Introduction

Introduction

In today's world of modern telecommunications, the need to extend networks is increasing rapidly. While a Local Area Network (LAN) may support a majority of communication and resource-sharing needs within an enterprise or campus setting, Wide Area Network (WAN) connectivity allows individuals and organizations to take further advantage of internetworking services such as the Internet, e-commerce, and video conferencing.

This lesson provides a brief introduction to the functional purpose of the WAN. It also provides an overview of the characteristics that define WAN network dimensions, compares and contrasts the major WAN traffic types (voice, data, and video) and explains the meaning and impact of convergence in emerging multi-service networks.

Objectives

Upon successful completion of this lesson, you should be able to do the following:

Define the purpose of a Wide Area Network (WAN).

Compare and contrast private and public WANs.

Compare and contrast the five major classifications of WAN service providers and define their general networking requirements: ILEC, IEC, CLEC, ISP, and PTT.

Compare and contrast voice and data networks.

Compare and contrast WAN connection and switching types.

WAN Overview

While the geographic distinctions of "local" and "wide" area networks imply a difference in the distance between network nodes, that is not always the case. By definition, a Wide Area Network (WAN) is a government-regulated public network or privately owned network that crosses into the public network environment. It doesn't matter whether the area being bridged is across the country or across the street. If the geographical separation crosses over a public thoroughfare, a WAN is required to make the connection.

The WAN is typically used to connect two or more local area networks (LANs). As you know, a LAN is a privately owned communications system that is designed to allow users to access and share resources (e.g., computers, printers, and servers) with other users. LANs that are interconnected by a WAN may be located in the same geographical area, such as an industrial park or campus setting, or in geographically separate areas, such as different cities or even different regions.

There are a number of transmission services that are used to support WAN communications, all running across the hardware components that physically connect different LANs. For the attached user, these services and components work in tandem to create the illusion of one large virtual network. Thus, the WAN is often represented as a cloud, as shown above. You will learn more about the specific WAN services and hardware that exist within this cloud as you continue through this course.

Metropolitan Area Network (MAN)

While the focus of this course is on the WAN, you should also be aware of another unique network type called a Metropolitan Area Network (MAN). Like the WAN facilities to which they are closely related, MANs are typically subject to government-regulation.

MANs interconnect LANs within a specific geographical area, such as a large urban center or campus environment. Typically configured as a ring topology, a MAN provides regional LANs with a high-speed backbone for the sharing of resources, as well as offering an efficient solution for connecting to the WAN.

While the focus of this course is on the WAN, you should also be aware of another unique network type called a Metropolitan Area Network (MAN). Like the WAN facilities to which they are closely related, MANs are typically subject to government-regulation.

MANs interconnect LANs within a specific geographical area, such as a large urban center or campus environment. Typically configured as a ring topology, a MAN provides regional LANs with a high-speed backbone for the sharing of resources, as well as offering an efficient solution for connecting to the WAN.

WAN Characteristics

The WAN concept has many different applications, which can be categorized by examining the differing characteristics of the network environment:

Is ownership of the network public or private?

Is the network designed primarily for voice or data traffic?

Are connections dedicated or switched?

Does the network perform circuit switching, packet switching, or cell switching?

As you proceed through this section, you will see how these characteristics differ from one WAN environment to another.

First, consider the difference between private and public WAN networks.

Private WAN

As stated previously, a WAN is a government-regulated public network or privately owned network that crosses into the public network environment. Consider the differences between these two network types, beginning with the private WAN environment.

By definition, private means "of, belonging to, or concerning a particular person or group." Likewise, a private WAN network is one where you have exclusive access to dedicated links, and are likely to pay well for the luxury. You will learn more about dedicated links later in this lesson.

While a majority of LANs connect to the WAN through a public interface, an increasing number of WAN connections are privately owned. Private network operators typically lease lines from public network provider while maintaining control and management of the network from their own facilities. Most private WANs also include a separate connection to the public WAN.

