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Second, consider the network layer from the point of view of the upper layer using it. The upper layer sees itself attached to an access point into a network supporting communication with multiple devices. The layer for assuring that data sent across a network is delivered to one of a number of other end systems is the network layer.

This argues for inclusion of a network layer. The lower sublayer is concerned with medium access control MAC , assuring that only one end system at a time transmits; the MAC sublayer is also responsible for addressing other end systems across the LAN. LLC performs traditional link control functions.

This would violate the principle of separation of layers. It breaks that PDU into fragments and reassembles them in the proper order. Each N-level PDU must retain its own header, for the same reason given in a. Three network packets are needed. These functions would not normally be performed by protocols above the transport layer. Thus UDP provides a useful, though limited, service. TCP does guarantee delivery. However, the technique that is used is a timeout. If the source does not receive an acknowledgment to data within a given period of time, the source retransmits.

The header in TCP is of variable length. A resends packet k, which B acknowledges. From now on, 2 copies of every data packet and ACK will be sent. The maximum throughput is therefore bytes divided by the round- trip time. Source: [STEV94]. This means that when the "netascii" transfer mode is employed, the file sizes of the local and the remote file may differ, without any implication of errors in the data transfer. This means that a given text file will usually occupy more space in a Windows host than in a UNIX system. Suppose host A receives an error packet from host B, and responds it by sending an error packet back to host B.

This packet could trigger another error packet from host B, which would again trigger an error packet at host A.

Data and Computer Communications

Thus, error messages would bounce from one host to the other, indefinitely, congesting the network and consuming the resources of the participating systems. For example, if both hosts are on the same local area network, a 5- second timeout is more than enough. On the other hand, if the transfer is taking place over a long delay satellite link, then a 5-second timeout might be too short, and could trigger unnecessary retransmissions. On the other hand, using a fixed value for the retransmission timer keeps the TFTP implementation simple, which is the objective the designers of TFTP had in mind.

Thus, reliability depends on the service provided by the underlying transport protocol UDP. While the UDP includes a checksum for detecting errors, its use is optional. Therefore, if UDP checksums are not enabled, data could be corrupted without being detected by the destination host. The internet protocol can be defined as a separate layer.

The functions performed by IP are clearly distinct from those performed at a network layer and those performed at a transport layer, so this would make good sense. The session and transport layer both are involved in providing an end-to-end service to the OSI user, and could easily be combined. If two devices transmit at the same time, their signals will be on the medium at the same time, interfering with each other; i.

Data and Computer Communications- Lecture #1- 9\1\2016

See discussion in Section Source: [MOSH89] 3. A real signal therefore is bandlimited, with frequencies above a certain point absent. However, most of the information is in the lower frequencies. This is not a problem if it is remembered that the object of the transmission is to send signals that represent binary 1s and 0s. Even though there will be some distortion because of the loss of higher frequencies, the shape of the original pulse is known by the specifications for the transmission system.

Thus, the receiver will usually be able to distinguish a binary 0 from a binary 1. Several shift lock codes were defined in various versions of TTS shift, supershift, unshift. These codes change the meaning of all codes that follow until a new shift lock code appears. The actual number is less, since some codes, such as space, are "don't-cares" with respect to shift locks. Retaining the vertical resolution of lines, each horizontal line occupies Due to the fact there is a distortion level as well as other potentially detrimental impacts to the rated capacity, the actual maximum will be somewhat degraded from the theoretical maximum.

A discussion of these relevant impacts should be included and a qualitative value discussed. Shannon addressed the question of what signaling rate can be achieved over a channel with a given bandwidth, a given signal power, and in the presence of noise. For example, it reduces crosstalk between wire pairs bundled into a cable.

Shielded twisted pair STP surrounds the wire with a metallic braid or sheathing that reduces interference. The glass or plastic itself consists of two concentric columns. The inner column called the core has a higher index of refraction than the outer column called the cladding.

It is affected by rainfall, however, especially above 10 GHz. It is also requires line of sight and is subject to interference from other microwave transmission, which can be intense in some places. In practice, there will be some beam spread. Nevertheless, it produces a highly focused, directional beam. The edge in effect become a source and waves radiate in different directions from the edge, allowing a beam to bend around an obstacle. If the size of an obstacle is on the order of the wavelength of the signal or less, scattering occurs.

