Nortel Networks Corporation

Aline Gateau

Cuthbert Cheung

Ian D Sinclair

Michel Thibon

Sandrine Millet

Erl&eacute; De Pillot

Methods and apparatus are described for maintaining established local calls at an access module in case of change of the call control for a distributed PBX over a communications network, e.g. WAN or LAN, especially an IP network. In accordance with a first main embodiment, the system is composed of one Call Server controlling the call processing of other PBX access modules in a normal situation. These access modules have the ability to operate in Stand Alone or Survival mode if the link to the Call Server is down for any reason. In a second main embodiment, two call servers may be used, an active call server and a hot-standby server. The active call server controls the call processing of the access modules. In case the link between one access module and the active call server fails, the hot-standby Call Server can take over the control of the access module. Therefore, each module can operate in normal mode of the active call server or in normal mode of the hot-standby call server or, as an alternative to either of these, in stand alone or survival mode.

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Vincennes, France

Aline Gateau