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 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

Physical layer

Encoding

Summary