WAN Characteristics 519
Figure 10-4 illustrates some of the common data link encapsulations associated with
synchronous serial lines.
Figure 10-4 Data Link Encapsulations
Figure 10-5 illustrates the synchronous serial communications model where services
offered to the router are made available through a modem or a CSU/DSU.
Figure 10-5 CSU/DSU
WAN Connection Options
You can use different WAN connection options to interconnect LANs. The following
subsections give a brief description of the most common WAN connection services:

■
Circuit-switched services

■ Packet-switched services

■
Cell-switched services

■
Dedicated digital services

■ Dialup, cable, and wireless services
Figure 10-6 depicts the different WAN connection services.
HDLC
PPP
Frame Relay
ISDN

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520 Chapter 10: WANs and Routers

Figure 10-6 WAN Connection Services
Circuit-Switched Services
Circuit switching is a WAN switching method in which a dedicated physical circuit
through a carrier network is established, maintained, and terminated for each communi-
cation session. ISDN is an example of a circuit-switched WAN technology. Figure 10-7
illustrates a network topology that supports circuit switching.
Figure 10-7 Circuit Switching
Table 10-3 describes the characteristics of two most common circuit-switched
services—plain old telephone service (POTS) and narrowband Integrated Services
Digital Network (ISDN).

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WAN Characteristics 521
POTS and ISDN are dialup services, which means that when the call is made, an end-
to-end physical path is set up and the bandwidth is reserved end to end. Both POTS
and ISDN use straight time-division multiplexing (TDM), which is sometimes referred
to as synchronous transfer mode (STM).
Packet-Switched Services
Packet-switched services route small units of data called packets through a network
based on the destination address contained within each packet. Figure 10-8 shows an
example network topology that supports packet-switched services.
Figure 10-8 Packet Switching
Table 10-3 Circuit-Switched Services
Circuit-Switched Service Characteristics
POTS Not a computer data service, but included because
many of its technologies are part of the growing data
infrastructure, and it is an incredibly reliable, easy-to-
use, wide-area communications network.
Typical medium is twisted-pair copper wire.
Narrowband ISDN A versatile, widespread, historically important

technology.
Was the first all-digital dialup service.
Cost is moderate.
Maximum bandwidth is 128 kbps for the lower cost
Basic Rate Interface (BRI) and about 3 Mbps for the
Primary Rate Interface (PRI).
Usage is fairly widespread, though it varies considerably
from country to country.
Typical medium is twisted-pair copper wire.
CSU/DSU
CSU/DSU
CSU/DSU
VC
Synchronous
Serial
Synchronous
Serial

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522 Chapter 10: WANs and Routers
Table 10-4 describes the characteristics of two most common packet-switched
services—X.25 and Frame Relay.
X.25 is both connection-oriented and reliable at both Layers 2 and 3, which is a big
part of why it is so much slower than Frame Relay.
Frame Relay is typically regarded as a much faster, sleeker version of X.25; it has no
defined Layer 3 protocol, nor is it reliable. However, it is connection-oriented.
Circuit-switched services use time-division multiplexing (TDM) and are said to be syn-
chronous (they use STM), whereas packet-switched services use statistical TDM and
are sometimes said to be asynchronous (similar to ATM).
Table 10-4 Packet-Switched Services

Packet-Switched
Service Characteristics
X.25 An older technology, but still widely used.
Has extensive error-checking capabilities from the days when
WAN links were more prone to errors. This makes it reliable
but limits its bandwidth.
Bandwidth can be as high as 2 Mbps.
Usage is fairly extensive.
Cost is moderate.
Typical medium is twisted-pair copper wire.
Frame Relay A packet-switched version of narrowband ISDN.
Has become an extremely popular WAN technology in its own
right.
More efficient than X.25, but with similar services.
Maximum bandwidth is 44.736 Mbps.
56 kbps and 384 kbps are extremely popular in the U.S.
Usage is widespread.
Cost is moderate to low.
Typical media include twisted-pair copper wire and optical
fiber.
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WAN Characteristics 523
Cell-Switched Services
Cell-switched services provide a dedicated-connection switching technology that
organizes digital data into cell units and transmits them over a physical medium using
digital signal technology.
Table 10-5 describes the characteristics of two most common cell-switched services—
Asynchronous Transfer Mode (ATM) and Switched Multimegabit Data Service (SMDS).
Dedicated Digital Services
Dedicated digital services also provide circuit-switched services but the connection is

