Wednesday, May 4, 2011

Calculating number of Mediation Servers and voice channels required for Lync Server 2010

I generally hate doing something I don't fully understand or haven't been taught so I've taken some time to try and grasp the mind bending, eye crossing, fascination that is capacity planning with respect to voice systems.

It all goes back to our Microsoft Certified Masters training for Lync Server 2010 in March/April of this year. Some of the pre-study content prescribed to us touches on what an "Erlang" is and why it's important to understanding voice systems design. In addition to this, the MCM program has us learn about applying factors such as Busy Hour Traffic (BHT), Blocking Percentage, Busy Hour Factor, and Erlangs against real world capabilities of Lync Server 2010.

So let's start with the basics. What is an Erlang? Well, if you look up the Wikipedia definition it states:

"The erlang (symbol E[1]) is a dimensionless unit that is used in telephony as a statistical measure of offered load or carried load on service-providing elements such as telephone circuits or telephone switching equipment."

Basically, an Erlang represents one voice path, or one channel, or one line in constant use (sorry Adam). The reason an Erlang is important is because we need to eventually determine the number of concurrent channels required for sizing T1/E1 capacity or even determining the number of Mediation servers we need.

The other important concept we need to understand is the Busy Hour Traffic (measured in Erlangs). BHT is the number of hours of call traffic during the busiest hour of the day. Said another way, BHT represents the maximum concurrent channels used during the busiest hour.

In addition to understanding line usage, we need to grasp the idea of a blocking percentage. This means the likelihood of a call being denied (blocked) due to insufficient channels or lines (capacity). When planning for capacity you need to determine the acceptable blocking percentage for an organization. Some will permit only 1% which means 1 out of every 100 calls will be blocked due to insufficient line capacity. Other organizations are willing to accept 2.5% or more. 

The last concept we need to cover is the Busy Hour Factor, represented in a percentage. The Busy Hour Factor is the percentage of minutes which are offered during the busiest hour of the day. The default is typically 17% for most businesses open during an 8 hour work window. We use the Busy Hour Factor to calculate the Erlangs based on a certain volume of minutes in a day.

Clear as mud? Let's look at the following scenario:

You are introduced to a customer who is looking to move to Lync Server 2010 and migrate from an existing PBX with 2 T1's. Rumors of an acquisition come true and the company plans to integrate more telephony capacity. You're given the phone statistics for both companies which works out to 37,000 minutes per day.

What is the Busy Hour Traffic (BHT, measured in Erlangs)?
What is the Busy Hour Factor (default is 17%)?
What is the Blocking Percentage?
How many T1's do you need?
How many Mediation servers do you need?

We actually can't answer these questions unless we have an "Erlang B" calculator which can be found here: But first let's solve what we can. For those of you who wish to solve without assistance, the formula is:

To calculate Busy Hour Traffic, we can multiply the Busy Hour Factor of 17% by the total number of minutes (37,000) then divide that by 60. The calculation looks like this: 

37,000 * 0.17 / 60 = 104.8 BHT (Erlangs)

Since the scenario didn't specify a blocking percentage, let's assume 1%. With this assumption and the calculated BHT value, we now have enough information to put into our "Erlang B" calculator to determine the number of lines or channels we need.

This produces 122 lines.

Knowing a T1 can handle 23 lines of voice traffic, we get 5.3 T1's being required. Now you can't have .3 of a T1 so maybe the client is willing to accept a higher blocking percentage to squeeze the traffic into 5 T1's. You can use the "Erlang B" calculator to determine what the blocking percentage would be in this case.

5 T1's can carry 115 channels and with 104.8 BHT this produces a blocking percentage of 2.7%.

Acceptable? Maybe...maybe not. It really depends on the customer.

Now there are other clever ways of squeezing out a few more channels. NFAS is one way in which you can forgo the D-channel on each T1 if you've trunked several of them together. For example, 3 T1's would typically have 3 D-channels whereas with NFAS, you can get away with 1 D-channel between the group of three. This gives you two more B-channels for voice. Multiply that by 5 T1's and you get four more B-channels increasing your capacity from 115 to 119. Using the "Erlang B" calculator again....

This produces a blocking percentage of 1.6%

Not bad at all!

Okay, so bringing things back to reality, we have a recommendation to the customer about how many T1's they need to plan for which is 5 using NFAS. The next question we need to answer is how many Mediation servers we need so let's look at some capacity numbers:

A stand-alone Mediation server with quad 1Gb NIC with dual quad-core CPU's can support 800 - 1200 concurrent calls (not including media bypass).

A collocated Mediation server with Front-End server can support 226 concurrent calls.

Based on our Busy Hour Traffic number of 104.8 Erlangs, even a single collocated Mediation server on a Front-End server can handle all the traffic.

Anyway, I hope this helps some of you understand the importance and complexity of sizing voice channels and servers. Microsoft has done an amazing job at increasing the capacity of concurrency with Lync Server 2010. Comments welcome.



  1. When you calculate the number of mediation servers needed, are you assuming media bypass is enabled for all calls?

  2. You don't typically calculate call volume for bypass calls since the media isn't traversing the server. You would use the above to calculate channels on the PRI though. You would also calculate non-bypass calls for external users since they wouldn't be able to bypass if they come through Edge.

  3. Hey man, great article, very helpful.

    I'm wondering if you wouldn't mind giving me some advice...

    I'm a rural Wyoming resident, and phone prices are extremely high here. Would you be the man to ask with questions concerning starting a small-town phone company? if so:

    I'd probably start with a max of 100 customers, so i'm thinking maybe 3 or 4 T1s, and it seems 1 collocated med. server. Now my main question is: do I NEED to have them in, say, a colo. center or telco hotel to connect to the telecom infrastructure, or can I do that from a well-connected office? Also, CenturyLink is the only provider here, so would I need to contact them for fees to send/receive over their lines in the area? I have many more questions, but I don't want to burden you, and this conversation would be better suited for e-mail.

  4. Hi Daniel, it might make sense to consider pairing up with a SIP trunk provider as opposed to T1's since you will typically pay for used capacity as opposed to buying capacity and anticipating it's use.

    I would also give some thought to the method your subscribers will use to connect to your system. Is Lync the right choice? I'm not so sure it would be. Have a look at SNOM's VoIP system and phone solution (

    As for more questions, you can fire away but I can't guarantee a timely response as I basically have two full time jobs not to mention a family.