SMS Units

SMS, or Short Message Service, is designed to carry only short messages. For plain text 7-bit ASCII characters, the maximum message length supported is 160 characters.

Longer text messages are supported, but they must be broken up into shorter chunks for transmission on the mobile network, then reassembled at the receiving end. Each chunk corresponds to the actual amount of text that a single SMS can support. Each chunk also require some space to contain multi-part information, which affects the overall message length.

Messages are charged according to the number of SMS units that actually need to be transmitted on the mobile network. A long text message that needs to be broken up into 3 chunks will be transmitted as 3 SMSes, and hence will be charged for 3 SMSes.

The following table illustrates the number of SMS units required to transmit plain text 7-bit ASCII character messages of various lengths.