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SONET/SDH Hardware

The hardware aspects of SONET are difficult to describe in discrete terms, as many of the functional elements overlap. Just as manufacturers of traditional voice and data equipment often build multiple functional elements under the skin of a single box, so do SONET/SDH equipment manufacturers. Given that caveat, the following is a description of discrete functional aspects of hardware devices.

Terminating Multiplexers
are Path Terminating Equipment (PTE) that provide user access to the SONET network (Figure 10.6), operating in a manner similar to a T3/E3 Time Division Multiplexer. Multiple DS-0’s and DS-1’s can be multiplexed to form an STS-1 or STS-3 frame, for instance. Terminal multiplexers also accomplish the conversion from electrical STS-N signals into Optical Carrier (OC-N) signals.


Figure 10.6  Terminal Multiplexer (TM) and Add/Drop Multiplexer (ADM) in SONET application.

Concentrators
perform the equivalent functions as traditional electrical concentrators and hubs. SONET concentrators combine multiple OC-3 and OC-12 interfaces into higher OC-N levels of transmission [10-15].
Add/Drop Multiplexers (ADMs)
do not have a exact equivalents in the electrical (DS) world, although they perform roughly the same functions as do traditional T-carrier TDMs. Generally found in the Central Office Exchange, they provide the capability to insert or drop individual DS1, DS2, or DS3 channels into a SONET transmission pipe (Figure 10.6). ADMs accomplish the process electrically, with the OC-N channel being converted prior to the process and being reconverted subsequently.
ADMs offer great advantage over traditional DS-N MUXs. The T-carrier approach, for instance, requires that a DS3 frame be de-multiplexed into its 28 component DS1 frames, which must be broken down into 24 DS0 channels in order to extract and route an individual channel. Once that has been accomplished, the process must be reversed in order to reconstitute the DS3, minus the extracted DS0, and send it on its way. ADMs perform the additional functions of dynamic bandwidth allocation, providing operation and protection channels, optical hubbing, and ring protection.
Digital Loop Carrier (DLC)
systems typically are located at the Local Exchange Carrier (LEC) office and are used to concentrate traffic in multiple DS0s into a single DS3 signal. Having performed this intermediate step, they hand off the STS-1 signal to a Terminating MUX which performs the OC-N conversion.
Digital-Cross Connects (DXCs)
perform approximately the same functions as their electrical equivalents (DACs/DCCSs), providing switching and circuit grooming down to the DS1 level. They provide a means of cross-connecting SONET/SDH channels through a software-driven, electronic common control cross-connect panel with a PC user interface. The routing of traffic through a DXC is accomplished through the use of payload pointers, which point to the payload in the OC-N frame and provide synchronization. Importantly, DXCs also serve to connect the fiber rings. DXCs perform the additional functions of monitoring and testing, network provisioning, maintenance and network restoral.
Regenerators
perform the same function as their traditional electrical equivalents. Often found under the skin of other SONET equipment, they are optoelectric devices which adjust the amplitude, timing, and shape of the signal.

Advantages of SONET

SONET offers a number of advantages in addition to the inherent advantages of fiber optic transmission systems. Certainly, the fact that SONET is highly standardized offers the benefits of interconnectivity and interoperability between equipment of different manufacturers. That translates into freedom of vendor choice and yields lower costs through competition. Additionally, SONET/SDH is extendible to the premise on a fully interoperable basis. While such extension currently is unusual for SONET, such deployment provides end-to-end advantages of enhanced bandwidth, error performance, dynamic bandwidth allocation, and network management. In an end-to-end deployment scenario, SONET is particularly attractive in support of broadband services such as SMDS, Frame Relay, ATM and, ultimately, B-ISDN.

In bandwidth-intensive applications, the high absolute cost of SONET can be offset by virtue of its extraordinarily high capacity. Whether deployed in a carrier network or extended to the user premise, SONET supports the aggregation of all forms of traffic, including voice, data, video, image, and facsimile. As a result, a SONET infrastructure can obviate the need for multiple transmission facilities in support of individual services. The simplicity of multiplexing and de-multiplexing via ADMs reduces costs, delay, and error. Clearly, SONET/SDH offers the considerable advantage of network resiliency through its inherent redundancy and self-healing capabilities.

Finally, SONET offers tremendous security, as is the case with fiber optic transmission systems. Fiber is difficult, if not impossible, to physically tap without detection. It is difficult to identify the one channel for detection out of the thousands of information channels supported in a SONET mode. As Francis Bacon said in 1625, “There is no secrecy comparable to celerity” (of Delay, Essays)—speed is a hallmark of SONET.

Applications of SONET/SDH

SONET primarily is deployed in backbone carrier networks where its many advantages can be put to full use. Particularly in a convergence scenario, the carriers have the potential to realize considerable cost savings by using a single fiber infrastructure in support of bandwidth-intensive video and image streams, in addition to voice, data, and facsimile traffic. That scenario ultimately will deliver SONET fiber to the premise—perhaps even to residences.


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