Video: PTP Over Wan

Work is ongoing in the IPMX project to reduce SMPTE ST 2110’s reliance on PTP, but the reality is that PTP is currently necessary for digital audio systems as well as for most ST 2110 workflows. There are certainly challenges in deploying PTP from an architectural standpoint with some established best practices, but these are only useful when you have the PTP signal itself. For the times when you don’t have a local PTP clock, delivery over a WAN may be your only solution. With PTP’s standards not written with a WAN in mind, can this be done and what are the problems?

 

 

Meinberg’s Daniel Boldt describes the work he’s been involved with in testing PTP delivery over Wide Area Networks (WANs) which are known for having higher, more variable latency than Local Area Networks (LANs) which are usually better managed with low latency which users can interrogate to understand exactly how traffic is moving and configure it to behave as needed. One aspect that Daniel focuses on today is Packet Delay Variation (PDV) which is a term that describes the difference in time between the packets which arrive the soonest and those that arrive last. For accurate timing, we would prefer overall latency to be very low and for each packet to take the same amount of time to arrive. In real networks, this isn’t what happens as there are queuing delays in network equipment depending on how busy the device is both in general and on the specific port being used for the traffic. These delays vary from second to second as well as throughout the day. Asymmetry can develop between send and receive paths meaning packets in one direction take half the time to arrive than those in the other. Finally, path switching can create sudden step changes in path latency.

Boundary Clocks and Transparent Clocks can resolve some of this as they take in to account the delays through switches. Over the internet, however, these just don’t exist so your options are to either build your own WAN using dark fibre or to deal with these problems at the remote site. If you are able to have a clock at the remote site, you could use the local GNSS-locked clock with the WAN as a backup feed to help when GNSS reception isn’t available. But when that’s not possible due to cost, space or inability to rack an antenna, something more clever is needed.

Lucky Packet Filter
Source: Meinberg

The ‘lucky packet filter’ is a way of cleaning up the timing packets. Typically, PTP timing packets will arrive between 8 and 16 times a second, each one stamped with the time it was sent. When received, its propagation time can be easily calculated and put in a buffer. The filter can look at the statistics then throw away any packets which took a long time to arrive. Effectively this helps select for those packets which had the least interference through the network. Packets which got held a long time are not useful for calculating the typical propagation time of packets so it makes sense to discard them. In a three-day-long test, Meinberg used a higher transmit rate of 64 packets per second saw the filter reduced jitter from 100 microseconds to an offset variation of 5 microseconds. When this was fed into a high-quality clock filter, the final jitter was only 300ns which was well within the 500ns requirement of ST 2059-2 used for SMPTE ST 2110.

Daniel concludes the video by showing the results of a test with WDR where a PTP Slave gateway device was fed with 16 packets a second from a master PTP switch over the WAN. The lucky packet filter produced a timing signal within 500ns and after going through an asymmetry step detection process in the clock produced a signal with an accuracy of no more than 100ns.

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Speaker

Daniel Boldt Daniel Boldt
Meinberg

Video: AES67 Beyond the LAN

It can be tempting to treat a good quality WAN connection like a LAN. But even if it has a low ping time and doesn’t drop packets, when it comes to professional audio like AES67, you can help but unconver the differences. AES67 was designed for tranmission over short distances meaning extremely low latency and low jitter. However, there are ways to deal with this.

Nicolas Sturmel from Merging Technologies is working as part of the AES SC-02-12M working group which has been defining the best ways of working to enable easy use of AES67 on the WAN wince the summer. The aims of the group are to define what you should expect to work with AES67, how you can improve your network connection and give guidance to manufacturers on further features needed.

WANs come in a number of flavours, a fully controlled WAN like many larger broadacsters have which is fully controlled by them. Other WANs are operated on SLA by third parties which can provide less control but may present a reduced operating cost. The lowest cost is the internet.

He starts by outlining the fact that AES67 was written to expect short links on a private network that you can completely control which causes problems when using the WAN/internet with long-distance links on which your bandwidth or choice of protocols can be limited. If you’re contributing into the cloud, then you have an extra layer of complication on top of the WAN. Virtualised computers can offer another place where jitter and uncertain timing can enter.

Link

The good news is that you may not need to use AES67 over the WAN. If you need precise timing (for lip-sync for example) with PCM quality and low latencies from 250ms down to as a little as 5 milliseconds do you really need AES67 instead of using other protocols such as ACIP, he explains. The problem being that any ping on the internet, even to something fairly close, can easily have a varying round trip time of, say, 16 to 40ms. This means you’re guaranteed 8ms of delay, but any one packet could be as late as 20ms. This variation in timing is known as the Packet Delay Variation (PDV).

