Alcatel Carrier Internetworking Solutions Switch/Router 用户手册

Page 29-4

The 

WSX

 automatically senses the cable type that you plug into one of its universal serial 

ports. It can sense whether the cable type is 

DCE

 or 

DTE

 and whether it is one of the follow-

ing interfaces:

• RS-232

• RS-449

• RS-530

• V.35

• X.21 (European)

All cable types, except 

RS-232

, are capable of access rates from 9.6 Kbps to 2 Mbps. The RS-

232 cable is not compatible with speeds greater than 64 Kbps. Each cable type is illustrated 
and described in Appendix D, “Custom Cables.”

The 

WSX

 serial port is normally considered a physical 

DTE

 device. It is possible to turn it into 

a physical 

DCE

 device simply by plugging in a 

DCE

 cable. The 

WSX

 board internally senses 

whether a 

DCE

 or 

DTE

 cable is connected.

The RS-232 protocol, which is employed at the physical level for all cable types, always 
defines Transmit and Receive pins in relation to the 

DTE

. So, the type of cable you attach 

(

DCE

 or 

DTE

) determines the direction of data flow on your connector’s Transmit and Receive 

pins.

If the 

WSX

 serial port is a physical 

DTE

, which is probably the most common configuration, 

then data is received on Receive pins and transmitted on Transmit pins. If you are using a 

WSX

 port as a physical 

DCE

, then data is transmitted on the Receive pins and received on the 

Transmit pins.

This chapter refers to “physical” and “logical” 

DTE

 (Data Terminal Equipment) and 

DCE

 (Data 

Communication Equipment) devices. A physical device operates on the network layer, and is 
normally an actual piece of hardware, such as a 

WSX

 or 

CSU/DSU

. Physical devices may 

further be differentiated as 

DTE

 and 

DCE

 devices. A physical 

DTE

 device would be a piece of 

hardware, such as a 

WSX

, that does not control the access rate for virtual circuits. The physi-

cal 

DTE

 device is a conduit for data traffic but not a controller of data traffic. A physical 

DCE

 

device is hardware, such as a 

CSU/DSU

, that does control access rates of Frame Relay traffic. 

Normally physical 

DTE

 and 

DCE

 devices are directly connected to one another.

Logical devices operate on the Frame Relay protocol layer, and are sometimes referred to as 
“Frame Relay logical” devices. Logical devices can also be broken down into 

DTE

 and 

DCE

 

devices. Logical 

DTE

 devices, again like the 

WSX

, do not have direct control over the Frame 

Relay network and the various congestion and control parameters that govern it. Logical 

DTE

 

devices do not control such actions as bringing up and tearing down virtual circuits; they act 
upon updates and commands generated by the Frame Relay network. Logical 

DCE

 devices, 

such as a Frame Relay switch, have a large span of control over Frame Relay network traffic. 
They bring up and tear down virtual circuits, set congestion control bits in packets, and 
communicate status to logical 

DTE

 devices.