4-10-4up.pdf data communication note by timon
- 1. 5: DataLink Layer 5a-11 Ethernet “dominant” LAN technology: ❒ cheap -- $20 for 100Mbs! ❒ first widely used LAN technology ❒ Simpler, cheaper than token rings and ATM ❒ Kept up with speed race: 10, 100, 1000 Mbps Metcalfe’s Etheret sketch 5: DataLink Layer 5a-12 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: ❒ 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 ❒ used to synchronize receiver, sender clock rates 5: DataLink Layer 5a-13 Ethernet Frame Structure (more) ❒ Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match ❒ Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) ❒ CRC: checked at receiver, if error is detected, the frame is simply dropped ❒ See Ethereal trace 5: DataLink Layer 5a-14 Ethernet: uses CSMA/CD A: sense channel, if idle then { transmit and monitor the channel; If detect another transmission then { abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; goto A } else {done with the frame; set collisions to zero} } else {wait until ongoing transmission is over and goto A}
- 2. 5: DataLink Layer 5a-15 Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Exponential Backoff: ❒ Goal: adapt retransmission attempts to estimated current load ❍ heavy load: random wait will be longer ❒ first collision: choose K from {0,1}; delay is K x 512 bit transmission times ❒ after second collision: choose K from {0,1,2,3}… ❒ after ten or more collisions, choose K from {0,1,2,3,4,…,1023} 5: DataLink Layer 5a-16 Ethernet Technologies: 10Base2 ❒ 10: 10Mbps; 2: under 200 meters max cable length ❒ thin coaxial cable in a bus topology ❒ repeaters used to connect multiple segments ❒ repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! 5: DataLink Layer 5a-17 10BaseT and 100BaseT ❒ 10/100 Mbps rate; latter called “fast ethernet” ❒ T stands for Twisted Pair ❒ Hub to which nodes are connected by twisted pair, thus “star topology” ❒ CSMA/CD implemented at hub 5: DataLink Layer 5a-18 10BaseT and 100BaseT (more) ❒ Max distance from node to Hub is 100 meters ❒ Hub can disconnect “jabbering adapter ❒ Hub can gather monitoring information, statistics for display to LAN administrators
- 3. 5: DataLink Layer 5a-19 Gbit Ethernet ❒ use standard Ethernet frame format ❒ allows for point-to-point links and shared broadcast channels ❒ in shared mode, CSMA/CD is used; short distances between nodes to be efficient ❒ uses hubs, called here “Buffered Distributors” ❒ Full-Duplex at 1 Gbps for point-to-point links 5: DataLink Layer 5a-20 Interconnecting LANs Q: Why not just one big LAN? ❒ Limited amount of supportable traffic: on single LAN, all stations must share bandwidth ❒ limited length: 802.3 specifies maximum cable length ❒ large “collision domain” (can collide with many stations) ❒ limited number of stations: 802.5 have token passing delays at each station 5: DataLink Layer 5a-21 Hubs ❒ Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces ❒ Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top 5: DataLink Layer 5a-22 Hubs (more) ❒ Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN ❒ Hub Advantages: ❍ simple, inexpensive device ❍ Multi-tier improves robustness: portions of the LAN continue to operate if one hub malfunctions ❍ extends maximum distance between node pairs (100m per Hub) ❍ What is the maximum number of chained hubs?
- 4. 5: DataLink Layer 5a-23 Hub limitations ❒ single collision domain results in no increase in max throughput ❍ multi-tier throughput same as single segment throughput ❒ Thus, limits on number of nodes in same collision domain and on total allowed geographical coverage ❒ cannot connect different Ethernet types (e.g., 10BaseT and 100baseT) 5: DataLink Layer 5a-24 Bridges ❒ Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination ❒ Bridge isolates collision domains since it buffers frames ❒ When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit 5: DataLink Layer 5a-25 Bridges (more) ❒ Bridge advantages: ❍ Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage ❍ Can connect different type Ethernet since it is a store and forward device ❍ Transparent: no need for any change to hosts LAN adapters 5: DataLink Layer 5a-26 Bridges: frame filtering, forwarding ❒ bridges filter packets ❍ same-segment frames not forwarded onto other segments ❒ forwarding: ❍ how to know which LAN segment on which to forward frame? ❍ looks like a routing problem!
- 5. 5: DataLink Layer 5a-27 Backbone Bridge 5: DataLink Layer 5a-28 Interconnection Without Backbone ❒ Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must path over CS segment