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Short Message Service: What, How and Where?
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Introduction 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 center (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 Center) 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 center, 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 Center) 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 + BTS’s), 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. To use the Short Message Service, users need the relevant subscriptions and hardware, specifically: Ø A subscription to a mobile telephone network that supports SMS Ø Use of SMS must be enabled for that user (automatic access to the SMS is given by some mobile network operators, others charge a monthly subscription and require a specific opt-in to use the service) Ø A mobile phone that supports SMS Ø Knowledge of how to send or read a short message using their specific model of mobile phone Ø Destination to send a short message to, or receive a message from. This is usually another mobile phone but may be a fax machine, PC or Internet address. 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 phone to any e-mail address via SMS. · Information services like news, weather, entertainment and stock prices etc. can be availed just by sending a keyword like NEWS, WEATH etc to the short message center number. · SMS can be used by the network operators to provide services like balance enquiry in case of prepaid cards using SMS. · Mobile chatting is one more hot application of SMS · SMS can be used to notify users that they have received new voice-mail or fax messages. · It provides an alternative to alphanumeric paging services · Using SIM-Toolkit, now a part of GSM specifications, SMS can be used to have on the air activation of features. By sending codes embedded in short messages from the server network operators can remotely provision the user's wireless terminal · Internet e-mail alerts. · Downloading new ring tones. Limitations of SMS There is no doubt that SMS has been very popular. The figures in the section above support this. What is more interesting to observe is that this popularity has been in spite of many limitations of SMS. Many of these limitations are the driving force behind the developments and initiatives being taken in the field of short messaging. Some of the limitations of SMS are: · Messages are plain vanilla in nature. You can only send simple text messages. There is no scope for any graphics or audio. However As mentioned in the next section EMS would help fill this gap. · The messages are limited by size. An SMS message can’t exceed 160 characters. (BTW this limitation is due to the limitation in the MAP protocol in GSM) In case of longer e-mails or information service messages like news, the messages need to broken down into more than one message.
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