Short Message Service (SMS) Terminology SCT Abstract The paper deals with the Short Message Services (SMS) terminology with end to end call flow. I think everyone in whole world know about SMS, they use it to tell something about their works, relationship etc. others. Every mobile operator in the world should have SMS services beside the voice. With voice they can real time communication in same time, but in SMS we just send our message and wait whether it delivered or not. Some question often arises is how SMS works? Why my message not delivered to my friend? Why sometimes the message just arrived to my friends in the next hour not real time? In this paper I just try to explain the basic SMS flow and works in our mobile operator to make understand us as a mobile subscriber. Keywords: Short Message Service; SMS; ESME; SMSE; Message Terminology 1. Introduction SMS or Short Message Service is a service provided by an operator of GSM / CDMA besides voice service. SMS and Voice is primary service in common mobile operator. The SMS is non-circuit related, while the circuit voice service is so important to understand in terms of the protocol used. It is better to know the basic protocol used in SMS that is SS7. I will explain it overview in other articles. Basically we can send SMS to other subscriber or to some services like short code which have digit less than prefix number. They work in different environment protocol they are using. The common protocols you have understand SMS are SS7 (Signaling System No 7) and SMPP (Short Message Peer to Peer Protocol). Those protocols representative two different domains : SS7 used to support communication between elements core network in mobile operator, while the SMPP used for communication in IP domain between SMSC ( SMS Center) and its external services like ESME ( External Short code Messaging Entity ) such as Service Provider or Content Provider. Sometimes you ever got message from a short code which consists of advertisement from your operator. It is one of services which operator can provide. It is using SMS blast server which do interconnection with SMS Centre through the SMS Gateway. So SMS Centre using SMPP protocol to communicate with SMS Gateway before sending SMS to ESME. How about Service Provider which using likes web application in their service? Who it can communicate with SMS gateway and SMSC instead of they are using SMPP Protocol. Here SMS Gateway should have ability to do conversion from SMPP Protocol to HTTP protocol which a Service Provider used. In this article, I just explain the basic overview about SMS. 2. Description The transmission of short text messages to and from a mobile phone, fax machine and/or IP address. Messages must be no longer than 160 alpha-numeric characters and contain no images or graphics. Why SMS text message limited to 160 characters? Somebody often ask to me why SMS text messages are limited to 160 characters for one message or one SMS. So I just share with this articles to explain what I know. Even you can send SMS until 960 201
characters but they are just sent in 6 separated text messages. Back to question why SMS text message limited to 160 characters. To answer that question, we should have basic concept of SMS text message. We have to know about SMS Protocol Description Unit ( PDU ) which is way sending SMS text mode. SMS text message is sent by SMSC ( SMS Center ) handset using SS7 protocol. If you see more in SS7, we need application layer called Mobile Application Part ( MAP ) which send SMS text message using application module Message Originating ( MO ) and Message Terminating ( MT ). In this protocol we can answer that limitation is in SMS development. When it was built, SMS just have Digital Coding Scheme ( DCS ) signaling protocol limited 140 octets, which means 140 * 8 bits = 1120 bits. For default alphabet, we use 7 bit / characters, and for another purpose like smart messaging like images and ringing tones or Chinese and Japanese we user 8 bit. For other purposes 16 bit characters encoding will be used like for Flash SMS which show blinking or alerting SMS. So for default alphabet we have 1120 bits / 7 bits = 160 characters. In same we way, we have 140 characters in 8 bit, and 70 characters in 16 bit. So we can see now that for one SMS text messages we have limited 160 characters for 7 bit alphabet, 140 characters for 8 bit alphabet and 70 characters for 16 bit characters. Even your mobile phone has capabilities to write more than 160 characters, the SMS system just see that in several messages. You can see the indicator in your text