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Hierarchical

In hierarchical topologies, each node communicates only with the node below and above it. Notice that the top node does not loop around to the bottom node. The top node can only communicate with the bottom node through the intervening nodes, just like a “chain of command.” And just like a chain of command, sometimes it isnt easy getting the attention of the top dog. Sorry, I mean top node.

A good example of a hierarchical network is the OSI protocol model. Each layer can only pass data to and from the layers above and below. From a networking perspective, a hierarchical topology is represented by MAN, WAN, and GAN gateways. For a user of a LAN to get access to another user on another LAN, located perhaps around the globe, the user must gain access to the intervening gateways. See Figure 5-8.

While a hierarchical topology might keep you from getting to the top (dog) node as fast as you might want, at least there is a path that does allow access. It is probably a practical impossibility (that is a legal defense in the technical world) to wire every LAN user directly to every other LAN user in the world. The amount of wiring alone would probably sink our ship. If users are grouped together by some meaningful criteria, such as geographical location, and each group has access to other groups via single entry points, then indirectly we can give every user in the world access to every other user. Hierarchical networks give us the ability to do just that in the form of MANs, WANs, and GANs. But, there are barriers regarding network commonality and interworking, transmission mediums, and access speed. ATM and ADSL are the enabling technologies that remove the barriers. We can fly, we can fly, we can...okay, we get the picture.


Figure 5-9  Full mesh topology

Mesh


Figure 5-10  Bus topology

Mesh topology is the direct connection of every network node to every other network node. Mesh technology is fast since every user can talk directly to every other user. But mesh topology is expensive due to the amount of wiring necessary to connect every node to every other node and the need to provide sophisticated address schemes (equals complex circuitry and software) so each user knows who they are talking to.


Figure 5-11  Ring topology

Figure 5-9 depicts a full mesh network. There is also a partial mesh topology that is the same as a full mesh except that not all the nodes are connected together. One or more nodes do not connect to another node. Mesh topology finds a use in connecting servers together, such as the Internet.

Bus

Bus topology is just like getting on a real bus through the front door. As you walk down the aisle, you look for an empty seat with your seat number that was previously assigned. When you find your empty seat, you sit down. But if you were assigned a seat number that does not exist, or you failed to recognize your seat when you passed by, you  continue down the aisle and pass out through the rear door.

Bus topology was common in the early days of networking but has been largely replaced by the ring topology.

Ring

The ring topology is a “bus” topology, except there are no front or rear doors. Any node can put data on the ring bus and the data will circulate around the bus until it finds the right seat. There are some issues regarding who puts data on the bus when (don&39t want two passengers up and walking about the bus at the same time), but all in all, it is an efficient topology for communicating.

The ring topology has found use as a metropolitan backbone network for interconnecting LANs into MANs. The service providers are installing metropolitan fiber-based rings around all the major cities of the U.S., Europe, and Asia. Not only does ATM provide an efficient transmission technology for the metropolitan rings, but it is also the enabling technology for connecting the MANs into WANs and GANs. Soon, the world will be one. One what? One communications network. One global community.

What’s Up?

There are at least nine different, and for the most part proprietary, communication protocols interconnecting computers and LANs today. The protocols listed in Table 5-1 are only a sample of the different protocols in use today. There are many more. Each protocol is intended to work on specific vendor equipment. When a company decides to purchase proprietary networking equipment from a vendor, the vendor will take the company in hand and provide all of that company’s networking needs. And we all know what happens when vendors have you by the...hand. These protocols are not intended to communicate directly with the other protocols. Even the one vendor (not really a vendor in this instance but rather a vendor specification) listed that is not a proprietary product (TCP/IP) has many vendors producing it with their own little quirks that affect the ability to efficiently network their flavors together.

Vendor Protocol
TCP/IP TFTP
Novell NetWare PCLANP
ISO FTAM
IBM SNA
3Com 3-Open
DecNet DAP
Zerox XNS
AppleTalk AFP
Banyan Vines StreetTalk
Hewlett-Packard RFA
Sun Network NFS

Table 5-1. Some communication protocols

AFP - Apple-Talk Filing System; DAP - Data Access Protocol; FTAM - File Transfer and Management; NFS - Network File System; PCLANP - PC Local Area Network Program; RFA - Remote File Access; SNA - Systems Network Architecture; TFTP - Trivial File Transfer Protocol; XNS - Xerox Network Systems

Loosen the Grip

Where does that leave us? Well, if you have a particular networked LAN implementation in South Dallas and the same networked configuration in North Dallas (or East St. Louis and West St. Louis, about the same difference) and you dont mind having your...hand held, you can easily connect the two networks. However, if you have a network in Dallas and one in London, well, let me say that ATM loosens the grip on the...hand considerably. Read on and liberate yourself from the grip.


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