The Internet Encyclopedia (Volume 3)

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170 PUBLICNETWORKS

Table 1Common Network Technologies

Service Usage Comments Transmission Speed Download

Voice-Grade Modem Modems are inexpensive, telephone

rates reasonable for modest

connect times

Upload: Up to 33.6 Kbps

Download: Up to 56 Kbps

3 min or more

Digital Subscriber Line More expensive than voice-grade

modems, downlink rates higher

than uplink

Upload: From 16 Kbps to 640 Kbps

Download: From 768 Kbps to

9 Mbps

1.1–13 s

Cable Modems Download rates depend on the

number of simultaneous

customers and configuration

Upload: From 64 Kbps to 256 Kbps

Download: From 10 Mbps to

30 Mbps

0.3–1 s

Satellite A cost-effective choice in remote

locations

Upload: From 56 Kbps to 256 Kbps

Download: From 150 Kbps to

1 Mbps

10–67 s

Integrated Services

Digital Network

Charges generally based on

duration of call

Basic rate: 128 Kbps, higher rates

available

1.3 min

Digital leased lines:

56 Kbps (DS0), T1

(DS1), T3 (DS3),...

Most common leased line for

high-traffic voice and data; fixed

price for a specific capacity

DS0: 56 Kbps T1, DS1: 1.54 Mbps

T3, DS3: 44.7 Mbps

56 Kbps: 3 min

T1: 6.5 s

T3: 0.22 s

SONET Specification for optical links,

highest speed

From 155.52 Mbps to 2.488 Gbps

leased

0.004–0.06 s

X.25 Older technology, still in use in

public networks

56 Kbps, but can be slower or faster 3 min or more

Frame Relay Fixed price per month for a specific

capacity, widely installed and

used

From 16 Kbps to 44.736 Mbps 0.22–625 s

ATM Universal technology for wide area

networking

From 1.544 Mbps to 2.5 Gbps for

access

0.004–6.5 s

The telephone company does not have to install any

special equipment to use voice-grade modems. However,

when the telephone company offers DSL service it has to

install digital subscriber line access multiplexers at the

end offices. Figure 3 illustrates the equipment used for

DSL (Panko, 2001). Because special equipment has to be

installed, DSL service is not available in all areas. One fac-

tor that determines the availability of ADSL is the distance

to the central office. In general, if the distance is greater

than 18,000 feet ADSL service is not available. Also, the

prices are fluctuating as DSL becomes available in more

and more areas.

Cable Modems

Cable modems are a service offered by cable televi-

sion companies. Often, the cable television or telephone

All digital

internally

Single twisted pair,

ordinary telephone

line

DSL Modem

DSU for computer

Codec for telephone

Telephone Company

Service Provider

Digital leased line,

Megabit speeds

DSL Access

Multiplexer

Figure 3: Asymmetric digital subscriber line. Source:Buisness Data Communications and Net-

working, 3/E(Panko, 2001). Reprinted by permission of Pearson Education Inc., Upper Saddle

River, NJ.