X21 Eia530

Network Connections at the CSU/DSU

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

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

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.

Summary of Understanding WAN Technologies

The following list summarizes the key points that were discussed in this lesson.

■ A WAN has three major characteristics: the connection of devices that are separated by wide geographical distances; the use of the services of carriers, such as telephone companies, cable companies, satellite systems, and network providers; and the use of serial connections of various types to access bandwidth over large geographic areas.

■ Many business and home needs require communication among remote users, including communication between users in remote company locations, data sharing among different organizations, access to corporate information by traveling workers, and access to Internet.

■ LANs connect computers, peripherals, and other devices in a single building or other small geographic area; WANs transmit data across broad geographic distances.

■ A company, organization, or individual must subscribe to an outside WAN service provider to use WAN network services, whereas LANs are owned typically by the company, organization, or individual that uses them.

■ WAN access functions in relation to the OSI reference model; the WAN function focuses primarily on Layer 1 and Layer 2.

■ The major types of devices used for WAN access environments include routers, communication servers, modems (DSU/CSUs).

■ Routers have both LAN and WAN interfaces, and whereas a router segments LANs, it is also used as the WAN connection device.

■ The data link layer protocols define how data is encapsulated for transmission toward remote sites in a WAN environment and the mechanisms for transferring the resulting frames.

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