D. 2 Standards and Regulation D. 3 Standards-Setting Organizations Appendix E Appendix F The International Reference Alphabet Proof of the Sampling Theorem 847 841 Appendix G Physical-Layer Interfacing G. 1 V. 24/EIA-232-F G. ISDN Physical Interface Appendix H The OSI Model H. 1 The Model H. 2 The OSI Layers CONTENTS xiii Appendix I Queuing Effects I. 1 Queuing Models I. 2 Queuing Results Appendix J Orthogonality, Correlation, and Autocorrelation J. 1 Correlation and Autocorrelation J. 2 Orthogonal Codes Appendix K The TCP/IP Checksum K. 1 Ones-Complement Addition K. 2 Use in TCP and IP Appendix L TCP/IP Example Appendix M Uniform Resource Locators (URLs) and Uniform Resource Identifiers (URIs) M. 1 Uniform Resource Locator M. 2 Uniform Resource Identifier M. 3 To Learn More Appendix N Glossary Augmented Backus-Naur FormThis page intentionally left blank PREFACE Begin at the beginning and go on till you come to the end; then stop. —Alice in Wonderland, Lewis Carroll OBJECTIVES This book attempts to provide a unified overview of the broad field of data and computer communications. The organization of the book reflects an attempt to break this massive subject into comprehensible parts and to build, piece by piec e, a survey of the state of the art. The book emphasizes basic principles and topics of fundamental importance concerning the technology and architecture of this field and provides a detailed discussion of leading-edge topics.The following basic themes serve to unify the discussion: †¢ Principles: Although the scope of this book is broad, there are a number of basic principles that appear repeatedly as themes and that unify this field. Examples are multiplexing, flow control, and error control. The book highlights these principles and contrasts their application in specific areas of technology. †¢ Design approaches: The book examines alternative approaches to meeting specific communication requirements. †¢ Standards: Standards have come to assume an increasingly important, indeed dominant, role in this field.An understanding of the current status and future direction of technology requires a comprehensive discussion of the related standards. INTENDED AUDIENCE The book is intended for both an academic and a professional audience. For the professional interested in this field, the book serves as a basic reference volume and is suitable for self-study. As a textbook, it can be used for a one-semester or two-semester course. It covers the material in Networking (NET), a core area in the Information Technology body of knowledge, which is part of the Draft ACM/IEEE/AIS Computing Curricula 2005.The book also covers the material in Computer Networks (CE-NWK), a core area in Computer Engineering 2004 Curriculum Guidelines from the ACM/IEEE Joint Task Force on Computing Curricula. PLAN OF THE TEXT The book is divided into six parts (see Chapter 0): †¢ Overview †¢ Data Communications †¢ Wide Area Networks xv xvi PREFACE †¢ Local Area Networks †¢ Internet and Transport Protocols †¢ Internet Applications In addition, the book includes an extensive glossary, a list of frequently used acronyms, and a bibliography. Each chapter inc ludes problems and suggestions for further reading.The chapters and parts of the book are sufficiently modular to provide a great deal of flexibility in the design of courses. See Chapter 0 for a number of detailed suggestions for both top-down and bottom-up course strategies. INSTRUCTIONAL SUPPORT MATERIALS To support instructors, the following materials are provided: †¢ Solutions Manual: Solutions to all end-of-chapter Review Questions and Problems. †¢ PowerPoint Slides: A set of slides covering all chapters, suitable for use in lecturing. †¢ PDF files: Reproductions of all figures and tables from the book. Projects Manual: Suggested project assignments for all of the project categories listed below. Instructors may contact their Pearson Education or Prentice Hall representative for access to these materials. In addition, the book’s Web site supports instructors with: †¢ Links to Webs sites for other courses being taught using this book †¢ Sign up i nformation for an Internet mailing list for instructors INTERNET SERVICES FOR INSTRUCTORS AND STUDENTS There is a Web site for this book that provides support for students and instructors.The site includes links to other relevant sites, transparency masters of figures in the book, and sign-up information for the book’s Internet mailing list. The Web page is at WilliamStallings. com/DCC/DCC8e. html; see the section, Web Site for Data and Computer Communications, preceding the Table of Contents, for more information. An Internet mailing list has been set up so that instructors using this book can exchange information, suggestions, and questions with each other and with the author. As soon as typos or other errors are discovered, an errata list for this book will be available at WilliamStallings. om. PROJECTS AND OTHER STUDENT EXERCISES For many instructors, an important component of a data communications or networking course is a project or set of projects by which the student gets hands-on experience to reinforce concepts from the text. This book provides an unparalleled degree of support for including a projects component in the course. The instructor’s supplement not only includes guidance on how to assign and structure the projects but also includes a set of User’s PREFACE xvii Manuals for various project types plus specific assignments, all written especially for this book.Instructors can assign work in the following areas: †¢ Practical exercises: Using network commands, the student gains experience in network connectivity. †¢ Sockets programming projects: The book is supported by a detailed description of Sockets available at the book’s Web site. The Instructors supplement includes a set of programming projects. Sockets programming is an â€Å"easy† topic and one that can result in very satisfying hands-on projects for students. †¢ Ethereal projects: Ethereal is a protocol analyzer that enables students to study the behavior of protocols. Simulation projects: