LIU (fax) functions and operation PSTN operation The public switched telephone network (PSTN) operates by having a central office (CO) that generates a constant voltage on the TIP and RING wires (48 V usually). When a device goes on-hook it does so by connecting impedance (such as 600 ohms for the US) across TIP and RING allowing a line current to flow. The CO can detect this current and it can send things like dial tones. The product generates more signaling tones such as dialing digits to tell the CO how to connect the call. The product also is able to detect tones like a busy tone from the CO to tell it how to behave.
When the call is finally connected, the CO behaves as if it were a piece of wire connecting the sender and receiver. This is the period during which all of the fax signaling and data transfer occurs. When a call completes, the circuit goes open again and line current ceases to flow, removing the CO connection from both sender and receiver. All of this is done with a two-wire arrangement. The two wires are called TIP and RING on all phone systems and on most phone systems they appear on pins 3 and 4 of the RJ-11 modular jack (the one on the LIU for HP LaserJet 3330mfp.
These two wires do not have to be polarized, as it is expected that all equipment works with either TIP or RING on pin 3 and the other one on pin 4. This allows the use of cables of either polarity to be interconnected and still work. These are the basic functions of PSTN operation that are assumed in the design of the fax subsystem for HP LaserJet 3330mfp. The product generates and detects the signaling tones, currents, and data signals required to transmit and receive faxes on the PSTN. The fax subsystem On the HP LaserJet 3330mfp, the formatter, LIU, firmware, and software all contribute to the fax functionality.
The designs of the formatter and LIU along with parameters in the firmware determine the majority of the regulatory requirements for telephony on the product. The HP LaserJet 3330mfp fax subsystem is designed to support V.34 fax transmission as well as lower speeds such as V.17 fax and older fax machines. 100 Chapter 4 - Operational overview EN