This article focuses on the impact of extreme PSTN conditions on echo canceller operation at VoIP gateways, which are interfacing on one side to the traditional telephone PSTN and on the other side to packet-based networks. With the migration of traditional telephone PSTNs to packet-based voice services, it is crucial to be able to provide the same services with this new technology without any degradation in quality of service or inconvenience to the end user. It presents an example of a national operator of traditional telephony, who encountered explicit echo problems in its network when upgrading it with packet-based technology. The reasons for an extensive echo are analysed and the guidelines for solving the problems are given. In particular, it explains the requirements of VoIP gateway echo canceller operation to work in network conditions that are much more demanding than described in the ITU-T G. 168 Recommendation and stresses how voice transmission over the packet-based network exposes the existing inconvenient or non-standard conditions of traditional telephone PSTN.
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