Tag Archives: Media Gateway

Collocated or Stand-alone Mediation Server

Most of the time, Lync Enterprise voice deployments need debate for collocated or stand-alone mediation server. Collocation of Mediation Server can reduce the TCO and data center footprints. Can Mediation server collocation be a wise option? To choose a wise option out of collocated or stand-alone mediation server depends on the following:

  1. Number of users enabled for UC-PSTN calls
  2. Number of UC-PSTN calls per user per hour
  3. Number of UC-PSTN calls at the time of peak load
  4. Connected gateway / SBC and mediation server
  5. Percentage of calls that support media bypass
  6. Branch sites configuration for UC-PSTN deployment

If I have missed any point here, please leave your comment so that I can add the same.

Any call which initiate from any Lync endpoint has two components signaling and media. For UC-PSTN calls, signaling always goes through Mediation server if stand-alone or Standard Edition / Front End server collocated with mediation server role.

No Media Bypass:

Media Bypass:

If your gateways, SBC or IP-PBX support media bypass, you can use collocated mediation server. But if you are planning for an option which do not support media bypass, I will advise you not to use collocated mediation server as collocated meditation server will increase load on front end servers which can cause of poor performance.

If you still want to use collocated meditation server, you can increase number of front end servers which can help you to distribute the load among front end servers.

A Stand-alone Mediation server deployment also depends on Branch sites which are connected to central site. If your branch sites don’t have dedicated PSTN connectivity then you should go with stand-alone mediation server pool. Again this topic requires more debate. As we know Lync 2013 use M:N trunk, in simple way it means if you have mediation server deployed in branch site and that can support media bypass you can still go with collocated option.

You can use Lync Planning tool to try all options and choose best out of that which provide you better ROI and best performance.


Branch Site Connectivity options for Lync

Good connectivity between sites is a basic requirement for every organization. To keep the same requirement in consideration Microsoft designed Lync with multiple branch connectivity options. Basically Lync 2013 supports four types of branch connectivity solutions to choose from which depends on your requirement. Branch sites can have few hundreds of users or few thousands of users. Therefore, solution also depends on usage, connectivity between the sites and number of users. Basically branch sites come into picture at the time of connectivity failure b/w central and branch site and provide resiliency to branch site users.

Only WAN connectivity: This solution is not called as a branch site connectivity option by Microsoft. Usually, it might be an option for small sites where organization has few users (1 – 25) but good WAN connectivity which can sustain the load of branch site users. Organization can have one or more sites which are directly connected to the central site with good WAN connectivity. All the users from this type of sites will directly login to central site and will use all the features including PSTN.

Survival Branch Appliance (SBA): SBA can have an option for those branch sites where organization has user’s b/w 25 to 1000 or maybe 2000 and don’t have local administrative support. SBA is an industry standard appliance which has Lync server registrar and mediation server component. SBA also contains PSTN Gateway for direct PSTN connectivity to branch sites. If you have users range b/w 25 to 1000 you can choose SBA accordingly or if you have more than 1000 users you can have two SBA, this is just an example but totally it depends on requirements and supported number of users by device. SBA provides resiliency to branch users at the time of WAN failure for Enterprise Voice but does not provide resiliency for other Lync features such as IM/Presence, conferencing etc.

Survival Branch Server (SBS): SBS can have an option for those branch sites where organization has more than 1000 users and have local administrative support. SBS is a Windows Server which has Lync server registrar and mediation server component installed on it. SBA does not contain PSTN Gateway for direct PSTN connectivity to branch sites. Therefore you need a separate PSTN gateway to connect PSTN service provider or you need SIP trunk connectivity with ITSP. Same like SBA, SBS provides resiliency to branch users at the time of WAN failure for Enterprise Voice but does not provide resiliency for other Lync features such as IM/Presence, conferencing etc.