The student can use the simulation package cnet to analyze network behavior. †¢ Performance modeling projects: Two performance modeling techniques are provided a tools package and OPNET. †¢ Research projects: The instructor’s supplement includes a list of suggested research projects that would involve Web and literature searches. †¢ Reading/report assignments: The instructor’s supplement includes a list of papers that can be assigned for reading and writing a report, plus suggested assignment wording. Writing assignments: The instructor’s supplement includes a list of writing assignments to facilitate learning the material. †¢ Discussion topics: These topics can be used in a classroom, chat room, or message board environment to explore certain areas in greater depth and to foster student collaboration. This diverse set of projects and other student exercises enables the instructor to use t he book as one component in a rich and varied learning experience and to tailor a course plan to meet the specific needs of the instructor and students. See Appendix B for details.WHAT’S NEW IN THE EIGHTH EDITION This eighth edition is seeing the light of day less than four years after the publication of the seventh edition. During that time, the pace of change in this field continues unabated. In this new edition, I try to capture these changes while maintaining a broad and comprehensive coverage of the entire field. To begin the process of revision, the seventh edition of this book was extensively reviewed by a number of professors who teach the subject. The result is that, in many places, the narrative has been clarified and tightened, and illustrations have been improved.Also, a number of new â€Å"field-tested† problems have been added. Beyond these refinements to improve pedagogy and user friendliness, there have been major substantive changes throughout the book . Every chapter has been revised, new chapters have been added, and the overall organization of the book has changed. Highlights include: †¢ Updated coverage of Gigabit Ethernet and 10-Gbps Ethernet: New details of these standards are provided. †¢ Updated coverage of WiFi/IEEE 802. 11 wireless LANs: IEEE 802. 11 and the related WiFi specifications have continued to evolve. viii PREFACE †¢ New coverage of IP performance metrics and service level agreements (SLAs): These aspects of Quality of Service (QoS) and performance monitoring are increasingly important. †¢ Address Resolution Protocol (ARP): This important protocol is now covered. †¢ New coverage of TCP Tahoe, Reno, and NewReno: These congestion control algorithms are now common in most commercial implementations. †¢ Expanded coverage of security: Chapter 21 is more detailed; other chapters provide overview of security for the relevant topic.Among the new topics are Wi-Fi Protected Access (WPA) and the secure hash algorithm SHA-512. †¢ Domain Name System (DNS): This important scheme is now covered. †¢ New coverage of multimedia: Introductory section in Chapter 2; detailed coverage in Chapter 24. Topics covered include video compression, SIP, and RTP. †¢ Online appendices: Fourteen online appendices provide additional detail on important topics in the text, including Sockets programming, queuing models, the Internet checksum, a detailed example of TCP/IP operation, and the BNF grammar.In addition, throughout the book, virtually every topic has been updated to reflect the developments in standards and technology that have occurred since the publication of the seventh edition. ACKNOWLEDGMENTS This new edition has benefited from review by a number of people, who gave generously of their time and expertise. The following people reviewed all or a large part of the manuscript: Xin Liu- (UC, Davis), Jorge Cobb, Andras Farago, Dr. Prasant Mohapatra (UC Davis), Dr. Jingxi an Wu (Sonoma State University), G. R.Dattareya (UT Dallas), Guanling Chen (Umass, Lowell), Bob Roohaprvar (Cal State East Bay), Ahmed Banafa (Cal State East Bay), Ching-Chen Lee (CSU Hayward), and Daji Qaio (Iowa State). Thanks also to the many people who provided detailed technical reviews of a single chapter: Dave Tweed, Bruce Lane, Denis McMahon, Charles Freund, Paul Hoadley, Stephen Ma, Sandeep Subramaniam, Dragan Cvetkovic, Fernando Gont, Neil Giles, Rajesh Thundil, and Rick Jones. In addition, Larry Owens of California State University and Katia Obraczka of the University of Southern California provided some homework problems.Thanks also to the following contributors. Zornitza Prodanoff of the University of North Florida prepared the appendix on Sockets programming. Michael Harris of the University of South Florida is responsible for the Ethereal exercises and user’s guide. Lawrie Brown of the Australian Defence Force Academy of the University of New South Wales produc ed the PPT lecture slides. Finally, I would like to thank the many people responsible for the publication of the book, all of whom did their usual excellent job.This includes the staff at Prentice Hall, particularly my editor Tracy Dunkelberger, her assistants Christianna Lee and Carole Snyder, and production manager Rose Kernan. Also, Patricia M. Daly did the copy editing. CHAPTER READER’S AND INSTRUCTOR’S GUIDE 0. 1 0. 2 0. 3 0. 4 Outline of the Book Roadmap Internet and Web Resources Standards 0 1 2 CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDE â€Å"In the meanwhile, then,† demanded Li-loe, â€Å"relate to me the story to which reference has been made, thereby proving the truth of your assertion, and at the same time affording n entertainment of a somewhat exceptional kind. † â€Å"The shadows lengthen,† replied Kai Lung, â€Å"but as the narrative in question is of an inconspicuous span I will raise no barrier against your flatter ing request, especially as it indicates an awakening taste hitherto unexpected. † —Kai Lung’s Golden Hours, Earnest Bramah This book, with its accompanying Web site, covers a lot of material. Here we give the reader some basic background information. 0. 