With the development of INTERNET technologies, IP facsimile technologies have also been developed rapidly. In the facsimile network structure shown in FIG. 1a, the facsimile machines each accesses the IP (Internet Protocol) network via respective gateways corresponding to the facsimile machines to realize facsimile communication. However, for a simple network, the gateways and facsimile machines may be connected only via telephone lines, without via telephone switching network. Real-time IP facsimile may be implemented in two modes, i.e., pass-through mode and relay mode. The relay mode is a facsimile conforming to T.38 protocol. Because resistance to network damage is poor in the pass-through mode, media gateways or relay gateways usually support facsimile in T.38 protocol. When a facsimile machine initiates a call, a gateway only takes the service as an ordinary telephone call, without identifying whether the call is a facsimile service; and signals are all transmitted in voice mode. The gateway considers that a facsimile service is present and starts to transfer facsimile data in the mode specified in T.38 protocol only when the gateway detects a special signal from the facsimile machine.
At present, there are mainly two types of facsimile machines on the market: one is ordinary facsimile machine with a highest transmission rate of 14400 bps, and the other is high-speed facsimile machine with a highest transmission rate of 33600 bps. These two types of facsimile machines conform to different facsimile processes, and their signal sending mode and operation mode are also different. The former conforms to T.30 protocol, while the latter conforms to the AnnexF part of the T.30 protocol and V.8 protocol. Generally, gateway products on the market can detect signals of an ordinary facsimile machine. With the market occupancy of high-speed facsimile machines becoming larger and larger, it is also required that signals of a high-speed facsimile machine may be detected correctly.
At present, the highest transmission rate supported by most of the gateway products which support facsimile in T.38 mode is usually 14400 bps. A method of performing a V.21(H)FLAG detection on a PSTN-side signal is used. Specifically, it is determined whether it is a facsimile service by detecting whether there is a FLAG signal in a receive Digital Identification Signal (DIS). A basic calling process of an ordinary facsimile machine may be understood with reference to FIG. 1. After a facsimile service is initiated, the sending facsimile machine sends a Calling tone (CNG) signal. The receiving party sends a called station identification (CED) signal and a DIS. The sending end sends a Digital Command Signal (DCS) as a response after detecting the DIS signal and then starts training and sending facsimile pages.
The DIS signal is modulated in V.21(H) modulation, the central frequency of its carrier frequency is 1750 Hz, and bit0 is modulated into a signal of 1850 Hz and bit1 is modulated into a signal of 1650 Hz. The content of the DIS signal is constructed with FLAGs (0×7e) for 1 second and a frame signal. Gateways generally perform V.21(H) detection. If it is determined that the demodulated data is a FLAG signal, it indicates that a facsimile service is present. The transmission rate of data modulated in V.21 will be 300 bps, equivalent to sending about 37 0×7e, because FLAGs are sent continuously for 1 second. Therefore, a gateway usually determines that a facsimile signal is present after detecting several 0×7e, and then switches to T.38 facsimile mode.
The method may correctly detect the FLAG signal of an ordinary facsimile machine. However, when the sending and receiving facsimile machines are both high-speed facsimile machines, no signal in V.21(H) FLAG form is sent, because the facsimile process for the high-speed facsimile machines is totally different from that of ordinary facsimile machines, and the calling process conforms to V.8 protocol. As a result, the gateway cannot detect the FLAG signal, and thus cannot switch from voice mode to facsimile mode. A basic process of high-speed facsimile is as shown in FIG. 2. The sending end sends a Calling tone (CNG) signal, and continuously sends Call Menu (CM) signals upon receipt of an answer tone with phase reversals (ANSam) signal fed back by the receiving end. If the receiving end sends a Joint Menu (JM) signal soon after it detects two complete CM signals, the sending party responds to the JM signal with a CM terminator (CJ) signal, and then the process enters training and facsimile procedures.
The CM is a signal modulated by V.21(L), and the central frequency of its carrier frequency is 1080 Hz. The CM signal contains parameters such as modulation rate of a facsimile machine, etc., and the CJ indicates that the sending of the CM signal completes. The ANSam is a single-frequency signal of 2100 Hz. Its longest transmission is 4 seconds, and its phase is reversed once every 450 ms. The JM is a signal modulated by V.21(H), which indicates that an available CM signal has been received. Although the JM is modulated using V.21(H), it does not contain the V.21(H) FLAG signal of an ordinary facsimile.
Because signals in V.21(H) FLAG form are not present in a calling procedure of a high-speed facsimile, the gateway cannot switch from voice mode to facsimile mode. Thus, a facsimile by a high-speed facsimile machine cannot be transferred via a gateway.
At present, after the market share of high-speed facsimile machines increases, some gateways also employ such a method for detecting a facsimile signal: detecting the ANSam and the V.21(H) FLAG at the same time. When a gateway detects an ANSam signal, the gateway does not package the voice data. If no available V.21(H) FLAG signal is received in a time-out period for the ANSam signal, the gateway will determine that the service is a voice service and starts packaging. If a V.21(H) FLAG signal is detected in the time-out period, which indicates that a facsimile service is present, the gateway switches to T.38 state and sets V.8BIT in the DIS signal to zero.
In this method, the ANSam signal is intercepted. In fact, the high-speed facsimile process is converted to an ordinary facsimile process by force. The sending facsimile machine cannot send a CM signal because it cannot receive an available ANSam signal. After sending the ANSam signal, the receiving facsimile machine sends a DIS signal because it cannot receive the CM signal from the sending facsimile machine. Thus, the facsimile process will proceed as an ordinary facsimile process.
In addition to voice and facsimile, the media or relay gateway services also include MDOEM service. A MODEM calling initiation process also conforms to V.8 protocol, which is similar to the calling process of high-speed facsimile. For the MODEM service, the above method will cause adverse influence and training failure as well as disconnection due to the interception of ANSam signal in the method.