Questions and Answers of Computer Network

A 12-bit Hamming code whose hexadecimal value is 0xE4F arrives at a receiver. What was the original value in hexadecimal? Assume that not more than 1 bit is in error.
One way of detecting errors is to transmit data as a block of n rows of k bits per row and adding parity bits to each row and each column. The lower-right corner is a parity bit that checks its row
A block of bits with n rows and k columns uses horizontal and vertical parity bits for error detection. Suppose that exactly 4 bits are inverted due to transmission errors. Derive an expression for
What is the remainder obtained by dividing x7 + x5 + 1 by the generator polynomial x3 + 1?
A bit stream 10011101 is transmitted using the standard CRC method described in the text. The generator polynomial is x3 + 1. Show the actual bit string transmitted. Suppose the third bit from the
Data link protocols almost always put the CRC in a trailer rather than in a header. Why?
A channel has a bit rate of 4 kbps and a propagation delay of 20 msec. For what range of frame sizes does stop-and-wait give an efficiency of at least 50 percent?
A 3000-km-long T1 trunk is used to transmit 64-byte frames using protocol 5. If the propagation speed is 6 µsec/km, how many bits should the sequence numbers be?
In protocol 3, is it possible that the sender starts the timer when it is already running? If so, how might this occur? If not, why is it impossible?
Imagine a sliding window protocol using so many bits for sequence numbers that wraparound never occurs. What relations must hold among the four window edges and the window size, which is constant and
If the procedure between in protocol 5 checked for the condition a < b < c instead of the condition a < b < c, would that have any effect on the protocol's correctness or efficiency? Explain your
In protocol 6, when a data frame arrives, a check is made to see if the sequence number differs from the one expected and nonak is true. If both conditions hold, a NAK is sent. Otherwise, the
Suppose that the three-statement while loop near the end of protocol 6 were removed from the code. Would this affect the correctness of the protocol or just the performance? Explain your answer.
Suppose that the case for checksum errors were removed from the switch statement of protocol 6. How would this change affect the operation of the protocol?
In protocol 6 the code for frame arrival has a section used for NAKs. This section is invoked if the incoming frame is a NAK and another condition is met. Give a scenario where the presence of this
Imagine that you are writing the data link layer software for a line used to send data to you but not from you. The other end uses HDLC, with a 3-bit sequence number and a window size of seven
Consider the operation of protocol 6 over a 1-Mbps error-free line. The maximum frame size is 1000 bits. New packets are generated 1 second apart. The timeout interval is 10 msec. If the special
In protocol 6, MAX_SEQ = 2n - 1. While this condition is obviously desirable to make efficient use of header bits, we have not demonstrated that it is essential. Does the protocol work correctly for
Frames of 1000 bits are sent over a 1-Mbps channel using a geostationary satellite whose propagation time from the earth is 270 msec. Acknowledgements are always piggybacked onto data frames. The
Compute the fraction of the bandwidth that is wasted on overhead (headers and retransmissions) for protocol 6 on a heavily-loaded 50-kbps satellite channel with data frames consisting of 40 header
Consider an error-free 64-kbps satellite channel used to send 512-byte data frames in one direction with very short acknowledgements coming back the other way. What is the maximum throughput for
A 100-km-long cable runs at the T1 data rate. The propagation speed in the cable is 2/3 the speed of light in vacuum. How many bits fit in the cable?
Suppose that we model protocol 4 using the finite state machine model. How many states exist for each machine? How many states exist for the communication channel? How many states exist for the
Give the firing sequence for the Petri net of Fig. 3-23 corresponding to the state sequence (000), (01A), (01—), (010), (01A) in Fig. 3-21. Explain in words what the sequence represents.
Given the transition rules AC → B, B → AC, CD → E, and E→CD, draw the Petri net described. From the Petri net, draw the finite state graph reachable from the initial state
PPP is based closely on HDLC, which uses bit stuffing to prevent accidental flag bytes within the payload from causing confusion. Give at least one reason why PPP uses byte stuffing instead.
What is the minimum overhead to send an IP packet using PPP? Count only the overhead introduced by PPP itself, not the IP header overhead.
For this problem, use a formula from this chapter, but first state the formula. Frames arrive randomly at a 100-Mbps channel for transmission. If the channel is busy when a frame arrives, it waits
A group of N stations share a 56-kbps pure ALOHA channels. Each station outputs a 1000-bit frame on an average of once every 100 sec, even if the previous one has not yet been sent (e.g., the
Consider the delay of pure ALOHA versus slotted ALOHA at low load. Which one is less? Explain your answer.
