Mumbai University > Information Technology > Sem4 > Computer Networks

Marks: 5M

Year: Dec 18

1. PREAMBLE – Ethernet frame starts with 7-Bytes Preamble. This is pattern of alternative 0’s and 1’s which indicates starting of the frame and allow sender and receiver to establish bit synchronization. PRE (Preamble) indicates the receiver that frame is coming and allow the receiver to lock onto the data stream before the actual frame begins.

2. Start of frame delimiter (SFD) – This is a 1-Byte field which is always set to 10101011. SFD indicates that upcoming bits are starting of frame, which is destination address. Sometimes SFD is considered the part of PRE, this is the reason Preamble is described as 8 Bytes in many places.

3. Destination Address – This is 6-Byte field which contains the MAC address of machine for which data is destined.

4. Source Address – This is a 6-Byte field which contains the MAC address of source machine. As Source Address is always an individual address (Unicast), the least significant bit of first byte is always 0.

5. Length – Length is a 2-Byte field, which indicates the length of entire Ethernet frame. This 16-bit field can hold the length value between 0 to 65534, but length can not be larger than 1500 because of some own limitations of Ethernet.

6. Data – This is the place where actual data is inserted, also known as Payload. Both IP header and data will be inserted here, if Internet Protocol is used over Ethernet. The maximum data present may be as long as 1500 Bytes. In case data length is less than minimum length i.e. 46 bytes, then padding 0’s is added to meet the minimum possible length.

7. Cyclic Redundancy Check (CRC) – CRC is 4 Byte field. This field contains 32-bits hash code of data, which is generated over Destination Address, Source Address, Length and Data field. If the checksum computed by destination is not same as sent checksum value, data received is corrupted.