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In this Eighth Edition we have retained the objectives and approaches for teach- ing materials science M Computer System Architecture-Morris Mano third. A detailed set of course notes in PDF format suitable for student handout or . Appendix B Projects for Teaching Data and Computer Communications B NOTICE This manual contains solutions to all of the review questions and homework problems in Data and Computer Communications, Eighth Edition.
A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message. Station B receives all three frames and cumulatively acknowledges with RR 3. Signal interference and noise immunity: An industry consortium that promotes IPv6-related products. The MLP sequence number is needed for duplicate detection in this case. The road provides the physical path for delivery.
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. Then the maximum effective data rate R is: 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. 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 The resultant accuracy is 2 minutes in 1 year or: 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. We have: 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. 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.
The beginning and end of each frame must be recognizable. 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. 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. Based on stop-and-wait flow control. A station retransmits on receipt of a duplicate acknowledgment or as a result of a timeout.
Go-back-N ARQ: Based on sliding-window flow control. 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. When an error is detected, only the frame in question is retransmitted.
Responsible for controlling the operation of the link. 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. Used with an unbalanced configuration.
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: 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. Thus, the total number of frames transmitted without an ACK is: 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. Also from the standard: This would contradict the intent of the SREJ frame or frames.
From the beginning of the transmission of the first frame, the time to receive the acknowledgment of that frame is: 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 same frame format as for LAPB is used, with one additional field: 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. In time-division multiplexing, the entire channel is assigned to the source for a fraction of the time.
If there is spare bandwidth, then the incremental cost of the transmission can be negligible. The new station pair is simply added to an unused subchannel. If there is no unused subchannel it may be possible to redivide the existing subchannels creating more subchannels with less bandwidth. If, on the other hand, a new pair causes a complete new line to be added, then the incremental cost is large indeed. What the multiplexer receives from attached stations are several bit streams from different sources.
What the multiplexer sends over the multiplexed transmission line is a bit stream from the multiplexer. As long as the multiplexer sends what can be interpreted as a bit stream to the demultiplexer at the other end, the system will work. The multiplexer, for example, may use a self-clocking signal.
The incoming stream may be, on the other hand, encoded in some other format. The multiplexer receives and understands the incoming bits and sends out its equivalent set of multiplexed bits.
In synchronous TDM, using character interleaving, the character is placed in a time slot that is one character wide. The character is delimited by the bounds of the time slot, which are defined by the synchronous transmission scheme. Thus, no further delimiters are needed. When the character arrives at its destination, the start and stop bits can be added back if the receiver requires these.
TDM's focus is on the medium rather than the information that travels on the medium. Its services should be transparent to the user. It offers no flow or error control. These must be provided on an individual-channel basis by a link control protocol.
The actual bit pattern is If a receiver gets out of synchronization it can scan for this pattern and resynchronize. This pattern would be unlikely to occur in digital data. Analog sources cannot generate this pattern. It corresponds to a sine wave at 4, Hz and would be filtered out from a voice channel that is band limited.
One SYN character, followed by 20 bit terminal characters, followed by stuff bits. The available capacity is 1. This is a practical limit based on the performance characteristics of a statistical multiplexer. If the receiver is on the framing pattern no searching , the minimum reframe time is 12 frame times the algorithm takes 12 frames to decide it is "in frame". Hence it must search the maximum number of bits 55 to find it.
Each search takes 12T f. Assuming the system is random, the reframing is equally to start on any bit position. Hence on the average it starts in the middle or halfway between the best and worst cases. Therefore, the channel cost will be only one-fourth, since one channel rather than four is now needed. The same reasoning applies to termination charges. The present solution requires eight low speed modems four pairs of modems.
The new solution requires two higher-speed modems and two multiplexers. The reliability of the multiplexed solution may be somewhat less. The new system does not have the redundancy of the old system. A failure anywhere except at the terminals will cause a complete loss of the system. Each multiplexer also acts as a buffer. It can accept bits in asynchronous form, buffer them and transmit them in synchronous form, and vice versa. Assume a continuous stream of STDM frames.
If a delimiter is used, bit or character-stuffing may be needed. Only a recipient who knows the spreading code can recover the encoded information. A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message. Each user uses a different spreading code.
The receiver picks out one signal by matching the spreading code. Thus, to achieve the desired SNR, the signal must be spread so that 56 KHz is carried in very large bandwidths. Thus a far higher SNR is required without spread spectrum. Period of the PN sequence is 15 b. MFSK c. Same as for Problem 9. This is from the example in Section 6. We need three more sets of 8 frequencies. The second set can start at kHz, with 8 frequencies separated by 50 kHz each. The third set can start at kHz, and the fourth set at kHz.
The first generator yields the sequence: The second generator yields the sequence: Because of the patterns evident in the second half of the latter sequence, most people would consider it to be less random than the first sequence. See [KNUT98], page 13 for a discussion. As discussed in the answer to Problem 9. Now, if we use a linear congruential generator of the following form: Often, a and c are chosen to create a sequence of alternating even and odd integers.
The simulation depends on counting the number of pairs of integers whose greatest common divisor is 1. With truly random integers, one-fourth of the pairs should consist of two even integers, which of course have a gcd greater than 1.
This never occurs with sequences that alternate between even and odd integers. For a further discussion, see Danilowicz, R. Subscriber line: Two stations of different data rates can exchange packets because each connects to its node at its proper data rate. On a packet-switching network, packets are still accepted, but delivery delay increases. Thus, if a node has a number of packets queued for transmission, it can transmit the higher- priority packets first.
These packets will therefore experience less delay than lower-priority packets. In the virtual circuit approach, a preplanned route is established before any packets are sent.
