By Navin R. Thadani, Cisco Systems, Inc.
As Cable MSOs start deploying PSTN replacement voice services (over IP), availability and reliability become important considerations. However, there is a lot more to high availability than the number of nines on a certain network device.
In addition to looking at device level availability, it is also important to understand the availability of the end-end network taking into consideration the various system level inter-dependencies. Further, it is even more important to understand, evaluate and design a network keeping in mind “service availability”. In the case of a voice service, two popular service availability metrics are the number of calls dropped and the number of ineffective attempts.
This paper reviews the availability requirements of a “primary line” (PSTN replacement) voice service, dispels some of the popular five 9s myths about the PSTN; and then establishes a framework by which to analyze and design a network to achieve the required level of service availability.
The paper also outlines some of the modifications in terms of redundancy as well as routing optimization that may be required on the edge, in the regional networks as well as the backbone networks in order to support PSTN equivalent voice over IP networks. In other words, it reviews some of the changes that need to be made for transport at the edges and between distribution networks in order to support highly available and reliable services such as voice over IP and digital broadcast video. It outlines the evolutionary path of the current High Speed Data IP networks to highly available service delivery platforms in the future.