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The synchronous serial port adapters (PA-8T-V35, PA-8T-X21, PA-8T-232, and PA-4T+) on Cisco 7200 series routers support half-duplex and binary synchronous communications (Bisync). Bisync is a character-oriented data link layer protocol for half-duplex applications. In half-duplex mode, data is sent one direction at a time. Direction is controlled by handshaking the RST and CTS control lines.
The following sections describe how to configure Bisync and half-duplex features on Cisco 7200 series routers.
To configure the Bisync feature on the synchronous serial port adapters (PA-8T-V35, PA-8T-X21, PA-8T-232, and PA-4T+) on Cisco 7200 series routers, refer to the "Block Serial Tunnelling (BSTUN)" section of the "Configuring Serial Tunnel (STUN) and Block Serial Tunnel (BSTUN)" chapter of the Bridging and IBM Networking Configuration Guide. All commands listed in the "Block Serial Tunnelling (BSTUN)" section apply to the synchronous serial port adapters on Cisco 7200 series routers. The only change is that any command syntax that specifies an interface number now supports the Cisco 7200 series slot/port syntax.
This section describes how to configure the synchronous serial port adapters (PA-8T-V35, PA-8T-X21, PA-8T-232, and PA-4T+) on Cisco 7200 series routers. To configure the half-duplex feature on synchronous serial port adapters, perform the tasks described in the following sections:
The following section describes the communication between half-duplex DTE transmit and receive state machines and half-duplex DCE transmit and receive state machines.
As shown in Figure 13, the half-duplex DTE transmit state machine remains in the ready state when it is quiescent. When a frame is available for transmission, the state machine transitions to the transmit delay state and waits for a time period, which is defined by the half-duplex timer transmit-delay command. The default is 0 ms. Transmission delays are used for debugging half-duplex links and assisting lower speed receivers that cannot process back-to-back frames.
After idling for a defined number of ms, the state machine asserts request to send (RTS) and transitions to the wait clear to send (CTS) state for the data communications equipment (DCE) to assert CTS. A timeout timer with a value set by the half-duplex timer rts-timeout command starts. This default is 3 ms. If the timeout timer expires before CTS is asserted, the state machine returns to the ready state and de-asserts RTS. If CTS is asserted prior to the timer's expiration, the state machine transitions to the transmit state and sends the frames.
When there are no more frames to transmit, the state machine transitions to the wait transmit finish state. The machine waits for the transmit first in first out (FIFO) in the serial controller to empty, starts a delay timer with a value defined by the half-duplex timer rts-drop-delay interface command, and transitions to the wait RTS drop delay state.
When the timer in the wait RTS drop delay state expires, the state machine de-asserts RTS and transitions to the wait CTS drop state. A timeout timer with a value set by the half-duplex timer cts-drop-timeout interface command starts, and the state machine waits for the CTS to de-assert. The default is 250 ms. When the CTS signal is de-asserted or the timeout timer expires, the state machine transitions back to the ready state. If the timer expires before CTS is de-asserted, an error counter is incremented, which can be displayed by issuing the show controllers command for the serial interface in question.
As shown in Figure 14, a half-duplex DTE receive state machine idles and receives frames in the ready state. A giant frame is any frame whose size exceeds the maximum transmission unit (MTU). If the beginning of a giant frame is received, the state machine transitions to the in giant state and discards frame fragments until it receives the end of the giant frame. At this point, the state machine transitions back to the ready state and waits for the next frame to arrive.
An error counter is incremented upon receipt of the giant frames. To view the error counter, enter the show interface command for the serial interface in question.
As shown in Figure 15, for a serial interface in DCE mode, the half-duplex DCE transmit state machine idles in the ready state when it is quiescent. When a frame is available for transmission on the serial interface, such as when the output queues are no longer empty, the state machine starts a timer (based on the value of the transmit-delay command, in milliseconds) and transitions to the transmit delay state. Similar to the DTE transmit state machine, the transmit delay state gives you the option of setting a delay between the transmission of frames; for example, this feature lets you compensate for a slow receiver that loses data when multiple frames are received in quick succession. The default transmit-delay value is 0 ms; use the half-duplex timer transmit-delay interface configuration command to specify a delay value not equal to 0.
After the transmit delay state, the next state depends on whether the interface is in constant-carrier mode (the default) or controlled-carrier mode.
If the interface is in constant-carrier mode, it passes through the following states:
If the interface is in controlled-carrier mode, the interface performs a handshake using the data carrier detect (DCD) signal. In this mode, DCD is de-asserted when the interface is idle and has nothing to transmit. The transmit state machine transitions through the states as follows:
As shown in Figure 16, 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 with the value cts-delay. 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.)
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 de-asserted by the DTE. The response of the DCE to the de-assertion of RTS is to de-assert CTS and go back to the ready state.
The half-duplex controlled-carrier command allows you to change between controlled-carrier and constant-carrier modes for serial DCE interfaces in half-duplex mode. Configure a serial interface for half-duplex mode by using the half-duplex command. Full-duplex mode is the default for serial interfaces. This interface configuration is available on Cisco 2520 through Cisco 2523 routers.
Controlled-carrier operation means that the DCE interface will have DCD de-asserted in the quiescent state. When the interface has something to transmit, it will assert DCD, wait a user-configured amount of time, then start the transmission. When it has finished transmitting, it will again wait a user-configured amount of time, then de-assert DCD.
