Serial Ports on Cisco Routers

A Cisco 2501 includes two synchronous DB-60 serial ports, which are used for the purpose of WAN connections. Synchronous communication relies on the use of timing in order to control (or synchronize) the transmission. Before synchronous systems communicate, they agree on parameters such as the time interval that will be used between sending bits of data.

A router is referred to as Data Terminal Equipment, or DTE. When DTE devices connect to a data network such as a service provider WAN link, they usually require an external device to handle their serial transmission timing, also referred to as clocking. In order to accomplish this, they connect to devices referred to as Data Communications Equipment (DCE). The function of the DCE is to provide access to the actual data network and provide clock synchronization. Two common examples of DCE equipment are modems and Channel Service Units / Data Service Units (CSU/DSU).

A CSU/DSU looks somewhat similar to an external modem, and it many ways, performs similar functions. The exception is that a modem converts digital signals to analog, while a CSU/DSU uses digital signals throughout. The DSU portion is responsible for timing, and actually connects to the DTE (in this case the router) via its serial port. The CSU portion is responsible for terminating the service provider’s link, and handles transmitting and receiving data over the WAN link. The figure below shows the connection between a Cisco 2501, CSU/DSU (which is assumed to have an EIA/TIA-232 interface), and a service provider’s data network.

Figure: CSU/DSU connection to a serial port on a Cisco 2501.

A variety of Physical Layer standards are supported over synchronous serial interfaces to connect to different types of DCE equipment. Some of the different signaling standards and connectors that might be found on DCE equipment include EIA/TIA-232, EIA/TIA-449, V.35, X.21, and EIA-530. Cisco and a variety of other vendors manufacture “transition” cables capable of connecting a router’s DB-60 DTE port to DCE equipment using these different standards. The list below outlines each standard, and for reference purposes provides information on the associated connector and connection properties.

Standard: EIA/TIA-232
Connector: DB-25
Properties: Formerly known as RS-232, this standard supports transmissions of up to 64kbps

Standard: EIA/TIA-449
Connector: DB-37
Properties: A faster version of the EIA/TIA-232 standard, not widely adopted. Speeds up to 2Mbps.

Standard: V.35
Connector: 34-pin
Properties: Speeds between 48kbps and 4 Mbps

Standard: X.21
Connector: 15-pin
Properties: Commonly used in the UK to connect to the public data network. Common speed of 64kbps, up to 2Mbps

Standard: EIA-530
Connector: DB-25
Properties: Similar to EIA/TIA-449, but uses a smaller DB-25 connector. Speeds up to 4Mbps possible.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.