Ethernet Frame Structure Ethernet frame PREAMBLE 7 bytes

Ethernet Frame Structure

Ethernet frame PREAMBLE 7 bytes INFORMATION 46 -1500 BYTES SD 1 byte PADDING 2 bytes 6 bytes 4 bytes

PREAMBLE (8 Bytes) • The Ethernet frame begins with an 8 -byte preamble field. • Each of the first 7 bytes of the preamble has a value of 1010. • The first 7 bytes of the preamble serve to “wake up” the receiving adapters and to synchronize their clocks to that of the sender’s clock. • The sender tries to transmit at a target rate (say 10 Mbps or 100 Mbps) but there is a drift in the target rate. So receiver locks on to first 7 bytes of the preamble to sync its clock with the sender’s clock. • The last byte (SD) is 10101011. • The last 2 bits of the eighth byte of the preamble (the first two consecutive 1 s) alert the reciever that the “important stuff” is about to come.

Destination Address (6 Bytes) • This field contains the MAC address of the destination (say BB-BB-BB-BB). • When receiver receives Ethernet frame with destination as its MAC address in the destination address field, it passes on the data frame to network layer. If the destination address is not for that receiver, it discards the frame.

Source Address (6 Bytes) • This field contains the MAC address of the adapter that transmits the frame onto the LAN (say AA-AA-AA-AA)

Type Field (2 Bytes) • Type Field tells the reciever to which network layer protocol should it pass the contents of the data field. • It helps receiver to know that to which protocol has the sender multiplexed the datagram so that it could demultiplex the datagram in the same network layer protocol. • Examples of Network Layer Protocol : IP, ARP, Apple. Talk, Novell IPX

Data Field (46 -1500 Bytes) • This field carries the IP datagram • The maximum transmission unit (MTU) of Ethernet is 1, 500 bytes. This means that if the IP datagram exceeds 1, 500 bytes, then the host has to fragment the datagram • The minimum size of the data field is 46 bytes. This means that if the IP datagram is less than 46 bytes, the data field has to be “stuffed” to fill it out to 46 bytes using ”PAD” field.

Cyclic redundancy check (CRC) (4 Bytes) • The purpose of the CRC field is to allow the receiving adapter to detect bit errors in the frame.

Minimum Header length is 22 Bytes (8+6+6+2)