Cisco Certified Expert

WAN Cabling Standards

Cisco offers a large variety of different WAN interface cards for its routers, including synchronous and asynchronous serial interfaces. For any of the point-to-point serial links or Frame Relay links in this chapter, the router uses an interface that supports synchronous communication.

Synchronous serial interfaces in Cisco routers use a variety of proprietary physcial connector types, such as the 60-pin D-shell connector shown in Figure 4-4. The cable connecting the router to the CSU uses a connector that fits the router serial interface on the router side, and a standardized WAN connector type that matches the CSU/DSU interface on the CSU/DSU end of the cable. Figure 4-4 shows a typical connection, with some of the serial cabling options listed.

Figure 4-4 Serial Cabling Options

End User

Router Connections

CSU/ DSU

CSU/ DSU

EIA/TIA-232 EIA/TIA-449 V.35 X.21 EIA-530 Network Connections at the CSU/DSU

The engineer who deploys a network chooses the cable based on the connectors on the router and the CSU/DSU. Beyond that choice, engineers do not really need to think about how the cabling and pins work—they just work! Many of the pins are used for control functions, and a few are used for the transmission of data. Some pins are used for clocking, as described in the next section. Table 4-2 summarizes the variety of standards that define the types of connectors and physical signaling protocols used on WAN interfaces.

NOTE The Telecommunications Industry Association (TIA) is accredited by the American National Standards Institute (ANSI) for the development of telecommunications standards. ANSI has the rights by U.S. federal law to represent the United States in work with international standards bodies, such as the International Telecommunicationss Union (ITU). For more information on these standards bodies, and for the opportunity to spend money to get copies of the standards, refer to the web sites www.tiaonline.org and www.itu.int.

These cables provide connectivity to the external DSU/CSU, as shown in Figure 4-4. The cable between the CSU/DSU and the telco CO typically uses an RJ-48 connector to connect to the CSU/DSU; the RJ-48 connector has the same size and shape as the RJ-45 connector used for Ethernet cables.

The cables and physical connector types each have differing limits on the speed of serial data transmission. Generally, the shorter the length of the cable is, the closer it can get to the maximum speed allowed for that cable and connector. From a practical perspective, this just means that you typically locate the CSU/DSU relatively close to the routers so that the cables can be kept short. Table 4-3 lists the speeds that can be used for certain cables and connectors, based on the lengths of the cables.

Many Cisco routers support serial interfaces that have an integrated DSU/CSU. With an internal CSU/DSU, the router does not need a cable connecting it to the CSU/DSU because the CSU/DSU is internal to the router. The line from the telco is connected to a receptacle on the router, typically an RJ-48 receptacle, in the router serial interface card.