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Configuring Serial Interfaces

 

Serial links are simple, bidirectional links that require very few control signals. The below topics discuss the overview, configuration and deleting serial interfaces, overview and configuration details of the 8-Port Synchronous Serial GPIM on security devices.

Serial Interfaces Overview

Serial links are simple, bidirectional links that require very few control signals. In a basic serial setup, data communications equipment (DCE) installed in a user's premises is responsible for establishing, maintaining, and terminating a connection. A modem is a typical DCE device.

A serial cable connects the DCE to a telephony network where, ultimately, a link is established with data terminal equipment (DTE). DTE is typically where a serial link terminates.

The distinction between DCE and DTE is important because it affects the cable pinouts on a serial cable. A DCE cable uses a female 9-pin or 25-pin connector, and a DTE cable uses a male 9-pin or 25-pin connector, and .

To form a serial link, the cables are connected to each other. However, if the pins are identical, each side's transmit and receive lines are connected, which makes data transport impossible. To address this problem, each cable is connected to a null modem cable, which crosses the transmit and receive lines in the cable.

This section includes the following topics:

Serial Transmissions

In basic serial communications, nine signals are critical to the transmission. Each signal is associated with a pin in either the 9-pin or 25-pin connector. Table 1 lists and defines serial signals and their sources.

Table 1: Serial Transmission Signals

Signal Name

Definition

Signal Source

TD

Transmitted data

DTE

RD

Received data

DCE

RTS

Request to send

DTE

CTS

Clear to send

DCE

DSR

Data set ready

DCE

Signal Ground

Grounding signal

CD

Carrier detect

DTR

Data terminal ready

DTE

RI

Ring indicator

When a serial connection is made, a serial line protocol—such as EIA-530, X.21, RS-422/449, RS-232, or V.35—begins controlling the transmission of signals across the line as follows:

  1. The DCE transmits a DSR signal to the DTE, which responds with a DTR signal. After this handshake, the link is established and traffic can pass.
  2. When the DTE device is ready to receive data, it sets its RTS signal to a marked state (all 1s) to indicate to the DCE that it can transmit data. (If the DTE is not able to receive data—because of buffer conditions, for example—it sets the RTS signal to all 0s.)
  3. When the DCE device is ready to receive data, it sets its CTS signal to a marked state to indicate to the DTE that it can transmit data. (If the DCE is not able to receive data, it sets the CTS signal to all 0s.)
  4. When the negotiation to send information has taken place, data is transmitted across the transmitted data (TD) and received data (RD) lines:
    • TD line—Line through which data from a DTE device is transmitted to a DCE device

    • RD line—Line through which data from a DCE device is transmitted to a DTE device

    The name of the wire does not indicate the direction of data flow.

The DTR and DSR signals were originally designed to operate as a handshake mechanism. When a serial port is opened, the DTE device sets its DTR signal to a marked state. Similarly, the DCE sets its DSR signal to a marked state. However, because of the negotiation that takes place with the RTS and CTS signals, the DTR and DSR signals are not commonly used.

The carrier detect and ring indicator signals are used to detect connections with remote modems. These signals are not commonly used.

Signal Polarity

Serial interfaces use a balanced (also called differential) protocol signaling technique. Two serial signals are associated with a circuit: the A signal and the B signal. The A signal is denoted with a plus sign (for example, DTR+), and the B signal is denoted with a minus sign (for example, DTR–). If DTR is low, then DTR+ is negative with respect to DTR–. If DTR is high, then DTR+ is positive with respect to DTR–.

By default, all signal polarities are positive, but sometimes they might be reversed. For example, signals might be miswired as a result of reversed polarities.

Serial Clocking Modes

By default, a serial interface uses loop clocking to determine its timing source. For EIA-530 and V.35 interfaces, you can set each port independently to use one of the following clocking modes. X.21 interfaces can use only loop clocking mode.

  • Loop clocking mode—Uses the DCE's receive (RX) clock to clock data from the DCE to the DTE.

  • DCE clocking mode—Uses the transmit (TXC) clock, generated by the DCE specifically to be used by the DTE as the DTE's transmit clock.

  • Internal clocking mode—Uses an internally generated clock. The speed of this clock is configured locally. Internal clocking mode is also known as line timing.

Both loop clocking mode and DCE clocking mode use external clocks generated by the DCE.

