You wouldn’t spend $300,000 on a Ferrari and then drive it to Discount Tires for the cheapest tires you could find, would you? I thought not. Aesthetics aside, the wrong set of tires would diminish the entire driving experience and lessen the value of your significant cash outlay. The same holds for your communications system. Why on earth would your spend that kind of money (or more) for the latest and greatest voice of IP (VoIP) system and then run it on an inadequate network? Voice quality would suffer, your users would hate it (and you), and like that Ferrari with the cheap tires, you would have wasted a lot of money.
All of which brings me to the topic of today’s blog – Quality of Service (Qos). QoS is the defined measure of performance of a VoIP communications system. Specifically, it deals with the factors that affect the transmission and reception of real-time media on a data network. QoS measurements are used to categorize good audio, and other forms of real-time media, from bad audio – a noise-free conversation from garbled gobblygook. A number of factors go into determining QoS and I would like to touch on the most important.
Clearly, you cannot hear something that was never received. A network that loses packets may slow down data activities such as file transfer, but it can be catastrophic to an audio transmission. That’s because file transfer is done with reliable protocols that sense lost data and ask the sender to resend it. That’s not the case with VoIP. Real-time date is never retransmitted. If a packet arrives then it’s used. If a packet doesn’t arrive then you just move on. Think about it for a second. If you’ve played the first syllable of a word and then receive the third syllable, do you really want to hold up the word midstream while you ask the sender to resend the lost packet. You don’t. You simply play the third syllable and move on.
Packet loss can be caused by packets not be received or by packets with errors that must be discarded. It can also be caused if packets are received out of order. For example, if you’ve already decoded and played packet 33 it doesn’t make sense to then decode and play packet 32.
Latency is the measurement of time between packets and clearly long delays between packets is not good. Latency can occur in many different places – the sender, the data network, or the receiver. In any case, excessive latency will make voice conversations at best frustrating and at worst unusable.
Low throughput can be categorized as excessive latency.
Jitter is what happens when packets are received with differing amounts of delay. This causes problems for the recipients VoIP hardware and software which runs best when the delay between packets is consistent.
Quality of Service
QoS is often measured by Mean Opinion Scores (MOS) where a score of 5.0 is perfect voice and 1.0 is impossible to communicate. Values between 4.0 and 4.5 are referred to as toll-quality and results in a satisfactory VoIP conversation. Values dropping below 3.5 are termed unacceptable by many users.
It’s important to know that MOS is subjective. It’s not calculated programmatically, but rather determined by what a human actually hears and how he or she categorizes it.
Replacing MOS is something called Perceptual Speech Quality Measure (PSQM) which is determined through hardware and software. PSQM is considered more reliable and its results are reproducible.
The field of QoS is fast moving and PSQM is now being replaced by Perceptual Evaluation of Speech Quality (PESQ) which is considered to be more accurate and informative.
QoS issued are resolved by a number of different factors, but high on the list is the ability to tag real-time data packets as being higher priority than other packet types. A QoS network will recognize those tags and process them before packets of a lesser priority. You may have heard the term DiffServ. DiffServ is short for Differential Services and is used to prioritize the IP packets of real-time media.
The difference between good voice transmission and lousy voice transmission is day and night and a network that has been poorly engineered for QoS can make the most expensive VoIP system sound like two tin cans with a string between them. It’s like putting those cheap tires on a new Ferrari. Spend the time and money (which may not be all that much) to do it right. Even if your users never thank you for it (since quality voice is expected from a telephone system), at least they won’t be yelling at you or sending nasty emails.