SME VoIP traffic considerations

We have had a couple of installs recently that are very typical scenarios in the SME market sector. Interestingly enough, though, the businesses are very different to each other, one being an office-based company in the internet sector, the other a retail outlet serving the building trade. They are similar in size, one having 7 extensions and the other 10.

Like a lot of businesses of this size, internet access is achieved using basic business-grade ADSL circuits. This can be a challenge for VoIP installs, as simply routing the VoIP traffic through the same circuit is simply begging for voice quality problems. Let’s have a quick look at some of those potential problems.

Priorities

If, for whatever reason, you need to use one circuit for data and voice traffic, your only real option is to prioritise the VoIP traffic. This will require you to apply Quality of Service (QoS) to the traffic within the network. In order to do so, you will need to ensure that all nodes on the network (routers, switches and network cards) support QoS and handle the traffic as you would like. A detailed post on VoIP is for another day, but suffice to say that care need to be taken or you will find that your efforts will not result in prioritised VoIP traffic after all.

However, even you you get the QoS configuration on your LAN right, you may still find that your call quality is not as good as you expected. How can this be? After all, you are ensuring that any VoIP packets are sent out before anything else. Well, the answer lies outside your control. Don’t forget that the routing of voice traffic from our handset to your ADSL router is only part of its journey. Once with your ISP it will travel through several more routers until it gets to one of the servers at your VoIP provider, and then through some more routers to get to the other party. No matter what happens inside your LAN, different rules apply outside. If your ITSP is prone to applying traffic-shaping to their network (basically, a form of QoS to ensure that a small number of users downloading large files don’t take most of the available bandwidth on the network) then your VoIP packets may have a much lower priority outside your LAN than inside. This is an issue particularly on incoming VoIP traffic, as it may well be competing with other inbound traffic for a share of your bandwidth without the benefit of QoS.

Contention

The other big problem with ADSL, is that even business-grade packages suffer from contention. This is where the bandwidth is shared between a number of ADSL circuits, and is the main reason why you can get internet connectivity so cheap. It is usually measured in terms of the number of users who share a single circuit, although ISP’s are reluctant to heavily publicise their contention ratios (for obvious reasons). Domestic ADSL will normally have a contention ratio of up to 50-1, whereas business services are usually lower, between 20-1 and 5-1. You can imagine, though, just what effect a bunch of heavy users sharing your circuit will have on your VoIP traffic!

What’s the answer?

There are a couple of approaches you can take to improving voice traffic quality. Since I’m not aware of an ISP that specifically offers a service that guarantees priority for VoIP traffic, instead you can utilise a low-contention service that offers an excess of bandwidth. This makes it more likely that VoIP traffic will traverse the network with no significant delays. QoS within the LAN should minimise the impact of data traffic too.

The preferred option

This isn’t the approach we took with the two customers mentioned at the start, though. Instead, the customers agreed that dedicating an ADSL circuit to VoIP was preferable. Whilst still not perfect, this has the effect of separating VoIP traffic from other data. There is still the potential for contention to cause disruption to the VoIP stream, but other possible issues are minimised or removed. Of course, this is likely to be a higher-cost solution than a single circuit, but can be justified as part of a business continuity solution, whereby all traffic can be diverted to a single circuit should the other fail.

Dual circuit configuration

Configuring your network to send VoIP traffic via a dedicated circuit is relatively easy, is a little care is taken. One approach is to keep all VoIP and data traffic separate within the LAN, using dedicated ports, switches, etc. on the physical infrastructure, and a dedicated subnet on the network level. If the network is heavily loaded, this can be a good solution, although the downside is that accessing devices on the VoIP network (e.g. for configuration purposes) can be awkward.

More typically, all devices will be on the same subnet and will share the same networking equipment. As a single DHCP server is always a good idea, configuring the VoIP devices to have a different default gateway can be most easily achieved by giving them static IP addresses. For relatively small installations this isn’t a problem anyway, and I like to give handsets an IP address that matches the extension no. (e.g. 192.168.0.201 for ext 201) to make configuration easier. Given a reasonable ADSL circuit, this is a pretty easy and cost-effective means of improving VoIP call quality.