Business Data
Communications and
Networking, 6th ed.
FitzGerald and Dennis
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Copyright © 1999 John Wiley & Sons, Inc.
All rights reserved. Reproduction or translation of this work
beyond that permitted in Section 117 of the 1976 United
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programs or from the use of the information contained
herein.
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Chapter 9
Metropolitan and
Wide Area Networks
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Objectives of Chapter 9
Become familiar with…
 the common carriers and the nature of
competition,
 how to improve MAN and WAN
performance,
 several factors in selecting MAN and WAN
services.
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Objectives of Chapter 9
Understand ...
 the role of common carriers in
organizational MANs and WANs,
 the four basic categories of MAN and WAN
circuits,
 dialed circuit services,
 dedicated circuit services,
 switched circuit services,
 packet network services.
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INTRODUCTION
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Introduction
Metropolitan area networks (MANs) typically
span from 3 to 30 miles and connect
backbone networks (BNs), and LANs.
Wide area networks (WANs) connect BNs
and MANs across longer distances, often
hundreds of miles or more.
Most organizations cannot afford to build their
own MANs and WANs, so they rent or lease
circuits from common carriers such as
AT&T, MCI, BellSouth, PACTEL or NYNEX.
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The Telephone Network
Most countries have a federal agency that
regulates data and voice communications.
In the United States, this agency is the
Federal Communications Commission
(FCC). Each state or province also has its
own public utilities commission (PUC) to
regulate communications within its borders.
The communications industry in North
America operates as a series of private
companies that are regulated by the
government.
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The Telephone Network
A common carrier is a private company that
sells or leases communications services and
facilities to the public. A common carrier that
also provides local telephone services is
called a local exchange carrier (LEC), while
one that provides long distance services is
called an interexchange carrier (IXC).
In the United States, 90 percent of the
telephone system used to be run by one
common carrier, AT&T.
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DIALED CIRCUIT SERVICES
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Dialed Circuit Services
Dialed circuit services are the simplest and
one of the most common types of MAN and
WAN connections. This type of connection
uses the normal telephone network. To use
dialed circuit services, the user simply lease
connection points into the common carrier’s
network, then dials the host computer using
a modem, and connects to the host system.
Dialed circuit services may use different
circuit paths between the two computers
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each time a number is dialed.
Dialed Circuit Services
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Direct Dialing
Direct dialing (also called dial-up) is the most
commonly used direct circuit service. Every
time you call your Internet service provider
from your home phone, you are using direct
dialing.
Charges for direct dialing are based on the
distance between the two telephones (in
miles) and the number of minutes the
connection is used.
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Wide Area Telephone Service
(WATS)
Wide Area Telephone Service (WATS) are
special rate service that allows direct circuit
calls for both voice and data transmission to
be purchased in large quantities.
WATS is limited to one direction only; it is
either outward dialing or inward dialing. In
general, inward WATS uses the toll free 800
and 888 area code series in North America,
and similar numbers in other countries.
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Wide Area Telephone Service
(WATS)
The United States and Canada are divided
into about 60 different WATS service areas.
The geographical coverage for WATS is
determined by the band of service to which
the customer subscribes.
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DEDICATED CIRCUIT
SERVICES
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Motown Café Network
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Dedicated Circuit Services
There are two main problems with dialed
circuits.
• Each connection goes through the regular
telephone network on a different circuit, which
may vary in quality.
• The data transmission rates on these circuits
are usually low 28.8 to 56 Kbps.
One alternative is to establish a private
dedicated circuit, which the user leases
from the common carrier for their exclusive
use, 24 hrs/day, 7 days/week.
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Dedicated Circuit Services
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Dedicated Circuit Services
Dedicate circuits are billed at a flat fee per
month and the user has unlimited use of the
circuit. Dedicate circuits therefore require
more care in network design than dialed
circuits.
There are five types of dedicated circuits:
•
•
•
•
•
Voice grade circuits
Wideband analog services
T Carrier circuits
SONET circuits
Digital subscriber line circuits
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Voice Grade Circuits
Voice grade circuits are analog circuits that
work in exactly the same manner as
traditional telephone lines, except that you
do not dial them.
Dedicated voice grade channels often have
conditioning (or equalization) done on them
to improve data transmission quality by
reducing noise and distortion.
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Wideband Analog Services
Wideband analog services are similar to voice
grade circuits but they provide much greater
bandwidth.
