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Configuring DS3 Interfaces

DS3 interfaces, also referred to as T3, is an high-speed data transmission medium formed by multiplexing DS1 and DS2 signals. The below topic discuss the functionality of T3 interfaces, configuration details and also deleting the T3 interface.

Understanding T3 and E3 Interfaces

T3 is a high-speed data-transmission medium formed by multiplexing 28 DS1 signals into seven separate DS2 signals, and combining the DS2 signals into a single DS3 signal. T3 links operate at 43.736 Mbps. T3 is also called DS3.

E3 is the equivalent European transmission format. E3 links are similar to T3 (DS3) links, but carry signals at 34.368 Mbps. Each signal has 16 E1 channels, and each channel transmits at 2.048 Mbps. E3 links use all 8 bits of a channel, whereas T3 links use 1 bit in each channel for overhead.

Multiplexing DS1 Signals

Four DS1 signals combine to form a single DS2 signal. The four DS1 signals form a single DS2 M-frame, which includes subframes M1 through M4. Each subframe has six 49-bit blocks, for a total of 294 bits per subframe. The first bit in each block is a DS2 overhead (OH) bit. The remaining 48 bits are DS1 information bits.

Figure 1 shows the DS2 M-frame format.

Figure 1: DS2 M-Frame FormatDS2 M-Frame Format

The four DS2 subframes are not four DS1 channels. Instead, the DS1 data bits within the subframes are formed by data interleaved from the DS1 channels. The 0n values designate time slots devoted to DS1 inputs as part of the bit-by-bit interleaving process. After every 48 DS1 information bits (12 bits from each signal), a DS2 OH bit is inserted to indicate the start of a subframe.

DS2 Bit Stuffing

Because the four DS1 signals are asynchronous signals, they might operate at different line rates. To synchronize the asynchronous streams, the multiplexers on the line use bit stuffing.

A DS2 connection requires a nominal transmit rate of 6.304 Mbps. However, because multiplexers increase the overall output rate to the intermediate rate of 6.312 Mbps, the output rate is higher than individual input rates on DS1 signals. The extra bandwidth is used to stuff the incoming DS1 signals with extra bits until the output rate of each signal equals the increased intermediate rate. These stuffed bits are inserted at fixed locations in the DS2 M-frame. When DS2 frames are received and the signal is demultiplexed, the stuffing bits are identified and removed.

DS3 Framing

A set of four DS1 signals is multiplexed into seven DS2 signals, which are multiplexed into a single DS3 signal. The multiplexing occurs just as with DS1-to-DS2 multiplexing. The resulting DS3 signal uses either the standard M13 asynchronous framing format or the C-bit parity framing format. Although the two framing formats differ in their use of control and message bits, the basic frame structures are identical. The DS3 frame structures are shown in Figure 2 and Figure 3.

M13 Asynchronous Framing

A DS3 M-frame includes seven subframes, formed by DS2 data bits interleaved from the seven multiplexed DS2 signals. Each subframe has eight 85-bit blocks—a DS3 OH bit plus 84 data bits. The meaning of an OH bit depends on the block it precedes. Standard DS3 M13 asynchronous framing format is shown in Figure 2.

Figure 2: DS3 M13 Frame FormatDS3 M13 Frame Format

A DS3 M13 M-frame contains the following types of OH bits:

  • Framing bits (F-bits)—Make up a frame alignment signal that synchronizes DS3 subframes. Each DS3 frame contains 28 F-bits (4 bits per subframe). F-bits are located at the beginning of blocks 2, 4, 6, and 8 of each subframe. When combined, the frame alignment pattern for each subframe is 1001. The pattern can be examined to detect bit errors in the transmission.

  • Multiframing bits (M-bits)—Make up a multiframe alignment signal that synchronizes the M-frames in a DS3 signal. Each DS3 frame contains 3 M-bits, which are located at the beginning of subframes 5, 6, and 7. When combined, the multiframe alignment patter for each M-frame is 010.

  • Bit stuffing control bits (C-bits)—Serve as bit stuffing indicators for each DS2 input. For example, C11, C12, and C13 are indicators for DS2 input 1. Their values indicate whether DS3 bit stuffing has occurred at the multiplexer. If the three C-bits in a subframe are all 0s, no stuffing was performed for the DS2 input. If the three C-bits are all 1s, stuffing was performed.

  • Message bits (X-bits)—Used by DS3 transmitters to embed asynchronous in-service messages in the data transmission. Each DS3 frame contains 2 X-bits, which are located at the beginning of subframes 1 and 2. Within an DS3 M-frame, both X-bits must be identical.

  • Parity bits (P-bits)—Compute parity over all but 1 bit of the M-frame. (The first X-bit is not included.) Each DS3 frame contains 2 P-bits, which are located at the beginning of subframes 3 and 4. Both P-bits must be identical.

