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Configuring DS1 Interfaces

 

T1 and E1 refer to the data transmission formats that carry DS1 signals across interfaces. The below topic discuss the functionality of T1 and E1, configuration details and also deleting the T1 interface.

Understanding T1 and E1 Interfaces

T1 and E1 are equivalent digital data transmission formats that carry DS1 signals. T1 and E1 lines can be interconnected for international use.

This topic contains the following sections:

T1 Overview

T1 is a digital data transmission medium capable of handling 24 simultaneous connections running at a combined 1.544 Mbps. T1 combines these 24 separate connections, called channels or time slots, onto a single link. T1 is also called DS1.

The T1 data stream is broken into frames. Each frame consists of a single framing bit and 24 8-bit channels, totaling 193 bits per T1 frame. Frames are transmitted 8,000 times per second, at a data transmission rate of 1.544 Mbps (8,000 x 193 = 1.544 Mbps).

As each frame is received and processed, the data in each 8-bit channel is maintained with the channel data from previous frames, enabling T1 traffic to be separated into 24 separate flows across a single medium. For example, in the following set of 4-channel frames (without a framing bit), the data in channel 1 consists of the first octet of each frame, the data in channel 2 consists of the second octet of each frame, and so on:

E1 Overview

E1 is the European format for DS1 digital transmission. E1 links are similar to T1 links except that they carry signals at 2.048 Mbps. Each signal has 32 channels, and each channel transmits at 64 Kbps. E1 links have higher bandwidth than T1 links because it does not reserve one bit for overhead. Whereas, T1 links use 1 bit in each channel for overhead.

T1 and E1 Signals

T1 and E1 interfaces consist of two pairs of wires—a transmit data pair and a receive data pair. Clock signals, which determine when the transmitted data is sampled, are embedded in the T1 and E1 transmissions.

Typical digital signals operate by sending either zeros (0s) or ones (1s), which are usually represented by the absence or presence of a voltage on the line. The receiving device need only detect the presence of the voltage on the line at the particular sampling edge to determine whether the signal is 0 or 1. T1 and E1, however, use bipolar electrical pulses. Signals are represented by no voltage (0), positive voltage (1), or negative voltage (1). The bipolar signal allows T1 and E1 receivers to detect error conditions in the line, depending on the type of encoding that is being used.

Encoding

The following are common T1 and E1 encoding techniques:

  • Alternate mark inversion (AMI)—T1 and E1

  • Bipolar with 8-zero substitution (B8ZS)—T1 only

  • High-density bipolar 3 code (HDB3)—E1 only

AMI Encoding

AMI encoding forces the 1s signals on a T1 or E1 line to alternate between positive and negative voltages for each successive 1 transmission, as in this sample data transmission:

When AMI encoding is used, a data transmission with a long sequence of 0s has no voltage transitions on the line. In other words, voice transmission does not use AMI encoding because it never encounters the “long string of zeroes” problem. In this situation, devices have difficulty maintaining clock synchronization, because they rely on the voltage fluctuations to constantly synchronize with the transmitting clock. To counter this effect, the number of consecutive 0s in a data stream is restricted to 15. This restriction is called the 1s density requirement, because it requires a certain number of 1s for every 15 0s that are transmitted.

On an AMI-encoded line, two consecutive pulses of the same polarity—either positive or negative—are called a bipolar violation (BPV), which is generally flagged as an error.

B8ZS and HDB3 Encoding

Neither B8ZS nor HDB3 encoding restricts the number of 0s that can be transmitted on a line. Instead, these encoding methods detect sequences of 0s and substitute bit patterns for the sequences to provide the signal oscillations required to maintain timing on the link.

The B8ZS encoding method for T1 lines detects sequences of eight consecutive 0 transmissions and substitutes a pattern of two consecutive BPVs (11110000). Because the receiving end uses the same encoding, it detects the BPVs as 0s substitutions, and no BPV error is flagged. A single BPV, which does not match the 11110000 substitution bit sequence is likely to generate an error, depending on the configuration of the device.

B8ZS uses bipolar violations to synchronize devices, a solution that does not require the use of extra bits, which means a T1 circuit using B8ZS can use the full 64 Kbps for each channel for data.

The HDB3 encoding method for E1 lines detects sequences of four consecutive 0 transmissions and substitutes a single BPV (1100). Similar to B8ZS encoding, the receiving device detects the 0s substitutions and does not generate a BPV error.

T1 and E1 Framing

T1 interfaces uses extended superframe (ESF). E1 interfaces use G.704 framing or G.704 with no CRC4 framing, or can be in unframed mode.

ESF Framing for T1

ESF extends the D4 superframe from 12 frames to 24 frames. By expanding the size of the superframe, ESF increases the number of bits in the superframe framing pattern from 12 to 24. The extra bits are used for frame synchronization, error detection, and maintenance communications through the facilities data link (FDL).

The ESF pattern for synchronization bits is 001011. Only the framing bits from frames 4, 8, 12, 16, 20, and 24 in the superframe sequence are used to create the synchronization pattern.

The framing bits from frames 2, 6, 10, 14, 18, and 22 are used to pass a CRC code for each superframe block. The CRC code verifies the integrity of the received superframe and detects bit errors with a CRC6 algorithm.

The framing bits for frames 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, and 23 are used for the data link channel. These 12 bits enable the operators at the network control center to query the remote equipment for information about the performance of the link.

T1 and E1 Loopback Signals

The control signal on a T1 or E1 link is the loopback signal. Using the loopback signal, the operators at the network control center can force the device at the remote end of a link to retransmit its received signals back onto the transmit path. The transmitting device can then verify that the received signals match the transmitted signals, to perform end-to-end checking on the link.

Two loopback signals are used to perform the end-to-end testing:

  • The loop-up command signal sets the link into loopback mode, with the following command pattern:

  • The loop-down signal returns the link to its normal mode, with the following command pattern:

While the link is in loopback mode, the operator can insert test equipment onto the line to test its operation.

Example: Configuring a T1 Interface

This example shows how to complete the initial configuration on a T1 interface.

Requirements

Before you begin, install a PIM, connect the interface cables to the ports, and power on the device. See the Getting Started Guide for your device.

Overview

This example describes the initial configuration that you must complete on each network interface. In this example, you configure the t1-1/0/0 interface as follows:

  • You create the basic configuration for the new interface by setting the encapsulation type to ppp. You can enter additional values for physical interface properties as needed.

  • You set the logical interface to 0. Note that 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.

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 T1 interface:

  1. Create the interface.

  2. Create the basic configuration for the new interface.

  3. Add logical interfaces.

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.

For brevity, this show interfaces command output includes only the configuration that is relevant to this example. Any other configuration on the system has been replaced with ellipses (...).

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

Verification

Confirm that the configuration is working properly.

Purpose

By using the ping tool on each peer address in the network, 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.

Meaning

Verifying Interface Properties

Purpose

Verify that the interface properties are correct.

Action

From the 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 t1-1/0/0] level of the configuration hierarchy.

    • In the J-Web configuration editor, clear the Disable check box on the Interfaces> t1-1/0/0 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 input and output bytes and packets matches expected throughput for the physical interface. To clear the statistics and see only new changes, use the clear interfaces statistics t1-1/0/0 command.

Example: Deleting a T1 Interface

This example shows how to delete a T1 interface.

Requirements

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

Overview

In this example, you delete the t1-1/0/0 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 the J-Web pages.

Configuration

Step-by-Step Procedure

To delete a T1 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.