There are four key steps to deploying QoS for VoIP, and if you don't follow them all, your VoIP implementation will not live up to its full potential. There's a lot more to implementing QoS than just switching on a router feature--so take the time to plan and design the right QoS deployment for your situation. Here are the four steps we have put together along with our colleague John Bartlett.
Step 1: Classify Your Voice Traffic
First you need to select the best scheme to mark packets for priority treatment. Classification involves deciding which streams get waved through and which wait. Everybody wants their applications to receive the highest priority--but if you do that you might as well not bother doing anything. QoS is a zero-sum game involving what gets highest priority and what gets demoted. Because VoIP is real-time and is finicky, you need to give it the highest priority classification.
We recommend mechanisms that enable the network to verify endpoints because when the network can verify that the device at a IP address is a sanctioned IP phone, the router can trust the traffic priority markings that phone provides. In this way, you can protect the network from inappropriate use and the endpoint gets to determine which packets get high priority and which get only best effort support.
Before you mark packets though, we suggest that you create a plan for what traffic will use which service levels, how many service levels there will be, and what markings you will apply to each traffic class. You also need to map your priority classes across the layer 2/layer 3 boundary, and map your priority classes to your carrier's service offerings.
Step 2: Turn On Class of Service (CoS) Mechanisms
Now turn on the mechanisms in your network routers and switches that will implement priority VoIP treatment. The main CoS mechanisms are DiffServ for routed infrastructure, and IEEE 802.1p for switched infrastructure. These mechanisms are embedded into network infrastructure, just waiting to be switched on if they are not already on. You should find turning them on is easy. You can enable CoS through commands in the router and switch configuration files.
Step 3: Plan and Manage Your Bandwidth
Next, you'll need to understand, plan for, and manage your bandwidth. You will find that DiffServ and IEEE 802.1p are not full fledged QoS mechanisms. They provide different levels of service, but they don't address oversubscription. For that you need other mechanisms to ensure that you don't oversubscribe your classes of service because voice quality will suffer.
Delivering good VoIP service quality requires you to estimate the number of simultaneous calls you expect. Then you need to assess whether the available bandwidth will support projected demand, or if you need more. At this point financial reality will inevitably kick in because your budget probably won't support all the bandwidth you need everywhere. So you'll have to manage the number of users who can simultaneously access the bandwidth. In addition, you will need the same type of feedback that the public switched telephone network (PSTN) gives when there is not enough bandwidth for a call. Think of it as the VoIP equivalent to a ‘trunk busy' signal.
IP-PBXs have this function built in. They know about the network topology and they have parameters to determine how many simultaneous voice calls are allowed between locations. When the limit is reached, the IP-PBX either gives the next caller a busy signal, or routes the call to another path (like the PSTN).
Step 4: Monitor VoIP Performance
In the final step, you need to monitor performance to know if your network is providing high quality transport for voice streams and high quality voice reproduction. If there are problems, you need to know before your users get irked. Because voice has special needs, you need new types of monitoring tools--so next week we will give you a thumbnail sketch of those tools.
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