|  | 8.2 | Understand Physical Layer
| 8.2.2 | Hubs, MAUs, switching hubs (switches), repeaters, transceivers
Hubs Hubs, which operate at the physical layer of the OSI Model, are the central location to which cabling from most topologies connect. Three types of hubs include passive, active, and intelligent. A passive hub receives information through one of its ports, then transmits the data out through another port to a destination location. It has no electrical power and does not possess any signaling processing capability. Passive hubs only allow communication from one location to another to flow across the network, and they absorb some signal energy, causing a signal to weaken. An active hub receives data through one of its ports, then works like a repeater; regenerating and re-timing the signal before sending it out through another port to a destination. Active hubs are also known as multi-port repeaters. Most hubs "share" bandwidth among users - more users, less bandwidth per user. Intelligent hubs have even more electronics than active hubs, and they allow network management (a "managed" hub) or even switching (a "switching hub", or more commonly a "switch"). MAUs A Multistation Access Unit (MAU) allows multiple workstations, which are connected on a Token Ring network, the ability to communicate with each other. Although, MAUs are not a UTP hub, they are commonly referred to as a token ring hub. Often, this device has eight ports and uses Universal Data Connectors (UDC) or RJ-45 connections. MAUs are not powered devices; however, occasional lights will flash when connected to the network. MAUs also adds fault tolerance to ring networks. Switching Hubs A switching hub, also called a multi-port bridge, is a device that automatically verifies the MAC addresses of each device connected to its ports. When a packet is sent to its network, the switching hub checks the MAC address before sending the data to the specified location. Unlike a standard passive or active hub, switching hubs do not broadcast signals to each segment on the network, but transmits data only to a specific destination. Switching Hubs (and switches) result in dedicated bandwidth per port; where as other hubs share the total bandwidth with the number of users. Repeaters When a repeater receives data from an Ethernet segment, it decodes/codes the binary information, and then retransmits the signal to the destination. Advantages of a repeater include; the ability to extend the network a greater distance, increase the number of devices connected to the network, added fault tolerance by isolating breaks on a network to only that cable segment, and the ability to link different cable types together. A disadvantage is that repeaters enlarge collision domains: If two computers send packets at the same time, a collision will occur and CSMA/CD is applied to the entire network, thereby slowing down the network. A repeater does not manage the flow of traffic, it only repeats signals. A maximum of four repeaters can be installed on a single-segment Ethernet network. Transceivers A transceiver (transmitter-receiver) is a device which transmits and receives data to and from the network This device attaches to the network interface card (NIC) in two different ways: On-Board and External. An On-Board Transceiver is usually "on-board" or attached to the adapter card, such as RJ-45 receptacles and BNC connectors. An External Transceiver makes a physical connection to the NIC using a small device, called an adapter unit interface (AUI) or a Digital-Intel-Xerox (DIX) connector, which is attached by an extension cable. A common external transceiver can also connect one side to an AUI interface and the other to an RJ-45 interface.