Source Address Translation (SAT) in EMG Print
Tuesday, 30 November 1999

When an SMS is sent to a recipient it is sometimes desirable for the recipient to be able to reply to the SMS by simply hitting the reply button. This is no problem for SMS that originates from another subscriberÂ’s mobile device but is a far greater problem for messages that originate from e-mail systems or other applications where there is no unique MSISDN source address for the sender.

Also the possibility to determine to which specific message a received reply relates poses a similar problem.

The solution to the problem is to perform source address translation on the originating address so that the real sender address is replaced with a valid MSISDN or short code before the message is sent out. When a reply is then received the original sender address can be retrieved by performing a lookup.

This procedure is equivalent to the procedure of network devices that perform NAT (Network Address Translation).

In order for one recipient to receive multiple messages from different senders with translated addresses a pool with multiple source addresses needs to be allocated. All addresses in the pool (usually short codes) must be configured in the receiving SMSC to be sent to the account where these messages will be received.

The combination of source address and destination address must be unique per sender for this procedure to work. The more messages with translated addresses a subscriber is likely to receive within a certain timeframe from different senders the larger the address pool must be. On the other hand in an environment where many different recipients receive few messages a relatively small pool can serve a large amount of users.

In order to determine which specific message a reply belongs to the combination of source address and destination address must be unique for each message which requires a pool large enough to accommodate one source address per sender per message.