messaging that your long SMS based on phone capabilities only the way to write SMS. SMS system only see it in several messages. For example if you can write until 400 characters, the system only see it in 3 SMS text messages and you will be charged for 3 messages by the mobile operators. But you have alternative way to send long messages to your mobile operators. To increase the maximum characters in one SMS text message we can concatenate SMS in larger content, it is called as long SMS. It is configured in your mobile operator not in your handset. In concatenated SMS, you will have additional User Data Header ( UDH ) contains segmentation information. Because of that the maximum capacity will decrease become 153 for 7 bit, 133 for 8 bit and 67 for 16 bit. Here some mobile operators have tariff scheme for this implementation. Most of them will charge additional UDH bits, because they use system resources. Once a message is sent, it is received by a Short Message Service Centre (SMSC), which must then get it to the appropriate mobile device. To do this, the SMSC sends a SMS Request to the home location register (HLR) to find the roaming customer. Once the HLR receives the request, it will respond to the SMSC with the subscriber's status: 1) inactive or active 2) where subscriber is roaming. If the response is "inactive", then the SMSC will hold onto the message for a period of time. When the subscriber accesses his device, the HLR sends a SMS Notification to the SMSC, and the SMSC will attempt delivery. The SMSC transfers the message in a Short Message Delivery Point to Point format to the serving system. The system pages the device, and if it responds, the message gets delivered. 202
The SMSC receives verification that the message was received by the end user, then categorizes the message as "sent" and will not attempt to send again. 3. End to End Call Flow of Short Message Service (SMS) SMS diagram network Fig-1 How SMS works According to the picture SMS have two domains: IP domain and SS7 domain. So we need protocol conversion between those domains in our SMS Centre. SMS delivery can be divided into 2 parts; they are Mobile Originated (MO) and Mobile Terminated (MT). We can say easily that MO is the process of sending SMS from a number until it reaches the SMSC. The indicator in mobile subscriber is they get notification message sent in their phone. So the message which is sent is just arrived in SMSC of his operator, not yet reached destination number. Next we can say that if message already delivered to destination number, the sender will get the notification which we call as delivery report. Commonly delivery report message is like message delivered / message successfully delivered, etc. MT is process of message delivery which already reached SMSC to destination number.it is the way how SMS Centre send message to destination number. 3.1 Mobile Terminated (MT) SMS Flow Here is a Simple Explanation of Mobile Terminated (MT) SMS Flow 203
Fig-2 MT Call flow 3.1.1 SMS MT Call Flow 1. The Short Message is transferred from SMSC to SMS-GMSC. 2. SMS-GMSC queries the HLR (SRI) & receives the routing information for the mobile subscriber (SRI-ACK). 3. The SMS-GMSC sends the short message to the MSC using "Forward Short Message" (FSM) operation. 4. The MSC retrieves the subscriber information from the VLR. This operation may include the Authentication Procedure. 5. The MSC transfers short message to the Mobile Station (MS). 6. The MSC returns the outcome of the "Forward Short Message" operation to the SMS-GMSC (FSM-ACK). 7. If requested by the SMSC, it returns a status report indicating Delivery of the Short Message. 8. 3.2 Mobile Originated (MO) SMS Flow Here is a simple explanation of Mobile Originated SMS flow- 204
3.2.1 SMS MO Call Flow 1. The mobile station transfers the short message to the MSC. 2. The MSC queries the VLR to verify that the message transfer does not violate the supplementary services invoked or the restrictions imposed on the subscriber. 3. The MSC sends the short message to the SMS-IWMSC (Inter-Working MSC for SMS) using the forward Short Message operation. 4. The SMS-IWMSC delivers the short message to the SMSC (Short Message Service Centre). 5. The SMSC acknowledges the successful outcome of the forward Short Message operation to the MSC. 6. The MSC returns the outcome of the short message operation to the mobile station. References 1. Short Message Peer-to-Peer Protocol Specification document. 2. http://www.wirelessdevnet.com/channels/sms/features/sms.html 3. http://en.wikipedia.org/wiki/short_message_service 205