tiframe of data bits. The n-bit code is arrived at by applying a synchronisation, effectively taking the circuit down momentarily. complex mathematical function to each group of 24 (ESF) or 8 Jitter, the cyclic offset of bits from their expected positions in (PCM-30) frames of data. The result of this calculation, the n-bit time, is one of the most ominous of all T1/E1 impairments. Jitter can code, is then transmitted in the CRC framing bit positions of the be intermittent and data dependent, which makes it difficult to following frame. isolate. Jitter occurs little by little, cumulatively over many bits, and At the other end of the circuit, the same mathematical function is can ultimately cause the missampling of the pulses resulting in performed on the same first group of frames. This newly calculated bipolar violations and bit error conditions. 21/12 Time division multiplexing The most common cause of jitter is the network equipment itself. The reason why lies in the construction and characteristics of such Jitter is inherent to the clock recovery timing used in transmission, cables. A transmission cable is made of a group of wires, having two and is typically added to the pulses at every regeneration point electrical properties. One is the d.c. resistance and the second is the within the network. As long as each network component adds only self inductance value of the wire. When a group of wires is placed a very small amount of jitter, the circuit will be unaffected. Prob- in a casing to form the cable, a third electrical property, capacitance lems arise when a failed or failing network component adds signi- between the wires, is brought to bear. The transmission cable now ficant amounts of jitter. Less typically, jitter can come from behaves like a low pass filter. crosstalk, electrical noise, and other types of interference. If a unipolar code is transmitted down the line, each high level Wander is an impairment very similar to jitter, and is defined as signal will inject energy into the capacitor/inductor 