2-3
2. Signalling Channel
The SES 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.
1234
K
282726
8.64 sec
UW
(64 Symbols)
Convolutional Encoded Data
(253 Symbols)
316 Symbols
(263.5ms)
370 Symbols
Figure 2-3 Signalling Channel Frame Format
(2nd Generation Satellite, 1200 symbols per second)
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.
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