Figure 1 shows the clock sources for loop, DCE, and internal clocking modes.

Figure 1: Serial Interface Clocking Modes
Serial Interface Clocking
Modes

Serial Interface Transmit Clock Inversion

When an externally timed clocking mode (DCE or loop) is used, long cables might introduce a phase shift of the DTE-transmitted clock and data. At high speeds, this phase shift might cause errors. Inverting the transmit clock corrects the phase shift, thereby reducing error rates.

DTE Clock Rate Reduction

Although the serial interface is intended for use at the default clock rate of 16.384 MHz, you might need to use a slower rate under any of the following conditions:

  • The interconnecting cable is too long for effective operation.

  • The interconnecting cable is exposed to an extraneous noise source that might cause an unwanted voltage in excess of +1 volt.

    The voltage must be measured differentially between the signal conductor and the point in the circuit from which all voltages are measured (“circuit common”) at the load end of the cable, with a 50-ohm resistor substituted for the generator.

  • Interference with other signals must be minimized.

  • Signals must be inverted.

Serial Line Protocols

Serial interfaces support the following line protocols:

EIA-530

EIA-530 is an Electronic Industries Association (EIA) standard for the interconnection of DTE and DCE using serial binary data interchange with control information exchanged on separate control circuits. EIA-530 is also known as RS-530.

The EIA-530 line protocol is a specification for a serial interface that uses a DB-25 connector and balanced equivalents of the RS-232 signals—also called V.24. The EIA-530 line protocol is equivalent to the RS-422 and RS-423 interfaces implemented on a 25-pin connector.

The EIA-530 line protocol supports both balanced and unbalanced modes. In unbalanced transmissions, voltages are transmitted over a single wire. Because only a single signal is transmitted, differences in ground potential can cause fluctuations in the measured voltage across the link. For example, if a 3-V signal is sent from one endpoint to another, and the receiving endpoint has a ground potential 1 V higher than the transmitter, the signal on the receiving end is measured as a 2-V signal.

Balanced transmissions use two wires instead of one. Rather than sending a single signal across the wire and having the receiving end measure the voltage, the transmitting device sends two separate signals across two separate wires. The receiving device measures the difference in voltage of the two signals (balanced sampling) and uses that calculation to evaluate the signal. Any differences in ground potential affect both wires equally, and the difference in the signals is still the same.

The EIA-530 interface supports asynchronous and synchronous transmissions at rates ranging from 20 Kbps to 2 Mbps.

RS-232

RS-232 is a Recommended Standard (RS) describing the most widely used type of serial communication. The RS-232 protocol is used for asynchronous data transfer as well as synchronous transfers using HDLC, Frame Relay, and X.25. RS-232 is also known as EIA-232.

The RS-232 line protocol is very popular for low-speed data signals. RS-232 signals are carried as single voltages referred to a common ground signal. The voltage output level of these signals varies between –12 V and +12 V. Within this range, voltages between –3 V and +3 V are considered inoperative and are used to absorb line noise. Control signals are considered operative when the voltage ranges from +3 V to +25 V.

The RS-232 line protocol is an unbalanced protocol, because it uses only one wire and is susceptible to signal degradation. Degradation can be extremely disruptive, particularly when a difference in ground potential exists between the transmitting and receiving ends of a link.

The RS-232 interface is implemented in a 25-pin D-shell connector and supports line rates up to 200 Kbps over lines shorter than 98 feet (30 meters).

Note

RS-232 serial interfaces cannot function error-free with a clock rate greater than 200 KHz.

RS-422/449

RS-422 is a Recommended Standard (RS) describing the electrical characteristics of balanced voltage digital interface circuits that support higher bandwidths than traditional serial protocols like RS-232. RS-422 is also known as EIA-422.

The RS-449 standard (also known as EIA-449) is compatible with RS-422 signal levels. The EIA created RS-449 to detail the DB-37 connector pinout and define a set of modem control signals for regulating flow control and line status.

The RS-422/499 line protocol runs in balanced mode, allowing serial communications to extend over distances of up to 4,000 feet (1.2 km) and at very fast speeds of up to 10 Mbps.

