The GSM Short Message Service


SMS is the ability to send and receive short messages of up to 160 characters at a mobile phone. SMS was conceived as part of the Global System for Mobile communications (GSM) digital standard, originally only at 900MHz but later also at 1800MHz (also known as DCS1800), 1900MHz (also known as PCS1900) and 800MHz (E-GSM). You can therefore in principle use these facilities on any GSM or "Personal Communications Networks" (PCN, which is also GSM). In the UK these networks are run by Cellnet, One2One, Orange and Vodafone. Cellnet and Vodafone also run analogue TACS networks.

SMS can be used for any purpose where voice is not the easiest or best communications medium. For example, it's very difficult to remember phone numbers and re-key them - you can easily give someone your phone number via SMS and usually the phones provide an easy way to extract the number from the Short Message and store it in memory.

Having established that you have a suitable digital phone, you need to check the exact facilities offered by the phone and the network.

SMS Mobile Terminated (MT) is pretty much ubiquitous unless you have a very old phone. This is the ability to receive text messages at the phone.

SMS Mobile Originated (MO) is only on a few phones. This is the ability to send text messages from the phone. Keying messages into a phone is a little arduous as you need to press several keys per character, but it can be mastered after a few attempts. You can also connect external devices to some phones, which have full QWERTY keypads and allow proper typing.

The networks can choose which (if any) of these services to provide, and whether you have to pay to have the use of the service and/or for each use. In the UK, the following approach has been adopted:

Cellnet - MT and MO are subscription free. MT is always free to the recipient. MO is charged at 10p+VAT per message.

One2One - MT is subscription free. MO is charged at 400p/month plus 4p/message.

Orange - MT and MO are subscription free. MO is charged at various rates according to the Orange "Talk" package.

Vodafone - MT and MO are subscription free. MT is always free to the recipient. MO is charged at 10p+VAT per message.

Having established that you have SMS facilities, the next thing is to look at how to get messages to and from your phone. Your phone instruction manual should explain how to deal with messages, but one important aspect is the Service Centre number. Sometimes called the "Message Centre", this is a store-and-forward machine which keeps messages until they can be delivered.

The GSM principle is that each network provides one or more Service Centres and each of these can send to any GSM mobile on any network. In practice, however, there are no agreements to allow SMS to travel between networks, although in many cases this is working at present.

So the ultimate goal is to set your Service Centre number to the one provided by your network, and forget it. You can then enter any number and send a message, just like making a voice call. Until that ultimate goal is achieved, it is still possible to contact users on other networks. To do this, refer to the SMS Connectivity Summary. There are two lists which should help.

* Service Centre -> Network connectivity (MT) (identified which Service Centres can send to subscribers of which networks)

* Service Centre <- Network connectivity (MO) (identified which Service Centres can receive from phones logged on to which networks)

If you want to send from Network A to Network B, you need to look down the second (MO) list and find network A on the right. This will show you the Service Centres (on the left) you can send to. You then need to find Network B on the first (MT) list, again on the right. This will show you the Service Centres (on the left) your correspondent can receive from. If the same Service Centre can receive from network A and send to Network B then you can contact your correspondent.

Having found the Service Centre you need to use, then look for the address under "Service Centre Addresses".

There are other ways to send SMS messages - 3 of the 4 UK networks have public dialup access, using either plain text or Telocator Alphanumeric Protocol (TAP). There is public dialup access to other GSM networks as listed, along with DTMF access where you select the appropriate message by pressing keys on the phone.


Technical

When you send a mobile originated SMS message, you tell your local cellular exchange where to send the SMS message. This is the "message centre" or Service Centre address. The SC address is normally stored on the phone and/or SIM card. The cellular exchange routes the SMS message in an SCCP packet. International SCCP messages are routed based on a Global Title. The Global Title used for SMS is the Service Centre address. The SCCP packet is passed from exchange to exchange until it reaches the destination Service Centre. Each exchange along the route inspects the Global Title and uses this to route the message to the next exchange in the chain. Some exchanges look at the first few numbers (e.g. 44 = UK) and simply pass the message on. Others expect to see valid ranges (e.g. 44802 is OK and 44860 is not OK). So not all messages reach their intended Service Centre because either one exchange along the route doesn't know where to pass the SCCP packet on, or an exchange thinks that the number is not valid and refuses to pass the packet on.

Once the message is received at the SC, it sends back a confirmation "I've received your message" . This is put into another SCCP packet, either at the SMSC or at the nearest exchange. This is routed in similar fashion back to the original cellular exchange and on to your mobile. Again, this can go astray for the above reasons, or occasionally rejected by the destination exchange because it is not understood (incompatible implementations). This results in a failed message indicator on your phone, but the message has been received by the SC and will be sent to its destination.

When the SC tries to send a message back to a mobile, it must first ascertain where that mobile is. It needs to ask the mobile's HLR for location information. It has no knowledge of where the HLR is, only the mobile number. So what it does is to send a location request SCCP packet, routed to the mobile number. The international SCCP network routes this packet not to the mobile, but the the appropriate HLR. Again, such packets can go astray because of incomplete routing. This has been evident recently when people have tried to send from overseas SCs to Orange mobile numbers - with 44973 reaching the mobile and 44976 not reaching the mobile. This failure is simply an absence of SCCP routing for 44976 between the overseas SC and the Orange network, at one or more exchanges along the way.

The HLR returns the location information in another SCCP packet. Again this can go astray or be incomprehensible for the reasons described above.

The SC then sends the message to the cellular exchange local to the mobile (as received from the HLR). This is addressed via another Global Title. The routing for this part is normally OK, as for roaming to work properly for voice, every cellular exchange global title has to be reachable. But of course the cellular exchange can in some cases be unhappy with the received packet and not pass the message on to the mobile.

Once the mobile has received the message, the local cellular exchange sends a confirmation SCCP packet back to the SC. Routing may again fail for this, or the SC may not understand the received packet format.

In summary, the sending and receiving of SMS across multiple networks is fraught with problems and it's a wonder that it works in the majority of cases!

SMS Technical information

SMS Routing - another explanation including diagram

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Last Updated: 26th January 1998