Usage Scenario

When two devices communicate by using asynchronous serial interfaces, the asynchronous serial interfaces work in protocol or flow mode. An asynchronous serial interface usually works in protocol mode. The flow mode is used for man-to-machine interactions such as dial-up.

• In protocol mode, serial interfaces on both ends establish a link based on the existing link layer protocol parameters after a physical connection is established between them.

• In flow mode, serial interfaces on both ends interact with each other, the local interface sends configuration information to the remote interface to set physical layer protocol parameters for the remote interface, and then the two interfaces establish a link based on the configured parameters.

Prerequisites

The serial interface has been configured to work in asynchronous mode by using the physical-mode command.

Precautions

• If you run the async mode command multiple times in the same serial interface view, only the latest configuration takes effect.

• After an asynchronous serial interface is configured to work in flow mode, the link layer protocol of this interface cannot be set to PPP.

Usage Scenario

Synchronous serial interfaces on both ends of a cable are of different types. One synchronous serial interface is a Data Terminal Equipment (DTE) interface, and the other is a Data Circuit-terminating Equipment (DCE) interface. A DTE refers to the device on which serial interfaces are connected to DTE cables; a DCE refers to the device on which serial interfaces are connected to DCE cables.

The baud rate determines the line transmission rate. You can set the baud rate on a DCE based on the cable type.

Prerequisites

The serial interface has been configured to work in synchronous mode by using the physical-mode command.

Precautions

If you run the baudrate command multiple times in the same serial interface view, only the latest configuration takes effect.

The baudrate command can be used only when the device functions as a DCE.

The baud rate range varies depending on the cable type.

• V.24DCE: 1200 bit/s to 64000 bit/s
• For the V.35DCE, RS449DCE, and RS530DCE, the baud rate range is as follows:
  • 1SA/2SA interface card: 1200 bit/s to 2048000 bit/s
  • 8SA interface card: 1200 bit/s to 8192000 bit/s

Ensure that the configured baud rate is the same as virtual baud rate (configured by the virtualbaudrate command) on the remote end (DTE). Otherwise, some packets will be discarded.

Usage Scenario

Two communicating parties are connected through serial interfaces. One party functions as the DTE, and the other functions as the DCE. To exchange data successfully, communicating parties must work in clock synchronization state. For example, the DCE receive clock must be synchronized with the DTE transmit clock to ensure that the DCE can correctly receive data from the DTE.

Figure 6-1 shows the connections of clocks on synchronous serial interfaces. RD indicates a pin for receiving data. TD indicates a pin for sending data. RC, TC, and ETC indicate pins for sending or receiving clock signals.

Figure 6-1 Connections of clocks on synchronous serial interfaces

Table 6-18 describes the clock modes of a device functioning as a DCE. TxClk indicates the DCE send clock, and RxClk indicates the DCE receive clock. RC and TC indicate external clocks, and Local indicates a local clock.

Prerequisites

The serial interface has been configured to work in synchronous mode using the physical-mode sync command.

Precautions

If you run the clock dce command multiple times in the same serial interface view, only the latest configuration takes effect.

The clock dce command can be used only when the device functions as a DCE.

Usage Scenario

Two communicating parties are connected through serial interfaces. One party functions as the DTE, and the other functions as the DCE. To exchange data successfully, communicating parties must work in clock synchronization state. For example, the DCE receive clock must be synchronized with the DTE transmit clock to ensure that the DCE can correctly receive data from the DTE.

Figure 6-2 shows the connections of clocks on synchronous serial interfaces. RD indicates a pin for receiving data. TD indicates a pin for sending data. RC, TC, and ETC indicate pins for sending or receiving clock signals.

Figure 6-2 Connections of clocks on synchronous serial interfaces

Table 6-19 describes the clock modes of a device functioning as a DTE. TxClk indicates the DTE send clock, and RxClk indicates the DTE receive clock. RC and TC indicate external clocks, and Local indicates a local clock.

Prerequisites

The serial interface has been configured to work in synchronous mode using the physical-mode sync command.

Precautions

If you run the clock dte command multiple times in the same serial interface view, only the latest configuration takes effect.

