Understanding WAN Technologies
As an enterprise grows beyond a single location, it becomes necessary to interconnect LANs in various locations to form a WAN. Several technologies are involved in the functioning of WANs, including hardware devices and software functions. This lesson describes the functions and characteristics of WANs and contrasts them with LANs. The lesson also explores how WANs relate to the Open Systems Interconnection (OSI) reference model in their design and function, which major hardware components are typically seen in WAN environments, and how data is managed in a WAN through multiplexing.
What Is a WAN?
A WAN is a data communications network that operates beyond the geographical scope of a LAN.
WANs use facilities provided by a service provider, or carrier, such as a telephone or cable company. They connect the locations of an organization to each other, to locations of other organizations, to external services, and to remote users. WANs generally carry a variety of traffic types, such as voice, data, and video. Figure 5-1 shows how the WAN provides interconnectivity between the campus LAN and remote sites.
Here are the three major characteristics of WANs:
- WANs connect devices that are separated by wide geographical areas.
- WANs use the services of carriers, such as telephone companies, cable companies, satellite systems, and network providers.
- WANs use serial connections of various types to provide access to bandwidth over large geographical areas.
Figure 5-1 WAN Connectivity
Why Are WANs Necessary?
LAN technologies provide both speed and cost-efficiency for the transmission of data in organizations in relatively small geographical areas. However, other business needs require communication among remote users, including the following:
- People in the regional or branch offices of an organization need to be able to communicate and share data.
- Organizations often want to share information with other organizations across large distances. For example, software manufacturers routinely communicate product and promotion information to distributors that sell their products to end users.
- Employees who travel on company business frequently need to access information that resides on their corporate networks.
In addition, home computer users need to send and receive data across increasingly larger distances. Here are some examples:
- It is now common in many households for consumers to communicate with banks, stores, and a variety of providers of goods and services via computers.
- Students do research for classes by accessing library indexes and publications located in other parts of their country and in other parts of the world.
Figure 5-2 shows the variety of ways that WAN users connect to corporate resources. Because it is obviously not feasible to connect computers across a country or around the world with cables, different technologies have evolved to support this need to connect these geographically diverse devices. WANs allow organizations and individuals to meet their wide-area communication needs.
How Is a WAN Different from a LAN?
WANs are different from LANs in several ways. Whereas a LAN connects computers, peripherals, and other devices in a single building or other small geographical area, a WAN enables the transmission of data across broad geographical distances. In addition, a company or organization must subscribe to an outside WAN service provider to use WAN carrier network services. LANs are typically owned by the company or organization that uses them. Table 5-1 compares the differences between the WAN and LAN.
Figure 5-2 WAN Connectivity
Table 5-1 LAN/WAN Comparisons
WAN Access and the OSI Reference Model
WANs function in relation to the OSI reference model. Their function focuses primarily on Layer 1 and Layer 2. WAN access standards typically describe both physical layer delivery methods and data link layer requirements, including physical addressing, flow control, and encapsulation. WAN access standards are defined and managed by a number of recognized authorities, including the ISO, the Telecommunications Industry Association (TIA), and the Electronics Industry Alliance (EIA).
The physical layer (OSI Layer 1) protocols describe how to provide electrical, mechanical, operational, and functional connections to the services of a communications service provider.
The data link layer (OSI Layer 2) protocols define how data is encapsulated for transmission toward a remote location and the mechanisms for transferring the resulting frames. A variety of different technologies are used, such as Frame Relay and ATM. Some of these protocols use the same basic framing mechanism, HDLC, an ISO standard, or one of its subsets or variants. Figure 5-3 shows where WAN technologies fall in the OSI reference model.
Figure 5-3 OSI Layers Where the WAN Operates
Several devices operate at the physical layer in a WAN. The following devices are used for WAN access:
- Routers: Routers provide internetworking and WAN access interface ports.
- Communication servers: Communication servers concentrate dial-in and dial-out user communications.
- Modems or digital service units (DSU)/channel service units(CSU): In analog lines, modems convert the digital signal of the sending device into analog format for transmission over an analog line and then convert this digital signal back to digital form so that it can be received and processed by the receiving device on the network. For digital lines, a DSU and a CSU are required. The two are often combined into a single piece of equipment, called the DSU/CSU. The DSU/CSU can also be built into the interface card in the router.
- WAN networking devices: Other devices, such as ATM switches, Frame Relay switches, public switched telephone network (PSTN) switches, and core routers, are also used within the cloud to support the access services.
Devices on the subscriber premises are referred to as customer premises equipment (CPE). The subscriber owns the CPE or leases the CPE from the service provider. A copper or fiber cable connects the CPE to the nearest exchange or central office (CO) of the service provider. This cabling is often called the local loop, or “last mile.” Transmission of analog data (such as a telephone call) is connected locally to other local loops, or nonlocally through a trunk to a primary center. Analog data then goes to a sectional center and on to a regional or international carrier center as the call travels to its destination.
For the local loop to carry data, however, a device such as a modem or DSU/CSU is needed to prepare the data for transmission. Devices that put data on the local loop are called data communications equipment (DCE). The customer devices that pass the data to the DCE are called data terminal equipment (DTE). The DCE primarily provides an interface for the
DTE into the communication link on the WAN cloud.