Not only do we need to find a way to transmit AES67, but also PTP. The Precise Time Protocol has ways of coping for jitter and delay, but these don’t work well on WAN links whether the delay in one direction may be different to the delay for a packet in the other direction. PTP also isn’t built to deal with the higher delay and jitter involved. PTP over WAN can be done and is a way to deliver a service but using a GPS receiver at each location is a much better solution only hampered by cost and one’s ability to see enough of the sky.

The internet can lose packets. Given a few hours, the internet will nearly always lose packets. To get around this problem, Nicolas looks at using FEC whereby you are constantly sending redundant data. FEC can send up to around 25% extra data so that if any is lost, the extra information sent can be leveraged to determine the lost values and reconstruct the stream. Whilst this is a solid approach, computing the FEC adds delay and the extra data being constantly sent adds a fixed uplift on your bandwidth need. For circuits that have very few issues, this can seem wasteful but having a fixed percentage can also be advantageous for circuits where a predictable bitrate is much more important. Nicolas also highlights that RIST, SRT or ST 2022-7 are other methods that can also work well. He talks about these longer in his talk with Andreas Hildrebrand

Nocals finishes by summarising that your solution will need to be sent over unicast IP, possibly in a tunnel, each end locked to a GNSS, high buffers to cope with jitter and, perhaps most importantly, the output of a workflow analysis to find out which tools you need to deploy to meet your actual needs.

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Speaker

Nicolas Sturmel Nicolas Sturmel
Network Specialist,
Merging Technologies

Video: What is IPMX? – The IPMX Stack

“The AV over IP market has really matured [giving us] great quality, low latency and the kind of stability and features that customers are looking for,” says Andrew Starks from Macnica Technology. If that’s the case, why do we need another standard by the name of IPMX? Intended to open up the AV-over-IP market and provide customers with a better deal, Andrew takes us through the motivations of AIMS, AMWA, VSF, SMPTE and the other organisations involved.

IPMX is a set of open standards and specifications which seek to bring a technology platform to the Pro AV industry on which all vendors can interoperate and innovate. Built on SMPTE’s ST 2110 suite of standards and the accompanying NMOS APIs from AMWA, IPMX adds essential capabilities such as HDMI, HDCP and USB support to create a complete and reliable foundation for AV events and installations.

 

 

Whilst there are a number of successful AV initiatives such as SDVoE, these are typically alliances built around a single-vendor hardware solution which is available to vendors in the alliance. This provides interoperability within that ecosystem but, explains Andrew, it prevents wider interoperability between vendors of different alliances. It also makes it hard to any vendor to innovate in the core feature set since that’s delivered from the single source relegating innovation to ‘plumbing’. For the vendors, at best, this means they have to contend with multiple, incompatible product lines and complicated support. Overall this results in a bad end user experience as they operate multiple islands which can have conflicting network requirements, i.e. 10GbE vs 1GbE.

IPMX can be implemented in software as well as hardware using compressed or uncompressed video with a focus on fully featured discovery as this has been identified as being as important as the ability to carry video. Timing has been made flexible such that it can operate with or without PTP which is one of a number of ways that it’s anticipated IPMX will be able to merge in with ST 2110 infrastructures.

Andrew finishes off his talk with a look at the tech stack of IPMX with layer 2 options from 1 to 100GbE connections supported on which RTP and PTP run. SMPTE’s ST 2110 standards feature heavily alongside a new standard for HDCP in 2110, a VSF spec for FEC and new specifications from AMWA for asynchronous control traffic like EDID, Serial, CEC, USB etc. Finally, there are the main APIs such as IS-04, -05 etc. as well as the application layer which uses OAuth2 for authenticating and has an RDS server for discovery. Lastly, there is a look at the JT-NM roadmap to see how the IPMX work will continue to advance throughout this year.

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Speakers

Andrew Starks Andrew Starks
Director of Product Management,
Macnica America’s Inc.

Video: PTP/ST 2059 Best Practices developed from PTP deployments and experiences

PTP is foundational for SMPTE ST 2110 systems. It provides the accurate timing needed to make the most out of almost zero-latency professional video systems. In the strictest sense, some ST 2110 workflows can work without PTP where they’re not combining signals, but for live production, this is almost never the case. This is why a lot of time and effort goes into getting PTP right from the outset because making it work perfectly from the outset gives you the bedrock on which to build your most valuable infrastructure upon.

