Mes Message Channel - Nera Inmarsat-c Service Manual

3. MES Message Channel

The message channels operate in TDMA mode and are controlled by the LES. Message
channels are used by MESs to transfer messages to an LES. Each LES has one or more
message channels assigned to it by the NCS.
Allocation of a message channel to an MES is performed by the LES using assignment
packets. Each message channel may be used by several MESs simultaneously engaged
in From-Mobile calls.
The message channel is quasi-continuous mode with variable frame duration depending
on the length of the message to be transferred.
A maximum frame length is 640 bytes or 5 packets. The first packet of a message
contains a mssage header, destination address and message data. The remaining packets
contain message data only. It takes 8.64 seconds to send 5 packets or one frame. The
message can be up to 32k bytes, that is, about 50 frames can be sent consecutirely.
8 7 6 5 4
1
Messagre Packet
No.1
127
Flush byte : all
128
Messagre Packet
No.2
Flush byte : all "0"
256
Messagre Packet
No.3
Flush byte : all "0"
512
Messagre Packet
No.4
Flush byte : all
640
Messagre Packet
No.5
Flush byte : all "0"
640
3 2 1
"0"
"0"
Fig. AP1.3.4 Message Channel
AP1-21
AP1.3 Channel types and Signal processing
8 7 6 5 4 3
Packet No. 1
C R Length
Logical Channel Number
Presentation Control
Last Count
Additional Information
Data
Check sum
8 7 6 5 4 3
Packet No. 2-5
Data
Check sum
2 1
1
C : Delivery Class
(Immediate/Deferred)
R : Confirmation Request
Message format (ITA2 or IA5)
Number of whole characters in
last packet of message
127
14
2 1
1
127