2.1 Modem Functions
2.1.5 Communication wiring and cabling
Depending on the type of connections, the cable used can be straight-through, crossover, or rolled; and it can have a DB-25 or RJ-45 connector.

DTE-to-DTE Wiring—Null Modem
When two DTE devices (for example, an access server and a terminal) are near each other, it makes sense to connect them directly without going through a telephone network and two modems. An ordinary EIA/TIA-232 cable does not work in this case because both DTE devices transmit on the TxD lead (pin 2), and both expect input on the RxD lead (pin 3). A null modem cable is required for the DTE-to-DTE connection.

Null modems crisscross DB-25 pins 2, 3, and other corresponding pins so that the two DTE devices can communicate, as shown in Figure . Some devices can be configured to operate either like a DTE or a DCE. Configuring a device as a DCE usually means that it receives data on pin 2 and transmits data on pin 3.

For example, many serial printers are configured as DCE devices so they can be connected directly to a DTE (for example, a PC or a terminal server) with an ordinary EIA/TIA-232 cable, eliminating the need for a null modem connection. On a null modem cable, as seen in Figure , the CD signal is on the same pin as DSR.

RJ-45 Wiring
Cisco uses RJ-45 ports and connectors for console, auxiliary, and asynchronous port connections. The specific pinout to be used on a RJ-45 interface for EIA-232 is not defined by any standards. As such, Cisco defines the RJ-45 as a DTE pinout, as shown in Figure .

Cabling from the access server port (RJ-45) to an external device (such as a modem or terminal) requires the use of two cabling components:

  • RJ-45 to RJ-45 cable—This can be either a rolled cable (reverse pins 1-8, 2-7, 3-6, 4-5) or a straight-through cable (1-1, 2-2, and so forth). To check whether a cable is straight or rolled, hold the two connectors (the two ends of the cable) side by side-with the keys at the back and the pins up-and compare them by inspecting the color-coded wires inside the connector. If the wires use the same colors on the same pins, it is a straight cable; if the wires are a mirror image of each other, it is a rolled cable. 
  • RJ-45 to DB-25 adapter—This can also be straight-through or rolled. A male or female DTE adapter (MDTE or FDTE) is straight. A male or female DCE adapter (MDCE or FDCE) is rolled. A male modem (MMOD) adapter is rolled. Only an MMOD adapter supports modems (modified from MDCE connectors by wiring DB-25 pin 8 to DSR, instead of pin 6).
Web Links
Cabling Guide for RJ-45 Console and AUX Ports