e second step an 11-bit Feedback Shift Register (FSR) produces a repeating pseudo-random bit pattern which is XOR'd with the 4B/5B output data stream. The effect of this pseudo-randomisation is to minimise high frequency harmonics in the final transmitted data signal. The same pseudo-random bit stream is used to recover the input data in a second XOR operation at the receiver. The final step uses an encoding method called Multi-Level Transition 3 (MLT-3) to shape the transmitted waveform in such a way that the centre frequency of the signal is reduced from 125 MHz to 31.25 MHz. MLT-3 is based on the repeating pattern 1, 0, -1, 0. As shown in Figure 2-8, an input 1-bit causes the output to transition to the next bit in the pattern while an input 0-bit causes no transition, i.e. the output level remaining unchanged. Compared to the Manchester Phase Encoding (MPE) scheme used in 10 Base-T Ethernet, the cycle length of the output signal is reduced by a factor of 4, giving a signal peak at 31.25 MHz instead of 125 MHz. On the physical UTP medium, the 1, 0 and -1 signal levels are represented by line voltages of +0.85, 0.0 and -0.85 Volts. 1111111111111111 Input bit stream MPE coded bit stream MLT-3 coded bit stream Figure 2-8: Ethernet MPE and Fast Ethernet MLT-3 Encoding Wireless Network Logical Architecture ISDN, which stands for Integrated Services Digital Network, allows voice and data to be transmitted simultaneously over a single pair of telephone wires. Early analogue phone networks were inefficient and error prone as a long distance data communication medium and, since the 1960s, have gradually been replaced by packet-based digital switching systems. The International Telephone and Telegraph Consultative Committee (CCITT), the predecessor of the International Telecommunications Union (ITU), issued initial guidelines for implementing ISDN in 1984, in CCITT Recommendation I.120. However, industry-wide efforts to establish a specific implementation for ISDN only started in the early 1990s when US industry members