QoS And VoIP - Birchills Blog
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QoS and VoIP

QoS and VoIP showing ethernet connector
A prospective customer asked us about QoS and how that might affect the performance of his phone system. We thought that it was about time that a simple guide to the issues was prepared. So here is QoS , VoIP and BT for the 21st century.


QoS stands for Quality of Service. It is usually used in VoIP telecoms, to indicate that VoIP voice calls are given priority over other data in a system. The idea is that the voice quality may be improved.  The internet works by transmitting packets of data from place to place – each place is called a node. Each node forwards the data on to each other node until the packets arrive at their destination.

Within each packet there are several pieces of data that describe the data packet, including where it should go, as well as the data actually being transmitted. One of these pieces of data is a priority tag, which tells the node how quickly it should handle the data.

So QoS with VoIP works by the IP phone setting a priority tag in it’s data packets. The data then hits a node which usually is on a router. If the router is congested, because it is receiving data faster than it can transmit it on, then it may choose to prioritise the data based on the priority tag and QoS would then have performed it’s function at that node.
The problem is that, for QoS to have any overall effect, as the data hops from node to node it requires that the QoS standard is implemented on each node.

Usually, when the node is not overloaded all packets are transmitted optimally, in any case. Where there is congestion then QoS can only have an effect if it is implemented at each node.


BT may try to ensure that QoS is implemented at the IP Phone and router and then onwards within their network. They can only do that because they insist on their broadband. However, when packets go outside their network there is no control over either congestion or QoS handling of the node. Likewise with incoming packets of data there is no control over wider network congestion. If the customer experience is bad because of this then the customer is locked in by the terms of the contract to carry on using  the system

Birchills supplies quality IP phones. To allow maximum flexibility and minimise customer costs, we do not tie customers down to a particular router or broadband.  We do provide information on the required broadband. In our experience the extra QoS data may cause problems with some routers and so, by default, it is not enabled.  As described above if the router is not congested then QoS has no effect. If the customer, unusually, experiences problems then the issue is far more probably with the wider network. Under these circumstances customers  are free to either swap or upgrade their broadband or terminate the service with no penalty. We will be happy to provide ongoing help and advice.

QoS is less of an issue than it used to be with ever increasing amounts of bandwidth and less congested connections/exchanges. Any quality issues, which are rare,  we see now are a result of congestion on the customer's own network, over which has QoS will have no effect.


The trade-off is reduced flexibility and choice and increased cost versus potential (but unlikely) quality issues if customers manage to saturate their connection. Ultimately however, now you know the facts, the choice is yours.


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