Um Interface - SMS Transfer On Um

SMS Transfer On Um

GSM 04.11 and 03.40 define SMS in five layers:

  1. L1 is taken from the Dm channel type used, either SDCCH or SACCH. This layer terminates in the BSC.
  2. 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.
  3. L3, the connection layer, defined in GSM 04.11 Section 5. This layer terminates in the MSC.
  4. L4, the relay layer, defined in GSM 04.11 Section 6. This layer terminates in the MSC.
  5. 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.

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