Once the route is established, all the packets between a pair of communicating parties follow this same route through the network. As a smaller packet size is used, there is a more efficient "pipelining" effect, as shown in Figure However, if the packet size becomes too small, then the transmission is less efficient, as shown in Figure The major differences are that frame relay uses out-of-channel signaling while X.
In frame relay there is no "hop-by-hop" flow control or error control as there is in X. If a frame error is detected it is just dropped rather than being retransmitted. Similarly, on an end-to-end basis, there is no error control or flow control except what is provided by higher level protocols outside of frame relay.
On the other hand, because of the lack of hop-by-hop flow control, the user of frame relay has fewer tools to manage network congestion. The effective use of frame relay also depends on the channels being relatively error free. For example, this is true for fiber optics, but probably not for most forms of broadcast, wireless transmission. Thus a telephone occupies a circuit for 3 minutes per hour. Each first stage matrix has n input lines and 2n — 1 output lines, so it has n 2n — 1 crosspoints.
By the same argument, there are N 2n — 1 crosspoints in the third stage.
For large n, we can approximate 2n — 1 by 2n. Circuit Switching vs. A large noise burst could create an undetected error in the packet. If such an error occurs and alters a destination address field or virtual circuit identifier field, the packet would be misdelivered.
Either can prevent the other from overwhelming it. The layer 3 flow control mechanism regulates the flow over a single virtual circuit. Thus, resources in either the DTE or DCE that are dedicated to a particular virtual circuit can be protected from overflow.
Errors are caught at the link level, but this only catches transmission errors. If a packet-switching node fails or corrupts a packet, the packet will not be delivered correctly. A higher-layer end-to-end protocol, such as TCP, must provide end-to- end reliability, if desired. Otherwise, there would have to be global management of numbers. In essence, the upper part of the fraction is the length of the link in bits, and the lower part of the fraction is the length of a frame in bits.
So the fraction tells you how many frames can be laid out on the link at one time. Multiplying by 2 gives you the round-trip length of the link. You want your sliding window to accommodate that number of frames so that you can continue to send frames until an acknowledgment is received.
Adding 1 to that total takes care of rounding up to the next whole number of frames. Adding 2 instead of 1 is just an additional margin of safety. See Figure 7. Additionally, error checking is only done on the header in ATM rather than on the whole cell or frame. Virtual channels of ATM that follow the same route through the network are bundled into paths. A similar mechanism is not used in frame relay.
In ATM, virtual channels, which have the same endpoints, can be grouped into virtual paths. All the circuits in virtual paths are switched together; this offers increased efficiency, architectural simplicity, and the ability to offer enhanced network services. Network transport functions can be separated into those related to an individual logical connection virtual channel and those related to a group of logical connections virtual path.
Increased network performance and reliability: The network deals with fewer, aggregated entities. Reduced processing and short connection setup time: Much of the work is done when the virtual path is set up. By reserving capacity on a virtual path connection in anticipation of later call arrivals, new virtual channel connections can be established by executing simple control functions at the endpoints of the virtual path connection; no call processing is required at transit nodes.
Thus, the addition of new virtual channels to an existing virtual path involves minimal processing. Enhanced network services: The virtual path is used internal to the network but is also visible to the end user.
Thus, the user may define closed user groups or closed networks of virtual channel bundles. A user of a VCC is provided with a Quality of Service specified by parameters such as cell loss ratio ratio of cells lost to cells transmitted and cell delay variation.
A chapter-by-chapter list of related web sites. This includes all active sites listed in the book, plus other useful sites. Suggestions welcome. Student Resource Site: Help and advice for the long-suffering, overworked student. Errata sheet: Latest list of errors, updated at most monthly. File name is Errata-DCC6e-mmyy. If you spot any errors, please report them to. PDF Viewgraphs: See several of the web sites listed below for additional slides and lecture notes.
On-line transparency masters of all the tables from the book in PDF format. PowerPoint Slides: Developed by Prof. PDF Slides: Hossam Hassanein of Queen's University, Canada. The cnet network simulator enables experimentation with various data link layer, network layer, routing and transport layer protocols, and with various network configurations. Tools Page: Projects Manual: This manual, developed by Prentice Hall, contains over 40 exercises that are suitable for use with Data and Computer Communications.
It is available at a discount for courses using this textbook. Data Link Simulator: This tool enables students to write connection oriented data link protocols and have them tested on a simulated communication channel. On-line Simulation: Ann Burroughs, an Associate Professor at Humboldt State University has created two simulations that may help you visualize some of the concepts in the book. They require ShockWave plug-ins and a reasonably up-to-date browser.
No password is required for any downloads. Downloading sometimes fails, either because your browser mistakenly assumes a password is needed or for other reasons. If so, try using another browser or an FTP package. If that doesn't work, there might be a problem at your end or at your ISP, perhaps a firewall issue. Then you would need to talk to a system manager on your end. Instructors might find these web sites for courses taught using this book useful. I would appreciate hearing about web sites for other courses.
Internets and Intranets. TEI of Crete. Includes PDF lecture slides in Greek. University of Idaho. Includes handouts and many interesting links. University College London. Includes PDF slides. Queen's U. Includes powerpoint slides. EG Communications Engineering. Includes lecture notes, an number of useful supplement pages. CS Computer Networks and Communications. Includes lecture notes.
University of Florida.. EEL Computer Networks. Lots of interesting material. CS Data Communications. Mount Union College, Alliance, Ohio. Some useful links and interesting concept demonstrations. CS Data Communication. Includes useful set of PDF and postscript lecture notes. Kasetsart University, Thailand.
Includes a very good set of student produced slides and notes in Powerpoint and PDF formats.. At Villanova. Rensselaer at Hartford. If you have any suggestions for site content, please contact me at. In particular, please pass along links to relevant web sites and links to course pages used by instructors teaching from this book. Network World: