Retrieval of the originating address (sender's number) by SMSC

Question

I'm trying to implement SMS transmission between two GSM devices. I've searched and read standards and articles a lot but couldn't find the answer to the following question. Does anyone know how does SMSC get aware of the sender's number and insert it in the TP-OA field of the SMS-Deliver packet stack?

Answer 1

Actually I'm not sure that either this is what you need or this would help you but anyways..

In GSM network there are a lot of nodes and each does certain things for which it is responsible. They communicate to each other with messages, that contain certain information needed for processing of required operation: establishing a call, sending SMS, doing Update Location etc.

So the information about sender's number comes to SMSC from another node called VLR/MSC together with a message saying to it "Yo, one guy wants to send SMS to another guy. So here is this message and information about these guys. Please, forward it".

Here is scenario of user A sending SMS to user B:

1. User A sends SMS to VLR/MSC together with information about its destination
2. VLR/MSC then invokes MAP message ForwardSM (Forward Short Message) to send received data further through the network to a node called SMS IMSC

SMS IMSC is a node that plays a role of interface between network and SMSC

3. SMS IMSC on receipt of ForwardSM message sends TPDU (Transfer Protocol Data Unit) message towards SMSC with the text of sent SMS and routing information

4. After this SMSC sends reply to user A with a result of receiving the SMS. If it's successful then SMSC starts routing this SMS through the network towards user B

So the message you are looking for is TPDU message from Point 3. Type of this message is SMS-SUBMIT according to TPDU types.

Edit:

Authentication is separate operation that takes place when subscriber starts his activity (eg turns on his phone) or roams to another area that is being served by another VLR/MSC. Anyway, VLR/MSC treats this user as new to this area. Skip this part if you know what is VLR/MSC:

HLR is a node that stores all the information about every subscriber in operator's network. So basically it's a database that has all the info about every single subscriber in the entire operator's network.

And network covers huge geographical area so it's divided is smaller areas called cells. VLR/MSC is a node similar to HLR (it also stores subscribers' data). But it is responsible for one certain cell, so it stores data of subscribers who currently operate in this certain cell. So when new subscriber comes to its area VLR/MSC downloads his data from HLR. And when subscriber leaves this area VLR/MSC removes his data.

And if some node would need to get subscriber's data, it won't "ask" HLR for it, but instead it will "ask" VLR/MSC in whose area the subscriber is currently active.

So when VLR/MSC sees a new subscriber coming to its area, before downloading his data from HLR it triggers authentication process to check if this user is valid and that someone else doesn't pretend to be this user:

1. VLR/MSC sends Send Authentication Info MAP message together with IMSI (mobile subscriber number which is uniquely identified by his SIM card) to HLR
2. HLR checks the given IMSI and validates subscriber. If subscriber exists and there is no troubles with him then HLR sends IMSI to AUC (Authentication Center)
3. On IMSI reception AUC generates a triplet (some encryption keys needed for subscriber identification) that contains SRES, RAND and Kc values and sends this data back to HLR
4. HLR forwards this data back to VLR/MSC which triggered authentication process
5. VLR/MSC after reception of SRES, RAND and Kc sends received RAND to mobile subscriber
6. After mobile subscriber receives the RAND he calculates SRES using given RAND and Ki (some value that is written in user's SIM card) and sends calculated SRES back to VLR/MSC
7. On the final step VLR/MSC compares SRES generated by AUC with SRES generated by user. And if they match then user is identified successfully, VLR/MSC fetches subscriber's data from HLR and authentication process is completed.

After VLR/MSC downloaded data from HLR every other node that would need data of this subscriber would fetch it from VLR/MSC not HLR. And after that authentication process won't start every time subscriber wants to do something (make a call, send SMS and so on).

So when user A sends SMS to user B the data about user A (his number for example) is stored in VLR/MSC already and it arrives to SMSC from VLR/MSC together with the SMS user A sent.