PSTN fallback using dial on demand routing scheme

Details PSTN fallback using dial on demand routing scheme PSTN fallback using dial on demand routing scheme

2001-07-09

2003-04-01

Nguyen, Chau (Department: 2663)

Multiplex communications

Pathfinding or routing

Combined circuit switching and packet switching

C370S238000, C370S435000

Reexamination Certificate

active

06542499

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to Voice over Internet Protocol (VoIP) calls and more particularly to a Dial On Demand Routing (DDR) scheme used when quality of service degrades on the VoIP calls.
Voice signals are transmitted over a packet network by first formatting the voice signal data stream into multiple discrete packets. In a Voice Over Internet Protocol call, an originating voice gateway quantizes an input audio stream into packets that are placed onto a packet network and routed to a destination voice gateway. The destination voice gateway decodes the packets back into a continuous digital audio stream that resembles the input audio stream. A codec uses a compression/decompression algorithm on the quantized digital audio stream to reduce the communication bandwidth required for transmitting the audio packets over the network.
The Quality of Service (QoS) of VoIP calls can degrade due to congestion on the packet network or failure of network processing nodes in the packet network. Quality of service can include anything from call sound quality to the ability and responsiveness of the VoIP network in establishing new VoIP calls. IP network reliability has not been proven to be in the same class as a traditional switched Public Services Telephone Network (PSTN). For this reason, many customers request features that place VoIP calls back out on the traditional circuit switched network (hairpinning) when there is IP network congestion or an IP network failure.
Hairpinning calls over the PSTN has several problems. The first is that hairpinning is expensive. A primary reason customers are attracted to VoIP calls is the cost savings over the PSTN network. Rerouting calls over the PSTN network eliminates a portion of that savings. Hairpinning also increases the number of PSTN channels that must be maintained for each customer by a factor of two (in the case of complete VoIP network failure).
Hairpinning is only used at call setup time. Once a VoIP call has gone into the active state, there is no way to then reroute the call through the PSTN network and then synchronize the PSTN call with the VoIP call. Thus, if the QoS of the IP network degrades during a VoIP call, that entire VoIP call will exhibit the degraded quality. If a QoS problem is detected before a new VoIP call is established, that new call can be hairpinned over the PSTN network. However, the remainder of that call continues to be hairpinned over the PSTN network even if the QoS of IP network improves. Thus, the customer continues to be charged for the more expensive PSTN call even though the call could have been reestablished over the IP network with acceptable QoS.
Accordingly, a need remains for a more effective way to provide VoIP call fallback.
SUMMARY OF THE INVENTION
A call fallback scheme is provided in a packet switched network. After receiving incoming calls, a Voice over IP (VoIP) link is established over a packet switched network with a destination endpoint. VoIP packets are generated from the incoming calls and sent over the VoIP link to the destination endpoint. When a low quality of service condition is detected on the VoIP link with the destination endpoint, a fallback call is established with the destination endpoint over a circuit switched network. The VoIP packets for the incoming calls are redirected from the VoIP link to the circuit switched data link.
As opposed to simply hairpinning a TDM voice call back over the PSTN network
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, the same VoIP packets for the incoming calls originally destine for the destination endpoint via the packet switched network are rerouted as a digital bitstream through the fallback call. This simplifies synchronization with VoIP packets sent over VoIP link. Because VoIP packets for more than one call can be sent over the fallback call, the cost of maintaining the fallback call is also substantially reduced.
The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings.


REFERENCES:
patent: 5953312 (1999-09-01), Crawley et al.
patent: 6064653 (2000-05-01), Farris
patent: 6078582 (2000-06-01), Curry et al.
patent: 6363065 (2002-03-01), Thornton et al.
patent: 6452922 (2002-09-01), Ho
“Internet Telephony”, Bob Emmerson, Byte Magazine, May 1997.

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