SMS in 2G/3G

I know, I know – this is not really the Rich communication related stuff. However it is good to understand the roots of Instant messaging in 4G network. For us it is also important because of the IP-SM-GW Transport Level Interworking.

Originally the core network elements supported services. SMS service was one of them and very important one. The application server is called SMSC.

2G and 3G network architecture

The main functionality of SMSC is to “Store-and-forward”. Basically the SMSC receives an SMS (MO-FWSM), stores it and acknowledges it back. Then it tries to deliver it. For that it needs to receive the routing information from HLR. If the delivery is not successful the message is scheduled for a retry.

The routing of MT message is done based on the information received from HLR. So firstly based on the message prefix we will route Send-Routing-Information-request (SRI-req) to a responsible HLR. HLR takes a look in its tables and finds out what MSC is currently handling the recipient. The address (Global Title) is returned as Network-Node-Number (NNN). It is possible to return both MSC and SGSN address and the preference how to deliver the message.

SMSC flow

There some more operations as AlertSC and RMDS which have to be supported. Alert-Service-Centre message is used to trigger the SMSC to deliver messages of previously Absent Subscribers. Report-SM-Delivery-Status is sent by SMSC to update the information about subscriber in HLR. Anyway both the architecture and the massage flows are much simpler than in case of IMS.

SMS Flow

Homerouting

In IMS we are used to apply services for both – originator and recipient. When we look at the original SMSC flow, we see that we can apply the services only for the originator. Around 2006 mobile operators realized that they are loosing a big money and came a concept of homerouting. (Actually technically it was present for a few years already as so called Foreign Subscriber Gateway – FSG.)

The idea was simple. Instead of direct delivery to the recipient’s MSC, the SMSC of the originator (SMSC-A) will forward the message to the recipient’s SMSC (SMSC-B). SMSC-B will apply the services for the recipient and will try to deliver the Short Message.

This should be done in a transactional mode and the SMSC-A is still responsible for the retries. That’s because the SMSC-A needs to know the delivery result. Hence it can generate the notification ‘delivered/deleted’.

Homerouting

Note, that the SMSC-B acts – from the SMSC-A point of view – as both, HLR and MSC. That means that the GT of SMSC-B has to be preconfigured for SRIs on the SMSC-A.

Btw. The homerouting can introduce very nice loops in the network (which some operators intentionally misused ;)).

Not all the operators use this call flow. Also mainly in the North America mobile operators prefer SMPP protocol instead of Sigtran in case of transfer between networks (SMSC-A to SMSC-B).

In the 4G of networks we reuse the homerouting scenario and the role of SMSC-B is played by IP-SM-GW.

Related posts:

• SMSC 30 years after
• IP-SM-GW Transport Level Interworking
• VoLTE Online Charging – SMS

Third Party Registration

The last time we went through the registration in the IMS. During the registration S-CSCF (the SIP Server handling a particular subscriber) authenticates the subscriber, learns her current point of presence, capabilities of her device, etc. But when it comes to multimedia sessions, S-CSCF can offer only some basic functions such as session routing or session management. For VoLTE or RCS service profiles we want to apply the service logic provided by Application Servers (TAS, RCS, IPSMGW..). The purpose of the Third party registration is to let the ASs know that the user is now connected and ready to communicate.

We ended up in the situation when a registrar (S-CSCF) authenticated the user. After the successful authentication S-CSCF downloads a user’s profile from the HSS. The profile contains information about what Application Servers (ASs) shall be triggered on behalf of the subscriber. The information is stored in a form of initial filter criteria (iFC).

Third Party Registration

Much Ado about Registration

Registration is something what was missing in the times of wires. Ok, not completely but it was done more or less once when an MSISDN was assigned to some line.

IMS Registration

In contrast to the previous types of the networks the 4G network is ‘user-centric’. It means the user can use multiple devices and identities and we have to deal with it. The main purpose of the registration is to create a binding between user (her public identity) and IP address of the device, so we know where we can send the data to. That’s why there is a Contact header in the SIP REGISTER message. The Contact header contains an address which identifies the current location of the user (Point-of-Presence – e.g. IP/FQDN of a particular client). During the registration is the Contact Address is linked to a Public Identity (IMPU, AOR in SIP terminology). The IMPU is an equivalent to MSISDN in GSM and has to be present in the To header of the SIP REGISTER. One identity can by used by more terminals and as each terminal can have different capabilities this has to be also taken into account. This information is part of the Contact header.

Contact: <sip:119560022547@152.67.235.234:49635;transport=tcp>;
   expires=3200;
   +g.oma.sip-im;
   language="en,fr";
   +u.asmc.apn="6a6044869e2aba3d23d4cfc0ef1384d00da28854c2408ddbbf";
   +sip.instance="<urn:uuid:D4196919-ED8A-4E00-9436-B9CDD1E76813>"

Messaging in RCS

Instant (multimedia) messaging is a key feature for RCS as it should replace SMS service. There are more types of messages which can be sent through the IMS network.

When we will take a look on the SIP indication of capabilities by Accept header there are quite a few of supported formats:

• Text/plain
• Message/CPIM
• Application/reginfo+xml
• Application/vnd.3gpp.sms
• Message/imdn+xml
• Application/3gpp-ims+xml

For common Instant Messages we typically use Common Profile for Instant Messaging (CPIM). This format allows compatibility among various messaging technologies. Notifications are then sent as in Instant Message Disposition Notification (IMDN) format.

Messages can be exchanged over SIP or MSRP protocol. Sometimes  also http/ws is being used for transfer of rich content.

Update: For the information about RCS Universal profile read the post GSMA Advanced Messaging – RCS Universal Profile.

RCS 5.2 defines two basic types of messaging. They are the Session Mode Messaging which is based on SIP INVITE and MSRP session and the Pager Mode Messaging which is based on SIP Message. The RCS refers to OMA CPM which is considered to be an evolution of the SMS/MMS messaging services.

Note that SIP MESSAGE can be used also for other than messaging purposes in IMS network (e. g. during eSRVCC registration)

 

Session Mode Messaging

Session mode does use SIP INVITE and SDP protocol to establish the session. Within the session MSRP protocol is used. This allows to split ones conversation into dialogs and use “..is typing” notifications. On the other hand more resources is needed.

IMS and MSRP

RCS – Rich Communication Services

As this is a blog, I’m not going to recap all the stuff you can find in wikipedia about RCS. Wikipedia is a great source of information but sometimes you can’t see the forest for the trees. The RCS is really a big theme so I guess it would be better to write several posts than to try put all the relevant information in one. I’ll probably use some simplifications, the details and exceptions will be described later.

 

So what is the RCS?

Update: For the information about RCS Universal profile read the post GSMA Advanced Messaging – RCS Universal Profile.

RCS is an attempt of mobile operators and their vendors to be more than just connectivity providers. With the 3rd generation of mobile networks many people started to believe that they’ll not need the operators soon – as long as they can connect to the Internet they have VoIP, Instant messaging, Emails, Facebook, etc (Over-the-top  applications OTT). Why should anyone pay for roaming or use SMS anymore? But for the operators Voice services and SMS were/are the main sources of income. VoLTE and RCS should give their customers the user experience they know from Skype, Whatsapp, Viber or Line and enable them to stay relevant.