A packet-based communication system comprising a first network element in a first network, wherein the first network element is configured to: establish context between the first network element and an originating communication device in the first network during registration of the originating communication device on the first network; receive a first compressed request message from the originating communication device in the first network to initiate a communication session between the originating communication device and a terminating communication device, said first compressed request message including parameter entries from a defined first parameter dictionary (325, 370) and a defined second parameter dictionary (320, 365), said first and second dictionaries containing compressed parameter data elements substituted for non-compressed data elements; and decompress the first compressed request message to generate a first decompressed request message for transmission to a communication server, said generation of said first decompressed message comprising the network element decoding a first parameter using the defined first parameter dictionary (325, 370) and a second parameter using the second defined parameter dictionary (320, 365), characterised in that: said first network element is configured to exchange an indexed list with the originating communication device in the context establishment process; and said first network element is further configured to decode a third parameter in the compressed request message using an encoding assistant function, wherein the encoding assistant function decodes at least one of a preferred identity parameter or an access network parameter based, at least in part, on an index value in the received first compressed request message, wherein the index value corresponds to an entry from the indexed list. establish context between the first network element and an originating communication device in the first network during registration of the originating communication device on the first network; receive a first compressed request message from the originating communication device in the first network to initiate a communication session between the originating communication device and a terminating communication device, said first compressed request message including parameter entries from a defined first parameter dictionary (325, 370) and a defined second parameter dictionary (320, 365), said first and second dictionaries containing compressed parameter data elements substituted for non-compressed data elements; and decompress the first compressed request message to generate a first decompressed request message for transmission to a communication server, said generation of said first decompressed message comprising the network element decoding a first parameter using the defined first parameter dictionary (325, 370) and a second parameter using the second defined parameter dictionary (320, 365), characterised in that: said first network element is configured to exchange an indexed list with the originating communication device in the context establishment process; and said first network element is further configured to decode a third parameter in the compressed request message using an encoding assistant function, wherein the encoding assistant function decodes at least one of a preferred identity parameter or an access network parameter based, at least in part, on an index value in the received first compressed request message, wherein the index value corresponds to an entry from the indexed list. said first network element is configured to exchange an indexed list with the originating communication device in the context establishment process; and said first network element is further configured to decode a third parameter in the compressed request message using an encoding assistant function, wherein the encoding assistant function decodes at least one of a preferred identity parameter or an access network parameter based, at least in part, on an index value in the received first compressed request message, wherein the index value corresponds to an entry from the indexed list.