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Cisco 819 Series Integrated Services Routers Software Configuration Guide
OL-23590-02
Chapter 8 Configuring the Serial Interface
How to Configure Serial Interfaces
4. The DCE transitions to the wait DCD drop delay state. This state causes a time delay between the
transmission of the last frame and the deassertion of DCD in the controlled-carrier mode for DCE
transmits.
5. When the timer expires, the DCE deasserts DCD and transitions back to the ready state and stays
there until there is a frame to transmit on that interface.
As shown in Figure 6, the half-duplex DCE receive state machine idles in the ready state when it is
quiescent. It transitions out of this state when the DTE asserts RTS. In response, the DCE starts a timer
based on the value specified using the cts-delay command. This timer delays the assertion of CTS
because some DTE interfaces expect this delay. (The default value of this timer is 0 ms; use the
half-duplex timer cts-delay interface configuration command to specify a delay value.)
Figure 6 Half-Duplex DCE Receive State Machine
When the timer expires, the DCE state machine asserts CTS and transitions to the receive state. It stays
in the receive state until there is a frame to receive. If the beginning of a giant frame is received, it
transitions to the in giant state and keeps discarding all the fragments of the giant frame and transitions
back to the receive state.
Transitions back to the ready state occur when RTS is deasserted by the DTE. The response of the DCE
to the deassertion of RTS is to deassert CTS and go back to the ready state.
Placing a Low-Speed Serial Interface in Constant-Carrier Mode
To return a low-speed serial interface to constant-carrier mode from controlled-carrier mode, use the
following command in interface configuration mode.
SUMMARY STEPS
1. no half-duplex controlled-carrier
DETAILED STEPS
Command or Action Purpose
Step 1
no half-duplex controlled-carrier
Example:
Router(config-if)# no half-duplex controlled-carrier
Places a low-speed serial interface in
constant-carrier mode.