An incoming signal is scattered into several weaker outgoing signals in unpredictable directions. From Figure 4. If the outer conductor of a coaxial cable is everywhere held at ground potential, no external disturbance can reach the inner, signal-carrying, conductor. Half of that is 5, km which is comparable to the east-to-west dimension of the continental U. While an antenna this size is impractical, the U. Defense Department has considered using large parts of Wisconsin and Michigan to make an antenna many kilometers in diameter. The received power will increase by a factor of 4 4. Thus, the higher the frequency, the higher power is needed to obtain a given SNR.

Power is much more readily available at earth stations than at satellites. Therefore, it makes more sense to put the higher power requirements on the earth stations than on the satellites. Using Equation 4. The available received signal power is 20 — In addition, lack of a direct-current dc component means that ac coupling via transformer is possible. The magnitude of the effects of signal distortion and interference depend on the spectral properties of the transmitted signal.

Clocking: Encoding can be used to synchronize the transmitter and receiver. Error detection: It is useful to have some error detection capability built into the physical signaling encoding scheme. Signal interference and noise immunity: Certain codes exhibit superior performance in the presence of noise. Cost and complexity: The higher the signaling rate to achieve a given data rate, the greater the cost.

Some codes require a signaling rate that is in fact greater than the actual data rate. The data themselves are encoded as the presence or absence of a signal transition at the beginning of the bit time. A transition low to high or high to low at the beginning of a bit time denotes a binary 1 for that bit time; no transition indicates a binary 0.

The binary 1 pulses must alternate in polarity. For pseudoternary, a binary 1 a is represented by the absence of a line signal, and a binary 0 by alternating positive and negative pulses. In the Manchester code, there is a transition at the middle of each bit period; a low-to-high transition represents a 1, and a high-to-low transition represents a 0.

In differential Manchester, the midbit transition is used only to provide clocking. The encoding of a 0 is represented by the presence of a transition at the beginning of a bit period, and a 1 is represented by the absence of a transition at the beginning of a bit period. The filling sequence must be recognized by the receiver and replaced with the original data sequence. The filling sequence is the same length as the original sequence, so there is no data rate penalty. This approach is susceptible to sudden gain changes and is rather inefficient. For PM, the phase is proportional to the modulating signal.

For FM, the derivative of the phase is proportional to the modulating signal. Map this level as indicated by the definition for 1 and 0 for each of the other codes. Second, under worst case, E-NRZ provides a minimum of one transition for every 14 bits, reducing the synchronization problem.

Third, the parity bit provides an error check. The disadvantages of E-NRZ are added complexity and the overhead of the extra parity bit. Bipolar-AMI 5. For AMI, positive and negative pulses are used alternately for binary 1. The pulse in position 1 represents the third binary 1 in the data stream and should have a positive value. First consider NRZ-L. For the remaining codes, one must first determine the average number of pulses per bit. For example, for Biphase-M, there is an average of 1.

These higher components cause the signal to change more rapidly over time. Hence, DM will suffer from a high level of slope overload noise. The demodulator portion of a modem expects to receive a very specific type of waveform e. Thus, it would not function as the coder portion of a codec. The case against using a codec in place of a modem is less easily explained, but the following intuitive argument is offered.

If the decoder portion of a codec is used in place of the modulator portion of a modem, it must accept an arbitrary bit pattern, interpret groups of bits as a sample, and produce an analog output. Some very wide value swings are to be expected, resulting in a strange-looking waveform. Given the effects of noise and attenuation, the digital output produced at the receiving end by the coder portion of the codec will probably contain many errors.

The actual step size, in volts, is: 0. Thus the actual maximum quantized voltage is: 0. The normalized step size is 2—8. The maximum error that can occur is one-half the step size. A stop binary one follows the character. One side transmitter or receiver pulses the line regularly with one short pulse per bit time. The other side uses these regular pulses as a clock.

Another alternative is to embed the clocking information in the data signal.

