1. MAC continued

2. Ethernet (802.3) 1-persistent CSMA/CD + binary exponential backoff
Carrier sense: station listens to channel first
1-persistent: If idle, station may initiate transmission
Collision Detection: continuously monitor channel and if collision, abort transmission immediately, and wait for a random time
To determine the back-off time of a frame, use binary exponential backoff :
each time slot is 51.2 us
first collision, retransmission interval = random number between [0,1]
second collision, interval = random number between [0,1,2,3]
kth collision, interval = random number between [0, 2^k-1]
upper bound 1023 slots.
Give up after 16 tries

3. 802.3

4. Why binary exponential backoff
Why not pick a random number from a fixed interval?
Why a fixed small interval not good?
Why a fixed large interval not good?

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

6. Minimum Frame Size Why a minimum frame size is needed?
How long does it take for a station to notice a collision?
The key is that you should be still transmitting when being aware of the collision, because if you finished before the collision, when the collision notifying signal comes, you might think it is for some other people’s frame.

7. Worst case

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

9. 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 transmission is
independent across stations, and
independent for one station at different times.
Find the average number of collisions before a frame is sent.
Three steps:
The probability that one station got the chance to send is A=kp(1-p)k-1.
Maximized when p=1/k. So A is bounded by (1-1/k) k-1.
Each contention is independent, so average number of collision is 1/A, which is e when k is large.
P Is the time to send a frame, contention is 2t, so channel efficiency is P/P+2et.

10. Switched Ethernet Stations connect to a switch using dedicated lines.
Input frames are buffered.
So no collision!

11. Ethernet Physical medium thin cable/thick cable/twisted pair/fiber
10Base meters thick (cable) Ethernet nodes/seg
10Base meters thin (cable) Ethernet nodes/seg
10BaseT meters twist pair nodes/seg
10BaseF meters fiber optics nodes/seg
10Base5/10Base2, cable connected to each machine
10BaseT -- connecting to a hub
10BaseF -- between building Connecting

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

13. Wireless LAN Basic structure: Design goal:
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

14. 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

15. 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.

16. 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

17. 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

18. 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

19. 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 , 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

20. 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

21. 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.

22. 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

23. DCF from a high level
When got a packet to send, sense the channel, if idle, send.
When channel is busy, wait until the channel is free. Start to backoff for a random time. If busy before reaching zero, freeze bo counter, and reactivate when idle again. If counted to 0 and channel is still idle, send.
After receives a packet, send ACK.
If no ACK received, double the window.

24. Simplified 802.11 DCF operation for unicast
(Automating Cross-Layer Diagnosis of Enterprise Wireless Networks, Sigcomm 2007 )

25. 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.

26. 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.

27. DCF

28. 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?

29. 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”

30. 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 g)
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

31. 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