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Demystifying Voice & Data -- Part 7


As an IT/telecom professional, you are likely to have a good understanding about the theory and principles of voice and data. However, even if you are an expert, at some point in time, you will need to discuss or explain voice, data and wireless principles to your nontechnical, line-of-business colleagues. The intent of this, and the other posts in this series, is to help you demystify technical concepts for nontechnical colleagues, customers, clients, and so on.

Each article covers a couple of topics, and tries to explain these in nontechnical terms. While the examples and analogies may not always be 100% applicable, they should provide some perspective -- i.e., highlight the differences and advantages.

For my previous posts in my "Demystifying Voice & Data" series, see:

In this article, I will demystify why we accept poor call quality and look into a problem with VoIP: Jitter.

About 10-15 years ago, poor voice quality would have been unacceptable. We expected calls to be clear, with no echoes, jitter, or dropped calls. However, our acceptance of voice quality standards has changed, primarily due to the cell phone phenomenon. When cell phones were rare, voice calls were exclusively land-based, and the quality was high.

Initially, cell phones were very pricey; only those who had money could afford cell phones and service. In these early days, when you received a cell phone call, it was from someone who was fairly important (or at least had a lot of money to spend). We tolerated the poor transmission and dropped calls, because it was a call from a VIP.

As prices dramatically dropped, however, more and more people started to obtain and use cell phones. Now instead of tolerating other cell phone calls, we started to use cell phones ourselves. In the trade-off between quality and convenience, convenience trounced quality and the degradation of voice calls became "OK."

When a product becomes "free" (or close to free), our ability to complain falls on deaf ears. The old adage, "you get what you paid for" applies. As more consumers (and businesses) convert to VoIP technology, the quality of calls have steadily dropped. The gradual decline is akin to the metaphor of boiling a frog. Rather than put a frog in a pot of boiling water, you put the frog in cold water and slowly turn up the heat. As the frog becomes used to the rising temperature, the frog will not jump out and consequently boil to death. Of course, in the real world, there may come a tipping point when the frog will realize that it needs to jump out.

Is there a tipping point with the poor quality of VoIP calls? There may be, according to a fellow SCTC consultant, Phil Edholm. Check out his February article, " VoIPmageddon: Is Quality Leading to a Telephony Meltdown?". As the number of VoIP endpoints reaches critical mass, we need to address the quality issues that threaten to obliterate the chance of a good call.

To illustrate the problem with VoIP, remember that the voice call is broken into packets. These packets are disassembled at the origin. Each individual packet is sent on different routes in the network over varied and diverse routing schemes. The packets are reassembled at the termination point back into the voice call.

An analogy for this would be sending a group of packages, in real time, from Boston to San Jose via commercial airlines. The nine packages (A, B, C, D, E, F, G, H, I) arrive at the Boston Airport every hour and are sent out on the next available flight. The packages are expected to arrive in San Jose, in the same sequence with approximately the same time difference between packages.

If all goes well, the packages leave Boston in a sequential order, and arrive in San Jose in the same sequence: A, B, C, D, E, F, G, H, I.

However, what happens if there is inclement weather on one of the routes and, say, for example, packages D and H are delayed by 2.5 hours? The arrival into San Jose and the new sequence now becomes; A, B, C, E, F, D, G, I, H. To illustrate the problem this causes in voice traffic, let's substitute specific words for packages.

A = The
B = Quick
C = Brown
D = Fox
E = Jumps
F = Over
G = The
H = Lazy
I = Dog

With no or minimal delays, you get the famous pangram: The Quick Brown Fox Jumps Over the Lazy Dog.

However, going back to the scenario where weather has severely delayed 2 of the packages, the sequence now becomes: The Quick Brown Jumps Over Fox The Dog Lazy (A, B, C, E, F, D, G, I, H).

This is an example of jitter, which is when the packets arrive in an out-of-sequence order.

A way to handle jitter problems is to add a jitter buffer, which will temporarily store arriving packets in order to reduce problems with delays. While this may help if delays are minor, it does not help if there are significant delays.

In the example where delays are significant, the jitter buffer will simply "drop" packets that are too late. Thus, delayed packets (words) D and H are simply dropped. The sequence now becomes: The Quick Brown Jumps Over the Dog. (A, B, C, E, F, G, I).

In VoIP calls, dropped packets are noticeable, as the quality of the call diminishes as some portion of the conversation is missing. If the number of dropped packets is minor, then your brain can compensate. However, there comes a point where the loss is significant, and it becomes difficult and annoying to understand what is being spoken.

"SCTC Perspectives" is written by members of the Society of Communications Technology Consultants, an international organization of independent information and communication technology professionals serving clients in all business sectors and government worldwide.