Mumbai University > Electronics Engineering > Sem7 > Optical Fiber Communication

a. DWDM:

• The widespread deployment of single-mode fiber has encouraged the investigation of WDM on this transmission medium. In particular, developments concerned with single-mode fiber WDM transmission can be distinguished into two broad categories, namely coarse WDM (CDWM) and dense WDM (DWDM).

• Although both categories use the same concept of multiple-wavelength channels on a single fiber, they differ in the channel spacing they employ. CWDM as implied by the terminology uses wider channel spacing and hence provides significantly fewer channels than DWDM.

• Dense WDM was originally concerned with optical signals multiplexed in the 1.55 μ m wavelength region using the capabilities of erbium-doped fiber amplifiers (EDFAs) to increase system capacity and therefore to reduce system cost.

• Figure 5.3 shows a block schematic for a DWDM system where a large number of channels N, each utilizing a single wavelength (i.e. from λ1 to λN), are multiplexed onto a single-fiber transmission medium.

• Both the deployment of EDFAs and dispersion compensation are required for long-haul DWDM systems to offset any optical signal power losses caused by optical wavelength multiplexers and other passive optical devices. Finally, a wavelength demultiplexer distributes each channel to the corresponding receiver.

• Dense WDM systems use narrow channel spacings and can therefore accommodate several hundred wavelength channels on a single optical fiber. The three possible channel spacings specified for DWDM systems are 1.6 nm (200 GHz), 0.8 nm (100 GHz) and 0.4 nm (50 GHz) while an even smaller channel spacing of 0.1 nm (12.5 GHz) is feasible in which case the system may also be referred to as super-DWDM .

DWDM has again advantages over general WDM:

• Channels are nearer together which leads to a higher possible capacity.

• If the different DWDM channels are chosen in a way that they lie all around the 1550 nm communication window, all channels can be amplified simultaneously using optical amplifiers as EDFA’s (erbium doped fibre amplifier). This saves additional equipment and therefore money.

b. SONET/SDH

Ans:

SONET/SDH SONET (Synchronous Optical Network) is the current transmission and multiplexing standard for high-speed signals within the carrier infrastructure in North America. A closely related standard, SDH (Synchronous Digital Hierarchy), has been adopted in Europe and Japan and for most submarine links.

Multiplexing SONET and SDH employ a sophisticated multiplexing scheme, which can, however, be easily implemented in today’s very large-scale integrated (VLSI) circuits. Although SONET and SDH are basically similar, the terms used in SONET and SDH are different. For SONET, the basic signal rate is 51.84 Mb/s, called the synchronous transport signal level-1 (STS-1). Higher-rate signals (STS-N) are obtained by interleaving the bytes from N frame-aligned STS-1s. Because the clocks of the individual signals are synchronized, no bit stuffing is required. For the same reason, a lower-speed stream can be extracted easily from a multiplexed stream without having to demultiplex the entire signal.

For SDH, the basic rate is 155 Mb/s and is called STM-1 (synchronous transport module-1). Note that this is higher than the basic SONET bit rate. The SONET bit rate was chosen to accommodate the commonly used asynchronous signals, which are DS1 and DS3 signals. The SDH bit rate was chosen to accommodate the commonly used PDH signals, which are E1, E3, and E4 signals. Higher-bit-rate signals are defined analogous to SONET, as shown in Table below.

A SONET frame consists of some overhead bytes called the transport overhead and the payload bytes. The payload data is carried in the so-called synchronous payload envelope (SPE). The SPE includes a set of additional path overhead bytes that are inserted at the source node and remain with the data until it reaches its destination node.

SONET/SDH Layers: The SONET layer consists of four sublayers—the path, line, section, and physical layers. Figure 5.4.shows the top three layers. Each layer, except for the physical layer, has a set of associated overhead bytes that are used for several purposes. These overhead bytes are added whenever the layer is introduced and removed whenever the layer is terminated in a network element.

• The path layer in SONET (and SDH) is responsible for end-to-end connections between nodes and is terminated only at the ends of a SONET connection. It is possible that intermediate nodes may do performance monitoring of the path layer signals, but the path overhead itself is inserted at the source node of the connection and terminated at the destination node.

Figure 5.4 SONET/SDH layers showing terminations of the path, line, and section layers for a sample connection passing through terminal multiplexers (TMs) and add/drop multiplexers (ADMs). The physical layer is not shown

Each connection traverses a set of links and intermediate nodes in the network.

• The line layer (multiplex section layer in SDH) multiplexes a number of path-layer connections onto a single link between two nodes. Thus the line layer is terminated at each intermediate line terminal multiplexer (TM) or add/drop multiplexer (ADM) along the route of a SONET connection. The line layer is also responsible for performing certain types of protection switching to restore service in the event of a line failure.

• Each link consists of a number of sections, corresponding to link segments between regenerators. The section layer (regenerator-section layer in SDH) is terminated at each regenerator in the network.

• Finally, the physical layer is responsible for actual transmission of bits across the fiber.

SONET Frame Structure: Figure 5.5 shows the structure of an STS-1 frame. A frame is 125 μs in duration (which corresponds to a rate of 8000 frames/s), regardless of the bit rate of the SONET signal. This time is set by the 8 kHz sampling rate of a voice circuit. The frame is a specific sequence of 810 bytes, including specific bytes allocated to carry overhead information and other bytes carrying the payload. We can visualize this frame as consisting of 9 rows and 90 columns, with each cell holding an 8-bit byte.

Figure 5.5 Structure of an STS-1 frame. B denotes an 8-bit byte.

The bytes are transmitted row by row, from left to right, with the most significant bit in each byte being transmitted first. The first three columns are reserved for section and line overhead bytes. The remaining bytes carry the STS-1 SPE. The STS-1 SPE itself includes one column of overhead bytes for carrying the path overhead.