UM0934 datasheet(30/113 Pages) STMICROELECTRONICS

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MAC layer

UM0934

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Doc ID 17389 Rev 2

4.6.2

MAC layer

At MAC layer, two further checks are performed in order to confirm or reject the time-slot

synchronization acquired by the PHY:

Frame integrity: a valid MAC frame (i.e. with a valid CRC) must be received within

“synchronization-confirmation-timeout”

(Section 4.7) from the reception of the

PRE+SSD causing the synchronization. This condition is aimed at avoiding the

acquisition of a fake synchronization, caused by the reception of a PRE+SSD

simulated by the noise on the line.

Address check: two operating modes are available:

–

unlocked mode: no checks on addresses are performed. Only condition 1 is used

to accept or refuse the synchronization;

–

locked mode: both source (SA) and destination (DA) addresses are checked and

compared against a local MIB object called “Initiator MAC address”, programmed

by upper layers (usually with the address of the network’s Client, also called

“initiator”). If at least one SA or DA matches with the “Initiator MAC address” value,

the synchronization is confirmed or otherwise rejected. This condition is aimed at

forcing a Server to accept only the time-slot synchronization coming from a

specific node (usually the network’s Client), by confirming the time-slot

synchronization only on frames originating from, or addressed to, that node.

After having confirmed the time-slot synchronization according to the conditions mentioned

above, the MAC layer keeps on monitoring the validity of the synchronization through the

“timeout-frame-not-OK”. At least one valid MAC frame must be received within this timeout.

The timeout counter is reset at any valid MAC frame reception. If no valid MAC frame is

received within this time interval, time-slot synchronization is rejected.

4.6.3

Initiator MAC address

The field “Initiator MAC address” in the MIB object 0001h can be used by upper layers to

force a Server to accept the time-slot synchronization coming only from a specific node, as

described in Section 4.6.2.

Valid values for the initiator MAC address are shown in Table 7.

Table 7.

Initiator MAC address

Initiator MAC address value

Mode

Description

NO-BODY (default value)

unlocked Any time-slot synchronization is confirmed.

Any value between FIMA and LIMA

locked

Only time-slot synchronization coming from the

specified node is confirmed.

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