Computer Communication & Networks: Datalink Layer: Local Area Network
Computer Communication & Networks: Datalink Layer: Local Area Network
Computer Communication & Networks: Datalink Layer: Local Area Network
IEEE Standards
In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers. Project 802 is a way of specifying functions of the physical layer and the data link layer of major LAN protocols.
WiMAX
Ethernet
Dominant LAN technology: Cheap $20 for 100Mbs! First widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 10, 100, 1000 Mbps
Preamble:
7 bytes with pattern 10101010 followed by one byte with pattern 10101011 Used to synchronize receiver, sender clock rates
8
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
10
Ethernet
Ethernet uses 1-persistent CSMA/CD on coaxial cable at 10 Mbps (802.3 allows other speeds & media) The maximum cable length allowed: 500m Longer distances covered using repeaters to connect multiple segments of cable No two stations can be separated by more than 2500 meters and 4 repeaters Including the propagation delay for 2500m and the store and forward delay in 4 repeaters, the maximum time for a bit to travel between any two stations is max=25.6se (one way)
11
12
Ethernets CSMA/CD
In order to ensure that every collision i s heard" by all stations, when a station detects a collision , it jams the channel for Example Two stations, A and B, are close together A third station, C, is far away A and B will detect each others transmission very quickly and shut off This will only cause a short blip which may not be detected by C but will still cause enough errors to destroy Cs packet
13
Ethernets CSMA/CD
When collisions occur, Ethernet uses a random retransmission scheme called exponential backoff: 1. If your packet is in a collision, set K=2 2. Pick a number k at random from {0, 1,..,K-1} 3. After max seconds, sense channel, transmit if idle 4. If collision occurs, let K=2 x K, go to step 2
After 10 repeats, stop doubling K After 16, give up and tell layer above I give up Fixes random access stability problem by passing it to the layer above!
14
Repeaters used to connect up to multiple segments Repeater repeats bits it hears on one interface to its other interfaces: physical layer device!
15
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
16
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
17
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
18
19
20
21