UM0934 datasheet(32/113 Pages) STMICROELECTRONICS

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MAC layer

UM0934

32/113

Doc ID 17389 Rev 2

When upper layers request a transmission through an M_Data.Request, according to the

node type:

●

Client and REPEATER: the node originates (IC+1) consecutive transmissions of the

same MAC frame

●

Server: whatever the value of the IC, the node performs only 1 transmission of the MAC

frame.

4.8.2

Current credit (CC)

The current credit (CC) is a 3-bit long field with an initial value equal to IC and automatically

decremented by the MAC layer after each repetition.

When receiving, the CC field is extracted and notified to the upper layers through the

M_Data.Indication upon receipt of the first valid MAC frame within the repetition burst.

Moreover, only in the case of REPEATER, does the node join the repetition by transmitting

CC frames in the next time-slots. CC field is automatically decremented by the MAC layer

after each repetition.

4.8.3

Delta credit (DC)

Delta credit (DC) is 2 bits long and used by the system management application entity

(SMAE) of the Client for credit management, while it has no meaning for a Server or a

REPEATER. It represents the difference (IC-CC) of the last communication originated by the

system identified by the DA address to the system identified by the SA address.

When the MAC builds a long MAC frame, it receives the IC and DC credit values from the

SMAE and they are usually specific to the destination address DA. The MAC fills the MAC

frame header with the passed IC and DC values, while CC is equal to IC for the first

transmitted frame.

When a MAC frame is received, the MAC automatically decodes the IC, CC, DC values and

notifies them to the upper layers (SMAE).

4.8.4

Repeater

The REPEATER is basically a Server being able, in addition, to perform repetitions.

A dedicated MIB object 000Bh (TIC selector and repeater state, 6.5.12) can be used to

configure a Server as a REPEATER.

4.8.5

Example

Figure 12

shows a repetition example.

Upper layers of the Client station requests a broadcast (DA=FFFh) transmission of a MAC

frame (made of a single subframe) with IC=CC=3. The Client performs the transmission of

the original frame (IC=CC=3) followed by 3 repetitions, for a total amount of 4 transmissions.

All the other nodes in the network (both Server and REPEATER) receive the transmissions

and provide an M_Data.Indication to the local host upon receipt of the first valid MAC frame

(i.e. with valid FCS), while the following transmissions are neglected (as they are detected

as being repetitions).

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