1. INMARSAT C SYSTEM OVERVIEW
1-4
Types of MES
There are three types of MES: class 1, class 2 and class 3. This FELCOM is a class
2 MES.
Section Description
NCS common channel The NCS has two major functions:
• Transmit information on a common channel.
• Transmit EGC messages to MESs.
NCS/LES signaling link The NCS/LES signaling link connects the NCS to all
LESs in its region. All EGC messages pass through
this link.
LES TDM channel The LES TDM channel carries the circuit control sig-
nal for MES and transmits messages from LES to
MES.
MES message channel The MES message channel carries messages from
MES to LES.
MES signaling channel The MES signaling channel transmits requests, dis-
tress alerts, data reports, etc. This channel also car-
ries login and logout from MES to NCS.
NCS/NCS signaling link This NCS/NCS signaling link connects all NCSs. This
link exchanges data among NCSs operating in differ-
ent ocean regions.
MES interface The MES consists of the Data Circuit Terminating
Equipment (DCE) and the Data Terminal Equipment
(DTE). The DCE consists of the antenna unit, and the
DTE consists of a terminal unit, a keyboard and print-
er.
Terrestrial network interface The major functions of the LESs are:
• Telex store-and-forward conversion
• Handling EGC messages
• Handling distress alerts
• Data reporting and polling
Class 1: • Transmits messages to LES.
• Receives messages from LES.
Class 2: • The functions of class 1 plus operation as an EGC receiver when not
transmitting or receiving.
• EGC-only receiver
Class 3: The function of class 1 plus simultaneous operation as an EGC-only receiv-
er.
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