In an RS-422/499-based system, a single master device can communicate with up to 10 slave devices in the system. To accommodate this configuration, RS-422/499 supports the following kinds of transmission:

  • Half-duplex transmission—In half-duplex transmission mode, transmissions occur in only one direction at a time. Each transmission requires a proper handshake before it is sent. This operation is typical of a balanced system in which two devices are connected by a single connection.

  • Full-duplex transmission—In full duplex transmission mode, multiple transmissions can occur simultaneously so that devices can transmit and receive at the same time. This operation is essential when a single master in a point-to-multipoint system must communicate with multiple receivers.

  • Multipoint transmission—RS-422/449 allows only a single master in a multipoint system. The master can communicate to all points in a multipoint system, and the other points must communicate with each other through the master.

V.35

V.35 is an ITU-T standard describing a synchronous, Physical Layer protocol used for communications between a network access device and a packet network. V.35 is most commonly used in the United States and Europe.

The V.35 line protocol is a mixture of balanced (RS-422) and common ground (RS-232) signal interfaces. The V.35 control signals DTR, DSR, DCD, RTS, and CTS are single-wire common ground signals that are essentially identical to their RS-232 equivalents. Unbalanced signaling for these control signals is sufficient, because the control signals are mostly constant, varying at very low frequency, which makes single-wire transmission suitable. Higher frequency data and clock signals are sent over balanced wires.

V.35 interfaces operate at line rates of 20 Kbps and above.

X.21

X.21 is an ITU-T standard for serial communications over synchronous digital lines. The X.21 protocol is used primarily in Europe and Japan.

The X.21 line protocol is a state-driven protocol that sets up a circuit-switched network using call setup. X.21 interfaces use a 15-pin connector with the following eight signals:

  • Signal ground (G)—Reference signal used to evaluate the logic states of the other signals. This signal can be connected to the protective earth (ground).

  • DTE common return (Ga)—Reference ground signal for the DCE interface. This signal is used only in unbalanced mode.

  • Transmit (T)—Binary signal that carries the data from the DTE to the DCE. This signal can be used for data transfer or in call-control phases such as Call Connect or Call Disconnect.

  • Receive (R)—Binary signal that carries the data from the DCE to the DTE. This signal can be used for data transfer or in call-control phases such as Call Connect or Call Disconnect.

  • Control (C)—DTE-controlled signal that controls the transmission on an X.21 link. This signal must be on during data transfer, and can be on or off during call-control phases.

  • Indication (I)—DCE-controlled signal that controls the transmission on an X.21 link. This signal must be on during data transfer, and can be on or off during call-control phases.

  • Signal Element Timing (S)—Clocking signal that is generated by the DCE. This signal specifies when sampling on the line must occur.

  • Byte Timing (B)—Binary signal that is on when data or call-control information is being sampled. When an 8-byte transmission is over, this signal switches to off.

Transmissions across an X.21 link require both the DCE and DTE devices to be in a ready state, indicated by an all 1s transmission on the T and R signals.

Example: Configuring a Serial Interface

This example shows how to complete the initial configuration on a serial interface.

Requirements

Before you begin, install a serial PIM in the SRX Series device. See SRX Series Services Gateways for the Branch Physical Interface Modules Hardware Guide.

Overview

In this example, you create the interface se-1/0/0. You create the basic configuration for the new interface by setting the encapsulation type to ppp. Then you set the logical interface to 0. The logical unit number can range from 0 through 16,384. You can enter additional values for properties you need to configure on the logical interface, such as logical encapsulation or protocol family. Finally, you set IPv4 address 10.10.10.10/24 on the serial interface.

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following command, paste it into a text file, remove any line breaks, change any details necessary to match your network configuration, copy and paste the command into the CLI at the [edit] hierarchy level, and then enter commit from configuration mode.

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For instructions on how to do that, see Using the CLI Editor in Configuration Mode.

To configure a serial interface:

  1. Create the interface.
  2. Create the basic configuration for the new interface.
  3. Add logical interfaces.
  4. Specify an IPv4 address for the interface.

Results

From configuration mode, confirm your configuration by entering the show interfaces se-1/0/0 command. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

[edit]
user@host# show interfaces se-1/0/0

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Purpose

Use the ping tool on each peer address in the network to verify that all interfaces on the device are operational.

Action

For each interface on the device:

  1. In the J-Web interface, select Troubleshoot>Ping Host.
  2. In the Remote Host box, type the address of the interface for which you want to verify the link state.
  3. Click Start. The output appears on a separate page.

