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Ethernet, Wireless LAN

Ethernet, Wireless LAN. Ethernet Frame Format. (a) DIX Ethernet, (b) IEEE 802.3. Minimum Frame Size. Why a minimum frame size is needed? How long does it take for a station to notice a collision?. Worst case. Minimum Frame Size.

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Ethernet, Wireless LAN

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  1. Ethernet, Wireless LAN

  2. Ethernet Frame Format (a) DIX Ethernet, (b) IEEE 802.3

  3. Minimum Frame Size • Why a minimum frame size is needed? • How long does it take for a station to notice a collision?

  4. Worst case

  5. Minimum Frame Size • So, if maximum delay is t, the minimum frame size is 2t*bit rate. • t is about 50us. • So the minimum frame size of 10M Ethernet is 512 bits. • What if the speed goes up?

  6. Ethernet Performance • Suppose there are k stations. Let p be the probability that a station has a frame to send when the channel is idle. Assume it is independent across stations, and is independent for one station at different times. Find the average number of collisions before a frame is sent. • First, the probability that one station got the chance to send is A=kp(1-p)^{k-1}. • Second, maximized when p=1/k. So A is bounded by (1-1/k)^{k-1}. • Third, each contention is independent, so average number of collision is 1/A, which is e when k is large. • Each contention is 2t, so channel efficiency is P/P+2et.

  7. Ethernet • Physical medium • thin cable/thick cable/twisted pair/fiber 10Base5 500 meters thick (cable) Ethernet 100 nodes/seg 10Base2 200 meters thin (cable) Ethernet 30 nodes/seg 10BaseT 100 meters twist pair 1024 nodes/seg 10BaseF 2000 meters fiber optics 1024 nodes/seg 10Base5/10Base2, cable connected to each machine 10BaseT -- connecting to a hub 10BaseF -- between building Connecting

  8. Ethernet Fast Ethernet • Keep everything in Ethernet, make the clock faster 100Mbps. • Cable • 100Base-T4 100m category 3 UTP, 4 lines. • 100Base-Tx 100m category 5 twisted pair • 100Base-Fx 2000m Fiber optic

  9. Application Presentation Session Transport Network 802.2 Logical Link Control (LLC) Data Link 802.4 Token Bus 802.5 Token Ring 802.11 Wireless 802.3 CSMA/CD Physical IEEE 802 LAN Protocol Stack ISO/OSI Reference Model Computer Science, FSU

  10. 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

  11. 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

  12. 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.

  13. 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

  14. Wireless communication • The received signal can be read 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

  15. Wireless communications • Hidden terminal A B C D • Exposed terminal A B C D

  16. Medium Access Control (MAC) Layer • 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 Computer Science, FSU

  17. 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

  18. 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.

  19. 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

  20. DCF • When got a packet to send, sense the channel, if idle, send immediately (not completely non-persistent! Why?) • When channel is busy, wait until idle, then backoff a random time. If still idle, send. (The non-persistent part. The CA feature). If busy before reaching zero, freeze it, and reactivate when idle again. • After receives a packet, send ACK. • If collision, use the exponential backoff.

  21. 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.

  22. 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.

  23. DCF

  24. 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?

  25. 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”

  26. 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” (often 1Mbit) • 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

  27. 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

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