TEL 355: Communication and
Information Systems in
Organizations
Digital Loop Carrier (DLC)
Professor John F. Clark
Definition of DLC
• The local loop is the physical connection
between the home or business user and the
telco service provider
• DLC uses digital technology to offer a wide
range of services via copper 2X wires
• The development of DLC technology is
vital to the future of new online consumer
services
History of Subscriber Carriers
• Loop carrier development originally
focused on providing higher quality over
longer distances
• Was originally deployed in suburban and
rural areas.
• Relatively few lines and subscribers
• Little attention paid to connection methods
and signal strength (pair-gain capabilities)
Analog Carrier Systems
• Designed to provide phone service at
distances greater than CO capabilities.
• POTS and carrier multiplexer were analog
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The advent of electronics in the 60s
Multiplexer connected to local exchange switch
Subscriber box served 1-8 customers
Limited technology, limited deployment
• Was difficult to maintain and performance
was inconsistent
Digital Loop Technology
• Early 70s – digital performance benefits
– Improved voice quality and higher gain (12 to 1)
– Added features: both analog and digital services
– Reliable, easy to install and maintain
• Architecture was digital on the network end
and analog on the subscriber end
• 96 line Remote Subscriber Terminal (RST)
– Connected to a Local Exchange Terminal
– T1 carrier between the terminals
Early Next Generation DLCs
• In the 80s – based on computer-driven Very
Large Scale Integration (VLSI) technology
• Primary purpose: deliver services such as
– basic and primary rate ISDN
– 56k and 64k data rates (DDS)
– T1 and other analog services
• Designed for fiber from the CO to the RST
– Service to 2000 customers per RST
• Widespread service was its downfall
NGDLC: Rural/Suburban Uses
• In the 90s – advanced computing and chip
technology makes it possible & economical
• Relies on software control and intelligence
• Works over any infrastructure – digital T1
or T3, analog, fiber or coax
• Based on a mixed network architecture and
mesh topology that reflects actual suburban
development patterns – its key to success
NGDLC Applications
• First and foremost – DLC emulation for
many remaining analog carriers
• Can be implemented a number of ways:
– Universal DLC Emulation – old analog carriers
– Drop and Insert Configuration – programmable
RSTs in any network medium
– Star Configuration – radiates T1, fiber, HDSL,
or coax facilities
– Tree/Mesh Configuration – a mixture of dropand-insert and star topologies
Special NGDLC Applications
• Integrating voice and video:
– Integrated Video/Telephony Point-to-Point
– Integrated Video/Telephony Point-to-Multipoint
• Hybrid systems that use RF technology to
multiplex voice and video over coax and
break out voice over 2X for the subscriber
• Network flexibility allows any of these
configurations to be used with any other for
very cost-effective solutions
Advantages of NGDLC
• Very economical for suburban/rural areas
– Cost effective for low line sizes
– Low amount of common control overhead
– Uses the existing infrastructure
• Customers want more service & bandwidth
– Means deploying more fiber
– Implement the system and upgrade as needed
– Pay for upgrades only when implemented
Future Directions
• Competition between HFC companies and
telcos and between different telcos is
driving development – especially in xDSL
• Increasing use of the Internet and even
higher bandwidth services is having an
affect on development and on economics
• Must supply increasing bandwidth while
supporting narrowband economically –
migration must be simple and cheap
Economics
• Equipment must be interchangeable and
modular; common control low
• Must have minimum frame-relay capability
– But not paid for until required
• Will move to a three-stage architecture:
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Traditional TDM bus for regular traffic
SONET bus: 4 or 5 Gbps drop and insert traffic
Cell relay bus (ATM) for high bandwidth
Common control will handle all three types
The Multiservice DLC
• Will become a network cloud that includes
multiple topologies and protocols
• Incorporation of frame relay and ATM
figures prominently: OC-12 and OC-48
• Because the high bandwidth infrastructure
for this does not exist in many places and
may never will, wireless local loop services
will be required