Voice VLANs isolate and prioritize voice traffic on a dedicated subnet to prevent the latency degradation that occurs beyond 150 milliseconds. This coverage includes the differences between data and voice traffic, QoS versus CoS prioritization, and how IP phones function as built-in switches to support dual VLAN access from a single network port.
Voice VLANs
One of the VLANs that's a little more unique is our voice VLAN. Voice traffic is a little more susceptible to delays, and there are some issues that happen when there's some latency that works into your voice traffic. So what we like to do is separate that out onto a separate network, a separate subnet, a separate VLAN, so that way we can prioritize the traffic that's within that VLAN.
Typically you have a lot of TCP and UDP traffic that's happening. TCP is that traffic that creates and establishes the connection, and if something goes missing then it goes and retrieves that information again, so it has those built-in error corrections and acknowledgments and all of that fun stuff. UDP is more designed for that faster connection, where it just needs to send data but it doesn't necessarily need the guarantee that TCP has with it.
Your computer will have both of those, but a lot of your traffic is that TCP traffic. You pull up a web browser and it's making connections via TCP, and you want to make sure you get the full web page, so it's going to download the full web page, and that's using TCP to do that. For that reason, a lot of the traffic that's coming from your computer doesn't have the latency problems with it. If things didn't arrive in a timely manner, then it'll eventually get there and you'll get the full web page.
Voice traffic is a little different. Typically we do a lot of voice conferencing and a lot of things on our computer nowadays, but on a phone it's definitely all voice traffic that we have when we're making connections with our phones, and so it primarily uses UDP. There are some connection things that may use a little bit of TCP to establish some connections, but the main data that's going back and forth on your phone lines is UDP.
We want the quality of that phone call to be very high, so we need to make sure that that voice traffic is being prioritized. In fact, we find that if there is a latency of more than 150 milliseconds, that voice traffic starts to really degrade. So we set up our networks to prioritize that traffic. What we can do is establish a voice VLAN and then prioritize the traffic that comes across there. But in order to prioritize this, we have to configure this network wide, so there's a lot of configuration that happens on these switches to make sure that it can prioritize that voice traffic.
The term QoS, or quality of service, is just how we prioritize that traffic. We label the traffic as it's being sent across there and then we prioritize that traffic. QoS is what we call on that layer 3 level, so QoS is set up on your routers and your layer 3 equipment, to prioritize the traffic that comes into it that is labeled with this QoS label.
Cost of service is something similar, but it's on layer 2. What we do is we assign a cost to this different traffic, and then that will prioritize the traffic according to that cost. That is from a layer 2 perspective. So when you hear QoS, think layer 3. When you hear CoS, think layer 2.
With this we have our voice VLAN. We've already set up the voice VLAN as being 150 on our network here, so when a phone connects into a switch, what we want it to do is be on this VLAN 150, and the cost of that is going to say this is prioritized traffic. Each of the switches is going to handle that traffic before it handles any of the other traffic that comes across it, provided that we've set this up correctly.
One of the things that's a little unique about a lot of the phones that are out there, and specifically the Cisco phones, is that the Cisco phone actually has a built-in switch to it. Just like you have a switch out here on your network, there is a mini switch that is inside of this phone. There's two ports: one of the ports is connected into that switch, the other port is connected into your laptop or whatever other device.
One of the big reasons for this is so that for each of your desks you only need one port to support that desk. Otherwise you would need a port to plug the phone into the switch and you'd need one for the laptop, so you would need double the amount of ports on your switches, and you would need double the amount of jacks out there for each of your desks. For that reason they have this switch built into phones now.
As part of this, what makes this really interesting is we want these on two different networks. We want the demo laptop to be on one of our — well, in this case right here it's on our management VLAN, but whatever device we have plugged in there, we probably want it on one of these different networks right here. And then we want our phone to be on this voice VLAN. So we want them to be on two different networks.
There's some special configurations that happen with this. What we do is we configure the port to be an access port and to be part of, in this case, VLAN 10, so it can support this computer. But we give it a little extra configuration and say, but we will accept some tagged traffic from this phone. So it's kind of a pseudo access port and trunk line. Then this phone will communicate using Cisco Discovery Protocol with this switch right here, to configure this little pseudo access trunk port so that way it can have this phone on VLAN 150 and this computer on VLAN 10.
It's a unique configuration that we have set up just for when you have this scenario right here, and Cisco equipment is all set up for it. As long as it is a Cisco switch and as long as the phone is a Cisco device, then all of this works out well and you can set up your configuration just like this. It's pretty fascinating how it works, and then it prioritizes that VLAN 150 traffic.
So what we talked about is data versus voice and how we want to prioritize voice. The way we do that on layer 3 is through that quality of service, versus on layer 2 it is cost of service. Then we finally wrapped it up with some of the differences with the phone and how the phone works: it works as this mini switch, and it sets up this pseudo access port, but it does have some trunk traffic that gets tunneled to that port onto that switch. So there's a little different configuration that happens with that.
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