Hitless Protection Switching: An OverviewPosted on October 10, 2017
Hitless protection switching was developed specifically to address both short-term and long-term outages that occur on wide area networks. In its most basic form, a hitless sender transmits two identical packet streams over two separate network paths. At the receiver, the two streams are re-aligned using adaptive buffers, and a single output is created using the good packets received from either one path or the other. As long as at least one copy of each packet is received from one of the two signal paths coming into the receiver, an error-free output can be produced.
SMPTE ST 2022-7 “Seamless Protection Switching of SMPTE ST 2022 IP Datagrams” provides an industry-accepted standard for implementing hitless protection switching. Publication of this standard was an important step forward, as it allowed hitless devices produced by different manufacturers to fully interoperate with each other. Interoperability testing of hitless technology was successfully performed between several manufacturers during the annual VidTrans conference hosted by the Video Services Forum industry consortium, and has also been featured at several Alliance for IP Media Systems (AIMS Alliance) interoperability demonstrations at recent broadcast industry trade shows. The packet formats defined in the newly-developed SMPTE ST 2110 standards for professional media over IP networks have also been carefully crafted to ensure compatibility with SMPTE ST 2022-7.
As shown in the diagram above, a hitless protection sender creates two (or more) packet streams with identical Real Time Protocol (RTP – defined in IETF RFC 3550) timestamps, sequence numbers, and payloads. Different UDP, IP, and Ethernet packet headers can be used on the two streams to enable them to be routed over different network segments. When the streams reach the receiver, the incoming packets are first fed into buffers to allow them to be time-aligned. This is accomplished by adjusting the data-reading pointers in the two buffers to both point to packets with the same RTP timestamps and sequence numbers. Each matching set of data packets is then retrieved simultaneously from the two buffers and then analyzed to determine if there are any errors present. Whenever missing packets are encountered, those positions in the buffer are skipped. A single output stream is created by combining the two streams and selecting the best packet available at each point in the sequence.
To obtain the maximum benefit of hitless protection switching, the two streams should be routed over completely separate networks. In other words, there should not be any common network devices or links anywhere along the path from the sender to the receiver. This restriction is important to ensure that no single point of failure can cause an interruption of both streams at the same time, which would lead to a loss of video at the system output.
In any hitless application, there is a limit to the size of the buffers that are used to accommodate the difference in arrival times between the two streams. This difference, called skew in the standard, has a direct effect on the size of the packet buffers in the receiver. Receivers that can support larger amounts of skew require larger amounts of expensive high-performance memory, particularly for high-speed, uncompressed signals such as 3 Gbps 1080P video signals. Plus, if large buffers are used when they are not needed, the overall signal end-to-end delay of the signal can be increased unnecessarily.
To learn more about SMPTE ST 2022-7 Seamless Protection Switching download our white paper, Broadcasters Can't Miss with Hitless Technology.
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