That’s not a rhetorical question I wrote for today’s blog post. It’s part of a question a prospective client asked us:

“Why is there a Mediation Server in Lync if you have Media Bypass? Doesn’t that make it (the server) redundant?”

Media Bypass Lets You “Go Around” a Mediation Server

Media Bypass allows you to call non-Lync phones without transcoding the media stream through Mediation Server. This reduces the load on Mediation Server. And it can preserve audio quality.

Media Bypass has 2 modes:

  • Always Bypass – Media Bypass will be attempted for every PSTN call.
  • Use Site and Region Information – Media Bypass creates a Bypass ID for each network region. Sites with lower bandwidth are given their own IDs; otherwise, sites inherit the same regional ID. This mode uses those IDs to determine when to bypass. (You have control over when bypass happens, in this mode.)

(Note: You cannot use Call Admission Control if you’re in Always Bypass mode. Just an FYI.)

If media can be sent through without transcoding though, why DO we need a Mediation Server? What does the Mediation Server bring to the table?

What a Mediation Server Does (Beyond Bypass)

Without the Mediation Server Role, you would not have Enterprise Voice. Period.

Mediation Servers handle traffic between Front End servers and your chosen gateway (an IP-PBX, SIP trunk, or PSTN gateway). It’s responsible for all SIP traffic, and all PSTN calls through its connected gateway/gateways.

Except for when Media Bypass routes PSTN calls around it.

Did Microsoft hamstring their own Lync Server roles? What’s missing from the picture here?

The Reason Mediation Server is Necessary: Wi-Fi

Is Mediation Server necessary? Only if one condition applies.

Your office uses a wireless network.

Why does that make Mediation Server necessary? Because of this paragraph right here:

“Wi-Fi networks typically experience more packet loss than wired networks. Recovery from this packet loss is not typically something that can be accommodated by gateways. Thus, we recommend that you evaluate the quality of a Wi-Fi network before determining whether bypass should be enabled for a wireless subnet. There is a tradeoff in latency reduction versus recovery from packet loss to consider, as well. RTAudio, a codec which is available for calls that do not bypass the Mediation Server, is better suited for handling packet loss.”

–From Planning for Media Bypass – TechNet

Transcoding media (phone calls) is why Mediation Server exists. One server role has to identify which network your call’s going to, and which codec will work best.

The G711 codec gives callers a better-quality signal. But RTAudio works better on networks where packet loss is higher. RTAudio for calls made via wireless; G711 for internal calls on the LAN.

And what decides which codec to use when you make a call? That’s right…your Mediation Server.

Media Bypass is a very useful feature of Lync Server. It lightens the load, reducing the number of Mediation Servers necessary in your deployment. But make no mistake. If you want to use Enterprise Voice, you will have at least one Mediation Server.

(Oh, and you can collocate it with your Front End pool!)

How’s your Mediation Server set up?

Why Does Lync Have a Mediation Server if We Have Media Bypass?

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