Link layer (Q.921)¶

The phone is a standard ISDN terminal on a basic-rate S0 bus. The PBX is the network side (NT). Layer 1 activation follows the normal ISDN procedure.

Addressing¶

An address field has two octets: SAPI<<2 | C/R<<1 | EA=0, then TEI<<1 | EA=1.

• Signalling uses SAPI 0. TEI management uses SAPI 63 / TEI 127.
• The phone drops frames with the wrong C/R bit, so a PBX that sends commands with C/R = 0 is not heard. ✅ Tested.
• Frames for other SAPIs, and point-to-point UI frames, are ignored by the phone. 📖 Read.

TEI assignment¶

The phone assigns itself a TEI automatically about 82 million emulated cycles after power-up, with an ID request (Ri is a random 16-bit reference):

phone -> network   fc ff 03 0f Ri Ri 01 ff        UI, SAPI 63, TEI 127: ID request, Ai = 127
network -> phone   fe ff 03 0f Ri Ri 02 <tei>     ID assigned, <tei> = (TEI << 1) | 1

The phone accepts TEIs 64 to 126. ✅ Tested. The author's stack assigns 64.

Establishing the link¶

The phone then sends SABME with P = 1 (00 81 7f for TEI 64). The network answers UA with F = 1 (00 81 73). ✅ Tested.

• Only modulo-128 operation exists: the phone knows SABME and has no SABM handling. 📖 Read.
• The window size is 1: the phone sends one I-frame and waits for the acknowledgement. The network should do the same. 📖 Read.
• I-frames from the network are accepted only as commands (C/R = 1). Otherwise the phone sends SABME to re-establish. 📖 Read.
• Maximum control plus information length 260 octets; longer frames are discarded. 📖 Read.

Keep-alive polling¶

When the link is idle the phone sends RR commands with P = 1 (for example 00 81 01 03: N(R) = 1, P = 1) at regular intervals. The network must answer each poll with an RR response with F = 1.

After four unanswered polls (the count is 📖 Read; the re-establishment was ✅ seen) the phone gives up, sends SABME again and repeats its registration (see Registration). ✅ Tested.

sequenceDiagram
  participant P as Phone
  participant N as PBX
  P->>N: UI ID request (SAPI 63, TEI 127)
  N->>P: UI ID assigned (TEI 64)
  P->>N: SABME (P=1)
  N->>P: UA (F=1)
  P->>N: I-frame 0 (registration)
  N->>P: RR
  N->>P: I-frame 0 (registration reply)
  P->>N: RR
  loop while idle
    P->>N: RR command (P=1)
    N->>P: RR response (F=1)
  end

Acknowledgements¶

• Acknowledge every I-frame with RR, setting F to the received P bit. Use REJ on a sequence error.
• An I-frame is acknowledged by the other side's N(R). With window 1, wait for N(R) = N(S) + 1 before sending the next.
• Retransmit an unacknowledged I-frame after a timeout. The phone's own timers are table values of 20, 40 and 200 (units ❓ Unknown), with N200 = 3 retransmissions.