Layer 3 framing (Q.931)¶
Evidence tags
✅ tested in emulation · 📖 read from code or tables, not exercised · 🔶 inferred · ❓ unknown · 🔌 hardware-verified (nothing yet). See the overview.
| Field | Value |
|---|---|
| Protocol discriminator | 08 (DSS1). The phone accepts 0f for SETUP only. |
| Call reference length | 01 (one octet). A zero length means the global call reference. |
| Call reference | 7-bit number plus a flag in bit 7 |
| Message type | Standard DSS1 values, see Messages |
Call reference flag¶
The flag is 0 on messages sent by the side that originated the call and 1 on messages sent by the other side.
| Call | Network messages carry | Phone messages carry |
|---|---|---|
| Originated by the network | flag 0 (01) |
flag 1 (81) |
| Originated by the phone | flag 1 (81) |
flag 0 (01) |
A message with the wrong flag is treated as belonging to an unknown call. The phone answers RELEASE COMPLETE with cause #81 (08 02 80 d1). ✅ Tested.
Information elements¶
Variable-length elements are id | length | content. Single-octet elements have bit 7 of the id set (a1 = sending complete).
Codeset 0 elements are standard Q.931:
| Id | Element | Id | Element |
|---|---|---|---|
04 |
Bearer capability | 6c |
Calling party number |
08 |
Cause | 6d |
Calling party subaddress |
10 |
Call identity | 70 |
Called party number |
14 |
Call state | 71 |
Called party subaddress |
18 |
Channel identification | 74 |
Redirecting number |
1c |
Facility | 76 |
Redirection number |
1e |
Progress indicator | 79 |
Redirection subaddress |
27 |
Notification indicator | 7c |
Low layer compatibility |
28 |
Display | 7d |
High layer compatibility |
29 |
Date/time | 7e |
User-user |
34 |
Signal | a1 |
Sending complete |
4c |
Connected number | 4d |
Connected subaddress |
Codeset shift¶
97 is a locking shift to codeset 7. Every element after it, up to the end of the message, is codeset 7. ✅ Tested.
The phone also decodes codeset 5 and 6 (one element, id 1a, content ❓ Unknown). Codeset 7 is the one that matters, see Codeset 7.
Frequently used elements¶
| Element | Bytes | Meaning |
|---|---|---|
| Bearer capability | 04 03 80 90 a3 |
speech, 64 kbit/s, A-law (a2 = mu-law) |
| Channel identification | 18 01 81 / 89 / 83 |
B1 preferred / B1 exclusive / any channel |
| Cause | 08 02 80 90 |
location user, cause 16 (normal clearing); the phone may add a diagnostic octet |
| Cause (invalid call reference) | 08 02 80 d1 |
cause 81 |
| High layer compatibility | 7d 02 91 81 |
telephony |
| Called party number | 70 02 81 31 |
digit "1" (octet after length: type and plan 81, then ASCII) |
| Calling party number | 6c <len> 21 80 <digits> |
as seen in the network's own SETUP |
| Date/time | 29 05 yy mm dd hh mi |
each octet binary, not BCD |
| Display | 28 <len> text |
the phone's LCD is 21 columns wide |