Government regulation of private WAN operations is relatively limited. Because many corporations and universities base their internal networks on leased lines, they may be making interstate and intrastate call connections that are subject to tariffs. While most private WAN connections are non-tariffed, WAN operators have been known to place calls that "hop off" at the tail end and connect to a local carrier, thus bypassing the long distance toll. The FCC regulates these "leaky" Private Branch Exchange (PBX) connections.

If leasing a physical WAN connection does not make economic sense, a private WAN operator has several other options to consider.

Wireless WAN connections, such as satellite or laser line-of-sight between buildings, are one common solution, but they are subject to potential environmental interference and breaches of security

Another non-lease WAN option is a Virtual Private Network (VPN), which connects distributed LAN locations across the Internet. Increasingly used as an alternative to traditional direct-dial remote access, the VPN uses an encrypted IP tunnel to provide remote and mobile users with a secure communications path to a network access device, such as a network server. In addition to security issues, VPN links do not offer the reliable service quality that some network users may demand.

If you want a secure, physical WAN connection that is dedicated and non-leased, the process can be a bit more complex. Consider the case of a large technology enterprise that wanted to connect two LAN facilities that were located on either side of an interstate highway. After measuring the long-term costs of running a dedicated line to the central office and back, they decided to explore the option of becoming a common carrier and creating their own physical connection. Common carriers are government-regulated organizations that offer telecommunications services for public use, although in this case the company simply needed the license, but had no plan to sell services. The company then offered to purchase a 6-inch swatch of the interstate from the Federal Highway Administration, assuming responsibility for the care and maintenance in exchange for being allowed to bury a fibre optic cable below the highway surface. The deal was approved, and the connection was made.

Now consider public WAN networks.

A key component of the public WAN is the presence of government regulation, which not only dictates the telecommunications services that are provided by public WAN operators, but also determines how which services are subject to tariff and at what rate.

In the United States, the Federal Communications Commission (FCC) is the primary regulator of the public network environment. In addition, there are a variety of national and international organizations that define the format standards for telecommunications. You will learn more about the organizations that regulate and create standards for the public WAN in the next lesson.

Public WAN

The best example of a public WAN designed for voice is the Public Switched Telephone Network (PSTN), while the Internet is the largest public WAN designed for data. You will learn more about voice and data networks later in this lesson.

In North America, telephone services across PSTN are provided by a telephone company, commonly referred to as known as the Telco. Telco may be used to refer to the local telephone companies only, or it may represent the telephone industry in general, including both local and long distance carriers.

In many European countries, a governmental agency known as the Postal, Telegraph and Telephone (PTT) is responsible for providing combined postal, telegraph, and telephone services. A similar centrally controlled system is used in China.

Another distinguishing characteristic of the public WAN is the tariff, which is the rate charged for a variety of telecommunication services that are provided to Telco customers. Unlike the private WAN, where the network owner pays the full cost of leased or owned connections, public WAN operators provide shared facilities where link ownership and payment is distributed among all connected users.

Both regulation and tariffs have a significant impact on the types of WAN services that are provided. While technology supports a large variety of WAN solutions, the offering of those services is subject to the approval of the regulating agencies. Even if a solution is approved, the service may still not be offered if it does not serve the business needs of the Telco.

Now consider the various parties that provide public and private WAN facilities and services.

WAN Providers

The Telco consists of a variety of Service Providers, who compete with each other to provide WAN services to both residential and non-residential customers. In general, a Service Provider is a company that provides a telecommunications service in exchange for some compensation from the customer. The collection of compensation is more commonly referred to as billing.

Billing is typically based on one of two methods:

Content-based billing is used by a Service Provider to collect compensation for a wide range of services and features, such as Internet access, Call Waiting, and Voice Mail.

