ines sensitivity to Worst case signal margin timing error Decision threshold tolerance Sample time variation (offsets and jitter) Figure 28.7 Construction of a binary eye: (a) equalised binary signal; (b) eye diagram; (c) interpretation of the eye diagram activity; see for example (Yamamoto and Wright, 1989) and refer- 5. Terminal equipment limitations and misettings, such as errors ences therein. in equalisation and decision timing. The principal causes of transmission errors in a communication 6. Network problems, including frame synchronisation errors and system are: lost bits. 1. Noise, in particular thermal noise for metallic cable and radio Various methods have been used to model these causes of degra- systems, and quantum noise for optical systems. dation and interference (Kanal and Sastry, 1978; Knowles and 2. Impulsive interference, often from nearby electro-mechanical Drukarev, 1988). Suffice it to note here that at the receiving terminal equipment. or regenerator in a digital link, the probability of making an erro- 3. Cross-talk and intermodulation, from other interfering trans- neous decision (the error probability) is critically dependent upon mission systems. the received signal-to-interference ratio. Asmall change in this ratio 4. Echoes and signal fading, arising from mismatches in trans- will result in a large change in error probability. It is thus vital to mission channels and from multi-path effects in radio systems. establish error performance measures SO that both users and trans- Transmission error performance 28/7 mission system providers can anticipate an end-to-end performance which is compatible with the requirements of the digital information Table 28.1 Some typical BER objectives to be carried. Such performance standards are now emerging, from the CCITT for example. Recommendations for error performance are often based on three performance measures, bit-error ratio, Digital service Transmission rates Long term mean available time and error-free seconds, as described in the following 