Network Communications News (NCN) December 2016 - Page 18

F E AT U R E PON PON: The evolution of data delivery to the home Paolo Novellini, system IO specialist and Antonello Di Fresco, strategic application engineer, at Xilinx, explore the next steps in home data delivery. F Paolo explores the next step in data deliver to the home ifteen years ago, the most common way of connecting to the Internet was through an analogue modem, sending data over a standard telephone voice channel. This technology quickly dominated the communications market because it was very cheap for the user and required no change to ‘last mile’ technology because it used existing twisted-pair wires already deployed for standard phone calls. No digging up roads; no changes in the central office (CO); it was quite attractive as you can imagine. Modem speeds peaked at 56Kbps. Why 56Kbps? Why not higher? The simple answer is: it was not ‘theoretically’ possible. This theoretical limit set the stage for ADSL technology. Analogue modems used the existing voice channel, strictly standardised by the ITU-T committee. This channel has a limited bandwidth (4KHz, including the guard band), which is hard-filtered at the CO (central office) before entering the Muldex (Multiplexer Demultiplexer). The Muldex is the device your telephone connects with in the CO. What is the maximum data rate you can transfer over a 4KHz analogue channel? The answer to this question is the key to understanding ADSL. The correct answer is: ‘It depends on the noise level of the channel.’ Famous American mathematician and electrical engineer Claude E Shannon gave us this answer back in 1948. You can transfer at any bit rate, provided the noise level is low enough. This result sometimes surprises us. Shannon was actually more precise in relating, in a quantitative way, the maximum bit rate with a given channel bandwidth and noise level. You use his famous formula: Where C is the maximum bit rate in bits/sec (capacity), B is the bandwidth in Hz, S/N is the signal-to-noise ratio of the channel. The ITU-T specifies the bandwidth and the noise level for the voice channel, limiting de facto its maximum bit rate on 18 18-20 PON – Xilinx.indd 18 02/12/2016 11:03