Conduit-based billing is used by a Service Provider to collect compensation for the physical WAN connection, such as the single phone wire that runs to a house or a high-capacity connection between two corporate LANs.

The actual compensation collected by the Service Provider for content or conduit may vary as well:

Flat Rate is a fixed charge that is collected for a customer's unmeasured usage of a Service Provider's content or conduit. For example, a residential customer might be charged a flat rate of $20 per month for unlimited access to the Internet.

Utilization is a variable rate that is collected based on a customer's measured usage of a Service Provider's content or conduit. Long distance telephone services are often billed in this manner, with rates such as 10 cents per minute.

Advertising is an increasingly common way that Service Providers are compensated for services provided. In this case, the user agrees to view or listen to advertisements in exchange for a service, and the Service Provider receives a fee from the advertiser. For example, you may be asked to listen to five recorded ads in exchange for 30 minutes of long distance service. An increasing number of free Internet services are supported by advertiser fees as well.

Public WAN services may be offered by a variety of different provider types. In some countries, the sole provider is a government monopoly such as the PTT.

There are several categories of WAN Service Providers, but the two most common groups provide a majority of WAN services:

Local Exchange Carrier (LEC) refers to the company that owns the "last mile" telephone wire that runs to your house and provides telephone service within the local exchange. Unless you dial 10-10-xxx, the LEC intercepts long distance calls and hands them off to your specified long distance carrier.

Long distance services are typically provided by an Inter Exchange Carrier (IEC or IXC). These are the Big Boys of the telecommunications industry, including AT&T, Sprint, MCI, and GTE, who own the interstate connections of the PSTN. In some cases, the IEC may also be your LEC.

WAN Providers (continue)

The LEC is better known as the Incumbent Local Exchange Carrier (ILEC), particularly when referring to the Regional Bell Operating Companies (RBOCs), or "Baby Bells," previously owned by AT&T, who faced little or no competition prior to the passage of the Telecommunications Act of 1996. This legislation had a significant impact on competition within the industry, opening the door for an increasing number of companies to enter the market and provide services using the existing PSTN infrastructure:

Competitive LEC (CLEC) refers to a company that provides competitive local exchange access by connecting to ILEC switches and purchasing space on existing ILEC lines. The CLEC may also operate as a reseller, purchasing telecommunications services from the ILEC at wholesale prices and selling them to customers at competitive rates.

Competitive Access Provider (CAP) refers to a companies that buy space on other IEC lines. CAPs may offer inter-exchange link access between two LEC locations, leased line access to the public WAN, or direct connections to a local Internet Service Provider.

Internet Service Provider (ISP) refers to a company that provides access to the Internet. The ISP might be a large organization such as AOL or CompuServe, which offer a variety of access services, or it may be your local cable TV company, your LEC, or perhaps some entrepreneur operating out of his garage.

Now that you are familiar with network ownership and the types of service providers offering WAN access and transport, consider the types of connections that are used to carry information across the WAN infrastructure.

WAN Connection Types

WAN connections come in two basic flavors:

Dedicated connections are links that is reserved for a single telecommunications purpose and available to the user at all times.

Switched connections are general purpose links that are available on demand and are usually paid for on a per usage basis.

The type of WAN connection that you use is dependent on several factors, including the type of information that needs to be sent, the average level of traffic, and the security requirements.

First consider dedicated connections.

Dedicated Connections

Permanent dedicated circuits, also known as leased lines or private lines, have the following characteristics:

Cost is high because the connection is used exclusively for one purpose, so they are primarily used for connections where large amounts of traffic need to be moved reliably between two network nodes.

Call setup is not required because the connections are point-to-point.

Connections are always available, 24 hours a day, seven days a week.

Dedicated connections are often deployed on high-capacity links including SONET/SDH, which are the standard North American and European fiber optic transmission systems for high-speed digital traffic, and T-1/E-1/J-1, which are the North American, European, and Japanese standards for low-speed transmission across canalized copper cable. You will learn more about transport media later in the course.

