Control plane protocol for the transport layer in 3rd Generation UMTS networks is called ALCAP ("Access Link Control Application Part"). ALCAP is defined by 3GPP as equivalent of ITU recommendation Q.2630.2. Basic functionality of ALCAP is multiplexing of different users onto one AAL2 transmission path using channel IDs (CIDs). It is used in the UMTS access network UTRAN along with ATM, while IPBCP is use for IP links in the core of the network.
ALCAP makes it possible for up to 248 channels to be multiplexed onto one AAL2 bearer.