A wide-area network (WAN)

Overview

A wide-area network (WAN) is a data communications network that connects user networks over a large geographical area. WANs have several important characteristics that distinguish them from LANs. The first lesson in this module will provide an overview of WAN technologies and protocols. It will also explain how WANs and LANs are different, and ways in which they are similar.

It is important to understand the physical layer components of a router. This knowledge builds a foundation for other i 939d38j nformation and skills that are needed to configure routers and manage routed networks. This module provides a close examination of the internal and external physical components of the router. The module also describes techniques for physically connecting the various router interfaces.

WANs

1.1.1 Introduction to WANs

A WAN is a data communications network that spans a large geographic area such as a state, province, or country. WANs often use transmission facilities provided by common carriers such as telephone companies.

These are the major characteristics of WANs:

• They connect devices that are separated by wide geographical areas.
• They use the services of carriers such as the Regional Bell Operating Companies (RBOCs), Sprint, MCI, and VPM Internet Services, Inc. to establish the link or connection between sites.
• They use serial connections of various types to access bandwidth over large geographic areas.

A WAN differs from a LAN in several ways. For example, unlike a LAN, which connects workstations, peripherals, terminals, and other devices in a single building, a WAN makes data connections across a broad geographic area. Companies use a WAN to connect various company sites so that information can be exchanged between distant offices.

A WAN operates at the physical layer and the data link layer of the OSI reference model. It interconnects LANs that are usually separated by large geographic areas. WANs provide for the exchange of data packets and frames between routers and switches and the LANs they support.

The following devices are used in WANs: 

• Routers offer many services, including internetworking and WAN interface ports.
• Modems include interface voice-grade services, channel service units/digital service units (CSU/DSUs) that interface T1/E1 services, and Terminal Adapters/Network Termination 1 (TA/NT1s) that interface Integrated Services Digital Network (ISDN) services.
• Communication servers concentrate dial in and dial out user communication.

WAN data link protocols describe how frames are carried between systems on a single data link. They include protocols designed to operate over dedicated point-to-point, multipoint, and multi-access switched services such as Frame Relay. WAN standards are defined and managed by a number of recognized authorities, including the following agencies:

• International Telecommunication Union-Telecommunication Standardization Sector (ITU-T), formerly the Consultative Committee for International Telegraph and Telephone (CCITT)
• International Organization for Standardization (ISO)
• Internet Engineering Task Force (IETF)
• Electronic Industries Association (EIA)

1.1.2 Introduction to routers in a WAN

A router is a special type of computer. It has the same basic components as a standard desktop PC. It has a CPU, memory, a system bus, and various input/output interfaces. However, routers are designed to perform some very specific functions that are not typically performed by desktop computers. For example, routers connect and allow communication between two networks and determine the best path for data to travel through the connected networks.

Just as computers need operating systems to run software applications, routers need the Internetwork Operating System (IOS) software to run configuration files. These configuration files contain the instructions and parameters that control the flow of traffic in and out of the routers. Routers use routing protocols to determine the best path for packets. The configuration file specifies all the information for the correct setup and use of the selected, or enabled, routing and routed protocols on a router.

The main internal components of the router are random-access memory (RAM), nonvolatile random-access memory (NVRAM), flash memory, read-only memory (ROM), and interfaces.

RAM has the following characteristics and functions:

• Stores routing tables
• Holds ARP cache
• Holds fast-switching cache
• Performs packet buffering as shared RAM
• Maintains packet-hold queues
• Provides temporary memory for the configuration file of a router while the router is powered on
• Loses content when a router is powered down or restarted

NVRAM has the following characteristics and functions:

• Provides storage for the startup configuration file
• Retains content when a router is powered down or restarted

Flash memory has the following characteristics and functions:

• Holds the IOS image
• Allows software to be updated without removing and replacing chips on the processor
• Retains content when a router is powered down or restarted
• Can store multiple versions of IOS software
• Is a type of electrically erasable programmable read-only memory (EEPROM)

ROM has the following characteristics and functions:

• Maintains instructions for power-on self test (POST) diagnostics
• Stores bootstrap program and basic operating system software
• Requires replacing pluggable chips on the motherboard for software upgrades

Interfaces have the following characteristics and functions:

• Connect routers to a network for packet entry and exit
• Can be on the motherboard or on a separate module

1.1.3 Router LANs and WANs

Routers have both LAN and WAN interfaces. WAN technologies are frequently used to connect routers. Routers use WAN connections to communicate with each other. Routers are the backbone devices of large intranets and of the Internet. They operate at Layer 3 of the OSI model, making decisions based on network addresses. The two main functions of a router are to select the best path for packets and to route packets to the proper interface. To accomplish this, routers build routing tables and exchange network information with other routers.

An administrator can configure static routes to maintain routing tables. However, most routing tables are maintained dynamically through the use of a routing protocol that exchanges network topology information with other routers.

A correctly configured internetwork provides the following:

• Consistent end-to-end addressing
• Addresses that represent network topologies
• Best path selection
• Dynamic or static routing
• Switching

1.1.4 Role of routers in a WAN

The standards and protocols or primary functions of a WAN operate at the physical layer and at the data link layer. This does not mean that the other five layers of the OSI model are not found in a WAN. It simply means that the standards and protocols that define a WAN connection are typically found at the physical and data link layers. In other words, the Layer 1 and Layer 2 WAN standards and protocols are different than the Layer 1 and Layer 2 LAN standards and protocols.

The WAN physical layer describes the interface between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE). Generally, the DCE is the service provider and the DTE is the attached device. In this model, the services offered to the DTE are made available through a modem or a CSU/DSU.

The main function of a router is to transmit data using Layer 3 addresses. This process is also called routing. Routing occurs at the network layer, which is Layer 3. If a WAN operates at Layers 1, 2, and 3, is a router a LAN device or a WAN device? The answer is both, as is so often the case in the field of networking. A router may be exclusively a LAN device, it may be exclusively a WAN device, or it may sit at the boundary between a LAN and a WAN and be a LAN and WAN device at the same time

When a router uses the physical and data link layer standards and protocols that are associated with WANs, it is operating as a WAN device. Therefore, the main role of a router in a WAN is not to route. It is to provide connections between the various WAN physical and data-link standards. These standards and protocols that define and structure a WAN connection operate at Layers 1 and 2. For example, a router may have an ISDN interface that uses PPP encapsulation and a serial interface at the end of a T1 line that uses Frame Relay encapsulation. The router must be able to move a stream of bits from one type of service, such as ISDN, to another, such as a T1, and change the data link encapsulation from PPP to Frame Relay.

Many of the details of WAN Layer 1 and Layer 2 protocols will be covered later in the course, but some of the key WAN protocols and standards are listed here for reference.

Here is a list of WAN physical layer standards and protocols:

• EIA/TIA-232
• EIA/TIA-449
• V.24
• V.35
• X.21
• G.703
• EIA-530
• ISDN
• T1, T3, E1, and E3
• xDSL
• SONET (OC-3, OC-12, OC-48, OC-192)

Here is a list of WAN data link layer standards and protocols:

• High-level data link control (HDLC)
• Frame Relay
• Point-to-Point Protocol (PPP)
• Synchronous Data Link Control (SDLC)
• Serial Line Internet Protocol (SLIP)
• X.25
• ATM
• LAPB
• LAPD
• LAPF