Common uses of dedicated connections include video conferencing, medical data imaging, and financial wire transfers. Commissioner Gordon's Batphone connection to the Batcave was also a dedicated connection!

Switched Connections

Switched connections, also known as virtual connections, have the following characteristics:

Cost is low because switched connections are made on demand over shared facilities.

Switched connections are set up as needed before information can be transmitted. The call setup might impart a slight delay, but you only pay for what you use.

Connections are point-to-multipoint, and routing decisions are made dynamically along the call path.

For voice connections, routing is performed only once per call.

For data connections, routing is more dynamic so that each and every packet might take a different route.

Switched connections are commonly found in the PSTN, as well as ISDN, Frame Relay, and ATM networks.

Hybrid connections may be required in some cases, where a leased line is needed to make the connection between the customer location and the service providers Point of Presence (POP). For example, X.25 networks are often accessed using a dedicated connection, and then a switched connection is used internally. You will learn more about X.25 later in this course.

Now consider the differences between the various types of switched networks.

Switched Connections

There are three basic types of WAN switching services:

Circuit-switched networks create a dedicated circuit, or channel, which is used for the duration of the transmission.

Packet-switched networks separate messages into variable-length segments and transmit them individually across dynamically created connections.

Cell-switched networks separate messages into fixed-length cells and transmit them individually across routed connections that are either dynamically or permanently created.

First consider circuit switching.

Circuit Switching

Circuit-switched networks were originally designed for the transmission of analog voice. As a result, they have the following characteristics:

Circuit-switched networks are connection-oriented because the receiving end must reply before transmission begins. Call setup is required prior to the exchange of information. This temporary point-to-point connection is known either as a circuit or channel.

The call path remains constant and bandwidth is dedicated throughout the duration of the call. Unused bandwidth is not recovered.

Traffic is transmitted at wire-speed with minimal delay and an emphasis on timing to minimize "jitter."

There is no error recovery because circuit-based switches maintain only small buffers.

Circuit-switched network operators typically charge customers for the duration of the connection, which includes any "transmitted silence."

The best example of a circuit-switched network is the PSTN. Later you will learn about ISDN and cellular networks, which also use circuit switching to establish connections.

Packet Switching

Packet switching, which divides messages into smaller packets for transmission, was originally developed for the purpose of sending data over analog circuits, which are subject to errors and noise. More recent developments for digital transmission include Frame Relay and Switched Multimegabit Data Service (SMDS). You will learn more about these network types later in this course.

Packet-switched networks have the following characteristics:

Packet-switched networks are connection-oriented because the receiving end must reply before transmission begins. However, no call setup is required because each packet contains a destination address that is used to route each packet through the network. The dynamically routed connection through the network is known as either a virtual circuit or virtual channel.

Dynamic routing results in flexible use of bandwidth and network resources.

Packet switches use a store-and-forward technique to carry voice and data through the network. The temporary storage of switched packets allows for error correction, as well as prioritization and more efficient use of bandwidth.

Packet-switched network operators charge based on the actual number of packets sent, which means you only pay for data that is transmitted.

In addition to the network types identified above, the Internet is a good example of a packet-based switched network.

Cell Switching

ATM networks employ cell switching, which combines the guaranteed bandwidth of a circuit-switched network with the efficient bandwidth-sharing and prioritization capabilities of a packet-switched network. ATM networking in the WAN environment is covered in more detail later.

Cell-switched networks have the following characteristics:

Cell-switched networks are connection-oriented because the receiving end must reply before transmission begins. However, no call setup is required because each 53-byte cell contains a destination address that is used to route it logically through the network. Dynamically routed connections are called Switched Virtual Circuits (SVC). Manually routed connections are referred to as Permanent Virtual Circuits (PVCs).

Logical routing allows for flexible use of bandwidth and network resources.

Logical circuits allow cell-switched networks to guarantee Quality of Service (QoS).

Cell switches use a store-and-forward technique to carry voice and data through the network. The temporary storage of switched cells allows for error detection (with no recovery), as well as prioritization and more efficient use of bandwidth.

Cell-switched network operators charge based on the actual number of cells sent, which means you only pay for data that is transmitted.

Now that you are able to differentiate between dedicated and switched connections, consider the traffic types.

Network Traffic

There are three major types of traffic that are found in the WAN environment:

Voice, which is the term collectively used to identify live uncompressed voice that is transported across the network.

Data, which refers to electronic information found in files, databases, documents, and images as well as digitally-encoded voice and video.

Video, which is the term collectively used to identify live uncompressed moving images that are transported across the network.

Theoretically, each of these traffic types could ride over any of the switched connections discussed previously. However, there are "historical preferences" that pair voice with circuit-switched networks, data with packet-switched networks, and so forth. But these pairings are becoming increasingly less meaningful as networks converge.

You will learn more about convergence in a moment after examining each of the three traffic types in more detail, beginning with voice.

Voice Traffic

Originally, networks were designed to carry one specific traffic type: voice or data. While modern telecommunications networks are increasingly carrying both voice and data (as well as video), it is important to consider the unique traffic requirements of voice and data networks separately.

As mentioned previously, the PSTN is the largest voice network in existence today. Originally designed to handle voice only, the PSTN is adapting rapidly to meet a variety of data needs. While the demand for data services will continue to increase, voice continues to be the largest source of revenue for WAN providers.

Voice networks have several common characteristics that are designed to meet the needs of voice traffic:

If voice transmission does not arrive in the proper order, then it is not likely to be of any use because it may not be understood. As a result, voice networks are typically circuit-switched because voice requires consistent timing to prevent jitter. In general, packet-switched networks are not suited for voice traffic because prioritization can cause result in delays.

Because timing restraints do not allow time to retransmit lost or damaged information, voice networks typically have minimal error control and no error recovery. Fortunately, voice traffic is very forgiving to errors.

Voice traffic is also supported by a wide variety of network types including SONET/SDH, wireless, frame relay, ATM, TCP/IP, and ISDN, but the vast majority of voice traffic is carried across the PSTN.

Data Traffic

Data is less sensitive to the timing constraints of voice traffic. Rather, data networks are more concerned about reliable transport of information. As a result, data networks are typically packet-switched or cell-switched to ensure the error control and recovery needed to ensure a reliable transfer.

Video Traffic

Video is similar to voice in that data in that it requires reliable end-to-end transport, while also sharing the timing requirements of voice traffic. The ideal network type for video information is a cell-switched ATM network which provides quality guarantees and fast delivery.

Network Traffic Convergence

As LANs interconnect the need to transmit all types of traffic across the WAN increases, resulting in the convergence of data and voice networks. Convergence refers to the combining of two or more technologies, in this case voice and data networks:

Transmission-level convergence of voice, video, and data across physical transport media, which may be copper cable, optical fiber, or wireless.

Network-level convergence of voice, video, and data across multi-protocol networks.

Application-level convergence of integrated software programs across application-aware networks.

Convergence at each level is having a dramatic impact on telecommunications, with network operators and service providers adapting their networks to carry all types of network traffic. Voice networks are carrying an increasing amount of data traffic, while voice traffic is migrating towards transport across data networks. You will learn more about convergence as you proceed through the remainder of this course.

Summary

A Wide Area Network (WAN) is a government-regulated public network or privately owned network that crosses into the public network environment. It is typically used to connect two or more local area networks (LANs).

There are three sets of characteristics that define WAN networks:

Ownership of the network is either public or private.

Connections are either dedicated (leased) or switched (call setup).

Connections may be circuit-switched, packet-switched, or cell-switched.

The primary traffic type for which the network was designed is either voice or data. Video is another predominant type of WAN traffic.