To place a serial interface in controlled-carrier mode, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Place a low-speed serial interface in controlled-carrier mode. | half-duplex controlled-carrier |
To return a serial interface to constant-carrier mode from controlled-carrier mode, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Place a low-speed serial interface in constant-carrier mode. | no half-duplex controlled-carrier |
To tune half-duplex timers, perform the following task in interface configuration mode:
The timer tuning commands permit you to adjust the timing of the half-duplex state machines to suit the particular needs of their half-duplex installation.
For examples of configuring the Bisync feature on the synchronous serial port adapters (PA-8T-V35, PA-8T-X21, PA-8T-232, and PA-4T+) on Cisco 7200 series routers, refer to the "BSTUN Configuration Example" section of the "Configuring Serial Tunnel (STUN) and Block Serial Tunnel (BSTUN)" chapter of the Bridging and IBM Networking Configuration Guide.
The following example shows how to set the RTS timeout and CTS delay timers to 10 ms on serial interface 1 in slot 3 of a Cisco 7200 series router. The RST timeout controls how long the DTE waits for CTS to be asserted after the assertion of RTS before giving up on its transmission attempt. The CTS delay specifies the delay introduced by the DCE interface between the time it detects RTS to the time it asserts CTS in response.
Router#configure terminalRouter(config)#interface serial 3/1Router(config-if)#half-duplex timer rts-timeout 10Router(config-if)#half-duplex timer cts-delay 10Router(config-if)#exitRouter(config)#exit
This section documents the half-duplex timer command. All other commands used with this feature are documented in the Cisco IOS Release 11.2 command references.
To tune half-duplex timers, use the half-duplex timer interface configuration command. The half-duplex timer cts-delay command replaces the sdlc cts-delay command. The half-duplex timer rts-timeout command replaces the sdlc rts-timeout command. Use the no form of this command, along with the appropriate keyword, to return to the default value for that parameter.
You can configure more than one of these options, but each option must be specified as a separate command.
half-duplex timer {cts-delay value | cts-drop-timeout value | dcd-drop-delay value | dcd-txstart-delay value | rts-drop-delay value | rts-timeout value | transmit-delay value} no half-duplex timer {cts-delay value | cts-drop-timeout value | dcd-drop-delay value | dcd-txstart-delay value | rts-drop-delay value | rts-timeout value | transmit-delay value}| cts-delay value | Specifies the delay introduced by the DCE interface between the time it detects RTS to the time it asserts CTS in response. The range is dependent on the serial interface hardware. The default value is 0 ms. |
| cts-drop-timeout value | Determines the amount of time a DTE interface waits for CTS to be de-asserted after it has de-asserted RTS. If CTS is not de-asserted during this time, an error counter is incremented to note this event. The range is 0 to 1140000 ms (1140 seconds). The default value is 250 ms. |
| dcd-drop-delay value | Applies to DCE half-duplex interfaces operating in controlled-carrier mode (see the half-duplex controlled-carrier command). This timer determines the delay between the end of transmission by the DCE and the de-assertion of DCD. The range is 0 to 4400 ms (4.4 seconds). The default value is 100 ms. |
| dcd-txstart-delay value | Applies to DCE half-duplex interfaces operating in controlled-carrier mode. This timer determines the time delay between the assertion of DCD and the start of data transmission by the DCE interface. The range is 0 to 1140000 ms (1140 seconds). The default value is 100 ms. |
| rts-drop-delay value | Specifies the time delay between the end of transmission by the DTE interface and de-assertion of RTS. The range is 0 to 1140000 ms (1140 seconds). The default value is 3 ms. |
| rts-timeout value | Determines the number of ms the DTE waits for CTS to be asserted after the assertion of RTS before giving up on its transmission attempt. If CTS is not asserted in the specified amount of time, an error counter is incremented. The range is dependent on the serial interface hardware. The default value is 3 ms. |
| transmit-delay value | Specifies the number of ms a half-duplex interface will delay the start of transmission. In the case of a DTE interface, this delay specifies how long the interface waits after something shows up in the transmit queue before asserting RTS. For a DCE interface, this dictates how long the interface waits after data is placed in the transmit queue before starting transmission. If the DCE interface is in controlled-carrier mode, this delay shows up as a delayed assertion of DCD.
This timer enables the transmitter to be adjusted if the receiver is a little slow and is not able to keep up with the transmitter. The range is 0 to 4400 ms (4.4 seconds). The default value is 0 ms. |
The default cts-delay value is 0 ms.
The default cts-drop-timeout value is 250 ms.
The default dcd-drop-delay value is 100 ms.
The default dcd-txstart-delay value is 100 ms.
The default rts-drop-delay value is 3 ms.
The default rts-timeout value is 3 ms.
The default transmit-delay value is 0 ms.
Interface configuration
This command first appeared in Cisco IOS Release 11.1.
The half-duplex timer command is used to tune half-duplex timers. These timer tuning commands permit you to adjust the timing of the half-duplex state machines to suit the particular needs of their half-duplex installation.
The range of values for the cts-delay and rts-timeout keywords are dependent on the serial interface hardware.
The following examples show how to set the cts-delay timer to 10 ms and the transmit-delay timer to 50 ms.
Router(config)#interface serial 2Router(config-if)#half-duplex timer cts-delay 10Router(config-if)#half-duplex timer transmit-delay 50
half-duplex controlled-carrier
physical-layer
For more information on the PA-8T-V35, PA-8T-X21, PA-8T-232, and PA-4T+ synchronous serial port adapters, refer to the following publications:
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