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Ethernet. Review Media Access Control – Broadcast media shared by all stations – MAC is used to determine who gets the right to send Developed protocol.

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Presentation on theme: "Ethernet. Review Media Access Control – Broadcast media shared by all stations – MAC is used to determine who gets the right to send Developed protocol."— Presentation transcript:

1 Ethernet

2 Review Media Access Control – Broadcast media shared by all stations – MAC is used to determine who gets the right to send Developed protocol (All contention based) – P0. Send at will. – P1. ALOHA. – P2. Slotted ALOHA – P3. Carrier Sense. (Ask which curve is for which) 1-persistent Non-persistent P-persistent – P4. CSMA+CD

3 Contention Free? The contention based protocols allows contention, so works well under light load. How to design contention-free protocols? The key is: let other people know that I am going to send

4 Contention Free Protocols – Bitmap bit-map method. – control frame contain N bits, each station send 1 bits to indicate whether it has a frame to send – at the end of the control frame, every station knows all stations that want to send, the station can send in order. – example: – Performance: d/(d+1) channel utilization rate for high load. N bits delay for low load. (d is the frame size).

5 Contention Free Protocols: Binary Count Down each station sends the address bits in some order The bits in each position from different stations are ORed. As soon as a station sees that a high-order bit position that is 0 is overwrite by 1, it gives up. Eventual, only one station (with largest station number among all the competitors) gets the channel. Performance: – channel utilization rate: d/(d+log(N)) for high load – log(N) bits delay for low load. – Contention field can serve as the address field. This protocol assumes that delays are negligible!

6 Contention Free Protocols – Token Ring Passes tokens among stations and only stations got the token is allowed to send

7 Limited Contention Protocols Want to be as contention-based protocols (ALOHA) under light load and as contention- free protocols under heavy load (bitmap) Can we achieve that? The trick is to dynamically control the size of the group that can contend for a slot – under light load, only one group including everyone and everyone can try for each slot – this is aloha – under heavy load, the group size is small and each group can only try for his slot – when the size of the group is 1, this is bitmap

8 Limited Contention Protocols So, how to determine the right size of the group? Each station monitors the load (because it is broadcast media). If there are N stations and n have frames to send, divide into N/n groups

9 Limited Contention Protocols What happens if there is a contention inside a group? Many ways to deals with it. One way we will talk about is to send all frames of this group, then move to the next group. Suggestions?

10 Limited Contention Protocols 0 1 Like a binary search. Example: ABC* D E*F*GH* Slot 0: C*, E*, F*, H* (all nodes under node 0 can try), conflict slot 1: C* (all nodes under node 1 can try), C sends slot 2: E*, F*, H*(all nodes under node 2 can try), conflict slot 3: E*, F* (all nodes under node 5 can try), conflict slot 4: E* (all nodes under E can try), E sends slot 5: F* (all nodes under F can try), F sends slot 6: H* (all nodes under node 6 can try), H sends.

11 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 binary exponential backoff (new, how to pick the random time): 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.

12 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?

13 Ethernet Frame Format (a) DIX Ethernet, (b) IEEE 802.3

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

15 Worst case

16 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?

17 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 indepedent, 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.

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

19 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

20 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


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