The quality of Voice over IP (VoIP) has improved considerably from past years, and VoIP has become the dominant technology for corporate telephone systems. Meanwhile, company VoIP is primarily imperfect towards the corporation intranet, and outside calls are converted to analogue and routed via the standard analogue telephone backbone. However, the expertise and bandwidth are now evolving to the point where peer-to-peer calls between VoIP phone systems across the internet have become viable in terms of quality, reliability, and value. VoIP phone systems have very high levels of reliability, and most modern generation systems are 99.999%+ reliable—provided they are correctly configured and managed. And VoIP has already proven its case as a cost-saving voice mode.
The key challenge in achieving this viability lies in managing voice quality. Call delay, variable delay, and packet loss are the primary factors that affect voice quality in a VoIP system. Let's address each of these:
- **Call delay or constant delay** refers to the consistent delay that can exist in calls, i.e., a time lag that remains the same throughout the call. This does not directly affect voice quality but does impact the way people communicate. At its worst, this leads to an awkward lag in the conversation or over-speaking, which can significantly affect the quality and flow of communication.
- **Variable delay**, also known as jitter, occurs when transmitted VoIP packets arrive at different time intervals at the distant end of the call—i.e., some of the packets are delayed. This is a common everyday situation for IP-based networks; after all, for data packets, it has little effect since they can simply be reordered and joined to recreate the file. However, it is critically important for voice packets, which must be reordered and joined to create a continuous and near real-time stream. Jitter causes choppiness and distortion in the analogue recreation that the listener receives. There are many causes of jitter, including router congestion, operating over parallel routers, changes to mid-stream in the physical infrastructure pathways between terminal customers, transmission issues, codec points, and processor issues. Different VoIP programs aim to correct jitter by buffering the incoming packets. The method holds several received packets in short-term memory so that any delayed packets can be inserted back into the stream before it is converted back to the analog voice pattern. If jitter is low, then the buffer period can be quite short. As long as jitter in the IP network is high, either the buffer period will need to be increased, or there may be perceptible gaps in the conversation. However, increasing the buffer significantly adds to the fixed delay discussed above.
- **Packet loss** occurs when a transmitted packet is not received on the receiving end. This packet loss can be due to an array of factors, notably line noise. The codecs in VoIP use advanced algorithms to compensate for minor packet loss, but they cannot fully regenerate or simulate the precise data contained in the lost packets. Therefore, this packet loss can result in audible gaps in the analog voice after being converted at the distant end of the VoIP phone systems.
Overlying all these points is the issue of fluctuating and often transient network conditions, which can occur anywhere along the transmission chain. To address these issues, it is essential to understand which one or more of these problems is causing the issue. It’s about knowing your enemy and protecting the voice from other functions running on your network. More sophisticated VoIP phone systems contain significant functionality to handle these problems. Additionally, there are various third-party diagnostic software tools available.