Short Message Service: What, How and Where?
Short message service is a mechanism of delivery of short messages over the mobile networks. It is a store and forward way of transmitting messages to and from mobiles. The message (text only) from the sending mobile is stored in a central short message centre (SMS) which then forwards it to the destination mobile. This means that in the case that the recipient is not available, the short message is stored and can be sent later. Each short message can be no longer than 160 characters. These characters can be text (alphanumeric) or binary Non-Text Short messages. An interesting feature of SMS is return receipts. This means that the sender, if wishes, can get a small message notifying if the short message was delivered to the intended recipient. Since SMS used signaling channel as opposed to dedicated channels, these messages can be sent/received simultaneously with the voice/data/fax service over a GSM network. SMS supports national and international roaming. This means that you can send short messages to any other GSM mobile user around the world. With the PCS networks based on all the three technologies, GSM, CDMA and TDMA supporting SMS, SMS is more or less a universal mobile data service. à Note: The actual limit of size of SMS is 160 characters if Latin alphabets are used. If non-Latin alphabets like Chinese or Arabic are used, the limit is 70 characters. How does SMS work
The figure below shows a typical organization of network elements in a GSM network supporting SMS.
The SMC (Short Message Centre) is the entity which does the job of store and forward of messages to and from the mobile station. The SME (Short Message Entity) which can be located in the fixed network or a mobile station, receives and sends short messages. The SMS GWMS (SMS gateway MSC) is a gateway MSC that can also receive short messages. The gateway MSC is a mobile network’s point of contact with other networks. On receiving the short message from the short message centre, GMSC uses the SS7 network to interrogate the current position of the mobile station form the HLR, the home location register. HLR is the main database in a mobile network. It holds information of the subscription profile of the mobile and also about the routing information for the subscriber, i.e. the area (covered by a MSC) where the mobile is currently situated. The GMSC is thus able to pass on the message to the correct MSC. MSC (Mobile Switching Centre) is the entity in a GSM network which does the job of switching connections between mobile stations or between mobile stations and the fixed network. A VLR (Visitor Location Register) corresponds to each MSC and contains temporary information about the mobile, information like mobile identification and the cell (or a group of cells) where the mobile is currently situated. Using information form the VLR the MSC is able to switch the information (short message) to the corresponding BSS (Base Station System, BSC + BTSs), which transmits the short message to the mobile. The BSS consists of transceivers, which send and receive information over the air interface, to and from the mobile station. This information is passed over the signaling channels so the mobile can receive messages even if a voice or data call is going on. Applications
Some of the common applications of SMS are:
* Exchanging small messages like "See you at 8.30 tonight at xyz". SMS is particularly suited for these kinds of short messages because SMS is much cheaper than calling some one and giving the same message. Calling some one to give the same message would invariably take more time and hence more cost. * Many operators offer e-mail service over SMS. Every user is assigned an e-mail address at signup and any message delivered to that email is converted to short messages and delivered to the mobile. * It is possible to send e-mail messages (less than 160 characters) from a mobile...
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