Wavecom W74PC V8.7.0 - Manual (Page 209)

WAVECOM Decoder W74PC, W-PCI/e, W-CODE, W-CLOUD Manual V8.7.0 Transmission Modes
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The control channel is divided into the Forward Control Channel from base station to units, and the Return
Control Channel from units to base.
The forward control channel may be dedicated (fixed), non-dedicated (any free channel) or the same for
all TSC's, which then access the channel in TDMA (time division multiple access).
The return control channel is randomly accessed by the mobiles in timeslots of 106.7 ms (128 bits).
The forward control channel is divided into time slots each carrying two 64-bit code words:
 A Control Channel System Codeword (CCSC), which identifies the system to radio units and pro-
vides synchronization to the following address codeword.
 An address codeword, which is the first codeword of any message and defines the nature of the
message.
A message consists of a codeword sync sequence, an address codeword and one or more data code
words. A codeword contains 48 information bits and 16 check bits. If bit 1 is logical one the codeword is
an address codeword, otherwise it is a data codeword.
The more important address code words are:
ALOHA (ALH)
TSC to mobile. Carries information on the number of time slots in the succeeding frame and the channel
number of the control channel.
REQUEST (RQS)
Mobile to TSC. Requests for action by the TSC.
AHOY (AHY)
TSC to mobile. General availability check.
ACKNOWLEDGEMENT (ACK)
TSC to mobile, mobile to TSC. Response to RQS or AHY. ACK from TSC also carries ALOHA time slot in-
formation.
GO-TO-CHANNEL (GTC)
TSC to mobile. Carries information on traffic channel number allocated for requested call.
The return control channel for the mobile units is accessed randomly using a particular form of the slotted
ALOHA protocol. The TSC continuously sends sync messages on the forward control channel to the mobiles
inviting random access messages. These TSC messages contain a parameter indicating the number of fol-
lowing timeslots - a frame - available. The mobile unit finds a random slot in the frame for its message. If
the messages of two or more units collide, the collision is detected by the TSC which now increases the
number of timeslots for the next frame and the mobile units retransmit.
If you select Fixed stations AHL messages will be filtered out except for the two following a non-AHL
message. This is to prevent the display being flooded by these messages which are the most numerous in
the system.
If you select Mobile stations all messages in the control channel will be displayed, and the messages
which are identical, but have different labels depending on their origin from TSC or mobile, will be labeled
with their mobile label.
Error correction may be enabled or disabled by toggling the ECC menu item.
A PC generated timestamp may be inserted ahead of the decoded message. Go to the Options menu and
enable the Time stamp function.
Here are a few examples of the most frequent MPT messages monitored on the Forward Control Channel
transmitted by the TSC. The most common message is ALH, a general invitation to transmit any single
codeword message:
ALH(P:42,I1:5461,CH:3,WT:6,M:0,N:4).
Invitation to all mobile units belonging to group 42-5461 (P:42,I1:5461) to transmit. The message is
sent on control channel 3 (CH:3), the TSC responses may be delayed by 6 slots (WT:6), no subdivision
of the radio population is enabled (M:0) and the next frame contains 4 random access timeslots (N:4).
A radio unit calls another unit with the same prefix: