The facsimile interface unit (FIU) is the software module that resides within a FoIP IWF. It demodulates voiceband signals from an analog interface and converts them to a digital format suitable for transport over a packet network. It also remodulates data received from the packet network and transmits it to the analog interface. In doing so, the FIU performs protocol conversion between Group-3 facsimile protocols and the digital facsimile protocol employed over the packet network.

The FIU, shown in Figure 10, consists of the following three units: fax-modem unit (FM)—This processes PCM samples based on the current modulation mode and supports the following functions:

• V.21 Channel 2 (300 bps) binary signaling modulation and demodulation
• high-level data link control (HDLC) framing (0 bit insertion/removal, cyclic redundancy check (CRC) generation/checking)
• V.27 ter (2400/4800 bps) high-speed data modulation and demodulation
• V.29 (7200/9600 bps) high-speed data modulation and demodulation
• V.17 (7200/9600/12000/14400 bps) high-speed data modulation and demodulation
• V.33 (12000/14400 bps) high-speed modulation and demodulation
• CED detection and generation
• CNG detection and generation
• V.21 Channel-2 detection

Figure 10. FoIP Module

fax protocol unit (FP)—This compensates for the effects of timing and lost packets caused by the packet network. The FP prevents the local fax machine from timing out while waiting for a response from the other end by generating HDLC flags. If, after a time out, the response from the remote fax machine is not received, it also sends a command repeat (CRP) frame to resend the frame. This unit monitors the facsimile transaction timing, the direction of current transmission, and the proper modem configuration. It performs the following functions:

• protocol processing (group-3 facsimile)
• examination/alteration of binary signaling messages to ensure compatibility of the facsimile transfer with the constraints of the transmission channel
• network channel interface data formatting
• line state transitions

fax network driver unit (FND)—This assembles and disassembles fax packets to be transmitted over the network and is the interface unit between the FP and network modules. The control information packets consist of header and time stamp information. In the direction of the PCM to the packet network, the FND collects the specified number of bytes and transmits the packet to the network. In the receive direction, the FND provides data with the proper timing (as generated on the transmit side and reproduced through the received time stamp information) to the rest of the FIU. The FND formats the network packets for transmission to the network based on the specific network protocol. The FND delays the data to remove timing jitter from the packet arrival times and performs the following functions:

• formatting of control information
• formatting of fax data
• properly timed playout of data
• elastic (slip) buffering
• lost-packet compensation