WAVECOM Decoder W74PC, W-PCI/e, W-CODE, W-CLOUD Manual V9.1.0
Transmission Modes
211
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.
