78 Part I • Information Technology
also available, in which the organization pays a monthly
fee for (typically) unlimited long-distance telephone serv-
ice using the ordinary voice circuits. WATS has the same
advantages and disadvantages as DDD. However, the cost
per hour of WATS is somewhat less than DDD, but the
customer pays for it whether it is used or not, while DDD
is paid for only when it is utilized. DDD is appropriate for
intermittent, limited-volume data transmission at relatively
slow speeds, while WATS is used for more nearly continu-
ous, somewhat larger volumes of data to be transmitted at
relatively slow speeds.
Leased Lines. Another, sometimes attractive,
alternative is to lease dedicated communications lines from
AT&T or another carrier. If a manufacturing company has
three plants geographically separated from corporate head-
quarters (where the mainframe computer or large servers
are located), it might make sense to lease lines to connect
each of the three plants to headquarters. These leased lines
are generally coaxial cables, microwave, or fiber-optic
cables of very high capacity, and they are less prone to data
errors than ordinary voice lines. The leased lines are
expensive, ranging from hundreds of dollars per month for
distances of a few miles up to tens of thousands of dollars
per month for cross-country lines.
The most common leased lines operate at a data
transmission rate of 1.544 mbps and are referred to as T- 1
lines. In order to effectively use this high data transmission
rate, organizations must employ multiplexers at each end
of a T-1 line to combine (or separate) a number of data
streams that are, individually, much less than 1.544 mbps.
Leased lines with capacities higher than T-1 are also
available. Four T-1 lines are combined to create a T-2 trunk,
with a capacity of 6.312 mbps, but T-2 trunks have largely
been bypassed in favor of T-3 trunks (consisting of seven
T-2s), with a data transmission capacity of nearly 45 mbps.
T-3 links are available between major cities, although the
costs are much higher than for T-1 lines. T-4 trunks also exist
(made up of six T-3s), with a huge capacity of 274 mbps.
The newest and highest capacity leased lines (and
also the most expensive) are fiber-optic transmission
lines, or SONET lines. Synchronous Optical Network
(SONET)is an American National Standards Institute
(ANSI)–approved standard for connecting fiber-optic
transmission systems. Data transmission rates for SONET
lines are shown in Table 3.3. Note that the slowest
SONET transmission rate (OC-1) of nearly 52 mbps is
faster than the T-3 rate of 45 mbps.
Satellite. Satellite microwave communication has
been used by a number of large organizations to set up
their WANs. The satellite or satellites involved are owned
by companies such as Hughes Communications, Intelsat
(Bermuda), Telesat (Canada), and SES (Luxembourg), and
the user organization leases a portion of the satellite’s
capacity. The user organization either provides its own
ground stations or leases time on a carrier’s ground
stations, as well as communication lines to and from those
ground stations. The use of Ku-band transmission with
relatively inexpensive VSAT—very small aperture termi-
nal (i.e., a small satellite dish)—ground stations made
satellite transmission very popular for organizations with
many remote locations. Among the many organizations
using VSAT networks are Kmart, Walmart, Walgreens
Pharmacy, CVS, Ford, and General Motors. The FordStar
VSAT network, for example, connects the thousands of
Ford dealerships with the manufacturer and is used for
transmitting and receiving sales figures and orders as well
as for receiving internal company communications and
service bulletins. The FordStar network is also used to
deliver interactive distance learning courses to Ford dealer-
ship personnel; One Touch Systems developed the one-
way video, two-way audio setup which runs on the
FordStar network, delivering more than 112 courses and
broadcasting over 70 hours of instruction per day (One
Touch Systems, 2010).
ISDN. Another way of implementing a WAN is an
Integrated Services Digital Network (ISDN). ISDN is a
set of international standards by which the public tele-
phone network is offering additional telecommunications
capabilities (including simultaneous transmission of voice
and data over the same line) to telephone users worldwide.
ISDN is available in many areas of the world, using the
same twisted pairs already used in the present telephone
network.
ISDN capabilities are made possible by hardware
and software at the local telephone company office and on
the organization’s premises that divide a single telephoneTABLE 3.3 SONET Circuits
SONET Level Data Transmission Rate
OC-1 51.84 mbps
OC-3 155.52 mbps
OC-12 622.08 mbps
OC-24 1.244 gbps
OC-48 2.488 gbps
OC-96 4.977 gbps
OC-192 9.953 gbps
OC-768 39.812 gbps
OC-3072 159.252 gbps
Key: mbps = million bits per second
gbps = billion bits per second