What is SMS :
Full form of sms is the Short message Service is the basic service provided by all GSM mobile operators. Service enables to send text from a mobile to other mobile. The 3GPP standard for service is GSM 03.40. When a mobile user buys a sim card. The mobile operator configures the sim card on HLR. The sim card configuration have teleservices configuration along with other services. To activate text , service, MO-SMS (teleservice code 33) and MT-SMS (teleservice code 34)services should be configured on HLR.
Upon location update, hlr sends the teleservice list, to the roaming VLR/MSC. While sending Mobile Originated sms, VLR checks if the subscriber have MO-SMS service is configured, then text go out from from VLR to the SMSC, else subscriber received error in sending.
For a mobile terminated (MT-SMS), sender smsc, do a lookup with the received HLR, if HLR , don't have MT-SMS configured for destination mobile subscriber, mobile terminated SMS fails.
SMS have the following components:
When a mobile subscriber sends a text to any other device. After press send , a MO-SMS originates from VLR and reaches to the SMSC. To deliver text to destination subscriber, smsc do a location look (uses SRI-SM MAP message over SS7/Sigtran). If response is an user error (e.g subscriber not reachable ), smsc stores and tries after an interval. If total time exceeds the validity time , message is dropped.
Message Waiting Data:
When HLR receives SRI-SM request from smsc and subscriber in not online or can not receive text (e.g memory is full). HLR returns user error and stores the GT of smsc, in message waiting data list. If subscriber becomes ready again , hlr sends Alert SC to the smsc.
More Messages to Send:
According to gsm standard, a text message can be max 160 bytes long. But a mobile subscriber may send a long (more than 160 bytes) message. This requires segmentation and reassembly of long message.
For MO-SMS, VLR segments long message into 160 bytes smaller messages. Each small message have segmentation number and indication to the receiving side that there are more small messages are yet to come. SMSC do reassembly and stores the full message.
For MT-SMS, smsc does segmentation and vlr does reassembly.
Service Center Time Stamp:
This is information which smsc , sends to the mobile device in a MT sms, the arrival time of smsc at service center. This should be unique in all messages delivering to same Mobile. If two or more messages arrives with in a second , unique time stamps are generated. The difference is kept minimum in time stamps.
SMSC uses this field, to attempt to deliver a message , which was failed earlier. The failure was temporary (e.g no more capacity in mobile). SMSC , don't wait ffrom HLR to send alert service center.
This is a one byte parameter. Define how the message will be handled by the MS. The OTA message uses different PID than normal text message.
SMS PDU Type:
SMS-Deliver : Sent from SMSC to the mobile device.
SMSC-Submit: Sent from mobile to smsc.