System : Digital Loop Carrier (DLC)

Digital Loop Carrier (DLC)
Digital loop carrier (DLC) is a high efficiency digital transmission system that uses existing distribution cabling systems to transfer digital information between the telephone system (central office) and a telephone or other communication device. There are two types of DLC: universal digital loop carrier (UDLC) and integrated digital loop carrier (IDLC).

The UDLC is a system that consists of RDTs and central office terminals (COTs). Optical systems such as synchronous optical network (SONET) can transfer signals transparently through the COT to the RDT. The RDT provides an interface between the digital transmission line (e.g., DS1) and the customer’s access line. The RDT can dynamically assign time slots from the communication line to customer access lines.

Integrated digital loop carrier (IDLC) is a digital line interface that has been re-engineered to integrate within a switch (usually as card) and shares the internal bus structure of the switch. This function (or card) is called an integrated digital terminal (IDT). Using the IDT, the switch can directly communicate with a remote digital terminal (RDT) that is closer to the end customer using an efficient multi-channel communication line. The RDT provides an interface between the high-speed digital transmission line (e.g., DS1) and the customer’s access line. The RDT can dynamically assign time slots from the communication line to customer access lines. Because customer access lines are not used at the same time, an RDT that interfaces to a DS1 line (24 channels) usually provides service to 96 customer access lines.

The key advantages to DLC carrier systems are the cost effective transmission and the ability to rapidly add, delete, or change customer services without having to dispatch an installation technician. The DLC system offers improved efficiency through the use of existing distribution cabling systems. DLC systems also offer the ability to extend the range of access lines from the central office to the end customer as the RDT effectively operates as a repeater.

An RDT is divided into three major parts: digital transmission facility interface, common system interface, and line interface. The digital transmission interface terminates the high-speed line and coordinates the signaling. The common system interface performs the multiplexing/de-multiplexing, signaling, insertion, and extraction. The line interface contains digital to analog conversions (if the access line is analog) or digital formatting (if the line is digital).

DLC initially allowed 40 analog telephone connections to be extended to the remote neighborhoods using a device called an SLC-40. Later an SLC-96 (known as a “slick 96”) was put into service that allowed 96 voice grade analog circuits to be extended from the CO on just ten (10) pairs thus reclaiming 86 pairs per installation. Still in use the SLC-96 has allowed the LEC’s to conserve much of their installed outside copper infrastructure.

Unfortunately, DLC systems are not transparent to other systems such as DSL systems. Although it is possible to install digital subscriber line network equipment (co-locate) along with RDT equipment, the RDT equipment housings and power supplies were not originally designed to hold additional equipment.

Figure 1 shows the deployment of an integrated digital loop carrier (IDLC) application in a local telephone distribution network. This diagram shows that a switching system has been upgraded to include an IDT and an RDT has been located close to a residential neighborhood. The IDT dynamically connects access lines (actually digital time slots) in the switching system to time slots on the communications line between the IDT and RDT. The RDT can connect to up to 96 residential telephone lines. When a call is to be originated, the RDT connects (locally switches) the residential line to one of the available channels on the DS1 interconnection line. The IDT communicates with the RDT using the GR-303 standard.


Figure 1: Integrated Digital Loop Carrier (IDLC)

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