Ericsson RBS 2106 Reference Manual page 252

Packet Data Services
The RBS initiates the setup and synchronization of the GSL, by issuing
PCU frames of type "synchronization". Upon reception of PCU frames
of type "synchronization" by RBS, the GSL is established.
Upon reception of an RF CHANNEL RELEASE from the BSC, Idle
pattern is started to be transmitted to the PCU. The GSL is stopped and
the RBS-PCU connection is released.
Upon reception of a PCU frame of type "synchronization", RBS
associates the downlink FN counter with the Air FN, and returns these
values in the next PCU frame of type "synchronization" to the PCU.
RBS supervises the frame synchronization of the GSL by checking the
PCU frames of type "traffic".
To minimize the downlink delay between the Air interface and the
PCU, RBS continuously monitors, and if necessary adjusts, the
alignment between the Radio Blocks sent on Air Interface and the PCU
Frames received on GSL.
The procedure runs autonomously as long as the GSL is active.
To minimize the uplink delay between the Air interface and the PCU,
RBS measures the difference in time between reception from Air
interface and transmission to PCU, and performs necessary adjustments.
The procedure runs autonomously as long as the GSL is active.
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EN/LZT 720 0008 P2A
2001-11-28