If the interface is operational, it generates an ICMP response. If this response is received, the round-trip time, in milliseconds, is listed in the time field.

Verifying Interface Properties

Purpose

Verify that the interface properties are correct.

Action

From operational mode, enter the show interfaces detail command.

The output shows a summary of interface information. Verify the following information:

  • The physical interface is Enabled. If the interface is shown as Disabled, do one of the following:

    • In the CLI configuration editor, delete the disable statement at the [edit interfaces se-1/0/0] level of the configuration hierarchy.

    • In the J-Web configuration editor, clear the Disable check box on the Interfaces> se-1/0/0 page.

  • The physical link is Up. A link state of Down indicates a problem with the interface module, interface port, or physical connection (link-layer errors).

  • The Last Flapped time is an expected value. It indicates the last time the physical interface became unavailable and then available again. Unexpected flapping indicates likely link-layer errors.

  • The traffic statistics reflect expected input and output rates. Verify that the number of inbound and outbound bytes and packets matches expected throughput for the physical interface. To clear the statistics and see only new changes, use the clear interfaces statistics se-1/0/0 command.

Example: Deleting a Serial Interface

This example shows how to delete a serial interface.

Note

Serial interfaces are no longer supported on SRX300, SRX320, SRX340, SRX345, SRX380, and SRX550HM devices.

Requirements

No special configuration beyond device initialization is required before configuring an interface.

Overview

In this example, you delete the se-1/0/0 interface.

Note

Performing this action removes the interface from the software configuration and disables it. Network interfaces remain physically present, and their identifiers continue to appear on J-Web pages.

Configuration

Step-by-Step Procedure

To delete a serial interface:

  1. Specify the interface you want to delete.
  2. If you are done configuring the device, commit the configuration.

Verification

To verify the configuration is working properly, enter the show interfaces command.

Understanding the 8-Port Synchronous Serial GPIM

A Gigabit-Backplane Physical Interface Module (GPIM) is a network interface card (NIC) that installs in the front slots of the SRX550 Services Gateway to provide physical connections to a LAN or a WAN.

Note

Serial interfaces, including the 8-port synchronous serial GPIM, are no longer supported on SRX300, SRX320, SRX340, SRX345, SRX380, and SRX550HM devices.

The 8-port synchronous serial GPIM provides the physical connection to serial network media types, receiving incoming packets from the network and transmitting outgoing packets to the network. Besides forwarding packets for processing, the GPIM performs framing and line-speed signaling. This GPIM provides 8 ports that operate in sync mode and supports a line rate of 64 Mbps or 8 Mbps per port.

Supported Features

Table 2 lists the features supported on the 8-port synchronous serial GPIM.

Table 2: Supported Features

Features

Description

Operation modes (autoselection based on cable, no configuration required)

  • DTE (data terminal equipment)

  • DCE (data communication equipment)

Clocking

  • Tx clock modes

    • DCE clock (only valid in DTE mode)

    • Baud clock (internally generated)

    • Loop clock (external)

  • Rx clock modes

    • Baud clock (internally generated)

    • Loop clock (external)

Clock rates (baud rates)

1.2 KHz to 8.0 MHz

Note: RS-232 serial interfaces might cause an error with a clock rate greater than 200 KHz.

MTU

9192 bytes, default value is 1504 bytes

HDLC features

  • Idle flag/fill (0x7e or all ones), default idle flag is (0x7e)

  • Counters—giants, runts, FCS error, abort error, align error

Line encoding

NRZ and NRZI

Invert data

Enabled

Line protocol

EIA530/EIA530A, X.21, RS-449, RS-232, V.35

Data cables

Separate cable for each line protocol (both DTE/DCE mode)

Error counters (conformance to ANSI specification)

Enabled

Alarms and defects

  • Rx clock absent

  • Tx clock absent

  • DCD absent

  • RTS/CTS absent

  • DSR/DTR absent

Data signal

Rx clock

Control signals

  • To DTE: CTS, DCD, DSR

  • From DTE: DTR, RTS

Serial autoresync

  • Configurable resync duration

  • Configurable resync interval

Diagnostic features

  • Loopback modes—local, remote, and dce-local loopback

  • Ability to ignore control signals

Layer 2 features

Encapsulation

  • PPP

  • Cisco HDLC

  • Frame Relay

  • MLPPP

  • MLFR

SNMP features

SNMP information receivable at each port

  • IF-MIB - rfc2863a.mib

  • jnx-chassis.mib

Anticounterfeit check

Enabled

Example: Configuring an 8-Port Synchronous Serial GPIM in Back-to-Back SRX650 Services Gateways

This example shows how to perform a basic back-to-back device configuration with an 8-port synchronous serial GPIM. It describes the most common scenario in which a serial GPIM is deployed.

