1. Ethernet

2. Problem In an Ethernet, suppose there are three stations very close to each other, A, B and C. Suppose at time 0, all of them have a frame to send, but the medium is busy. After the medium is free (for the inter-frame gap, 9.6us in some Ethernet), A, B, and C will all send, which results in a collision. They will perform the random back-off algorithm. What is the probability that they will collide again in the next attempt? a)2/3. b)3/4. c)5/8. d)None of the above.

3. Problem In an Ethernet, suppose there are three stations very close to each other, A, B and C. Suppose at time 0, all of them have a frame to send, but the medium is busy. After the medium is free (for the inter-frame gap, 9.6us in some Ethernet), A, B, and C will all send, which results in a collision. They will perform the random back-off algorithm. What is the probability that they will collide again in the next attempt? a)2/3. b)3/4. c)5/8. d)None of the above. Answer: c. There are 8 possibilities and three will not cause collision – one selects 0, the other two selects 1.

4. Problem Hypothetically, suppose it turns out that there cannot be more than 8 stations in any Ethernet. Which of the following statement is true? a)The minimum size of the Ethernet frame can be significantly reduced. b)The back-off algorithm should be modified. c)Both of the above. d)None of the above.

5. Problem Hypothetically, suppose it turns out that there cannot be more than 8 stations in any Ethernet. Which of the following statement is true? a)The minimum size of the Ethernet frame can be significantly reduced. b)The back-off algorithm should be modified. c)Both of the above. d)None of the above. Answer: b. never has to choose a large window size.

6. Wireless LAN

7. Basic structure: – Stations plus an access point – Stations talk to the access point, then to outside – Access point talks to stations – Stations talk to stations Design goal: – A MAC protocol to determine who talks next

8. Wireless communications Signal decays according to a power law with the distance, at least to the power of -2 with distance Comparing to Ethernet, what is the difference (as far as MAC is concerned)? When a station is sending, not all stations can hear. No real 100% carrier sense. – In Ethernet, everybody can hear everybody

9. Wireless communications When a station is sending, he cannot hear other stations – cannot decide if there is a collision. No CD in wireless LAN. – In Ethernet, the sender can determine if there is collision and abort immediatelly.

10. Wireless communications Being able to sense the carrier does not mean that you can decode the data If received signal having power P means that you can decode the data, it may be true that at power P/2 you can realize that there is something going on

11. Wireless communication The received signal can be decoded if the signal to noise ratio is larger than a certain threshold. Whether there is a collision depends on the signal to noise ratio at the receiver. You may allow two transmissions at the same time without collision. – In Ethernet, two simultaneous transmission means collision ADCB ADCB A->B, C->D A->B, D->C

12. Wireless communications Hidden terminal, A->B, C->D. C did not hear A. ADCB Exposed terminal. A->B, C->D. C hears A. A D C B

13. Medium Access Control (MAC) Layer 802.11 Asynchronous Data Service – DCF (Distributed Coordination Function) Contention-Based Medium Access Control CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance For elastic applications like email, file transfer Time-Bounded Service – PCF (Point Coordination Function) Contention Free Medium Access Control Optional access method works like polling For time-sensitive voice/video applications

14. Goals How to design an efficient contention-based MAC protocol for wireless LAN? Goals – Collision avoidance to reduce wasted transmissions – Reasonable fairness – Cope with hidden terminals – Allow exposed terminals to talk

15. Problems What problems will occur if apply Ethernet MAC? – No CD, does not know whether there is a collision – No CD, channel waste could be large using 1- persistent – Cannot hear all other people means the sender cannot be sure that he can reserve the whole channel.

16. Fixes No CD, use ACK. If there is no ACK, assume there is collision No CD, has to use non-persistent to reduce collision by AVOIDING COLLISION, CA Cannot hear other people, so devise some channel reservation technique

17. DCF Idea When get a packet to send, sense the channel. If channel is busy, wait until the channel is free for DIFS. Start to backoff for a random time. If busy before reaching zero, freeze bo counter, and reactivate when idle for DIFS again. If counted to 0 and channel is still idle, send. After receives a packet, send ACK. If no ACK received, double the window and retry.

18. Simplified 802.11 DCF operation for unicast in implementation (Automating Cross-Layer Diagnosis of Enterprise Wireless Networks, Sigcomm 2007 ) The first packet does not have to experience the backoff before it is sent; backoff after a successful packet transmission. So if there is a packet following the first packet, it will go through the backoff process before transmission.

19. DCF Do you want the ACK to have the same priority as data packets? How do you make sure that ACK has higher priority? Use time. You have to wait for a certain amount time before you can send. High priority packets wait shorter.

20. DCF The SIFS, DIFS. SIFS is for control packets. DIFS is for data packets. When a station wants to send, if it is a control packet, sense the channel for SIFS, then send. If it is a data packet, sense the channel for DIFS, then send.

21. Research Challenge Any problem do you see in the design of 802.11? Hint: wireless packets are subject to random loss, e.g., if you just walk by and blocked the line-of-sight path, the packet may be lost. In this case, what will 802.11 do? What should be done?

22. Further improvement Further improvement by improving carrier sense The problem is other people cannot hear me sending, so they will send. So, how to make sure that they will know I am sending?

23. RTS/CTS RTS/CTS in the place for carrier sense – RTS – reserves channel for a bit of time, if sender hasn’t heard other CTSes – CTS – sender replies if it hasn’t heard any other RTSes – Both messages include time. Network Allocation Vector (NAV) – If no CTS, exponential backoff – “RTS-CTS-DATA”

24. RTS/CTS 802.11 standardized both CSMA/CA and RTS/CTS In practice, most operators disable RTS/CTS – Very high overhead! RTS/CTS packets sent at “base rate” (6Mbps for 802.11g) – Avoid collisions regardless of transmission rate – Most deployments are celluar (base stations), not ad hoc. Neighboring cells are often configured to use non-overlapping channels, so hidden terminals on downlink are rare Hidden terminal on uplink possible, but if clients mostly d/l, then uplink packets are small. THIS MAY CHANGE. And is likely not true in your neighborhood! – When CS range >> reception range, hidden terminal less important

25. PCF The AP acts as the master and sends out beacon signals for polling stations and stations can sign up for certain amount of bandwidth use Co-exists with DCF. How to make sure that beacon signals have higher priority? – PIFS