Protocol Stack used for SMS:

1. The Short Message Application Layer (SM-AL):
   1. Works between MS and SME
   2. Used to send and interpret the SMS
2. The Short Message Transfer Layer (SM-TL)
   1. Offers reliable transport service to SM-AL
   2. works between MS and SC
3. Short Message Relay Layer (SM-RL):
   1. Used to transfer messages through different equipment’s using Store and Forward
   2. In GSM 2 protocols are specified for this layer:
      1. Short Message Relay Protocol (SM-RP): it’s used between the mobile terminal and the VMSC/VLR
      2. Mobile Application Protocol (MAP): it’s used between the VMSC/VLR and the SMS-IWMSC
      3. The protocol between the SMS-GMSC/SMS-IWMSC and the SC does not concern GSM standards
4. The Short Message Control Protocol (SM-CP)
   1. works between the MS and the VMSC/VLR
   2. protects against loss caused by changing the dedicated channel