Mocomtech CIM-550 - Manual (Page 133)

CiM-550 IP Enabled Satellite Modem Rev. 2
EDMAC CD/CIM550.IOM
107
8.2 M&C C ONNECTION
Data to be transmitted to the distant-end is sent to a local unit via the remote control port.
A message for the distant-end is indistinguishable from a ‘local’ message - it has the
same structure and content, only the address will identify it as being for a distant-end
unit.
Before the M&C data can be successfully transmitted and received, pairs of units must
be split into EDMAC Masters and EDMAC Slaves. Masters are local to the M&C
Computer, and Slaves are distant-end.
Now, a unit which has been designated an EDMAC master not only responds to its own
unique bus address, but it will also be configured to listen for the address which
corresponds to its EDMAC Slave. When a complete message packet has be received by
the EDMAC Master, it will begin to transmit this packet over the satellite channel, using
the overhead bytes which become available.
Note: The ‘normal’ protocol for the message packet is not used over the satellite path, as it is
subject to errors. For this reason, a much more robust protocol is used which
incorporates extensive error checking.
At the distant-end, the EDMAC slave, configured for the correct address, receives these
bytes, and when a complete packet has been received, it will take the action requested,
and then send the appropriate response to the EDMAC Master, using the return overhead
path on the satellite link. The EDMAC Master assembles the complete packet, and
transmits the response back to the M&C Computer.
Apart from the round-trip satellite delay, the M&C Computer does not see any difference
between local and distant-end units - it sends out a packet, addressed to a particular unit,
and gets back a response. It can be seen that the EDMAC Master simply acts as
forwarding service, in a manner which is completely transparent.
This approach does not require any additional cabling - connection is made using the
normal M&C remote port. Furthermore, the user does not have to worry about
configuring the baud rate of the M&C connection to match the lowest data rate modem in
the system. The M&C system can have mixed data-rate modems, from 2.4 kbps to 2048
kbps, and still run at speeds in excess of 19,200 baud. It should be pointed out that at 2.4
kbps, the effective throughput of the overhead channel is only 11 async
characters/second. For a message of 24 bytes, the time between sending a poll request
and receiving a response will be around 5 seconds.
Note: When either of the BPSK Turbo rates are in use, the overhead rate is reduced by a factor
of three, and therefore the response time will be around 15 seconds.)