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SPA2102 - QOS Settings?

I have a Linksys SPA2102-NA.


I want to reserve the proper amount of bandwith on my broadband connection when making a VoIP phone call.


So, my QOS Policy is "On when phone in use", QOS QDisc is "TBF", and Maximum Uplink Speed is ??


I'm using G711u as my preferred codec (the most demanding byte count wise, I think).


I need to reserve 64 Kbps, up and down, so that's 128 Kbps. So if my average uplink speed (as measured at a site like speedtest.net) is 950 Kbps, then the value for the Maximum Uplink Speed would be (950 - 128) = 822 Kbps, right? If my measured, averaged speed is 1367 Kbps then my Maximum Uplink Speed would be (1367 - 128) = 1239 Kbps, right?


I think I might be missing something because other posts I've read sometimes advise a value like 10-20% of the average uplink speed. I'm confused as to why it would ever be a percentage because is not the max byte count always known when using a particular codec like G711u?


What am I missing to properly configure my SPA2102 for a particular codec and the known performance of my WAN connection?

1 ACCEPTED SOLUTION

Accepted Solutions

Re: SPA2102 - QOS Settings?

TBF limits data traffic from LAN to WAN of the SPA to reserve Internet bandwidth for voip.

Consider this scenario:
- Internet upload link 512kbps
- you want make 2 simultaneus calls in g711 (2x64 kbps=128kbps)

You can set the Maximum Uplink Speed to 512 - 128 = 384kbps

In this way the SPA drops the traffic exceeded 384kbps and preserves 128kbps for voice services.

Another example:

- Internet upload link 2Mbps

- you want make 1 simultaneus calls in g711 (1x64 kbps=64kbps)

You can set the Maximum Uplink Speed to 2048 - 64 = 1984kbps

In this way the SPA drops the traffic exceeded 1984kbps and preserves 64kbps for voice services.

If you want be more accurate you must considere also packetization period of voice samples, RTP and UDP overhead: http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml

Regards.

4 REPLIES

Re: SPA2102 - QOS Settings?

These are the QoS options of Linksys ATA:

None or TBF (token bucket filter)

Always On or On whenPhone is Use

Maximum Uplink Speed The maximum bandwidth for LAN to WAN throughput. The default is 128 kbps

TBF is very complex. It is also a shaper and it is in that aspect that it really doesn't work very well. A bad configuration of link speed can give negative effects.

You can find more details on this link:http://lartc.org/howto/lartc.qdisc.classless.html

Regards.

New Member

Re: SPA2102 - QOS Settings?

Thank you for trying to help.

Forgive me though, it's still not clear what my setting should be for the Maximum Uplink Speed. I do not understand what "

The maximum bandwidth for LAN to WAN throughput" is? How would one determine that?

Is the value something that is calculated based on the performance of my internet connection in conjunction with a specific codec?

Is the value based on the bandwidth needed by the codec?

Or something else?

Re: SPA2102 - QOS Settings?

TBF limits data traffic from LAN to WAN of the SPA to reserve Internet bandwidth for voip.

Consider this scenario:
- Internet upload link 512kbps
- you want make 2 simultaneus calls in g711 (2x64 kbps=128kbps)

You can set the Maximum Uplink Speed to 512 - 128 = 384kbps

In this way the SPA drops the traffic exceeded 384kbps and preserves 128kbps for voice services.

Another example:

- Internet upload link 2Mbps

- you want make 1 simultaneus calls in g711 (1x64 kbps=64kbps)

You can set the Maximum Uplink Speed to 2048 - 64 = 1984kbps

In this way the SPA drops the traffic exceeded 1984kbps and preserves 64kbps for voice services.

If you want be more accurate you must considere also packetization period of voice samples, RTP and UDP overhead: http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml

Regards.

New Member

SPA2102 - QOS Settings?

Wow. Perfect. Much thanks.

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