Carrier System : Optical Carrier (OCx), ISDN Digital Subscriber Line (IDSL), Integrated Digital Loop Carrier (IDLC),

Optical carrier (OCx) transmission is a hierarchy of optical communication channels and lines that range from 51 Mbps to more than 39 Gbps. Lower level OC structures are combined to produce higher-speed communication lines. There are different structures of OC. The North American optical transmission standard is called synchronous optical network (SONET) and the European (world standard) is synchronous digital hierarchy (SDH). OCx has been used to represent the digital transmission standards where the “x” denotes the multiple of 51.84 Mbps service. Optical carrier standards continue up through OC768 but some definition at the higher levels is still lacking. The following is an abbreviated list of the optical carrier systems:

Optical Carrier 1 (OC1) – Operates at 51.84 Mbps;

Optical Carrier 3 (OC3) – Operates at 155.52 Mbps (3 X OC1)

Optical Carrier 9 (OC9) – Operates at 466.56 Mbps (9 X OC1);

Optical Carrier 12 (OC12) – Operates at 622.08 Mbps (12 X OC1).

Optical Carrier 192 (OC192) – Operates at 9.95 Gbps

Optical Carrier 256 (OC256) – Operates at 13.27 Gbps.

Optical Carrier 768 (OC768) – Operates at 39.81 Gbps

Synchronous Digital Hierarchy (SDH) is an international digital transmission format used in optical (fiber) networks standardized that is similar (but not identical) to the title Synchronous Optical Network (SONET) used in the United States. SDH uses standardized synchronous transmission according to CCITT standards G.707, G.708 and G.709. These standards define data transfer rates, defined optical interfaces and signal structure formats.

ISDN Digital Subscriber Line (IDSL)
ISDN Digital Subscriber Line (IDSL) is a hybrid of ISDN and DSL technologies. It uses the same data formatting as ISDN devices on the copper wire pair and delivers up to 144 kilobits per second bandwidth through two 64 kbps channels and one 16 kbps channel. The key difference for IDSL systems is that the IDSL system only uses the 64 kbps DS0 channels and the ISDN control channel (D channel) is ignored. The IDSL system effectively multiplies the number of channels on a single copper pair by 2x. The ability to avoid using ISDN signaling is very important as software upgrades for switching systems, to allow ISDN operation can cost more than $500,000 per switch.

Integrated Digital Loop Carrier (IDLC)
Integrated digital loop carrier (DLC) is a digital transmission technology that is used between the central office and groups of customers. The IDLC system is composed of two primary parts: an integrated digital terminal (IDT) and a remote digital terminal (RDT). The IDT concentrates up to 96 lines on to a single 24 channel T1 line. It does this by assigning central office channels to time slots on the IDLC line (between the IDT and RDT) as needed. The RDT reverses the process by assigning a time slot to an access line. The RDT also changes the format of the time slot to the access technology of choice (e.g., ISDN or analog).

The key advantages to the DLC carrier system is that some of the switching function is moved closer to the customer (in the RDT) and increased cost effective transmission through the increased sharing of local loop copper lines. Because the RDT in the DLC system acts as a repeater, this also extends the range of access lines from the central office to the end customer.

Unfortunately, DLC systems are not transparent to 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.

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).

Figure 4.23 shows an integrated digital loop carrier system. The DLC carrier system is composed of two basic parts: the Integrated Digital Terminal (IDT) and the Remote RDT. 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 reverses the process and converts the high speed DLC channel into independent POTS channels (DS0s) that are connected via local loop lines to homes or businesses.


Figure 1: Digital Loop Carrier (DLC) System

The most common multiplexer is the SLC (subscriber line carrier)-96. This system, routinely referred to as a “Slick96”, can deliver 96 voice circuits (the equivalent 4 T-1’s) to the customer site. This methodology overcomes distance problems associated with providing voice services to remote customers as well as free additional twisted pair for future use.