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

CSMA/CD 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)

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)

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

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

CSMA / CD : backoff algorithm  example: suppose 2 stations collide. What happens?  what is Prob [another collision]?  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 : ; pad : 0-46; FCS : 4

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

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)

CSMA / 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);  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

CSMA / CD : standard s IEEE : 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  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 machines (PCs, Macs, etc), which is located in a single building. (eqmt, dollars)

CS3505: Bridges / LAN internetworks

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

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 offices in different towns 4. reduce collisions --increase efficiency 5. security --help restrict traffic to one LAN

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

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

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)

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.

bridge connecting 2 ethernets

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

LAN internet

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

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

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

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.

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.

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 CS