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CS3502: Data and Computer Networks Local Area Networks - 2 CSMA/CD : IEEE 802.3

CS3502: Data and Computer Networks Local Area Networks - 2 CSMA/CD : IEEE 802.3 . LANs : CSMA deficiency . when collisions occur, how much time is wasted? what is approximate relative likelihood of repeating the collision, with CSMA, 1-persistent CSMA, 0.1 persistent

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CS3502: Data and Computer Networks Local Area Networks - 2 CSMA/CD : IEEE 802.3

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  1. CS3502:Data and Computer NetworksLocal Area Networks - 2CSMA/CD : IEEE 802.3

  2. LANs : CSMA deficiency • when collisions occur, how much time is wasted? • what is approximate relative likelihood of repeating the collision, with • CSMA, 1-persistent • CSMA, 0.1 persistent • CSMA, nonpersistent • How can time wasted be reduced?

  3. LANs : CSMA/CD • adds collision detection capability to CSMA; greatly reduces time wasted due to collisions • standardized as IEEE 802.3; most widespread LAN, perhaps most widespread network protocol; estimate: more that 50% of all LANs on the Internet use this protocol... implemented by many different companies • developed by Robert Metcalfe, XEROX PARC, early 1970s..... led to founding of “3COM” company, (Santa Clara). [later Metcalfe sold his company for $400M)

  4. CSMA / CD : basic protocol • big difference from CSMA is that transmissions are stopped when collisions are detected. • first version used bus -broadcast topology when the MAC receives a packet to transmit: (1) sense the carrier; {LISTEN} if no signal is detected then begin Xmitting message & continue sensing; if collision detected then Xmit jam, stop Xmitting, wait(BACKOFF), goto (1); when end-of-packet Xmitted, END. else {carrier is busy} go to (1)

  5. CSMA / CD : basic protocol • the “wait random time” is precisely defined: uses the binary exponential backoff algorithm • physical encoding: digital signals, Manchester encoding. Broadcast medium. • if collisions are detected, then wasted time is relatively short Q : what if the packet length is very short? (consider propagation delay) How long should the packet be?

  6. CSMA / CD : backoff algorithm binary exponential backoff (BACKOFF) • 1 slot = 51.2 s • time following collision measured in slots • after ith collision (i = 1, ..., 10) chose a random slot in [0, 2i - 1]. • for i = 11..16, choose from [0..1023] • when i reaches 16, give up.

  7. CSMA / CD : backoff algorithm • example: suppose 2 stations collide. What happens? • what is Prob [another collision]? • if 2nd collision occurs, what is Prob [3rd]? • why measure time in 51.2 s slots?

  8. CSMA / CD : frame format length: 64 to 1518 bytes preamble : 7 bytes; SOF : 1 byte; DA,SA : 2/6 bytes; length : 2 bytes; data : 0-1500; pad : 0-46; FCS : 4 exercise: specify the protocol using a CFSM model

  9. CSMA / CD : topologies, media , etc. • media • coaxial cable • twisted pair • fiber (less common) • topologies • bus - original design; widespread for many years • star with hub in middle; now becoming common • data rates • 10 Mbps • 100 Mbps • Gbps on the way

  10. CSMA / CD : network components • medium (coax, tp, fiber) • transceivers • AUI cable (drop cable; station to coax) • NIC (network interface card -MAC protocol logic) • repeaters (needed to extend coax) • test equipment • hub (multiport repeater) : for star configuration • bridges (to connect to other LANs)

  11. CSMS / CD : physical layout coax. cable, physical(and logical) bus CSMA/CD

  12. CSMA / CD : physical layout star/hub configuration. physical star, logical bus 2-twisted pair connections; hub is a repeater

  13. CSMA / CD : some specifics • prop speed 0.77 c on coax, 0.59 c on t.p. • at most 4 repeaters between 2 stations; so at most 5 cable segments • 500 m /segment max, or 2500 with repeaters (coax); 185 m /segment on thin cable. • drop cables 25 m max (thick coax only) • max 100 stations per segment on thick coax • at least 2.5 m between adjacent receivers on coax • at most 30 stations per segment on thin coax • at most 1024 stations per ethernet

  14. CSMA / CD : standard s IEEE 802.3 : several physical configurations: • 10BASE5 : baseband coaxial cable; original • 10BASE2 : thin coaxial cable; cheaper alternative • 10BASE-T : twisted pair, hub configuration • 10BROAD36 : uses broadband coax (TV cable) • 10BASE-F : fiber • 100BASE-X : fiber OR twisted pair NOTE: all use the same frame format and basic MAC protocol

  15. CSMA / CD : practical considerations For the following: consider only the LAN hardware needed; don’t yet consider internet connections • you have 3 PCs in your house; what will it take (equipment/dollars) to connect them together ? • design a network for a small business with 30-40 machines (PCs, Macs, linux), which is located in a single building. (eqmt, dollars) • ditto, but it’s in 2 different buildings 10 miles apart

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