The Internet Encyclopedia (Volume 3)

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FACILITIES ANDINFRASTRUCTURE 779

a) Lost link

b) Lost node

X

X

Figure 3: SONET ring survivability, a) lost link

and b) lost node.

by looping traffic back on each side of the break, and Fig-

ure 3b shows how loss of a network node can be handled

similarly. SONET has been deployed extensively by ser-

vice providers in metropolitan areas to create highly re-

liable and scalable transport capabilities. Once the fiber

and switching equipment are in place, transport capac-

ity can be increased by installing higher-speed signaling

interfaces.

Another approach to increasing the capacity of fiber

systems has become available with advances in optical

component technology. Rather than using the entire range

of wavelengths that can be carried over fiber as a sin-

gle transmission channel, newer equipment allows us to

divide the range into multiple channels for simultane-

ous transmission using wavelength-division multiplexing

(WDM). This is quite similar to sending multiple television

channels over a coaxial cable. Channels must be spaced

far enough apart to limit the interference between adja-

cent signals that would degrade signal quality. Incoarse

WDM (CWDM) the channels are widely spaced; fordense

WDM (DWDM), they are very close together (spacing≤

25–50 GHz). By combining WDM and high-speed sig-

naling, transmission capacities of OC-192, OC-768, and

greater become possible, limited primarily by the quality

of existing fiber installations.

Access Technologies

In order to get traffic in and out of a MAN or WAN, sub-

scribers must have physical connections, oraccess,tothe

appropriate service provider’s network resources. In the

regulated telecommunications environment of the United

States, this typically means acquiring connectivity from a

LEC to tie the subscriber’s physical premises to a WAN

service provider’s (i.e., IXC’s) equipment as physically lo-

cated in a point of presence (POP). In a metropolitan

area, a single company may be allowed to provide both

local exchange connections and MAN services. The pri-

mary means of accessing MAN and WAN service provider

networks are described below.

Dial-up

Dial-up access is appropriate for occasional connections

of limited duration, as for making a telephone call. Where

the physical facilities used for dial-up were designed and

installed to support analog voice traffic, two characteris-

tics are particularly important for data networking:

Digital data must be converted to analog using a modem

at the subscriber end and reconverted to digital by a

modem at the provider end of the connection.

Data rates are limited by the analog frequency range ac-

cepted at provider receiving equipment and by the sig-

nal modulation techniques of the modem devices. The

most widely accepted standards today support maxi-

mum data rates of 56 Kbps.

Leased Line

Leased-line access is more appropriate for connections

that need to be continuous and/or of better quality for

higher-speed transmission. Such facilities are dedicated

to the use of a specific subscriber. For example, a business

may lease a T1 access line as its basic unit of connection

capacity (1.544 Mbps), particularly for access to Internet

service providers. Fractional-T1 and multiple-T1 lines are

also available in some areas. A newer technology designed

to be digital from end to end over copper cabling, called

digital subscriber line (DSL), is beginning to be offered

as a lower-cost alternative to the traditional T-carrier.

Leased-line access requires matching equipment at each

end of the line (subscriber and service provider) to ensure

transmission quality suitable to the desired data rates.

Wireless

Wireless access is growing in popularity among mobile

individuals who do not work from a fixed desktop in a sin-

gle building location (e.g., salespeople, customer service

representatives, and travelers). Rather than having to find

a suitable “land-line” telephone connection with an ana-

log data port to connect the modem, wireless users have

either wireless network interface cards or data interface

cables that connect their modems to cellular telephones.

Both approaches require proximity to a wireless receiving

station of matching technology that is then connected to

the wired resources making up the remainder of the MAN

or WAN.

Cable Modem

Cable modem access is provided by cable television com-

panies who have expanded their business into data net-

working. A modem designed to transmit data signals over

coaxial, broadband television cable is connected, usually

via Ethernet technology, to the subscriber’s internal net-

work or computer equipment. In residential applications,