2.7 GPRS Interfaces
The GPRS standards define a number of interfaces between components. Apart from the PCU, which has to be from the same manufacturer as the BSC, all other components can be selected freely. Thus, it is possible, for example, to connect a Nokia-Siemens Networks PCU to an Ericsson SGSN, which is in turn connected to a Cisco GGSN.
The Abis Interface
The Abis interface connects the BTS with the BSC. The protocol stack as shown in Figure 2.23 is used on all timeslots of the radio network which are configured as (E)GPRS PDTCHs. Usually, all data on these timeslots is sent transparently over the nonstandardized interface between the BSC and PCU. However, as the link is also used to coordinate the BSC and PCU with each other, it is still not possible to connect the BSC and PCUs of two different vendors. On the lower layers of the protocol stack, the RLC/MAC protocol is used for the radio resource management. On the next protocol layer, the Logical Link Control (LLC) protocol is responsible for the framing of the user data packets and signaling messages of the mobility management and session management subsystems of the SGSN. Optionally, the LLC protocol can also ensure a reliable connection between the mobile device and the SGSN by using an acknowledgment mechanism for the correctly received blocks (acknowledged mode). On the next higher layer, the Subnetwork-Dependent Convergence Protocol (SNDCP) is responsible for framing IP user data to send it over the radio network. ...