Wavecom W61PC V7.5.0 - Manual (Page 199)

WAVECOM Decoder W61PC/LAN Manual V7.5 Transmission Modes  189
ceivers that receive short bursts from the SCs at 2400 bps. The ORBCOMM System will be capable of
providing near real-time wireless data communications service around the world.
All communications within the ORBCOMM System must pass through an ORBCOMM Gateway. An ORB-
COMM Gateway consists of one Gateway Control Center (GCC)—the facility that houses the computer
hardware and software that manages and monitors message traffic—and a GES. The GES provides the link
between the Satellite constellation and an ORBCOMM GCC.
Using the ORBCOMM System, a typical messaging scenario will proceed, as shown in the following se-
quence:
 An ORBCOMM System subscriber creates a message, which is intended for receipt by another sub-
scriber’s home computer. Using an e-mail program on a laptop PC, the subscriber downloads the
message to a SC.
 The SC transmits the message to the Satellite that receives, reformats and relays the message to
a GES.
 The GES transmits the message over a dedicated line to the GCC that places the message on the
public switched network for delivery to the receiver subscriber’s PC Internet provider.
 The receiver subscriber downloads the message once the computer makes a connection via mo-
dem to its Internet service provider.
 A message from the home base to the subscriber follows the reverse route: PC to the Internet
over a public switched network to the GCC, GCC to GES, GES to Satellite, and finally Satellite to
SC and SC to the user display. Even “direct” subscriber-to-subscriber transmissions must pass
through an ORBCOMM Gateway.
The software is able to decode the continuous 4800 bps stream of packet data coming from a satellite. Not
all messages could be shown as text, but all messages are available in hexadecimal notation.
PACKET-1200
PACKET-1200 can be monitored in the amateur VHF and UHF bands. This mode uses indirect FM (sub car-
rier) modulation.
Parameter Value
Frequency range VHF/UHF
Operation modes Duplex FEC TDMA
Modulation FM, SUB FSK
Symbol rate 600 and 1200 Bd
Center frequency 1700 Hz
Shift 1000 Hz
Receiver settings FM BW = 12 kHz
Input format(s) AF, IF
Additional Info ITA-5 with block coding, amateur AX.25 System
To start PACKET-1200, select 1200 Bd or 600 Bd. The correct shift or center may be set in the Demodula-
tor menu. Standard values for the center frequency and the shift are 1700 Hz and 1000 Hz.
The packet radio protocol is a derivative of the X.25 and HDLC computer network protocols. Packet radio
is a synchronous system in which data is transmitted in ASCII character packets.
At the start and end of each block a control character or flag (01111110) is sent. The address can consist
of up to 80 characters, but 16 or 24 character addressing is common when using direct connections or a
single repeater.
The packet protocol distinguishes between three frame types: I (Information), S (Supervisory) and U (Un-
numbered). Payload data transfer uses I frames or very rarely U frames. S and U frames are used for
transmission control.