1 OUTLINE OF THE BOOK The book is organized into five parts: Part One. Overview: Provides an introduction to the range of topics covered in the book.This part includes a general overview of data communications and networking and a discussion of protocols, OSI, and the TCP/IP protocol suite. Part Two. Data Communications: Concerned primarily with the exchange of data between two directly connected devices. Within this restricted scope, the key aspects of transmission, interfacing, link control, and multiplexing are examined. Part Three. Wide Area Networks: Examines the internal mechanisms and user-network interfaces that have been developed to support voice, data, and multimedia communications over long-distance ne tworks.The traditional technologies of packet switching and circuit switching are examined, as well as the more recent ATM and wireless WANs. Separate chapters are devoted to routing and congestion control issues that are relevant both to switched data networks and to the Internet. Part Four. Local Area Networks: Explores the technologies and architectures that have been developed for networking over shorter distances. The transmission media, topologies, and medium access control protocols that are the key ingredients of a LAN design are explored and specific standardized LAN systems examined.Part Five. Networking Protocols: Explores both the architectural principles and the mechanisms required for the exchange of data among computers, workstations, servers, and other data processing devices. Much of the material in this part relates to the TCP/IP protocol suite. Part Six. Internet Applications: Looks at a range of applications that operate over the Internet. A more detailed, chapte r-by-chapter summary of each part appears at the beginning of that part. 0. 2 / ROADMAP 3 0. 2 ROADMAP Course EmphasisThe material in this book is organized into four broad categories: data transmission and communication; communications networks; network protocols; and applications and security. The chapters and parts of the book are sufficiently modular to provide a great deal of flexibility in the design of courses. The following are suggestions for three different course designs: †¢ Fundamentals of Data Communications: Parts One (overview) and Two (data communications) and Chapters 10 and 11 (circuit switching, packet switching, and ATM). Communications Networks: If the student has a basic background in data communications, then this course could cover Parts One (overview), Three (WAN), and Four (LAN). †¢ Computer Networks: If the student has a basic background in data communications, then this course could cover Part One (overview), Chapters 6 and 7 (data communication techniques and data link control), Part Five (protocols), and part or all of Part Six (applications). In addition, a more streamlined course that covers the entire book is possible by eliminating certain chapters that are not essential on a first reading.Chapters that could be optional are Chapters 3 (data transmission) and 4 (transmission media), if the student has a basic understanding of these topics; Chapter 8 (multiplexing); Chapter 9 (spread spectrum); Chapters 12 through 14 (routing, congestion control, cellular networks); Chapter 18 (internetworking); and Chapter 21 (network security). Bottom-Up versus Top-Down The book is organized in a modular fashion. After reading Part One, the other parts can be read in a number of possible sequences.Figure 0. 1a shows the bottom-up approach provided by reading the book from front to back. With this approach, each part builds on the material in the previous part, so that it is always clear how a given layer of functionality is supporte d from below. There is more material than can be comfortably covered in a single semester, but the book’s organization makes it easy to eliminate some chapters and maintain the bottom-up sequence. Figure 0. 1b suggests one approach to a survey course.Some readers, and some instructors, are more comfortable with a top-down approach. After the background material (Part One), the reader continues at the application level and works down through the protocol layers. This has the advantage of immediately focusing on the most visible part of the material, the applications, and then seeing, progressively, how each layer is supported by the next layer down. Figure 0. 1c is an example of a comprehensive treatment and Figure 0. 1d is an example of a survey treatment. 4 CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDEPart One Overview Part Two Data Communications Part Three Wide Area Networks Part Four Local Area Networks Part Five Internet and Transport Protocols Part Six Inter net Applications (a) A bottom-up approach Part One Overview Chapter 18 The Internet Protocol Part Six Internet Applications Part Five TCP/IP Part Three WANs Part Four LANs Part Two Data Communications (c) A top-down approach (d) Another top-down approach (b) Another bottom-up approach Part One Overview Chapter 18 The Internet Protocol Part Six Internet Applications Part Five TCP/IP Part Three WANs (10, 12) Part Four LANs (15) Part One Overview (1, 2) Part Two Data Communications (3, 6, 7, 8) Part Three WANs (10, 12) Part Four LANs (15) Part Five TCP/IP (18, 20) Figure 0. 1 Suggested Reading Orders Finally, it is possible to select chapters to reflect specific teaching objectives by not sticking to a strict chapter ordering. We give two examples used in courses taught with the seventh edition.One course used the sequence Part One (Overview); Chapter 3 (Data Transmission); Chapter 6 (Digital Data Communications Techniques); Chapter 7 (Data Link Control); Chapter 15 (LAN Overview); Cha pter 16 (High-Speed LANs); Chapter 10 (Circuit and Packet Switching); Chapter 12 (Routing); Chapter 18 (Internet Protocols); and Chapter 19 (Internet Operation). The other course used the sequence Part One (Overview); Chapter 3 (Data Transmission); Chapter 4 (Guided and Wireless Transmission); Chapter 5 (Signal Encoding Techniques); Chapter 8 (Multiplexing); Chapter 15 (LAN 0. 3 / INTERNET AND WEB RESOURCES 5 Overview); Chapter 16 (High-Speed LANs); Chapter 10 (Circuit and Packet Switching); Chapter 20 (Transport Protocols); Chapter 18 (Internet Protocols); and Chapter 19 (Internet Operation). 