An apparatus comprising means configured for: transmitting, to a positioning system (304, 404, 504, 604), a message for initiating a positional data stream associated with at least one audio source, the message comprising one or more requirements for positional characteristics of the positional data stream; wherein: the message for initiating the positional data stream is a session initiation protocol invite message; the one or more positional characteristics are encoded as a session description protocol offer that is transmitted as part of the session initiation protocol invite message; and the session description protocol offer comprises a position stream identifier identifying the at least one audio source; receiving a response from the positioning system comprising an indication of a position update frequency the positioning system supports and one or more of the positional characteristics that the positioning system supports for the at least one audio source, wherein the position update frequency is the frequency of an updated position data of the at least one audio source, wherein the supported position update frequency is based on the capabilities of the positioning system, wherein the positioning system tracks the position of the at least one audio source; and initiating the positional data stream with the positioning system based on the received response. transmitting, to a positioning system (304, 404, 504, 604), a message for initiating a positional data stream associated with at least one audio source, the message comprising one or more requirements for positional characteristics of the positional data stream; wherein: the message for initiating the positional data stream is a session initiation protocol invite message; the one or more positional characteristics are encoded as a session description protocol offer that is transmitted as part of the session initiation protocol invite message; and the session description protocol offer comprises a position stream identifier identifying the at least one audio source; receiving a response from the positioning system comprising an indication of a position update frequency the positioning system supports and one or more of the positional characteristics that the positioning system supports for the at least one audio source, wherein the position update frequency is the frequency of an updated position data of the at least one audio source, wherein the supported position update frequency is based on the capabilities of the positioning system, wherein the positioning system tracks the position of the at least one audio source; and initiating the positional data stream with the positioning system based on the received response. the message for initiating the positional data stream is a session initiation protocol invite message; the one or more positional characteristics are encoded as a session description protocol offer that is transmitted as part of the session initiation protocol invite message; and the session description protocol offer comprises a position stream identifier identifying the at least one audio source; receiving a response from the positioning system comprising an indication of a position update frequency the positioning system supports and one or more of the positional characteristics that the positioning system supports for the at least one audio source, wherein the position update frequency is the frequency of an updated position data of the at least one audio source, wherein the supported position update frequency is based on the capabilities of the positioning system, wherein the positioning system tracks the position of the at least one audio source; and initiating the positional data stream with the positioning system based on the received response.