Administrator Guide

Audio Video Bridging 1675
field in the SYNC and FOLLOW_UP messages. The master sends the
FOLLOW_UP message with the same sequence ID as the SYNC message.
The value (t2 – t1) gives the (offset + link delay) between the master and
slave. The link delay is calculated as described below. Assuming that the link
delay is symmetric, the offset value can be derived from (t2 – t1). This
sequence of SYNC and FOLLOW_UP messages is repeated at every SYNC
transmission interval.
A slave device can, in turn, act as a master for the downstream nodes by
forwarding incoming synchronization information to the downstream nodes.
The received SYNC/FOLLOW_UP messages are not forwarded as-is; a new
set of SYNC/FOLLOW_UP messages is generated based on the downstream
master port properties (port identity, port sequence ID, etc.). Also, the
correction field in the FOLLOW_UP packet is updated with the residence
time before transmitting the message on master port(s). The rate at which
SYNC messages are transmitted from the master port is limited. For example,
if SYNC messages are received on a bridge's slave port at a very high rate, the
SYNC messages are transmitted out of the bridge's master port at a rate
greater than or equal to one half of its configured SYNC interval. Ideally a
SYNC message is transmitted from a master port whenever a SYNC message
is received on its slave port. If a bridge acts as a grandmaster, SYNC messages
are generated based on the CPU clock speed; however, the rate at which
SYNC messages are transmitted out of the master interface is always greater
than or equal to half of the configured SYNC interval.
Link Delay Measurement
The peer delay mechanism measures the port-to-port propagation time (link
delay) between two ports. A delay measurement is made on all ports and can
be initiated independently of the 802.1AS port state. The transmission of
PDELAY_REQ message is the first step in the measurement process. The
delay requestor captures the time stamp of the transmission time of
PDELAY_REQ packet (t1). When the PDELAY_REQ message is received by
the delay responder, the packet's RX time stamp is captured (t2). The delay
responder issues two packets in response to the PDELAY_REQ: a delay
response PDELAY_RESP and a delay response follow up
PDELAY_RESP_FOLLOWUP. The receive timestamp of PDELAY_REQ
(t2) and transmit timestamp of PDELAY_RESP (t3) is captured and is