A service in GSM mobile telephony systems that allows the user to send and receive short (maximum 160-character) messages independently of voice calls; a nearly real-time service that stores messages in message centers if the receiving mobile telephone cannot be contacted. Note: SMS is both the handset function and the network service. SMS is used to inform users of pending voice messages, network outages, etc. , but the principal use is in user-to-user messaging. Addressing is by telephone numbers (GSM only. ) Networks usually relay messages over network boundaries. There are gateways from e-mail and Web to SMS and from SMS to e-mail, but only on an experimental-service basis. The cost of SMS messages is usually fixed and is lower than for a short voice call.