In this video, Gerard Phillips from Arista, Leigh Whitcomb from Imagine Communications and Telestream’s Mike Waidson join forces to run down their top 15 best practices of building a PTP infrastructure you can rely on.

Gerard kicks off underlining the importance of PTP but with the reassuring message that if you ‘bake it in’ to your underlying network, with PTP-aware equipment that can support the scale you need, you’ll have the timing system you need. Thinking of scale is important as PTP is a bi-directional protocol. That is, it’s not like the black and burst and TLS that it replaces which are simply waterfall signals. Each endpoint needs to speak to a clock so understanding how many devices you’ll be having and where is important to consider. For a look a look at PTP itself, rather than best practices, have a look at this talk free registration required or this video with Meinberg.

 

 

Gerard’s best practices advice continues as he recommends using a routed network meaning having multiple layer 2 networks with layer 3 routing between This reduces the broadcast domain size which, in turn, increases stability and resilience. JT-NM TR-1001 can help to assist in deployments using this network architecture. Gerard next cautions about layer 2 IGMP snoopers and queriers which should exist on every VLAN. As the multicast traffic is flooded to the snooping querier in layer 2, it’s important to consider traffic flows.

When Gerard says PTP should be ‘baked in’, it’s partly boundary clocks he’s referring to. Use them ‘everywhere you can’ is the advice as they bring simplicity to your design and allow for easier debugging. Part of the simplicity they bring is in helping the scalability as they shed load from your GM, taking the brunt of the bi-directional traffic and can reduce load on the endpoints.

It’s long been known that audio devices, for instance, older versions of Dante before v4.2, use version one of PTP which isn’t compatible with SPMTE ST 2059’s requirement to use PTP v2. Gerard says that, if necessary, you should buy a version 1 to version 2 converter from your audio vendor to join the v1 island to your v2 infrastructure. This is linked to best practice point 6; All GMs must have the same time. Mike makes the point that all GMs should be locked to GPS and that if you have multiple sites, they should all have an active, GPS-locked GM even if they do send PTP to each other over a WAN as that is likely to deliver less accurate timing even if it is useful as a backup.

Even if you are using physically separate networks for your PTP and ST 2110 main and backup networks, it’s important to have a link between the two GMs for ST 2022-7 traffic so a link between the two networks just for PTP traffic should be established.

The next 3 points of advice are about the ongoing stability of the network. Firstly, ST 2059-2 specifies the use of TLV messages as part of a mechanism for media notes to generate drop-frame timecode. Whilst this may not be needed day 1, if you have it running and show your PTP system works well with it on, there shouldn’t be any surprises in a couple of years when you need to introduce an end-point that will use it. Similarly, the advice is to give your PTP domain a number which isn’t a SMPTE or AES default for the sole reason that if you ever have a device join your network which hasn’t been fully configured, if it’s still on defaults it will join your PTP domain and could disrupt it. If, part of the configuration of a new endpoint is changing the domain number, the chances of this are notably reduced. One example of a configuration item which could affect the network is ‘ptp role master’ which will stop a boundary clock from taking part in BCMA and prevents unauthorised end-points taking over.

Gerard lays out the ways in which to do ‘proper commissioning’ which is the way you can verify, at the beginning, that your PTP network is working well-meaning you have designed and built your system correctly. Unfortunately, PTP can appear to be working properly when in reality it is not for reasons of design, the way your devices are acting, configuration or simply due to bugs. To account for this, Gerard advocates separate checklists for GM switches and media nodes with a list of items to check…and this will be a long list. Commissioning should include monitoring the PTP traffic, and taking a packet capture, for a couple of days for analysis with test and measurement gear or simply Wireshark.

Leigh finishes up the video talking about verifying functionality during redundancy switches and on power-up. Commissioning is your chance to characterise the behaviour of the system in these transitory states and to observe how equipment attached is affected. His last point before summarising is to implement a PTP monitoring solution to capture the critical parameters and to detect changes in the system. SMPTE RP 2059-15 will define parameters to monitor, with the aim that monitoring across vendors will provide some sort of consistent metrics. Also, a new version of IEEE-1588, version 2.1, will add monitoring features that should aid in actively monitoring the timing in your ST 2110 system.

This Arista white paper contains further detail on many of these best practices.

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Speakers

Gerard Phillips Gerard Phillips
Solutions Engineer,
Arista
Leigh Whitcomb Leigh Whitcomb
Principal Engineer.
Imagine
Michael Waidson Mike Waidson
Application Engineer,
Telestream