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Giving VoIP Traffic the Green Light, Part 1

Bandwidth is Not Speed

  • June 26, 2006
  • By Carla Schroder

VoIP call quality isn't always what it should be. Sometimes it is plagued by jitter, echo, lag--even dropped calls. In the next two articles you're going to learn how to prioritize your VoIP traffic to get the best quality. Linux has all the tools you need to do this. All that's required from you is an understanding of TCP/IP networking fundamentals--which we'll talk about today--and a traffic-shaping utility. Next week we'll get into the hands-on implementation, using the excellent utility Wondershaper.

Usually, when it comes to computer networking speed and capacity we think of bandwidth, as in the more bandwidth the faster our network will be. But bandwidth is merely one of several different factors affecting network performance.

Some definitions are in order:

Bandwidth is the maximum number of bits per second that your LAN or Internet account supports. Think of it as pipeline diameter: the bigger the pipeline, the more bits it can carry. For example, Fast Ethernet has an upper bandwidth limit of 100 megabits (Mbps) per second. That's bits, not bytes, so when it's expressed in bytes it doesn't look so impressive: 12.5 megabytes (MBps) per second.

(As a side note, ATA disk transfer speeds are so far beyond Fast Ethernet speeds these days you might want to consider upgrading your LAN to Gigabit Ethernet. For example, ATA 133 hard drives are rated at data transfer speeds of 133 MBps, while SATA2 is 300 MBps. See Resources for more information.)

Latency, or delay, is the real bugaboo of VoIP quality. It doesn't matter how fat your pipeline is if you are plagued by high latency. You can think of latency as staring down your nice big pipeline waiting for the bits to come out. While there are many possible causes of latency, one is the basic architecture of today's computer networking, which we'll get to in a minute.

Throughput is the intersection of bandwidth and latency; the actual number of bits transferred in a specific length of time.

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