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For digital signals, this can be accomplished with Manchester or differential Manchester encoding. For analog signals, a number of techniques can be used; for example, the carrier frequency itself can be used to synchronize the receiver based on the phase of the carrier. That is, it provides more information that can be used to detect errors. You could design a code in which all codewords are at least a distance of 3 from all other codewords, allowing all single-bit errors to be corrected. Suppose that some but not all codewords in this code are at least a distance of 5 from all other codewords.

Then for those particular codewords, but not the others, a double- bit error could be corrected. For 10, characters, there are 20, extra bits. The file takes 10 frames or additional bits. Ten times as many extra bits and ten times as long for both. There are 7 data bits, 1 start bit, 1. Write down a few dozen characters. Since some 1's will intervene before you find that zero, you will have moved the starting point of the framing process.

Eventually, you will achieve proper framing. The stop bit is needed so that the start bit can be recognized as such. The start bit is the synchronization event, but it must be recognizable. The start bit is always a 0, and the stop bit is always a 1, which is also the idle state of the line. When a start bit occurs, it is guaranteed to be different from the current state of the line.

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Then a frame is 12T long. Let a clock period be T'. The last bit bit 12 is sampled at For a fast running clock, the condition to satisfy is T There are more bits that can be in error since the parity bit is now included. The parity bit may be in error when there are no errors in the corresponding data bits. Therefore, the inclusion of a parity bit with each character would change the probability of receiving a correct message. The modulo 2 scheme is easy to implement in circuitry. It also yields a remainder one bit smaller than binary arithmetic. Each 1 bit will merge with a 1 bit exclusive-or to produce a 0; each 0 bit will merge with a 0 bit to produce a zero.

The CRC bits are The string is sent. The errors are detected. The errors are not detected. The HDLC standard provides the following explanation.


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The addition of XK L X corresponds to a value of all ones. This addition protects against the obliteration of leading flags, which may be non-detectable if the initial remainder is zero.

The addition of L X to R X ensures that the received, error- free message will result in a unique, non-zero remainder at the receiver. The non-zero remainder protects against the potential non-detectability of the obliteration of trailing flags. The implementation is the same as that shown in Solution 6. At both transmitter and receiver, the initial content of the register is preset to all ones. The final remainder, if there are no errors, will be For a codeword w to be decoded as another codeword w', the received sequence must be at least as close to w' as to w.

Therefore all errors involving t or fewer digits are correctable. Data transmitted by one side are received by the other. In order to operate a synchronous data link without a modem, clock signals need to be supplied. The Transmitter and Receive Timing leads are cross-connected for this purpose. Flow control: The sending station must not send frames at a rate faster than the receiving station can absorb them.

Error control: Bit errors introduced by the transmission system should be corrected. Addressing: On a multipoint line, such as a local area network LAN , the identity of the two stations involved in a transmission must be specified. Control and data on same link: The receiver must be able to distinguish control information from the data being transmitted. Link management: The initiation, maintenance, and termination of a sustained data exchange require a fair amount of coordination and cooperation among stations.

Procedures for the management of this exchange are required. With smaller frames, errors are detected sooner, and a smaller amount of data needs to be retransmitted. The window changes dynamically to allow additional packets to be sent. The sliding window flow control technique can send multiple frames before waiting for an acknowledgment. Efficiency can be greatly improved by allowing multiple frames to be in transit at the same time. A station retransmits on receipt of a duplicate acknowledgment or as a result of a timeout.

When an error is detected, the frame in question is retransmitted, as well as all subsequent frames that have been previously transmitted. Selective-reject ARQ. Based on sliding-window flow control. When an error is detected, only the frame in question is retransmitted. Frames issued by the primary are called commands. Secondary station: Operates under the control of the primary station. Frames issued by a secondary are called responses. The primary maintains a separate logical link with each secondary station on the line. Combined station: Combines the features of primary and secondary.

A combined station may issue both commands and responses. The primary may initiate data transfer to a secondary, but a secondary may only transmit data in response to a command from the primary. Asynchronous balanced mode ABM : Used with a balanced configuration. Either combined station may initiate transmission without receiving permission from the other combined station. Asynchronous response mode ARM : Used with an unbalanced configuration. The secondary may initiate transmission without explicit permission of the primary. The primary still retains responsibility for the line, including initialization, error recovery, and logical disconnection.

