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.