Solutions to Exercise Session 4,

Transcription

1 Solutions to Exercise Session 4, I.1. Chapter 14: 2, 4, 5, 7, 8, 21, 23, a 4. b, c 5. c 7. b 8. a 21. a 23. a

2 26. Token bus networks are intended for factory automation; hence they are not likely candidates to connect/extend with other networks. I.2. Chapter 15: 2, 5, 12-14, 16-19, c 5. c 12. c 13. d 14. d 16. a 17. d 18. b 19. c 20. Types of mobility Movement inside BSS Movement between BSSs No transition yes no no Movement between ESSs BSS Transition yes yes no ESS Transition yes yes yes 21. CSMA/CD CSMA/CA 1. Both sense the medium to see if it s idle before transmitting. 2. If idle, it depends on persistence strategy if frame transmitted immediately or not. 3. If busy, it depends on persistence strategy if medium continuously sensed for idleness or a random amount of time is waited and then sense again. 4. If after a predefined amount of time a collision is detected, then jam signal, wait for corresponding backoff time and resend frame. 22. DCF 2. If idle, a random amount of time is waited, then transmission takes place. 3. If busy, it waits a random amount of time, then senses medium and if idle, it waits for a further IFS, then if still idle it waits for corresponding back-off time and if medium still idle, it transmits. 4. If no acknowledgement comes in due time, an error of some sort is assumed and the frame is retransmitted.

3 PCF - distributed coordination function, mandatory - basic access method, to be implemented on all stations - contention method based on CSMA/CA - includes as basics RTS, CTS, frame, and ACK - for error and flow control it implements timers - fragmentation supported, each fragment has own sequence number and FCS - transmitting the fragments of a frame has priority over starting transmitting a new frame; this is implemented with SIFS and DIFS - point coordination function, optional - implemented on AP station, on top of DCF, to handle time-sensitive transmissions - contention-free access method - based on polling stations for data - SIFS < PIFS < DIFS - can handle power save modes 23. Fields field size field size DA 6 SA 6 Address 1 6 Address 2 6 Address 3 6 Address 4 6 FC 2 D/ID 2 SC 2 PDU length 2 Data and padding Frame body FCS (CRC) 4 4 II Why does ATM use small, fixed-size cells? Because hardware can be programmed to route small, fixed-size cells and doing routing in hardware is faster (and cheaper?) than in software. Having small cells does not occupy the line too much, ensuring some quality of service and since these cells are guaranteed to arrive in order, this facilitates the multimedia transmissions. III Wireless networks are easy to install, which makes them inexpensive (usually installation costs far overshadow equipment costs). Yet, they also have disadvantages. List some of these disadvantages. - Small data rate compared to current wired networks

4 - Unreliable and noisy - Wireless protocols are more complex than wired correspondent IV In the following figure from Lecture 6, stations A, B, C, D are shown. Which of the last two do you think it s closer to A and why? Obs: we assume that these are the only stations in our network, for the purpose of this exercise. C is closer to A because it sets the NAV after the RTS is sent. This implies that it hears the RTS, so it is in A s range. D does not hear the RTS (sets its NAV only after hearing the CTS from B), so it s further from A than C. V Assume we have the following WLAN Stations A, B, C are within one BSS with AP1 and stations D, E are within another BSS with AP2, where access point stations AP1 and AP2 belong to a wired infrastructure network. Station A successfully sends a frame to C, B successfully sends a frame to D and then, AP1 polls stations A, B, C for data. B and C have some data to send while A has nothing at that time. Show the steps of these communications (including as much details as you consider necessary). We have this WLAN: AP1 AP2 A B C D E a) A successfully transmits to C A first sends an RTS to C: τ1 addressc addressa FCS, where τ1 is the time in milliseconds to send the RTS + the time to get the CTS (plus a SIFS) + the time to send the data frame (plus a SIFS) + the time to get an ACK back (plus a SIFS) ; τ1 occupies 2 bytes, the addresses 6 bytes each and the FCS 4 bytes

5 C then sends a CTS to A: τ2 addressa FCS, where τ2 is the time in milliseconds to send the CTS + the time to get the data frame (plus a SIFS) + the time to send an ACK back (plus a SIFS) ; it is derived from τ1 A then sends the data to C: τ3 addressc addressa addressap1 seq_nr 6 unused bytes payload FCS, where τ3 is the time in milliseconds to send the data frame + the time to get an ACK back (plus a SIFS) ; it is derived from τ1 Finally C sends back the ACK for correctly receiving the data: τ4 addressa FCS, where τ4 is 0 b) B successfully transmits to D B and D are in different BSSs so there are 2 possibilities. If no PCF is implemented, then a similar RTS/CTS sequence as above is exchanged between B and AP1. Then, the following data frame goes from B to AP1: τ addressap1 addressb addressd seq_nr 6 unused bytes payload FCS. Next, after transferring the data to AP2, a similar RTS/CTS exchange takes place between AP2 and D, then AP2 sends the following data frame to D: τ addressd addressap2 addressb seq_nr payload FCS. D sends the ACK to AP2, which then communicates with AP1, which then sends the ACK to B. If PCF is implemented, then, when AP1 is polling station B, this one can send the data during that contention-free interval, much like in point c) below. c) AP1 is polling A,B,C for data For this, AP1 is first sending a beacon frame, after waiting a PIFS interval. The FC field of the beacon is Then, the situation can be a similar one to that in next page. Here Vivian and Martha stand for stations B and C, while George (with nothing to send) models station A. The FC fields of the exchanged frames are as follows: Data+CF-Poll: Data+CF-ACK: Data+CF-ACK+CF-Poll: CF-Poll: Data(null): CF-End:

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