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Review: Medium Access Control Sublayer What is the problem to be addressed in this sublayer? Protocols that allow collis

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.

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Review: Medium Access Control Sublayer What is the problem to be addressed in this sublayer? Protocols that allow collis

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  1. 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

  2. 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.

  3. 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.

  4. 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.

  5. Example: 0 2 1 3 6 4 5 D A B C* E* F* G H* 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.

  6. 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

  7. Ethernet: • Use shared medium instead of switched-based • cost: one adaptor/machine + link • performance: all hosts sharing one link. • first Ethernet: • 3 Mbps • PDP-11 0.25 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.

  8. 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

  9. 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.

  10. 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

  11. 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 2/6 2 0-1500 0-46 4 • Header: 14 Bytes, CRC: 4 Bytes • Minimum data (+ pad) length: 46 Bytes • Maximum data length: 1500 Bytes

  12. 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

  13. How to find out your Ethernet address: "arp” • /usr/sbin/arp xi --> xi (128.186.121.41) 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

  14. 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.

  15. 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 200 - 300 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.

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