The "numerical line current differential relay" has proven to be one of the most successful types of transmission line protective relay system, and advances in communication technology have facilitated its ever wider application. The adoption of SDH and SONET as standards for high speed data communication over optical fiber and microwave has further encouraged the use of this type of protection. However, the impact of these standards has given rise to the possibility of communication links with characteristics unsuitable for conventional line current differential relays. Even where utility communication networks are mostly dedicated, public telephone operator provided links may be used at the periphery and these are likely to be the weak link in the chain. More