Microsoft Communicator Mobile Edition 2007 R2 has introduced number of performance improvements over its predecessor related to battery power consumption and over the air bandwidth savings. This post looks at the concept of low vs. high fidelity subscription and how it helps in saving battery power in mobile devices and saving over the air traffic. Let’s first look at the problem with traditional event subscription model. Typically, clients subscribe to an ad hoc list of contacts (buddy list) presence information at the server with non-zero expiration value. The message body of a SIP SUBSCRIBE message lists all the contacts user is subscribing to for their presence information. It conveys to the server that client is interesting in keeping up to date with the real time presence state updates of buddies.[…]

More info:  OCS Team blog