0. 3 INTERNET AND WEB RESOURCES There are a number of resources available on the Internet and the Web to support this book and to help one keep up with developments in this field.Web Sites for This Book A special Web page has been set up for this book at WilliamStallings. com/DCC/ DCC8e. html. See the two-page layout at the beginning of this book for a detailed description of that site. As soo n as any typos or other errors are discovered, an errata list for this book will be available at the Web site. Please report any errors that you spot. Errata sheets for my other books are at WilliamStallings. com. I also maintain the Computer Science Student Resource Site, at WilliamStallings. com/StudentSupport. html. The purpose of this site is to provide documents, information, and links for computer science students and professionals.Links and documents are organized into four categories: †¢ Math: Includes a basic math refresher, a queuing analysis primer, a number system primer, and links to numerous math sites †¢ How-to: Advice and guidance for solving homework problems, writing technical reports, and preparing technical presentations †¢ Research resources: Links to important collections of papers, technical reports, and bibliographies †¢ Miscellaneous: A variety of useful documents and links Other Web Sites There are numerous Web sites that provide informa tion related to the topics of this book. In subsequent chapters, pointers to specific Web sites can be found in the Recommended Reading and Web Sites section. Because the addresses for Web sites tend to change frequently, I have not included URLs in the book. For all of the Web sites listed in the book, the appropriate link can be found at this book’s Web site. Other links not mentioned in this book will be added to the Web site over time.The following are Web sites of general interest related to data and computer communications: †¢ Network World: Information and links to resources about data communications and networking. †¢ IETF: Maintains archives that relate to the Internet and IETF activities. Includes keyword-indexed library of RFCs and draft documents as well as many other documents related to the Internet and related protocols. 6 CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDE †¢ Vendors: Links to thousands of hardware and software vendors who currently have Web sites, as well as a list of thousands of computer and networking companies in a phone directory. †¢ IEEE Communications Society: Good way to keep up on conferences, publications, and so on. ACM Special Interest Group on Communications (SIGCOMM): Good way to keep up on conferences, publications, and so on. †¢ International Telecommunications Union: Contains a listing of ITU-T recommendations, plus information on obtaining ITU-T documents in hard copy or on DVD. †¢ International Organization for Standardization: Contains a listing of ISO standards, plus information on obtaining ISO documents in hard copy or on CD-ROM. †¢ CommWeb: Links to vendors, tutorials, and other useful information. †¢ CommsDesign: Lot of useful articles, tutorials, and product information. A bit hard to navigate, but worthwhile. USENET Newsgroups A number of USENET newsgroups are devoted to some aspect of data communications, networks, and protocols.As with virtually all USENET groups, there is a high noise-to-signal ratio, but it is worth experimenting to see if any meet your needs. The most relevant are as follows: †¢ comp. dcom. lans, comp. dcom. lans. misc: General discussions of LANs †¢ comp. dcom. lans. ethernet: Covers Ethernet, Ethernet-like systems, and the IEEE 802. 3 CSMA/CD standards †¢ comp. std. wireless: General discussion of wireless networks, including wireless LANs †¢ comp. security. misc: Computer security and encryption †¢ comp. dcom. cell-relay: Covers ATM and ATM LANs †¢ comp. dcom. frame-relay: Covers frame relay networks †¢ comp. dcom. net-management: Discussion of network management applications, protocols, and standards †¢ comp. rotocols. tcp-ip: The TCP/IP protocol suite 0. 4 STANDARDS It has long been accepted in the telecommunications industry that standards are required to govern the physical, electrical, and procedural characteristics of communication equipment. In the past, this view has not been embraced by the computer industry. Whereas communication equipment vendors recognize that their 0. 4 / STANDARDS 7 equipment will generally interface to and communicate with other vendors’ equipment, computer vendors have traditionally attempted to monopolize their customers. The proliferation of computers and distributed processing has made that an untenable position.Computers from different vendors must communicate with each other and, with the ongoing evolution of protocol standards, customers will no longer accept special-purpose protocol conversion software development. The result is that standards now permeate all of the areas of technology discussed in this book. There are a number of advantages and disadvantages to the standards-making process. We list here the most striking ones. The principal advantages of standards are as follows: †¢ A standard assures that there will be a large market for a particular piece of equipment or software. Thi s encourages mass production and, in some cases, the use of large-scale-integration (LSI) or very-large-scale-integration (VLSI) techniques, resulting in lower costs. A standard allows products from multiple vendors to communicate, giving the purchaser more flexibility in equipment selection and use. The principal disadvantages are as follows: †¢ A standard tends to freeze the technology. By the time a standard is developed, subjected to review and compromise, and promulgated, more efficient techniques are possible. †¢ There are multiple standards for the same thing. This is not a disadvantage of standards per se, but of the current way things are done. Fortunately, in recent years the various standards-making organizations have begun to cooperate more closely. Nevertheless, there are still areas where