Windows – What does Desktop Composition mean in Remote Desktop

Caching bitmap means that images and other bitmap resources are locally stored on the client computer for reusing them later. This way, the remote server or PC doesn't send images twice reducing the amout of data sent and saving your bandwidth usage.

The option makes particularly sense for slow (low bandwidth) connections, less if you connect to a machine in the same local area network.

If you enable the option the Remote Desktop client caches bitmaps into a BMC file located on the client hard disk in (example for Windows XP)

C:\Documents and Settings\<username>\Local Settings\
     Application Data\Microsoft\Terminal Server Client\Cache 

folder.

Note (interesting if your computer is part of a domain): since the cache folder is stored in a "Local Settings" folder, it won't be replicated as part of a roaming user profile.

With the recent advancements to the Task Manager, incorporating Resource Monitor, you can easily see the bandwidth usage from Terminal Services.

In short, it all depends on the settings you have used and what you're doing in the remote session. More actions require more repaints. Below are two shots I've taken while in an RDP session to a Windows 2008 R2 server.

The first screenshot, only Resource Monitor open (~3.6KBps):

The second, a YouTube video streaming (~951KBps):

Hourly bandwidth consumption at rates like these can vary from ~12M to 3.4G+. Unless you are watching video over RDP, the higher end of the ladder is highly unlikely. For normal activity at 1024x768 resolution with standard settings I'd estimate about ~25M per hour.