The WAN access physical layer describes the interface between the DTE and the DCE. Figure 5-4 shows the location of DTE and DCE equipment and some Layer 1 connectivity supported between those devices.
Cisco routers support the EIA/TIA-232, EIA/TIA-449, V.35, X.21, and EIA/TIA-530 standards for serial connections.
When you order the cable, you receive a shielded serial transition cable that has the appropriate connector for the standard you specify. For some Cisco routers, the router end of the shielded serial transition cable has a DB-60 connector, which connects to the DB-60 port on a serial WAN interface card (WIC). Because five different cable types are supported with this port, the port is sometimes called a five-in-one serial port. The other end of the serial transition cable is available with the connector that is appropriate for the standard you specify. The documentation for the device to which you want to connect should indicate the standard for that device.
Figure 5-4 DTE and DCE
Your CPE, in this case a router, is the DTE. The data DCE, commonly a modem or a DSU/ CSU, is the device that converts the user data from the DTE into a form acceptable to the WAN service provider. The synchronous serial port on the router is configured as DTE or DCE (except EIA/TIA-530, which is DTE only) depending on the attached cable, which is ordered as either DTE or DCE to match the router configuration. If the port is configured as DTE (the default setting), it requires external clocking from the DCE device. Figure 5-5 shows the variety of WAN connectors that can be used on a Cisco router.
Figure 5-5 WAN Cables
NOTE To support higher densities in a smaller form factor, Cisco introduced a smart serial cable. The serial end of the smart serial cable is a 26-pin connector. It is much smaller than the DB-60 connector that connects to a five-in-one serial port. These transition cables support the same five serial standards, are available in either DTE or DCE configuration, and are used with two-port serial connections and two-port asynchronous and synchronous WICs.
The Role of Routers in WANs
An enterprise WAN is actually a collection of separate but connected LANs, and routers play a central role in transmitting data through this interconnected network. Routers have both LAN and WAN interfaces, and whereas a router segments LANs, it is also used as the WAN access connection device. The functions and role of a router in accessing the WAN can be best understood by looking at the types of connections available on the router.
Three basic types of connections on a router exist: LAN interfaces, WAN interfaces, and management ports. LAN interfaces enable the router to connect to the LAN media through Ethernet or some other LAN technology, such as Token Ring or ATM.
WAN connections are made through a WAN interface on a router to a service provider to a distant site or to the Internet. These might be serial connections or any number of other WAN interfaces. With some types of WAN interfaces, an external device such as a DSU/ CSU or modem (such as an analog modem, cable modem, or DSL modem) is required to connect the router to the local point of presence (POP) of the service provider. The physical demarcation point is the place where the responsibility for the connection changes from the user to the service provider. This is important because when problems arise, both sides of the link need to prove that the problem either resides with them or not.
Figure 5-6 illustrates how a router interconnects different LANs through a WAN segment. The management ports provide a text-based connection that allows for configuration and troubleshooting of a router. The common management interfaces are the console and auxiliary ports. These ports are connected to a communications port on a computer. The computer must run a terminal emulation program to provide a text-based session with the router, which enables you to manage the device.
Figure 5-6 Routers Use the WAN to Connect Remote LANs
WAN Data Link Layer Protocols
In addition to physical layer devices, WANs require data link layer protocols to establish the link across the communication line from the sending to the receiving device.
Data link layer protocols define how data is encapsulated for transmission to remote sites and the mechanisms for transferring the resulting frames. A variety of different technologies, such as ISDN, Frame Relay, or ATM, are used. Many of these protocols use the same basic framing mechanism, HDLC, an ISO standard, or one of its subsets or variants. ATM is the most different because it uses small, fixed-size cells of 53 bytes (48 bytes for data).
The WAN data link layer protocols are as follows:
- Frame Relay (Link Access Procedure for Frame Relay [LAPF])
WAN Communication Link Options
WANs are accessed in a number of ways, depending on the data transmission requirements for the WAN. Figure 5-7 maps out the different WAN connectivity options.
Figure 5-7 WAN Connectivity Options
Two major categories of communication links for WANs exist: dedicated and switched. Within each category, individual types of communication link options exist, as follows:
- Dedicated communication links: When permanent dedicated connections are required, point-to-point lines are used with various capacities that are limited only by the underlying physical facilities and the willingness of users to pay for these dedicated lines. A point-to-point link provides a pre-established WAN communications path from the customer premises through the provider network to a remote destination. Point-to-point lines are usually leased from a carrier and are also called leased lines.
- Circuit-switched communication links: Circuit switching dynamically establishes a dedicated virtual connection for voice or data between a sender and a receiver. Before communication can start, you need to establish the connection through the network of the service provider.
- Packet-switched communication links: Many WAN users do not make efficient use of the fixed bandwidth that is available with dedicated, switched, or permanent circuits because the data flow fluctuates. Communications providers have data networks available to more appropriately service these users. In packet-switched networks, the data is transmitted in labeled cells, frames, or packets.