Ses Message Channel - Furuno FELCOM 12 Service Manual

3. SES 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 3 2 1
1
Messagre Packet
No.1
127
Flush byte : all "0"
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 "0"
640
Messagre Packet
No.5
Flush byte : all "0"
640
C R
Logical Channel Number
Additional Information
Figure 2-5 Message Channel
2-5
8 7 6 5 4 3 2 1
1
Packet No. 1
Length
Presentation Control
Last Count
Data
Check sum
127
14
8 7 6 5 4 3 2 1
1
Packet No. 2-5
Data
Check sum
127
C : Delivery Class
(Immediate/Deferred)
R : Confirmation Request
Message format (ITA2 or IA5)
Number of whole characters in
last packet of message