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A satellite modem or satmodem is a modem used to establish
data transfers using a communications satellite as a relay. There
is a wide range of satellite modems from cheap devices for
home internet access to expensive multifunctional equipment
for enterprise use.

A "modem" stands for "modulator-demodulator". A satellite
modem's main function is to transform an input bitstream to a
radio signal and vice versa. There are some devices that include
only a demodulator (and doesn't comprise any modulator) that
are also referred to as "satellite modems". These devices are
used in satellite Internet access (in this case data are
transferred in other direction through a conventional PSTN
modem or an ADSL modem). The main functions of a satellite
modem are modulation and demodulation. Satellite
communication standards also define error correction codes
and framing formats.

There are two main issues that impact the desirability of
satellite modems as a means of Internetaccess:
1. Bandwidth available through the satellite is shared among
a large number of users. While each transponder supports from
6 to 38 Mbps of data transfer capacity, the DirecPC service only
guarantees 144 Kbps to each user.
2. Satellite modems are only capable of receiving data.
Consequently, the subscriber mustuse a standard phone line
for return data. This arrangement considerably complicates the
network architecture, and provides only a comparatively slow
upstream connection to the Internet information provider.

The functional blocks that make up a satellite modem include: A
satellite interface containing the satellite-specific link functions;
A CPU complex consisting of the CPU plus RAM and ROM,
responsible for configuring and managing the system; A host
interface used to connect a modem to the host computer (or to
a local area network if the modem includes router functionality).

Each one of these blocks is typically implemented by a small
number of ASSPs. In most cases there are mismatches between
the ASSPs used to implement each of these blocks. The system
levelglue needed to interface these blocks while delivering a
product early to market is a key benefitof Xilinx’s high-volume
FPGA and CPLD products.

To understand how a modem does what it does, you should
understand how satellite internet works. A satellite internet
modem connects your computer to a Network Operations
Center (NOC). The NOC is your gateway to the Web. When
your web browser requests a web page, the request is sent
up to a satellite 22,3000 miles above the Equator. The satellite
retransmits the request down to the NOC. The NOC uses high
speed internet connections to contact the web server. The
server sends the requested data to the NOC, where the NOC
sends the data to the satellite and down to your satellite
modem. These steps do not vary much from any other Internet
Service Provider (ISOP). If you use a traditional dial up phone
modem to connect the internet, your modem dials to your ISP's
modem and they serve the same function as the NOC. With a
NOC, the connection is made via satellite as opposed to a
phone line.

There are some critical differences between dial up, DSL, or a
cable modem. A satellite signal traveling 22,300 miles up and
down and then back up and back down takes about 480
milliseconds. This is called signal latency. This same effect can
be seen when you talk to someone on a phone call which uses
satellites. Most phone calls today use fiber optics thus greatly
reducing this lag effect.

When you add up the satellite signal latency to the normal
signal latency between the Net NOC and the Web, you will
have an average overall latency (in internet terms this called
ping times) of at least 600ms and common ping times up to
800ms. This compares to 100ms to 250ms for other, non-
satellite, broadband methods. As long as a user understands
that satellite internet will appear to have slower page loads
then other broadband options of the same download speeds,
most users will accept this as normal. Upload speeds are
affected by location and weather. Just like a flashlight beam
drops intensity as you look from the center of the beam to
outer edges, a satellite signal becomes weaker as you move
to the fringes of the beam. Yet the efficiency of a satellite to
receive signals from your satellite dish also varies from region
to region.

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