In the original voice design for OCS, all traffic bound for the PSTN had to transit a mediation server. The mediation server provided three key features: it removed the encryption of SIP packets that go to the gateway (and added it for signaling traffic received form the Media gateway), it similarly managed the encryption of the RTP packets sent to and from the PSTN and finally transcoded the media traffic between Microsoft’s Real Time Audio codec and G.711 (used on the PSTN).
In the early days of OCS, this made sense as the then-available gateways could not really handle the encryption and de-encryption and the early clients did not natively send/receive using G711. The downside to this design was the need for extra servers (and associated OS/Software licenses) and the processing delay encountered particularly when transcoding RTP traffic (i.e. the actual audio). And since transcoding was never going to be perfect, there was also a bit of a reduction in call quality.
Lync introduced an excellent new feature, Media bypass, which enables the client to send all media traffic directly to the media gateway thus removing most of the traffic through the mediation server. This was possible only with new gateways that supported Media Bypass. With Media Bypass, the call setup (signaled by SIP traffic sent from the client) was done as in OCS, but all the media traffic could a) be initially encoded using G.711 and b) sent directly to the Media gateway. This approach delivers several benefits: first, it reduces significantly the traffic traversing the Mediation Server allowing that role to co-exist on a front end server. Since the RTP traffic never transits the mediation server, RTT times and other factors are improved. And finally, since the client natively encodes audio into G.711, fidelity is at least as good and probably better than with OCS. Assuming your gateways were up to the job, media bypass represents a big reduction in the number of servers needed to support PSTN interconnection.
While Media Bypass is designed mainly for use in PSTN-interconnection scenarios, it can also be used if you are routing calls to an internal IP-PBX (assuming of course that that IP-PBX supports media bypass) and can be used with connecting to some SIP Providers.
In a great article on Windows IT Pro’s site, Lync super-star Byron Spurlock explains how media bypass works and shows how relatively simple it is to turn on. Byron makes the point that while Media Bypass is not quite out of the box, it’s fairly simple to configure assuming your infrastructure is capable of handling it.