5-15 MINI-LINK BAS
EN/LZB 111 0542 P2B Technical Description
The downlink TDM frame is made of several timeslot, where the
number of slots can be set to 80 or 128 (default value). The ATM cell
is carried together with the permit field, needed in the MAC protocol,
in the TDM slot structure.
The permit field is checked by a CRC-8, named CRC-P, and discarded
if errors are detected. The whole information, included permits, CRC-
P and the complete ATM cell, is error checked by a CRC-8. This code
is used to allow fault handling and performance monitoring, the CRC-
8 errors are counted per NU.
Furthermore 24 bits are dedicated to the FEC function. The actual
way, according to which FEC bits are processed, is explained in the
physical layer relevant section.
The frame length must be chosen together with the MAC polling
period as depicted in the following table, where the time duration of
the frame is reported too.
MAC Polling Period Downlink TDM Frame
20 Slots 80 Slots (1ms)
32 Slots (Default) 128 Slots (1.6ms)
80 Slots 80 Slots (1ms)
The polling of all NUs is done in consecutive slots, which means that
64 NUs are polled in 8 slots. The above-mentioned length of radio
frame will give a period time of 1-2 ms.
The Frame Alignment Word (FAW) is used for frame
synchronisation. It is composed by the first group of polling permits
plus the correspondent CRC-P code.
5.4.1.2 Uplink TDMA Frame
The uplink TDMA frame has basically the same structure of the
downlink TDM frame. It is made of several timeslots, where the
number of slots can be set to 80 or 128 (default value), each timeslot
is 480 bits wide.
There are different kinds of slots in order to support MAC protocol
and ATM cell transport. Structure of timeslots is depicted in Figure
5-7.
Some fields of timeslot structure are common to all kind of timeslots,
name guard and preamble fields.
A guard is made up of 10 bits and is inserted at the beginning of the
upstream slot respectively. Guards are meant to reserve time to the
terminal transmitter for switching on/off.
A preamble is made of 20 bits and is inserted at the slot beginning. It
is meant to allow the RN receiver to get carrier phase locking in order
to demodulate properly the useful part of the timeslot.
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