CS3505: Loca Area Networks CSMA/CD : IEEE 802.3 and LAN Internetworks

1. CS3505:Loca Area Networks CSMA/CD : IEEE 802.3andLAN Internetworks

2. CSMA/CD • basic LAN architecture and protocol • most widespread LAN; estimate: more that 50% of all LANs on the Internet... 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)

3. CSMA / CD : basic protocol • broadcast medium • first version used bus when the MAC receives a packet to transmit: 1. sense carrier (listen); if no signal detected then begin Xmitting message, & continue sensing; if collision detected then Xmit jam, stop Xmitting, wait, then goto (1); when end-of-packet Xmitted, END. else {carrier is busy} go to (1)

4. CSMA / CD : basic protocol • the “wait random time” is precisely defined: binary exponential backoff • physical encoding: digital signals, manchester encoding. Broadcast medium. • because collisions are detected, wasted time is short

5. CSMA / CD : backoff algorithm binary exponential backoff (BACKOFF) • 1 slot = 51.2 s • time following collision measured in slots • a random slot between 1 and 1023 is chosen, and the packet is retransmitted then

6. CSMA / CD : backoff algorithm • example: suppose 2 stations collide. What happens? • what is Prob [another collision]? • why measure time in 51.2 s slots?

7. 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

8. CSMA / CD : topologies, media , etc. • transmission media • coaxial cable (decreasing) • twisted pair (Cat 5, widely used) • fiber (less common but increasing) • topologies • bus - original design; used for many years • star with dumb or smart hub; now usual • data rates • 10 Mbps • 100 Mbps • Gbps where needed available

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

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

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

12. 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); • drop cables 25 m max • max 100 stations per segment on coax • at least 2.5 m between adjacent receivers on coax • at most 1024 stations per ethernet

13. 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

14. CSMA / CD : practical considerations • you have 3 PCs in your house; what will it take (equipment/dollars) to connect them together ? • design or plan a network for a small business with 30-40 machines (PCs, Macs, etc), which is located in a single building. (eqmt, dollars)

15. CS3505:Bridges / LAN internetworks

16. Bridges : connecting LANs together • why do we need to connect LANs • what is a bridge? • types of bridges • routing in LAN internetworks • comparison: bridges, routers, repeaters • connecting similar LANs • connecting dissimilar LANs

17. why LANs need to be connected 1. connect 2 existing LANs (CS, math) 2. LAN too big; split it, but stay connected -- too many stations or traffic for one LAN 3. connect geographically separate LANs. -- 2 offices in different towns 4. reduce collisions --increase efficiency 5. security --help restrict traffic to one LAN

18. bridge : what is it? • low level “switch” that connects two or more LANs. “low level” => “MAC layer” • transparent : there is no change in the LANs or in the protocols of the networks • able to do simple routing • retains the simplicity and flexibility of LANs • faster than “software” switches (routers) • reasonable cost; cheaper than routers

19. bridge or switch? terminology • 1st bridges sold in 1984, were 2-ports • early 1990s, multiport bridges appeared; were called “switches” by marketing vendors; technically no difference between a “switch” (layer 2) and a bridge • multiple LANs connected by high port density bridges commonly called “switched LANs” - actually an internet of LANs • “switching hub”, “LAN switch” - other terms for a multiport bridge

20. bridges & LAN connectors :types • local bridge • remote (2 half bridges) • same LAN, different LAN • two port, multiport • hub (not a bridge) • repeater (not a bridge) • router(not a bridge)

21. bridge : basic function suppose a bridge B connects networks X & Y. Then B : 1. reads all packets on X and Y, noting the destination, source addresses (DA, SA) 2. each packet on Y with DA on X is copied and transmitted on X. 3. each packet on X with DA on Y is copied, transmitted on Y. • the bridge operates on X, Y using the MAC protocol of those LANs.

22. bridge connecting 2 ethernets

23. notes on bridges • all stations have unique MAC addresses • bridge must “know” which LAN station is on • multi-port bridges - similar; extends to multiple LANs • no change or adjustment in NIC needed; bridge completely transparent • bridge operates on each LAN using the MAC protocol • remote (half bridge) - may use another protocol between the 2 half bridges, while using MAC on each LAN

24. LAN internet

25. half - bridge, connecting 2 LANs • 2 halves communicate through some other protocol, e.g., PPP, HDLC.

26. bridges - routing • how do bridges “know” which packets to forward, and in which direction? 2 basic techniques : 1. fixed routing - the information is loaded manually into the bridge (typing it in, etc.). This info is then stored in a routing table. 2. dynamic routing: “learning bridges” - the bridge “learns” where the stations are by watching the traffic on its ports

27. bridges - routing • for fixed routing, many topologies possible • dynamic routing - the internet must be configured as a tree; this simplifies routing • tree : LANs and bridges are the nodes, and the links between them are the edges, and • LANs can be connected only to bridges, not (directly) to other LANs • if a cycle exists, the bridges will detect it and remove one from the active network, so that a tree structure is maintained

28. bridges - dynamic routing • bridge has a routing table, 3 fields : [ dest.address, next port # , time] when bridge receives a packet [DA,SA] on port X: 1. if SA found in table, reset timer, else add [SA, port#, time ] to table. 2. if [DA] found in table send packet out on next port indicated; else send packet on all ports except X.

29. bridge dynamic routing • timer: typical value : 300 seconds (why have the timer? is this a good default value?) • given the tree structure, bridges will learn a station’s direction (explain how?) • MAC addresses could be divided into (network, station) parts. If so, tables can be made smaller, but same algorithm used.

30. bridges, routers, repeaters, hubs • repeaters : simply connect 1 cable to another, repeat the bits. No routing decisions or filtering. • hubs : serve to extend the ethernet. No routing or filtering of messages. • bridges - connect LANs together at the MAC layer; filter and rout messages at the MAC layer. • routers -. Layer 3/3.5 (internet). Software, IP protocol.. Usually more expensive. Discussed in CS4550 ....