SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
The protocol defined by 802.1AS automatically selects a device to be the master clock, and then distributes this clock throughout the bridged LAN / IP subnet to all other network devices using link-specific transmit/receive time-stamping. However, we only use a two-step solution only on transmit. That is, we do not modify a packet with the timestamp on the way out. The timestamp packet is sent out and then a separate message with the timestamp is sent by the host afterward. Receive can be one or two step.
The 802.1AS-distributed clock is not used as a media clock. Rather, the shared 802.1AS clock reference is used to regenerate the media clock at the listener/renderer. Such a reference removes the need to force the latency of the network to be constant, or compute long running averages in order to estimate the actual media rate of the transmitter in the presence of substantial network jitter. IEEE 802.1AS is based on the ratified IEEE 1588 standard.
Based on IEEE 1588:2002, PTP devices exchange standard Ethernet messages that synchronize network nodes to a common time reference by defining clock master selection and negotiation algorithms, link delay measurement and compensation, and clock rate matching and adjustment mechanisms.
Designed as a simplified profile of IEEE 1588, a primary difference between 1588 and IEEE 802.1AS is that PTP is a layer 2--in other words, a non-IP routable protocol. Like IEEE 1588, PTP defines an automatic method for negotiating the network clock master, the Best Master Clock Algorithm (BMCA). PTP nodes can be assigned one of eight priority levels, presumably based on clock quality. BMCA defines the underlying negotiation and signaling mechanism whose purpose is to identify the AVB LAN Grandmaster. Once a Grandmaster has been selected, synchronization automatically begins.
At the core of 802.1AS synchronization is time-stamping. In short, during PTP message ingress/egress from the 802.1AS-capable MAC, the PTP Ether type triggers the sampling of the value of a local real-time counter (RTC). Slave nodes compare the value of their RTC against the PTP Grandmaster and, by use of link delay measurement and compensation techniques, match their RTC value to the time of the AVB LAN PTP domain. After network time throughout the AVB LAN has converged, periodic SYNC and FOLLOW_UP messages provide the information that enables the PTP rate matching adjustment algorithms. The result is all PTP nodes are then synchronized to the same "Wall Clock" time. PTP assures 1-µs accuracy over seven network hops.
The media transport protocols that work within the AVB framework are: