Lesson 3:WAN Technologies

Wide area networking is a variation on the remote network access concepts introduced in Lesson 1 of this chapter. Technically speaking, a computer accessing the Internet with a modem and a PSTN line is using a WAN connection, but the term WAN is more commonly employed when referring to connections between two networks at different locations. For example, a company with branch offices located in several cities might maintain individual LANs in each branch, all of which are connected by WAN links.


After this lesson, you will be able to

  • Describe the characteristics of a leased line
  • Understand how frame relay provides flexible WAN solutions
  • List the optical carrier (OC) carrier levels provided by the Synchronous Optical Network (SONET)
  • Describe the characteristics of the Asynchronous Transfer Mode (ATM) protocol

Estimated lesson time: 15 minutes


WAN connections are, in most cases, point-to-point links that do not use shared media like a LAN. A PSTN connection is a perfect example of a WAN link, albeit a slow one. WANs can also use the ISDN and DSL technologies discussed in Lesson 1. To connect LANs at remote locations, install a router at each site and connect them using a WAN link. Many of the WAN links described in this lesson use PPP at the data-link layer, just like a PSTN connection. The following sections describe some of the most popular high-speed WAN technologies used today.

When discussing the transmission capabilities of WAN links, "high-speed" is a relative term. All of the WAN technologies discussed in this lesson run at much faster speeds than PSTN connections, but they are also substantially slower than even the most modest LAN. This is because a WAN link nearly always involves an outside service provider that charges a fee based on the bandwidth used, whereas LANs are wholly owned by their operators and incur no bandwidth charges.

Leased Lines

A leased line is a permanent telephone connection between two locations that provides a predetermined amount of bandwidth at all times. Leased lines can be analog or digital, although most of the lines used today are digital. The most common leased line configuration in the United States is called a T1, which runs at 1.544 Mbps. The European equivalent of a T1 is called an E1, which runs at 2.048 Mbps. Many organizations use T1s to connect their networks to the Internet or to connect remote networks. For applications requiring more bandwidth, a T3 connection runs at 44.736 Mbps and an E3 runs at 34.368 Mbps.

Leased line services are split into 64-Kbps channels. A T1, for example, consists of 24 channels that can be used as a single data pipe or as individual 64-Kbps links. It's also possible to install a leased line that uses part of a T1. This fractional T1 service enables you to specify exactly the amount of bandwidth you need. For data transmission purposes, a leased line is typically left as a single channel utilizing all of the available bandwidth. However, T1s and other leased line services are used for standard telephone communications as well. When a large organization installs its own telephone system, the PBX or switchboard is connected to one or more T1 lines, split into the 64-Kbps channels, each of which is capable of functioning as one voice telephone line. The PBX allocates the channels to the various users of the telephone system as needed.

A T3 connection is the equivalent of 672 channels of 64 Kbps each, or 28 T1s. This much bandwidth is usually required only by ISPs and other service providers with a need for huge amounts of bandwidth.

To install a leased line, you contract with a telephone provider to furnish a link between two specific sites, running at a particular bandwidth. Prices depend on the amount of bandwidth and the distance between the sites, but a T1 can easily cost $1000 to $2000 per month. At each end of the connection, you must have a device called a channel service unit/data service unit (CSU/DSU), which functions as the terminus for the link and provides testing and diagnostic capabilities. To use the line, you connect the CSU/DSU to your network using a router, in the case of a data network, or a PBX, in the case of a telephone network.

Leased lines are a popular WAN solution, but they do have some significant drawbacks. Because the link is permanently connected, you are paying for a specific amount of bandwidth 24 hours a day. If your applications are not running around the clock, you might end up paying premium prices for bandwidth you're not using. Also, the bandwidth of a leased line is capped at a particular rate. If your bandwidth needs ever exceed the capacity of the line, the only way to augment your connection is to install another line. As a result, leased lines are excellent solutions for some applications but can be less cost-effective for others.

Frame Relay

Frame relay is a WAN solution that provides bandwidth similar to that of a leased line, but with greater flexibility. Frame relay services range from 56 Kbps all the way up to T3 speeds, but you're not permanently locked into a specific transmission rate, as you are with a leased line. When you enter into a contract with a frame relay provider, you agree on a specific amount of bandwidth called the committed information rate (CIR), which is the base speed of your link. However, the frame relay service can also furnish you with additional bandwidth (called bursts) during your high-traffic periods by borrowing it from other circuits that are not operating at full capacity. In addition to the CIR, you also negotiate a committed burst information rate (CBIR), which is the maximum amount of bandwidth that the provider agrees to furnish during burst periods. Your contract also specifies the duration of the bursts you are permitted. If you exceed the bandwidth agreed on, extra charges are levied.

A frame relay connection is not a permanent link between two points, like a leased line. Instead, each of the two sites is connected to the service provider's network, usually using a standard leased line. The provider's network takes the form of a frame relay cloud, which enables the leased line at one site to be dynamically connected to that at the other site. Because each of the sites uses a local telephone provider for its leased line to the cloud, the cost is generally less than it would be to have a single leased line connecting the two different sites.

The hardware device that provides the interface between the LAN at each site and the connection to the cloud is called a frame relay assembler/disassembler (FRAD). A FRAD is a network layer device that strips off the LAN's data-link layer protocol header from each packet and repackages it for transmission through the cloud. One of the main advantages of frame relay is that you can use a single connection to a frame relay provider to replace several dedicated leased lines. For example, if a corporation has 5 offices located in different cities, it would take 10 leased lines to connect each office to every other office. With frame relay, you only need a single leased line running from each office to the cloud, and the service can provide separate virtual circuits through the cloud, interconnecting all of the offices.

SONET/Synchronous Digital Hierarchy

The Synchronous Optical Network (SONET) is a physical layer standard that defines a method for building a synchronous telecommunications network based on fiber optic cables. First ratified by the American National Standards Institute (ANSI), SONET was then adapted by the International Telecommunications Union (ITU), which called it the Synchronous Digital Hierarchy (SDH). Intended as a replacement for the T-carrier and E-carrier services used in the United States and Europe, respectively, SONET provides connections at various optical carrier (OC) levels running at different speeds. The idea behind SONET is to create a standardized series of transmission rates and formats, eliminating the problems that currently affect connections between different types of carrier networks. The OC levels are listed in Table 12.2.

Table 12.2  SONET OC Levels

OC Level Data Transmission Rate (in Mbps)

OC1

51.84

OC3

155.52

OC6

311.04

OC9

466.56

OC12

622.08

OC18

933.12

OC24

1244.16

OC36

1866.24

OC48

2488.32

OC96

4976.640

OC192

9953.280

ATM

Asynchronous Transfer Mode (ATM) is a protocol that was originally designed to carry voice, data, and video traffic both on LANs and WANs. Today, ATM is sometimes used for network backbones, but it is more commonly found in WAN connections. Unlike most data-link layer protocols, ATM uses fixed-length, 53-byte frames (called cells) and provides a connection-oriented, full-duplex, point-to-point service between devices. There are no broadcast transmissions, and data is relayed between networks by switches, not routers. ATM speeds range from a 25.6-Mbps service, intended for desktop LAN connections, to 2.46 Gbps. Physical media include standard multimode fiber optic and unshielded twisted pair (UTP) cables on LANs, and SONET or T-carrier services for WAN connections.

On an internetwork where ATM is implemented on both the LANs and the WAN connections, cells originating at a workstation can travel all the way to a destination at another site through switches without having to be reencapsulated in a different data-link layer protocol. ATM never gained popularity on the desktop, however, because at the time of its introduction, Fast Ethernet provided better transmission rates and a simpler upgrade procedure. In the same way, Gigabit Ethernet is becoming the predominant high-speed backbone protocol. Today, therefore, ATM has largely been relegated to use on WANs.

FDDI

Fiber Distributed Data Interface (FDDI) is unusual in that it is essentially a LAN protocol, but it is also sometimes grouped with WAN technologies. FDDI runs at 100 Mbps and uses token passing on a shared network medium, which puts it into the LAN protocol category. However, because FDDI uses fiber optic cable, it can span much longer distances than traditional copper-based networks. While FDDI cannot provide truly long distance links, as leased lines and other WAN technologies can, you can use it to connect LANs located in nearby buildings, forming a campus internetwork.

For more information on FDDI, see Lesson 3: FDDI in Chapter 5, "Data-link Layer Protocols."

Exercise 1: WAN Concepts

Match the WAN technologies in the left column with the appropriate concepts in the right column.

  1. Frame relay
  2. T1
  3. ATM
  4. SDH
  5. E3
  1. International equivalent of SONET
  2. Uses 53-byte cells
  3. Provides bursts of additional bandwidth
  4. Consists of 24 channels providing 64 Kbps of bandwidth each
  5. Runs at 34.368 Mbps

Lesson Review

  1. What is the name of the device that connects a leased line to a frame relay cloud?
    1. A CSU/DSU
    2. A FRAD
    3. A cell
    4. An OC3
  2. For which of the following services do you negotiate a CBIR?
    1. T3
    2. E3
    3. ATM
    4. Frame relay
  3. A SONET network uses which type of cable at the physical layer?
    1. UTP
    2. Shielded twisted pair
    3. Coaxial
    4. Fiber optic

Lesson Summary

  • Leased lines are dedicated, permanent, point-to-point connections between two sites that are provided by telephone carriers.
  • Frame relay is a service that provides flexible amounts of bandwidth between sites using a cloud of virtual circuits.
  • SONET is a fiber optic telecommunications network standard consisting of a series of optical carrier levels.
  • ATM is a switched, connection-oriented service that was designed for use on both LANs and WANs.



Network+ Certification Training Kit
Self-Paced Training Kit Exam 70-642: Configuring Windows Server 2008 Network Infrastructure
ISBN: 0735651604
EAN: 2147483647
Year: 2001
Pages: 105

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