UM0934 datasheet(31/113 Pages) STMICROELECTRONICS

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UM0934

MAC layer

Doc ID 17389 Rev 2

31/113

4.7

Timeouts

The MAC layers of a Server node handle 3 timeouts aimed at:

●

Managing the time-slot synchronization, as described in Section 4.7.1, through the

“synchronization-confirmation-timeout” and “timeout-frame-not-OK”

●

Checking the validity of the local MAC address through the “timeout-not-addressed”.

The time-outs can be programmed with the desired values through dedicated MIB objects

0002h (Timeout Synchro Confirmation, Section 6.5.3), 0003h (Timeout Frame Not OK,

Section 6.5.4

), and 0004h (Timeout Frame Not Addressed, Section 6.5.5).

4.7.1

Synchronization-confirmation-timeout

Refer to Section 4.6.2

4.7.2

Timeout-frame-not-OK

Refer to Section 4.6.2

4.7.3

Timeout-not-addressed

This timeout is aimed at periodically checking the validity of the local MAC address and, if

necessary, at releasing it.

Once the Server has achieved the time-slot synchronization, it expects to receive at least

one valid MAC frame addressed to it (i.e. a frame with field DA equal to the local MAC

address of the node) within “timeout-not-addressed”. The timeout counter is refreshed at

any valid MAC frame with DA equal to MAC local address reception.

If the timeout-not-addressed has elapsed, the local MAC address is automatically reset to

the default value NEW (FFEh) and the time-slot synchronization is rejected.

4.8

Credit management and repetitions

In a real power line network several undesired electrical phenomena such as signal

attenuation, noise, and narrow band interferers may impact on communication performance,

preventing a message being delivered to a destination node. In order to overcome this

problem, in accordance with [2], the MAC layer provides a repetition mechanism based on

“credits” (IC, CC and DC) and on specific Server nodes called REPEATERs, behaving

exactly as a Server but being able, in addition, to perform repetitions according to IC and CC

values. Credit information is embedded in the header of each long MAC frame, with the

meaning described in the following sections.

4.8.1

Initial credit (IC)

The initial credit (IC) is a 3-bit long field (with a maximum value equal to 7) and counts how

many times the frame must be repeated.

When receiving, the field IC 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. Such

information is not used by the MAC layer.

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