Cisco Systems 2950 Guidelines for Using LRE Profiles

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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter13 Configuring LRE
Configuring LRE Ports
•Age and type of wiring—You can estimate the type of wiring you have based on your site’s age and
type.
– Newer installations less than 15 years old often use Category 3 cable in bundles of 25 pairs.
There is no significant difference between 25-pair bundles and larger bundles.
– Older installations (hotel, school, hospital, commercial—Nort h Ame ric a) 15 t o 30 ye ars old
often use 24 American Wire Gauge (AWG) wiring with between 1 and 12 twists per f oot
(similar to Category 1) in bundles of 25 or more.
– Older installations (residential—North America) 15 to 30 yea rs old ofte n u se 26 AWG wiring
with between 1 and 12 twists per foot (possibly type-2) in bundles of 1 00 or mo re.
– Older installations (Europe) 15 to 30 years old often use 0.4millimeter (similar to 26 AWG)
wiring with between 1 and 12 twists per foot in bundles of 100 o r m or e.
– Older installations (Asia) 15 to 30 years old often use 0.4millimeter (similar to 26 AWG)
wiring with between 1 and 12 twists per foot in bundles of 100 o r m or e.
– 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 cab le 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 freque ncy respon se of th e c ab le at th e hi gh er
frequencies. LRE signals are more susceptible to interference at higher freque ncies. The LRE
upstream signal operates at the high end of the frequency spec trum. Cables have higher attenuation
at higher frequencies and also interfere with other pairs in the bundle at higher frequencies. Thi s
interference or cross talk can significantly impact the signal quality.
Guidelines for Using LRE Profiles
When assigning a profile to a switch LRE port, keep these considerations in mind:
•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 to 3.5 MHz. The ups tream
transmission runs in a high-frequency band from approximately 4 to 8 MHz. Higher frequen cies 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 th e asymm etr ic por t p r ofiles. These profiles use
a low upstream rate but provide a high downstream rate.
Note All POTS telephones not directly connected to the CPE device require microfilters with a
300-ohm termination. Microfilters improve voice call quality when voice and data
equipment are using the same telephone line. They also prevent nonfiltered telephone rings
and nonfiltered telephone transitions (such as on-hook to off-hook) from interrupting the
LRE connection.