Our fifth entry in the “How It Fits” series is…the Mediation Server!

Mediation is a central element within Skype for Business. It’s arguably the most versatile Server Role in the Skype for Business topology too. There’s almost no end to the number of configurations you can deploy for it…collocate, standalone, or pool. SIP trunk or PSTN gateway. Multiple gateways. Multiple trunks. Call routes and bypasses.

The one thing all of these configurations have in common…is listening. Mediation Server listens and translates. Routes and connects. If you use Skype for Business at all for voice, you’re talking through a Mediation Server.

This post, like the previous posts in my “How it Fits” series, will give an overarching take on the Mediation Server’s function and value. I took a more agnostic approach, since we now have two versions of Skype for Business Server to consider (2015 and 2019).

How does Mediation Server work in both of them? Any differences between versions? Let’s find out.

The Mediation Server’s Primary Role

Mediation servers translate signals between your Skype for Business’ Enterprise Voice infrastructure, and the gateway your topology uses to reach the PSTN: either a PSTN gateway, a SIP trunk, or even a PBX. “Mediating” your voice communications, basically.

Mediation Server Signal Processing
The signaling protocols Mediation Server handles. Photo courtesy of Microsoft Docs.

Because of this critical function, Mediation Server is a required Server Role. It also helps facilitate E911, Call Admission Control, and Media Bypass.

This is one of the Server Roles for whom hardware quality matters. The higher the server’s processing capacity & available RAM, the more calls a Mediation Server can handle.

Main Components of the Mediation Server

  1. Signal Translation: The reason you must have a Mediation Server for Enterprise Voice. Without signal translation, nobody could understand each other on the phone. You’d either sound like 80s-era robots, or brain-scrambled demons!
  2. Call Routing: The server coordinates with your gateway of choice to route calls where they need to go. Peer-to-peer inside the network, out to a branch site, or out to a customer three states away on their cellphone.
  3. Media Bypass: Not really a component, but a capability. Skype for Business admins can configure a call route to flow AROUND the Mediation Server! The call route would travel directly between a user’s device and a PSTN Gateway. Why do this? It can reduce lag without traversing the Mediation Server. Media bypass improves call quality by reducing latency, unnecessary translation, possibility of packet loss, and the number of potential points of failure.
  4. Call Admission Control (CAC): A bandwidth management tool. Based on available bandwidth, the Mediation Server determines the best use for existing calls. The idea is to automatically prevent poor call quality as often as possible.NOTE: Media Bypass and CAC are mutually exclusive. If one’s in use for a particular call, the other is not.
Digital Voice Traffic VoIP
Basically, Mediation Server helps you avoid the digital voice equivalent of this.
Photo by Jens Herrndorff on Unsplash

Other Servers a Mediation Server Communicates With

Front End. Of course, Mediation communicates with the Front End Servers all the time. It employs Front End’s database for call routing, and performs a similarly-central role in voice communications Site-wide.

PSTN Gateway / SIP Trunk / IP-PBX. These are the gateway mechanisms, or “peers” for bringing calls to & from Skype for Business. This is where your defined call routes meet the Mediation Server.

Load Balancers. I mentioned in the How the Load Balancer Fits post that load balancers must communicate with servers they’re balancing AND the servers sending them traffic. Since almost all voice traffic must go through the Mediation Server, they’ll talk with load balancers frequently.

(The peers performing call routing to/from Mediation Server also act as load balancers, particularly when you deploy a Mediation Pool.)

How a Mediation Server Works in a Hybrid Deployment

What does a Mediation Server do in a hybrid topology with Office 365?

Fundamentally the same thing. If you’re hybridizing an existing Skype for Business Server deployment, you’ll enable synchronization for Active Directory and change call routes. You’ll have to reflect such changes in your on-prem Mediation Server.

There are too many options to the hybridization process to cover in 1 post. Suffice to say, it all depends on your gateways/SIP trunks, and how much of Office 365’s calling services you use.

Should You Collocate with Front End, or Use a Separate Mediation Pool?

By default, Skype for Business wants to collocate a Mediation Server with the Front End Server. Which is fine for smaller topologies.

If you’re using a SIP trunk though, I recommend the standalone approach. At least one Mediation Server, or a small pool. Microsoft also recommends this approach, but we’ve seen it borne out in the field. Each time we deployed a standalone Mediation Server for a customer location with a SIP trunk, we fielded fewer calls about latency issues (if any).

One caveat for you Skype for Business Server 2019 deployers: According to Brian Siefferman at Perficient, if you’re migrating your Skype4B topology from an existing deployment, it’s a good idea to collocate the legacy Mediation Server during initial deployment. Then you can decide whether to keep it collocated, or move to standalone, later in the process.

Will the Mediation Server Change in Skype for Business Server 2019?

Not fundamentally. It continues its role of call routing/media processing.

We even get a performance boost for Mediation’s call capacity. Paul Lange points out that that a standalone Mediation Server in 2019 will handle 2,000 concurrent calls, with hyper-threading enabled (it can handle 1,500 calls in Skype4B 2015).

Makes sense, since a few deprecated elements deal with messaging—XMPP Gateways, Persistent Chat. Mediation Server won’t need communications with them now, freeing up more processing power for concurrent calls.

Dog Licking Mediation Server
Still reliably doing what it’s ‘trained’ to do.
Photo by James Barker on Unsplash

A Good Listener to Facilitate Voice Calls

The Mediation Server has existed since the OCS 2007 days. Of course, It has grown as more VoIP options came into being. But like its Front End partner, it has continued to provide the same fundamental service for over 10 years.

As long as it has sufficient bandwidth & a reliable gateway available, Mediation Server makes voice calls happen. Which type of gateway you use with it, depends on your network and Site needs.

If you’d like further reference on deploying Mediation Server, try this guide: Mediation Server Deployment Guidelines – MS Docs

What kind of gateway does your Skype for Business’ Mediation Server talk to?

How the Mediation Server Fits into Skype for Business
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