Understanding IEEE's deterministic AV bridging standards
IEEE 802.1AS Precision Time Protocol (PTP):1 Based on IEEE 1588:2002,2 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 timestamping. In short, during PTP message ingress/egress from the 802.1AS-capable MAC, the PTP Ethertype 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 guarantees 1-µs accuracy over seven network hops.
IEEE 802.1Qat Stream Reservation Protocol (SRP):3 Legacy IEEE 802 Ethernet standards are characterized--and limited--by their inability to deterministically prioritize time-sensitive streaming media ahead of legacy asynchronous messages. To provide guaranteed QoS, the Stream Reservation Protocol ensures end-to-end bandwidth availability before an A/V stream starts. If bandwidth is available, it is "locked down" along the entire path until implicitly or explicitly released. SRP utilizes IEEE 802.1ak Multiple Registration Protocol as the messaging protocol to pass stream descriptors and resource reservation requests/ results.
Switches implementing SRP can reserve--and defend--up to 75% of the available capacity on a given LAN link within an AVB "Defended Cloud."4
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