The clock dte command can be used only when the device functions as a DTE.

Usage Scenario

NRZ and NRZI are encoding and decoding modes for synchronous serial interfaces.

• In NRZ, the positive level and negative level represent different logic values (1 or 0). The NRZ signal does not have a transition if the bit being transmitted is a logic 0.
• In NRZI, level inversion represents a logic value (1 or 0), and an unchanged level represents the other logic value (1 or 0). The NRZI signal does not have a transition if the bit being transmitted is a logic 0.

The default encoding and decoding mode of a synchronous serial interface is NRZ. When the encoding and decoding mode of the remote synchronous serial interface is NRZI, the encoding and decoding mode of the local synchronous serial interface needs to be changed to NRZI.

Prerequisites

The serial interface has been configured to work in synchronous mode by using the physical-mode command.

Precautions

If you run the code command multiple times in the same serial interface view, only the latest configuration takes effect.

If two devices communicate by using synchronous serial interfaces, the two devices must have the same encoding and decoding mode. Otherwise, received data frames will be decoded incorrectly and discarded as error frames.

Usage Scenario

CRC is a commonly used method to detect transmission errors in a communication system. It checks for transmission errors by adding redundancy codes to the original data. Transmission errors are more likely to be detected when more redundancy codes are added, but data is transmitted at a lower efficiency. Use the 16-bit or 32-bit CRC mode as required.

Prerequisites

The serial interface has been configured to work in synchronous mode by using the physical-mode command.

Configuration Impact

If you run the crc command multiple times in the same serial interface view, only the latest configuration takes effect.

Usage Scenario

DCD signals monitor the working status of lines and DCEs. The detect dcd command enables DCD signal detection on a synchronous serial interface. DCD signal detection is used together with detection of Data Set Ready (DSR) and Data Terminal Ready (DTR) signals to determine the status of the synchronous serial interface.

The status of a synchronous serial interface is determined based on the following items:

• If detection of DSR and DTR signals is enabled on the interface, the device determines the status (Up or Down) of the interface by detecting DSR and DTR signals and checking whether the interface is connected to an external cable. The device considers the interface Up only when DSR and DCD signals are valid and the interface is connected to an external cable.

• If detection of DSR and DTR signals is disabled on the interface, the device considers that the interface is in Up state as long as it detects that the interface is connected to an external cable.

Prerequisites

The serial interface has been configured to work in synchronous mode by using the physical-mode command.

A DCE sends DSR signals to tell a DTE whether it is working, and a DTE sends DTR signals to tell a DCE whether it is working. The detect dsr-dtr command enables DSR and DTR signal detection on a serial interface, helping to determine the status of the serial interface.

The status of an asynchronous serial interface is determined based on the following items:

• If detection of DSR and DTR signals is enabled on the interface, the device considers the interface Up only when valid DSR signals are detected and the interface is connected to an external cable.
• If detection of DSR and DTR signals is disabled on the interface, the device automatically notifies users that the interface is in Up state without detecting whether the interface is connected to an external cable.

When monitoring the serial interface status or locating faults on a serial interface, you can run the display interface serial command to view the running status of and statistics about the serial interface. You can collect traffic statistics and locate faults on the serial interface according to the command output.

Usage Scenario

After configuring the TDM service, you can run this command to view service statistics of a TDM interface.

Precautions

• The service statistics of a TDM interface can be displayed only after the TDM service is configured.

• TDM interfaces include the synchronous/asynchronous serial interfaces that work in synchronous mode on the 8SA interface card, the interfaces formed by CE1/PRI or CT1/PRI interfaces (excluding ISDN PRI interfaces) on the 8E1T1-M interface card, and the interfaces formed by E1-F or T1-F interfaces on the 8E1T1-F interface card, interfaces on the 6E&M interface card, and interfaces on the 8E1T1-M or 8E1T1-F interface card (on the device using SRU-200H and with the PCB field displaying VER.B).

Usage Scenario

A synchronous serial interface transmits electrical signals, and therefore the interface line keeps transmitting data. When the interface line is idle, the line idle code is used to indicate that the line is idle.

