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PCM3 Trunk Technology and Channel Associated Signaling (CAS)
0:00:07 Introduction: The presenter (Harald) clarifies he's not an expert on PCM3/CAS, his knowledge comes from theoretical research, not operational experience. He welcomes corrections.
0:00:58 Background: The presentation originates from the osmocom community, where E1 is commonly known for ISDN, frame relay, and cellular interfaces (Abis, Gb). These typically use Central Channel Signaling (CCS), unlike the Channel Associated Signaling (CAS) that PCM3 uses.
0:02:04 Classic PSTN: Components of a classic Public Switched Telephone Network (PSTN) include telephone switches/exchanges, the local loop (connecting subscribers to the switch), and trunks (connecting switches).
0:03:34: Trunks used transmission technology using baseband and carrier frequency.
0:06:38 PCM History: PCM technology's history goes back to 1926 (fax system patent). Key developments include Alec Reeves' work in the 1930s and Shannon & Oliver's 1946 patent. Commercial PCM systems (like T1 in 1961) appeared in the 1960s.
0:08:53 E1 vs. T1: E1 (32 channels) was standardized around 1968 and used globally, except in North America and Japan, which use T1 (24 channels).
0:09:53 Economic Viability: In 1970, PCM3 was economically advantageous for distances over 10 km in Germany.
0:10:23 German vendors: Four vendors in Germany are mentioned: SEL, Siemens, Tekade, and Telefonbau Normalzeit.
0:11:13 PCM3 System Components: A block diagram shows the key components: KZU/KET (signaling converter), multiplexer, line transmission equipment (LE), and regenerators.
0:13:06 Digital Regeneration Advantage: Digital regenerators allow lossless signal regeneration over long distances, unlike analog amplifiers.
0:13:37 E1 Time Slots: The E1 frame structure is explained: 32 time slots, with time slot 0 for framing and time slot 16 for CAS.
0:15:22 CAS Details: CAS uses a continuous stream of 4-bit signaling words (ABCD bits) for each voice time slot within time slot 16. A multiframe of 16 E1 frames is used.
0:16:39 Signaling Rate: Signaling bits change at a 500 Hz rate (every 2 milliseconds).
0:17:11 CAS Function: The ABCD bits directly reflect/control the state of the analog line (on-hook, off-hook, dial pulses, etc.).
0:18:04 KZU/KET Role: The KZU/KET converts analog line conditions to/from the CAS signaling words.
0:19:17 Vendor Interoperability: PCM3 equipment from different vendors was standardized (electrically and signaling) for interoperability.
0:20:53 Use Cases (Modern): Two modern use cases are discussed:
Carrying PCM3 over IP using ISDN USB devices (TDM over IP).
Interfacing an analog switch to a digital softswitch using a single PCM3 termination and a computer.
0:23:11 Alignment Challenge: The E1 multiframe alignment needs to be known for CAS, but ISDN USB doesn't preserve this alignment.
0:23:59 TS16 Alignment Pattern: A workaround is to look for a repeating 0000 pattern in the upper nibble of time slot 16 every 16 frames. This is possible because the all-zero pattern should not be used for ABCD signaling (per G.704).
0:24:59: The PCM3 equipment doesn't look at this bit pattern and needs the multiframe alignment.
0:25:38 Interfacing with Softswitches (Dahdi): Dahdi supports CAS, but the implementation relies on the hardware of Digium TDM cards to handle the bit manipulation. There are potential timing issues with reliably detecting/generating dial pulses in user space.
0:28:42: There exists Dahdi code that handles pulse counting and generation, keeping the time-sensitive functions in kernel space.
0:30:37 i1 USB with Dahdi: The Dahdi driver for i1 USB needs to be updated to support CAS, as it currently only supports CCS protocols (ISDN, SS7).
0:31:22 Book References: Two German books on PCM3 technology are recommended.
0:34:05 G.704 standard: G.704 indicates that Time Slot 16, Frame 0, should have the specified bit pattern, allowing multiframe recovery by examining TS16.
0:35:58: The receive direction is aligned but the transmit is not.
0:36:35 i1 USB Firmware. i1 USB firmware's multiframe handling: The firmware and FPGA are aware of multiframes.
0:38:20: The 7 or 9 frame issue is caused by compensating clock differences between the PC and E1.
0:39:10: The hardest part is handling lost packets.
0:40:58: Storing 16 frames, or a complete multi-frame, can be a solution for retransmissions.
0:42:55: The FIFO buffer is 256 multiframes, or 65 kilobytes.
0:43:05 osmo-e1d Discussion. osmo-e1d client needs connection between frame numbers.
0:43:42: Use Dahdi for analog switch to soft switch.
0:45:39: Rifle handles missing packets on the protocol level.