Black Box MEG821AE, Руководство пользователя

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Manual of VDSL2 Solution
http://www.black-box.eu
8
Single-Ended Loop Testing (SELT) : The most convenient tests to perform are those that only require connection to one end
of the line, because only one tester is required, and one person is required, and no travelling needs to be done. Some tests
require the line to be correctly terminated and it may be possible to set the installed equipment (eg modem) to do this without
having to go to the end of the line.
Double-Ended Loop Testing (DELT) : Double-ended testing involves dispatching a technician to the customer's location to
install a modem or test equipment that communicates with the reference modem in the CO. If the service doesn't work, a work
order is issued to clean up the pair.
Trellis modulation : The name trellis was coined because a state diagram of the technique, when drawn on paper closely
resembles the trellis lattice used in rose gardens. The scheme is basically a convolutional code of rates (r,r+1). Ungerboeck's
unique contribution is to apply the parity check on a per symbol basis instead of the older technique of applying it to the bit
stream then modulating the bits. The key idea he termed Mapping by Set Partitions. This idea was to group the symbols in a
tree like fashion then separate them into two limbs of equal size. At each limb of the tree, the symbols were further apart.
Although in multi-dimensions, it is hard to visualize, a simple one dimension example illustrates the basic procedure. Suppose
the symbols are located at [1, 2, 3, 4, ...]. Then take all odd symbols and place them in one group, and the even symbols in the
second group. This is not quite accurate because Ungerboeck was looking at the two dimensional problem, but the principle is
the same, take every other one for each group and repeat the procedure for each tree limb. He next described a method of
assigning the encoded bit stream onto the symbols in a very systematic procedure. Once this procedure was fully described,
his next step was to program the algorithms into a computer and let the computer search for the best codes. The results were
astonishing. Even the most simple code (4 state) produced error rates nearly 1,000 times lower than an equivalent uncoded
system. For two years Ungerboeck kept these results private and only conveyed them to close colleagues. Finally, in 1982,