sábado, 13 de febrero de 2010
It is the nature of modern communications networks to be in a state of ongoing evolution. Factors such as new applications, changing patterns of usage, and redistribution of content make the definition of networks a work in progress. Nevertheless, we can broadly define the larger entities that make up the global network based on variables such as transport technology, distance, applications, and so on.
One way of describing the metropolitan area network (MAN) would be to say that it is neither the
long-haul nor the access parts of the network, but the area that lies between those two (see Figure 1-1).
Long-haul networks are at the core of the global network. Dominated by a small group of large transnational and global carriers, long-haul networks connect the MANs. Their application is transport, so their primary concern is capacity. In many cases these networks, which have traditionally been based on Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) technology, are experiencing fiber exhaust as a result of high bandwidth demand.
At the other end of the spectrum are the access networks. These networks are the closest to the end users, at the edge of the MAN. They are characterized by diverse protocols and infrastructures, and they span a broad spectrum of rates. Customers range from residential Internet users to large corporations and institutions. The predominance of IP traffic, with its inherently bursty, asymmetric, and unpredictable nature, presents many challenges, especially with new real-time applications. At the same time, these networks are required to continue to support legacy traffic and protocols, such as IBM's Enterprise System Connection (ESCON).
Metropolitan Area Networks
Between these two large and different networking domains lie the MANs. These networks channel traffic within the metropolitan domain (among businesses, offices, and metropolitan areas) and between large long-haul points of presence (POPs). The MANs have many of the same characteristics as the access networks, such as diverse networking protocols and channel speeds. Like access networks, MANs have been traditionally SONET/SDH based, using point-to-point or ring topologies with add/drop multiplexers (ADMs).
The MAN lies at a critical juncture. On the one hand, it must meet the needs created by the dynamics of the ever-increasing bandwidth available in long-haul transport networks. On the other hand, it must address the growing connectivity requirements and access technologies that are resulting in demand for high-speed, customized data services.
Metropolitan and Long-Haul Networks Compared
There is a natural tendency to regard the MAN as simply a scaled-down version of the long-haul network. It is true that networks serving the metropolitan area encompass shorter distances than in the long-haul transport networks. Upon closer examination, however, these differences are superficial compared to other factors. Network shape is more stable in long-haul, while topologies change frequently in the MAN. Many more types of services and traffic types must be supported in MANs, from traditional voice and leased line services to new applications, including data storage, distributed applications, and video. The long-haul, by contrast, is about big pipes.
Another important way in which metropolitan networks today differ from trunk-oriented long haul networks is that they encompass a collection of low bit-rate asynchronous and synchronous transmission equipment, short loops, small cross-sections, and a variety of users with varying bandwidth demands. These fundamental differences between the two types of networks have powerful implications for the requirements in the metropolitan domain. Protocol and speed transparency, scalability, and dynamic provisioning are at least as important as capacity, which rules in the long-haul market.
An Alternative View
The preceding breakdown of the global network represents a somewhat simplified view. In reality, the lines between the domains are not always so clear-cut. Long-haul and metropolitan networks are sometimes not clearly delineated; the same holds true for the access and metropolitan domains.
Furthermore, other views of the global network exist. One, for example, defines the access network as part of, rather than separate from, the MAN, while also including enterprise connectivity in the metropolitan domain. In this view, the metropolitan market breaks down as follows:
• Core—These are essentially scaled-down long-haul systems. They are considered the core of the
MAN, because they interconnect carrier POPs and do not directly interface with end users.
• Metropolitan access—This is the segment between carrier POPs and access facilities, which could be equipment at customer premises or at an aggregation point.
• Enterprise—This is the part of the network dedicated to serving the needs of enterprises. Using owned or leased fiber (or leased fiber capacity), connectivity is provided between geographically disparate enterprise sites and for new applications, such as storage area networks (SANs).
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