A method for compressing a signalling message in a compressor in a communication system. The compressor comprising a compression memory, wherein the compression memory comprises a first memory in a virtual machine and a second memory external to the virtual machine. The method comprising the steps of: receiving a part of the signalling message; searching the second memory for a copy of the received part of the signalling message; and determining a reference to the received part of the signalling message on the basis of the searching step; outputting the reference as a part of the compressed signalling message representing the received part of the signalling message.