BRX-XLR User Guide
36
180-0145-001-R01
Annex A
Excerpt on Impulse Noise from Broadband Forum
TR-176: ADSL2Plus Configuration Guidelines for IPTV
Impulse noise is defined as electrical interference that occurs in short bursts. It may be caused by any
number of sources, from large motors to arc welders, improper AC power and grounding to consumer
electronic devices not performing to normal EMC design requirements. These types of disturbers
cause an electrical impulse that is brief but powerful and may temporarily interfere with transmission
on the DSL circuit.
ADSL2plus Profiles offer a parameter for defining the minimum amount of Impulse Noise Protection.
At the transmission layer, DMT symbols are of fixed duration of 250 microseconds. The INPMin
parameter defines the minimum number of DMT symbols that will be protected from impulse noise
and thus the minimum duration of impulse noise from which error correction should be able to recover.
To provide maximum error protection, INPMin should be set as high as possible without unduly
compromising bit-rates and latency. It should be noted that
Service Providers have discovered that
8ms delay may not adequately protect against Repetitive Electrical Impulse Noise REIN in 60Hz
regions due to sub-optimal conditions including but not limited to imperfect waveforms and variance in
the repetition of REIN. Under these circumstances, 7ms may be more appropriate.
There is a direct relation between INPMin and symbol rate such that higher values of INPMin will
restrict the DSL circuit to a lower maximum bit rate. This relationship is dependent in part on the
interleaving capabilities of the DSL chipsets at both ends of the DSL line (S, D, framing parameters
and interleaving memory). There is also a relationship between INPMin and the delay incurred as
higher INPMin values require more buffering and thus incur longer delay. INP defines the maximum
number of successive corrupted DMT symbols that can be corrected within the duration
corresponding to the delay. As a result, an INP of 2 can correct up to two successive DMT symbols
during one delay period. As an impulse of 250μs duration can occur randomly compared to DMT
symbols, it will generally corrupt two DMT sym
bols. So an INP of 2 will fully protect against 250μs max
impulsive noise. An INP of 1 or lower will give some protection but without a guarantee concerning the
duration of pulses.
As can be expected, there is an interaction between fixed FEC parameters (interleaving depth and
delay) and INP setting.
Low delay and high INP can actually help stabilize a DSL connection (the
low delay being counter intuitive)
. However, such a setting forces the FEC parity ratio (R/N) to
values like 1/3 or 1/2, so lots of errors in every code word are corrected (so if the line is not extremely
long it is possible to use the extra bits it nominally could carry without impulse to actually counteract
the impulse). Alternative ranges of such INP/delay can be useful but should be tested since there can
be a wide variation of support between vendors.
