Alcatel Omni Switch/Router, Руководство пользователя

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Universal Serial Port Cable Interfaces
Page 29-4
Universal Serial Port Cable Interfaces
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.
DTE/DCE Type and Transmit/Receive Pins
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.
“Physical” and “Logical” Devices
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.