SST SD and S-NSSAI in 5G

SST - Slice/Service Type

SD - Slice Differentiator. Also referred as sst-sd in 38.331 5gnr rrc specification .
Usage: This is optional entity. This is to complement SST. Which helps to differentiate among multiple network slices.




S-NSSAI - Single Network Slice Selection Assistance Information 
Usage: In-order to identify a network slice end to end

An S-NSSAI is comprised of:

- A Slice/Service type (SST),

- A Slice Differentiator (SD), which is optional information that complements the Slice/Service type(s) to differentiate amongst multiple Network Slices.

SST is 8 bit information. SST field value can be standardized (3GPP defined 0 to 127) or non-standardized (Operator-Reserved  128 to 255).
 
SD is 24 bit information.
S-NSSAI can consit both SST and SD or only SST.
So S-NSSAI can can be of 32bit or 8 bit only.

Example of S-NSSAI inside AMF Configuration is as below

      <s_nssai>
        <id>0</id>
        <sd>198153</sd>
        <sst>3</sst>
      </s_nssai>

Where as inside CU configuration it's as below.

                   <sNSSAI>50529801</sNSSAI>

  

But if you see that number 3 , 198153 and 50529801 - it's quite difficult at first glance to relate them.
Now let's understand the relation between them.

sst is 3 here and it has 8 bit as per the structure.
So the binary format of 3 in 8 bit comes as 0000 0011 .

sd is 198153  here and it has 24 bit as per the structure.

So the binary format of 198153   in 24 bit comes as  0000 0011 0000 0110 0000 1001 .
Now if we form the s-NSSAI with available sst and sd then it becomes like below - 

8 bit sst     -  24 bit sd

0000 0011 -  0000 0011 0000 0110 0000 1001

s-NSSAI -> 0000 0011 0000 0011 0000 0110 0000 1001 -> which is in decimal 50529801.

SST value for 

eMBB - 1

URLLC - 2

MIoT - 3
V2X - 4

Source : 

23.003 : https://www.etsi.org/deliver/etsi_ts/123000_123099/123003/16.05.00_60/ts_123003v160500p.pdf
23.501 : https://www.etsi.org/deliver/etsi_ts/123500_123599/123501/16.06.00_60/ts_123501v160600p.pdf

38.331 : https://www.etsi.org/deliver/etsi_ts/138300_138399/138331/15.06.00_60/ts_138331v150600p.pdf