Wavecom W61PC V7.5.0 - Manual (Page 285)

WAVECOM Decoder W61PC/LAN Manual V7.5 SAT System  275
Channel types
The channel types for Aero are:
P-channel
A packet switched continuous channel in the forward direction ground-to-air from each GES carrying user
data and signaling, e.g. system tables broadcast.
R-channel
A random access (slotted Aloha) channel in the return direction carrying user data and signaling if the sig-
naling unit is 33 bytes or less.
T-channel
A TDMA channel in the return direction carrying user data and signaling. The GES allocates time slots ac-
cording to requests from the AES.
C-channel
Circuit-switched SCPC voice channel in both directions.
A satellite region contains one or more Ground Earth Stations (GES). The GES manages the control chan-
nels, allocation of traffic channels, channel resources, interfacing to terrestrial networks (PSTN, data net-
works, Internet) and session control. The user terminals called an Aeronautical Earth Stations, AES, com-
municate with the GES. The GES communicate with other GES within and outside its region and broad-
casts system information.
The only frequencies the AES initially knows are the frequencies of the GES in all regions - all other fre-
quency information is dynamically extracted from the system broadcasts or in session oriented signaling
messages. After being switched on, the AES must be told (or remembers) in which region it is currently
located, and will automatically tune to the corresponding frequency of the GES and will identify the spot
beam ID where it is located.
AES Identification
Each AES is identified by an ICAO assigned 24 bit identification code. For voice an IMN (Inmarsat Mobile
Number) is available.
Session Signaling
In its idle state, the MES continuously listens to the GES P-channel updating its internal network status
database (control channels, frequencies, etc) through the system table broadcasts.
There are two possibilities for starting a session:
 GES initiates the session;