barcode generator in vb.net codeproject Figure 5-5 A subscriber loop carrier system. in Software

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network model of the future: it is more efficient and places both the services and the bandwidth that they require where they belong. What does this have to do with ATM and SONET A lot, as it turns out. Let s consider one simple example of the distributed central office that I allude to in the previous paragraph. DSL is a marvelous technology that offers customers the ability to enjoy broadband access to the network, yet its level of penetration remains depressingly low. Part of the reason for this is the fact that most residential local loops in the U.S. are deployed over digital-loop carrier systems, as illustrated in Figure 5-5. This design makes sense for service providers for several reasons. First of all, consider the drawing. In the drawing, 96 telephone customers are provided with voice service over five T-1 carrier facilities, requiring a total of 20 conductors (wires) between the central office terminal (COT) and the remote terminal (RT). If it were not for the carrier system, those same 96 customers would require 192 conductors, a significant increase and cost. The only problem with this model is that each customer is assigned a single 64-Kbps time slot, which effectively eliminates his or her ability to enjoy higher bandwidth services. To counter this service limitation, some service providers have chosen to move the central office-based Digital Subscriber Loop Access Multiplexer (DSLAM) from the CO to the residential neighborhood by hanging it on the end of an optical fiber and using ATM protocol over that fiber to carry out QoS mandates and high-speed switching requirements as they crop up. The customer can then attach directly to the DSLAM, thus
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Figure 5-5 A subscriber loop carrier system.
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96 customers 64 Kbps each
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5 T-1s (4 active, 1 spare) 20 conductors
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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Asynchronous Transfer Mode (ATM)
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eliminating the legacy bottleneck that occurs with carrier- system service. This is shown in Figure 5-6. Of course, the network management model must now change in response to the new network architecture. Instead of managing centralized network elements (a relatively simple task) the management system must now manage in a distributed environment. This is more complex, but if done properly results in far better customer service because of the immediacy that results from managing at the customer level.
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ATM Technology Overview
Because ATM plays such a major role in SONET networks today, it is important to develop at least a rudimentary understanding of its functions, architectures, and offered services. We begin this discussion of ATM with the overall structure of a typical ATM network, starting from the perspective of the user and working toward the network cloud. The access interface between the user s equipment and the first ATM switch is called the User-to-Network Interface (UNI) (see Figure 5-7). The user s equipment may be something as simple as a DSU, a router behind a DSU, or a private, local ATM switch. If the user s equipment is another
Figure 5-6 Remote DSLAM as a way to improve customer service.
Multiple customers Variable bandwidth
Optical facility; ATM on back side of DSLAM DSLAM
Central Office
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Asynchronous Transfer Mode (ATM)
Figure 5-7 The ATM network protocol model.
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Public ATM Switch B-NT2 UNI DSU UNI DSU UNI DXI Central Office
B-TE B-TE
Router
switch or an ATM-capable PBX, the device is referred to as a BroadbandNT2. Devices that access this local switch are known as Broadband Terminal Equipment (B-TEs). In the event that the user s equipment is a router and the router is not capable of interfacing directly to the ATM network because of a protocol incompatibility, then the router will communicate with a DSU over an ATM Data Exchange (DXI) Interface. The DSU will in turn communicate with the network. In some cases, for cost and complexity reasons, the router will send its data directly to the ATM switch in the cloud without first converting it to cells. In those cases, the data will be sent to the switch inside a layer-two frame, across what is known as a Frame-Based UNI (FUNI). In the event that the ATM network is being used to emulate LAN services across a wider area, a LAN Emulation (LANE) server may be deployed. In this case, the Broadband-TEs will access the server via a LANE UNI (LUNI). The LANE server, in turn, will access the local ATM switch (which is probably privately owned) across a Private Network Node Interface (PNNI). Inside the cloud, the ATM switches communicate with one another across a Network Node Interface (NNI). Public ATM clouds communicate with each other across a standardized interface in the same way that traditional public packet-switched networks communicate with each other. The Broadband Interexchange Carrier Interface (B-ICI) is used to interconnect discrete networks.
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Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode (ATM)
These interfaces, particularly those installed to interconnect public and private networks, must implement signaling and routing functions between the two and must therefore rely on the appropriate standards to do so. Organizations such as the ITU-T, the ATM Forum, ANSI, Telcordia, and the IETF have all contributed greatly to the development of standards that address these and other issues.
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