How to prioritize RTP Packets for VOIP Audio on RV180

[Keith7RWA]

Hi There,

I'm a relative newbie to more advanced networking but have managed to get our small office IP PBX running over a SIP Trunk. The only real problem we are having is choppy outgoing audio when there is other heavy outgoing traffic on the network.

My understanding is that I need to set some QoS parameters, which I have played with but it didn't seem to help much. I mostly dealt with allocating bandwidth. I now think I need to somehow prioritize the outgoing RTP packets from our PBX (which runs on a PC on our LAN) to help avoid the choppy audio. My research shows this can maybe be done with something called DSCP 46 and my router does support that -- I'm just a little confused on how to exactly set the configuration.

Our router is a Cisco rv180w. I'm thinking it should be pretty straightforward, but any guidance would be appreciated (and feel free to let me know if I'm barking up the entirely wrong tree, too!)

Thanks so much.

[Tom Watts]

Hi Keith, generally, every device will attach it's DSCP value when transmitting a packet. This is contained within the actual packet. Based off the packet, a value is assigned to it. VOIP is usually a value of 46.  These is layer 2 CoS (class of service queue) that can map to layer 3 DSCP tags (0-63).

The only thing that should be needed is to map the CoS to DSCP.

STEP 1 Choose QoS > CoS Settings > Cos Settings.

STEP 2 In the CoS to Queue field, check Enable.

STEP 3 For each CoS priority level in the CoS to Traffic Forwarding Queue Mapping

Table, choose a priority value from the Traffic Forwarding Queue drop-down

menu.

-Tom
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[Keith7RWA]

OK, thank you. So specifically -- if I want to prioritize all of the RTP traffic flowing out through the router, can I do it ALL with just COS and not set any QoS, profile binding etc?

So far I have enabled the COS Queue, left the default settings (where COS Priorities 6 and 7 are set to highest), then on the COS to DSCP page I have entered the value 46 into the Priority 6 and 7 boxes. All the rest I left at 0.

Unfortunately this didn't seem to solve the issue. The way I have been testing is to call our PBX from an outside line, then put myself on hold so I can hear the hold music (effectively an audio stream from the PBX server). Then I listen carefully while I run a bandwidth test from speedtest.net.

During the download test the audio (music on hold) is pretty smooth. But during the upload test (lots of data flowing outbound) the audio gets very choppy. The COS settings I've tried don't seem to improve or even change that

I assume I'm doing something wrong and/or need to involve QoS somehow?

- Keith

[Tom Watts]

Hi Keith, if you're having audio issues over the LAN, CoS and DSCP can address this. If you're having issues from the LAN to the WAN, this is generally more of an issue with either the internet provider or with the ITSP (SIP provider). Once the packets hit the WAN, all traffic is equal as the ISP doesn't generally trust QoS nor apply it unless it is a paid-for service.

As far as I know, the WAN QoS profiles affect upload speeds only and will garauntee a minimum amount of uplink allocation based on the profile.

-Tom
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[Keith7RWA]

Thank you Tom. Now that I think about it that makes total sense. I can control the priority of the packets up to the modem but from there on they are subject to however Comcast decides to treat them.

So, I will do the best I can on the LAN side and if audio is still choppy I guess I'll need to shop for an ISP that provide QoS for VOIP. It's just driving me crazy because I know how little bandwidth VOIP audio uses -- it ain't much!

- Keith