Standard Edition Server: Standard Edition Server is an option to provide all the Lync functionality to branch site users or you can say that is a small central site. Standard Edition Server need a PSTN connectivity same as central site using a Media Gateway or SBC to connect ITSP/PSTN service provider. Standard Edition Server provides resiliency to branch users at the time of WAN failure for Enterprise Voice as well as for other Lync features such as IM/Presence, conferencing etc.

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Bandwidth Requirements for Lync 2013

Lync is a real-time communication platform to provide rich communication experience to end users. To accomplish rich experience for Lync users, Network assessment and planning is a most important phase of your overall design and implementation. This article can help architects, consultants and system administrators at the time of Network planning for Lync infrastructure.

Before going deep into the bandwidth requirements, I would like to explicate Lync call flow scenarios. Any IP based UC solution (here Lync), used diverse media paths for different scenarios such as Peer to Peer sessions, Conference sessions and PSTN/PBX sessions. During peer to peer and conference sessions, there is a possibility to share the content such as entire desktop or any specific application etc.

Lync uses different codecs/protocols for different modalities. Below are the tables which will demonstrate the bandwidth requirement for Lync 2013 infrastructure.

Audio Codec Bandwidth:

Audio codec Scenarios Maximum bandwidth (Kbps) w/o FEC Maximum bandwidth (Kbps) with FEC Typical bandwidth (Kbps)
RTAudio Wideband Peer-to-peer, default codec 62 91 39.8
RTAudio Narrowband Peer-to-peer, PSTN 44.8 56.6 30.9
G.722 Default conferencing codec 100.6 164.6 46.1
G.722 Stereo Peer-to-peer, Conferencing 159 73.1
G.711 PSTN 97 161 64.8
Siren Conferencing 52.6 68.6 25.5


Shown Bandwidth in exceeding table includes IP header, UDP header, RTP header, and SRTP headers. The stereo version of the G.722 codec is used by systems that are based on the Lync Server 2013 Meeting Room Edition, which enables stereo microphone capture so that listeners to can more easily distinguish between multiple talkers in the meeting room.

Video Codec Bandwidth:

Video codec Resolution and aspect ratio Maximum video payload bit rate (Kbps) Minimum video payload bit rate (Kbps) Typical bit rate (Kbps)
H.264 320×180 (16:9)212×160 (4:3) 250 15 200
H.264/RTVideo 424×240 (16:9))320×240 (4:3 350 100 280
H.264 480×270 (16:9)424×320 (4:3) 450 200 350
H.264/RTVideo 640×360 (16:9)640×480 (4:3) 800 300 640
H.264 848×480 (16:9) 1500 400 1200
H.264 960×540 (16:9) 2000 500 1600
H.264/RTVideo 1280×720 (16:9) 2500 700 2000
H.264 1920×1080 (16:9) 4000 500 3200
H.264/RTVideo 960×144 (20:3) 500 15 400
H.264 1280×192 (20:3) 1000 250
H.264 1920×288 (20:3) 2000 500


Forward error correction (FEC) for video is included in the video payload bit rate.

Neither in peer to peer nor in conference sessions, audio or video packets stream continuously. Only sessions activity define that how often packets are sent for a stream.

Streaming activity in a peer-to-peer scenario:

  • When the users speak in peer to peer call then only endpoints send audio streams.
  • Audio streams received by both participants.
  • In peer to peer video calls, both endpoints send and receive video streams during the entire call.
  • In peer to peer video call if there are little or no movement in video scenes, the actual bit rate may temporarily be very low, because the video codec skips encoding regions of the video with no changes.

Streaming activity in a conferencing scenario:

  • When the users speak in conference calls then only endpoints send audio streams.
  • In a conference call, all participants in conference receive audio streams.
  • In a conference call if video is used, all participants can receive up to five receive video streams and one panoramic (for example, aspect ratio 20:3) video stream. By default, the five receive video streams are based on active speaker history, but users can also manually select the participants from which they want to receive a video stream. Users also can fixed video by using pin option which they want to see without based on active participants.

