FDDI mac Frame

In FDDI reverse double loop or other structure, the data unit transmitted on the fiber is called the media access Control (MAC) frame.

As shown in Figure 11. PA SD FC DA SA I FCS ED FS

PA: Frame First sequence I: information

SD: Frame First delimiter FCS: Frame test sequence

FC: Frame Control (2 symbols) ED: Frame tail delimiter

DA: Endpoint address FS: Frame state

SA: Source Point Address

(a) Frame format

PA SD FC ED

(b) Token format

Frames are made up of 9 fields, each of which has its own functionality. These features are briefly described below:

1, leading code (PA)

Used to synchronize frames with each station's clock. The preamble issued by the frame start station consists of 16 idle symbols of 64 bits

Composition Subsequent stations can change the length of the field to suit the clock's requirements.

2, Frame first delimiter (SD)

Indicates the beginning of a frame, which is always composed of the signal code type of the data of the people. The code is JK, where J and K are non-number

According to the symbol.

3, Frame control (FC)

Its bit format is clffzzzz. C indicates the frame type, L indicates 16 bits or 48 addresses, FF indicates that the frame is an LLC frame or

is the Mac control frame.

4, the end of the address (DA)

Indicate the station to which the frame wishes to be sent. The address can be a single station address, multicast address or broadcast address.

5, Source point address (SA)

The station address where the frame is issued.

6, including the LLC data and operation-related information.

7. Frame Inspection Sequence

32-bit length for protection of Fc,da,sa and information fields.

8, Frame tail delimiter (ED)

Consists of a number of non-data symbols. For tokens, Ed has a length of 8 bits and 4 bits for other frames.

9, Frame State (FS)

By the error bit, address recognition, frame copy and other indicators. Each instruction is represented by a symbol. FS can also be wrapped

Encloses the additional control bits, which are determined by the implementing person.