Network Topology
The term network topology refers to the shape of how the computers and other network components are connected to each other. There are several different types of network topologies, each with advantages and disadvantages. In the following discussion of network topologies, I use two important
terms:
✦ Node: A node is a device that is connected to the network. For our purposes here, a node is the same as a computer. Network topology deals with how the nodes of a network are connected to each other.
✦ Packet: A packet is a message that is sent over the network from one node to another node. The packet includes the address of the node that sent the packet, the address of the node the packet is being sent to, and data.
Bus topology
The first type of network topology is called a bus, in which nodes are strung together in a line, as shown in Figure 1-1. Bus topology is commonly used for LANs.
The key to understanding how a bus topology works is to think of the entire network as a single cable, with each node “tapping” into the cable so that it can listen in on the packets being sent over that cable. If you’re old enough to remember party lines, you get the idea. In a bus topology, every node on the network can see every packet that’s sent on the cable. Each node looks at each packet to determine whether the packet is intended for it. If so, the node claims the packet. If not, the node ignores the packet. This way, each computer can respond to data sent to it and ignore data sent to other computers on the network. If the cable in a bus network breaks, the network is effectively divided into two networks. Nodes on either side of the break can continue to communicate with each other, but data can’t span the gap between the networks, so
nodes on opposite sides of the break can’t communicate with each other.
Star topology
In a star topology, each network node is connected to a central device called a hub or a switch, as shown in Figure 1-2. Star topologies are also commonly used with LANs. If a cable in a star network breaks, only the node connected to that cable is isolated from the network. The other nodes can continue to operate without
interruption — unless, of course, the node that’s isolated because of the break happens to be the file server.
You should be aware of the somewhat technical distinction between a hub and a switch. Simply put, a hub doesn’t know anything about the computers that are connected to each of its ports. So when a computer connected to the hub sends a packet to a computer that’s connected to another port, the hub sends a duplicate copy of the packet to all its ports. In contrast, a switch knows which computer is connected to each of its ports. As a result, when a switch receives a packet intended for a particular computer, it sends
the packet only to the port that the recipient is connected to. Strictly speaking, only networks that use switches have a true star topology. If the network uses a hub, the network topology has the physical appearance of a star, but is actually a bus. That’s because when a hub is used, each computer on the network sees all the packets sent over the network, just like in a bus topology. In a true star topology, as when a switch is used, each computer sees only those packets that were sent specifically to it, as well
as broadcast packets that were specifically sent to all computers on the network.
Ring topology
A third type of network topology is called a ring, shown in Figure 1-3. In a ring topology, packets are sent around the circle from computer to computer. Each computer looks at each packet to decide whether the packet was intended for it. If not, the packet is passed on to the next computer in the ring.
Mesh topology
A fourth type of network topology, known as mesh, has multiple connections between each of the nodes on the network, as shown in Figure 1-4. The advantage of a mesh topology is that if one cable breaks, the network can use an alternative route to deliver its packets. Mesh networks are not very practical in a LAN setting. For example, to network eight computers in a mesh topology, each computer would have to have seven network interface cards, and 28 cables would be required to connect each computer to the seven other computers in the network. Obviously, this scheme isn’t very scalable. However, mesh networks are common for metropolitan or wide area networks. These networks use devices called routers to route packets from network to network. For reliability and performance reasons, routers are usually arranged in a way that provides multiple paths between any two nodes on the network in a mesh-like arrangement.
The term network topology refers to the shape of how the computers and other network components are connected to each other. There are several different types of network topologies, each with advantages and disadvantages. In the following discussion of network topologies, I use two important
terms:
✦ Node: A node is a device that is connected to the network. For our purposes here, a node is the same as a computer. Network topology deals with how the nodes of a network are connected to each other.
✦ Packet: A packet is a message that is sent over the network from one node to another node. The packet includes the address of the node that sent the packet, the address of the node the packet is being sent to, and data.
Bus topology
The first type of network topology is called a bus, in which nodes are strung together in a line, as shown in Figure 1-1. Bus topology is commonly used for LANs.
The key to understanding how a bus topology works is to think of the entire network as a single cable, with each node “tapping” into the cable so that it can listen in on the packets being sent over that cable. If you’re old enough to remember party lines, you get the idea. In a bus topology, every node on the network can see every packet that’s sent on the cable. Each node looks at each packet to determine whether the packet is intended for it. If so, the node claims the packet. If not, the node ignores the packet. This way, each computer can respond to data sent to it and ignore data sent to other computers on the network. If the cable in a bus network breaks, the network is effectively divided into two networks. Nodes on either side of the break can continue to communicate with each other, but data can’t span the gap between the networks, so
nodes on opposite sides of the break can’t communicate with each other.
Star topologyIn a star topology, each network node is connected to a central device called a hub or a switch, as shown in Figure 1-2. Star topologies are also commonly used with LANs. If a cable in a star network breaks, only the node connected to that cable is isolated from the network. The other nodes can continue to operate without
interruption — unless, of course, the node that’s isolated because of the break happens to be the file server.
You should be aware of the somewhat technical distinction between a hub and a switch. Simply put, a hub doesn’t know anything about the computers that are connected to each of its ports. So when a computer connected to the hub sends a packet to a computer that’s connected to another port, the hub sends a duplicate copy of the packet to all its ports. In contrast, a switch knows which computer is connected to each of its ports. As a result, when a switch receives a packet intended for a particular computer, it sends
the packet only to the port that the recipient is connected to. Strictly speaking, only networks that use switches have a true star topology. If the network uses a hub, the network topology has the physical appearance of a star, but is actually a bus. That’s because when a hub is used, each computer on the network sees all the packets sent over the network, just like in a bus topology. In a true star topology, as when a switch is used, each computer sees only those packets that were sent specifically to it, as well
as broadcast packets that were specifically sent to all computers on the network.
Ring topologyA third type of network topology is called a ring, shown in Figure 1-3. In a ring topology, packets are sent around the circle from computer to computer. Each computer looks at each packet to decide whether the packet was intended for it. If not, the packet is passed on to the next computer in the ring.
Mesh topologyA fourth type of network topology, known as mesh, has multiple connections between each of the nodes on the network, as shown in Figure 1-4. The advantage of a mesh topology is that if one cable breaks, the network can use an alternative route to deliver its packets. Mesh networks are not very practical in a LAN setting. For example, to network eight computers in a mesh topology, each computer would have to have seven network interface cards, and 28 cables would be required to connect each computer to the seven other computers in the network. Obviously, this scheme isn’t very scalable. However, mesh networks are common for metropolitan or wide area networks. These networks use devices called routers to route packets from network to network. For reliability and performance reasons, routers are usually arranged in a way that provides multiple paths between any two nodes on the network in a mesh-like arrangement.
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