Meeting Bandwidth Challenges with RIST

Ciro A. Noronha, Ph.D.

If you have a good, decent IP network (or Internet connection) with a lot of bandwidth, pretty much any video transport protocol will work for you (because it does not need to work very hard). There may be some occasional packet loss, which the protocol will promptly correct, but in general life is good.

However, here in the real world, you will face the following realities:

• Bursts of packet losses.

• Restricted (and variable network bandwidth).

• Occasional network outages.

If this is your situation, the RIST suite of protocols can help, courtesy of its advanced features. Let’s look at each of these challenges.

Dealing with Burst Packet Loss

The first challenge is dealing with the situation where a whole block of packets is dropped by the network (i.e., a burst loss). This is the most common type of loss, and it is typically due to congestion somewhere in the network. Well, no big deal – if the receiver detects that, say, 50 packets are lost, it just asks for them to be retransmitted, and all is well. Not quite the case if your network has restricted bandwidth. Adding the 50 retransmitted packets all at once together with the content that is still going will cause further packet losses in this case. Now, besides some of the 50 packets you didn’t get, you have more losses induced by the retransmissions. On more primitive protocols, this process can snowball and bring your network to a halt.

RIST can help you here by optionally throttling the retransmission flow. This has been an integral feature of the protocol since RIST Simple Profile was released in 2018. If your network has limited or restricted bandwidth, you can avoid this snowball effect by (possibly) accepting additional latency.

The other useful RIST feature for networks with limited bandwidth is NULL Packet Deletion (NPD), introduced in RIST Main Profile. In typical transport streams, NULL packets take about 5% of the bandwidth, but carry no data – they are there to keep the timing. RIST includes a mechanism to remove these packets, flag their locations, and re-insert them at the other end of the link so that there are no timing issues. The reclaimed bandwidth can be used for retransmissions.

Dealing with Variable Network Bandwidth

Sometimes, the instantaneous network capacity falls below the stream requirements. In a situation like this, no amount of throttling or retransmission can fix it. You simply cannot transmit, say, a 5 Mb/s stream on a 4 Mb/s pipe. That is where RIST Source Adaption saves the day. RIST has a mechanism whereby the receiver can provide network quality feedback to the sender. If the network degrades, the sender may be able to reduce the encoder bit rate on-the-fly to adapt to network conditions. Having blockier video is better than having no video at all! RIST Source Adaptation can be combined with NPD to provide a fully compliant CBR stream at the receiver, even though the network transport is VBR.

Dealing with Occasional Network Outages

Well, it happens. You have retransmission throttling and you have source adaptation, but if the network link goes down, you lose your feed. Can RIST do anything about that? Yes, by borrowing from something that existed before, namely SMPTE ST 2022 7 Seamless Switching.

If your signal absolutely cannot go down, you need to have redundant network links. More specifically, get a link from one ISP, and another from a different ISP. This way, the likelihood that both links will go down at the same time is minimized.

As the name indicates, Seamless Switching is, well, seamless. The RIST sender transmits every packet on all links. The RIST receiver normally gets two copies of each packet and ignores the duplicates. If one of the links goes down, the RIST receiver simply uses the packets from the other link, and there is no glitch. Another desirable feature here is that only the packets that are lost on both links need to be retransmitted; RIST takes care of that automatically.

Conclusion

When the going gets tough, RIST is what you need.

For more information about how RIST-Enabled products can empower you to deliver broadcast content reliably and seamlessly, or to find out about becoming a RIST member, get in touch.