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98 From GSM to LTE-Advanced Pro and 5G

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 Huawei 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 2.22 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 non‐
standardized 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 SM 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. Optionally, SNDCP can also compress the user data
stream. The LLC layer and all layers above are transparent for the PCU, BSC and BTS as
they are terminated in the SGSN and the mobile device, respectively.

The Gb Interface
The Gb interface connects the SGSN with the PCU as shown in Figure 2.22. On layer 1,
2 Mbit/s E‐1 connections were mostly used in the past. An SGSN is usually responsible
for several PCUs in an operational network and they were thus usually connected by
several 2 Mbit/s connections to the SGSN. On layers 2 and 3 of the protocol stack, the
Frame Relay protocol was used, which was a standard packet‐switched protocol used in

Um Abis Gb

User data User data
SNDCP SNDCP
LLC LLC
RLC BSSGP
MAC FR or IP

E-1 E-1, Ethernet

MS BTS PCU SGSN

Figure 2.22 GPRS protocol stacks in the radio network.