Nera Inmarsat-c, Service Manual, Page: 140 / 228

AP1.3 Channel types and Signal processing
AP1-19
2. Signaling Channel
The MES signalling channel packet is always a fixed length of 15 bytes (120 bits). This
channel is used for signalling from MES to LES and NCS. For example, the packet
"Assignment Request" contains MES ID, LES ID, message size, and information of
network to be connected. Further, short messages such as data reporting (ship's position,
speed and course, etc.) is sent to LESs and NCSs on this channel.
The signalling channel is based on the frame length of 8.64 seconds. Each frame is
divided into 28 slots and the transmission rate for a burst within a slot is 1200 symbols/s
for second generation satellite. There are two types of access to the signalling channel:
reserved and unreserved. For reserved access the slot that is to be used by the MES is
pre-allocated by the LES (Bulletin Board). For unreserved access the MES selects
slot(s) unreserved for data transmission.
(2nd Generation Satellite, 1200 symbols per second)
Fig. AP1.3.2 Signalling Channel Frame Format
1 2 3 4
K
28 27 26
8.64 sec
UW
(64 Symbols)
Convolutional Encoded Data
(253 Symbols)
316 Symbols
(263.5ms)
370 Symbols