Tektronix SPG8000A, User Manual

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SPG8000A User Manual 161
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Two masters can be used on different domains to serve slaves on different
communication modes
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For systems with a large number of end points, multicast mode will create
a significant message processing burden on the slaves and switch. Mixed
mode is intended to reduce this overhead and is recommended on large
systems.
Multicast system messages: Multicast messages are broadcast and so they can be
received without knowing the address of the sender. This allows a slave to hook
up to the network, receive the announce messages and discover the identity of the
master. Multicast in some networks requires the devices to use IGMP to join and
leave the multicast group.
In Multicast PTP systems, the master sets the rate of the announce and sync
messages. The master also sends the maximum allowed delay request rate in the
delay response message. Ideally, the slave will use that value to set the delay
request rate although some slave set this rate independently. The delay request
rate is typically the same as the sync rate but may be higher or lower.
Unicast system messages: Unic ast messages require the address of the master be
entered into the Acceptable Master Table (AMT) in each slave. If there are
multiple masters, the IP address for each master must be entered in the AMTs of
all the slaves.
In Unicast PTP systems, the slave starts the messaging process. The slave sends
several grant requests to the master requesting specific rates for each message
type. If the master accepts the grant requests, it will send grant acknowledges. If
the master denies the grant, the slave may send a new grant request at a lower rate.
This process may continue for several iterations until the master and slave agree
on a rate. The master and slave will then start exchanging the PTP messages. If
the master cannot support any of the rates requested by the slave, then the slave
will not be able to lock to the master.
Mixed mode system messages: In Mixed Mode systems, some messages are
Multicast and some are Unicast. For the SMPTE ST2059 profile, the Announce
and Sync messages are sent as Multicast. This allows devices to discover the
active master. The Delay Request rate is configured for Unicast, so the slave and
master must exchange grant messages before the system will start the full range of
messages.
In Mixed mode without negotiation, the delay request rate set by the slave must
be equal to or less than the Sync rate set by the master. Otherwise, the slave will
not be able to connect to the master.
SMPTE profile system messages: For SMPTE profile systems, a given domain
must use one type of message for all Announce, Sync and Follow - up messages.
Therefore, all masters and slaves on that domain must be configured for either
Multicast or they all must be configured for Unicast.