Support QoS (Quality of Service) (kb2340)
The information in this article applies to:
- SwyxIt! from version 3
- SwyxWare from version 3
How does SwyxWare support QoS (Quality of Service) and what does QoS mean ?
Voice over IP does transmit voice packets over IP Networks. This means, that voice packets are in competition with data packets, which use the same shared media for transmission.
With the help of Quality of Service (QoS) it's possible to classify packets and therefore assign and guarantee special services (like delay, jitter, packet loss - better avoidance of packet loss, etc.).
There are 2 possibilities to assign services to network flows.
- First one is to reserve bandwidth at all active network components (routers etc.). This is known as an Integrated Service (IntServ).
- Second possibility is to mark all packets with special priority bits inside their headers. All active network components will look at these bits and handle packets requesting prio with higher prio than other ones (e.g. routers will have different queues to route packets, depending on their priority). This mechanism is known as Differentiated Service (DiffServ).
IntServ (best known implementation is RSVP) has the disadvantages of fast increasing overhead and if one network component does not support IntServ, the whole reservation of bandwidth for network flows will fail. As DiffServ is much better scaleable and more flexible, this one is most commonly used.
One of the first implementations is ToS (Type of Service). ToS did reserve 1 byte inside the IP-headers for prioritization, but only 6 bits were used. Unfortunately manufacturers of network equipment did not use the same bits with the same meaning. Therefore the IETF did redefine the meaning of the prio bits and named them DSCP (DiffServ CodePoint).
SwyxWare does support the DiffServ mechanism. This means, that all packets of layer 2 and layer 3 containing audio are marked with priority bits.
IP-voicepackets (layer 3) are marked with the DiffServ CodePoint (DSCP) Expedited Forwarding (EF) (101110 bin = 2E hex = 46 dec).
Since SwyxWare v6.12 the DiffServ CodePoint (DSCP) 26 is set for SIP callcontrol messages, too.
Ethernet frames (layer 2) are marked with priority bits, too. In layer 2 there will be 16 priority bits added to the ethernet frame. Please make sure, that your network equipment does support these enlarged frames (older equipment does fail sometimes on calculating the checksum of the frames and deletes them).
The layer 2 prioritization is described in the standard 802.1p. To use the mechanism you have to enable it at your network card of your PC. Further informations on this can be found in the Knowledgebase article
With the help of a network monitor you can check the prio bits at the packets. A good choice is Wireshark. It's for free and does interpret the bits in the right way (in contrast to the Microsoft Netmon).
Important hint for operating systems since Windows Vista / Server 2008:
Pleae note, that Windows does not use the usal value 0x2E hex / 46 dec any more since Windows Vista / Server 2008 for marking of packets as Expedited Forwarding (EF), instead a value of 0x28 hex / 40 dec is used!
To have a consistent marker for all packages of all components, which is sensefull for filtering at routers and switches, you should instruct Windows by a Group Policy to use the value of 0x2E hex / 46 dec again. Detailed information about this is available at Microsoft.
Quick instruction: run gpedit.msc, Computer Configuration, Administrative Templates, Network, QoS Paket Scheduler, DSCP Value of confirming packets, Guaranteed service type: Enable and set DSCP value to 46.
- Description of 802.1P Signaling
- Description of the Resource Reservation Protocol (RSVP) (Q227261)
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