Voice communication is a real-time communication, which means it has severe delay constraints . Dedicated systems for voice do not face any problems considering delay but in a system made for asynchronous data transmission it will become a big issue. In the following subsections the challenges that will be faced during VoWLAN product development and the requirements of voice transmission are given.
The goal is relatively simple: add telephone calling capabilities (both voice transfer and signaling) to WLANs with backbone IP-based networks and interconnect these to the public telephone network and to the private voice networks in such a way as to maintain current voice quality standards and preserve the features everyone expects from the telephone. The challenges for the product developer arise in five specific areas:
- Voice communication quality should be comparable to what is available using the PSTN, even over networks having variable levels of QoS.
- The underlying network must meet strict performance criteria including minimizing call refusals, network latency, packet loss, and disconnects. This is required even during congestion conditions or when multiple users must share the network resources.
- Call control (signaling) must make the telephone calling process transparent so that the callers need not know what technology is actually implementing the service.
- PSTN/VoIP/VoWLAN service interworking (and equipment interoperability) involves gateways between the voice and the wireline data network environments and the wireline and the wireless data networks.
- System management, security, addressing (directories, dial plans) and accounting must be provided, preferably consolidated with the PSTN operation support systems (OSSs).
Providing a level of quality that equals that of the PSTN (this is usually referred to as “toll quality voice”) is viewed as a basic requirement. It has been found that there are three factors that can profoundly impact the quality of the service:
- Delay: Talker overlap (the problem of one caller stepping on the other talker’s voice) becomes significant if the one-way delay becomes greater than 250 milliseconds. The end-to-end delay budget is therefore the major constraint and driving requirement for reducing the delay through a packet network.
- Jitter (delay variability): Jitter is the variation in inter-packet arrival time as introduced by the variable transmission delay over the network. Removing the jitter requires collecting packets and holding them long enough to allow the slowest packets to arrive in time to be played in the correct sequence, which causes additional delay. The jitter buffers add delay, which is used to remove the packet delay variation that each packet is subjected to as it transits the packet network.
- Packet loss: Wireless and IP networks cannot provide a guarantee that packets will be delivered at all, much less in order. The packets will be dropped under peak loads.