Now we will study the fourth part, short message sequence. The mobile originated short message transfer procedure is separated from mobile terminated. The basic short message sequence consists of two parts: MO SMS Transfer and MT SMS Transfer.
It’s very similar to the call sequence.
At first let’s see MO SMS transfer sequence.
Like the call sequence, the subscriber pressing the “send” key initiates a “Channel Request” message from the MS to the BSS. This is followed by the assignment of dedicated control channel by the BSS and the establishment of the signaling link between the MS and BSS.
The message “Request for Service” is passed to the MSC which relays it to the VLR.
The VLR will carry out the authentication process if the MS has been previously registered on this VLR, if not, the VLR will have to obtain authentication parameters from the HLR.
The message “RP_MO_DATA” is sent by the mobile to the MSC accompanied by the SM information, such as type of SM, the SMC number. This message is forwarded from the MSC to the VLR, that is, the MSC should query to the VLR whether the mobile subscriber has authority of sending short message via “SIF_MO_SMS” message, and then VLR returns the results for query.
If the mobile has the authority to send short message, the MSC will then transfer the short message to the interworking-MSC via the message ”MO_Forward_SM” tagged with SMC number. The SMC number is set on the mobile station. It’s the home SMC of the mobile.
The interworking-MSC will then forward the short message to the corresponding SMC. Once responds from SMC,
And the MSC will then send the message ”RP_ACK” to MS in response to the message ”RP_MO_DATA”.
At this time “Send Successfully" is displayed on the mobile. It’s terminated the MO sequence.
Ok, now let’s see the MT SMS transfer sequence. It consists of two cases: for one short message and several messages.
At first I’ll introduce the MT SMS transfer sequence for one message.
A message “short message” arrives at a gateway-MSC. The MS to be called is identified by its MSISDN. The GMSC will then request routing information from HLR using the message “SRI_for_SM”, still tagged by the MS’s MSISDN. The HLR then responds with the message “SRI_for_SM_ACK” which includes the VLR number. This information will enable the GMSC to identify the MSC to which the “Forward_SM” must be directed.
Then the “visitor” MSC requests call Set-up information from the VLR using the message “SIF_MT_SMS”. The VLR response is the “page” message back to the MSC, containing the required information. The MSC then sends “Paging Request ”to the MS via the appropriate BSS.
If the mobile is powered on, and in the service area, the MS responds and requests a dedicated control channel from the BSS. Once the air interface signaling link is established, the dedicated control channel carries the “Paging Response” to the BSS which relays it to the VLR via the MSC.
Then the MS is authenticated and cipher mode is set. Of course it’s optional by the operator.
The servicing MSC will then transfer the short message to the MS. The MS acknowledges this and sends “Short_Message_Ack” to the MSC. The servicing MSC then responds to the gateway MSC using the message “MT_Forward_SM_Ack” and in turn the gateway MSC sends acknowledgement to the SMC in response to the message “Short_Message”.
Ok, let’s see the sequence for MT transfer several messages.
At first it’s same to the sequence for one message, a message “short message” arrives at a gateway-MSC. The MS to be called is identified by its MSISDN. The GMSC will then request routing information from the HLR using the message “SRI_for_SM”, still tagged by the MS’s MSISDN. The HLR then responds with the message “SRI_for_SM_ACK” which includes the VLR number. This information will enable the GMSC to identify the MSC to which the “Forward_SM” must be directed.
Actually, in the message “MT_Forward_SMS”, there is a flag for more message, in this case, this flag is true. If only one message will be transferred, the flag is false.
The “visitor” MSC then requests call set-up information from the VLR, the VLR responds with “page” message. The MSC then sends “Paging Request” to the MS via the BSS.
The MS responds and requests a dedicated control channel from the BSS. Once the air interface signaling link is established, the dedicated control channel carries “Paging Response” to the BSS which relays it to the VLR via the MSC.
The MS is then authenticated and cipher mode is set. Of course, it can be optional by the operator.
Then the MSC transfers the short message to the MS through the BSS. Once the MS sends the acknowledgement to the servicing MSC, the MSC will then respond to the message “MT_Forward_SM” to the gateway MSC which in turn sends response to the SMC. Now one message has been transferred to the MS.
Then the SMC sends the second message to the gateway MSC. The gateway MSC will transfer the message via “MT_Forward_SM” to the servicing MSC without request routing information to the HLR. The “visitor” MSC then sends the message to the MS. Once the MS responds, the MSC sends acknowledgement to the gateway MSC using the message “MT_Forward_SM_Ack”. In turn the gateway MSC responds to the SMC.
If the flag in the message “MT_Forward_SM” is false, after the message is received by the MS, the sequence for several messages is closed.
Ok, after the basic SMS sequence, let’s answer a question.
What is the sequence when the originating subscriber sets the wrong SC No. in the mobile station.
MO_Forward_SM Short_Message MO_Forward_SM_Ack Short_Message_Ack RP_ACK (SC_No.) MS BSS MSC VLRInterworking MSC SC
"Send Not Successfully" is displayed on the mobile
Answer
Answer
Illegal Subscriber Illegal Subscriber
From the MO SMS sequence, we know, after the establishment of the air interface signaling link and authentication, the MS sends “RP_MO_Data” to the MSC. The MSC will then transfer the short message to the interworking MSC, tagged with SMC number.
The SMC number is set on the mobile, and sent from the mobile to the network. This SMC is the home SMC of the mobile. If the mobile set the wrong number, and if the number is other SMC number, then the gateway MSC sends short message to the other SMC. In this SMC, the mobile does not exist, so the SMC returns the acknowledgement with the cause of illegal subscriber, and then “Send Not Successfully" is displayed on the mobile.
Ok, that’s all for the fourth part.