“always-up.”
Table 10-6 describes the characteristics of the most common dedicated digital services—
T1, T3, E1, E3; digital subscriber line (xDSL); and Synchronous Optical Network
(SONET).
Table 10-5 Cell-Switched Services
Cell-Switched Service Characteristics
ATM Closely related to broadband ISDN.
Is becoming an increasingly important WAN
technology.
Uses small, fixed 53-byte length cells to carry data.
Maximum bandwidth is currently 622 Mbps, though
higher speeds are being developed.
Typical media are twisted-pair copper wire and
optical fiber.
Usage is widespread and increasing.
Cost is high.
SMDS Closely related to ATM, and typically used in
metropolitan-area networks (MANs).
Maximum bandwidth is 44.736 Mbps.
Typical media are twisted-pair copper wire and
optical fiber.
Usage is not very widespread.
Cost is relatively high.
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524 Chapter 10: WANs and Routers
Table 10-6 Dedicated Digital Services
Dedicated Digital
Service Characteristics
T1, T3, E1, E3 The T series of services in the U.S. and the E series of services in
Europe are extremely important WAN technologies.

They use TDM to slice up and assign time slots for data
transmission.
Bandwidths for the T and E series lines are 1.544 Mbps for T1,
44.736 Mbps for T3, 2.048 Mbps for E1, and 34.368 Mbps
for E3. Other bandwidths are also available.
The media used are typically twisted-pair copper wire and opti-
cal fiber.
Usage is extremely widespread.
Cost is moderate.
xDSL A new and developing family of WAN technologies intended
primarily for home use.
xDSL indicates the entire family of DSL technologies, including
high-bit-rate DSL (HDSL), single-line DSL (SDSL), asymmetric
DSL (ADSL), and very-high-data-rate DSL (VDSL).
Bandwidth decreases with increasing distance from the phone
company’s equipment.
Top speeds of 51.84 Mbps are possible near a phone company
office; however, most bandwidths are much lower from hun-
dreds of kbps to several Mbps.
Cost is moderate and decreasing.
SONET A family of very high-speed physical layer technologies.
Designed for optical fiber, but can also run on copper cables.
Has a series of data rates available with special designations.
Implemented at different Optical Carrier (OC) levels, ranging
from 51.84 Mbps (OC-1) to 9,952 Mbps (OC-192).
Can achieve high data rates by using wavelength division multi-
plexing (WDM). WDM is when lasers are tuned to slightly
different colors, or wavelengths, to send huge amounts of data
optically.
Usage is widespread among Internet backbone entities.

The cost is expensive. This is not a technology that connects to
homes.
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WAN Characteristics 525
Dialup, Cable, and Wireless Services
Table 10-7 describes the characteristics of other WAN services that do not fall into any
of the previously covered categories of WAN technologies. Among these miscellaneous
technologies are dialup modems, cable modems, and terrestrial and satellite wireless.
Figure 10-9 illustrates different WAN technologies connected by routers.
Table 10-7 Dialup, Cable, and Wireless Services
WAN Service Characteristics
Dialup modem
(switched analog)
Limited in speed, but quite versatile.
Works with existing phone network.
Maximum bandwidth approximately 56 kbps.
Cost is low.
Usage is still very widespread.
Typical medium is the twisted-pair phone line.
Cable modem
(shared analog)
Puts data signals on the same cable as television signals.
Increasing in popularity in regions that have large amounts of
existing cable TV coaxial cable. Ninety percent of homes in
United States have existing cable TV coaxial cable.
Maximum bandwidth can be 10 Mbps, though this decreases
as more users attach to a given network segment. Behaves like
an unswitched (shared) LAN.
Cost is relatively low.
The medium is coaxial cable.

Wireless Wireless requires no medium since the signals are electromag-
netic waves that are transmitted through the air. A variety of
wireless WAN links exist, including the following:
Terrestrial has bandwidths typically in the 11 Mbps range (for
example, microwave). Cost is relatively low, line of sight (LOS)
is usually required, and usage is moderate.
Satellite can serve mobile users such as those in a cellular tele-
phone network and remote users that are too far from any
wires or cables. Usage is widespread, and the cost is high.
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526 Chapter 10: WANs and Routers
Figure 10-9 WAN Technologies Connected by Routers
Comparison of WAN Technologies
Table 10-8 shows an overview of the previously discussed WAN technologies.
Table 10-8 WAN Technologies
WAN Acronym WAN Name
Maximum
Bandwidth Comments
POTS plain old telephone
service
4 kHz analog Standard for
reliability
ISDN Integrated Services
Digital Network
128 kbps Data and voice
together
X.25 X.25 2 Mbps Old reliable
workhorse
Frame Relay Frame Relay Up to 44.736 Mbps New workhorse
ATM Asynchronous

