Improving VoIP call quality

There's more to improving VoIP calls than buying more bandwidth. Andrew Collins explains how organisations can make their voices speed across the network.

So you’re barking insults into your new VoIP handset, admonishing an incompetent subordinate for their recent substandard performance. Suddenly, your minion’s feeble excuses start cutting in and out, taking longer and longer to come back each time. What happened to your voice call?

Unbeknown to you, the very employee you were berating began playing a game over the network the minute you began yelling at them. Or perhaps they began to download last week’s episode of Dancing with the Stars via BitTorrent. Or maybe every one of the 30 co-workers you forwarded that humorous email to clicked the YouTube link at precisely the same moment.

In any case, the packets making up this clearly non-critical traffic flooded the network, drowning your own VoIP packets. The VoIP traffic therefore took a few hundred extra milliseconds to transmit, ultimately disrupting the flow of conversation. This meant you had to walk down the hallway to resume your yelling session in person, effectively rendering your new VoIP system useless.

Wish you could have delivered your tirade from your chair? Meet quality of service (QoS).

Quality of service

Quality of service can mean one of two things: a technology that tells your network device which packets are more important to you, or general assurance from a vendor that traffic will roll smoothly down your network.

According to Steve Woff, business manager for applications at NEC, this general assurance is simply an attempt to guarantee that a customer can get whatever they want down the network promptly and reliably, for the duration of that conversation.

The QoS technology itself is rather simple, in theory. It informs your network that certain packets - such as VoIP packets - are more important than packets representing other services, such as web or FTP. When the VoIP packets in our scenario arrive at a switch that’s already got a few thousand web packets lined up for transmission, the VoIP traffic leaps to the head of the queue, speeding its way through the network.

Mitch Radomir, of the Enterprise Solutions Marketing group at Nortel Asia, explains with a simple analogy: “It’s just like a bus lane. The bus lane is always open for the buses and they can keep going while the other traffic has to work on the road.”

So it’s not a matter of allotting extra bandwidth - or lanes - so much as simply allowing certain packets ahead of others in a queue.

As such, it’s an amazingly simple way of making real-time services move more swiftly through the network than services that aren’t so time critical, such as email. After all, unless you’re the world’s biggest CrackBerry addict, you won’t care if an email takes five more seconds to wing its way into your inbox. But you would notice a five-second delay on a VoIP call.

In fact, QoS is so good at doing what it does in an efficient manner that some vendors wouldn’t even think of outlaying a VoIP system for a customer without QoS enabled.

“QoS is essential to the overall performance of VoIP,” says Matt Miller, systems engineer at Juniper. “As VoIP is based upon User Datagram Protocol, it will be impacted due to its non-connection-based, best effort nature, unlike other protocols such as HTTP and email.”

In addition to this, VoIP uses 64K packets, which are smaller than packets of other services, making it more susceptible to latency and jitter. QoS is therefore essential, Miller says, otherwise VoIP will be pushed aside by TCP-based protocols with larger packet sizes.

Radomir agrees, saying: “If you start to run voice over an IP data network you must have the ability to provide these latency-type services with a guaranteed service.”

But this sentiment is not mirrored by NEC’s Woff, who says QoS is “optional”.

“We prefer not to have a customer running without any form of QoS,” Woff says. “But it is definitely doable, by scaling your networks big enough so they can accommodate voice, then it doesn’t become an issue if there’s enough bandwidth and enough reliability in the network.”

The only downside to prioritising your VoIP traffic, then, is if your office makes a bunch of VoIP calls at once, flooding your network and prohibiting any non-VoIP traffic from being sent. In that case, Radomir says, you would definitely need more bandwidth.

First steps

Many organisations approach the problem of low VoIP call quality or jitter by simply purchasing links with greater bandwidth - which often doesn’t help the problem at all.

“Some people just chuck a lot of bandwidth at it, but bandwidth doesn’t mean latency-prevention,” says Radomir. “It just means I’ve got big pipes. It doesn’t actually prioritise the traffic through the network.”

In other words, you might shell out for some mega-bandwidth links in your LAN or WAN only to find your staff’s craving for YouTube has increased along with your bandwidth, leading you straight back to low VoIP call quality and jitter all over again.

