SMS Transfer On Um
GSM 04.11 and 03.40 define SMS in five layers:
- L1 is taken from the Dm channel type used, either SDCCH or SACCH. This layer terminates in the BSC.
- L2 is normally LAPDm, although GPRS-attached devices may use Logical link control (LLC, GSM 04.64). In LAPDm SMS uses SAP3. This layer terminates in the BTS.
- L3, the connection layer, defined in GSM 04.11 Section 5. This layer terminates in the MSC.
- L4, the relay layer, defined in GSM 04.11 Section 6. This layer terminates in the MSC.
- L5, the transfer layer, defined in GSM 03.40. This layer terminates in the SMSC.
As a general rule, every message transferred in L(n) requires both a transfer and an acknowledgment on L(n-1). Only L1-L4 are visible on Um.
Read more about this topic: Um Interface
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