Typically wideband analog services provide
one 48,000 hertz bandwidth channel for use
with frequency division multiplexing or as 12
individual voice grade channels (4000 Hz
each).
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T Carrier Circuits
T Carrier circuits are dedicated digital circuits
and are the most commonly used form of
dedicated circuit services in North America
today.
Instead of a modem, a channel service unit
(CSU) or data service unit (DSU) are used
to connect the circuit into the network.
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T Carrier Circuits
A T-1 circuit (a.k.a. a DS-1 circuit) provides a data rate
of 1.544 Mbps. T-1’s allow 24 simultaneous 64
Kbps channels (with TDM) which transport data, or
voice messages using pulse code modulation.
A T-2 circuit (6.312 Mbps) is basically a multiplexed
bundle of four T-1 circuits.
A T-3 circuit (44.376 Mbps) is equal to the capacity of
28 T-1 circuits.
A T-4 circuit (274.176 Mbps) is equal to the capacity of
178 T-1s.
Fractional T-1, (FT-1) offers portions of a 1.544 Mbps
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T-1 for a fraction of its full costs.
T Carrier System
T-Carrier Designation
DS Designation
Speed
DS-0
64 Kbps
T-1
DS-1
1.544 Mbps
T-2
DS-2
6.312 Mbps
T-3
DS-3
33.375 Mbps
T-4
DS-4
274.176 Mbps
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Synchronous Optical Network
(SONET)
The synchronous optical network (SONET)
has recently been accepted by the U.S.
standards agency (ANSI) as a standard for
optical (fiber) transmission at gigabits per
second speed.
The international telecommunications
standards agency (ITU-T) also recently
standardized a version of SONET under the
name of synchronous digital hierarchy
(SDH). The two are very similar and can be
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easily interconnected.
Synchronous Optical Network
(SONET)
SONET transmission speeds begin at the
OC-1 level (optical carrier level 1) of 51.84
Mbps. Each succeeding rate in the SONET
fiber hierarchy is defined as a multiple of
OC-1.
Several common carriers (e.g. MCI) now use
OC-12 circuits at 622.08 Mbps to carry
digitized voice traffic.
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SONET
SONET DesignationSDH Designation
OC-1
Speed
51.84 Mbps
OC-3
STM-1
155.52 Mbps
OC-9
STM-3
466.56 Mbps
OC-12
STM-4
622.08 Mbps
OC-18
STM-6
933.12 Mbps
OC24
STM-8
1.244 Gbps
OC-36
STM-12
1.866 Gbps
OC-48
STM-16
2.488 Gbps
OC-192
9.952 Gbps
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NY Information Technology Center
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Digital Subscriber Line (DSL)
Digital Subscriber Line (DSL) is one of the
most promising proposals now under
consideration by the ITU-T to significantly
increase the data rates over traditional
telephone lines.
The reason for the limited capacity on voice
telephone circuits lies with the telephone
and the switching equipment at the end
office. The actual cable is capable of
providing much higher data rates.
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Digital Subscriber Line (DSL)
DSL services are quite new and not all
common carriers offer them.
Two general categories of DSL services have
emerged in the marketplace.
• Symmetric DSL (SDSL) provides the same
transmission rates (up to 128 Kbps) in both
directions on the circuits.
• Asymmetric DSL (ADSL) provides different data
rates to (up to 640 Kbps) and from (up to 6.144
Mbps) the carrier’s end office. It includes an
analog channel for voice transmissions.
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Digital Subscriber Line (DSL)
A new version of ADSL called Very high rate
Digital Subscriber Line (VDSL) has been
designed for use over local loops of 1000
feet or less. It uses FDM to provide three
channels:
• Normal analog channel
• Upstream digital 1.6 Mbps channel
• Downstream digital 51.84 Mbps channel.
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Digital Subscriber Line (DSL)
One potential competitor to DSL is the “cable
modem” a digital service offered by cable
television companies which offers an
upstream rate of 1.5-10 Mbps and a
downstream rate of 2-30 Mbps.
A few cable companies offer downstream
services only, with upstream
communications using regular telephone
lines.
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CIRCUIT SWITCHED
SERVICES
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Circuit Switched Services
The major problem with dedicated circuit
services it that the user must carefully plan
all circuits needed.
In contrast, switched circuits work much like
dialed circuits. The user buys a connection
into the common carrier’s network from the
end points of the WAN, without specifying
all the interconnecting circuits needed.
