The Internet Encyclopedia (Volume 3)

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DeNoia WL040/Bidgolio-Vol I WL040-Sample.cls June 20, 2003 17:57 Char Count= 0

Wide Area and Metropolitan Area NetworksWide Area and Metropolitan Area Networks

Lynn A. DeNoia,Rensselaer Polytechnic Institute

Introduction 776

History and Context 776

Definitions 776

Challenges 776

Functional Requirements 777

Evolution and Coexistence 777

Facilities and Infrastructure 778

Digital Transmission 778

Optical Fiber Systems 778

Access Technologies 779

Management 780

Differences around the World 780

Switching, Routing, and Signaling 781

Network Architecture 781

Switching Technologies 782

Routing Technologies 785

Signaling and Interworking 787

Providers and Services 788

Carriers and Service Providers 788

Class of Service, Quality of Service 789

Vir tual Private Networks 789

Management 789

Conclusion 790

Glossary 790

Cross References 790

Further Reading 790

INTRODUCTION

In today’s social, political, and economic environment,

individuals and organizations communicate and operate

over ever-increasing geographic distances. This means

that access to and sharing of information and resources

must extend beyond the “local” office, building, or cam-

pus out across cities, states, regions, nations, continents,

and even beyond the planet. Bridging this diversity of dis-

tances in ways that satisfy application requirements for

speed, capacity, quality, timeliness, etc. at reasonable cost

is no simple challenge, from either a technical or a busi-

ness perspective. In this chapter we concentrate on the

main elements required to meet such a challenge in wide

area and metropolitan area networks.

HISTORY AND CONTEXT

Definitions

The public networking arena has typically been divided

into two segments with the following characteristics:

Metropolitan area networks (MANs) are built and op-

erated by service providers (SPs) who offer network ser-

vices to subscribers for a fee, covering distances up to

tens of miles, often within or surrounding a major city.

MANs are often built by telecommunication companies

such as local exchange carriers (LECs) or by utility com-

panies. A recent alternative using Ethernet for the MAN

has spawned a new category of companies called Ethernet

LECs or ELECs.

Wide area networks (WANs) are built and operated

by SPs who offer network services to subscribers for a

fee, covering distances up to hundreds or thousands of

miles, such as between cities, across or between coun-

tries, across oceans, etc. WANs designed for voice are usu-

ally built by telecommunication companies known in the

United States as interexchange carriers (IXCs). WANs for

data are also called public data networks (PDNs).

By contrast, local area networks (LANs) are typically

built and operated as private networks, by individuals

or enterprises, for their own use. In addition, landlords

operating as building LECs (BLECs) may offer LAN ser-

vices to tenants. In either case, the geographic scope of

a LAN is usually limited to a building or campus envi-

ronment where all rights of way for cabling purposes be-

long to the individual/enterprise/landlord. The boundaries

between LANs and MANs and WANs began to blur as ge-

ographic limitations of networking technologies were ex-

tended with increasingly capable implementations over

fiber-optic cabling. Even the distinctions between private

and public networks became more difficult to draw with

the advent of “virtual private network” equipment and

services.

Challenges

The number of options and choices available to network

designers in both the subscriber and provider communi-

ties continues to grow for both MANs and WANs. Multi-

ple technologies and standards, increasing numbers and

types of applications, higher expectations for MAN and

WAN performance comparable to (or at least approach-

ing) that found in a LAN environment, and pressure to

keep unit costs low, all combine to create enormous chal-

lenges for MAN and WAN builders. Infrastructure choices

must last long enough, not just for cost recovery, but

to achieve return on investment. Service providers must

marry new technologies to their existing installed base,

create smooth transitions (e.g., for network upgrades, new

service roll-outs) with minimal disruption to customer

services, and add or enhance services to meet advancing

customer expectations, all in an environment of increas-

ing economic and competitive pressure. Many providers

have begun to recognize that their long-term survival de-

pends on a strategy of simplification—reducing the com-

plexity (to have fewer technologies, fewer equipment ven-

dors, fewer equipment types, fewer management systems,

etc.) of their infrastructure while maintaining the flexibil-

ity to adapt to changing application, user, and competitive

requirements.

776