Short Messaging Service (SMS)

Short messaging service (SMS) is a mechanism of delivery of short messages over the mobile phone networks. It is a store and forward way of transmitting messages to and from mobiles. The message (test only) from the sending mobile is stored in a central short message center (SMS) which then forwards it to the destination mobile. This means that in case that the recipient is not available, the short message is stored and can be sent later. But it is worthy noting that that there is a limit to how long the message can be stored. Some networks store the messages for 24 hours and others 48 hours. It differs from one mobile phone network provider to another. After the elapse of this time the message is automatically deleted.

Each short message can be no longer than 160 characters and contain no images or graphics. These characters can be text (alphanumeric0 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 uses 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.

SMS appeared on the wireless scene in 1991 inEurope. The European standard for digital wireless, now known as the Global System for Mobile Communications (GSM) included short messaging services from the outset. The first message was sent in December 1992 from a Personal Computer (PC) to a mobile phone on the Vodafone GSM network in . 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 SMC (ShortMessageCenter) is the entity which does the job of storing and forwarding 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 networks point of contact with other networks. On receiving the short message from the short message center, GMSC uses the SS7 network to interrogate the current position of the mobile station from 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 (Mobil SwitchingCenter) 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 from the VLR the MSCC 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 transreceivers, 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.