In this example, the SRX650 devices are shown as both data communication equipment (DCE) and data terminal equipment (DTE). In certain deployment scenarios, the DTE can be a serial modem or an encryptor or decryptor.

Note

Serial interfaces, including the 8-port synchronous serial GPIM, are no longer supported on SRX300, SRX320, SRX340, SRX345, SRX380, and SRX550HM devices.

Requirements

This example uses the following hardware and software components:

  • Junos OS Release 12.1 R2 or later for SRX Series Services Gateways.

  • Two SRX650 devices connected back-to-back.

  • Two 8-port synchronous serial GPIMs.

  • Four pairs of DCE and DTE cables. The cable can be any type as mentioned in 8-Port Serial GPIM Interface Cables.

Before you begin:

Overview and Topology

In this scenario, the configuration is done on two interfaces. All ports are configured with different encapsulations, such as Cisco High-Level Data Link Control (HDLC), Frame Relay, and Point-to-Point Protocol (PPP). When Frame Relay is set, then the data link connection identifier (in this example, 111) must also be set.

In this example, all eight ports on Device 1 (SRX650) are configured in DTE mode and their respective eight ports on Device 2 (SRX650) are configured in DCE mode.

For Device 1, you set the encapsulation type to ppp. Then you set the logical interface to 0. The logical unit number can range from 0 through 16,384. You can enter additional values for properties you need to configure on the logical interface, such as logical encapsulation or protocol family. Finally, you set the IPv4 address to 10.10.10.1/24 on the serial port. For Device 2, you follow a procedure similar to Device 1, but you set the clocking mode to dce.

Figure 2 shows the topology used in this example.

Figure 2: Basic Back-to-Back Device Configuration
Basic Back-to-Back Device
Configuration

Configuration

CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Device 1

Device 2

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For instructions on how to do that, see Using the CLI Editor in Configuration Mode.

To configure the interfaces on Device 1:

  1. Specify the maximum transmission unit (MTU) value for the interface.
  2. Configure the encapsulation type.
  3. Configure the serial options, such as the clocking mode.
  4. Set the IPv4 address on the serial port.
  5. Configure the static route information.
    Note

    Repeat the same configuration for the other seven ports on Device 1.

  6. If you are done configuring the device, commit the configuration.

Step-by-Step Procedure

To configure the interfaces on Device 2:

  1. Specify the MTU value for the interface.
  2. Configure the encapsulation type.
  3. Configure the serial options, such as the clocking mode.
  4. Set the IPv4 address on the serial port.
  5. Configure the static route information.
    Note

    Repeat the same configuration for the other seven ports on Device 2.

  6. If you are done configuring the device, commit the configuration.

Results

From configuration mode, confirm your configuration by entering the show interfaces command. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

Device 1

If you are done configuring the device, enter commit from configuration mode.

Device 2

If you are done configuring the device, enter commit from configuration mode.

Verification

Confirm that the configuration is working properly.

Purpose

Verify that the interface link status is up.

Action

From operational mode, enter the show interface terse se-7/0/* command.

Meaning

The output displays a list of all interfaces configured. If the Link column displays up for all interfaces, the configuration is working properly. This verifies that the GPIM is up and end-to-end ping is working.

Verifying Interface Statistics for DCE

Purpose

Verify that the interfaces are configured properly for DCE.

Action

From operational mode, enter the show interface se-7/0/0 extensive | no-more command.

Meaning

The output displays a list of all DCE verification parameters and the mode configured. If the local mode displays DCE, the configuration is working properly.

Verifying Interface Statistics for DTE

Purpose

Verify that the interfaces are configured properly for DTE.

Action

From operational mode, enter the show interfaces se-3/0/0 extensive | no-more command.

Meaning

The output displays a list of all DTE verification parameters and the mode configured. If the local mode displays DTE, the configuration is working properly.