Questions and Answers of Computer Communications

One key design decision for ATM was whether to use fixed- or variable-length cells.Let us consider this decision from the point of view of efficiency. We can define transmission efficiency as N
Although ATM does not include any end-to-end error detection and control functions on the user data, it is provided with a HEC field to detect and correct header errors. Let us consider the value of
Assuming no malfunction in any of the stations or nodes of a network, is it possible for a packet to be delivered to the wrong destination?
What value of P, as a function of N, L, and H, results in minimum end-to-end delay on a datagram network? Assume that L is much larger than P, and D is zero.
Define the following parameters for a switching network:N = number of hops between two given end systems L = message length in bits B = data rate, in bits per second (bps), on all links P = fixed
Explain the flaw in the following reasoning: Packet switching requires control and address bits to be added to each packet. This introduces considerable overhead in packet switching. In circuit
Consider a TDM system with 8 I/O lines, and connections 1-2, 3-7, and 5-8. Draw several frames of the input to the TSI unit and output from the TSI unit, indicating the movement of data from input
Consider a TSI system with a TDM input of 8000 frames per second. The TSI requires one memory read and one memory write operation per slot. What is the maximum number of slots per frame that can be
Consider a three-stage switch such as in Figure 9.6. Assume that there are a total of N input lines and N output lines for the overall three-stage switch. If n is the number of input lines to a stage
a. If a crossbar matrix has n input lines and m output lines, how many crosspoints are required?b. How many crosspoints would be required if there were no distinction between input and output lines
Consider a simple telephone network consisting of two end offices and one intermediate switch with a 1-MHz full-duplex trunk between each end office and the intermediate switch. Assume a 4-kHz
List and briefly explain the fields in an ATM cell.
What are the characteristics of a virtual path connection?
What are the characteristics of a virtual channel connection?
What types of delay are significant in assessing the performance of a packet-switching network?
What is the significance of packet size in a packet-switching network?
What are the advantages of packet switching compared to circuit switching?
What is the principal application that has driven the design of circuit-switching networks?
Why is it useful to have more than one possible path through a network for each pair of stations?
In statistical TDM, there may be a length field. What alternative could there be to the inclusion of a length field? What problem might this solution cause and how could it be solved?
For a statistical time-division multiplexer, define the following parameters:F = frame length, bits OH = overhead in a frame, bits L = load of data in the frame, bps C = capacity of link, bpsa.
Assume that you are to design a TDM carrier, say DS-489, to support 30 voice channels using 6-bit samples and a structure similar to DS-1. Determine the required bit rate.
In synchronous TDM, the I/O lines serviced by the two multiplexers may be either synchronous or asynchronous although the channel between the two multiplexers must be synchronous. Is there any
A company has two locations: a headquarters and a factory about 25 km away. The factory has four 300-bps terminals that communicate with the central computer facilities over leased voice-grade lines.
kbps input may be treated as two 4.8-kbps inputs).b. What is the percentage overhead in the multiplexed bit stream?c. What is the multiplexed output bit rate?d. What is the minimum reframe time? What
A synchronous nonstatistical TDM is to be used to combine four 4.8-kbps and one 9.6-kbps signals for transmission over a single leased line. For framing, a block of 7 bits (pattern 1011101) is
Ten 9600-bps lines are to be multiplexed using TDM. Ignoring overhead bits in the TDM frame, what is the total capacity required for synchronous TDM? Assuming that we wish to limit average TDM link
Find the number of the following devices that could be accommodated by a T1-type TDM line if 1% of the T1 line capacity is reserved for synchronization purposes.a. 110-bps teleprinter terminalsb.
A character-interleaved time-division multiplexer is used to combine the data streams of a number of 110-bps asynchronous terminals for data transmission over a 2400-bps digital line. Each terminal
for a TDM PCM system that will accommodate four 300-bps, synchronous, digital inputs and one analog input with a bandwidth of 500 Hz. Assume that the analog samples will be encoded into 4-bit PCM
Draw a block diagram similar to Figure
Twenty-four voice signals are to be multiplexed and transmitted over twisted pair.What is the bandwidth required for FDM? Assuming a bandwidth efficiency (ratio of data rate to transmission
In the DS-1 format, what is the control signal data rate for each voice channel?
One of the 193 bits in the DS-1 transmission format is used for frame synchronization.Explain its use.
Explain in terms of data link control and physical layer concepts how error and flow control are accomplished in synchronous time-division multiplexing.
Why is it that the start and stop bits can be eliminated when character interleaving is used in synchronous TDM?
In synchronous TDM, it is possible to interleave bits, one bit from each channel participating in a cycle. If the channel is using a self-clocking code to assist synchronization, might this bit
Consider a transmission system using frequency-division multiplexing. What cost factors are involved in adding one more pair of stations to the system?
and relate the preceding to the figure.
To paraphrase Lincoln: “. . . all of the channel some of the time, some of the channel all of the time. . . .” Refer to Figure
The information in four analog signals is to be multiplexed and transmitted over a telephone channel that has a 400- to 3100-Hz bandpass. Each of the analog baseband signals is bandlimited to 500 Hz.
as a guide, indicate the major difference between North American and international TDM carrier standards.
Using Table
Why is a statistical time-division multiplexer more efficient than a synchronous timedivision multiplexer?
Explain how synchronous time-division multiplexing (TDM) works.
Define upstream and downstream with respect to subscriber lines.
What is echo cancellation?
How is interference avoided by using frequency-division multiplexing?
Why is multiplexing so cost-effective?
