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Successful VOIP Over DSL, Part 1: Some Installation Basics

Here are a few fundamental considerations when planning a VOIP implementation using DSL.

  1. What is your actual available bandwidth inbound & outbound?
  2. How many simultaneous calls do you need to sustain?
  3. What voice codecs are you using? And so, how much total bandwidth do you require?
  4. Do you have managed QoS on your network?
  5. Can you also implement traffic shaping to reserve bandwidth for VOIP purposes? Especially outbound bandwidth as this is typically the most scarce.

To begin to answer these questions let’s start with some basic facts taken from my home office installation.

  • My DSL service provides 2.2 M x 768k of measured, confirmed bandwidth.
  • I need to sustain a minimum of four (4) simultaneous calls
  • I will prefer G.729a encoded calls
    • Requires approx 20k per call leg (inbound & outbound) including IP overheads
    • The codec typically used by ITSPs that have a reduced bandwidth feature
  • I must accommodate G.711 encoded calls
    • Requires approx 80k per call leg (inbound/outbound)
    • The baseline codec used on the PSTN

Based upon these simple facts my VOIP activity could require from 80-320k bandwidth in each direction, depending upon codec mix. Since I expect calls incoming from several providers, some which don’t support G.729a, I cannot force all calls into G.729a. However, my primary provider will handle G.729a calls so the 320k figure is likely a worst case number.

In part 2 I’ll examine the basics network QoS.

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