[Ethernet] Packet formet

Media Access Control (MAC) frame and packet

*octet = 8bit = 1byte

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1. Preamble field

7-octet field that is used allow the PLS circuitry to reach its steady-state synchronization with the received packet’s timing.

A bit sequence used for physical medium stabilization and synchronization,

 

10101010 10101010 10101010 10101010 10101010 10101010 10101010

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2. SFD

Start Frame Delimiter(SFD)

 

10101011

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3. Address fields

Destination Address field

The Destination Address field shall specify the destination addressee(s) for which the MAC frame is intended.

 

Source Address field

The Source Address field shall identify the station from which the MAC frame was initiated.

Address field format

3-1. Address format

Individual Address

The address associated with a particular station on the network.

 

Group Address

A multidestination address, associated with one or more stations on a given network.

1. Multicast-Group Address

2. Broadcast Address

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4. Length/Type field

This two-octet field takes one of two meanings.

 

1. Less than or equal to 1500 decimal (05DC hexadecimal)

Length/Type field indicates the number of MAC client data octets contained in the subsequent MAC Client Data field of the basic frame (Length interpretation).

 

2.Greater than or equal to 1536 decimal (0600 hexadecimal)

Length/Type field indicates the Ethertype of the MAC client protocol (Type interpretation). The Length and Type interpretations of this field are mutually exclusive.

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5. MAC Client Data field

Data

1. 1500 decimal—basic frames

2. 1504 decimal—Q-tagged frames

3. 1982 decimal—envelope frames

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6. Pad field

A minimum MAC frame size is required for correct CSMA/CD protocol operation. If necessary, a Pad field (in units of octets) is appended after the MAC Client Data field prior to calculating and appending the FCS field. 

 

The length of the Pad field required for MAC Client Data that is clientDatasize/8 octets long is 
max [0, minFrameSize – (clientDatasize + 2 * addressSize + 48)] bits.

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7. Frame Check Sequence (FCS) field

A cyclic redundancy check (CRC) is used by the transmit and receive algorithms to generate a CRC value for the FCS field. The FCS field contains a 4-octet (32-bit) CRC value. 

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8. Extension field

The Extension field follows the FCS field, and is made up of a sequence of extension bits, which are readily distinguished from data bits. The length of the field is in the range of zero to (slotTime-minFrameSize) bits, inclusive. The contents of the Extension field are not included in the FCS computation.