Nera Inmarsat-c, Руководство по обслуживанию

Страница: 142 / 228

AP1.3 Channel types and Signal processing
AP1-21
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.
Fig. AP1.3.4 Message Channel
8 7 6 5 4 3 2 1
1
127
640
Messagre Packet
No.1
14
C : Delivery Class
(Immediate/Deferred)
R : Confirmation Request
Number of whole characters in
last packet of message
Flush byte : all “0”
Messagre Packet
No.2
Flush byte : all “0”
Messagre Packet
No.3
Flush byte : all “0”
Messagre Packet
No.4
Flush byte : all “0”
Messagre Packet
No.5
Flush byte : all “0”
128
256
512
640
8 7 6 5 4 3 2 1
Packet No. 1
Length
Logical Channel Number
Last Count
Additional Information
Data
Check sum
1
C R
Presentation Control
8 7 6 5 4 3 2 1
Packet No. 2-5
Data
Check sum
1
Message format (ITA2 or IA5)
127
127