I imagine most ILECs reading this still have a significant TDM infrastructure – consisting of a pair of SS7 A-links on 56K/64K circuits, plus a variety of ISUP trunk groups to tandems or other local telcos. I wrote an article 6 years ago that looked at how we might move away from TDM networks – but frankly, not much has changed in those 6 years.
However, that may be about to change… at least in a small way.
In recent weeks we have heard from several clients that the fees they’re being charged for their TDM SS7 links has doubled (or more).
I’m not a mind reader, but if an A-link provider is doubling the price of something they either think you have no choice but to pay it, or else they’re trying to force you to switch to a different option.
Remind me, what are SS7 links?
Before I go any further, let me take a quick moment to explain what an SS7 link is, and how it fits into your network.
- The above diagram shows a very simple scenario with your switch in the middle, a tandem to the left, another local ILEC’s switch to the right, and two STPs (used by all three switches).
- In this diagram the blue lines represent SS7 links, which are used for signaling messages.
- The thicker red lines are your trunk groups, where the media flows.
- So when a call is set up between your subscriber and the tandem, your switch sends call set-up messages over the blue links (SS7 A-links) to one of the STPs, which then transfers that call set-up message to the tandem. Those call set up messages select (e.g.) circuit 7 on your trunk group, and notify both switches that the media for this call will use that particular circuit.
So when I say the cost of A-links is going up, I’m talking about the monthly price you pay for the physical 56K/64K circuit between your switch and the two STPs.
What is the other option?
As you can see from the diagram, SS7 links are a critical piece of your TDM infrastructure. If you can’t send call set-up messages then you can’t process phone calls. So SS7 connectivity is not optional as part of a TDM network.
However, you don’t have to pay for physical SS7 links. The alternative is a technology called SIGTRAN, which basically allows the same call set-up messages to be sent over an IP network rather than using dedicated circuits. This is not the same as moving to SIP trunks – you’re still doing SS7, ISUP and TCAP – it’s just that you’re using the IP network to send the signaling messages (while maintaining your TDM ISUP circuits for the media).
Of course, SIGTRAN is not free. You will still need to pay for these SIGTRAN links, but because there’s no longer a dedicated circuit, the costs for SIGTRAN are generally lower than for traditional SS7 links – even before the price hikes.
Does my voice switch support SIGTRAN?
If you have a Metaswitch, the answer is yes.
If you have an older generation voice switch (e.g. a DMS-10/DMS-100) then it will not support SIGTRAN natively. However, if you find yourself in that situation you could potentially purchase the smallest TelcoBridges media gateway, and use that as a bridge between a traditional circuit-based link on your old switch and a SIGTRAN connection to the outside world. Please contact me if you want to explore this, and I can get you connected with the right people at TelcoBridges.
I want to learn more about SIGTRAN…
If you want a deeper dive into this technology, I wrote a helpful primer last year.
If you need help setting up SIGTRAN links on your Metaswitch, please contact us and we can set up a support retainer and help you out with that.
I’m sick of all this TDM infrastructure – can I just move my voice entirely to the cloud?
I’m not a regulatory consultant, but if you’re a CLEC, the answer is probably yes.
If you’re an ILEC then… it’s complicated. There are a various regulations (NECA is big here) requiring you to have a media gateway physically within your LATA, not to mention the fact that your RBOC tandem provider (AT&T, Lumen, etc) probably only offers you connectivity via SS7.
So there are some challenges, but we certainly know of people who are exploring it. I’ll be writing more about this in the near future, and we will be sending out a survey to our readers soon, to get a sense of your views on this. Watch this space.
