Questions and Answers of Computer Network

Q:

Imagine that you have trained your St. Bernard, Bernie, to carry a box of three 8mm tapes instead of a flask of brandy. (When your disk fills up, you

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An alternative to a LAN is simply a big timesharing system with terminals for all users. Give two advantages of a client-server system using a LAN.

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The performance of a client-server system is influenced by two network factors: the bandwidth of the network (how many bits/sec it can transport) and

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Besides bandwidth and latency, what other parameter is needed to give a good characterization of the quality of service offered by a network used for

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A factor in the delay of a store-and-forward packet-switching system is how long it takes to store and forward a packet through a switch. If

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A client-server system uses a satellite network, with the satellite at a height of 40,000 km. What is the best-case delay in response to a request?

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In the future, when everyone has a home terminal connected to a computer network, instant public referendums on important pending legislation will

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A collection of five routers is to be connected in a point-to-point subnet. Between each pair of routers, the designers may put a high-speed line, a

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A group of 2n - 1 router is interconnected in a centralized binary tree, with a router at each tree node. Router i communicate with router j by

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A disadvantage of a broadcast subnet is the capacity wasted when multiple hosts attempt to access the channel at the same time. As a simplistic

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What are two reasons for using layered protocols?

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The president of the Specialty Paint Corp. gets the idea to work with a local beer brewer to produce an invisible beer can (as an anti-litter

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What is the principal difference between connectionless communication and connection-oriented communication?

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Two networks each provide reliable connection-oriented service. One of them offers a reliable byte stream and the other offers a reliable message

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What does ''negotiation'' mean when discussing network protocols? Give an example.

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In Fig. 1-19, a service is shown. Are any other services implicit in this figure? If so where? If not, why not?

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In some networks, the data link layer handles transmission errors by requesting damaged frame to be retransmitted. If the probability of a frame's

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Which of the OSI layers handles each of the following? (a) Dividing the transmitted bit stream into frames. (b) Determining which route through the

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If the unit exchanged at the data link level is called a frame and the unit exchanged at the network level is called a packet, do frames encapsulate

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A system has an n-layer protocol hierarchy. Applications generate messages of length M bytes. At each of the layers, an h-byte header is added. What

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List two ways in which the OSI reference model and the TCP/IP reference model are the same. Now list two ways in which they differ.

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What is the main difference between TCP and UDP?

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The subnet of Fig. 1-25(b) was designed to withstand a nuclear war. How many bombs would it take to partition the nodes into two disconnected sets?

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The Internet is roughly doubling in size every 18 months. Although no one really knows for sure, one estimate put the number of hosts on it at 100

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When a file is transferred between two computers, two acknowledgement strategies are possible. In the first one, the file is chopped up into packets,

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Why does ATM use small, fixed-length cells?

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How long was a bit on the original 802.3 standard in meters? Use a transmission speed of 10 Mbps and assume the propagation speed in coax is 2/3 the

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An image is 1024 x 768 pixels with 3 bytes/pixel. Assume the image is uncompressed. How long does it take to transmit it over a 56-kbps modem

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Ethernet and wireless networks have some similarities and some differences. One property of Ethernet is that only one frame at a time can be

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Wireless networks are easy to install, which makes them inexpensive since installation costs usually far overshadow equipment costs. Nevertheless,

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List two advantages and two disadvantages of having international standards for network protocol?

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When a system has a permanent part and a removable part (such as a CD-ROM drive and the CD-ROM), it is important that the system be standardized, so

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Compute the Fourier coefficients for the function f (t) = t (< t < 1).

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A noiseless 4-kHz channel is sampled every 1 msec. What is the maximum data rate?

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Television channels are 6 MHz wide. How many bits/sec can be sent if four-level digital signals are used? Assume a noiseless channel.

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If a binary signal is sent over a 3-kHz channel whose signal-to-noise ratio is 20 dB, what is the maximum achievable data rate?

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What signal-to-noise ratio is needed to put a T1 carrier on a 50-kHz line?

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What is the difference between a passive star and an active repeater in a fiber network?

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How much bandwidth is there in 0.1 micron of spectrum at a wavelength of 1 micron?

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It is desired to send a sequence of computer screen images over an optical fiber. The screen is 480 x 640 pixels, each pixel being 24 bits. There are

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Is the Nyquist theorem true for optical fiber or only for copper wire?

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In Fig. 2-6 the left hand band is narrower than the others. Why?

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Radio antennas often work best when the diameter of the antenna is equal to the wavelength of the radio wave. Reasonable antennas range from 1 cm to

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Multi path fading is maximized when the two beams arrive 180 degrees out of phase. How much of a path difference is required to maximize the fading

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A laser beam 1 mm wide is aimed at a detector 1 mm wide 100 m away on the roof of a building. How much of an angular diversion (in degrees) does the

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The 66 low-orbit satellites in the Iridium project are divided into six necklaces around the earth. At the altitude they are using, the period is 90

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Consider a satellite at the altitude of geostationary satellites but whose orbital plane is inclined to the equatorial plane by an angle. To a

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How many end office codes were there pre-1984, when each end office was named by its three-digit area code and the first three digits of the local

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Using only the data given in the text, what is the maximum number of telephones that the existing U.S. system can support without changing the

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A simple telephone system consists of two end offices and a single toll office to which each end office is connected by a 1-MHz full-duplex trunk.

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A regional telephone company has 10 million subscribers. Each of their telephones is connected to a central office by a copper twisted pair. The

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Is an oil pipeline a simplex system, a half-duplex system, a full-duplex system, or none of the above?

