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snom 720/760/821 QoS Configuration


xinity
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hy,

 

we are planning to add QoS on our network switches and firewall to maximize VoIP quality.

it seems that snom phones have builtin QoS capabilities.

My question is, should i only have to enable 802.1p QoS features on my switches/routers or do i need to add anything else ?

 

Thanks for your help,

 

Best Regards,

 

xinity

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There is layer 2 and layer 3 QoS.

 

Layer 2 is practically VLAN, where you give the packets in that VLAN a higher priority than packets in the default VLAN. The Ethernet switch is here the device that you need to control. If you do it right, you can have a ISDN-like quality level in your LAN, even if there is very heavy traffic in the LAN. You can set that up manually, but better is to use LLDP which is a kind of DHCP for the VLAN.

 

Layer 3 is using some bits in the IP header to tell the router with what priority the packet needs to be routed. The point here is that you need to have the router under your control. Those QoS bits are practically never used when you send a packet over the public Internet. But you can use then for example if you have a large corporate network where you control the routers.

 

From the PBX perspective, the PBX stays out of the discussion. You need to make sure that the PBX has a interface that has the VLAN set up right. The PBX tags like the phone the packets with reasonable bits on layer 3, so that the subsequent routers can process the packet accordingly.

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in my case i have control over my switches and routers.

it seems (correct me if i'm wrong), that snom phones uses tag DSCP 46 for QoS , so i put DSCP 46 on highest priority in my switches and routers.

i've also reserved 3mb of BW for our SIP traffic.

 

i'll take further test, but my goal is to have the lower jitter possible, so i can assure better audio quality and stability for my users.

 

have i done something wrong, have i missed something ?

 

Regards,

 

Xinity

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Ops... Well then you are in the position to make this a rock-solid installation ^_^ . What exact DiffSrv you are using IMHO does not really matter, as long as it is not the default QoS tag for the other IP traffic.

 

Maybe you can set up a test where you have a phone call and send a packet storm for example downloading a huge file from an internal server, without the QoS rule on the router and then with it. It should make a difference.

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