Link Qualification And Snr Margins - Cisco 2950G 24 - Catalyst Switch Software Configuration Manual

Configuring LRE Ports

Link Qualification and SNR Margins

When rate selection is running, the SNR is used as an indicator of link quality. The switch does 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
environment would require a higher SNR to be able to provide a stable link. A lower bit-error rate would
require a higher SNR. Typically a 6-dB margin provides an error rate of 10
To provide link stability, you should add a margin to the required SNR. You can configure your margins
to an amount that is appropriate for the noise level of your environment. Increasing the margin
requirement can cause the system to choose a lower profile, which would in turn translate to a lower rate
but with a higher reach.
The switch does not guarantee any margins after a link is activated; margins are only guaranteed 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 means the remote end of the link, and upstream the local end. The link has to satisfy both
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
Table 10-3 SNR Requirements for Downstream Rates
Gross Data
Profile Rate
LRE-4-1 4.17
LRE-7 8.333
LRE-8 9.375
LRE-5 6.25
LRE-10 12.5
LRE-15 16.667
LRE-10-5 12.5
LRE-10-3 12.5
LRE-10-1 12.5
LRE-15-5 16.667
LRE-15-3 16.667
LRE-15-1 16.667
LRE-998-15-4 16.667
LRE-997-10-4 12.5
LRE-2 2.08
LRE-3 3.13
LRE-4 4.17
Table 10-4
Catalyst 2950 Desktop Switch Software Configuration Guide
10-12
lists the SNR requirements for downstream rates for different profiles.
Theoretical
QAM Minimum SNR
16 19
16 19
64 25
8 16
64 25
256 31
64 25
64 25
64 25
256 31
256 31
256 31
256 31
256 31
4 13
4 13
4 13
lists the SNR requirements for upstream rates for different profiles.
-21
Medium Noise
Low Noise SNR SNR
21 23
21 23
27 29
19 21
27 29
33 35
27 29
27 29
27 29
33 35
33 35
33 35
33 35
33 35
15 17
15 17
15 17
Chapter 10 Configuring LRE
bits.
High Noise
SNR
26
26
32
24
32
39
32
32
32
39
39
39
39
39
20
20
20
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