The transmission route of SMS is not different from that of conversation. Whenever a person sends SMS through his mobile, first of all that data is received by the SMS center of the sender’s service provider’s network, which is nothing but the server meant for directing SMS traffic. It is the job of this server to contact the network of the receiver’s service provider and through it the SMS message ultimately reaches the receiver’s mobile. For this process to take place it is necessary that the receiver’s mobile should be switched-on. This is the normal procedure but its noteworthy feature is that the sender’s network does not relay the message to the receiver’s network straightway. The sender’s network first ascertains from the receiver’s network whether the receiver’s mobile is switched-on or not, through the latter’s Home Location Register (HLR).
Suppose the receiver’s mobile phone is not switched-on. In that case the sender’s network requests the HLR of the receiver’s network to pickup the SMS message when the receiver’s mobile is switched-on and in the meanwhile it keeps the message pending. How long the SMS message will be kept pending depends upon service provider’s policy. There is no uniform practice among the service providers. According to the business policy of individual service providers the period of pendency ranges from 2 hours to 2 days.