Cisco 2950G 24 Software Configuration Guide - Page 260

Configuring LRE Ports Chapter 10 Configuring LRE 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 10-21 bits. 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 lists the SNR requirements for downstream rates for different profiles. Table 10-3 SNR Requirements for Downstream Rates Profile LRE-4-1 LRE-7 LRE-8 LRE-5 LRE-10 LRE-15 LRE-10-5 LRE-10-3 LRE-10-1 LRE-15-5 LRE-15-3 LRE-15-1 LRE-998-15-4 LRE-997-10-4 LRE-2 LRE-3 LRE-4 Gross Data Rate 4.17 8.333 9.375 6.25 12.5 16.667 12.5 12.5 12.5 16.667 16.667 16.667 16.667 12.5 2.08 3.13 4.17 QAM 16 16 64 8 64 256 64 64 64 256 256 256 256 256 4 4 4 Theoretical Minimum SNR 19 19 25 16 25 31 25 25 25 31 31 31 31 31 13 13 13 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 High Noise SNR 26 26 32 24 32 39 32 32 32 39 39 39 39 39 20 20 20 Table 10-4 lists the SNR requirements for upstream rates for different profiles. 10-12 Catalyst 2950 Desktop Switch Software Configuration Guide 78-14982-01