Labo English

Tele- and Datacommunications Labo: Problems The original English tasks Frederik Colle - 4FI Academiejaar 2005-2006

1

1

Session 1

1.1

Page 46, problem 2.4

Two blue armies are each poised on opposite hills preparing to attack a single red army in the valley. The red army can defeat either of the blue armies seperately but will fail to defeat both blue armies if they attack simultaneously. The blue armies communicate via an unreliable communications system (a foot soldier). The commander with one of the blue armies would like to attack at noon. His problem is this: If he sends a message to the other blue army, ordering the attack, he cannot be sure it will get through. He could ask for acknowledgment, but that might not get through. Is there a protocol that the two blue armies can use to avoid defeat?

1.2

Page 88, problem 3.4

Sound may be modeled as sinusoidal functions. Compare the relative frequency and wavelength of musical notes. Use 330m/s as the speed of sound and the following frequencies for the musical scale. Note Frequency (Hz)

1.3

C 264

D 297

E 330

F 352

G 396

A 440

B 495

C 528

Page 88, problem 3.5

If the solid curve below represents sin(2πt), what does the dotted curve represent? That is, the dotted curve can be written in the form A sin(2πf t + φ); what are A, f and φ? 2

1

0

−1

−2 −0.5

1.4

0

0.5

Not found in the book

This problem could not be found in the book.

2

1.5

Page 89, problem 3.13

1. Suppose that a digitized TV picture is to be transmitted from a source that uses a matrix of 480 × 500 picture elements (pixels), where each pixel can take on one of 32 intensity values. Assume that 30 pictures are sent per second. (This digital source is roufhly equivalent to broadcast TV standards that have been adopted.) Find the source rate (bps). 2. Assume that the TV picture is to be transmitted over a channel with 4.5M Hz bandwith and a 35 − dB signal-to-noise ratio. Find the capacity of the channel (bps). 3. Discuss how the parameters given in part (1) could be modified to allow transmission of color TV signals without increasing the required value for R.

1.6

Page 125, problem 4.1

Suppose that data are stored on 1.4 − M byte floppy diskettes that weigh 30g each. Suppose that an airliner carries 104 kg of these floppies at a speed of 1000km/h over a distance of 5000km. What is the data transmission rate in bits per second of this system?

1.7

Page 126, problem 4.4

Coaxial cable is a two-wire transmission system. What is the advantage of connecting the outer conductor to the ground?

1.8

Page 126, proble 4.8

Stories abound of people who receive radio signals in filling in their teeth. Suppose you have one filling that is 2.5mm (0.0025m) long that acts as a radio antenna. That is, it is equal to one-half the wavelength. What frequency do you receive?

3

2

Session 2

2.1

Page 167, problem 5.6

For the bit stream 01001110, sketch the wareforms for each of the codes of Table 5.2. Assume that te signal level for the preceeding bit for NRZI was high; the most recent preceding 1 bit (AMI) has a negative voltage; and the most recent preceding 0 bit (pseudoternary has a negative voltage.

2.2

Page 167, problem 5.2

Develop algorithms for generating each of the codes of Table 5.2 from NRZ-L.

2.3

Page 168, problem 5.7

The waveform below belongs to a Manchester encoded binary data stream. Determine the beginning and the end of bit periods (i.e., extract clock information) and give the data sequence.

2.4

Page 204, problem 6.1

Suppose a file of 10.000 bytes is to be sent over a line at 2400bps. 1. Calculate the overhead in bits and time in using asynchronous communication. Assume one start bit and a stop element of length one bit, and 8 bits to send the byte itself for each character. The 8-bit character consists of all data bits, with no parity bit. 2. Calculate the overhead in bits and time using synchronous communication. Assume that the data are sent in frames. Each frame consists of 1000 characters = 8000 bits and an overhead of 48 control bits per frame. 3. What would the answers to parts (1) and (2) be for a file of 100.000 characters? 4. What would the answers to parts (1) forthe original file of 10.000 characters except at a data rate of 9600bps?

2.5

Page 205, problem 6.4

Suppose that a sender and receiver use asynchronous transmission and agree not to use any stop elements. Could this work? If so, explain any necessary conditions.