multiple conflicting standards exist.Throughout this book, we describe the most important standards in use or being developed for various aspects of data and computer commun ications. Various organizations have been involved in the development or promotion of these standards. The following are the most important (in the current context) of these organizations: †¢ Internet Society: The Internet SOCiety (ISOC) is a professional membership society with more than 150 organizational and 6000 individual members in over 100 countries. It provides leadership in addressing issues that confront the future of the Internet and is the organization home for the groups responsible for Internet infrastructure standards, including the Internet Engineering Task Force (IETF) and the Internet Architecture Board (IAB).All of the RFCs and Internet standards are developed through these organizations. †¢ IEEE 802: The IEEE (Institute of Electrical and Electronics Engineers) 802 LAN/MAN Standards Committee develops local area network standards and metropolitan area network standards. The most widely used standards are for the Ethernet family, wireless LAN, bridging, a nd virtual bridged LANs. An individual working group provides the focus for each area. 8 CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDE †¢ ITU-T: The International Telecommunication Union (ITU) is an international organization within the United Nations System where governments and the private sector coordinate global telecom networks and services.The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors of the ITU. ITU-T’s mission is the production of standards covering all fields of telecommunications. †¢ ATM Forum: The ATM Forum is an international nonprofit organization formed with the objective of accelerating the use of ATM (asynchronous transfer mode) products and services through a rapid convergence of interoperability specifications. In addition, the Forum promotes industry cooperation and awareness. †¢ ISO: The International Organization for Standardization (ISO)1 is a worldwide federation of national standards bod ies from more than 140 countries, one from each country.ISO is a nongovernmental organization that promotes the development of standardization and related activities with a view to facilitating the international exchange of goods and services, and to developing cooperation in the spheres of intellectual, scientific, technological, and economic activity. ISO’s work results in international agreements that are published as International Standards. A more detailed discussion of these organizations is contained in Appendix D. 1 ISO is not an acronym (in which case it would be IOS), but a word, derived from the Greek, meaning equal. PART ONE Overview The purpose of Part One is to provide a background and context for the remainder of this book. The broad range of topics that are encompassed in the field of data and computer communications is introduced, and the fundamental concepts of protocols and protocol architectures are examined.ROAD MAP FOR PART ONE Chapter 1 Data Communicati ons, Data Networks, and The Internet Chapter 1 provides an overview of Parts Two through Four of the book, giving the â€Å"big picture. † In essence, the book deals with four topics: data communications over a transmission link; wide area networks; local area networks; and protocols and the TCP/IP protocol architecture. Chapter 1 provides a preview of the first three of these topics. Chapter 2 Protocol Architecture, TCP/IP, and Internet-Based Applications Chapter 2 discusses the concept protocol architectures. This chapter can be read immediately following Chapter 1 or deferred until the beginning of Part Three, Four, or Five.After a general introduction, the chapter deals with the two most important protocol architectures: the Open Systems Interconnection (OSI) model and TCP/IP. Although the OSI model is often used as the framework for discourse in this area, it is the TCP/IP protocol suite that is the basis for most commercially available interoperable products and that is the focus of Parts Five and Six of this book. 9 CHAPTER DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET 1. 1 1. 2 1. 3 1. 4 1. 5 1. 6 Data Communications and Networking for Today’s Enterprise A Communications Model Data Communications Networks The Internet An Example Configuration 1 10 The fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point. The Mathematical Theory of Communication, Claude Shannon KEY POINTS †¢ The scope of this book is broad, covering three general areas: data communications, networking, and protocols; the first two are introduced in this chapter. Data communications deals with the transmission of signals in a reliable and efficient manner. Topics covered include signal transmission, transmission media, signal encoding, interfacing, data link control, and multiplexing. Networking deals with the technology and architecture of the communications networks used to interconnect communicating devices. This field is generally divided into the topics of local area networks (LANs) and wide area networks (WANs). †¢ †¢The 1970s and 1980s saw a merger of the fields of computer science and data communications that profoundly changed the technology, products, and companies of the now combined computer-communications industry. The computercommunications revolution has produced several remarkable facts: †¢ There is no fundamental difference between data processing (computers) and data communications (transmission and switching equipment). †¢ There are no fundamental differences among data, voice, and video communications. †¢ The distinction among single-processor computer, multiprocessor computer, local network, metropolitan network, and long-haul network has blurred.One effect of these trends has been a growing overlap of the computer and communications industries, from component fabrication to system integration. Another resul t is the development of integrated systems that transmit and process all types of data and information. Both the technology and the technical standards organizations are driving toward integrated public systems that make virtually all data and information sources around the world easily and uniformly accessible. This book aims to provide a unified view of the broad field of data and computer communications. The organization of the book reflects an attempt to break this massive subject into comprehensible parts and to build, piece by piece, a survey of the state of the art.This introductory chapter begins with a general model of communications. Then a brief discussion introduces each of the Parts Two through Four of this book. Chapter 2 provides an overview to Parts Five and Six 11 12 CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET 1. 