To make data transmission more extensible and efficient than a simple peer-to-peer network, network designers use specialized network devices, such as hubs, bridges and switches, routers, and wireless access points, to send data between devices.
Hubs, shown in Figure 1, extend the range of a network by receiving data on one port and then regenerating the data and sending it out to all other ports. A hub can also function as a repeater. A repeater extends the reach of a network because it rebuilds the signal, which overcomes the effects of data degradation over distance. The hub can also connect to another networking device, like a switch or router that connects to other sections of the network.
Hubs are used less often today because of the effectiveness and low cost of switches. Hubs do not segment network traffic, so they decrease the amount of available bandwidth for all devices connected to them. In addition, because hubs cannot filter data, a lot of unnecessary network traffic constantly moves between all the devices connected to it.
Bridges and Switches
Files are broken up into small pieces of data, called packets, before they are transmitted over a network. This process allows for error checking and easier retransmission if the packet is lost or corrupted. Address information is added to the beginning and end of packets before they are transmitted. The packet, along with the address information, is called a frame.
LANs are often divided into sections called segments, similar to the way a company is divided into departments, or a school is divided into classes. The boundaries of segments can be defined using a bridge. A bridge filters network traffic between LAN segments. Bridges keep a record of all the devices on each segment to which the bridge is connected. When the bridge receives a frame, the bridge examines the destination address to determine if the frame is to be sent to a different segment or dropped. The bridge also helps to improve the flow of data by keeping frames confined to only the segment to which the frame belongs.
Switches, shown in Figure 2, are sometimes called multiport bridges. A typical bridge has two ports, linking two segments of the same network. A switch has several ports, depending on how many network segments are to be linked. A switch is a more sophisticated device than a bridge.
In modern networks, switches have replaced hubs as the central point of connectivity. Like a hub, the speed of the switch determines the maximum speed of the network. However, switches filter and segment network traffic by sending data only to the device to which it is sent. This provides higher dedicated bandwidth to each device on the network.
Switches maintain a switching table. The switching table contains a list of all MAC addresses on the network, and a list of which switch port can be used to reach a device with a given MAC address. The switching table records MAC addresses by inspecting the source MAC address of every incoming frame, as well as the port on which the frame arrives. The switch then creates a switching table that maps MAC addresses to outgoing ports. When a frame arrives that is destined for a particular MAC address, the switch uses the switching table to determine which port to use to reach the MAC address. The frame is forwarded from the port to the destination. By sending frames out of only one port to the destination, other ports are not affected.
Power over Ethernet (PoE)
A PoE switch transfers small amounts of DC current over Ethernet cable, along with data, to power PoE devices. Low voltage devices that support PoE, such as Wi-Fi access points, surveillance video devices, and NICs, can be powered from remote locations. Devices that support PoE can receive power over an Ethernet connection at distances up to 330 ft (100 m) away.