Entry #7 in the “How it Fits” series is…the PSTN Gateway!

Like the Reverse Proxy, a PSTN Gateway isn’t a dedicated Server Role in Skype for Business. However, that doesn’t mean it’s optional. In fact, it’s critical if you want to use Enterprise Voice.

Without one of these three options – PSTN Gateway, IP-PBX, or SIP Trunk – you can’t call out of the office. Nor can anyone calling you reach you.

PSTN Gateway Guard
I don’t let anyone pass unless they have a PSTN Gateway. Or a treat.
Photo by Szymon Fischer on Unsplash.

This post will explain why, and how to deploy a PSTN gateway for your Skype for Business Server.

The PSTN Gateway’s Primary Role

In a Skype for Business topology, the PSTN Gateway translates signals between VoIP and PSTN networks. This allows internal VoIP phones to connect out into the vast worldwide analog phone network. And vice versa.

Why would you need to do that? It’s due to the signal types used for voice calls.

The PSTN, or “Public Switched Telephone Network” uses analog signals to transmit your voice. However, Skype for Business uses a digital signal for its transmissions. Same with every other “Voice over IP” system.

These signal types are markedly different. If you tried to listen to a digital IP signal as-is, you’d get an ear-splitting howl!

That’s where the PSTN Gateway steps in. By converting one signal type to other, it allows for seamless voice communications.

It’s not the only solution—you can also use a SIP Trunk for the same purpose. I may do a post on SIP trunks as well, but for now, we’re focusing on the PSTN Gateway.

Main Components of the PSTN Gateway

  1. PSTN Interface: The necessary hardware/software to communicate with the external PSTN network.
  2. VoIP Interface: The necessary hardware/software to communicate with the internal IP network.
  3. Listening Port: The gateway has to listen for signals from the Mediation Server. When creating a topology, you set the port for said listening. Default installs use port 5066 for TCP, and port 5067 for TLS.
  4. DNS Load Balancing – In order to work in Skype for Business Server, a PSTN gateway must implement DNS load balancing. Since it may connect to a pool of Mediation Servers, it has to load-balance calls across the pool evenly.

Other Servers a PSTN Gateway Communicates With

Mediation Server. PSTN Gateways and Mediation Servers have a peer relationship. They’re both translating signals, within the topology and outside the network, to facilitate your conversations.

PBX. If you still have a legacy PBX, the Gateway can inter-operate with it. The Gateway essentially links the VoIP-enabled users into the PBX.

Skype for Business Voice Topology with PSTN Gateway
Example of PSTN Gateway working with Mediation Server. Illustration courtesy of Microsoft Docs.

How a PSTN Gateway Works in a Hybrid Environment

Let’s say you want to move users to Skype for Business Online, but you’ve already invested in an on-prem PSTN connection. Like a SIP Trunk or PSTN Gateway. Can you re-use that investment in any way?

Yes! You can configure Skype for Business to home users in the cloud, while still routing their voice calls through your existing PSTN connection. There are two ways: Use Cloud Connector Edition (CCE), or modify the on-prem deployment for hybrid PSTN.

The FlinchBot blog has done a good job outlining these scenarios: Skype for Business Hybrid deployment with On-Premise PSTN using Cloud PBX. Part: 3

I realize that Skype for Business Online has a retirement date. This option will not be viable very soon. Still, it’s useful to know, in case you need to take a similar approach with regard to Teams in the future.

The PSTN Gateway in Skype for Business & Teams

Obviously, the PSTN Gateway comes into play in an on-prem deployment. What’s the gateway’s equivalent in Teams? It’s Direct Routing: Voice Calling in Teams

From the Teams page:

“Microsoft Direct Routing enables people to use existing phone numbers with Direct Routing in Teams Phone System for a complete calling experience that includes dial tone.”

Software performing the role, as you’d expect in a cloud service. Now, you don’t have to use Direct Routing while using Teams; you can use one of Microsoft’s Calling Plans to make/receive calls too. Direct Routing exists if you have existing numbers and want to stay with your current telecom provider.

Where to Get a PSTN Gateway Appliance

As it’s not a Server Role, you’ll have to install an appliance to act as your PSTN Gateway. However, “where to get one” isn’t as easy a question to answer as it once was.

Why? With Teams rising and more businesses moving to cloud-based VoIP, the need for PSTN gateway devices has dropped. As such, some manufacturers have stopped making them.

Not all though. Sangoma makes VoIP gateways, as does Audiocodes. We’ve used both in deployments, and they will do the job.

PSTN Gateways Plug You Into the Global Phone Network

“Do you want to use a SIP trunk or a PSTN gateway?” I remember a co-worker asking one of our earlier Skype4B customers this, back in late 2015. Of course the customer didn’t know the difference.

After we explained though, they opted for the gateway. That customer is still on Skype for Business, in the same topology with the same gateway, today. Without the PSTN gateway, they’d have gone out of business years ago…because no one could ever call them!

The Mediation Server facilitates voice calls for Skype for Business users. A PSTN gateway makes sure those callers can understand each other.

PSTN gateway connectivity
The gateway is open! Go forth into the wide open PSTN!
Photo by Ágatha Depiné on Unsplash.

What do you think will happen to technology like the PSTN Gateway, as the cloud expands?

How the PSTN Gateway Fits into Skype for Business
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