1 DATA COMMUNICATIONS AND NETWORKING FOR TODAY’S ENTERPRISE Effective and efficient data communication and networking facilitie s are vital to any enterprise. In this section, we first look at trends that are increasing the challenge for the business manager in planning and managing such facilities. Then we look specifically at the requirement for ever-greater transmission speeds and network capacity. TrendsThree different forces have consistently driven the architecture and evolution of data communications and networking facilities: traffic growth, development of new services, and advances in technology. Communication traffic, both local (within a building or building complex) and long distance, both voice and data, has been growing at a high and steady rate for decades. The increasing emphasis on office automation, remote access, online transactions, and other productivity measures means that this trend is likely to continue. Thus, managers are constantly struggling to maximize capacity and minimize transmission costs. As businesses rely more and more on information technology, the range of services expand s. This increases the demand for high-capacity networking and transmission facilities.In turn, the continuing growth in high-speed network offerings with the continuing drop in prices encourages the expansion of services. Thus, growth in services and growth in traffic capacity go hand in hand. Figure 1. 1 gives some examples of information-based services and the data rates needed to support them [ELSA02]. Finally, trends in technology enable the provision of increasing traffic capacity and the support of a wide range of services. Four technology trends are particularly notable: 1. The trend toward faster and cheaper, both in computing and communications, continues. In terms of computing, this means more powerful computers and clusters of computers capable of supporting more demanding applications, such as multimedia applications.In terms of communications, the increasing use of optical fiber has brought transmission prices down and greatly increased capacity. For example, for long-d istance telecommunication and data network links, recent offerings of dense wavelength division multiplexing (DWDM) enable capacities of many terabits per second. For local area networks (LANs) many enterprises now have Gigabit Ethernet backbone networks and some are beginning to deploy 10-Gbps Ethernet. 2. Both voice-oriented telecommunications networks, such as the public switched telephone network (PSTN), and data networks, including the Internet, are more â€Å"intelligent† than ever. Two areas of intelligence are noteworthy.First, today’s networks can offer differing levels of quality of service (QoS), which include specifications for maximum delay, minimum throughput, and so on. Second, today’s networks provide a variety of customizable services in the areas of network management and security. 1. 1 / DATA COMMUNICATIONS AND NETWORKING FOR TODAY’S ENTERPRISE Speed (kbps) Transaction processing Messaging/text apps Voice Location services Still image t ransfers Internet/VPN access Database access Enhanced Web surfing Low-quality video Hifi audio Large file transfer Moderate video Interactive entertainment High-quality video Performance: Poor Adequate Good 9. 6 14. 4 28 64 144 384 2000 13 VPN: virtual private network Figure 1. 1 Services versus Throughput Rates 3.The Internet, the Web, and associated applications have emerged as dominant features of both the business and personal world, opening up many opportunities and challenges for managers. In addition to exploiting the Internet and the Web to reach customers, suppliers, and partners, enterprises have formed intranets and extranets1 to isolate their proprietary information free from unwanted access. 4. There has been a trend toward ever-increasing mobility for decades, liberating workers from the confines of the physical enterprise. Innovations include voice mail, remote data access, pagers, fax, e-mail, cordless phones, cell phones and cellular networks, and Internet portals.T he result is the ability of employees to take their business context with them as they move about. We are now seeing the growth of high-speed wireless access, which further enhances the ability to use enterprise information resources and services anywhere. 1 Briefly, an intranet uses Internet and Web technology in an isolated facility internal to an enterprise; an extranet extends a company’s intranet out onto the Internet to allow selected customers, suppliers, and mobile workers to access the company’s private data and applications. 