Data Communication Networks Lec 18 Wired LAN Ethernet

Data Communication Networks Lec 18

Wired LAN: Ethernet • Datalink layer – Logical link control(LLC) – MAC • Physical layer

IEEE standard for LANs

Data link layer: LLC • Framing, flow control , error control • Provide one single data link control protocol for IEEE standards. • MAC provide different protocols for different LANs. • Makes MAC sublayer transparent

Need of LLC • Provide flow and error control for upper layer protocols that demand services. • LLC can be used to provide flow and error control for application layer protocols.

Media Access Control • CSMA /CD method for Ethernet. • Token

Standard Ethernet

MAC Sublayer • Govern the operations of access method. • It frames data received from the upper layer and passes them to the physical layer.

Frame format • Seven fields – Preamble – SFD – DA – SA – Length and type of protocol data unit(PDU) – Upper- layer data – CRC

Frame format

Frame format

Frame format

Frame length

Maximum and minimum length • Minimum length restriction is required for correct operation of CSMA/CD. • Minimum length is 512 bits or 64 bytes. • Header and trailer. • 18 bytes of header and trailer • Minimum length of data from the upper layer is 64 -18=46 bytes. • If upper-layer packet is less than 46 bytes , padding is added.

Maximum and minimum length • Maximum length of the frame (without preamble and SFD)as 1518 bytes. • 18 bytes of header and trailer , the maximum length of the payload is 1500 bytes. – Memory was expensive , help to reduce size of buffer – Prevent one station from monopolizing the shared medium, blocking other stations that have data to send.

Maximum and minimum length

Addressing • NIC • 6 byte physical address.

Unicast, Multicast and broadcast addresses • A source address is always a unicast address – from one station • Destination can be unicast, multicast or broadcast. • If LSB of the first byte in a destination address is 0, adddress is unicast, otherwise , it is muticast.

Unicast, Multicast and broadcast addresses

Unicast, Multicast and broadcast addresses

Physical layer

Encoding

Summary