HVTA [cnttbachkhoa]

Network engineers, analysts and administrators characterize network capacity as the amount of traffic the network is designed to handle. When discussing voice over Internet Protocol (VoIP), network capacity becomes more a measure of how many simultaneous calls the network can process. This concept of "peak load", the maximum assumed volume that the network should be able to handle, will be the basis of your VoIP capacity planning.

During your VoIP capacity planning process you should consider the following:

• Your local area network (LAN) and/or wide area network (WAN) design
• Existing data traffic on the network
• The voice codecs your VoIP solution will use
• Connectivity to the Public Switched Telephone Network (PSTN)
• Your network's hardware infrastructure
• VoIP and network redundancy

Mapping the current network and gathering baseline network data

Begin your VoIP capacity planning process by developing a clear picture of your existing network infrastructure. Identify each network link that will transmit voice traffic and document the link's bandwidth. This information will help you recognize and overcome potential bottlenecks. Without knowing your current network utilization, you can't accurately forecast how adding voice traffic will impact that utilization.

Whether a bandwidth problem impacts your VoIP implementation or not depends largely on your existing network's layout. Network capacity is unlikely to block a VoIP deployment on a single-site network with high-speed infrastructure. Common networking problems, such as duplex settings and broadcast storms, will impact overall network bandwidth, but most modern LANs can easily pass a high number of VoIP calls. Bandwidth can be a roadblock when deploying VoIP over a WAN with multiple locations. WAN links are largely serial-based connections operating on T1 or fractional T1 lines. These are bandwidth bottlenecks on many networks and will impact VoIP communications.

Once you accurately document your existing network's maximum capacity, you'll then need to determine the bandwidth you're network applications are currently using. A remote site connected via a 768Kbps leased line can easily support 10 VoIP phones, but if that link is oversubscribed, it many not be able to handle both voice and data traffic. For example, standard VoIP quality of service (QoS) guidelines specify that the network transmit voice traffic ahead of data traffic. This improves voice service, but can negatively impact other network applications. Large file transfers that once took five minutes, may take 20 minutes or more during heavy call periods.

VoIP Capacity Planning Talking Points

• Map your existing network infrastructure
• Identify each network link's bandwidth and potential bottlenecks
• Gather baseline network traffic data
• WAN links can be VoIP traffic bottlenecks
• VoIP QoS in a tight bandwidth environment can negatively impact data traffic
• Measure capacity needs on average and peak usage data
• Remote office usage may warrant increasing the speed of existing circuits or adding new circuits
• Simulate VoIP traffic to collect baseline VoIP data
• Remote locations may need local PSTN gateways
• Ensure the network hardware can handle the increase in traffic after VoIP deployment

Using standard PRI or voice T1 lines, you will likely consume all available channels and be unable to place additional VoIP calls. Users will receive a fast-busy or an operator error. If enough bandwidth isn't available, heavy VoIP traffic can cause data circuit to constantly send data at their maximum bandwidth and eventually begin dropping packets. Even with QoS measures and compression in place, having more VoIP calls than the network can handle will also negatively impact call quality. Some vendors support admission controls that monitor bandwidth usage and generate an alert end users when traffic is too heavy to place a call. Needless to say, waiting for an ebb in network traffic is a poor user experience.

To determine each connection's average and peak usage you can usually pull call records from your existing PBX and/or request historical call records from your telephone company. You average and peak usage data will help you determine how much PSTN connectivity you should retain and how much bandwidth you may need to add.

With you network mapping and baseline data in hand, you should carefully consider increasing the speed of existing circuits or adding additional lines to remote locations. You should base this decision on each location's number of users and peak usage. As metropolitan-area network (MAN) technologies, like Metro Ethernet, proliferate, adding the higher bandwidth lines these services allow may cost less than adding more traditional circuits.

Performing VoIP baselines

Once you have a general VoIP traffic forecast, you can begin to test how the forecasted traffic will actually impact your network. Standard bandwidth monitoring techniques and protocol analyzers can provide good, basic information on much bandwidth you're using and help identify potential problems. You can perform a more detailed analysis using a variety of VoIP-specific tools. These monitoring tools will accurately replicate different types of voice traffic and watch for errors. They can monitor for VoIP problems such as jitter and delay. Jitter is the measurement of transit delay in voice packets and can be caused by many reasons. The end result is lower quality communications. You can select different VoIP codecs and simulate different call loads during real-time tests. Different codecs, such as G.711 and G.729, provide different sampling rates that affect packet size. Selecting a lower quality codec can dramatically affect the bandwidth usage on network links. During your test you should also test compression, which can increase available network resources and allow for more simultaneous phone calls.

PSTN Connectivity

During your VoIP capacity planning, you'll need to determine if remote locations will continue to have PSTN connectivity or if that connectivity will be consolidated at a central location. All phone calls would pass through WAN links to a central location where the calls would proceed out the associated PSTN gateway. Centralizing PSTN connectivity can simplify VoIP network planning and consolidating hardware can decrease costs and increase redundancy. PSTN consolidation can also impact VoIP network capacity. The remote location's size and bandwidth requirements will determine the impact's magnitude.

When locations reach a specific end user number, your best option may be a local connection to the PSTN for local and offnet, long distance calls. For example, if your remote locations incur long distance charges to call each other or a home office, it might be better to pass internal voice traffic through your WAN but pass local, outside calls through a local PSTN gateway. You can easily accomplish this by routing calls based on destination area code. Again, your PBX records and telephone company history can help you determine how much call traffic falls into each category.

Hardware considerations

Hardware directly influences network capacity. During your VoIP capacity planning process, you should carefully evaluate your existing hardware infrastructure and future hardware needs. Adding VoIP to your network may double overall traffic. You must ensure the network's core infrastructure and distribution-level points can handle the increase. You should establish 100Mbps, full duplex connections to every phone. Most modern Ethernet hardware can currently support this connection type. If your network hardware is modular and lets you easily increase port density, you will likely be able to use your existing equipment. If not, you may need to purchase new hardware. You must also guarantee that your uplinks to the network core can handle the additional traffic load. The PSTN gateway is another hardware consideration. Adding additional voice modules to you PSTN gateway may require an expensive chassis upgrade.

Your hardware will also influence your network's capability for QoS, compression, and voice support. As these tools and techniques can increase network bandwidth, consider them as carefully as you would fat pipes and fast backbones. The ability to grow is key, and whenever possible VoIP should be rolled out in stages and capacity evaluated at each step.

The bottom line

Planning for VoIP capacity involves a number of different steps, from the bandwidth available on LAN and WAN links, to PSTN connectivity and hardware choices. VoIP, while not exactly “plug and play”, is a very feasible option given proper planning. WAN links will almost always be choke points and should be evaluated to make sure they can support the necessary voice traffic. Connections to the PSTN should be intelligently placed where they can be most beneficial. Finally, you should thoroughly examine the network hardware infrastructure to ensure support for both immediate and future VoIP capacity needs.