When transferring or forwarding calls between remote subscribers over different SIP lines, the SIP proxy in the LANCOM VoIP router attempts to connect the two callers by means of a REFER or a Re-INVITE. The two external subscribers are not always able to reach one another directly and so the connection may fail. This is because the SIP providers do not make the necessary adaptations, e.g. translation of the destination IP addresses. To improve performance in these situations, the SIP proxy in the LANCOM VoIP routers has been additionally equipped with a media proxy.
The media proxy helps to transfer and forward calls between subscribers who are reachable over different types of telephone line (e.g. SIP PBX line and SIP provider line). The media streams, generally RTP connections, remain unchanged. The media proxy changes the ports and IP addresses in the data packets and it adapts special media end points to the corresponding destination networks (ARF networks, interface and IP address).
Multiple media streams in one SIP connection
The SIP protocol can negotiate multiple data streams in a session, e.g.separate media streams for audio and video. Each stream is handled separately. A data stream initially terminates at the media proxy and continues from the "other side". This provides the data stream with end points at the LAN and WAN sides of the media proxy.
All of the connection information in the direction of the SIP provider can be maintained and all of the necessary changes to IP addresses, ports, etc., are handled by the media proxy.
The data streams are all fed through the firewall individually, which enables a differentiated control of the QoS settings, among other things.
Connection management by the media proxy enables all subscribers to be connected to one another, whatever type of line they are using. This makes it possible to connect between SIP, ISDN and analog subscribers, something that a pure SIP connection is not capable of. Furthermore, the monitoring of individual media streams in the firewall allows certain types of application to be permitted or prevented depending on the connection's end point.
Management of media streams in case of an upstream SIP PBX
Even for two subscribers in the same network behind the LANCOM VoIP router, when connected to an upstream SIP PBX the media proxy generates data streams with separate media end points on the LAN side and on the WAN side (towards the SIP PBX).
In this case it is not necessary to pass the media streams through the upstream PBX, so the SIP signaling helps the LANCOM VoIP router to make a new decision about the path to be taken by the connection data. Using the end points in the media proxy the data streams can be connected directly, making a diversion via the SIP PBX unnecessary.
This decision is also made again in the media proxy if a local and an external subscriber are connected in such a way that, ultimately, two local subscribers are connected to one another. The media proxy re-assigns the end points when making the connection, so enabling the direct transmission of the data streams between the local participants.
Managing the media streams in the firewall
The media streams are monitored in the firewall as a matter of principle. A firewall rule is generated for each media stream (audio, video). This rule opens a connection for the corresponding IP addresses and ports for each side (LAN-WAN) and carries out a translation according to the IP-port relationships as specified by the media proxy.
Automatic QoS rules for media streams
The QoS mechanism in the firewall reserves the maximum possible amount of connection bandwidth as agreed during the SDP negotiation (SDP, Session Description Protocol) and the packets are prioritized accordingly.
Handling subscribers using different codecs
When connecting different subscribers, the situation can arise where the codecs available to the subscribers do not match together—there are no common codecs due to the SDP negotiation.
The following situations are to be observed here:
- Connections with different media streams, e.g. a video-telephone call (audio + video) and a traditional telephone call (audio only): This connection will be rejected with the message "Codec mismatch".
- Similar media types (audio-audio, video-video) with codecs that do not match: This connection will be rejected with the message "Codec mismatch".
The media proxy can only connect different subscribers if the media type and the codec type match.
