Successful VOIP Over DSL, Part 3: QoS Features In Small Routers

SOHO users like myself are unlikely to have high-end networking gear supporting their home office setups. My initial experience with VOIP over broadband involved using Vonage and a Linksys BEFSR-41 broadband router. At the time Vonage was the leading phone-over-broadband service and the BEFSR-41 was the leading SOHO router.

It helps to frame this up in time. I used Vonage back in 2002. At that time Vonage was providing customers with a Cisco ATA-186 analog terminal adapter as the device to bridge the network to a couple of FXS jacks. The ATA 186 was the very first widely deployed ATA.

Cisco ATA-186 Analog Terminal Adapter
Cisco ATA-186 Analog Terminal Adapter

The Cisco ATA 186 is a single function device. As such it had very basic networking features. It could assign DiffServ TOS tags but relied upon the upstream network devices to manage traffic priority. This is typical of many low cost ATA devices even today.

Some time later Vonage shifted to providing a Motorola ATA device. This new device had the added feature of inserting itself into your network between the router and the rest of the LAN. Positioned in this manner it could itself ensure that the VOIP traffic received priority. This type of hardware is now the more common way that consumer phone-over-broadband companies address the unknown network circumstances of their customers.

The combination of the ATA 186 and the Linksys BEFSR-41 ultimately proved troublesome. Actually, they worked well enough together, but any other network activity could potentially interfere with the VOIP traffic causing choppy sounding calls. This was especially likely to happen if I had Outlook connected to my employers VPN. Outlook could easily saturate the outbound side of the DSL and the VOIP calls would suffer. The BEFSR-41 simply provided no mechanism to prioritize one sort of traffic over another.

After suffering this problem some time I found that Linksys had released a new router with built-in QoS features, the BEFSR-81. Since I had been happy enough with the first Linksys device until the VOIP troubles I bought the newer one immediately.

It was a simple matter to put the Cisco/Vonage ATA on port 1 of the router and assign that port the highest priority. This took care of my QoS issue as long as I used Vonage.

As to its internal workings according to Linksys literature the BEFSR-81 provides, “QoS Capabilities Based on IEEE 802.1p and IP TOS/DS Greatly Reduces the Chance of Data Loss Based on Weighted Round-Robin (WRR)

The port based QoS feature is a little too simplistic to effectively support an Asterisk server as it doesn’t allow you to assign QoS to a range of ports as required for SIP media. You can see in the screen shot where I had this setup for SIP signaling (port 5060) and IAX2 (port 4569.)

There was a period where Linksys let firmware development for the BEFSR-81 lapse. At that point the current firmware release that I tried had a serious bug that caused the device to lock up with startling regularity. At the time Linksys didn’t seem at all interested in providing a resolution even though the fault was well known on public forums like Broadband Reports. This motivated my eventual migration to m0n0wall.

It is interesting that the BEFSR-81 remains a current product as of December 2007. Its cost/feature combination makes it in some ways it is an ideal device for non-technical SOHO users.

An Alternative To QoS In The Router

What if you can’t or simply don’t want to replace your router? There are other options. Both Dlink and Hawking Technology (amongst others) make “voip accelerator” devices that are designed to add QoS to a network that otherwise lacks such capability. Both of these devices are installed between the existing router and the rest of the users network. All traffic passes through the device, which can thus identify RTP streams and prioritize traffic accordingly.

There are threads on the Vonage users forums that would indicate that these devices really do help some people with QoS issues. YMMV.

Recommending A Router

There are about three short of a zillion low cost consumer routers available. It is not my intention to make specific recommendations about hardware routers. Instead I’d rather convey my experience with those that I have actually used.

However, I will make one exception to this. If you are interested in learning about routers, feel like supporting an open source project, and cost is not your major concern, then I suggest you consider m0n0wall.

In part 4 I’ll examine network traffic shaping in addition to QoS

Addendum: I just happened to learn that the Linksys BEFSR-41 v4 router does in fact have a QoS feature like the BEFSR-81 that is the basis of much of this article. Apparently this become possible in the v4 hardware revision.

2 thoughts on “Successful VOIP Over DSL, Part 3: QoS Features In Small Routers”

  1. One solution to the QOS question is the use of an internal ADSL card in a Linux router/firewall instead of a separate box.

    As you have noted. the QOS has to take place at the confluence of all the IP streams, and also, to work properly, it has to throttle the traffic on the lowest speed link. An ADSL card in a Linux server allows you to finely traffic shape the data stream as it goes into the ADSL transmit buffer in such a way that VoIP traffic is never more than one 1500 byte Ethernet frame away from transmission.

    This post is a little self-serving because Sangoma does supply such crd. But it is nonetheless true!

  2. That is a neat idea and certainly seems a valid alternative to the traditional freestanding router. I wonder if such a solution fits the profile of most SOHO situations? That is, how many folks who work from home professionally would be comfortable with such an arrangement?

    At least in my case my ISP wants me to use their provided ADSL interface, which is a laughable modem/router combo device. I only put it back into service if I need to call their tech support.

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