EBI 400 tracks

[PJW]

hi there, I studied IRSE news issue 147 july/august in that it had been given about EBI TRACK 200,300,400. I understood about EBI track 200 and 300. But it is difficult to understand about EBI Track 400 given in that can anyone explain about this content
"A prime development exercise related
to the size of the code word so that
wrong-side failure integrity could be
mathematically proven. The choice of a
256 bit telegram in turn dictated a bit rate
of 128 bits per second which the
communication channel would have to
support given that code recognition was
required within two seconds to stay close
to the tried and tested performance
parameters of the previous TI21 track
circuit. The 256 bit code word has meant
that by optimising the hamming distance
between code words to cover both
wrong-side and right-side failure
conditions at least 16 000 exceptionally
safe unique codes per carrier frequency
can be generated."
given in page 7 of that news letter
Thanks in advance

In essence what it means is that, in addition to the "frequency" (actually the two alternating for ac immunity) additional security is being given by including a code superimposed upon the carrier frequency. One of the problems with jointless track circuits is that there can be frequency leaking from one frequency A track circuit, perhaps via traction bonding or in London Underground case via the air main to another frequency A track circuit. In the extreme this can give rise to wrongside failure in the presence of various fault conditions.
Hence by adding a particular code to each of the freuency A track circuits then even if enough voltage is received by another track circuit in the area then it will not respond as the code wouldn't match the one it was expecting.
On the one hand the more different codes that there are the further away that two tracks can be before needing to share the same one; on the other hand the more bits of data need to be included on the track circuit. TI21s were deliberately 2 seconds slow to pick (partly a result of needing to check the alternate reception of the two separate ones of th "warble" but also advantageous to give a level of protection against rusty rails etc) but obviously if this time had to become longer then it would be more operationally restrictive and the delay would impact unduly use of the layout. Therefore this time consraint plus the baud rate limit the level of complexity of the code and thus the number of combinations available.