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laforge, 07/17/2017 08:33 PM


Titan TTCN3 Notes

Some notes about developing in Titan TTCN3, specifically regarding test cases in the Osmocom universe.

RAW coder

A lot of the messages (GSMTAP, LAPDm, L1CTL, ...) are described using the Titan extensions to the TTCN-3 type language and the associated RAW coder.

Section 4.23 of the Programmer's Technical Reference Guide for the TITAN TTCN-3 Toolset contains the description of this RAW coder

FIELDORDER

This determines the order of fields within a record or set type.

In (not only) GSM specs, typically the messages are described from first field at top to the last field at the bottom, so the logical choice here is to set "FIELDORDER" as we do in the GMS_Types.ttcn and other files. However, there's one CAVEAT: In GSM 04.08 (and derived specs), Information Elements with 1/2 (4bit) length are ordered un-intitively.

Example: TS 44.018 SYSTEM INFORMATION TYPE 4:

  • L1 Pseudl Length (1)
  • protocol descriptor (1/2)
  • skip indicator (1/2)
  • Message Type (1)

which then translates to:

        type record RrHeader {
                L2PseudoLength  l2_plen,
                uint4_t         skip_indicator,
                uint4_t         rr_protocol_discriminator,
                RrMessageType   message_type
        } with { variant "FIELDORDER(msb)" };

Note the skip_indicator and rr_protocol_discriminator fields are swapped in their order compared to the spec!