on November 14, 2009 by denpras in Messaging / SMS, Comments (1)

SMS wireless delivery ..

Its better to understand first about the flow and basic SMS concept before read this posting. We refresh again about the method used for sending text message SMS to mobile subscriber. We have two method : sending using mobile device through SS7 network and using application ESME ( External Short Messaging Entity) which using SMPP ( Short Message Peer to Peer Protocol ) in IP based.
Both of them using Short Messaging Service Center (SMSC) for delivery system which control routing and determine store and forward mechanism.
SMSC need HLR (Home Location Register ) to query about status of B number or destination number. HLR will return the status based on report from MSC/VLR (Mobile Switching Center / Visitor Location Register ) about the B number by sending MSC B number current information to A number or sender.

Please see again about flow of SMS network here :

sms_1
You can see at the picture that we have two source : Public Internet / VPN and Wireless Network to reach SMSC. From SMSC point of view we have SS7 domain and IP domain. Further, we can discuss about IP domain in SMSC for support SMPP and HTTP protocol. That is basic concept to implement ESME application and shortcode services from Service Provider or Content Provider.

When A number request sending SMS, SMSC will ask MSC, HLR to determine status of B number. SMSC will determine whether message can be delivered immedietly or pending items. The latest we know as schedulling and retry mechanism of SMSC.

The complexity and power of SMSC can be seen from how complexity method of retry mechanism can be supported. The most common error SMSC faced that the way of sending SMS or message until deliver to B number.

After SMSC asking the HLR, and HLR return status of B number, SMSC will ask MSC to deliver message to B number based on information sent by HLR.
MSC will deliver message using method SMDPP ( Short Message Delivery Point to Point ) to appropiate destination in one BTS ( Base Transceiver Station ) coverage.

So how MSC can find location of B number ? MSC have to do “paging” to find location of B number. We will discuss briefly about it related to wireless delivery of SMS.
The air interface or wireless is divided into two parts :
Control Channels (CCH) and Traffic Channels (TCH). The CCH is further divided into two types of channels : Common CCH and Dedicated
CCH. SMS will be sent using Control channel. In a linkset MTP2, we just need signalling link. Not like Voice transfer, it need voice trunk. So for SS7 mode, we just need TCAP for realibility sending, not use ISUP.

The Common CCH consists of logical channels including
Paging Channel (PCH) and Random Access Channel
(RACH). The Common CCH is the mechanism used by the base station to initiate the delivery of voice and SMS data. Accordingly, all connected mobile devices are constantly listening to the Common CCH for voice and
SMS signaling.

BTS sends a message on the PCH which contain Temporary Mobile Subscriber ID (TMSI) of B number. When B number hear its TMSI, it will contact BTS over RACH and tell to network that it can receive text message SMS.

After BTS get confirmation from B number, BTS will ask B number to listen spesific Standalone Dedicated Control Channel (SDCCH).
Using the SDCCH, the base station is able to facilitate authentication of the destination device (via the subscriber information at the MSC), enable encryption, deliver a fresh TMSI and then deliver the SMS message itself.

We can see the basic flow in this picture :
Paging

BS ( Base Station ) or BTS notifies A number or mobile host MH1 and Mobile host MH2 of new message coming. When MH2 send SMS to MH1, MH1 hears its identifier and responds. After authenticating and establishing an ncrypted channel, the text message is delivered over a dedicated control channel.

That is the basic mode wireless interface of sending SMS. We will continue in advance for sending the wireless SMS. Thank You !