Ethernet

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? • 2/3. • 3/4. • 5/8. • 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? • 2/3. • 3/4. • 5/8. • 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? • The minimum size of the Ethernet frame can be significantly reduced. • The back-off algorithm should be modified. • Both of the above. • 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? • The minimum size of the Ethernet frame can be significantly reduced. • The back-off algorithm should be modified. • Both of the above. • None of the above. Answer: b. never has to choose a large window size.

6. Wireless LAN

7. Wireless LAN • 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 A B C D A B C D A->B, D->C A->B, C->D

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

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