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Implementing a PBX for the CS410

Evidence tags

✅ tested in emulation · 📖 read from code or tables, not exercised · 🔶 inferred · ❓ unknown · 🔌 hardware-verified (nothing yet). See the overview.

A checklist for building the network side. Everything here is from the notes; nothing was tested against real hardware.

1. Layer 1

  • Act as the NT on a basic-rate S0 interface and activate the line.
  • Power: the CS410 may take its power from the S0 bus. The current it draws is not known to the author; check the hardware or measure before choosing an interface card. The card must also support NT mode.

2. Layer 2

  1. Answer the TEI ID request with a TEI between 64 and 126.
  2. Answer SABME with UA (F = P). Reset sequence numbers.
  3. Use C/R = 1 on commands and C/R = 0 on responses from the network.
  4. Window size 1: wait for the acknowledgement of each I-frame.
  5. Answer RR P = 1 with RR F = 1. Without this the phone drops the link after four polls.
  6. Accept SABME at any time. The phone re-establishes whenever it loses patience, and then re-registers.

On Linux, an ISDN stack such as mISDN is expected to handle steps 1 to 6 and deliver layer 3 frames (not verified by the author).

3. Registration

  1. Wait for message 75 with 97 01 08 "PS130357".
  2. Reply with RELEASE COMPLETE: call reference of the phone with the flag set, cause, date/time, 97, 01 08 "DW130958", 26 02 82 80. See Registration.
  3. Expect a STATUS (7d, cause 95, call state 0, 97 02 00) a little later. Do not treat it as an error.
  4. Repeat for every new link establishment.

4. Calls

  • Respect the call reference flag. Your messages for a network-originated call carry flag 0, for a phone-originated call flag 1.
  • Incoming: SETUP with bearer capability, channel B1, calling and called number, optional display text. Expect ALERTING, then CONNECT when the handset is lifted.
  • Outgoing: be ready for SETUP without a called number. Answer SETUP ACKNOWLEDGE, collect the digits from INFORMATION messages and decide yourself when the number is complete.
  • Clear with DISCONNECT and answer RELEASE with RELEASE COMPLETE.
  • Audio is on the B channel (64 kbit/s A-law). Connect it to your own audio path (for example a SIP gateway).

5. Pitfalls found

Symptom Cause
Phone never sees your frames wrong C/R bit
Phone repeats the registration message every few seconds reply not type 5a, flag clear, or element 01 missing
Link drops after a while polls not answered with F = 1
RELEASE COMPLETE cause 81 from the phone wrong call reference flag, or the call no longer exists
FACILITY has no visible effect system FACILITY needs a call context the notes do not yet describe

6. Testing without hardware

The phone's behaviour can be reproduced with a software emulation of the phone, if you have your own copy of its firmware. These notes do not include any.