Description
PSM-500/500L/500LT SCPC Satellite Modem
PSM-500/500L/500LT - Rev. 0.91
Page 1-4
Maximum data rates are dependent on many factors besides the basic capabilities of the interface,
including cabling, interface type, features and options installed. The TPC4K hardware codec is
limited to 5Mbps.
1.1.3 Applications
Following are just a few representative forms of satellite communications links and networks in
which the PSM-500 modem series may be used.
1.1.3.1 SCPC Point-to-Point Links
The most straightforward application for a satellite modem is to serve as the Data Communications
Equipment (DCE) for a point-to-point data link. When used in this mode, two modems located at two
different sites are tuned to complementary transmit and receive frequencies. Each direction of the
communications link may have the same or entirely different transmission parameters. In this
application it is typical that the link is established and maintained on a continuous basis, although a
special “on demand” case is described later.
In SCPC point-to-point links the power required from the satellite or the size of the receive antenna
is dependent upon the modem receive performance. The PSM-500 modem uses the most rigorous
methods to maintain performance as close to the theoretical “waterfall” curves as possible. In most
cases the modem will perform at 0.1 to 0.2 dB from the curve (although we say “typically” 0.3 dB).
This consistent performance, plus advanced technology such as TPC results in the absolute
minimum power requirements, which equates to the minimum operating.
Ku Band satellite systems are subject to changing performance due to rain at one or more sites. The
PSM-500 contains built in software to perform Automatic Uplink Power Control (AUPC). If the
modems at each link end are provided with an external asynchronous channel of 300 bps they can
be set to automatically maintain a constant Eb/No within programmable limits. This can result in
significantly lower satellite power requirements in a large system in addition to maintaining proper
performance in any system. The optional Multiplexer/interface card can provide this low rate channel
in addition to an Earth Station to Earth Station overhead service channel.
1.1.3.2 SCPC Point to Multi–Point Links in a Broadcast Application
A broadcast application might involve the necessity of sending continuous or intermittent data from
one source and
“broadcasting” the
information to many
remote locations. For
instance, constant
pricing information and
updates may be sent by
a central location to
many store locations.
There may be minor
return information from
the remotes
acknowledging receipt.
Another broadcast
application could be
transmitting background
music from a central
location to many store
sites. In this case there
would be no return path.
The topology of the
network in both of these broadcast examples would typically be called a “Star” network. As shown in
Figure 1-1 Simple Star Network
Hub Station
Remote A
Remote B
Remote C
Remote D
Satellite
