Albert Mitchell Archives – The Broadcast Knowledge

Part II in this Cisco series on PTP, Precision Time Protocol, focuses on Boundary Clocks and Transparent Clocks. Last week we heard how PTP maintains accurate time by calculating the delay between clocks and the grandmaster clock which is the source of time for the network. This video summarises how to distribute that source of time to all your devices and how to choose between the two methods.

Albert Mitchell from Cisco explains that transparent clocks are just that, they transparently let the timing data flow through. All they do is update the timestamps on the outgoing packets to compensate for the extra time getting through the switch. A boundary clock (BC), however, is a source of time of itself but gets its time from the grandmaster like any other clock. Acting in this dual way, it creates the boundary it’s named after. It’s a boundary because it provides time to other end devices on the network, These devices never see the grandmaster, they only see the BC. Likewise, the grandmaster only sees the BC acting like any ordinary clock sending delay requests. This means that the boundary clock can shield the grandmaster from the rest of the devices on the network. A grandmaster with 10 boundary clocks can deliver time to over a thousand endpoints without a problem. Without the boundary clocks, the grandmaster may not be able to handle the two-way conversations necessary with so many clocks.

For broadcast networks, boundary clocks are preferred as they enable easier diagnosis and can reduce the blast radius of problems. Importantly they can span multiple VLANs. Other benefits are that they filter packet delay variation and shields the downstream/following clocks from any transient changes in the grandmasters. The downside of BCs is that they do add small errors to the timing which can add up if multiple BCs are concatenated.

Transparent clocks, on the other hand, don’t help with scalability like BCs and are limited to single VLANs. On the plus side, they require no configuration and provide faster convergence.

Lastly, Albert looks at the Best Master Clock Algorithm (BMCA) which is the method used to determine which grandmaster is providing timing to the whole network. For a deeper dive into the BMCA, have a look at this Arista video on PTP timing. Albert gives a good starting overview of how the algorithm works, the data it needs to operate and advice on settings to make sure you know which clock will win in each instance.

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Speakers

Albert Mitichell
Technical Marketing Engineer,
Cisco

As we’ve seen in so many videos, PTP is fundamental to large-scale SMPTE ST 2110 and pro-audio installations. On The Broadcast Knowledge we’ve looked at a large range of talks on PTP on architecture, scaling, and how it fits in to the broadcast industry. Few of these break down PTP into the fundamentals like today’s article about a video from Albert Mitchell from Cisco.

The key to a PTP network is having one grandmaster clock which can provide time for the rest of the network. In this article, the clocks running in the end devices are called ‘ordinary clocks’. Whilst there are ways to avoid using PTP with uncompressed video such as ST 2110, for live, studio-style productions where you will be bringing them together in a video mixer or similar, keeping these videos effectively zero latency is important and frame syncs on every input of the mixer are discouraged. A grandmaster clock can provide the timing the whole network needs to make this work, usually fed by GPS time.

SMPTE’s ST 2110 suite has built itself on the timing mechanism of PTP in form of IEEE-1588. SMPTE ST 2059 standards suite provides a method to accommodate all legacy reference and media signals using IEEE-1588 Precision Time Protocol (PTP), delivered over an IP network.

Albert moves on to how this all works. He keeps it simple explaining that there are two measurements needed to get the timing right. You need to know how long it takes to get a message from the grandmaster to the clock and how long it takes to get a message from the clock to the grandmaster. If the grandmaster sends a message with the time in it, it’s trivial for the ordinary clock to look at the time when the message arrived and work out the time it took. It can do the same; an ordinary clock can put the time into a message and send it to a grandmaster. The grandmaster will look at the current time and reply saying how long the transmission delay was. The ordinary clock averages these two measurements and can use the result and the time from the grandmaster to correct its own clock.

Albert finishes by explaining that if there are other switches between the grandmaster and the ordinary clock, those switches should be expected to identify the ‘residence time’ and add this extra delay of simply going through the switch to the time message. Changes in network delay due to congestion or path changes are the reason this timing calculation happens once a second.

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Speakers

Albert Mitchell
Technical MArketing Engineer,
Cisco