QoS For Small Networks

August 29, 2008 by Garrett Smith

Today we’re going to configure a Linksys Router for VoIP QoS. Most modern routers offer some sort of this feature. I just picked Linksys because it is so common. First, let’s explain what it is and what it does. So,what is QoS? It stands for Quality of Service and it is a way of prioritizing network traffic by what is deemed as most important. Obviously, this is what we want in any mixed network that has voice applications. It works by giving preference to traffic deemed most important. It can do this in a number of ways . (See screenshot below)

  1. MAC Address: this allows you to simply plug in the Ethernet hardware address of each device you want to give priority to. This is usually listed under the status information on a router. Look around in your router or ATA and you’ll find it. This method is a very easy setup for phones or ATA adaptors. The downside is the don’t give you many entries.
  2. Ethernet Port on the Router: If you have a device directly connected to that port then just give it the priority you’d like. That simple. Could be an ATA , a Phone or even a larger switch that y ou connect devices to.
  3. TCP/IP Ports: You can enter the ports that VoIP uses, usually 5060 for SIP and 10000-20000 for RTP. Your router will then listen for any traffic on these ports and give preferential treatment to those ports. On the other hand you could decrease bandwidth for BitTorrent type applications.

QoS as found on any consumer router running on a standard Internet Service Provider will ONLY work on upstream/outbound data (data going from you to your ISP). You cannot realistically control the priority of data coming TO you FROM your ISP, since you can only control the data on your side of the modem.

It is true that slowing down the download of data will slow the acknowledgments of that data in a TCP/IP connection, and will therefore slow down (eventually) the transmission of data from the remote. However, this is not a function of QoS. This is a function of TCP/IP. And it will not solve contentions between VoIP and other data, since VoIP data still doesn’t get priority. You CAN help this out by limiting your bandwidth to downloading applications if theiy support it, but this is throttling and not setting priorities.

So, QoS can help a bit and may be worth the effort if you think your network is causing problems with your VoIP.

1 Comment

  • Mihai

    Can you recommend something simillar to a bigger office such as 50 users or so ?

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