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Tuesday, July 17, 2007

Networking Architecture

Networking Basics: Architecture

This document will discuss three network architectures in exploring the basics of networking:

along with some of their differences and similarities.

These architectures are labeled as 'OPEN' since you can run a variety of network operating systems on them.


Ethernet is by far and large the most common network architecture encountered today.

Ethernet usually runs on 10-Base-T (UTP) wiring but in older installation might be found on 10-BASE-2 or 10-BASE-5 (Coax cable).

Ethernet can be deployed on either a star or linear-bus wiring topology.

Linear-bus (coax) topology

Ethernet can be run on a linear-bus topology using coax (RG-58) cabling. This type of network will employ a point-to-point structure where every node will speak over the same cabling segment.

Basically there will exist a single string of cable that will 'touch' each node of the network. (A Node is defined as any fileserver, workstation, or peripheral device attached to the network).

Each end of the string will be terminated to form an electrical circuit for communications to travel over. This topology is a simple two-wire circuit - the coax core and the wire mesh beneath the outer casing.

A linear bus topology relies on the integrity of the entire length of its coax line. The major drawback to this structure is that if any point on the communications string develops a fault, the entire network goes down.

There are no communication Hubs in this structure to use in isolating problems with specific segments of the network; the network is all one segment.

Note: Coax has a distance limitation of about 600 feet, before the signal degrades. Since a linear-bus topology treats the entire network as one segment, the distance between its two nodes furthest from each other cannot exceed 600 ft.

The way around this limitation is to compensate for signal degradation by installing repeaters along the network.

Repeaters will boost the signal and send it off ready to travel another 600 feet.

This changes the scenario to one where the node must be less than 600 ft. from the nearest repeater instead of the furthest node.

Star/Complex Star (UTP) topology

A star topology involves hubs. Hubs come in various forms and, depending on their complexity/abilities, may be called concentrators, switches, or ethernet switches.

The hub serves as a central connection point for the population of the network. Nodes, (fileservers, workstations, and shared network devices), will all plug into a hub to communicate with the other nodes on the network.

Looking at the lines connecting the nodes to the hub from above you might see a star shape.

Working with this star topology has many benefits in the areas of trouble shooting and fault tolerance. Instead of a single network segment touching each node, linear-bus, each node has a separate cable segment that touches the hub.

This segment will take form, at least in part, as a patch cable. Patch cables are lengths of network wire with connectors on both ends. Workstations will typically have a patch cable running between the computer and a wall outlet.

Commercial/Industrial installations will also have patch panels in the computer room. Patch Panels represent the other end of the cable running from the back of the node's wall outlet.

Yet another patch cable connects a node's port on the patch panel to a hub. Because nodes have their own cable segment and speak to each other via a hub, any individual connection can be interrupted without affecting other nodes on the network.

The disconnected node will obviously be unable to reply to connected nodes but connected nodes can continue talking to each other.

This is a significant advantage when trouble shooting network problems and isolating traffic problems.

For example, network cards or connectors will sometimes become faulty. A faulty NIC or connector can cause normal data packets to become distorted and be broadcast as garbage.

In the case of a faulty NIC, it may continuously broadcast these garbage packets - flooding its own cabling segment, and probably those of its neighbors, to the point where normal traffic is unable to travel.

This condition is commonly known as packet storm or data storm. Network administrators are often tipped off to this problem by finding that communication problems happen only at certain times of the day or only in certain departments.

Many administrators have stayed late or come in early to diagnose problems and found nothing. They later determine that the problem only occurs when Jack/Jill is at work and their computer is on. Its also a good idea to take note of who is on vacation and who doesn't turn off their computer when they go home.

The advantage for star networks in this case is that nodes can be connected/disconnected one by one to find the offending node. Connecting/disconnecting nodes on a hub is a simple as unplugging a phone cord.

The normal scenario here would be to have two stations connected in the problem area, generate traffic between them, and add stations until the communication problem is duplicated.

Note: While IP nodes may let you plug and unplug them at will, IPX/SPX (Ex. Novell) networks need you to re-boot the node after disconnecting it. This is because IP is a packet-oriented technology and IPX is a broadcast technology.

In IP, the packet is more or less self-contained and addressed to a specific destination.

In broadcast technology, IPX, every node basically listens to all traffic and only pays attention to the traffic addressed to it. In ethernet, a collision detection system ensures that if two nodes broadcast data at the same time and data collides, the data gets rebroadcast.

IPX/SPX networks have several layers of drivers loaded to establish this listening environment. When the physical connection is disrupted in IPX, the drivers need to be reloaded to re-establish the listening environment.

Complex Star topologies

Complex Star topologies consist of multiple Star networks connected to each other. Complex Stars typically develop out of formerly isolated departmental networks. The consolidation of the individual star networks may be dictated or desired to provide one bigger/better central fileserver serving all departments in lieu of deploying smaller, less expensive, fileservers for each department.

Consolidating networks also helps departments share information and physical resources, like printers. Individual stars of a complex star network are connected via a backbone.

