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VISM Clocking

This section describes the options and procedures for setting up the clocking on a VISM-equipped MGX 8850 shelf.

VISM Clocking Options

VISM cards and MGX 8850 PXM cards each provide multiple clocking options. To avoid conflicts and to ensure proper operation, it is important that the settings for clocking options in both card types be considered together.

An overriding principle is that an MGX 8850 shelf consisting of PXM and VISM cards should have one and only one primary clocking source.

A second principle is that at the VISM/MGX 8850 PXM interface there are two options:

  • The MGX 8850 PXM card provides clock for all the VISM cards in the shelf.

  • One of the VISM cards on the shelf provides clock for the PXM (and hence the remainder of the shelf).

The user must choose one of these options.

Using the first option, in which the clock source originates at the PXM side of the VISM/PXM interface, the source stems from one of the following:

  • An external BITS clock on the PXM's T1 or E1 backcard port

  • An external OC3 signal on a PXM SONET backcard port

  • The PXM's internal crystal

The internal crystal is the default and is automatically set as the primary clock source at power-on. The user can use one of the other two clock sources by executing the cnfclksrc (configure clock source) command.

Also, in this situation, the PXM becomes the clock source for the entire shelf. As such, it uses its clock source to provide clocking for all the VISM cards in the shelf. The VISM cards, in turn, use this clock to provide clocking for their T1 or E1 lines. In order for this situation to operate correctly, all the VISM lines must be configured for local clocking using the cnfln (configure line) command.

Moving on to the second option, the clock source originates on the VISM side of the VISM/PXM interface. It stems from one of the T1 or E1 lines on one of the VISM cards (the line receiving the clock signal on the selected VISM card must be line number 1). The line number 1 that is receiving the clock source must be configured for loop clocking using the cnfln command. All the remaining T1 or E1 lines on all the VISM cards in the shelf must be configured for local clocking.

Also, in this situation, the VISM becomes the clock source for the PXM and hence the entire shelf, including the remaining VISM cards. In order for this situation to operate correctly, the PXM must be configured for a service module as the clocking source with the selected VISM and its clock line specified in the cnfclksrc command.

Configuration Procedures

Follow these steps to set up clocking on a VISM-equipped MGX 8850 shelf:

Step 1

Examine the entire configuration of the MGX 8850 shelf and determine the single clock source. The type of equipment connected to the VISM's T1 or E1 lines might dictate this choice. If the selected clock source is from one of the VISM's T1 or E1 lines, that line must be connected to the physical port 1 on the VISM backcard.

Step 2

Configure the clocking option on the PXM card using the cnfclksrc command:

cnfclksrc <slot.port> <clktype>

The slot.port parameter specifies the clock source. The clktype parameter is either p for primary, s for secondary, or null for no external clock source. The following configurations show the criteria used to specify these parameters:

  1. If the clock source is the external BITS clock (a T1 or E1 port on the PXM backcard), specify the configuration as

    cnfclksrc 7.35 p

    Use slot 7 whether the PXM is in slot 7 or 8. The BITS port is always numbered port 35.

  2. If the clock source is an external signal on one of the PXM OC3 ports, specify the configuration as

    cnfclksrc 7.n p

    Use slot 7 whether the PXM is in slot 7 or 8. n is the OC3 port number in the range 1 to 4.

  3. If the clock source is the PXM's internal crystal and no other clock source has been specified, there is no need to configure the clock source, because the crystal is the automatic default. However, if another clock source is specified and you want to change to the crystal, specify the configuration as

    cnfclksrc 7.x null

    Use slot 7 whether the PXM is in slot 7 or 8. x is either 35 or the OC3 port number, depending on which is the currently specified source. The null value cancels the previous configuration and returns the clocking source to the default crystal.

  4. If the clock source is from the a line on a VISM card, specify the configuration as

    cnfclksrc y.z p

    y is the slot number of the VISM card, and z is the line number (which must be 1).

Step 3

Configure the clocking option on the VISM card(s) using the cnfln command:

cnfln <line_num> <line_code> <line_len> <clk_src>
  <line_type> <loopback_detection>
You must enter all the parameters, but for clocking purposes, only line_num and clk_src are relevant. The line number is the physical port number on the VISM backcard. The clock source can be specified as either loop clock or local. Loop clock is clocking from a T1 or E1 line, and local clock is clocking from the PXM.
  1. If the clock source to the VISM is from the PXM, all lines on all VISM cards must be configured as local using the cnfln command.

  2. If the clock source is from a line on a VISM card, specify that line (which must be line 1) as loop clock, and configure all remaining lines on the VISM and all lines on all the other cards as local.


You have successfully set up clocking on a VISM-equipped MGX 8850 shelf.

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