Clock Rates DCE and DTE

When a network engineer needs to add a point-to-point leased line between two routers, he contacts a service provider and orders the circuit. As part of that process, the customer specifies how fast the circuit should run, in kilobits per second (kbps). While the circuit is being set up by the telco, the engineer purchases two CSU/DSUs, installs one at each site, and configures each CSU/DSU. He also cables each router to the respective CSU/DSU using the cables shown in the previous section. Eventually, the telco installs the new line into the customer premises, and the line can be connected to the CSU/DSUs, as shown in Figure 4-3. (Note: In some countries, the telco owns the CSU/DSU, so it orders, installs, and configures the CSU/DSUs.)

The terms clock rate and bandwidth both refer to the speed of the circuit. You will also hear the speed referred to as the link speed. When you order a circuit that runs at a particular speed, the two CSU/DSUs are configured to operate at that same speed. The CSU/DSUs provide a clocking signal to the routers so that the routers simply react, sending and receiving data at the correct rate. So, the CSU/DSU is considered to be clocking the link.

A couple of other key WAN terms relate to the process of clocking. The device that provides clocking, typically the CSU, is considered to be the data communications equipment (DCE). The device receiving clocking, typically the router, is referred to as data terminal equipment (DTE).

On a practical note, when purchasing serial cables from Cisco, you can pick either a DTE or a DCE cable. You pick the type of cable based on whether the router is acting like a DTE or a DCE. If the router is a DTE, with the CSU providing the clocking, you need a DTE cable. If the router was clocking the CSU/DSU, which can be done, you would need a DCE cable— but that almost never happens.

However, DCE cables do have an important practical use. When building a lab to study for any of the Cisco exams, you do not need to buy DSU/CSUs. You can buy two routers, a DTE serial cable for one router, and a DCE serial cable for the other and connect the two cables together. The router with the DCE cable in it can be configured to provide clocking— meaning that you do not need a CSU/DSU. So, you can build a WAN in your home lab, saving hundreds of dollars by not buying CSU/DSUs. The DTE and DCE cables can be connected to each other and to the two routers. (The DCE cable has a female connector, and the DTE has a male connector, so they can be connected.) With one additional configuration command on one of the routers (the clock rate command), you have a point-to-point serial link. This type of connection between two routers sometimes is called a back-to-back serial connection.

Figure 4-5 shows the cabling for a back-to-back serial connection and also shows that the combined DCE/DTE cables reverse the transmit and receive pins, much like a crossover Ethernet cable allows two directly connected devices to communicate.

Figure 4-5 Serial Cabling Uses a DTE and a DCE Cable

Figure 4-5 Serial Cabling Uses a DTE and a DCE Cable

DTE Cable DCE Cable

As you see in the figure, the DTE cable, the same cable that you typically use to connect to a CSU/DSU, does not swap the Tx and Rx pins. The DCE cable swaps transmit and receive, so the wiring with one router's Tx pin connected to the other router's Rx, and vice versa, remains intact.

+1 0

Post a comment