Ten thousand airline reservation stations are competing for the use of a single slotted ALOHA channel. The average station makes 18 requests/hour. A slot is 125 µsec. What is the approximate total
A large population of ALOHA users manages to generate 50 requests/sec, including both originals and retransmissions. Time is slotted in units of 40 msec. (a) What is the chance of success on the
Measurements of a slotted ALOHA channel with an infinite number of users show that 10 percent of the slots are idle. (a) What is the channel load, G? (b) What is the throughput? (c) Is the channel
In an infinite-population slotted ALOHA system, the mean number of slots a station waits between a collision and its retransmission is 4. Plot the delay versus throughput curve for this system.
How long does a station, s, have to wait in the worst case before it can start transmitting its frame over a LAN that uses (a) The basic bit-map protocol? (b) Mok and Ward's protocol with permuting
A LAN uses Mok and Ward's version of binary countdown. At a certain instant, the ten stations have the virtual station numbers 8, 2, 4, 5, 1, 7, 3, 6, 9, and 0. The next three stations to send are 4,
Sixteen stations, numbered 1 through 16, are contending for the use of a shared channel by using the adaptive tree walk protocol. If all the stations whose addresses are prime numbers suddenly become
A collection of 2n stations uses the adaptive tree walk protocol to arbitrate access to a shared cable. At a certain instant, two of them become ready. What are the minimum, maximum, and mean number
The wireless LANs that we studied used protocols such as MACA instead of using CSMA/CD. Under what conditions, if any, would it be possible to use CSMA/CD instead?
What properties do the WDMA and GSM channel access protocols have in common? See Chap. 2 for GSM.
Six stations, A through F, communicate using the MACA protocol. Is it possible that two transmissions take place simultaneously? Explain your answer.
A seven-story office building has 15 adjacent offices per floor. Each office contains a wall socket for a terminal in the front wall, so the sockets form a rectangular grid in the vertical plane,
What is the baud rate of the standard 10-Mbps Ethernet?
Sketch the Manchester encoding for the bit stream: 0001110101.
Sketch the differential Manchester encoding for the bit stream of the previous problem. Assume the line is initially in the low state.
A 1-km-long, 10-Mbps CSMA/CD LAN (not 802.3) has a propagation speed of 200 m/µsec. Repeaters are not allowed in this system. Data frames are 256 bits long, including 32 bits of header, checksum,
Two CSMA/CD stations are each trying to transmit long (multi frame) files. After each frame is sent, they contend for the channel, using the binary exponential back off algorithm. What is the
Consider building a CSMA/CD network running at 1 Gbps over a 1-km cable with no repeaters. The signal speed in the cable is 200,000 km/sec. What is the minimum frame size?
An IP packet to be transmitted by Ethernet is 60 bytes long, including all its headers. If LLC is not in use, is padding needed in the Ethernet frame, and if so, how many bytes?
Ethernet frames must be at least 64 bytes long to ensure that the transmitter is still going in the event of a collision at the far end of the cable. Fast Ethernet has the same 64-byte minimum frame
Some books quote the maximum size of an Ethernet frame as 1518 bytes instead of 1500 bytes. Are they wrong? Explain your answer.
The 1000Base-SX specification states that the clock shall run at 1250 MHz, even though gigabit Ethernet is only supposed to deliver 1 Gbps. Is this higher speed to provide for an extra margin of
How many frames per second can gigabit Ethernet handle? Think carefully and take into account all the relevant cases. Hint: the fact that it is gigabit Ethernet matters.
Name two networks that allow frames to be packed back-to-back. Why is this feature worth having?
In Fig. 4-27, four stations, A, B, C, and D, are shown. Which of the last two stations do you think is closest to A and why?
Suppose that an 11-Mbps 802.11b LAN is transmitting 64-byte frames back-to-back over a radio channel with a bit error rate of 10-7. How many frames per second will be damaged on average?
An 802.16 network has a channel width of 20 MHz. How many bits/sec can be sent to a subscriber station?
IEEE 802.16 supports four service classes. Which service class is the best choice for sending uncompressed video?
Give two reasons why networks might use an error-correcting code instead of error detection and retransmission.
From Fig. 4-35, we see that a Bluetooth device can be in two Pico nets at the same time. Is there any reason why one device cannot be the master in both of them at the same time?
Figure 4-25 shows several physical layer protocols. Which of these is closest to the Bluetooth physical layer protocol? What is the biggest difference between the two?
Bluetooth supports two types of links between a master and a slave. What are they and what is each one used for?
Beacon frames in the frequency hopping spread spectrum variant of 802.11 contain the dwell time. Do you think the analogous beacon frames in Bluetooth also contain the dwell time? Discuss your answer.
Consider the interconnected LANs shown in Fig. 4-44. Assume that hosts a and b are on LAN 1, c is on LAN 2, and d is on LAN 8. Initially, hash tables in all bridges are empty and the spanning tree
One consequence of using a spanning tree to forward frames in an extended LAN is that some bridges may not participate at all in forwarding frames. Identify three such bridges in Fig. 4-44. Is there
Imagine that a switch has line cards for four input lines. It frequently happens that a frame arriving on one of the lines has to exit on another line on the same card. What choices is the switch
A switch designed for use with fast Ethernet has a backplane that can move 10 Gbps. How many frames/sec can it handle in the worst case?