    If the previous DS3 frame contained an odd number of 1s, both P-bits are set to 1. If the previous DS3 contained an even number of 1s, both P-bits are set to 0. If, on the receiving side, the number of 1s for a given frame does not match the P-bits in the following frame, it indicates one or more bit errors in the transmission.

C-Bit Parity Framing

In M13 framing, every C-bit in a DS3 frame is used for bit stuffing. However, because multiplexers first use bit stuffing when multiplexing  DS1 signals into DS2 signals, the incoming DS2 signals are already synchronized. Therefore, the bit stuffing that occurs when DS2 signals are multiplexed is redundant.

C-bit parity framing format redefines the function of C-bits and X-bits, using them to monitor end-to-end path performance and provide in-band data links. The C-bit parity framing structure is shown in Figure 3.

Figure 3: DS3 C-Bit Parity FramingDS3 C-Bit Parity Framing

In C-bit parity framing, the X-bits transmit error conditions from the far end of the link to the near end. If no error conditions exist, both X-bits are set to 1. If an out-of-frame (OOF) or alarm indication signal (AIS) error is detected, both X-bits are set to 0 in the upstream direction for 1 second to notify the other end of the link about the condition.

The C-bits that control bit stuffing in M13 frames are typically used in the following ways by C-bit parity framing:

  • Application identification channel (AIC)—The first C-bit in the first subframe identifies the type of DS3 framing used. A value of 1 indicates that C-bit parity framing is in use.

  • Na—A reserved network application bit.

  • Far-end alarm and control (FEAC) channel—The third C-bit in the first subframe is used for the FEAC channel. In normal transmissions, the FEAC C-bit transmits all 1s. When an alarm condition is present, the FEAC C-bit transmits a code word in the format 0xxxxxxx 11111111, in which x can be either 1 or 0. Bits are transmitted from right to left.

    Table 1 lists some C-bit code words and the alarm or status condition indicated.

    Table 1: FEAC C-Bit Condition Indicators

    Alarm or Status Condition

    C-Bit Code Word

    DS3 equipment failure requires immediate attention.

    00110010 11111111

    DS3 equipment failure occurred—such as suspended, not activated, or unavailable service—that is non-service-affecting.

    00011110 11111111

    DS3 loss of signal.

    00011100 11111111

    DS3 out of frame.

    00000000 11111111

    DS3 alarm indication signal (AIS) received.

    00101100 11111111

    DS3 idle received.

    00110100 11111111

    Common equipment failure occurred that is non-service-affecting.

    00011101 11111111

    Multiple DS1 loss of signal.

    00101010 11111111

    DS1 equipment failure occurred that requires immediate attention.

    00001010 11111111

    DS1 equipment failure occurred that is non-service-affecting.

    00000110 11111111

    Single DS1 loss of signal.

    00111100 11111111

  • Data links—The 12 C-bits in subframes 2, 5, 6, and 7 are data link (DL) bits for applications and terminal-to-terminal path maintenance.

  • DS3 parity—The 3 C-bits in the third subframe are DS3 parity C-bits (also called CP-bits). When a DS3 frame is transmitted, the sending device sets the CP-bits to the same value as the P-bits. When the receiving device processes the frame, it calculates the parity of the M-frame and compares this value to the parity in the CP-bits of the following M-frame. If no bit errors have occurred, the two values are typically the same.

  • Far–end block errors (FEBEs)—The 3 C-bits in the fourth subframe make up the far-end block error (FEBE) bits. If a framing or parity error is detected in an incoming M-frame (via the CP-bits), the receiving device generates a C-bit parity error and sends an error notification to the transmitting (far-end) device. If an error is generated, the FEBE bits are set to 000. If no error occurred, the bits are set to 111.

Example: Configuring a T3 Interface

This example shows how to complete the initial configuration on a T3 interface.


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.


This example describes the initial configuration that you must complete on each network interface. In this example, you configure the t3-1/0/0 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 to 16,384. You can enter additional values for properties you need to configure on the logical interface, such as logical encapsulation or protocol family.



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 T3 interface:

  1. Create the interface.

  2. Create the basic configuration for the new interface.

  3. Add logical interfaces.


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.


Confirm that the configuration is working properly.

Verifying the Link State of All Interfaces


By using the ping tool on each peer address in the network, verify that all interfaces on the device are operational.


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


Verify that the interface properties are correct.


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 t3-1/0/0] level of the configuration hierarchy.

    • In the J-Web configuration editor, clear the Disable check box on the Interfaces> t3-1/0/0 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 t3-1/0/0 command.

Example: Deleting a T3 Interface

This example shows how to delete a T3 interface.


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


In this example, you delete the t3-1/0/0 interface.


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.



Step-by-Step Procedure

To delete a T3 interface:

  1. Specify the interface you want to delete.

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


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