Page 126 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 6: Data Communication

Cable modems use a range of frequencies originally intended to carry RF television channels. Notes
Multiple cable modems attached to a single cable can use the same frequency band, using a low-
level media access protocol to allow them to work together within the same channel. Typically,
‘up’ and ‘down’ signals are kept separate using frequency division multiple access.

Figure 6.10: DSL Modem

New types of broadband modems are beginning to appear, such as doubleway satellite and
power line modems.
Broadband modems should still be classed as modems, since they use complex waveforms to
carry digital data. They are more advanced devices than traditional dial-up modems as they are
capable of modulating/demodulating hundreds of channels simultaneously.
Many broadband modems include the functions of a router (with Ethernet and WiFi ports) and
other features such as DHCP, NAT and firewall features.
When broadband technology was introduced, networking and routers were unfamiliar to
consumers. However, many people knew what a modem was as most internet access was through
dial-up. Due to this familiarity, companies started selling broadband modems using the familiar
term modem rather than vaguer ones like adapter or transceiver, or even “bridge”.
Many broadband modems must be configured in bridge mode before they can use a router.

6.3.5 Home Networking
Although the name modem is seldom used in this case, modems are also used for high-speed
home networking applications, specially those using existing home wiring. One example is
the G.hn standard, developed by ITU-T, which provides a high-speed (up to 1 Gbit/s) Local
area network using existing home wiring (power lines, phone lines and coaxial cables). G.hn
devices use orthogonal frequency-division multiplexing (OFDM) to modulate a digital signal
for transmission over the wire.

The phrase “Null modem” was used to describe attaching a specially wired cable between the
serial ports of two personal computers. Basically, the transmit output of one computer was wired to
the receive input of the other; this was true for both computers. The same software used with
modems (such as Procomm or Minicom) could be used with the null modem connection.

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