Cisco WS-C2960S-24PD-L Software Guide - Page 217

Chapter 7 Configuring the Switch Ports Configuring the LRE Ports - Older installations (Asia) 15 to 30 years old often use 0.4 mm (similar to 26 AWG) wiring with between 1 and 12 twists per foot in bundles of 100 or more. - Older installations over 30 years old often use heavy gauge wire (22 or 20 AWG) with no significant twist. In many cases, the cabling is set into the fabric of the building. The cables might be tightly or loosely bundled. For this estimate, assume that they are tightly bundled in groups of 25 or more. • Cross talk (noise) and interference-LRE operates with any number of wires in a cable binder carrying the LRE signal. Anywhere from one wire pair to every wire pair in the cable can carry LRE signals at the same time. LRE operates in full cable binders and adjusts power levels on each LRE link to maximize the performance of all connections. The greatest impact on LRE performance is from the frequency response of the cable at the higher frequencies. LRE signals are more susceptible to interference at higher frequencies. The LRE upstream signal operates at the high end of the frequency spectrum. Cables have higher attenuation at higher frequencies and also interfere with other pairs in the bundle at higher frequencies. This interference or cross talk can significantly impact the signal quality. Considerations for Using LRE Profiles When assigning a profile to a switch LRE port, keep these considerations in mind: • Each switch LRE port always has a private profile assigned to it. The LRE-10 profile is the default. Public profiles have priority over private profiles. If you assign a public profile to the switch, the switch uses the public profile and ignores any private profile assigned to the switch LRE ports. If a public profile is configured on the switch and you want the switch LRE ports to use private profiles, you must first disable the public profile by using the no lre profile global global configuration command. When you assign a different profile to a switch LRE port, the port immediately resets and uses the newly assigned profile. • Before you add an LRE switch to a cluster, make sure that you assign it the same public profile that is used by other LRE switches in the cluster. A configuration conflict occurs if a switch cluster has LRE switches using both private and public profiles. If one LRE switch in a cluster is assigned a public profile, all LRE switches in that cluster must have that same public profile. A cluster can have a mix of LRE switches using different private profiles. For more information about clusters, see Chapter 5, "Clustering Switches." • Phone lines typically operate at a frequency of up to 3.4 kHz. On the LRE link, the downstream transmission runs in a low-frequency band from approximately 1 MHz to 3.5 MHz. The upstream transmission runs in a high-frequency band from approximately 4 MHz to 8 MHz. Higher frequencies are more susceptible to interference. Consequently, upstream signals are susceptible to cross talk and disruption on the link. To maintain the quality of the LRE connection, use the asymmetric private profiles. These profiles use a low upstream rate but provide a high downstream rate. We recommend configuring all switch LRE ports with the LRE-10-5 profile rather than the default LRE-10 profile. • Use the LL profiles (LRE-5LL, LRE-10LL, and LRE-15LL) with care. These profiles have the low-latency (LL) feature enabled and the interleaver feature turned off. The LL feature does not delay data transmission, but it makes data more susceptible to interruptions on the LRE link. All other profiles, public and private, have the interleaver feature enabled and the LL feature disabled. The interleaver feature provides maximum protection against small interruptions on the LRE link but delays data transmission. 78-6511-08 Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide 7-19