4

2.6

Page 205, problem 6.6

Suppose that a synchronous serial data transmission is clocked by two clocks (on at the sender and one at the receiver) that each have a drift of 1 minute in one year. How long a sequence of bits can be sent before possible clock drift could cause a problem? Assume that a bit waveform will be good if it is sampled within 40% of its center and that sender and reveiver are synchronized at the beginning of each frame. Note that the transmission rate is not a factor, as both the bit period and the absolute timing error decrease proportionaly at higher transmission rates.

5

3

Session 2bis

3.1

Page 205, problem 6.14

1. In CRC error detection scheme, choose P (x) = x4 + x + 1. Encode the bits 10010011011. 2. Suppose the channel introduces an error pattern 100010000000000 (i.e., a flip from 1 to 0 or from 0 to 1 in position 1 and 5). What is received? Can the error be detected? 3. Repeat with error pattern 1100110000000000.

3.2

Page 206, problem 6.19

Draw a timing diagram showing the state of all EIA-232 leads between two DTE-DCE pairs during the course of a data call on the switched telephone network.

3.3

Page 206, problem 6.20

Explain the operation of each null modem connection in Figure 6.14.

6

4

Session 3

4.1

Page 230, problem 7.1

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. 1. What is the effect on line utilization of increasing the message size so that fewer messages will be required? Other factors remain constant. 2. What is the effect on line utilization of increasing the number of frames for a constant message size? 3. What is the effect on line utilization of increasing frame size?

4.2

Page 230, problem 7.2

A channel has a data rate of 4kbps and a propagation delay of 20ms. For what range of frame sizes does stop-and-wait give an efficiency of at least 50%?

4.3

Page 230, problem 7.3

Consider the use of 1000-bit frames on a 1-Mbps satelite channel with a 270-ms delay. What is the maximum link utilization for 1. Stop-and-wait flow control? 2. Continuous flow control with a window size of 7? 3. Continuous flow control with a window size of 127? 4. Continuous flow control with a window size of 255?

4.4

Page 205, problem 6.12

For P = 110011 and M = 11100011, find the CRC.

4.5

Page 232, problem 7.18

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 (selective reject, go-back-N) would provide less of a burden to a particulary popular WWW server.

7

5

Session 4

5.1

Page 230, problem 7.3

Consider the use of 1000-bit frames on a 1-Mbps satelite channel with a 270-ms delay. What is the maximum link utilization for 1. Stop-and-wait flow control? 2. Continuous flow control with a window size of 7? 3. Continuous flow control with a window size of 127? 4. Continuous flow control with a window size of 255?

5.2

Page 205, problem 6.8

What is the purpose of using modulo 2 arithmetic rather than binary arithmetic in computing an FCS?

5.3

Page 231, problem 7.7

Suppose that a selective-reject ARQ is used where W=4. Show, by example, that a 3-bit sequence number is needed.

5.4

Page 273, problem 8.3

Consider a transmission system using frequency division multiplexing. What cost factors are involved in adding one more pair of stations to the system?

5.5

Page 273, problem 8.5

Why is it that the start and stop bits can be eliminated when character interleaving is used in synchronous TDM?

5.6

Page 274, problem 8.15

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. The company is considering installing TDM equipment so that only one line will be needed. What cost factors should be considered in the decision?

8

Opmerkingen Gebruik Sommigen onder ons vinden engelse teksten eenduidiger dan de vertaalde versies ervan. Bij deze de opgaven letterlijk overgetypt uit het originele boek (Data and Computer Commuications, Seventh Edition by William Stallings). Uiteraard kan ik foutjes gemaakt hebben bij het (blind) overtypen uit het boek. Indien dit het geval zou zijn mag je mij dit altijd laten weten, dan corrigeer ik dat zo snel mogelijk.

Publicatie Dit document werd opgemaakt met als doel mezelf en mijn medestudenten het leven iets makkelijker te maken. Indien een derde zich eventueel benadeeld zou voelen door de publicatie van dit document, aarzel dan zeker niet om contact met mij op te nemen. Dit document werd door mij persoonlijk aangemaakt het spreekt dan ook voor zich dat ik geen verantwoordelijkheid neem als het op juistheid van de inhoud aankomt. Indien er zich fouten in zouden bevinden dan mag je mij dat altijd gerust laten weten. Alle vragen en opmerkingen zijn nog altijd steeds welkom op het mailadres [email protected]

9