Chapter 4

A group of \(\mathrm{N}\) stations share a \(56-\mathrm{kbp}\) s pure ALOHA channel. Each station outputs a 1000 -bit frame on average ance every \(100 \mathrm{sec}\), even if the previous one has not yet been sent (e.g., the stations can buffer outgoing frames). What is the maximum value of N?

Consider the delay of pure ALOHA versus slotted ALOHA at low load. Which one is less? explain your answer.

What is the length of a contention slot in CSMALCD for (a) a 2-km twin-lead cable (where signal propagation speed is \(82 \%\) of the signal propagation speed in vacuum)?, and (b) a \(40-k m\) multimode fiber optic cable (signal propagation speed is \(65 \%\) of the signal propagation speed in vacuum)?

In the binary countdown protocol, explain how a lower-numbered slation may be starved from sending a packet.

Sixteen stations, numbered I 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 ready at ance, bow many bit slots are needed to resolve the contention?

A group of friends gets togcther to play highly interactive CPU- and network- intensive video games. The friends play together using a high-bandwidth wireless network. The wireless signal cannot propagate through walls, but the friends are all in the same room. In such a setup, would it be best to use nonpersistent CSMA or the token ring protocol? Please explain you answer.

Consider five wireless stations, \(A, B, C, D\), and \(E\), Station \(A\) can communicate with all other stations. B can communicate with A, C and E. C can communicate with A.B and D. D can communicate with \(A, C\) and \(E\). E can communicate \(A\), D and B. (a) When \(A\) is sending to \(\mathrm{B}\), what other communications are possible? (b) When IB is sending to A what other communications are possible? (c) When \(\mathrm{B}\) is sendiag to \(\mathrm{C}\), what other coenmunications are possible?

A 1-km-long, 10-Mbps CSMA.CD 1.AN (not \(802.3\) ) has a propagation speed of \(200 \mathrm{~m} / \mathrm{jecc}\). Repeaters are not allowed in this system. Data frames are 256 bits long. including 32 bits of header, checksum, and other overhead. The first bit stot after al saccessful transmission is reserved for the receiver to capture the channel in order lo send a 32-bit acknowledyement frame. What is the effective data rate, excluding overhead, assuming that there are no collisaons?

An IP packet to be transmitted by Ethemet 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 transmiitter 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 size but can get the bits out ten times faster. How is it possible to maintain the same minimum frame size?

Name a network that allow frames to be packed back-to-buck. Why is this feature worth having?

Give an example to show that the RTS/CTS in the \(802.11\) protocol is a litule different than in the MACA protocal.

Suppose that an 11-Mbps \(802.11 \mathrm{~b}\) LAN is transmitting 64-byte frames back-lo-back over a radio channel with a bit error rate of \(10^{-7}\). How many frames per second will be damaged on average?

Two devices connected to the same \(802.11\) network are both downloading a large file from the Intemet. Explain bow one device could obtain a higher data rate than the other by (ab)using a \(802.11\) mechanism intended to provide quality of service.

Give two reasons why networks might use an crror-correcting code instcad of error detection and netransmission.

A disadvantage of Bluetooth's profiles is that they add significant complexity to the protocol. How can these profiles be an advantage from the perspective of the applicattions?

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.

Briefly deseribe the difference between store-and-forward and cut-through switches.

Consider two Ethemet networks. In network (a), stations are connected to a hub via full-duplex cables. Ia network (b), stations are connected to a switch using half-duplex eables. For cach of these networks, why is CSMANCD (not) needod?

Store-and-forward switches have an advantage over cut-through switches with respoct to damaged frames. Fxplain what it is.

It is mentioned in Section \(4.8 .3\) that some bridges may not even be present in the spasning tree. Outline a scesario where a bridge may not be present in the spanning tree.

Write a program to simulate the behavior of the CSMA/CD protocol over Ethernet when there are \(\mathrm{N}\) stations ready to transmit while a frame is being transmitted. Your program should report the times when cach statioa successfully starts sendang its frame. Assume that a clock tick occurs once every slot time (512 \musec) and a collision detection and sending of a jamming sequence takes one slot time. All frames are the maximum length allowed.