14 CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET Data Transmission and Network Capacity RequirementsMomentous changes in the way organizations do business and process information have been driven by changes in networking technology and at the same time have driven those changes. It is hard to separate chicken and egg in this field. Similarly, the use of the Internet by both businesses and individuals reflects this cycli c dependency: the availability of new image-based services on the Internet (i. e. , the Web) has resulted in an increase in the total number of users and the traffic volume generated by each user. This, in turn, has resulted in a need to increase the speed and efficiency of the Internet. On the other hand, it is only such increased speed that makes the use of Web-based applications palatable to the end user.In this section, we survey some of the end-user factors that fit into this equation. We begin with the need for high-speed LANs in the business environment, because this need has appeared first and has forced the pace of networking development. Then we look at business WAN requirements. Finally we offer a few words about the effect of changes in commercial electronics on network requirements. The Emergence of High-Speed LANs Personal computers and microcomputer workstations began to achieve widespread acceptance in business computing in the early 1980s and have now achieved virtu ally the status of the telephone: an essential tool for office workers.Until relatively recently, office LANs provided basic connectivity services—connecting personal computers and terminals to mainframes and midrange systems that ran corporate applications, and providing workgroup connectivity at the departmental or divisional level. In both cases, traffic patterns were relatively light, with an emphasis on file transfer and electronic mail. The LANs that were available for this type of workload, primarily Ethernet and token ring, are well suited to this environment. In the 1990s, two significant trends altered the role of the personal computer and therefore the requirements on the LAN: 1. The speed and computing power of personal computers continued to enjoy explosive growth. These more powerful platforms support graphics-intensive applications and ever more elaborate graphical user interfaces to the operating system. . MIS (management information systems) organizations hav e recognized the LAN as a viable and essential computing platform, resulting in the focus on network computing. This trend began with client/server computing, which has become a dominant architecture in the business environment and the more recent Webfocused intranet trend. Both of these approaches involve the frequent transfer of potentially large volumes of data in a transaction-oriented environment. The effect of these trends has been to increase the volume of data to be handled over LANs and, because applications are more interactive, to reduce the acceptable delay on data transfers.The earlier generation of 10-Mbps Ethernets and 16-Mbps token rings was simply not up to the job of supporting these requirements. The following are examples of requirements that call for higher-speed LANs: †¢ Centralized server farms: In many applications, there is a need for user, or client, systems to be able to draw huge amounts of data from multiple centralized servers, called server farms. An example is a color publishing operation, in 1. 1 / DATA COMMUNICATIONS AND NETWORKING FOR TODAY’S ENTERPRISE 15 which servers typically contain tens of gigabytes of image data that must be downloaded to imaging workstations. As the performance of the servers themselves has increased, the bottleneck has shifted to the network. Power workgroups: These groups typically consist of a small number of cooperating users who need to draw massive data files across the network. Examples are a software development group that runs tests on a new software version, or a computer-aided design (CAD) company that regularly runs simulations of new designs. In such cases, large amounts of data are distributed to several workstations, processed, and updated at very high speed for multiple iterations. †¢ High-speed local backbone: As processing demand grows, LANs proliferate at a site, and high-speed interconnection is necessary. Corporate Wide Area Networking Needs As recently as the ear ly 1990s, there was an emphasis in many organizations on a centralized data processing model.In a typical environment, there might be significant computing facilities at a few regional offices, consisting of mainframes or well-equipped midrange systems. These centralized facilities could handle most corporate applications, including basic finance, accounting, and personnel programs, as well as many of the business-specific applications. Smaller, outlying offices (e. g. , a bank branch) could be equipped with terminals or basic personal computers linked to one of the regional centers in a transaction-oriented environment. This model began to change in the early 1990s, and the change accelerated through the mid-1990s. Many organizations have dispersed their employees into multiple smaller offices.There is a growing use of telecommuting. Most significant, the nature of the application structure has changed. First client/server computing and, more recently, intranet computing have funda mentally restructured the organizational data processing environment. There is now much more reliance on personal computers, workstations, and servers and much less use of centralized mainframe and midrange systems. Furthermore, the virtually universal deployment of graphical user interfaces to the desktop enables the end user to exploit graphic applications, multimedia, and other data-intensive applications. In addition, most organizations require access to the Internet.When a few clicks of the mouse can trigger huge volumes of data, traffic patterns have become more unpredictable while the average load has risen. All of these trends means that more data must be transported off premises and into the wide area. It has long been accepted that in the typical business environment, about 80% of the traffic remains local and about 20% traverses wide area links. But this rule no longer applies to most companies, with a greater percentage of the traffic going into the WAN environment [COHE 96]. This traffic flow shift places a greater burden on LAN backbones and, of course, on the WAN facilities used by a corporation.Thus, just as in the local area, changes in corporate data traffic patterns are driving the creation of high-speed WANs. Digital Electronics The rapid conversion of consumer electronics to digital technology is having an impact on both the Internet and corporate intranets. As these new gadgets come into view and proliferate, they dramatically increase the amount of image and video traffic carried by networks. Two noteworthy