Comtech EF Data SNM-1002 - Manual (Page 96)

SNM-1002 LinkSync
 Modem
Revision 2
Theory of Operation MN/SNM1002.OM
4–2
The AFC process consists of three steps:
1. Removing the receive offset of the Network Control Modem at the MIDAS
Controller.
2. Removing the receive offset of the Node Control Modems and the traffic
modems at the traffic sites.
2. Removing the transmit offset of the Node Control Modems and the traffic
modems at the traffic sites.
These three steps are performed as follows:
1. The LinkSync™ Modem measures the receive frequency of the outbound
control channel and provides this information to the MIDAS Controller
periodically, the period being user-configurable. The MIDAS Controller then
calculates the offset from the nominal, which is used to correct the Network
Control Modem’s TX frequency.
2. The Node Control Modem determines the receive offset by measuring the
receive frequency of the outbound control channel.
a. This offset is used to calibrate the RX frequency of the Node Control
Modem and all the traffic modems (during call setup) at the node.
b. The traffic modems that have active calls are not adjusted. Calibrations
are periodically performed where the period is user –configurable.
3. MIDAS Controller periodically polls a Node Control Modem to transmit
exclusively, and measures the receive frequency for the incoming bursts from
the Node Control Modem on the inbound control channel.
a. It then estimates the offset and provides this information to the Node
Control Modem.
b. The Node Control Modem then applies this offset to correct its TX
frequency and that of all the traffic modems (during call setup).
c. The traffic modems that have active calls are not adjusted. This process
is repeated periodically, the period being user-configurable.