Ethernet Frame explained.

Actually,Ethernet frames look like

The function of the various parts is as follows: 

Preamble/Start of Frame Delimiter, 8 Bytes - Alternate

ones and zeros for the preamble, 11010101 for the SFD

(Start of Frame Delimiter). This allows for receiver

synchronisation and marks the start of frame.

Destination Address, 6 Bytes - The MAC destination

address of the frame, usually written in hex, is used to

route frames between devices. Some MAC addresses are

reserved, or have special functions. For example

FF:FF:FF:FF:FF:FF is a broadcast address which would go

to all stations.

Sources Address, 6 Bytes - The MAC address of the

sending station, usually written in hex. The source

address is usually built into a piece of equipment at

manufacture. The first three bytes would identify the

manufacturer and the second three bytes would be

unique to the equipment. However there are some

devices, test equipment for example, in which the

address is changeable.

VLAN Tag, 4 Bytes (optional) - The VLAN tag is

optional. If present it provides a means of separating

data into “virtual” LANs, irrespective of MAC address. It

also provides a “priority tag” which can be used to

implement quality of service functions.

Length/Type, 2 Bytes - This field is used to give either

the length of the frame or the type of data being carried

in the data field. If the length/type value is less than

05DC hex then the value represents the length of the

data field. If the value is greater than 0600 hex then it

represents the type of protocol in the data field, for

example 0800 hex would mean the frame was carrying

IP. 809B hex would mean the frame was carrying

AppleTalk.

Data, 46 to 1500 Bytes - The client data to be

transported. This would normally include some higher

layer protocol, such as IP or AppleTalk.

Frame Check Sequence, 4 Bytes - The check sequence

is calculated over the whole frame by the transmitting

device. The receiving device will re-calculate the

checksum and ensure it matches the one inserted by the

transmitter. Most types of Ethernet equipment will drop a

frame with an incorrect or missing FCS.

The minimum legal frame size, including the FCS but

excluding the preamble, is 64 bytes. Frames below the

minimum size are known as “runts” and would be

discarded by most Ethernet equipment.

The maximum standard frame size is 1522 bytes if VLAN

tagging is being used and 1518 bytes if VLAN is not being

used. It is possible to use frames larger than the

maximum size. Such frames are called “Jumbo Frames”

and are supported by some manufacturer’s equipment in

various sizes up to 64 Kbyte. Jumbo frames are identical

in form to standard frames but with a bigger data field.

This produces a better ratio of “overhead” bytes to data

bytes and hence more efficient transmission. Jumbos are

non-standard and manufacturer specific and therefore

interoperability cannot be guaranteed.

The frames are transmitted from left to right, least

significant bit first. The frames are separated by an

“inter-packet gap”. The minimum length of the interpacket

gap is 12 bytes. The inter-packet gap exists

because in a half duplex system time is needed for the

medium to go quiet before the next frame starts

transmission. The inter-packet gap is not really needed

for full duplex operation but is still used for consistency.

Auto-Negotiation

Most Ethernet devices support auto-negotiation. When

two devices are first connected together they will send

information to each other to “advertise” their

capabilities. The devices will then configure themselves

to the highest common setting. The capabilities

negotiated are speed, full or half duplex operation and

the use of flow control.