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The lines connecting the various elements pictured in Figure 5-3 seem simple enough. But, thousands of miles of twisted pair wires, hundreds of miles of coaxial cable, tens of thousands of miles of optical fiber, hundreds of microwave transmission facilities, and several satellite links may be required to provide the geographical coverage shown in Figure 5-3 by the Interexchange Carriers and Local Exchange Carriers.
Why do we care about LECs and Interexchange Carriers? Because one provides the ATM permanent virtual circuit (PVC) that hauls the ATM data from source to destination, and the other provides the local ADSL access. One charges for the ATM PVC and Quality of Service (QoS) and the other charges for the local and ADSL access. The two, or more in some cases, carriers must work together to bring ATM and ADSL technology to the marketplace.
Currently, there typically is more than one Interexchange Carrier providing ATM service to or from a particular geographical region. Not all Interexchange Carriers service all geographical regions. And their service rates differ dramatically. The ATM Interexchange Carriers, their service areas, and typical rates are given in a later chapter.
LECs install and maintain the local access switches that must be provisioned with ADSL modems for ATM/ADSL to be a viable communication system. The LEC receives the ATM cells from the Interexchange Carrier ATM network and converts the cells to an ADSL signal, then transmits the ADSL signal over the local access phone lines to the customer (consumers and small businesses) via the ADSL modem.
Local area networks are typically building and/or campus based. Generally, campus based means the server and the remote desktop computers served are physically located in a small geographical area, such as a cluster of several colocated buildings. When two or more LANs are interconnected through their respective servers in the same general geographical area, the network becomes known as a metropolitan area network. A MAN could be two separate corporate locations within the same metropolitan area that have their LANs networked together. A LAN can be interconnected to its network elements with wiring confined to the customer premises. However, a MAN generally requires access to the public communications network.
Figure 5-4 ATM network with LAN topology
Topology means a way of doing something. Network topologies are various ways of connecting the network elements, also called nodes, such as hubs, bridges, routers, switches, gateways, and users, to accomplish the intended function, whatever that intended function is. The common network topologies are point-to-point, star, hierarchical, mesh (full and partial), bus, and ring. Each network topology has specific attributes that make it especially useful for accomplishing specific functions. The following discussion centers around the topologies utilized by LAN networks. Why do we need to look at LAN topologies? ATM and ADSL will find a large market in the consumer video segment, but they will also find a huge market empowering LANs as MANs, WANs, and GANs. Sort of like a goodly dose of castor oil mixed with growth hormone for LANs. Take them from childhood to adulthood. Give them some muscle. Allow them to mature. Let us pray they do not overdose and become the technological monsters of yesterdays B movies. By the way, where is Hal nowadays?
Figure 5-5 Point-to-point network topology
Point-to-point is the most simple of the various network topologies. Only two nodes are involved in direct communications in the point-to-point network, so each node always knows who it is talking to. There is no need for any sophisticated addressing and switching schemes. In Figure 5-5 Node A can only talk to Node B and Node B can only talk to Node C. There are no other choices. There is little delay in the signal path due to routing issues (is this my packet or do I send it on to the next node?). Just fire it up and go! A point-to-point network example can be represented by your desktop computer and mouse. The mouse is hardwired to the computer. There is no place else for the signals to go, except from the mouse to the computer. A good example of an RF point-to-point network is microwave links.
Figure 5-6 Star network topology
Star network topology is characterized as having one boss and many workers. In Figure 5-6, the boss node is represented by Node A and the worker nodes are Nodes B, C, D, and E. All the worker nodes must communicate directly with the boss node. As long as the worker nodes only want to discuss something with the boss, network efficiency remains reasonable, unless all of the workers want to talk at the same time. Then line up outside the door and wait your turn. If any worker wants to talk to another worker, then the message must be relayed through the boss. Slow, slow, slow. Imagine if you must pass all workplace communications through your boss. Your boss knows everything you say and do. Scary. And everyone works at something less than peak efficiency, especially the boss, if there are many workers involved in the network.
Figure 5-7 Hierarchical topology
Figure 5-8 A networking hierarchical topology
A good example of the star topology is the workplace LAN. All the users are tied to a server which acts as the boss of the LAN. All the LAN users must go through the boss to get to the other users, which may be desktop computers, printers, etc. An e-mail server is a prime example of this type of topology.
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