Media gateways (MG) allow analog terminal equipment (for example, telephones, facsimiles, telephone modems, textphones, etc.) to communicate over digital packet networks. A regular gateway usually is connected to PSTN (Public Switched Telephone Network) terminal equipment via an analog signal interface and is linked to IP network via a packet interface.
A normal voice-over-IP (VoIP) path of voice signals through the gateway comprises of: (1) a near speaker audio signal is sampled and entered the gateway via signal interface, (2) the input sample stream is cleared of echo and forwarded to a speech encoder for compression, (3) the compressed stream is packetized and transmitted to the IP network via packet interface, (4) packets received by gateway from IP, are de-packetized and forwarded through de-jitter (play out) buffer to a speech decoder, (5) the decoder de-compresses the received bit stream and (6) sends the output signal samples to the gateway signal interface.
Turning now to FIG. 1, there is shown an exemplary embodiment of a communication link according to the prior-art as explained hereinabove.
Non-speech signals may pass through a gateway over different paths. For example, modem signals may pass a gateway via a voice-band-data (VBD) path using low compression codecs, or may be relayed by means of modem termination or modem-over-IP (MoIP) method. Fax calls usually pass through gateways via a fax relay or fax-over-IP (FoIP) path including a fax demodulation/modulation processing, a FoIP packetization and other special processing.
Gateways supporting both fax-over-IP (FoIP) and voice-over-IP (VoIP) calls establish a connection in voice mode. The signals transferred over IP in voice or voice-band-data mode, are encoded by voice/VBD encoder and played by means of voice/VBD decoder. Traditional procedures of fax/modem call discrimination are based on detecting fax/modem signals at voice encoder side. This detection corresponds to the signal propagation in direction from fax/modem terminal equipment to IP network. The ITU-T Recommendations T.38, V.150.1, V.152 and V.151 define fax/modem/textphone signals which may be used as stimuli of transitioning to FoIP, MoIP, VBD and ToIP operation.
Traditional gateways initiate a transition from voice to fax relay (FoIP) when a fax signal is detected at a sample input of the gateway (it is a voice encoder side). Communicating gateways may setup FoIP call with the help of media gateway controller (MGC) or autonomously without any negotiation in real time.
In case of MGC management, a gateway reports all signals detected to MGC. The MGC performs a call discrimination taking into account detection events received from near and remote gateways. The MGC management introduces transition delays and involves additional (MGC level) protocols not required during fax relay. Currently, the MGC procedures are defined for regular fax relay call setups (fax rates up to 14.4 kbps) but are not available for V.34 fax relay. The MGC transitioning method is not considered in this invention, but this does not mean that the invention methods or autonomous behavior of invention gateway cannot be used in MGC environment.
In a more reliable case, when MGC's are not involved, gateways may require a MG-to-MG signaling protocol for a time of call discrimination. The goal of MG-to-MG signaling protocol is to allow a gateway staying in VoIP or VBD mode to make a call discrimination with the help of MG-to-MG information about signals detected by a remote gateway from a connected terminal equipment. It is obvious, that a gateway making the autonomous call discrimination and call establishment depends on MG-to-MG signaling capabilities and performance of a remote gateway. There is a need in the field of communication for improved methods and circuits for autonomous call establishment between Media Gateways.