Transmission Plane

Not only does the SGSN interface with a
BSC for packet transfer to and from a given MS, direct logical interfaces are also used between an MS and an SGSN—for signaling (signaling plane)
and for packet data transfer (transmission plane), even though the interfaces
pass physically through the BSS. The overall interface structure for
the transmission plane is shown in Figure 5-3.

At the MS, we first have the RF interface, above which are the Radio Link
Control (RLC) and Medium Access Control (MAC) functions. Above these,we
find the Logical Link Control (LLC), which provides a logical link and framing
structure for communication between the MS and the SGSN. Any data
between the MS and SGSN is sent in Logical Link Protocol Data Units (LLPDUs).
The LLC supports the management of this transfer, including mechanisms
for the detection and recovery from lost or corrupted LL-PDUs,
ciphering, and flow control. It is worth noting that ciphering in GPRS is
somewhat more extensive than in standard GSM. In standard GSM, only
the radio link between the MS and BTS is ciphered. In GPRS, ciphering is
applied between the MS and the SGSN, such that information is encrypted
across the radio interface, the Abis interface, and the Gb interface.
Above the LLC, we find the SubNetwork Dependent Convergence Protocol
(SNDCP), which resides between the LLC and the network layer (such
as IP or X.25). The purpose of SNDCP is to enable support for multiple network
protocols without having to change the lower layers such as LLC. Not
only does SNDCP provide a buffer between the higher and lower layers, it
enables several packet streams to be multiplexed onto a single logical link between the MS and SGSN. It also optionally performs compression (such
as TCP/IP header compression and/or V.42bis data compression). Such
compression, in particular V.42bis, can make a noticeable difference to the
throughput.

At the BSS, a relay function relays LL-PDUs from the Gb interface to the
air interface (the Um interface). Similarly, at the SGSN, a relay function
relays PDP PDUs between the Gb interface and the Gn interface.
When one looks at Figure 5-3 initially, one finds that the IP layer appears
to be repeated. In fact, it can be. Recall that GTP is a tunneling protocol. As
far as the applications at either end are concerned, only one IP connection
exists—the one directly below the application layer, as shown in Figure 5-3.
GTP effectively places this connection and its associated packets in a wrapper
for transmission through the IP network between GGSN and SGSN.
Thus, the IP network nodes (routers) between SGSN and GGSN consider
the GTP packets to be the application, and those routers do not examine the
contents of the GTP layer. At the SGSN, the wrapper is removed and the
packet is passed to the MS using SNDCP, LLC, and lower layers. For packets
from the MS to the external network (such as the Internet), the GGSN
removes the wrapper and forwards the IP packets. 181