Is your network PhaseReady?

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Phase Ready?

As you think about the evolution of your network, don't limit your thoughts to the frequency stability you need now. We want to help ensure that network roll-outs today will still be relevant come the explosion of HetNets and Small Cells.

Edge Microwave - Transparent or Boundary Clock?

We've been fortunate enough at Chronos to work with several microwave equipment manufacturers in the last year or so, helping them develop and test their implementations of On Path Support for Edge Phase applications. We have seen some quite astonishing improvements in Packet Delay Variation (PDV) of PTP packets when implementing a Transparent Clock; actually in turning the radio link in to a virtual Transparent Clock.

All the manufacturers we have spoken to (of course this is not ALL manufacturers!) are implementing a version of this Transparent Clock. They have come to this conclusion independently, I think because they realise that the many variations that the radio link can encounter due to loading, bandwidth use, scheduling, weather or other external factors means that simply terminating the PTP flow at the link "Ingress" and generating a fresh flow at the "Egress" will not deliver the standard of output clock needed to support a living and breathing network.

I was surprised therefore when I discovered that manufacturers were under external pressure to deliver Boundary Clock rather than Transparent Clock. The main two reasons seem to be that use of Boundary Clock for Phase is now standardised whereas Transparent Clock is not, and that Transparent Clock breaks OSI Layer boundaries.

We are actually beginning some tests in the next week or so to start to characterise some microwave links' PDV and their effect on Boundary Clock performance. In my opinion the fact the the standards bodies can't keep up with technology should not be a brake on development, and is no surprise to any of us in any case! The Layer Violation argument seems a bolt on excuse to me - if the manufacturers believe Transparent Clock is the best way to deliver On Path Support then we should get on with supporting them in the standards!

A poorly working compromise will simply not do in the urban Small Cells space. We're in a different world now and what works should be what is implemented.

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Small Cells Summit Roundup

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Donut or Doughnut? Where will your Phase come from?

I've spent a lot of time since the beginning of the year talking to carriers and equipment vendors about the problems using PTP to get phase sync to the edge of networks. This includes talking to both the carrier and vendor about a network that is being installed this year and will need edge phase sync down the line. I've also spoken to two mobile carriers with very different philosophies of likely network rollout to achieve adequate phase sync, and another operator planning a network for rollout in a couple of years still to make such choices.

The fundamental choice seems to boil down to this - are you a donut or doughnut person? In much of the world a donut has a ring shape - hollow in the centre (center if we're following the spelling of donut!). Many believe they can deliver phase sync from "the edge of the core" or access layer. This would involve a layer of GPS PRTC / Boundary Clocks a minimal number of hops from the network edge - a phase sync "donut".

Here in the UK a doughnut is a disc shaped confection with no hole but a large amount of jam (jelly?) in the middle. In this case a smaller number of larger capacity and more resilient GPS PRTC devices would be deployed in the network core; disseminating phase sync through a network path that would probably demand full PTP awareness and on-path support.

Of course many factors in your network design could tip you from the donut to the doughnut - will you be deploying in largely legacy sites or is this a "green field" project; is it planned to have some or all of your traffic carried through a third party Managed Service; will you be using third party "dumb pipes" to transport your PTP packets for at least part of their journey to the edge?

Even if your LTE-A plans are a year or two away, you need to be thinking whether you are a donut or doughnut network. We at Chronos are developing "Phase Ready" tools to help carriers make these decisions, then implement and monitor them.

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An Independent Sceptic?

Very early in the development of PDH and SDH networks, carriers removed the timing functions from the network elements and implemented independent systems for time and timing dissemination and management. Interesting therefore that as timing requirements for edge applications are probably more stringent and difficult to achieve than ever, a large amount of timing dissemination and management is being taken back in to the network equipment!

What does that mean for you as a carrier? If you're a "bell head" traditional transmission person its a headache; if your a "net head" then perhaps not so much so. Fundamentally though to move microsecond phase coherence to the edge of your network you will need on-path support throughout (although don't jump to the conclusion that if you have on-path support that you will necessarily be successful in meeting all applications requirements).

But how well do these transparent and boundary clocks perform? What about the network in between them, especially if it is not in your control? Will the equipment vendor continue to support and improve their clock implementations for the long term? How do you manage failure scenarios? If you are using a managed service' phase implementation, how do you monitor its performance and hold them to account?

