Review: Medium Access Control Sublayer –What is the problem to be addressed in this sublayer? –Protocols that allow collision Pure ALOHA Slotted ALOHA CSMA CSMA/CD –Collision free protocols: bitmap method, binary countdown and token

Collision free protocols: Token pass. –There is only one token in the network. –The token is passed through every node in the network. –Only the node that has the token can transfer data.

Limited contention protocols: –collision based protocols (ALOHA,CSMA/CD) are good when the network load is low. –collision free protocols (bit map, binary countdown) are good when load is high. –How about combining their advantages -- limited contention protocols. Behave like the ALOHA scheme under light load Behave like the bitmap scheme under heavy load.

Limited contention protocols: –adaptive tree walk protocol trick: dynamic partition the stations into groups and limit the contention for each slot. –under light load, every one tries for each slot like ALOHA –under heavy load, only a small group can try for each slot –how do we do it »treat stations as the leaves of a binary tree. »first slot, all stations (under the root node) can try to get the slot. »if no conflict, repeat. »if conflict, use depth first search to traverse the tree, only nodes of a sub-tree get to try for the next slot.

0 1 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.

Ethernet: –Invented at Xerox by Robert Metcalfe (founder of 3Com) and Dave Boggs –background: ARPANet in late 60's, linking computers at different sites to central mainframe computers. By early 70's, the cost of computers went down, introduction of mini-computers PDP, which means each school can have more than one computer! – Applications: share printers, share files, share cycles Factory automation: many computers on factory floor Need local area networks to link the computers

Ethernet: –Use shared medium instead of switched-based –cost: one adaptor/machine + link –performance: all hosts sharing one link. –first Ethernet: 3 Mbps –PDP MIPS, 0.1 Mbps peak –no all computers transmit at peak all the time meaning, easily support up to 100 computers at that time – Now: –500MHz Pentium, around 200MIPS, 100Mbps –10Mbps cannot support as many machines.

Medium Access Problem: –multiple stations may transmit on the medium at the same time, which may result in collisions – Two solutions guarantee that only one station transfers at one time: (contention free protocol) FDDI, token ring, token bus use the first approach try the luck and re-transmit if there is a collision (contention based protocol) –need algorithm to reduce the probability of collision Ethernet uses CSMA/CD + binary exponential backoff to reduce the probability of collisions

CSMA/CD + binary exponential backoff –sense before send (CSMA) –abort sending upon detecting collision (CD). –adjust retransmission interval (binary exponential backoff) each time slot to be 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.

Important design parameters –Bandwidth: 10 Mbps Propagation Delay: limit the frame size. 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

–Multiple segments can be connected through the repeaters (hubs). –All segments connected by the repeaters are in the same collision domain. constraint: no two transceivers may be 2.5km apart and separated by 4 repeaters. frame format | Preamble | Start| Dst Addr | Src Addr | length | Data |Pad |Checksum| 7 1 2/6 2/ –Header: 14 Bytes, CRC: 4 Bytes – Minimum data (+ pad) length: 46 Bytes – Maximum data length: 1500 Bytes

Minimum frame size = ?? Why? To run CSMA/CD, each frame must be large enough to detect collision. –2 * max propagation delay? »standard: 2500m, 500m per segment, 4 repeaters. »speed of light: 3*10^8m/s »speed of signal propagation: 2*10^8m/s »propagation delay: about 25us (on wire) +25 us in repeaters, total delay = 51.2us »How many bytes do we need in each frame? Maximum frame size = ? Why? –larger is better for bandwidth utilization

How to find out your Ethernet address: "arp” –/usr/sbin/arp xi --> xi ( ) at 8:0:20:92:43:b1 Ethernet Switch: Increase the bandwidth, segments connected by switch have different collision domain. –Ethernet switch: data link layer device –Ethernet hub (repeater): physical layer device –Fast Ethernet Keep everything in Ethernet, make the clock faster 100Mbps. What are the problems? –Cable »100Base-T4 100m category 3 UTP, 4 lines. »100Base-Tx 100m category 5 twisted pair »100Base-Fx 2000m Fiber optic

What are the problems? –Cable –CSMA/CD? minimum frame size = 64byte = 512 bits, 5.12us using 100Mbps transmission rate. What can you do about this? –Increase the minimum frame size. –Reduce cable length –Faster Ethernet: »Reduce the cable length by a factor of 10, maximum length = 200 meters (100-Base-T, 100 meter cable). Full duplex mode: point to point connection, no contention. No CSMA/CD needed, can have longer cable.

–Gigabit Ethernet: make it even faster at 1Gbps. Cable: mainly fiber optics. CSMA/CD domain –Shortening the cable? 20 meters Alternative: increase the minimum frame size to 512 bytes, CSMA/CD domain 200 meters (not much error margin) –Experimental studies say that typical frame size are bytes. backward compatibility: –carrier extension -- short packet, stuff extra bits to make to 512 bytes improve performance: packet bursting -- transmit a burst of small frames, only the first one need carrier extension.