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TIHE Encyclopedia

Swtiches and Networks
Taken Fromhttp://www.cisco.com/warp/public/473/lan-switch-cisco.shtml

A typical network consists of nodes (computers), a connecting medium (wired or wireless), and specialized network equipment like routers or hubs. In the case of the Internet, all these pieces working together allow your computer to send information to another computer that could be on the other side of the world!

Switches are a fundamental part of most networks. They make it possible for several users to send information over a network at the same time without slowing each other down. Just like routers allow different networks to communicate with each other, switches allow different nodes (a network connection point, typically a computer) of a network to communicate directly with each other in a smooth and efficient manner.

Image courtesy Cisco Systems, Inc. An illustration of a Cisco Catalyst switch.

There are a lot of different types of switches and networks. Switches that provide a separate connection for each node in a company's internal network are called LAN switches. Essentially, a LAN switch creates a series of instant networks that contain only the two devices communicating with each other at that particular moment. We will focus on Ethernet networks using LAN switches. You will learn what a LAN switch is and how transparent bridging works. You will also learn about VLANs, trunking and spanning trees.

Adding Switches
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: 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 are waiting 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 wrong speed settings (100Mbps on a 10Mbps hub) or excessive broadcasts. Switches can be configured to limit broadcast levels.

Collisions: Ethernet uses a process called Carrier Sense Multiple Access with Collision Detection (CSMA/CD) 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.

Imagine that each vehicle is a packet of data waiting for an opportunity to continue on its trip.

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. But a switch is like a cloverleaf intersection. Each car can take an exit ramp to get to their destination without having to stop and wait for other traffic to go by. 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 if every car has to check all the other roads before proceeding. 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 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 it has to share that intersection with everyone else. But a cloverleaf allows all of the traffic to continue at full speed from one road to the next.

In a fully switched network, switches replace all the hubs of an Ethernet network with a dedicated segment for every node. These segments connect to a switch, which supports multiple dedicated segments (sometimes in the hundreds). Since the only devices on each segment are the switch and the node, the switch picks up every transmission before it reaches another node. The switch then forwards the frame over the appropriate segment. Since any segment contains only a single node, the frame only reaches the intended recipient. This can allow many conversations to occur simultaneously on a switched network.

Image courtesy Cisco Networks An example of a network using a switch.

Switching allows a network to maintain full-duplex Ethernet. Before switching, Ethernet was half-duplex, which means that only one device on the network can transmit at any given time. In a fully switched network, nodes only communicate with the switch and never directly with each other. Using our road analogy, half-duplex is similar to the problem of a single lane, like when road construction closes down the use of one lane of a two lane road. Traffic is trying to use the same lane in both directions. This means that traffic coming one way must wait until traffic from the other direction stops. Otherwise, they will hit head-on!

Fully switched networks employ either twisted pair or fiber optic cabling, both of which use separate conductors for sending and receiving data. In this type of environment, Ethernet nodes can forgo the collision detection process and transmit at will, since they are the only potential devices that can access the medium. In other words, traffic flowing in each direction has a lane to itself. This allows nodes to transmit to the switch at the same time the switch transmits to them, achieving a collision free environment. Transmitting in both directions also can effectively double the apparent speed of the network when two nodes are exchanging information. For example, if the speed of the network is 10 Mbps then each node can transmit at 10Mbps at the same time.

A mixed network with two switches and three hubs.

Most networks are not fully switched because of the costs incurred in replacing all of the hubs with switches. Instead, a combination of switches and hubs are used to create an efficient yet cost-effective network. For example, a company may have hubs connecting the computers in each department and a switch connecting all of the department-level hubs together.
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