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====== Signalling of non-fiber based Ethernet ====== Based on documentation listed as sources, written and assembled by Jon Langseth. ===== 10Base ===== 10Base2, 10Base5 og 10BaseT are using Manchester coding on the physical layer. Thus, the electrical signalling on the wire is a binary signal. Manchester code (also known as Phase Encoding, or PE) is a line code in which the encoding of each data bit has at least one transition and occupies the same time. It is, therefore, self-clocking, which means that a clock signal can be recovered from the encoded data. ===== 100Base ===== The IEEE-802.3u specification enables Fast Ethernet's compatibility by supplanting 10BaseT's binary-level Manchester encoding with a three-level symbol-transmission system. This scheme uses a 4B/5B code that was originally developed for the FDDI (fibre-distributed-data-interface) system. The coding translates 4-bit data nibbles into a 5-bit code that enables error detection, clocking rate control and adds control codes, such as start- and end-of-stream delimiters. For example, a run of 4 bits such as 0000 contains no transitions and that causes clocking problems for the receiver. 4B/5B solves this problem by assigning each block of 4 consecutive bits an equivalent word of 5 bits. These 5 bit words are pre-determined in a dictionary and they are chosen to ensure that there will be at least two transitions per block of bits. Upping the symbol rate to 125 Mbps compensates for the 4B/5B's inherent 20% data-transmission inefficiency, but this bandwidth increase creates a spectrum that Manchester encoding would extend into hundreds of megahertz. Attenuation losses and EMC issues prohibit this approach, so 100BaseTX uses a MLT-3 (multiple-level-transition, three-level) carrier. Like Manchester coding, MLT-3 encodes a bit according to transitions, but it packs its output into a three-level waveform that crudely simulates sine-wave-energy distribution. These three levels continuously alternate from +1 to 0 to –1 and back again, with a logic zero effectively halting the sequence, and a one restarting it . A long sequence of ones creates the output signal's highest frequency, when the signal repeats the 1, 0, –1, 0 pattern. This pattern's cycle length is one-fourth the clock rate, which reduces the worst-case energy component to 32.5 MHz. ===== 1000Base ===== To achieve gigabit speeds, 1000BaseT uses all four of the cable's wire pairs and can simultaneously send and receive over each wire pair. In this approach, transmission and reception signals simultaneously occupy the same low-frequency portion of the channel to minimize attenuation problems. But their spectra then overlap to create interference in the form of echoes, when the near-end transmission signal reflects off the line and acts as a linear filter on the transmitted signal. Because each end's receiver "knows" what it's just sent, it can subtract its transmission patterns from the composite signal to recover the opposite end's sent data. This technique, full-duplex echo-cancelling transmission, implemented using DSP FIR (finite-impulse-response)-filter techniques. Deriving from RF communications practice, 1000BaseT uses a PAM (pulse-amplitude-modulation) scheme, PAM-5, that features a 5×5 constellation with 2 bits per symbol and a 125-MHz symbol rate. Combining five-level coding across four wire pairs permits 1000BaseT to send 1 byte per signal pulse (four wire pairs times 125 million symbols/sec times 2 bits/symbol equals 1 Gbps). One effect of increasing transmission levels from Fast Ethernet's three to Gigabit Ethernet's five and maintaining the same overall voltage swing is to increase sensitivity to noise disturbances by some 50%, or 6 dB. Because only four levels are necessary to transmit the symbol's 2 bits, Gigabit Ethernet uses the fifth level for FEC (forward-error-correction) to compensate for this SNR loss. ===== Wireless ===== WiFi systems use two primary radio transmission techniques. * 802.11b (<=11 Mbps): The 802.11b radio link uses a direct sequence spread spectrum technique called complementary coded keying (CCK). The bit stream is processed with a special coding and then modulated using Quadrature Phase Shift Keying (QPSK). * 802.11a and g (<=54 Mbps): The 802.11a and g systems use 64-channel orthogonal frequency division multiplexing (OFDM). In an OFDM modulation system, the available radio band is divided into a number of sub-channels, and some of the bits are sent on each. The transmitter encodes the bit streams on the 64 subcarriers using Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), or one of two levels of Quadrature Amplitude Modulation (16, or 64-QAM). Some of the transmitted information is redundant, so the receiver does not have to receive all of the sub-carriers to reconstruct the information. The original 802.11 specifications also included an option for frequency hopping spread spectrum (FHSS), but that has largely been abandoned. ===== Sources ===== * * * * *