The test instrumentation consists principally of a jitter measurement function and a jitter generation function. Measurements can be performed at the physical layer of XG-PON systems. A bit-error rate test (BERT) set may also be required for certain types of measurements; this may be part of the same instrumentation or it may be physically separate. Test instrumentation for the generation and measurement of jitter in digital systems based on synchronous digital hierarchy (SDH) is specified in [ITU-T O.172]. It is recommended that [ITU-T G.987.2] should be read in conjunction with this Recommendation.