Overview

• Type: Digital Loop Carrier (DLC) / Subscriber Loop Carrier (SLC)
• Architecture: Central Office (CO) Terminal and Remote Terminal (RT) pair.
• Primary Function: Multiplexes multiple analog phone lines onto a standard T1 digital line (1.544 Mbps DS1 signal) for more efficient transmission between the telco’s central office and a remote neighborhood cabinet.
• Technology: Based on the VMEbus (VersaModule Eurocard) architecture for its card shelves, which was an industry standard for rugged, real-time computing.

Key Technical Parameters

1. System Capacity & Architecture

• Line Capacity: A single D200 VME shelf could typically support up to 96 subscriber lines.
• Shelves per Bay: Multiple shelves could be mounted in a single equipment bay, significantly increasing the total line capacity per bay compared to older systems.
• Channel Units: The subscriber line interfaces were provided by plug-in “channel unit” cards, each typically handling 8, 12, or 16 individual subscriber lines.

2. Transmission & Interfaces

• Line Rate: 1.544 Mbps (Standard DS1 / T1).
• Line Code: B8ZS (Bipolar with 8-Zero Substitution) was standard, with AMI as a legacy option. B8ZS ensures ones density and provides clear-channel capability.
• Framing Format: ESF (Extended Superframe) was standard, with D4 (SF) framing available for backward compatibility. ESF provides enhanced error checking and maintenance capabilities.
• Span Lines: The system used standard T1 spans to connect the CO and Remote terminals. It could support multiple T1 spans for redundancy (protection switching) and additional capacity.

3. Subscriber Line Interfaces (Channel Units)

The system supported a wide variety of plug-in channel units to provide different services:

• POTS: Standard analog telephone service (loop start or ground start).
• Coin Phone: For payphones.
• ISDN-BRI: Basic Rate Interface (2B+D) for digital data and voice.
• DDS: Digital Data Service (e.g., 2.4/4.8/9.6 kbps subrate, 56/64 kbps).
• Ground Start: For trunk lines or special services.
• Loop Extender: To extend the range of a subscriber loop beyond the normal limits.

4. Common Equipment & Cards (VME Cards)

The system was built around a series of interchangeable VME cards:

• Common Equipment Card (CEC): The master controller for the shelf. It provided timing, alarm processing, and control functions.
• Line Interface Unit (LIU): Provided the physical interface (DSX-1) for the T1 span lines. It handled line build-out (LBO), equalization, and B8ZS/AMI line coding.
• Protection Switch Unit (PSU): Provided automatic (1:1 or 1:N) protection switching between a working T1 span and a protection (backup) T1 span.
• Channel Unit Cards: Various types (e.g., 8-channel POTS, 12-channel POTS, ISDN, DDS).

5. Power Requirements

• Input Voltage: Typically -48V DC (standard telco central office battery plant voltage). It could also be configured for +24V DC in certain applications.
• Power Consumption: Varied based on the number and type of cards installed. A fully loaded shelf typically consumed between 100-200 watts.

6. Physical Specifications

• Shelves: Constructed of heavy-gauge steel for EMI shielding and durability.
• Card Size: Standard single-height or double-height VME format cards.
• Mounting: Designed to be mounted in standard 19-inch or 23-inch telco equipment racks.

7. Environmental Specifications

• Operating Temperature: 0°C to +50°C (32°F to 122°F) for commercial environments. Hardened versions existed for outside plant (OSP) applications in controlled environmental enclosures.
• Relative Humidity: 0% to 95% non-condensing.

8. Alarm & Maintenance

• Alarms: Comprehensive alarm reporting for major (critical) and minor (warning) conditions:
  • Loss of Signal (LOS)
  • Out of Frame (OOF)
  • Alarm Indication Signal (AIS)
  • Remote Alarm Indication (RAI/Yellow Alarm)
  • Card Failures
  • Power Failures