The primary differences from dialed circuits is
that the circuits are entirely digital and that
they offer higher data transmission rates.
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Circuit Switched Services
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Narrowband Integrated
Services Digital Network
The first generation of Integrated services
digital network (ISDN), commonly called
narrowband ISDN, combines voice, video,
and data over the same digital circuit.
ISDN has long been more of a concept than a
reliable service in North America.
Acceptance has been slowed because
equipment vendors and common carriers
conflicting interpretations of ISDN
standards.
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Narrowband Integrated
Services Digital Network
Narrowband ISDN offers two types of service:
• Basic rate interface (BRI, basic access service or
2B+D) provides two 64 Kbps bearer (B) channels
and one 16 Kbps control signaling (D) channel.
One advantage of BRI is it can be installed over
existing telephones lines. (if less than 3.5 miles).
• Primary rate interface (PRI, primary access
service or 23B+D) provides 23 64 Kbps ‘B’
channels and one 64 Kbps ‘D’ channel. (basically
T-1 service)
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Broadband Integrated
Services Digital Network
The second generation of ISDN is called
Broadband ISDN (B-ISDN). B-ISDN is a
circuit switched service and is backwardly
compatible with ISDN.
B-ISDN is currently offered in three services:
• Full duplex channel at 155.2 Mbps.
• Full duplex channel at 622.08 Mbps.
• Asymmetrical service with two simplex
channels (Upstream at 155.2 Mbps,
downstream at 622.08 Mbps).
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PACKET SWITCHED
SERVICES
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Packet Switched Services
Packet switched services enable multiple
connections to exist simultaneously
between computers.
With packet switching users buy a connection
into the common carrier network, and
connects via a packet assembly/
disassembly device (PAD).
Packet switching splits messages into small
segments called packets (usually 128
bytes).
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Packet Switched Services
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Packet Switched Services
Packets from separate messages are
interleaved with other packets for
transmission.
Although the packets from one data stream
may mix (interleave) with several other data
streams during their journey, it is unlikely
that packets from two different data streams
will travel together during the entire length
of their transmission.
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Packet Switched Services
There are two methods used to route packets:
 A Datagram
is a connectionless service which
adds a destination and sequence number to each
packet, in addition to information about the data
stream to which the packet belongs. Packets may
follow a different route, and are reassembled at
the destination.
 In a Virtual circuit the packet switched network
establishes an end-to-end circuit between the
sender and receiver. All packets for that
transmission take the same route over the virtual
circuit that has been set up for that transmission.
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Packet Switched Services
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Packet Switched Services
Packet switched services are often provided
by different common carriers than the one
from which organizations get their usual
telephone and data services.
Therefore, organizations often lease a
dedicated circuits from their offices to the
packet switched network point-of-presence
(POP).
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X.25
The oldest packet switched service is X.25, a
standard developed by ITU-T. X.25 offers
datagram, switched virtual circuit, and
permanent virtual circuit services.
Although widely used in Europe, X.25 is not
widespread in North America. The primary
reason is transmission speed, now 2.048
Mbps (up from 64 Kbps).
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Frame Relay
Frame relay is a newer packet switching
technology that transmits data faster than
X.25. It differs from X.25 and traditional
networks in three important ways:
1. Frame relay only operates at the data link layer.
2. Frame relay networks do not perform error
control.
3. Frame relay defines two connection data rate
that are negotiated per connection and for each
virtual circuit as it is established. (Committed
information rate and Maximum allowable rate).
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Frame Relay
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Frame Relay
Different common carriers offer frame relay
networks with different transmission
speeds: 56 Kbps to 45 Mbps.
At present, frame relay suffers from the same
problems as ISDN - a lack of standards.
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Asynchronous Transfer Mode
(ATM)
Asynchronous transfer mode (ATM) is one of
the fastest growing new technologies, and
is similar to frame relay.
All data are packet-switched, and there is no
error control at the intermediate computers
within the network; error control is the
responsibility of the source and destination.
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Asynchronous Transfer Mode
(ATM)
ATM has three important difference from
frame relay:
• ATM uses fixed packet lengths of 53 bytes (5
bytes of overhead and 48 bytes of user data),
which is more suitable for voice transmissions.
• ATM provides extensive quality of service
information that enables the setting of very
precise priorities among different types of
transmissions (i.e. voice, video & e-mail).
