Wide Area Network (WAN) Connection

The Wide Area Network (WAN) link is usually the bottleneck for data and voice traffic. On the LAN side, we have lots of bandwidth running 100 Mbps or 1Gpbs Ethernet. The link to the outside world, the WAN link, is where the bandwidth suffers. It is usually significantly lower than the LAN bandwidth.

There are many options available for the WAN link but be aware that some provide asynchronous bandwidth between uploads and downloads. Cable modems and ADSL are examples. Typically, the upload bandwidth is a fraction of the download bandwidth. For example, basic ADSL provides 3 Mbps download but only 500 kbps upload! A better solution is Synchronous DSL which has equal upload and download bandwidths.

Often the bandwidth that you purchase, is not what you actually end up. There's a lot of reasons why the bandwidth at your receiving end is different. For ADSL, it can be the number of line taps, the quality and age of the cable running to your premise. For cable modems, it can be the number of users on the cable path. The best solution is to test your line using one of the online speed testers line like SpeedTest.net to see what your actual upload and download speed is.

How much bandwidth is needed?

You must take into account the data and voice traffic that you are using. The simple answer to how much bandwidth is needed is "as much as you can afford"! Still, we would like to know what the minimum requirements are. A good process to find this out is:

  1. Monitor your data traffic on the WAN to determine the peak traffic
  2. Monitor your voice traffic to find out the peak number of concurrent calls

From the above information, you can determine how much bandwidth is used and you can determine how big a WAN pipe you need. On some routers, you can allocate a portion of the available bandwidth specifically for voice traffic.

How to Calculate Voice Bandwidth

If it is a new installation, you may not be able to monitor the voice traffic to find out the bandwidth for the peak calls. There is a way to calculate the number of channels using Erlang B tables. Erlangs are a unit of communication that will be used to look-up the number of channels from a Erlang B table. The table will indicate the number of channels required for a certain percentage of blocking. The percentage of blocking refers to how often an incoming call will be busy.

The standard blocking percentages are 1% for 1 in 100 calls will receive a busy signal - this would be for emergency services or a company that absolutely depends on their phone system for their revenue. For most businesses, 5% blocking would be sufficient where 1 in 20 calls would report as busy. Just a note, this blocking not only affects incoming calls but outgoing calls also. If you can afford the bandwidth, go with 1% blocking!

  1. The first step is to monitor or estimate the number of incoming and outging calls during peak hours. For this example, we'll say that there 51 concurrent calls (simulatenous calls) during an hour (51 calls/hour).

  2. Next monitor or estimate the average time per call in seconds. For this example, the average call will last 10.5 minutes or 630 seconds.

  3. Now we run these two numbers through this formula to calculate the number of Erlangs:

    In case you were wondering, the number 3600 is the number of seconds in an hour and is used to make sure that the units hours and seconds cancel.

  4. Next we need to determine the number of voice channels that are needed to meet the required blocking. To do this we use the Erlang B table (pdf) under the % blocking column. For our example, we select the closest value to 8.9 Erlangs from the 5% blocking column. That would require a minimum of 13 channels. For 1% blocking, 16 channels would be the minimum number of channels required.

  5. Now we can determine the bandwidth required. To do this we need to know the voice codec that is going to be used. Voice codecs are selected for two reasons: voice quality and bandwidth used. On the LAN, there is plenty of bandwidth available so the g.711 (alaw or ulaw) codec is used. The codec uses 64 kbps of BW just for transporting the voice communications. In addition to the bandwidth used by the voice codec, there is the bandwidth overhead of the voice protocols used, the WAN protocol, and the TCP/UDP and IP protocols!

    The Asterisk Guru has a nice online bandwidth calculator for determining the total bandwidth used based on the number of incoming and outgoing channels using common codecs and VoIP protocols.

    In our example, for 5% blocking we needed 15 channels using the SIP protocol and the g.711 codec. We would need a minimum of 2,070 kbps or roughly 2 Mbps, of our WAN link's bandwidth, dedicated to voice traffic.

Control of QoS

You have complete control over Quality of Service (QoS) on the LAN, because you are the master of the LAN and can configure and tune it. But what about the WAN connection? You can control the traffic going out but what about the traffic coming in?

To control the incoming traffic, you must have a QoS agreement with Service Provider for the WAN connection. Otherwise, you can only control the QoS on outgoing but not incoming!

The WAN link is the big problem!

WAN QoS Solutions

On some routers, the physical Ethernet ports can have priorities. You can use one port for the voice traffic and give it priority over the data traffic port. Some companies have two WAN connections, one dedicated specifically for voice traffic and the other for data traffic.

Application Layer Gateways

Another option is an Application Layer Gateway (ALG). An ALG can examine the packet for the Application layer information. It can make decisions based on the presence of an Application layer voice protocol such as SIP, IAX, SCCP, H.323, RTP, RTCP, etc. And give the voice protocols priority on the WAN link over the data traffic.

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Copyright July 2013 Eugene Blanchard