The typical stream bandwidth for panoramic video is based on currently available devices that stream only up to 960×144 panoramic video. After devices with 1920.x.288 panoramic video become available, the typical stream bandwidth is expected to increase.

Network Bandwidth Requirements for Media Traffic:

Modality Description Maximum bandwidth Typical bandwidth
IM, presence, and signaling Nonmedia elements 2 Kbps 1.6 Kbps
Voice Default = RTAudio Wideband 62 Kbps 39 Kbps
Conference voice Default = G.722 100.6 Kbps 46.1 Kbps
Video – small Uses H.264 at 320×180 250 Kbps 200 Kbps
Video – medium Uses H.264 at 640×480 800 Kbps 640 Kbps
Video – high Uses H.264 at 1280×1080 4 Mbps 3.2 Mbps


  • All figures are based around the industry standard of 20ms packetization for UC applications, which is 50 packets per seconds.
  • Bandwidth figures include the protocol overheads for IP, UDP, RTP, and SRTP. This is why the Microsoft bandwidth figures for standard codecs are different from those quoted by other VoIP suppliers, who only state the raw codec figure, and not the entire packet overhead. For Microsoft, this figure includes encrypting all communications.
  • Figures in the following table for audio bandwidths do not include a Forward Error Correction (FEC) overhead. FEC is a mitigation technique that is enabled when the network suffers unusually high packet loss. The assumption is that, for planning, the network administrator will estimate traffic use without mitigations.
  • For video, the default codec is the H.264/MPEG-4 Part 10 Advanced Video Coding standard, coupled with its scalable video coding extensions for temporal scalability. To maintain interoperability with Lync 2010 or Office Communicator 2007 R2 clients, the RTVideo codec is still used for peer-to-peer calls between Lync Server 2013 and legacy clients. In conference sessions with both Lync Server 2013 and legacy clients, the Lync Server 2013 endpoint may encode the video by using both video codecs, and may send the H.264 bit stream to the Lync Server 2013, and send the RTVideo bit stream to Lync 2010 or to Office Communicator 2007 R2 clients.

Default aspect ratio for Lync Server 2013 has been changed to 16:9. The 4:3 aspect ratio is still supported for webcams that don’t allow capture in 16:9 aspect ratio.

The network bandwidth numbers in all preceding tables represent one-way traffic only, and include 5 Kbps for RTCP traffic overhead for each stream.

For all bandwidth tables, the maximum bandwidth figures should generally be used in network planning. Lync Server depends entirely on the underlying network for the user-perceived quality of its communications, particularly voice. In addition, sites with fewer than 100 users should always use the maximum figures because, statistically, the network peaks for Lync Server occur more frequently. For sites with more than 100 users, the typical figures can be used.


Disclaimer: In this article data has taken form Microsoft Lync Server Networking guideline which has been modified a bit to understand better instead of going through a more than 100 page document.

Lync Enterprise Voice Connectivity

World is changing and moving towards rich collaboration and corporates can’t avoid these changes. As well as, we also can’t neglect our traditional way of communication. It means still we need phone to communicate with people. Microsoft did excellent job in this field as earlier only mailing solution was the key application for formal communication. But to understand better, we were using traditional phone. Microsoft Lync as an application can cater all your corporate needs. Microsoft Lync is a rich communication medium and easily can be integrated with other business applications such as Exchange & SharePoint.

Lync provides many options to enable Enterprise Voice. Usually organization uses traditional PBX or IP PBX which involves lots of cost and need distinct administration to manage the whole solution. Lync provides an Enterprise solution which can be deployed with or without your traditional PBX’s. In general there are three ways to enable enterprise voice with Lync.

1. Traditional PBX connectivity with Lync mediation server using media gateway.

2. Direct SIP, Advance IP PBX connectivity directly with Lync mediation server.

3. Direct SIP/PSTN Gateway/VOIP only deployment option directly with Lync mediation server.

4. SIP Trunk, Direct connectivity to Lync Mediation server from ITSP using SBC.