This is achieved by bit stuffing. Additionally, flow and error control data, using the ARQ mechanism, are piggybacked on an information frame. Supervisory frames S-frames provide the ARQ mechanism when piggybacking is not used. Unnumbered frames U-frames provide supplemental link control functions. Because only one frame can be sent at a time, and transmission must stop until an acknowledgment is received, there is little effect in increasing the size of the message if the frame size remains the same.

All that this would affect is connect and disconnect time. This would lower line efficiency, because the propagation time is unchanged but more acknowledgments would be needed. For a given message size, increasing the frame size decreases the number of frames. This is the reverse of b. Then, using Equation 7. Using Equation 7. The first frame takes 10 msec to transmit; the last bit of the first frame arrives at B 20 msec after it was transmitted, and therefore 30 msec after the frame transmission began.

It will take an additional 20 msec for B's acknowledgment to return to A. Thus, A can transmit 3 frames in 50 msec. B can transmit one frame to C at a time. The REJ improves efficiency by informing the sender of a bad frame as early as possible. Station A sends frames 0, 1, 2 to station B.

Station B receives all three frames and cumulatively acknowledges with RR 3. Because of a noise burst, the RR 3 is lost. A times out and retransmits frame 0. B has already advanced its receive window to accept frames 3, 0, 1, 2. Thus it assumes that frame 3 has been lost and that this is a new frame 0, which it accepts. The sender never knows that the frame was not received, unless the receiver times out and retransmits the SREJ.

This would contradict the intent of the SREJ frame or frames. However, for simplicity, bit stuffing is used on this field. When a flag is used as both an ending and starting flag that is, one 8-bit pattern serves to mark the end of one frame and the beginning of the next , then a single-bit error in that flag alters the bit pattern so that the receiver does not recognize the flag.

Accordingly, the received assumes that this is a single frame. If a bit error somewhere in a frame between its two flags results in the pattern , then this octet is recognized as a flag that delimits the end of one frame and the start of the next frame. Any discrepancies result in discarding the frame. Bit-stuffing at least eliminates the possibility of a long string of 1's. This is the number of the next frame that the secondary station expects to receive. The LAPB control field includes, as usual, a sequence number unique to that link.

The MLC field performs two functions. First, LAPB frames sent out over different links may arrive in a different order from that in which they were first constructed by the sending MLP. Second, if repeated attempts to transmit a frame over one link fails, the DTE or DCE will send the frame over one or more other links. The MLP sequence number is needed for duplicate detection in this case. In essence, a transmitter must subtract the echo of its own transmission from the incoming signal to recover the signal sent by the other side. This explains the basic difference between the 1. A scheme such as depicted in Figure 8.

Each Hz signal can be sampled at a rate of 1 kHz. If 4-bit samples are used, then each signal requires 4 kbps, for a total data rate of 16 kbps. This scheme will work only if the line can support a data rate of 16 kbps in a bandwidth of Hz. The text clearly indicates where the animations should be used. Strong pedagogical support — The liberal use of figures and tables; glossary; list of acronyms; recommended reading list and Websites; and a bibliography provide students with convenient study tools. IRC — This Instructor Resource Center to accompany Data and Computer Communications, 9e is a password-protected area of the Prentice Hall online catalog that enables instructors to download presentation and assessment resources quickly.

The chapter organization has been changed somewhat so that now the material is organized into two Units, with Unit Two containing more advanced material and an expansion of the material related to the Internet. Beyond this organizational revision, the most noteworthy changes include the following:. With a focus on the most current technology and a convenient modular format, this best-selling text offers a clear and comprehensive survey of the entire data and computer communications field.

Emphasizing both the fundamental principles as well as the critical role of performance in driving protocol and network design, it explores in detail all the critical technical areas in data communications, wide-area networking, local area networking, and protocol design. Features A modular format — This structure allows instructors to easily design a course to meet their individual needs. This text features: Solutions Manual — Solutions to all the homework problems and review questions in the book.

Lecture PowerPoints — This chapter-by-chapter outline highlights the key points from each chapter using the text illustrations for an effective and visual presentation. Wireshark Exercise Set — Contains a set of homework assignments for use with the text. Includes the Ethereal Exercises, and Answers for instructors.