Transfer Mode
622 Mbps High-powered
networks
SMDS Switched Multi-
megabit Data
Service
1.544 Mbps and
44.736 Mbps
MAN variant of
ATM
T1, T3 T1, T3 1.544 Mbps and
44.736 Mbps
Widely used tele-
communications
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WAN Characteristics 527
xDSL digital subscriber
line
384 kbps Technology over
phone lines
SONET Synchronous Opti-
cal Network
9,992 Mbps Fast optical fiber
transmission
Dialup modem modem 56 kbps Mature technology
using phone lines
Cable modem cable modem 10 Mbps Technology using
cable TV
Terrestrial
wireless

wireless 11 Mbps Microwave and
laser links
Satellite wireless wireless 2 Mbps Microwave and
laser links
More Information: Costs of the WAN
Tempering the various performance criteria of the WAN is cost. The costs of owning and oper-
ating a WAN include the initial startup costs, as well as the monthly recurring expenses. Not
surprisingly, the larger and more powerful network components are much more expensive
than smaller, less-robust components. Therefore, designing a WAN becomes an economic
exercise in which a careful balance of performance and cost is achieved.
Achieving this balance can be painful. No one wants to design a WAN that disappoints the
users with its performance, but no one wants to design a WAN that blows the budget, either!
Fortunately, the following suggestions can help guide the design of a WAN that satisfies exist-
ing requirements, provides flexibility for future growth, and doesn’t exceed the budget:
■ The capital investments in routers and other network hardware become a fixed part of the
network. After the hardware components are placed in operation, the logistics of replacing
hardware become quite complicated. And, depending on your depreciation schedule for
capital equipment, you might find yourself obligated to use the hardware for five or more
years! It might behoove you to purchase a larger router that is relatively low in port den-
sity. You can add hardware (memory, CPUs, and interfaces) in the future as the need for
them arises. This method allows future expansion at modest incremental costs and little (if
any) operational downtime.
■ The transmission facilities are relatively easy to replace with other transmission facilities.
They are an expense item, not a capital investment, so there is no depreciation expense to
retire. These can be replaced with other facilities as often as your lease agreement with
the carrier permits. Therefore, you might want to explore your options for meeting perfor-
mance requirements with the various available transmission facilities and technologies.
Table 10-8 WAN Technologies (Continued)
WAN Acronym WAN Name
Maximum

Bandwidth Comments
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528 Chapter 10: WANs and Routers
More Information: Resource Utilization Rates
The degree to which the various physical resources of the WAN are being utilized is also a good
indicator of how well, or how poorly, the WAN is performing relative to the performance
requirements. Two main categories of resource utilization rates should be monitored carefully:
■ Router CPU and memory utilization rates
■ Transmission facility utilization rates
Router Physical Resource Rates
Routers are one of the most vital components of any WAN. And, unlike the transmission facili-
ties, they are outside the view of the telecommunications carrier. Therefore, they are distinctly
the responsibility of the customer. Fortunately, routers are intelligent devices that contain their
own CPU and memory. These physical resources are indispensable in the calculation of WAN
routes and the forwarding of packets. They can also be used to monitor the performance of the
router.
If either CPU or memory utilization rates approach 100 percent, performance suffers. Numer-
ous conditions can result in either utilization rate temporarily spiking upward with subsequent
performance degradation. One example is a sudden increase in transmissions from the LAN to
the WAN. LANs can operate at speeds of up to 1 Gbps, but they usually operate only at 10, 16,
or 100 Mbps. Any of these speeds is a gross mismatch against the typical WAN transmission
facility, which offers a paltry 1.544 Mbps of bandwidth. This mismatch in bandwidth must be
buffered by the router’s memory. It won’t take long for a router to become resource constricted
given a sustained period of heavy LAN transmissions.
If such situations are rarely experienced, they should be considered aberrations. Aberrations
should be monitored, but they shouldn’t drive physical upgrades. If these resource constrictions
recur or constitute a trend, however, something needs to be done. Usually this requires an
upgrade, either to the next larger router or via an expansion of memory. If a router is chronically
at or near 100 percent of capacity with its memory, it is time to purchase additional memory.
However, responding to chronically high CPU utilization rates might not be as simple as a

memory upgrade. Really, only three options exist for improving high CPU utilization rates:
■ If possible, add another CPU to the router.
■ Upgrade to a more powerful router.
■ Investigate the WAN’s traffic patterns to see if the load on the problematic router can be
reduced.
Manipulating traffic patterns is really only a viable option in larger WANs with complex topolo-
gies that can afford route redundancy.
Transmission Facility Rates
Transmission facilities can also be monitored for utilization. Typically, this utilization rate is
expressed in terms of the percentage of consumed bandwidth. If you are using a T1, for exam-
ple, a given sample might indicate that 30 percent of its 1.544 Mbps of available bandwidth is
currently being utilized.
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