> For the complete documentation index, see [llms.txt](https://prosbcdocs.telcobridges.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://prosbcdocs.telcobridges.com/telecom-references/ss7-and-signalling/sls.md).

# SLS

## Signaling Link Selector

### Description

The **signaling link selection** (or *SLS*) is a field in the Message Transfer Part level 3 (MTP3) frame of [SS7](https://telcobridges.com/media-gateways/solutions/signaling/migrating-ss7-sigtran/).

The SLS is used as a label to route MTP level 3 messages in the SS7 network.

SLS can also be used in reference to the algorithm that an SS7 node uses for selection of the appropriate signaling link to use for communicating with an adjacent SS7 node. Factors such as load balancing and link availability are used to determine the appropriate link.

| SLS lenght | Variants      | Description                                 |
| ---------- | ------------- | ------------------------------------------- |
| 4 bits     | ITU,TTC,CHINA | The SLS is 4 bits wide                      |
| 5 bits     | ANSI          | Prior to ANSI96, this bit is 5 bits wide    |
| 8 bits     | ANSI96        | This field was extended to 8 bits in ANSI96 |

### Example

#### ITU MTP3 SLS field

![SCCP MTP3 SLS](https://docs.telcobridges.com/w/images/2/22/SCCP_MTP3_SLS.png)

### Reference

[Ref: SLS on Wikipedia](http://en.wikipedia.org/wiki/Signaling_Link_Selection)


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