Serial cables are most commonly used for Wide Area Network (WAN) connections and are available in a wide variety of shapes, sizes and specifications depending your router’s interface specs and connection type. For instance, DB60 cables are most widely used for home networks and beginning network professionals who are practicing on their own equipment. It is suitable for basic connections that require a Data Terminal Equipment (DTE) end for customer equipment and Data Circuit-terminating Equipment (DCE) end that determines the speed of the connection.
Serial ports on Cisco devices use a 60-pin connector or smaller “smart serial” device that is proprietary to Cisco. Two WAN serial connection standards are EIA/TIA-232 and EIA/TIA-449.
For serial connections, the Data Terminal Equipment (DTE) connects to the end user’s device on a WAN link and the Data Circuit-terminating Equipment (DCE) connects to the service provider. In some conditions, such as a test environment, the DCE will connect to another router in a back-to-back router configuration, also called serial connectivity. Some specially designed cables have both DTE and DCE connectors for back-to-back routing. The serial connectivity can be set up using either fixed or modular ports, which affects the configuration you use when choosing settings for the IOS. Fixed serial ports usually have a label such as serial 0. Modular ports might have a label such as serial 1/0, which indicates that the module is installed in Slot 1 and the module’s port is Port 0.
ISDN BRI Connections
Cables run from an ISDN BRI port should only be connected to devices equipped for ISDN, such as an ISDN switch, because they tend to run at higher voltages and can damage devices that aren’t equipped for ISDN. When working with ISDN BRI, you should first determine whether you or the service provider provides a Network Termination (NT1) device, which provides an intermediate connection between the router and the service provider’s cloud. The “cloud” is an ISDN switch that acts as an intermediary between the four-wire subscriber wiring and the two-wire local loop. This determines whether you should use a BRI S/T or a BRI U interface.
In North America, the customer usually provides the NT1 switch, while the service provider will usually provide the switch in the rest of the world. If the NT1 device is integrated in your router, the BRI connection is labeled as BRI U. If not, the BRI connection is labeled as BRI S/T. When you purchase a router, the port labeling should specify the type of ISDN port. Routers can have more than one type of ISDN interface.
Digital Subscriber Lines (DSL) provide WAN connectivity over telephone lines and require phone cables with RJ-11 connectors, which are slightly smaller than RJ-45 connectors and possess only two pins. The RJ-11 connector fits into a typical phone jack. The other end of the cable should be connected to the Asymmetric Digital Subscriber Line (ADSL) port on the router. A typical router for DSL connections would be the Cisco 827 ADSL router, which has one ADSL port.
Cable connections are designed to run over the same line as cable television. A typical cable connection will run over a router like the Cisco uBR905 cable access router, which uses an F-connector coaxial cable interface to link with the cable system. A cable splitter or directional coupler and a high-pass filter is recommended to prevent the television and Internet signals from interfering with each other.
Asynchronous router connections are used for managing the router. These connections are made by connecting a rollover cable to the console port and configuring terminal emulation software with the COM settings, 9600 bps, 8 data bits, no parity, 1 stop bit, no flow control. If the laptop you use to manage routers and switches does not have a 25- or 9-pin serial connector, you may need to obtain a USB-to-DB-9 converter cable.
The AUX port on a router is an asynchronous port that can be used to provide out-of-band management that doesn’t use the network bandwidth. A modem is usually used to make the connection, and then dialing the modem accesses the AUX port and the executive process Command Line Interface (CLI).