A synchronous serial interface supports two line idle codes: 0x7e and 0xff. The default line idle code 0x7e is recommended.

Prerequisites

The serial interface has been configured to work in synchronous mode by using the physical-mode command.

Precautions

If you run the idlecode command multiple times in the same serial interface view, only the latest configuration takes effect.

Two devices can communicate properly only when the same line idle code is set for serial interfaces on both devices.

To set attributes for a serial interface, run the interface serial command to enter the serial interface view.

Usage Scenario

In some special scenarios, clock delay occurs on a link. Then clocks on two ends are not synchronized or a large number of packets are discarded. To resolve this problem, configure the serial interface to invert clock signals that the serial interface sends and receives.

Prerequisites

The serial interface has been configured to work in synchronous mode by using the physical-mode command.

Precautions

You can configure the device to invert clock signals received by a synchronous serial interface as required.

Usage Scenario

When line signals are frequently interfered, clock delays may occur on the line from time to time. The device having the invert receive-clock command configured inverts clock signals received by a serial interface only once and does not invert clock signals for subsequent delays. In addition, when the delay disappears, the received clock signals are not automatically inverted back. As a result, clock signals are still not synchronized on the two interworking parties.

To resolve the problem, you can run the invert receive-clock auto command on the serial interface to configure the device to automatically invert clock signals. In this way, the received clock signals are automatically inverted upon clock delays on the line. When the delays disappear, the received clock signals are automatically inverted back, preventing impact of the delays and ensuring that clock signals are synchronized on the two interworking parties.

Prerequisites

The serial interface has been configured to work in synchronous mode using the physical-mode sync command.

Precautions

• The invert receive-clock auto command takes effect only on serial interfaces of the 1SA and 2SA interface cards.

• The invert receive-clock auto and invert receive-clock command cannot be configured simultaneously on the same interface.

Usage Scenario

In some special scenarios, clock delay occurs on a link. Then clocks on two ends are not synchronized or a large number of packets are discarded. To resolve this problem, configure the serial interface to invert clock signals that the serial interface sends and receives.

Prerequisites

The serial interface has been configured to work in synchronous mode by using the physical-mode command.

Precautions

You can configure the device to invert clock signals transmitted by a synchronous serial interface as required.

Usage Scenario

An interframe filling tag is a code sent when an interface does not send any service data, and allows the receiver to have a period of time for processing the received data frame and getting ready to receive the next frame.

Interframe filling tags generate extra costs and affect the transmission rate of the interface. You can use the itf number command to manually set a proper number of interframe filling tags between two neighboring frames, ensuring a proper transmission rate.

• When you increase the number of interframe filling tags, the rate of sending valid data packets decreases.
• When you decrease the number of interframe filling tags, the rate of sending valid data packets increases.

  Since interframe filling tags bring extra costs, the transmission rate of a serial interface usually does not reach the bandwidth requirement. To increase the transmission rate of the serial interface, set a small number of interframe filling tags using the itf number command.

Prerequisites

The serial interface has been configured to work in synchronous mode using the physical-mode sync command.

Precautions

• Interfaces on both ends can communicate only when the same number of interframe filling tags is set for them.

• The number of interframe filling tags cannot be set to 2 on the 1SA and 2SA interface cards.

• Serial interfaces formed by E1/T1 interfaces do not support the itf number command.

Usage Scenario

To enable a serial interface to send and receive TDM data, run the link-protocol tdm command to set the encapsulation protocol to TDM for the serial interface.

• When changing the encapsulation protocol to TDM, you can directly enable TDM without disabling the original encapsulation protocol.

• After the encapsulation protocol is set to TDM, the interface does not support subinterface creation.

Precautions

• The configuration of the link-protocol tdm command overrides the configurations of previous link-layer protocols, interrupts services of the protocols, and releases resources occupied by the protocols.

• The following serial interfaces support this command: synchronous/asynchronous serial interfaces on the 8SA interface card, interfaces formed by CE1/PRI, CT1/PRI interfaces (excluding ISDN PRI interfaces) on the 8E1T1-M interface card, interfaces formed by E1-F or T1-F interfaces on the 8E1T1-F interface card and E&M interfaces.

