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SONET Basics
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SONET Evolution
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SONET was initially designed to provide multiplexed point-to-point transport. However, as its capabilities became better understood and networks became mission-critical, its deployment became more innovative, and soon it was deployed in ring architectures, as shown in Figure 2-1. These rings, which are described later, represent one of the most commonly deployed network topologies. For the moment, however, let s examine a point-to-point deployment. As it turns out, rings don t differ all that much. If we consider the structure and function of the typical point-to-point circuit, we find a variety of devices and functional regions, as shown in Figure 2-2. The components include end-devices, multiplexers in this case, which provide the point of entry for traffic originating in the customer s equipment and seeking transport across the network; a full-duplex circuit, which provides simultaneous two-way transmission between the network components; a series of repeaters or regenerators, responsible for periodically reframing and regenerating the digital signal; and one or more intermediate multiplexers, which serve as nothing more than passthrough devices. When non-SONET traffic is transmitted into a SONET network, it is packaged for transport through a step-by-step, quasi-hierarchical process
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Figure 2-1 SONET ring architectures.
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Figure 2-2 SONET overhead in the network.
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Mux R R Intermediate Mux R R Mux
Line Overhead
Line Overhead
Path Overhead
that attempts to make reasonably good use of the available network bandwidth and ensure that receiving devices can interpret the data when it arrives. The intermediate devices, including multiplexers and repeaters, also play a role in guaranteeing traffic integrity, and to that end, the SONET standards divide the network into three regions: path, line, and section. To understand the differences between the three, let s follow a typical transmission of a DS-3, probably carrying 28 T-1s, from its origination point to the destination. When the DS-3 first enters the network, the ingress SONET multiplexer packages it by wrapping it in a collection of additional information, called Path Overhead, which is unique to the transported data. For example, it attaches information that identifies the original source of the DS-3, so that it can be traced in the event of network transmission problems; a bit-error control byte; information about how the DS-3 is actually mapped into the payload-transport area (and unique to the payload type); an area for network performance and management information; and a number of other informational components that have to do with the end-to-end transmission of the unit of data. The packaged information, now known as a payload, is inserted into a SONET frame, and at that point, another layer of control and management information is added, called Line Overhead. Line Overhead is responsible for managing the movement of the payload from multiplexer to multiplexer. To do this, it adds a set of bytes that enable receiving devices to find the payload inside the SONET frame. As you will learn a bit later, the payload can occasionally wander around inside the frame due
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SONET Basics
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to the vagaries of the network. These bytes enable the system to track that movement. In addition to these tracking bytes, the Line Overhead includes bytes that monitor the integrity of the network and have the ability to effect a switch to a backup transmission span if a failure in the primary span occurs. It also includes another bit-error checking byte, a robust channel for transporting network-management information, and a voice communications channel that enables technicians at either end of the line to plug in with a handset (sometimes called a butt-in or buttinski) and communicate while troubleshooting. The final step in the process is to add a layer of overhead that enables the intermediate repeaters to find the beginning of and synchronize a received frame. This overhead, called the Section Overhead, contains a unique initial-framing pattern at the beginning of the frame, an identifier for the payload signal being carried, another bit-error check, a voice communications channel, and another dedicated channel for network management information, similar to but smaller than the one identified in the Line Overhead. The result of all this overhead, much of which seems like overkill (and in many peoples minds it is), is that the transmission of a SONET frame containing user data can be identified and managed with tremendous granularity from the source all the way to the destination. So, to summarize, the hard little kernel of DS-3 traffic is gradually surrounded by three layers of overhead information, as shown in Figure 2-3, that help it achieve its goal of successfully transiting the network. The Section Overhead is used at every device the signal passes through, including multiplexers and repeaters; the Line Overhead is only used between multiplexers; and the information contained in the Path Overhead is only used by the source and destination multiplexers the intermediate multiplexers don t care about the specific nature of the payload because they don t have to terminate or interpret it. One final point for the protocol purists out there: the SONET overhead is often described as shown in Figure 2-4. This layered model can be a bit misleading because it seems to imply a hierarchy of functionality or intelligence. Make no mistake about it, though: SONET is purely a physical-layer standard. A functional hierarchy of sorts may exist among the three overhead types, but they are all mired in the primordial ooze of the transmission layer. Enough about protocol. On to the SONET frame structure.
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