Optical Transmission System

Optical transmission systems use strands of glass or plastic fiber to transfer optical energy between points. For most optical transmission systems, the transmitting end-node uses a light amplification through stimulated emission of radiation (LASER) device to convert digital information into pulsed light signals (amplitude modulation). The light signals travel down the fiber strand by bouncing (reflecting) off the sides of the fiber (called the cladding) until they reach the end of the fiber. The end of the fiber is connected to a photo-detector that converts these light pulses back into their electrical signal form.

Synchronous optical transmission systems used a specific frame structure and the data transmission through the transmission line is synchronized to a precise clock. This eliminates the signaling overhead requirement for framing or timing alignment messages. The basic frame size used in optical transmission systems is 125 usec frames.

Optical transmission systems are characterized by their carrier level (OCx) where the basic carrier level 1 is 51.84 Mbps. Lower level OC structures are combined to produce higher-speed communication lines. There are different structures of OC used in the world. The North American optical transmission standard is called synchronous optical network (SONET) and the European (world standard) is synchronous digital hierarchy (SDH).

Signals are applied to and are extracted from optical transmission systems using an optical add/drop multiplexer (OADM). The OADM is a network element that provides access to all or some subset synchronous transport signal (STS) line signals contained within an optical carrier level N (OC-N). The process used to direct a data signal or packet to a payload of an optical signal is called mapping. The mapping table is contained in the OADM. A copy of the OADM mapping is kept at other locations in the event of equipment failure. This allows the OADM to be quickly reprogrammed.

Synchronous digital hierarchy (SDH) is an international digital transmission format used in optical (fiber) networks standardized that is similar (but not identical) to SONET. 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.

Some of the key differences between SONET and SDH include differences in overhead (control) bits and minimum transfer rates. The first level available in the SONET system is OC1 and is 51.84 Mbps. The first level in the SDH system starts at STM-1 and has a data transmission rate of 155.52 Mb/s. SONET also multiplexes synchronous transport signal level 1 (STS-1s) to form multiple levels of STS. The SDH system divides the channels into multiple DS0s (64 kbps channels). This is why the overhead signaling structures are different.

Figure 1 shows the optical standards for both SONET and SDH. This table shows that the first common optical level between SONET and SDH is OC3 or STS-1. STS-x and STM-x are the standards that specify the electrical signal characteristics that are input to the respective optical encoding/multiplexing processes.

Figure 1: Optical Transmission Systems

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