Page 124 - DCAP104_EXPOSURE_TO_COMPUTER_DISCPLINES

Unit 6: Data Communication

33.6k Modem (3429 baud) (V.34) 33.6 Notes
56k Modem (8000/3429 baud) (V.90) 56.0/33.6

56k Modem (8000/8000 baud) (V.92) 56.0/48.0
Bonding modem (two 56k modems)) (V.92) 112.0/96.0

Hardware compression (variable) (V.90/V.42bis) 56.0-220.0
Hardware compression (variable) (V.92/V.44) 56.0-320.0

Server-side web compression (variable) (Netscape ISP) 100.0-1,000.0
6.3.2 Radio Modems

Direct broadcast satellite, WiFi, and mobile phones all use modems to communicate, as do most
other wireless services today. Modern telecommunications and data networks also make extensive
use of radio modems where long distance data links are required. Such systems are an important
part of the PSTN, and are also in common use for high-speed computer network links to outlying
areas where fibre is not economical.

Even where a cable is installed, it is often possible to get better performance or make other parts of
the system simpler by using radio frequencies and modulation techniques through a cable. Coaxial
cable has a very large bandwidth, however signal attenuation becomes a major problem at high
data rates if a digital signal is used. By using a modem, a much larger amount of digital data can
be transmitted through a single piece of wire. Digital cable television and cable Internet services
use radio frequency modems to provide the increasing bandwidth needs of modern households.
Using a modem also allows for frequency-division multiple access to be used, making full-duplex
digital communication with many users possible using a single wire.
Wireless modems come in a variety of types, bandwidths, and speeds. Wireless modems are often
referred to as transparent or smart. They transmit information that is modulated onto a carrier
frequency to allow many simultaneous wireless communication links to work simultaneously
on different frequencies.

6.3.2.1 WiFi and WiMax

Wireless data modems are used in the WiFi and WiMax standards, operating at microwave
frequencies.
WiFi is principally used in laptops for Internet connections (wireless access point) and wireless
application protocol (WAP).
6.3.3 Mobile Modems and Routers
Modems which use a mobile telephone system (GPRS, UMTS, HSPA, EVDO, WiMax, etc.),
are known as wireless modems (sometimes also called cellular modems). Wireless modems
can be embedded inside a laptop or appliance or external to it. External wireless modems are
connect cards, usb modems for mobile broadband and cellular routers. A connect card is a PC
card or ExpressCard which slides into a PCMCIA/PC card/ExpressCard slot on a computer.
USB wireless modems use a USB port on the laptop instead of a PC card or ExpressCard slot.
A cellular router may have an external datacard (AirCard) that slides into it. Most cellular
routers do allow such datacards or USB modems. Cellular Routers may not be modems per
se, but they contain modems or allow modems to be slid into them. The difference between
a cellular router and a wireless modem is that a cellular router normally allows multiple

LOVELY PROFESSIONAL UNIVERSITY 117