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DTE-to-DTE Wiring
This section examines the DTE-to-DTE wiring functions.
When the DTE-to-DTE Devices Are in the Same Vicinity
If two DTE devices such as a terminal and an access server are located close to one another, it makes more sense to link them back to back instead of using a telephone network and two DCEs. A regular EIA/TIA-232 cable cannot be used for such DTE-to-DTE links, because both DTE devices send on TxD pin 2 and receive on RxD pin 3. In such instances, a null modem can accomplish direct DTE-to-DTE connections. With null modems, pins 2 and 3 are crisscrossed, as well as other corresponding pins of the DB-25 connector, and thus allow the DTEs to communicate with one another.
Alternatively, you can configure some devices, such as serial printers, to act as either a DTE or a DCE. If a device is configured as a DCE, it transmits data on pin 3 and receives data on pin 2. Such configuration forgoes a null modem connection and allows a DTE (such as a PC or server) to be directly connected to a printer with a regular EIA/TIA-232 cable.
RJ-45 Wiring and Cables
RJ-45 connectors are used for the following ports:
Console
Asynchronous
Auxiliary
No standards define RJ-45 interface pinouts, but Cisco defines them as DTE. If you were to cable the access server port (RJ-45) to an external device (modem or terminal), you would need RJ-45-to-RJ-45 cable and an RJ-45-to-DB-25 adapter. An RJ-45-to-RJ-45 cable can be rollover or straight-through. A rollover cable has its pins reversed, as in 1 to 8, 2 to 7, and so on. A straight-through cable, on the other hand, has the pins going straight in a 1 to 1, 2 to 2 fashion.
To find out which of the two types of cable you have, hold the two connector ends of the same cable side by side. Check the color-coded wires inside the connector. Straight-through cable wires are the same color for the same pins on both connectors. A rolled cable has the wire colors on the two connectors flipped, as shown in Figure 3-1.
Figure 3-1. Identifying Rollover Cable
The octal cable used for the asynchronous port connections functions as a rolled cable.
An RJ-45-to-DB-25 adapter can be either rollover or straight-through. For instance, a male or female DTE adapter (MDTE or FDTE) is straight-through. A male or female DCE adapter (MDCE or FDCE) is rolled. A male modem (MMOD) adapter is rolled and is the only one that supports modems. In it, the MDCE connectors are changed so that DB-25 pin 8 instead of pin 6 is wired to DSR.
The auxiliary and console ports are configured as DTE devices on Cisco access servers. Terminals (such as PCs) are also DTE devices. Two DTE devices cannot be directly connected unless the signals are rolled exactly once. So you must either roll the pins in the cable or in the DB-25 adapter, but not both. To directly connect two DTE devices, you can use either of these formulas:
DTE + rolled RJ-45 cable + straight DB-25 adapter + DTE
DTE + straight RJ-45 cable + rolled DB-25 adapter + DTE
DTE-to-DCE Wiring
A DTE-to-DCE connection should not have rolls. The same effect can be achieved with having two rolls and the connector. Cisco routers come with a kit for console and auxiliary port cabling. The kit includes the following:
RJ-45-to-RJ-45 rollover cable
RJ-45-to-DB-9 female DTE adapter (labeled "TERMINAL")
RJ-45-to-DB-25 female DTE adapter (labeled "TERMINAL")
RJ-45-to-DB-25 male DCE adapter (labeled "MODEM")
The RJ-45-to-DB-9 female DTE adapter is typically used to connect a PC being used as a console terminal. The RJ-45-to-DB-25 female DTE adapter is used to connect a terminal to the console or auxiliary port. The RJ-45-to-DB-25 male DCE adapter is used to connect the auxiliary port to a modem. Table 3-2 describes the port types for console and auxiliary ports on Cisco routers.
Routers | ||
|---|---|---|
DB-25 | RJ-45 | |
DCE | DTE | |
Auxiliary port | DTE | DTE |
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