[asterisk-dev] How is an SLS assigned?

[Pavel Troller]

> Dear all,
> 
> I am learning SS7 standards recently. I got confused with the Signaling Link 
> Selection (SLS) field in MSU's. I know an SLS is used for traffic 
> distribution among signaling links, but how does an SSP assign SLS's to its 
> outgoing messages?
> 
> Is an SLS of only local significance or is it necessary for two adjacent 
> signaling points to used the same SLS value for messages exchanged on the 
> same signaling link (If this is the case, how do they negotiate the SLS 
> value to be used)?
> 
> Hope you guys will kindly give me some hints or refer me to some materials 
> that clarify my question, thank you very much and I do appreciate your help!
> 
> Tian 
> 

Hi!
 AFAIK, ITU Q.763 standard specifies, that SLS should contain the lowest four
bits of CIC. As I live in Europe, I don't know, how it's done in ANSI standard.
 However, it looks that libss7 is using different approach (incrementing SLS
for every call modulo 16), which is definitely not correct, at least for ITU.
SLS can have any value, even for less than 16 signalling links in linkset; in
this case, the receiving SP obviously computes modulo by the actual number of
links, which then selects the particular siglink to be used (but another level
of sharing may occur at MTP3 with routesets, which also depend on SLS, so
maybe even more computation is needed).
  With regards, Pavel Troller