10-12
Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 10 Configuring LRE
Configuring LRE Ports

Link Qualification and SNR Margins

When rate se lection is ru nning, th e SNR is used as an ind icator of li nk quality. The switch doe s not
provide any internal mechanism to ensure link quality. There can be different requirements for link
quality, depending on the required bit-error rate and the noise level of the environment. A noisier
en viro nment would req uire a higher SNR to be a ble to provide a stable link. A lo wer bit- error rate w ould
requir e a higher SNR. Typi cally a 6-dB margin provides an error ra te of 10-21 bits.
To provide link stability, you should add a margin to the required SNR. You can configure your margins
to an amou nt that is ap propri ate fo r the noise level of your environment . Increa sing the m argin
requi rement ca n cause the system to choo se a lo wer prof ile, whic h would in turn translate to a low er rate
but with a higher reach.
The switc h does not gua rantee any margin s after a link is a ctivated; ma rgins are only guarant eed only
when the link is established. When a link is activated, if the SNR requirements do not match the
configured margin level, the link is not established.
Downstream m eans t he r em ote en d of the lin k, an d upst r eam the l o cal en d. The li nk has to sa tisf y bot h
the local and remote margin requirements. If either one is not met, the link is advertised as down. This
command has no significance if rate selection is disabled on the interface.
Table 10-3 lists the SNR requirements for downstream rates for different profiles.
Table 10-4 lists the SNR requirements for upstream rates for different profiles.
Table 10-3 SNR Requirements for Downstream Rates
Profile Gross Data
Rate QAM Theo retical
Minimum SNR Low No is e SNR Medium No ise
SNR High Noise
SNR
LRE-4-14.171619212326
LRE-7 8.333 16 19 21 23 26
LRE-8 9.375 64 25 27 29 32
LRE-5 6.25 8 16 19 21 24
LRE-10 12.5 64 25 27 29 32
LRE-15 16.667 256 31 33 35 39
LRE-10-512.56425272932
LRE-10-312.56425272932
LRE-10-112.56425272932
LRE-15-5 16.667 256 31 33 35 39
LRE-15-3 16.667 256 31 33 35 39
LRE-15-1 16.667 256 31 33 35 39
LRE-998-15-4 16.667 256 31 33 35 39
LRE-997-10-4 12.5 256 31 33 35 39
LRE-2 2.08 4 13 15 17 20
LRE-3 3.13 4 13 15 17 20
LRE-4 4.17 4 13 15 17 20