Administrator Guide

Audio Video Bridging 1603
A device that can issue or receive IEEE 802.1AS communications is termed a
“time-aware system”. A time-aware system can either be an end station device
attached to a network or a bridge that interconnects end stations. Typically,
an end station device has single port and a bridge has multiple ports. The
segment of an 802.1AS network that enables direct communication between
two time-aware systems is defined as an 802.1AS communication path. The
port on time-aware end station can be a master or slave. A time-aware bridge
can have at most one port in slave state and all other ports in master state. If
the time-aware system is the grandmaster, all ports on that system are in the
master state and there are no slave ports.
The main functions of 802.1AS component are:
• Best Master selection (BMCA)
• Time Synchronization
• Link delay measurement
Best Master Selection
An 802.1AS domain consists of one of more time-aware systems that
communicate with each other as defined by the 802.1AS protocol. An
802.1AS network consists of single 802.1AS domain and a single grandmaster
clock.
The best master clock algorithm (BMCA) determines the best master for an
802.1AS domain and establishes a tree structured master-slave hierarchy with
the best master as the root. Best master clock selection information is
periodically exchanged between time-aware systems via ANNOUNCE
messages. Every time-aware system invokes the BMCA to compare data
associated with the announcing clocks to determine the better clock. If a
master port receives an ANNOUNCE message from a better clock, then the
port ceases to be a master and becomes a slave port. Likewise, if a clock with a
port acting as slave determines that it is a better master than the current
master clock, it becomes a master port and starts sending ANNOUNCE
messages. In a steady state, once the best master is elected, the ANNOUNCE
messages are sent only on master port(s), and the slave port receives
ANNOUNCE messages sent by its master port. The mean default time
interval between successive ANNOUNCE message transmissions is 1 second.
The default ANNOUNCE receipt timeout is 3 times the ANNOUNCE
message transmission interval. If ANNOUNCE receipt timeout occurs, the
time-aware station invokes the BMCA to determine the new best master