Bandwidth Considerations for VoIP

By Robin Wilding
Updated: February 10, 2011

If you are an accountant you will love the numbers below. Former Mathletes ditto. If you are one of those IT geeks that hates numbers—delegate this and run away quickly.

Before you implement a VoIP solution you need to have the Internet backbone to support it. Bandwidth is a crucial component to deploying a VoIP solution, especially if you are running an SIP trunk device thereby running your VoIP over the same line as your Internet connection.

The good news is that while it takes a lot of numbers and some simple math, your bandwidth needs should be easy to calculate. For the sake of not having your eyes glaze over, let’s measure the bandwidth necessary solely for VoIP and not consider your Internet needs for now.

VoIP Bandwidth is determined by 4 factors:

  1. The CODEC used for voice compression.
  2. The underlying network path (Ethernet, frame relay, ATM).
  3. The number of users or call volume.
  4. The position of the Interaction Center Server in relation to your end stations.

Numbers 1, 2 and 4 will be easy enough to figure out based on your specific configuration, so let’s focus on number 3. A single non-compressed VoIP call uses 80k of bandwidth, but when CODECs are applied that number can reduce to as little as 25K. If you have 10 users making calls at the same time then you will need 800K of bandwidth (without compression). You can figure out what the volume is based on your compression tool and the size of a single call. This can be done through various Internet-based tools that can be found with a Google search.

If you have 10 users you need 800K…100 users 8,000K….1,000 users needs 80,000K…and so on.
Once you have figured out your compression rates, number of users and call size you can figure out exactly the bandwidth that you need for VoIP (not counting Internet-based activities).

Now compare this number with the data speeds of different Internet connections:

  • ISDN: 64 Kbps to 128 Kbps
  • DSL: 128 Kbps to 8Mbps
  • CABLE: 512 Kbps to 20 Mbps
  • T-1 line: 64 Kbps per channel or up to 1.544 Mbps
  • Bonded T-1: 3 Mbps
  • T-3 line: 43 to 45 Mbps
  • OC3: 155 Mbps

If that OC3 fiber optic network is just out of your reach, as are the T3, T1, and even Cable lines, then consider using a strong compression tool and re-evaluating your call volume. After all studies have proven that at any given time an average of only 1/3 of your users will be on the phone at one time, and if you are looking to save money then you may want to factor this in.

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