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

LANs : Aloha/CSMA , summary describe these using a CFSM model: pure Aloha slotted Aloha 1-persistent CSMA p-persistent CSMA nonpersistent CSMA Compare approximate utilization of the above

LANs : CSMA 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?

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)

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)

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?

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.

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?

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

CSMA / CD : topologies, media , etc. 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

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)

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

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

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 2000 with repeaters (coax); 185 m /segment on thin cable. drop cables 25 m max max 100 stations per segment on coax at least 2.5 m between adjacent receivers on coax at most 30 stations per segment on thin cable at most 1024 stations per ethernet

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

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