Network Topologies

Some of the most common topologies used today include:

• Bus - Each node is daisy-chained (connected one right after the other) along the same backbone, similar to Christmas lights. Information sent from a node travels along the backbone until it reaches its destination node. Each end of a bus network must be terminated with a resistor to keep the signal that is sent by a node across the network from bouncing back when it reaches the end of the cable.

• Ring - Like a bus network, rings have the nodes daisy-chained. The difference is that the end of the network comes back around to the first node, creating a complete circuit. In a ring network, each node takes a turn sending and receiving information through the use of a token. The token, along with any data, is sent from the first node to the second node, which extracts the data addressed to it and adds any data it wishes to send. Then, the second node passes the token and data to the third node, and so on until it comes back around to the first node again. Only the node with the token is allowed to send data. All other nodes must wait for the token to come to them.

• Star - In a star network, each node is connected to a central device called a hub. The hub takes a signal that comes from any node and passes it along to all the other nodes in the network. A hub does not perform any type of filtering or routing of the data. It is simply a junction that joins all the different nodes together.

• Star bus - Probably the most common network topology in use today, star bus combines elements of the star and bus topologies to create a versatile network environment. Nodes in particular areas are connected to hubs (creating stars), and the hubs are connected together along the network backbone (like a bus network).

 Quite often, stars are nested within stars, as seen in the example below:

The Problem: Traffic

In the most basic type of network found today, nodes are simply connected together using hubs. As a network grows, there are some potential problems with this configuration:

• Scalability - In a hub network, limited shared bandwidth makes it difficult to accommodate significant growth without sacrificing performance. Applications today need more bandwidth than ever before. Quite often, the entire network must be redesigned periodically to accommodate growth.

• Latency - This is the amount of time that it takes a packet to get to its destination. Since each node in a hub-based network has to wait for an opportunity to transmit in order to avoid collisions, the latency can increase significantly as you add more nodes. Or, if someone is transmitting a large file across the network, then all of the other nodes have to wait for an opportunity to send their own packets. You have probably seen this before at work -- you try to access a server or the Internet and suddenly everything slows down to a crawl.

• Network failure - In a typical network, one device on a hub can cause problems for other devices attached to the hub due to incorrect speed settings (100 Mbps on a 10-Mbps hub) or excessive broadcasts. Switches can be configured to limit broadcast levels.

• Collisions - Ethernet uses a process called CSMA/CD (Carrier Sense Multiple Access with Collision Detection) to communicate across the network. Under CSMA/CD, a node will not send out a packet unless the network is clear of traffic. If two nodes send out packets at the same time, a collision occurs and the packets are lost. Then both nodes wait a random amount of time and retransmit the packets. Any part of the network where there is a possibility that packets from two or more nodes will interfere with each other is considered to be part of the same collision domain. A network with a large number of nodes on the same segment will often have a lot of collisions and therefore a large collision domain.

While hubs provide an easy way to scale up and shorten the distance that the packets must travel to get from one node to another, they do not break up the actual network into discrete segments. That is where switches come in. In the next section, you'll find out how switches assist in directing network traffic.

The Solution: Adding Switches

Think of a hub as a four-way intersection where everyone has to stop. If more than one car reaches the intersection at the same time, they have to wait for their turn to proceed.

Now imagine what this would be like with a dozen or even a hundred roads intersecting at a single point. The amount of waiting and the potential for a collision increases significantly. But wouldn't it be amazing if you could take an exit ramp from any one of those roads to the road of your choosing? That is exactly what a switch does for network traffic. A switch is like a cloverleaf intersection -- each car can take an exit ramp to get to its destination without having to stop and wait for other traffic to go by.

A vital difference between a hub and a switch is that all the nodes connected to a hub share the bandwidth among themselves, while a device connected to a switch port has the full bandwidth all to itself. For example, if 10 nodes are communicating using a hub on a 10-Mbps network, then each node may only get a portion of the 10 Mbps if other nodes on the hub want to communicate as well. But with a switch, each node could possibly communicate at the full 10 Mbps. Think about our road analogy. If all of the traffic is coming to a common intersection, then each car it has to share that intersection with every other car. But a cloverleaf allows all of the traffic to continue at full speed from one road to the next.