2.12 SLS Rotation
The Signaling Link Selection(SLS) Rotation feature facilitates a proper distribution of SLS values to provide a good distribution of traffic and load sharing across links and linksets.
In many cases, MSCs, switches and other originating nodes do not send an adequate distribution of SLS values, which results in a poor distribution of traffic across links.
For example, in case of ITU ISUP messages, SLS is obtained from the lower 4 bits of CIC field representing the circuit that is being used. CIC selection can be determined based on an odd or even method where SSP uses either all the odd CICs or all the even CICs to help prevent glaring. This causes Least Significant Bit (LSB) of the SLS to be fixed (0 or 1), which means SSP sends either odd or even SLS. As a result, the transit nodes (STPs) do not achieve a good distribution of traffic across links.
For combined linkset in ANSI and ITU MTP protocols, the LSB of the SLS is used to load share between linksets of a combined linkset and the remaining SLS bits are used to distribute traffic across different links within a linkset. Since, STP receives improper distribution of SLS values (LSB either 0 or 1) the STPs cannot perform proper load sharing across linksets and links of a linkset.
Similarly for single linkset, STPs cannot perform proper load sharing across all links of a linkset, because of receiving improper distribution of SLS values (LSB either 0 or 1).
To overcome this problem, the SLS Rotation feature provides the following SLS Rotation options to users: