All IP Networks …special challenges
ISDN was probably introduced before the market was ready for it, and while it achieved reasonable penetration in Europe, it failed to become universally popular. However it did introduce the concept of integrated digital access for voice and data, and effectively achieved what transmission planners had been trying to achieve for about a century. With a lossless network and the 0dBr point could be put in the handset itself, so that a uniform OLR of 10dB could be practically realised for every call between ISDN customers.
VoIP over all IP network also allows this goal to be achieved. However while ISDN was very much an evolution of the telephone network, with the data being carried on voice circuits. In an IP world, the situation is the reverse with voice being carried on what was originally a network designed specifically for data. The biggest difference is that in an ISDN, when party A calls party B, a dedicated circuit is set up between the two parties for the duration of the call. So resources are allocated to that call, and barring faults or natural disasters, the call has the full use of that resource until it is completed.
In a VoIP call, the A party's speech is digitised and then every 20 or so milliseconds packetised and sent to Party B. In a simple IP network each packet makes its way through the network independently of any other packet. Depending on the network loading at the time, each packet may arrive by a different route, and may even arrive in a different order from which they are sent. This variable delay is a significant problem for VoIP, and is dealt with by buffering the packets at the receive end, so that the buffer length is the proportional to the maximum likely delay through the network. This of course adds further to the end to end delay which can degrade the perceived quality of the speech. The other degradation often occurs for economic reasons is due to low bit rate encoding. By using low bit rate encoding, transmission costs are reduced, but at the expense of increased distortion and usually additional delay.
The VoIP networks which network operators will use to replace the existing voice networks will minimise delay by using very fast switches and routers and using special protocols to reduce the variability of the delay. It is likely that the need for low bit rate coding will be unnecessary, as the difference in cost between 8kbps and 64kbps will be insignificant in networks designed to switch bandwidths in the Mbps.

Private VoIP networks
Where private VoIP networks are interconnected with public networks everything is simplified if the 0dBr reference point in the private network is the same as the public one, and standards compliant phones are used. If a non standard phone with say a high SLR e.g. 18dB is used in a private network, the easy fix would appear to be to add 10dB gain in the gateway to the public network, which would certainly fix any calls between that phone and the public network. However two undesirable situations result.
- No standard IP phone can be connected to that network, as it will sound too loud (by 10dB) to public network users.
- The non standard gateway setting corrects the send levels to Public network telephones but not to the other phones on the private IP network.
The second effect could in theory be corrected by introducing a 10dB loss at the gateway for the signal path from the public network to the private one. This would also necessitate the phones having an RLR of -8dB so that calls within the private network as well as beyond it would sound the same.
A further difficulty with running non standard levels in private networks is that the analogue to digital conversion processes will be forced to operate outside their normal dynamic range introducing noise and distortion.

Summary
VoIP allows the easy implementation of the optimal end to end loudness rating (OLR). SLR/RLR values of +8/+2 for handsets has been standardised by the major standards bodies. As far as speech levels are concerned, there are no compromises required if these loudness ratings are used, the user of any IP phone will be able to hear and be heard by any the user of any other IP phone. If IP phones do not have uniform LRs, then the IP world ends up no better than the analogue world. That is a public network surround by a collection of proprietary private networks with a limited range of proprietary (and expensive) handsets, and variable performance….i.e. back where we were 30 years ago!