• ATM is scaleable. It is easy to multiplex basic
ATM circuits into much faster ATM circuits.
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Home Depot Network
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Switched Multimegabit Data
Service (SMDS)
Switched multimegabit data service (SMDS)
is an unreliable packet service like ATM and
frame relay.
Like ATM and frame relay, SMDS does not
perform error checking; the user is
responsible for error checking.
SMDS is not yet a widely accepted standard.
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Hacienda La Puente Unified School District network
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IMPROVING MAN/WAN
PERFORMANCE
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Improving MAN/WAN
Performance
Improving MAN/WAN performance is handled
in the same way as improving LAN
performance.
You begin by checking the devices in the
network, by upgrading the circuits between
computers, and by changing the demand
placed on the network.
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MAN/WAN Performance
Performance Checklist
Increase Computer and Device Performance
• Upgrade devices
• Change to a more appropriate routing protocol (either
static or dynamic)
Increase Circuit Capacity
• Analyze message traffic and upgrade to faster circuits
where needed
• Check error rates
Reduce Network Demand
• Change user behavior
• Analyze network needs of all new systems
• Move data closer to users
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Improving Device Performance
One way to improve network performance is
to upgrade the devices and computers that
connect backbones to the WAN.
Another strategy is to examine the routing
protocol, either static or dynamic. Dynamic
routing will increase performance in
networks which have many possible routes
from one computer to another, or those in
which message traffic is “bursty.”
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Improving Circuit Capacity
The first step is to analyze the message traffic in the
network to find which dedicated point-to-point
circuits are approaching capacity.
The capacity may be adequate for most traffic, but
not for meeting peak demand. One solution may
be to add a circuit switched or packet switched
service that is only used when demand exceeds
circuit capacity.
Sometimes a shortage of capacity may be caused
by a faulty circuit. Before installing new circuits,
monitor the existing ones to ensure that they are
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operating properly.
Reducing Network Demand
One step to reduce network demand is to
require a network impact statement for all
new application software developed or
purchased by the organization.
Another approach is to shift network usage
from peak or high cost times to lower
demand or lower cost times.
The network can be redesigned to move data
closer to the applications and people who
use them.
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SELECTING MAN/WAN
SERVICES
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Selecting MAN/WAN Services
A 1995 survey of network managers found that:
• 45 percent of WAN costs were for network
management (primarily support staff salaries).
• 35 percent was spent on services (leasing data
circuits from common carriers).
• Only 20 percent was spent on equipment.
The most expensive part of the WAN will be the
people require to plan, install, and operate it, so
pick one that is easy to manage.
It costs more to lease services from common carriers
than to buy hardware, so selection decisions should
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be driven more by the services.
Commonly Available Services
Type of Service
Dialed Circuit Services
Voice-grade
WATS
Approximation Data Rates
28.8 Kbps to 56 Kbps
28.8 Kbps to 56 Kbps
Dedicated Circuit Services
Voice-grade
Wideband analog
T-carrier
SONET
28.8 Kbps to 56 Kbps
288 Kbps to 274 Mbps
65 Kbps to 274 Mbps
52 Mbps to 622 Mbps
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Commonly Available Services
Type of Service
Approximation Data Rates
Circuit Switched Services
Narrowband ISDN
Broadband ISDN
128 Kbps to 1.5 Mbps
155 Mbps to 622 Mbps
Packet-Switched services
X.25
Frame relay
ATM
SMDS
56 Kbps to 2 Mbps
56 Kbps to 45 Mbps
1.5 Mbps to 622 Mbps
56 Kbps to 45 Mbps
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Key Issues
 Vendor
capabilities
 Capacity
 Flexibility
 Control
 Reliability
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Value Added Networks and
Virtual Private Networks
Several companies offer value added
networks (VANs) that are alternatives to
building networks by leasing circuits from
common carriers. VANs provide additional
services over and above those provided by
common carriers.
A new type of VAN, called a virtual private
network (VPN), or software defined
networks, provide circuits that run over the
Internet but appear to the user to be private
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networks.
Value Added Networks and
Virtual Private Networks
The primary advantage of the VPN is low
cost.
There are two important disadvantages of
VPNs:
• Traffic on the Internet is unpredictable.
• There are several competing standards for
Internet-based CPN, so not all vendor’s
equipment and services are compatible.
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End of Chapter 9
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