  The encapsulation protocol can be set to TDM on the 8E1T1-M and 8E1T1-F interface cards of the device only when the SRU can deliver the 8KHZ clock to the interface cards. The SRU-200H can deliver the 8KHZ clock to the 8E1T1-M and 8E1T1-F interface cards.

  The encapsulation protocol can be set to TDM only on the VB interface cards among 8E1T1-M and 8E1T1-F interface cards. You can run the display version slot slot-id command in the diagnosis view to check the version number of the 8E1T1-M or 8E1T1-F interface card on the specified slot. If the PCB field value is VER.B, the 8E1T1-M or 8E1T1-F interface card is the VB interface card.

Usage Scenario

The size of data frames is limited at the network layer. Any time the IP layer receives an IP packet, it checks to which local interface the packet needs to be sent and obtains the MTU configured on the interface. Then the IP layer compares the MTU with the packet length. If the packet length is longer than the MTU, the IP layer fragments the packet into smaller packets, which are shorter than the MTU.

If forcible unfragmentation is configured, some packets may be discarded during data transmission at the IP layer. Therefore, large packets must be forcibly fragmented to prevent packet loss on the network. In this case, you can run the mtu command to set the size of a fragment.

Configuration Impact

If you run the mtu command multiple times in the same serial interface view, only the latest configuration takes effect.

Follow-up Procedure

Run the shutdown and undo shutdown/restart commands to restart the related physical interface to make the configuration take effect.

Usage Scenario

A serial interface works in synchronous or asynchronous mode. When a serial interface is used to establish a Digital Data Network (DDN) leased line or connects to a terminal, the serial interface works in synchronous mode. When a serial interface is used to establish an asynchronous leased line or is used for modem dial-up, data backup, or terminal access, the serial interface works in asynchronous mode.

Precautions

If you run the physical-mode command multiple times in the same serial interface view, only the latest configuration takes effect.

Serial interfaces on both ends must work in the same mode.

Synchronous serial interfaces on both ends of a cable are of different types. One synchronous serial interface is a Data Terminal Equipment (DTE) interface, and the other is a Data Circuit-terminating Equipment (DCE) interface. The device where a DTE interface is located is a DTE, and the device where a DCE interface is located is a DCE. The device functions as a DTE or DCE.

When changing the working mode of a serial interface, run the reset counters interface serial interface-number command to clear the statistics on the interface to ensure the statistics accuracy.

Usage Scenario

The MRU is the maximum size of a packet that can be received by an interface. The MTU of a serial interface cannot be greater than the MRU of the interface, ensuring that both ends are capable of receiving packets from each other.

Prerequisites

The serial interface has been configured to work in asynchronous mode by using the physical-mode command.

Configuration Impact

If you run the phy-mru command multiple times in the same serial interface view, only the latest configuration takes effect.

Usage Scenario

A synchronous serial interface works in full duplex mode by default. To make the device compatible with devices working in half duplex mode, run the reverse-rts command to configure the device to invert RTS signals of a synchronous serial interface so that RTS signals become invalid. Then the remote interface does not send data when the local interface is sending data.

Prerequisites

The serial interface has been configured to work in synchronous mode by using the physical-mode command.

Usage Scenario

The virtualbaudrate command sets the baud rate for a serial interface of a DTE.

Prerequisites

The serial interface has been configured to work in synchronous mode by using the physical-mode command.

Precautions

If you run the virtualbaudrate command multiple times in the same serial interface view, only the latest configuration takes effect.

The virtualbaudrate command can be used only on a DTE.

The virtual baud rate range varies depending on the cable type.

• V.24DCE: 1200 bit/s to 64000 bit/s
• For the V.35DCE, RS449DCE, and RS530DCE, the baud rate range is as follows:
  • 1SA/2SA interface card: 1200 bit/s to 2048000 bit/s
  • 8SA interface card: 1200 bit/s to 8192000 bit/s

Ensure that the configured virtual baud rate is the same as baud rate (configured by the baudrate command) on the remote end (DCE). Otherwise, some packets will be discarded.