Generally, vendors agree that your first step to improving VoIP call quality should be finding out exactly why it has degraded.

Robbie Kruger, CTO of Avaya Asia Pacific, explains the process: “We normally try to encourage customers to actually undertake an audit. We call it Network Assessment and Network Analysis.”

The assessment is about understanding the network, its overall quality and the constituent components, Kruger says. The analysis is a tool that runs into the network and examines the network’s components and bandwidth, then simulates calls across the network to identify any potential bottlenecks.

A typical audit will reveal traffic comprising web browsing, email, network games, VoIP and even some peer-to-peer downloading. However, an audit can reveal some surprises.

“I’ve seen a couple of companies where they’ve said 'No, we know what’s on our network’, and the next minute you get voice quality degrading and then you see the handset reregister,” Kruger says.

“When we start to analyse what’s happening on the network we find misconfigurations on parts of the Windows environment like the DNS, or there are applications just misconfigured. And when they are misconfigured they operate the wrong way, and they start to transmit thousands of packets on the network, which acts like a denial of service attack on real-time applications.”

Now that you understand the innards of your network, you can grade your traffic in order of priority, and assign a QoS rating to each.

“What you really want to do is try to get rid of all the unwanted traffic and then prioritise all the real-time traffic and mission-critical traffic over the other traffic,” Kruger says.

What’s more, with modern devices it’s possible to prioritise traffic at multiple points, including the application layer, the PC, the VoIP phone, the switch and the router.

“If you can support it at the end-point layer, that’s great. And if you can’t, then you need to do it at the closest point: the data switch. If you can’t do it at the data switch you have to do it at the router, when it goes between subnets or over the network to another remote site,” Kruger says.

Once you’ve told all your network devices what to do with your packets, you’re good to go and should see a marked improvement in call quality.

And you can actually realise some cost savings with QoS - but only on the WAN. By making the most of your existing connection, Radomir says, you negate the need to spend big on increasing your wide area bandwidth.

But WAN traffic is where some organisations experience problems with QoS, thanks to the unpredictable nature of the public internet.

The public internet

If your business relies on the public internet for your communications, you could be in trouble when it comes to QoS.

This is thanks to two reasons: the lack of control over traffic after it leaves your LAN, and the general unpredictability of the internet.

Once your traffic leaves your network, by default it loses any QoS-related tagging you’ve attached to it. So while QoS might do wonders for your intra-office communications, once your VoIP packets hit the public internet they are no longer prioritised and are awash in a sea of YouTube and peer-to-peer traffic.

But don’t cry for your VoIP packets just yet: there is hope. Some carriers will keep your packet tagging intact. In this scenario, your VoIP traffic leaves your headquarters as priority traffic, travels across the internet as priority traffic and is treated as priority traffic by your remote office’s LAN.

That is, if you can get your carrier to agree to honour your tagging. While Radomir says he doesn’t hear many complaints from users, Woff notes that “some carriers play fair; some don’t”.

The second issue is potentially more crippling: even if you can get your carrier to respect your tagging, your traffic must contend with other customers’ prioritised packets, as well as the general waves of internet congestion. This is especially true if your carrier offers tiered service levels, says Kruger: if you are at one of the lower tiers, your traffic will be placed all the way behind all the other businesses with enough cash to get themselves onto the higher grades of service.

“The people that pay the money get the service; the people that don’t, don’t get the service,” he says.

And congestion can get nasty. Kruger recounts the Taiwanese earthquake which severed internet connectivity in the Asia Pacific, resulting in the rerouting of internet traffic through India. All internet traffic in the region was subjected to massive delays, meaning that anyone relying on the internet for telephony was out of luck, as VoIP calls were severely hampered.

But precluding natural disasters, things aren’t so bad.

“The internet’s pretty good anyway,” Kruger says, going on to say that his duties as CTO demand he travel for around 80% of the time. When he’s out of the office, he uses a VoIP softphone across the public internet, and generally has few troubles with VoIP call quality.

But that’s the story of one man and his softphone. If you try and put more than a few people on your average Australian broadband connection, according to Radomir, you’ll have problems.

“Once you start to go to multiple users, well then you really need to have a dedicated IP connection. So if you’re going to have five people on the phone at one time, using the public internet infrastructure, it’s dicey.

“We don’t recommend that,” he says.


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