8.5 Unbundling and DLC-Fed Lines

Nearly one-third of subscriber lines in the United States were served from digital loop carrier remote terminals (DLC-RTs) or fiber-fed next generation DLC (NGDLC) in the year 2000. DLC and NGDLC shorten the copper telephone line by using fiber, T1, or HDSL to link a remote line multiplexer (the DLC-RT) to the CO, and then using individual copper telephone lines to connect each customer to the DLC-RT located within their neighborhood. The Telecommunications Act of 1996 required the unbundling of lines served from DLC-RTs, and it further required the unbundling of spare fibers connecting the CO to the remote sites. Using these facilities, a CLEC could place their own NGDLC-RT or remote DSLAM at the remote site once they had obtained the rights to locate equipment on the property. The high cost of outdoor cabinets , construction, power access, and the difficulty of obtaining public easement rights make CLEC overbuilds generally unattractive for neighborhood remote serving sites. A more attractive opportunity that is widely practiced is the placement of a remote DSLAM within an office building or multiple dwelling unit (MDU, e.g., an apartment building) to serve many customers within the building.

Being less than 12,000 feet in length and lacking loading coils, the lines fed from DLC-RT sites are ideal for DSL transmission. However, each DLC-RT must first be upgraded with the addition of DSL line cards, new common equipment plug-ins to perform statistical multiplexing, and, in some cases, an additional fiber- optic link to the central office.

CLECs can lease unbundled DLC transport for POTS and ISDN that provides a path from a DLC-COT (central office terminal) port to an RT-fed telephone line connecting to the customer's premises. The FCC's Second Memorandum Opinion and Order released September 8, 2000, permitted SBC Corporation to provide DSL access via ILEC owned NGDLC equipment (including the DSL line cards) and the associated optical concentration devices (OCDs) in the central office that aggregate the traffic from all the NGDLC systems. The FCC's Order required SBC to lease unbundled DSL access to CLECs at a price approximately equal to its cost. With this, the CLEC can lease access at the OCD to the DSL data bit-stream to and from a customer. This can be provided in a line sharing arrangement where the ILEC provides POTS on the same line. Though this FCC Order was specific to SBC, it clearly establishes a precedent for similar arrangements by other ILECs.

With line sharing and unbundled access to NGDLC-fed DSL lines, data carriers can provide service to most customer lines. The principal exceptions are lines exceeding 18,000 feet and lines using digital added main line (DAML, also know as universal digital carrier) technology. The attenuation of the lines beyond about 18,000 feet prohibit DSL operation at data rates needed for many applications. High bit-rate operation on these long lines could be enabled by a midspan repeater, but ILECs have not made a practice of providing repeaters for the use of CLECs. Midspan repeaters are expensive, primarily due to the labor cost of splicing the repeater into the cable and the equipment cost for the apparatus case to protect the repeater from the harsh outside environment. The cost of the repeater can be reduced by encapsulating the repeater in epoxy with a wire stub enclosed ; this configuration is known as a repeater brick. Mounting repeater bricks can be a problem if there are more than a few of them.

DAML systems convey two or more digitally derived POTS channels via one copper telephone line. DSL-like transmission is used on the line, with the traditional analog voice interfaces re-created at both the customer and the central office ends of the line. Unlike voice-over DSL systems, traditional DAML systems convey only derived voice channels and do not also convey data (except for data modulated within the voice band using V.34 or V.90 modems). Most DAML systems use 2B1Q modulation in a band from zero to a few hundred kHz . Because most DSL transmission systems also operate within this frequency band , DSL may not be used on lines with traditional DAML systems. Some new DSL products combine a DAML-like multiline voice function and a high speed data channel in an integrated system.

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