examples of this trend are digital versatile disks (DVDs) and digital still cameras. With the capacious DVD, the electronics industry has at last 16 CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET found an acceptable replacement for the analog VHS videotape.The DVD has replaced the videotape used in videocassette recorders (VCRs) and replaced the CD-ROM in personal computers and servers. The DVD takes video into the di gital age. It delivers movies with picture quality that outshines laser disks, and it can be randomly accessed like audio CDs, which DVD machines can also play. Vast volumes of data can be crammed onto the disk, currently seven times as much as a CDROM. With DVD’s huge storage capacity and vivid quality, PC games have become more realistic and educational software incorporates more video. Following in the wake of these developments is a new crest of traffic over the Internet and corporate intranets, as this material is incorporated into Web sites. A related product development is the digital camcorder.This product has made it easier for individuals and companies to make digital video files to be placed on corporate and Internet Web sites, again adding to the traffic burden. 1. 2 A COMMUNICATIONS MODEL This section introduces a simple model of communications, illustrated by the block diagram in Figure 1. 2a. The fundamental purpose of a communications system is the exchange of data between two parties. Figure 1. 2b presents one particular example, which is communication between a workstation and a server over a public telephone network. Another example is the exchange of voice signals between two telephones over the same network. The key elements of the model are as follows: †¢ Source.This device generates the data to be transmitted; examples are telephones and personal computers. Source system Destination system Source Transmitter Transmission System (a) General block diagram Receiver Destination Workstation Modem Public telephone network (b) Example Modem Server Figure 1. 2 Simplified Communications Model 1. 2 / A COMMUNICATIONS MODEL 17 †¢ Transmitter: Usually, the data generated by a source system are not transmitted directly in the form in which they were generated. Rather, a transmitter transforms and encodes the information in such a way as to produce electromagnetic signals that can be transmitted across some sort of transmission system .For example, a modem takes a digital bit stream from an attached device such as a personal computer and transforms that bit stream into an analog signal that can be handled by the telephone network. †¢ Transmission system: This can be a single transmission line or a complex network connecting source and destination. †¢ Receiver: The receiver accepts the signal from the transmission system and converts it into a form that can be handled by the destination device. For example, a modem will accept an analog signal coming from a network or transmission line and convert it into a digital bit stream. †¢ Destination: Takes the incoming data from the receiver.This simple narrative conceals a wealth of technical complexity. To get some idea of the scope of this complexity, Table 1. 1 lists some of the key tasks that must be performed in a data communications system. The list is somewhat arbitrary: Elements could be added; items on the list could be merged; and some items repr esent several tasks that are performed at different â€Å"levels† of the system. However, the list as it stands is suggestive of the scope of this book. The first item, transmission system utilization, refers to the need to make efficient use of transmission facilities that are typically shared among a number of communicating devices.Various techniques (referred to as multiplexing) are used to allocate the total capacity of a transmission medium among a number of users. Congestion control techniques may be required to assure that the system is not overwhelmed by excessive demand for transmission services. To communicate, a device must interface with the transmission system. All the forms of communication discussed in this book depend on the use of electromagnetic signals propagated over a transmission medium. Thus, once an interface is established, signal generation is required for communication. The properties of the signal, such as form and intensity, must be such that the signal is (1) capable of being propagated through the transmission system, and (2) interpretable as data at the receiver.Not only must the signals be generated to conform to the requirements of the transmission system and receiver, but also there must be some form of synchronization Table 1. 1 Communications Tasks Transmission system utilization Interfacing Signal generation Synchronization Exchange management Error detection and correction Flow control Addressing Routing Recovery Message formatting Security Network management 18 CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET between transmitter and receiver. The receiver must be able to determine when a signal begins to arrive and when it ends. It must also know the duration of each signal element.Beyond the basic matter of deciding on the nature and timing of signals, there is a variety of requirements for communication between two parties that might be collected under the term exchange management. If data are to be exchanged in both directions over a period of time, the two parties must cooperate. For example, for two parties to engage in a telephone conversation, one party must dial the number of the other, causing signals to be generated that result in the ringing of the called phone. The called party completes a connection by lifting the receiver. For data processing devices, more will be needed than simply establishing a connection; certain conventions must be decided on.These conventions may include whether both devices may transmit simultaneously or must take turns, the amount of data to be sent at one time, the format of the data, and what to do if certain contingencies such as an error arise. The next two items might have been included under exchange managem