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The cost of a fast microprocessor has dropped to the point where it is now possible to put one in each modem. How does that affect the handling of

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A modem constellation diagram similar to Fig. 2-25 has data points at the following coordinates: (1, 1), (1, -1), (-1, 1), and (-1, -1). How many bps

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A modem constellation diagram similar to Fig. 2-25 has data points at (0, 1) and (0, 2). Does the modem use phase modulation or amplitude modulation?

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In a constellation diagram, all the points lie on a circle centered on the origin. What kind of modulation is being used?

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How many frequencies does a full-duplex QAM-64 modem use?

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An ADSL system using DMT allocates 3/4 of the available data channels to the downstream link. It uses QAM-64 modulation on each channel. What is the

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In the four-sector LMDS example of Fig. 2-30, each sector has its own 36-Mbps channel. According to queuing theory, if the channel is 50% loaded, the

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Ten signals, each requiring 4000 Hz, are multiplexed on to a single channel using FDM. How much minimum bandwidth is required for the multiplexed

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Why has the PCM sampling time been set at 125 µsec?

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What is the percent overhead on a T1 carrier; that is, what percent of the 1.544 Mbps are not delivered to the end user?

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Compare the maximum data rate of a noiseless 4-kHz channel using (a) Analog encoding (e.g., QPSK) with 2 bits per sample. (b) The T1 PCM system.

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If a T1 carrier system slips and loses track of where it is, it tries to resynchronize using the 1st bit in each frame. How many frames will have to

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What is the difference, if any, between the demodulator part of a modem and the coder part of a codec? (After all, both convert analog signals to

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A signal is transmitted digitally over a 4-kHz noiseless channel with one sample every 125 µsec. How many bits per second are actually sent for each

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A pure sine wave of amplitude A is encoded using delta modulation, with x samples/sec. An output of +1 corresponds to a signal change of +A/8, and an

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SONET clocks have a drift rate of about 1 part in 109. How long does it take for the drift to equal the width of 1 bit? What are the implications of

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In Fig. 2-37, the user data rate for OC-3 is stated to be 148.608 Mbps. Show how this number can be derived from the SONET OC-3 parameters.

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To accommodate lower data rates than STS-1, SONET has a system of virtual tributaries (VT). A VT is a partial payload that can be inserted into an

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What is the essential difference between message switching and packet switching?

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What is the available user bandwidth in an OC-12c connection?

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Three packet-switching networks each contain n nodes. The first network has a star topology with a central switch, the second is a (bidirectional)

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Compare the delay in sending an x-bit message over a k-hop path in a circuit-switched network and in a (lightly loaded) packet-switched network. The

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Suppose that x bits of user data are to be transmitted over a k-hop path in a packet-switched network as a series of packets, each containing p data

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In a typical mobile phone system with hexagonal cells, it is forbidden to reuse a frequency band in an adjacent cell. If 840 frequencies are

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The actual layout of cells is seldom as regular that as shown in Fig. 2-41. Even the shapes of individual cells are typically irregular. Give a

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Make a rough estimate of the number of PCS micro cells 100 m in diameter it would take to cover San Francisco (120 square km).

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Sometimes when a mobile user crosses the boundary from one cell to another, the current call is abruptly terminated, even though all transmitters and

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D-AMPS has appreciably worse speech quality than GSM. Is this due to the requirement that D-AMPS be backward compatible with AMPS, whereas GSM had no

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Calculate the maximum number of users that D-AMPS can support simultaneously within a single cell. Do the same calculation for GSM. Explain the

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Suppose that A, B, and C are simultaneously transmitting 0 bits, using a CDMA system with the chip sequences of Fig. 2-45(b). What is the resulting

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In the discussion about orthogonality of CDMA chip sequences, it was stated that if S•T = 0 then is also 0. Prove this.

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Consider a different way of looking at the orthogonality property of CDMA chip sequences. Each bit in a pair of sequences can match or not match.

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A CDMA receiver gets the following chips: (-1 +1 -3 +1 -1 -3 +1 +1). Assuming the chip sequences defined in Fig. 2-45(b), which stations transmitted,

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At the low end, the telephone system is star shaped, with all the local loops in a neighborhood converging on an end office. In contrast, cable

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A cable TV system has 100 commercial channels, all of them alternating programs with advertising. Is this more like TDM or like FDM?

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A cable company decides to provide Internet access over cable in a neighborhood consisting of 5000 houses. The company uses a coaxial cable and

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Using the spectral allocation shown in Fig. 2-48 and the information given in the text, how many Mbps does a cable system allocate to upstream and

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How fast can a cable user receive data if the network is otherwise idle?

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An upper-layer packet is split into 10 frames, each of which has an 80 percent chance of arriving undamaged. If no error control is done by the data

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The following character encoding is used in a data link protocol: A: 01000111; B: 11100011; FLAG: 01111110; ESC: 11100000 Show the bit sequence

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The following data fragment occurs in the middle of a data stream for which the byte-stuffing algorithm described in the text is used: A B ESC C ESC

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One of your classmates, Scrooge, has pointed out that it is wasteful to end each frame with a flag byte and then begin the next one with a second

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A bit string, 0111101111101111110, needs to be transmitted at the data link layer. What is the string actually transmitted after bit stuffing?

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When bit stuffing is used, is it possible for the loss, insertion, or modification of a single bit to cause an error not detected by the checksum? If

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Can you think of any circumstances under which an open-loop protocol, (e.g., a Hamming code) might be preferable to the feedback-type protocols

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To provide more reliability than a single parity bit can give, an error-detecting coding scheme uses one parity bit for checking all the odd-numbered

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Sixteen-bit messages are transmitted using a Hamming code. How many check bits are needed to ensure that the receiver can detect and correct single

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An 8-bit byte with binary value 10101111 is to be encoded using an even-parity Hamming code. What is the binary value after encoding?

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