A serial connection has a data terminal equipment (DTE) device at one end of the connection and a data communications equipment (DCE) device at the other end. The connection between the two DCEs is the WAN service provider transmission network. The CPE, which is generally a router, is the DTE. Other DTE examples could be a terminal, computer, printer, or fax machine. The DCE, commonly a modem or CSU/DSU, is the device used to convert the user data from the DTE into a form acceptable to the WAN service provider transmission link. This signal is received at the remote DCE, which decodes the signal back into a sequence of bits. This sequence is then signaled to the remote DTE.

Many standards have been developed to allow DTEs to communicate with DCEs. The Electronics Industry Association (EIA) and the International Telecommunication Union Telecommunications Standardization Sector (ITU-T) have been most active in the development of these standards.

The DTE-DCE interface for a particular standard defines the following specifications:

• Mechanical/physical – Number of pins and connector type
• Electrical – Defines voltage levels for 0 and 1
• Functional – Specifies the functions that are performed by assigning meanings to each of the signaling lines in the interface
• Procedural – Specifies the sequence of events for transmitting data

If two DTEs must be connected together, like two computers or two routers in the lab, a special cable called a null-modem is necessary to eliminate the need for a DCE. For synchronous connections, where a clock signal is needed, either an external device or one of the DTEs must generate the clock signal.

The synchronous serial port on a router is configured as DTE or DCE depending on the attached cable, which is ordered as either DTE or DCE to match the router configuration. If the port is configured as DTE, which is the default setting, external clocking is required from the CSU/DSU or other DCE device.

The cable for the DTE to DCE connection is a shielded serial transition cable. The router end of the shielded serial transition cable may be a DB-60 connector, which connects to the DB-60 port on a serial WAN interface card. The other end of the serial transition cable is available with the connector appropriate for the standard that is to be used. The WAN provider or the CSU/DSU usually dictates this cable type. Cisco devices support the EIA/TIA-232, EIA/TIA-449, V.35, X.21, and EIA/TIA-530 serial standards.

To support higher densities in a smaller form factor, Cisco has introduced a smart serial cable. The serial end of the smart serial cable is a 26-pin connector significantly more compact than the DB-60 connector.   

Interactive Media Activity PhotoZoom: DCE/DTE Cable In this PhotoZoom, the student will view DCE and DTE cable.

Web Links High-Speed Serial Interface (HSSI) http://www.cisco.com/univercd/cc/ td/doc/ cisintwk/ito_doc/ hssi.htm