Voice over IP basiscs (3): Quality of Service

Bart Teunis

Introduction:

The most difficult step in voice-enabling your network with VoIP is providing the Quality of Service (QoS) users expect form the telephone. To outfit your network to handle VoIP with an acceptable quality of service, you will have to alter your network configuration. At this moment when a user picks up the telephony he hears a noise free dialing tone and can make his call, in some cases you cannot guarantee this with VoIP. The problem is that TCP/IP is a very polite protocol, slowing down when it sees congestion. It is singularly unsuited to voice, which has to be able to hog bandwidth and push its way through the crowd. When you compare this to the traditional PSTN switching, which is circuit based and dedicates the amount of bandwidth for each conversation and thus the quality is guaranteed. To make sure you get the dedicated bandwidth you need for a call in a IP network you have to use QoS.

What is QoS

The most difficult step in voice-enabling your network with VoIP is providing the Quality of Service (QoS) users expect form the telephone. To outfit your network to handle VoIP with an acceptable quality of service, you will have to alter your network configuration. At this moment when a user picks up the telephony he hears a noise free dialing tone and can make his call, in some cases you cannot guarantee this with VoIP.

Quality of Service can be described as : "The collective effect of service performance which determine the degree of satisfaction of a user of the service" Or a more accurately definition is to consider the viewpoints of the different parties of the communication process:

1. QoS requirements of the user (customer)
2. QoS offered by the service provider
3. QoS achieved by the service provider
4. QoS perceived by the user
5. QoS requirements of the ISP.

The primary technical difference between the IP based networks and the PSTN is their switching architectures. The Internet uses dynamic routing (based on non-geographic addressing) versus the PSTN which uses static switching (based on geographic telephone numbering). Furthermore, the internet's "intelligence" is very much decentralized, or distributed, versus the PSTN which bundles transport and applications resulting in the medium's intelligence residing at central points in the network.

What are the problems with guaranteeing QoS

The problem is that when you guarantee QoS there are several factors, which can give you difficulties guaranteeing QoS.
Out on the WAN guaranteeing QoS is still a problem in the Public Internet where service providers can't control the infrastructure from end to end. Some of the bigger players can constantly ping the network to check latency and choose routes path accordingly, bumping some voice traffic over the PSTN as needed. But we may never see toll-quality over the public Internet until the entire infrastructure has been upgraded to Ipv6.
The "private internet" is a very different matter, because service providers can tune backbone networks and use various tricks of trade including peering relationships, to assure QoS.
On the LAN, enterprises have total end-to end control and can just throw in more bandwidth at the problem.However, VoIP implementations to date have been too small to really test the infrastruture.

What is available to improve the QoS

With the points named before in this article it is hard to believe that is possible to bring a VoIP implementatie to a good end, but there are a lot ways to improve the QoS.
In conclusion we can say that VoIP is the largest for QoS on IP networks. It is easy to provide services like voice over ATM and to some degree voice over frame relay, because of the circuit emulation capabilities and continuous-bit-rate-like guarantees for bandwidth, jitter ab latency. But it is extremely difficult to replicate the true CBR-like features on IP-based networks.

The question remains what is useable for guaranteeing QoS at the moment?

• Rescoure Reservation Protocol (RSVP), is a protocol that allows channels or paths on the internet to be reserved for the multicast transmissions of video and high-bandwidth messages.
• Multi Protocol over ATM (MPOA), one of the first industry standards based solution that allows routed advantage of the benefits of the ATM networks.
• Ipv6, which delivers for VoIP the following improvements: - The header of the IP packet is simpler reducing overhead and thus giving more bandwidth for user data - Flow labeling makes real-time prioritization possible - Larger address space allows to add the new VoIP client equipment and assign them a static IP address. - Security options gives possibility to protect VoIP traffic