Consider the network of Fig. 4-49(a). If machine J were to suddenly become white, would any changes be needed to the labeling? If so what?
Briefly describe the difference between store-and-forward and cut-through switches.
Store-and-forward switches have an advantage over cut-through switches with respect to damaged frames. Explain what it is.
To make VLANs work, configuration tables are needed in the switches and bridges. What if the VLANs of Fig. 4-49(a) use hubs rather than multi drop cables? Do the hubs need configuration tables, too?
In Fig. 4-50 the switch in the legacy end domain on the right is a VLAN-aware switch. Would it be possible to use a legacy switch there? If so, how would that work? If not, why not?
Give two example computer applications for which connection-oriented service is appropriate. Now give two examples for which connectionless service is best.
Are there any circumstances when connection-oriented service will (or at least should) deliver packets out of order? Explain.
Datagram subnets route each packet as a separate unit, independent of all others. Virtual-circuit subnets do not have to do this, since each data packet follows a predetermined route. Does this
Give three examples of protocol parameters that might be negotiated when a connection is set up.
Consider the following design problem concerning implementation of virtual-circuit service. If virtual circuits are used internal to the subnet, each data packet must have a 3-byte header and each
Assuming that all routers and hosts are working properly and that all software in both is free of all errors, is there any chance, however small, that a packet will be delivered to the wrong
Consider the network of Fig. 5-7, but ignore the weights on the lines. Suppose that it uses flooding as the routing algorithm. If a packet sent by A to D has a maximum hop count of 3, list all the
Give a simple heuristic for finding two paths through a network from a given source to a given destination that can survive the loss of any communication line (assuming two such paths exist). The
Consider the subnet of Fig. 5-13(a). Distance vector routing is used, and the following vectors have just come in to router C: from B: (5, 0, 8, 12, 6, 2); from D: (16, 12, 6, 0, 9, 10); and from E:
If delays are recorded as 8-bit numbers in a 50-router network, and delay vectors are exchanged twice a second, how much bandwidth per (full-duplex) line is chewed up by the distributed routing
In Fig. 5-14 the Boolean OR of the two sets of ACF bits are 111 in every row. Is this just an accident here, or does it holds for all subnets under all circumstances?
For hierarchical routing with 4800 routers, what region and cluster sizes should be chosen to minimize the size of the routing table for a three-layer hierarchy? A good starting place is the
In the text it was stated that when a mobile host is not at home, packets sent to its home LAN are intercepted by its home agent on that LAN. For an IP network on an 802.3 LAN, how does the home
Looking at the subnet of Fig. 5-6, how many packets are generated by a broadcast from B, using (a) Reverse path forwarding? (b) The sink tree?
Consider the network of Fig. 5-16(a). Imagine that one new line is added, between F and G, but the sink tree of Fig. 5-16(b) remains unchanged. What changes occur to Fig. 5-16(c)?
Compute a multicast spanning tree for router C in the following subnet for a group with members at routers A, B, C, D, E, F, I, and K.
In Fig. 5-20, do nodes H or I ever broadcast on the lookup shown starting at A?
Suppose that node B in Fig. 5-20 has just rebooted and has no routing information in its tables. It suddenly needs a route to H. It sends out broadcasts with TTL set to 1, 2, 3, and so on. How many
In the simplest version of the Chord algorithm for peer-to-peer lookup, searches do not use the finger table. Instead, they are linear around the circle, in either direction. Can a node accurately
Consider the Chord circle of Fig. 5-24. Suppose that node 10 suddenly goes on line. Does this affect node 1's finger table, and if so, how?
As a possible congestion control mechanism in a subnet using virtual circuits internally, a router could refrain from acknowledging a received packet until (1) it knows its last transmission along
A datagram subnet allows routers to drop packets whenever they need to. The probability of a router discarding a packet is p. Consider the case of a source host connected to the source router, which
Describe two major differences between the warning bit method and the RED method.
Give an argument why the leaky bucket algorithm should allow just one packet per tick, independent of how large the packet is.
The byte-counting variant of the leaky bucket algorithm is used in a particular system. The rule is that one 1024-byte packet, or two 512-byte packets, etc., may be sent on each tick. Give a serious
Give three examples of protocol parameters that might be negotiated when a connection is set up. Discuss.
A computer on a 6-Mbps network is regulated by a token bucket. The token bucket is filled at a rate of 1 Mbps. It is initially filled to capacity with 8 megabits. How long can the computer transmit
Imagine a flow specification that has a maximum packet size of 1000 bytes, a token bucket rate of 10 million bytes/sec, a token bucket size of 1 million bytes, and a maximum transmission rate of 50

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