Ethernet Frame Format

Ieee 802.3 Frame Format(Ethernet):

The ethernet frame format has seven filelds :preamble,start frame delimiter(SFD), destination address(DA),source address(SA),length or type,data, cyclic redundancy check(CRC). In ethernet their no acknownledgement mechanism in order to intemate received frame,well acknowledgement is implemented in higher layers.

Preamble (7 bytes)

SFD (1bytes)

Destination Address (6 bytes)

Source address (6 bytes)

Length or type (2 bytes)

Data and              padding

CRC (4 bytes)

Fig: frame format of ieee 802.3

Preamble:

It is the first field in the ethernet frame format which contains 7 bytes of alternating 0 and 1 that informs the receiving system to the coming frame and enables it to synchronize its input timing.this is actually added at the physical layer and is not part of the frame.

Start Frame Delimiter:

It is the second filed in the Ethernet frame format which contains 1 byte. It determines the beginning of the frame. SFD warns the stations or station about the chance for synchronization.

Destination Address:

This field has 6 bytes which contains the physical address of the destination station or stations to receive the packet.

Source Address:

This field has 6 bytes which contains the physical address of the sender of the packet.

Length or type:

This filed is defined as a type or length field. The original Ethernet used this field because the type filed to define the upper-layer protocol using the MAC frame. The IEEE standard used as the length filed to define the number of bytes in the data field. It is of 2 bytes field.

Data:

This field carries data encapulsated from the upper layer protocal. It is a minimum of 46 bytes andmaximum of 1500 bytes.

CRC:

This is of 4 bytes field used for error detection.

Data field:

Destination Address (6 bytes)

Source address (6 bytes)

Length or type (2 bytes)

Data and              padding

CRC (4 bytes)

                                                                       Minimum payload length:46 bytes

                                                                     Maximum payload length:1500 bytes

The Minimum frame length has: 512 bits or 64 bytes

The Maximum frame length has:12,144 bits or 1518 bytes

Fig: Minimum and maximum lengths

Addressing:

Each station on ethernet network conisting of its own network interface card(NIC). NIC is fixed inside the station and provides the station with a 6 bytes physical address. Let consider an example of ethernet in hexadecimal notation.

06:01:02:01:2C:4B

6 bytes =12 hex digits =48 bits

Unicast, multicast and broadcast address:

The least significant bit(LSB) of the first byte will defines the type of address. If the bit is determined as 0, the address is unicast otherwise it is defined as multicast.

The broadcast destination address is a special case of the multicast address in which all its are 1s.

Let us consider  an example in order to define destination address.

4A:30:10:21:01:1A

To find the type of address, we need to look at the second hexadecimal digit from the left.if it even the address is unicast. If it is odd the address is multicast.if all the digits are F’s the address is broadcast.

Given example is unicast address because A in binary is 1010(even)

47:20:1B:2E:08:EE

Given example is multicast address because 7 in binary is 0111(odd)