Page 286 - Anatomy of a Robot

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COMMUNICATIONS 271
WIRED SYSTEMS
A few wired communication systems are widespread at this time. They can all support
higher-level protocols, so we’ll start with just the physical layers.
Phone Network
The common carrier phone systems can be used to transmit data, which can be done in
a couple of different ways:

Dialed services Several companies (like AOL) have phone numbers that a
robotic computer could dial up to access the Internet. Modems are required to
support connections to the phone line. These modems have a top end of 33 to 56
Kbps. The top speed will usually depend on the quality of the phone connection.
Service can be denied if the line is busy. (See www.driverzone.com/guides/
modem/intro/modemguide_p3.html and www.v90.com/v90magic.htm.)
Digital subscriber line (DSL) Given that one phone wire already comes into the
house, the phone companies use frequency division to put DSL signals on the
wire. Voice traffic only uses frequencies below 10 kHz (at most). DSL signals typ-
ically use the same wire to carry QAM signals at bit rates around 1 Mbps. The
service is continuous and largely based on the Internet. Service may be interrupted
if the robot does not exercise the communications link now and then. The phone
company takes away the robot’s IP address if the robot goes idle for too long. Just
make sure the robot is active now and then so it keeps its IP (DHCP) reservation
intact. More info can be found at www.howstuffworks.com/dsl.htm and at
www.dslforum.org/.

Cable Networks
Many homes also have cable TV coming into the house. Although the cable system was
originally designed as a one-way system, many of the cable systems now have reverse
channels capable of taking information from the homes back to the cable company.
The standard that most cable TV companies use is the Data Over Cable Structured
Interface Standard (DOCSIS), which provides time division access to home sub-
scribers. The cable system is a closed system so the physical layer can be proprietary.
The modulation methods used are Quadrature Phase Shift Keying (QPSK), 16 QAM
(upstream), and 64 QAM (downstream), backed up with RS coding, as discussed
earlier.