Most organizations do not build their own metropolitan or long-distance communication circuits, preferring instead to lease them from common carriers or to use the Internet. Therefore, this topic focuses on the MAN/WAN architectures and telecommunications services offered by common carriers for use in MANs and WANs, not the underlying technology that the carriers use to provide them. We discuss the four principal types of MAN and WAN services that are available: circuit-switched services, dedicated-circuit services, packet-switched services, and virtual private network (VPN) services. We conclude by discussing how to improve MAN and WAN performance and how to select services to build MANs and WANs.
Objectives
■ Understand circuit-switched services and architectures
■ Understand dedicated-circuit services and architectures
■ Understand packet-switched services and architectures
■ Understand VPN services and architectures
■ Understand the best practice recommendations for MAN/WAN design
■ Be familiar with how to improve MAN and WAN performance
Introduction
Metropolitan area networks (MANs) typically span between 3 and 30 miles and connect BNs and LANs. MANs also provide dial-in and dial-out capability to LANs, BNs, and mainframes and access to the Internet. WANs connect BNs and MANs across longer distances, often hundreds or thousands of miles.
The communication media used in MANs and WANs were described in next topic (e.g., twisted-pair, wire coaxial cable, fiber optics, microwave, satellite, infrared). Although some organizations build their own MANs and WANs using these media, most do not. Most organizations cannot afford to lay long stretches of cable, build microwave towers, or lease satellites. Instead, most rent or lease circuits from common carriers, private companies such as AT&T, Bell Canada, Sprint, BellSouth, and so on that sell or lease communication services and facilities to the public. As a customer, you do not lease physical cables per se; you simply lease circuits that provide certain transmission characteristics. The carrier decides whether it will use twisted-pair, coaxial, fiber optics, or other media for its circuits.
In this topic, we examine the MAN and WAN architectures and technologies from the viewpoint of a network manager, rather than that of a common carrier. We focus less on internal operations and how the specific technologies work, and more on how these services are offered to network managers and how they can be used to build networks because network managers are less concerned with how the services work and more concerned with how they can use them effectively.
Likewise, we will focus on MAN and WAN services in North America because the majority of our readers are in North America. Although there are many similarities in the way data communications networks and services have evolved in different countries, there also are many differences. Most countries have a federal government agency that regulates data and voice communications. In the United States, the agency is the Federal Communications Commission (FCC); in Canada, it is the Canadian Radio-Television and Telecommunications Commission (CRTC). Each state or province also has its own public utilities commission (PUC) to regulate communications within its borders.
Common carriers are profit oriented, and their primary products are services for voice and data transmissions, both over traditional wired circuits as well as cellular services. Common carriers often supply a broad range of computer-based services, such as the manufacturing and marketing of specialized communication hardware and software. A common carrier that provides local telephone services (e.g., BellSouth) is commonly called a local exchange carrier (LEC), whereas one that provides long-distance services (e.g., AT&T) is commonly called an interexchange carrier (IXC). As the LECs move into the long-distance market and IXCs move into the local telephone market, this distinction may disappear.
