Abbreviation for Packet Control Unit. The PCU is one
of the new network elements that need to be integrated to the existing GSM infrastructure
to enable GPRS.
The PCU serves the very important function to
interface the new GPRS core network to the existing GSM BSS. This is done by converting
packet data coming from the SGSN in
so called PCU-frames that have the same format as TRAU-frames (figure 1). These PCU-frames
are transparently routed through the BSC and towards the BTS. The BTS needs to determine
the respective coding scheme
and other options before processing a PCU-frame.
Additionally, the PCU takes over all GPRS radio
related control functions from the BSC. This means that the PCU is in charge for all
functions that are related to the Radio Link Control (RLC) and Medium Access Control (MAC)
Protocols (RLC/MAC) on the network side.
Figure 1: Functions of the
PCU
- Please note that the standards allow for different locations of the
PCU (figure 2). In fact, there is no real choice where to locate the PCU. It rather
depends on the location of the TRAU where a network operator needs to be place the PCU,
too.
- This is obvious, considering that the PCU issues data blocks
(<=> PCU-frames) in downlink direction that look like TRAU-frames.

Figure 2: Different Locations of the PCU
[3GTS 23.060 / 2GTS 04.60]
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