Every device connected to a network must have a network adapter. Part of the function of the network adapter makes it a Media Access Controller. It must listen for silence on the cable before sending a signal, as well as deal with collision of data. Wired network adapters must conform with the requirements of the physical cable. The most common standard for network cable is Ethernet. Local Area Networks (LANs) operate with media access control (MAC) addressing. Every network adapter has a unique serial number which is also used to provide the MAC address. Wireless network adapters operate in the same manner as a regular network adapter, except they have a radio transmitter/receiver instead of a cable socket.
Despite the consumer popularity of wireless networks, most networks in the world are connected by cable. The most common standard for network cable is the Ethernet standard was originally created by Xerox but is not managed by the Institute of Electrical and Electronics Engineers. The IEEE defines a variety of standards with different types of media including fiber optic and unshielded twisted pair. UTP is the most common cable type in use in networks.
Hubs and Switches
The simplest device for transferring data from one computer to another over a network is the hub. The hub is a box with an array of sockets. Each socket hosts a cable and each cable leads to a device such as a computer, printer or fax machine. Any message coming in on one of the sockets is copied and sent out on all other sockets. The hub does not use addresses but merely repeats what arrives on a socket. Originally hubs were popular because they were much cheaper than switches. The falling price of switches is now making hubs obsolete, since switches offer greater functionality. The switch is the same as a hub except it only copies incoming data out to the socket it has associated with the MAC address it finds in the header of the incoming data packet.
The task of the Medium Access Controller requires it to wait for silence on a cable before sending data. This means that if there are many computers connected to the same cable, each must wait a long time to get a turn sending data packets. Segmenting the network reduces this wait time, since each computer only competes with other computers on the same segment. Network segments are connected via bridges, which are simple switches with only two sockets: one for each segment. Bridges record the MAC addresses of the computers on each segment and pass data from one side to the other if the source and destination of the data packets reside on different segments.
Switches and Bridges deal with MAC addresses, while routers deal with Internet protocol (IP) addresses. The MAC address is like the street address of a house and the IP address is like the telephone number. The Address Resolution Protocol (ARP) maps between the two addressing systems. The MAC address applies across one link, which is usually enough to get data across a Local Area Network. Routers send data across many links and the overall path is dictated by the IP address.