1. Virtual Signaling Transfer Point User Guide
  2. vSTP Features
  3. SLS Rotation
  4. Outgoing Bit Rotation

2.12.1 Outgoing Bit Rotation

If the Outgoing Bit Rotation option is configured, the vSTP rotates the 4 bits of SLS according to the outgoing linkset. Thus, changing the LSB of the SLS.

This option can be used as a solution to the problem of vSTP selecting same linkset of a combined linkset. Bit rotation can be used on a per linkset basis to ensure that vSTP does not use static LSB (always 0 or always 1) in the received SLS for linkset selection. It is applicable to all ITU messages.

The Outgoing Bit Rotation option enables a user to select the SLS field bit (from 1-4) that must be used as LSB for the linkset selection, while defining a linkset. This rotation during linkset selection affects the 4 bits of SLS selection in the following manner:

  • If bit position 4 is selected (slsrsb =4) for the outgoing linkset, then bit locations 4 3 2 1 are rotated to positions 3 2 1 4.

    For example, SLS = 0110 becomes Rotated SLS = 1100

  • If bit position 3 is selected (slsrsb =3) for the outgoing linkset, then bit locations 4 3 2 1 are rotated to positions 2 1 4 3.

    For example, SLS = 0110 becomes Rotated SLS = 1001

  • If bit position 2 is selected (slsrsb =2) for the outgoing linkset, then bit locations 4 3 2 1 are rotated to positions 1 4 3 2.

    For example, SLS = SLS = 0110 becomes Rotated SLS = 0011

  • If bit position 1 is selected (slsrsb =1) for the outgoing linkset, then no rotation is performed since bit 1 is the existing LSB. Bit 1 is the default value.

    For example, SLS = 0110 remains 0110 only.

Outgoing Bit Rotation Example:

The following figure shows an example of Outgoing Bit Rotation:

Figure 2-10 Example: SLS Outgoing Bit Rotation


img/outgoing_bit_rotation_example.png

Note:

  • After the SLS is rotated then the existing algorithm for selecting a linkset and signaling link is performed and the message is sent out on the selected link. Note that the SLS is modified only for the link selection algorithm and is not modified in the outgoing message.

  • For ITU ISUP messages, SLS is obtained from the lower 4 bits of the CIC field representing the circuit being used. Use of Outgoing bit rotation alone does not guarantee an even distribution of ITU-ISUP messages across all links within a linkset. The vSTP uses all 4 bits of the SLS to determine the actual link to route messages. Since the static bit is simply rotated within the SLS, all possible values of the SLS field will still not be realized. A second option, "Use of Other CIC Bit", must be applied to guarantee even distribution across all links within the linkset.