Backbones are simply another cabling segment of the network. Backbones differ from other segments by the fact that they normally connect hub to hub instead of node to hub. Characteristically, backbones are often longer than other cabling segments since they might provided connectivity to stars that are hundreds or thousands of feet apart.


In ethernet, wires 1,2,3,6 are used for communications. Two of the wires, or pins, are used to transmit data and two are used to receive. At some point, the transmissions of one node have to end up on the receiving pins of another node. This crossover happens in the hub.

Hypothetically the crossover should happen only an odd number of times. In practice, it only happens once. For this reason, you cannot simply connect one hub to another without turning the crossover off on one of the connecting ports.

A backbone simply combines two hubs into one large hub - typically over some significant distance.



For connecting networks IN THE SAME BUILDING, running 10-Base-T is a nice, quick networking solution.

However, the UTP cable should not be run through harsh environments, like extremely hot/cold warehouses or places where birds, squirrels, or pests might chew on or damage the cable.

In short, UTP is very fragile. All an end user has to do to ruin a cable is run the wheels of a desk chair over it a few times. Signals will degrade after traveling over 300' of UTP wire and become unreliable.

Repeaters can be installed to compensate for the 300' distance limitation.

Note (repeaters): Although similar in appearance to hubs, repeaters serve a different function.

Repeaters will take the fading signal from either side and boost it to full strength, enabling it to travel potentially another 300', over CAT5/UTP.

A repeater used in a coax backbone would allow the signal to travel another 600'. This difference in distance is a property of the cable more than of the repeater.


Coax backbones are quite common due to the greater lengths they can carry a signal before it degrades. In addition to having twice the range of UTP, they are less sensitive to their environments and fairly sturdy.

Coax backbones can be buried, preferably in metal conduit, or run in the open air between buildings, ideally attached to a structure or rigid line.

Coax lines can also employ repeaters to cover distances greater than 600' but it's important to note that repeaters installed outdoors would have to live in a weatherproof work box and be secured to a structure or tower.

Its generally a bad idea to install a work box at ground level unless its immediately next to a building - vehicles tend to run over them otherwise.

Many hubs will have a BNC connector on the back to support coax backbones. In this case the crossover mentioned above is not an issue.

If a hub does not have a native BNC connector, a balen may be used to convert the RJ45 port to a BNC connector, with one each end of the backbone. As implied just now, a balen will translate an RJ45 connector to a BNC connector.

Fiber-Optic Cable

Fiber will connect sites that are miles apart, but fiber is expensive.

The expenses include: hardware on both ends to connect to the fiber, paying the telco to lay the cable - if leaving the company grounds, and the fiber itself.

Fiber is fragile (its essentially just glass) and should be professionally installed if going any great distance. The cost of fiber usually prohibits its use for short distances. This leaves a local network administrator choosing professional installation for most fiber.

Its not uncommon, however, to connect hub/switches in wiring closets with fiber when convenient. A ten foot, prefabricated fiber patch cable might cost about $35.

Higher-end hubs will have slots that you can fill with cards that support fiber cabling.

The overall advantage of fiber is the high quality of the transmitted signal over great distances.

Wireless backbones

There are means of connecting buildings with line-of sight wireless backbones. Wireless backbones are basically a radio-frequency link between hubs.

This type of backbone is reliable, until you have bad weather or a bird takes a liking to your transceiver or a tree grows a branch in your line-of-sight.

However, sometimes the customer just wants one because they know someone who has one or saw it in a magazine. Wireless backbones tend to need more technical support that standard backbones.


ARCnet is an old, old networking architecture.

It is coax-based but employs a specific type of coax. Where it follows linear-bus wiring topologies similar to ethernet it has intricate rules regarding its cabling.

Cable segments in an ARCnet network can vary form 100' to 20,000 feet - if you follow the rules. These rules, the Hierarchy of Hubs involves both active and passive hubs.

ARCnet is really much more trouble than it's worth considering that ARCnet is rated at about 2.5 mbps. vs. Ethernet at 10 or 100 mbps (megabits per second).

In addition to the wiring rules, ARCnet uses 93 Ohm coax vs. the 50 Ohm coax of Ethernet.

Currently there are few, if any, 'new' users of ARCnet. Those that do have it already are phasing it out.

Token Ring

Token ring was a product for large companies, with deep pockets, that needed complicated networks. Basically there was one token. The token was passed from node to node.

If you had the token you could talk to the other nodes. If someone else had the token, you just had to wait until the token got back to you.

This is the exact opposite of a broadcast technology (Ex. IPX/SPX). Token ring networks are built on the star topology.

However, Token ring networks use SHIELDED-twisted-pair (STP) vs. UTP (Unshielded Twisted Pair).

They also use hubs but not the same hubs found in Ethernet networks. Networks running Token ring will have MAU's (Multi-station Access Units).

Token ring still has a presence in the industry, but you won't see many new token ring installations. This goes hand in hand with all the AS400s still in use because of the software they run.

Many AS400 systems can today be replaced with powerful PCs for a fraction of the cost. In short, they were the best in their time, but technology got smaller, faster, and cheaper

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