UM0934 datasheet(25/113 Pages) STMICROELECTRONICS

25 / 113 page

UM0934

MAC layer

Doc ID 17389 Rev 2

25/113

Figure 9.

MAC synchronization services

4.4

Framing

The framing process performed by the MAC layer is depicted in Figure 11.

Upper layers (running in the external host) request a transmission through the primitive

M_Data.Request(), passing to the MAC the service data unit M_sdu to be transmitted and

additional information regarding addresses and repetition credits. Such information is used

by the MAC to build the “long MAC frame” made up of the following fields:

●

Header:

–

NS: number of subframes building the long frame

–

SA: source address (12-bit)

–

DA: destination address (12-bit)

–

Repetition credits (information used for repetition management, 8 bits)

●

M_sdu (from 1 up to 242 bytes)

●

FCS (24 bit CRC).

The “long MAC frame” is then divided into several 38-byte fixed-length protocol data units

(M_pdu) called “MAC subframes”, the structure is depicted in Figure 10. The frame indicator

(FI) is always equal to 0000h to mark it as a subframe of a long MAC frame.

Power Line

time-slot

synchronization

change

(found or loss)

time-slot

synchronization

reject

P_Sync.request

P_Sync.indication

PHY

Layer

MAC

Layer

Local Node

M_Sync.request

M_Sync.indication

Upper

Layers

(ext. Host)

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