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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 12 Configuring LRE
Configuring LRE Ports
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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.
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 upstream
transmission runs in a high-frequency band from approximately 4 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 port profiles. 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.
• When the link between the LRE switch and the CPE device must co-exist in the same cable bundle
as an asymmetric digital subscriber line (ADSL), we recommend that you use either the ANSI
profile (LRE-998-15-4) or the ETSI profile (LRE-997-10-4). For details on which profile to use
elsewhere, consult the regulations for connecting to the PSTN in your area.
• LRE signaling can co-exist with ADSL signaling in one cable bundle. However, LRE signaling is
not compatible with T1 signals in the same cable bundle.
Use the show controllers lre status link privileged EXEC command to display the LRE link statistics
and profile information on the LRE ports. For information about this command, see the switch command
reference.
CPE Ethernet Link Guidelines
Follow these guidelines when configuring CPE Ethernet links:
• Guidelines for Configuring Cisco 575 LRE CPEs and 576 LRE 997 CPEs, page 12-11
• Guidelines for Configuring Cisco 585 LRE CPEs, page 12-11