SMS flow in UMTS

[fon909]

Hi,

Can anyone please explain how does SMS flow happen in detail in a 3G network. Various network components used, channels used etc. I recently read one document about this but it only explains different network entities, it doesn't mention channels used.



Thanks.

[wolverine]

SMS in UMTS is sent using the SRB. At a logical level this is the DCCH. From a transport level this can be mapped to FACH or DCH (or if you are using SRB over HS on your network even the HS-DSCH can be used). At a physical level this is mapped on to the DPDCH (in case of DCH transport channel) or the SCCPCH (in case the UE happens to be in CELL_FACH and listening to the FACH).

I am not really a core network guy, but from a core network perspective the SMSC is connected to a MSC(s). The SMS is forwarded to the MSC which then uses the IuCS interface to forward the SMS to the RNC. From there on the paragraph above applies.

If the UE is IDLE then it will have to be paged to set up the RRC connection (with the associated SRB). If the UE is in FACH it will be paged with a Paging message Type 2 to send the Paging Response required to set-up the IuCS connection. If the UE is in DCH state with an ongoing CS connection (i.e. speech) it will be sent using the existing IuCS connection and no paging is required. If the UE is in DCH state with a PS connection the same as FACH applies.

Hopefully this makes sense..

[fon909]

@wolverine: could you please share some doc on this if you have? reason being if there is a surplus of sms in network at new year's eve, then what backup measures we need to take. Am sure even your network must be planning to cope with traffic/load for 31st Dec.

Like in 2G, we can increase no. of SDCCH channel for additional load. Similarly, in 3G which component or resource needs upgrading for additional traffic/load.


Thanks.

[dholama]

In 2G we had SDCCH which would carry the SMS hence you needed to add more SD's for overload. However now DCH carries the SMS like other data so the criteria for overload would be similar to other data type that would cause oveload. Check for CE, COde and Power Utilization.

[wolverine]

Hi fon909,

I don't have any documents to share on the subject, what I wrote was pretty much my own knowledge. There are a couple of steps you could take in terms of relieving congestion due to SMS. I have written some ideas below. Whether they work or not and how much benefit they deliver very much depends on the type of traffic on your network etc.

- To relieve paging congestion you could use a PCH of 24kbps (if you are not already using it). Some networks today still use 8kbps PCH which gets congested very easily.
- Some network vendors (e.g. NSN, ALU) allow you to choose per RRC establishment type if you are going to set up the RRC connection on FACH or DCH. To save DCH resources you could select "Low Priority Signalling" (i.e. SMS) to use FACH
- If you are setting up RRC on DCH you could use the 3.4kbps rate (as opposed to 13.6kbps) if your vendor allows it. This will save you some codes and power.

These are the ones that come to mind.. Hopefully it helps..

@wolverine: could you please share some doc on this if you have? reason being if there is a surplus of sms in network at new year's eve, then what backup measures we need to take. Am sure even your network must be planning to cope with traffic/load for 31st Dec.

Like in 2G, we can increase no. of SDCCH channel for additional load. Similarly, in 3G which component or resource needs upgrading for additional traffic/load.


Thanks.