A World Wide Web server is usually set up to receive relatively small messages from its clients but to transmit potentially very large messages to them. Explain, then, which type of ARQ protocol
Consider that several physical links connect two stations. We would like to use a“multilink HDLC” that makes efficient use of these links by sending frames on a FIFO basis on the next available
Assume that the primary HDLC station in NRM has sent six I-frames to a secondary.The primary’s N(S) count was three (011 binary) prior to sending the six frames. If the poll bit is on in the sixth
Using the example bit string of Figure 7.8, show the signal pattern on the line using NRZ-L coding. Does this suggest a side benefit of bit stuffing?
Suggest improvements to the bit stuffing-algorithm to overcome the problems of single-bit errors described in the preceding problem.
Because of the provision that a single flag can be used as both an ending and a starting flag, a single-bit error can cause problems.a. Explain how a single-bit error can merge two frames into one.b.
It is clear that bit stuffing is needed for the address, data, and FCS fields of an HDLC frame. Is it needed for the control field?
Two stations communicate via a 1-Mbps satellite link with a propagation delay of 270 ms. The satellite serves merely to retransmit data received from one station to another, with negligible switching
The ISO standard for HDLC procedures (ISO 4335) includes the following definitions:(1) an REJ condition is considered cleared upon the receipt of an incoming I-frame with an N(S) equal to the N(R) of
Out-of-sequence acknowledgment cannot be used for selective-reject ARQ. That is, if frame i is rejected by station X, all subsequent I-frames and RR frames sent by X must have N1R2 = i until frame i
Two neighboring nodes (A and B) use a sliding-window protocol with a 3-bit sequence number. As the ARQ mechanism, go-back-N is used with a window size of 4. Assuming A is transmitting and B is
Suppose that a selective-reject ARQ is used where W = 4. Show, by example, that a 3-bit sequence number is needed.
No mention was made of reject (REJ) frames in the stop-and-wait ARQ discussion.Why is it not necessary to have REJ0 and REJ1 for stop-and-wait ARQ?
A channel has a data rate of R bps and a propagation delay of t s/km. The distance between the sending and receiving nodes is L kilometers. Nodes exchange fixed-size frames of B bits. Find a formula
frames are generated at node A and sent to node C through node B. Determine the minimum data rate required between nodes B and C so that the buffers of node B are not flooded, based on the
In Figure
The number of bits on a transmission line that are in the process of actively being transmitted (i.e., the number of bits that have been transmitted but have not yet been received) is referred to as
Consider a half-duplex point-to-point link using a stop-and-wait scheme, in which a series of messages is sent, with each message segmented into a number of frames.Ignore errors and frame overhead.a.
What are the three frame types supported by HDLC? Describe each.
Define data transparency.
What is the purpose of the flag field?
What are the transfer modes supported by HDLC? Describe each.
What are the station types supported by HDLC? Describe each.
List and briefly define three versions of ARQ.
Describe automatic repeat request (ARQ).
List common ingredients for error control for a link control protocol.
Define error control.
What is piggybacking?
What is the advantage of sliding-window flow control compared to stop-and-wait flow control?
Describe sliding-window flow control.
What are reasons for breaking up a long data transmission into a number of frames?
Describe stop-and-wait flow control.
Define flow control.
List and briefly define some of the requirements for effective communications over a data link.
A common technique for implementing CRC is to use a table lookup algorithm. The document site at box.com/dcc10e contains several papers describing this approach.Write a short paper that summarizes
states that for a given positive integer t, if a code satisfies dmin Ú 2t + 1, then the code can correct all bit errors up to and including errors of t bits. Prove this assertion. Hint: Start by
Section
and show that p1v|w12 7 p1v|w22 if and only if d1v,w12 6 d1v,w22. This proves that the codeword w that gives the largest value of p1v|w2 is that word whose distance from v is a minimum.
discusses block error-correcting codes that make a decision on the basis of minimum distance. That is, given a code consisting of s equally likely codewords of length n, for each received sequence v,
Section
Calculate the Hamming pairwise distances among the following codewords:a. 00000, 10101, 01010b. 000000, 010101, 101010, 110110
A modified CRC procedure is commonly used in communications standards. It is defined as follows:and k is the number of bits being checked (address, control, and information fields).a. Describe in
a. In a CRC error-detecting scheme, choose P1X2 = X4 + X + 1. Encode the bits 10010011011.b. Suppose the channel introduces an error pattern 100010000000000 (i.e., a flip from 1 to 0 or from 0 to 1
A CRC is constructed to generate a 4-bit FCS for an 11-bit message. The generator polynomial is X4 + X3 + 1.a. Draw the shift-register circuit that would perform this task (see Figure 6.4).b. Encode
For P = 110011 and M = 11100011, find the CRC.
Explain in words why the shift-register implementation of CRC will result in all 0s at the receiver if there are no errors. Demonstrate by example.
Using the CRC-CCITT polynomial, generate the 16-bit CRC code for a message consisting of a 1 followed by 15 0s.a. Use long division.b. Use the shift-register mechanism shown in Figure 6.4.
What is the purpose of using modulo 2 arithmetic rather than binary arithmetic in computing an FCS?
The high-speed transport protocol XTP (Xpress Transfer Protocol) uses a 32-bit checksum function defined as the concatenation of two 16-bit functions: XOR and RXOR are illustrated in Figure 6.10. The
One nice property of the Internet checksum is that it is endian-independent. Little Endian computers store hex numbers with the least significant byte last (Intel processors, for example). Big Endian
Compute the Internet checksum for the data block E3 4F 23 96 44 27 99 F3. Then perform the verification calculation.

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