I think we at Chronos, and specialists like us, have a role to play in developing and maintaining your phase networks going forward. We can be that independent sceptic you need; one who understands the issues, can help with phase network design, verification, dimensioning and commissioning.

I will be developing these themes in the coming weeks and months. This is the biggest change in timing networks for at least twenty years and I believe we have the expertise and tools to help you make this process as pain free as possible!

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Is the "Phase Ready" message getting through - Part Two

We're engaged in some excellent work with several Wireless Backhaul manufacturers on PTP for phase and SyncE delivery at the moment. Fundamentally we have a simple message for those planning to roll out networks that require phase at the edge - if the radios have no On Path Support your network WILL NOT deliver microsecond phase coherence at the edge.

Interesting then that it looks like Phase is closer than some of us thought! We're currently assisting with the sync design of a Carrier Ethernet network that will require phase alignment at the edge, and frankly there is still a lot of engineering time and effort needed to find a solution that will deliver network requirements. Here's a good mantra to take with you in to any design exercise - "Don't think that On Path Support at every node ALONE will deliver adequate edge phase performance!"

We're also helping a Carrier with early planning of a Phase Ready network with lots of wireless backhaul that is probably three or four years away. Problem is their radio manufacturer currently does not have any product with On Path Support, but without this they can't engineer a network that will deliver! They're trying to be Phase ready but the market hasn't got there yet.

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Is the "Phase Ready" message getting through?

A couple of contrasting situations in big European Mobile Carriers demonstrates that the message is starting to get through - but there's still a lot of work to be done!

One major carrier who work closely with us at Chronos on sync and timing are busily engineering their edge network to be as symmetrical as possible to make sure their PTP for phase performance is a good as it can be. They currently have NO projects that require phase, although they use both SyncE and PTP for edge frequency.

I also heard this week of a carrier that committed only last year to a large rollout of Ethernet Microwave radios for backhaul, but selected a platform that cannot support PTP on path support. This is a supported product and so no criticism on selecting this for the project they had at the time, but they are now considering a trial of LTE-A requiring phase at the edge via a backhaul pipe that will impair phase performance with no upgrade path to help!

It really does make sense to think of the phase supporting future when considering frequency only projects today. This future is not as far away as you may think, and is likely to be in the lifetime of equipment you are currently considering for use.

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If the clock delivers, who cares about packets?

The most direct, and often least complicated, way to give visibility of whether packet timing technologies are delivering the expected synchronisation performance is to measure the produced or synthesised signals at the slave clock. Whether sync is delivered across a Carrier Ethernet, Microwave, or IP/MPLS network using IEEE 1588v2 (PTPv2) the ‘business end’ and proof of slave clock performance can be ascertained via the frequency, pulse, or time-code produced by it.

Connecting to a frequency, 1PPS or time-code output of a slave clock is often much quicker to perform and the, subsequent results simpler to analyse, than the more complicated procedures required to measure the performance or Packet Delay Variation (PDV) of an Ethernet or IP timing flow.

If the measured output performance is within specification for that part of the network, it doesn’t matter that PTPv2 packets may be occasionally dropped in the network or that the PDV sometimes strays outside the nominal quality levels because as long as the application is getting the quality of clock it requires then packet performance is of secondary importance.

Packet Delay Variation of the PTPv2 flow worries some engineers and they can, and do, invest valuable time and resources trying to unnecessarily find and solve perceived issues. From experience and sync monitoring projects, Chronos experts know that well engineered networks and PTPv2 clients will produce in-specification clock in many types of network conditions and this can be verified by monitoring the frequency, pulse or time-code from a test point. Of course if the measured clock is not within specification then investigation into the incoming flow is required but it does not need to be the first step.

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First Small Cells Backhaul Summit a success

Just back from the Small Cells World Summit and decided to sit in on the whole of the inaugural Backhaul Summit track. Had to sit on my hands at times to not ask the "what about phase sync?" question, but I was pleasantly surprised that there was some awareness at least of the challenges coming our way.

Some very interesting battles to come in the sub 6GHz non line of sight area if the debates at this summit were anything to go by!

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