We all have a network, every system we use runs on a network but yet it is something we often don’t think about in terms of data, sure you think of your wan links but now most people will put in 1GB switches and essentially fire and forget! Bringing Lync 2010 on to your network forces you to take a step back. The key thing with lync is to ensure a good user experience over anything else and while we can plan for servers and scale them appropriately people often forget about the network and what state that is in!

Before diving into to planning rates etc… I want to discuss of how robust certain things are in lync and why that although we do need to think and plan bandwidth etc… we have pretty resilient mechanisms with the media stack that’s built in to lync.

For Example

Our SIP implementation is done over TCP which is a reliable protocol and you have to wonder if the likes of QoS are required. App Sharing Also uses TCP.

We have an adaptive algorithm which monitors the available bandwidth on the channel and adjusts the codecs around to suit the channel and to give the best possible experience to the end user.

We also use FEC which can handle up to 10-15% (really ideal conditions) packet loss.

Video will adapt in terms of resolution and frames per second to the channel…

Finally as part of our resiliency of things worth noting is the fact we can suffer loss of the signalling channel and still maintain a call between endpoints! Its true.. it does limit functionality like putting the call on hold etc.. but the call itself will stay up and until one endpoint disconnects the media session will stay alive, but even better if the signalling channel comes up again the session will “repair” itself and continue on…..

Remember one of the main goals is to sustain voice so video will drop / app sharing will drop to keep the voice sessions alive.

One of the nice things I find in lync is the visual indicators of the network quality, this is a pre warning to the user to tell them they might suffer bad quality due to conditions that lync cannot control.

We also can use CAC which is controlled centrally and applies policies to the subnets and logical connection between points rather than a user. So a user can travel around and no matter where they are, they will only be affected by the correct CAC policy for that particular site which proves to be very useful and easy to manage.

So knowing all this information

Lets take a look at some of the bandwidth requirements, please use the maximum rates were possible for planning if you think FEC might happen on your network plan for that.

Slides taken from Tech-Ed Content!




The above information is great for video and audio but what about app sharing I hear you saying well…

App sharing is bursty (very bad english!), as in it will use a very small amount of bandwidth most of the time but when you get a screen change or something like that it will require a large amount of bandwidth for a short time frame. So you need to understand that and measure it on your networks.

As mentioned we can use CAC to help control bandwidth, for those who don’t know what CAC is, CAC stands for Call Admission Control and what you can do is set minimums on links between sites so that if the bandwidth is not there that a session can’t be established instead of a session being established and the user has a bad experience.

If you want to use QoS to help control call flow think about what we said above about our media stack, sip uses TCP but our audio uses UDP so by all means Prioritize the Audio over the network but there is no real need to do it for SIP.

All in all you need to observe that the network is doing and have visibility, please run network monitors and bandwidth monitors and eliminate the bottle necks after you have used the planning tools to design and identify the links. Use the bandwidth calculator available to give you an idea as well of what you will need based on your ideal profile versus what you have deployed currently in your environment.

Understanding our technology and planning appropriately is the key to a successful lync implementation!