1 Computer Networks Local Area Networks

2 A LAN is a network: –provides Connectivity of computers, mainframes, storage devices, etc. –spans limited geographical area (up to ~ 2.5 km) LAN topologies Medium Access Control

3 LAN Topologies

4 Ethernet Segments

5 Two-Level Hub Topology

6 Ring Topology

7 IEEE802 Protocol Suite v.s. OSI

8 802 standards MAC + physical layers –802.3 Bus/tree/star topologies CSMA/CD –802.5 Token Ring – Wireless

9 Encapsulation Application data header TCP IP LLC MAC trailer TCP segment IP datagram LLC PDU MAC frame

10 IEEE802.3: CSMA/CD Ethernet uses CSMA/CD – listens to line before/during sending CSMA/CD: Carrier sense, multiple access with collision detection –collisions detected within short time –colliding transmissions aborted –Persistent or non-persistent retransmission Collision Detection: – On baseband bus, collision produces much higher signal voltage than transmitted signal – For Hub-topology activity on more than one port

11 Transmit Process in IEEE802.3 Assemble frame Channel Busy? Start Transmission Collision Detected? T/x done? Send jamming signal Increment # of attempts Too many attempts? Compute Back-off / Wait Backoff time T/x successful T/x aborted Yes No Yes No Yes No

Collision Detection Node ANode B Node A starts transmission at time 0 At time almost T, node A’s message has almost arrived How can we ensure that A knows about the collision? Node B starts transmission at time T 

Collision Detection contd. Node ANode B Node A starts transmission at time 0 At time almost T, node A’s message has almost arrived Node B starts transmission at time T At time 2T, A is still transmitting and notices a collision 

Collision Detection contd. AB AB AB time = 0 time = T time = 2T

Collision Detection contd. How can A know that a collision has taken place? –There must be a mechanism to insure retransmission on collision –A’s message reaches B at time T –B’s message reaches A at time 2T –So, A must still be transmitting at 2T IEEE specifies max value of 2T to be 51.2 micro.sec. –This relates to maximum distance of 2500m between hosts –At 10Mbps it takes 0.1 micro.sec. to transmit one bit so 512 bits (64B) take 51.2 micro.sec. to send –So, Ethernet frames must be at least 64Bytes long Padding is used if data is less than 64Bytes Send jamming signal after collision is detected to insure all hosts see collision

16 IEEE802.3 Frame Format preamble 7 SFD 12 or 62 destination address source address 2 or 6 lengthdata unit pad FCS 4octets IEEE bit CRC used to synchronize receiver, sender clocks SFD: Start-of-frame delimiter

17 IEEE802.3 MAC Addresses Source and destination MAC addresses. These are the hardware addresses or physical addresses. They are 6 Octets each IEEE Organizationally Unique Identifier (OUI) - allows vendor to build hardware with unique addresses XX XX XX XX XX XX bytes Ethernet MAC Address Vendor Part 24 bits Vendor Assigned 24 bits Windows 2000 > ipconfig /all

18 IEEE802.5 : Token Ring Frames flow in one direction Special bit pattern (token) rotates around ring. The token is 24-bit long Node having a frame to transmit must capture token first Node must release token after done transmitting Node remove frame when it comes back around Round-robin service

19 Token Ring Operation

20 IEEE802.5:Token Ring Token Holding Time (THT) Token Rotation Time –TRT <= # of Active Nodes * THT + Ring Delay –Active nodes denotes the number of nodes that have data to transmit. –Ring latency denotes how long it takes the token to circulate around the ring when no one has data to send. Monitor Station –Introduce additional delay –Detect missing token # of Nodes * THT + Ring Delay –Ring delay- token circulation time

Fiber Distributed Data Interface protocol As opposed to Token Ring's single ring, FDDI, uses two to achieve better results and less chance of failure. In a basic Token Ring network, at any instant there is a single active ring monitor which supplies the master clock for the ring, whereas in FDDI this approach isn't ideal because of the high data rates. Instead, each ring interface has its own local clock, and outgoing data is transmitted using this clock. Due to its dual ring architecture, FDDI has the ability to recover from link and station failures. If a station goes down, the signals are routed around it by a loop formed from the rings. 21