Study Unit 4 computer systems and data communication


Network media access methods



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Network media access methods


How computing devices access the network cable and send data is known as the media access method. There are four commonly used media access methods: Ethernet, Token Ring, Local Talk, and FDDI.

Ethernet


Ethernet is the most popular media access method which can allow for linear bus, star, or tree topologies. It use access method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection). It is able to transmit data over UTP, thin-coaxial, thick-coaxial and fiber optic cables at rates of 10 Mbps. Fast Ethernet supports 100 Mbps, but requires faster hubs and network interface cards
Token Ring

Token ring was developed by IBM in the mid 1980s. It uses a star-wired ring topology over shielded and unshielded twisted-pair wiring. A central hub (referred to as a "MAU" Multi-station Access Unit) is at the center of the ring.


There are two versions of Token Ring, namely 4 Mbps and 16 Mbps. They both use a token passing process to circulate packets around the ring. These are more expensive than Ethernet, but are a good option for PC networks.
Local Talk

Local talk was developed by Apple Computer, Inc. for Macintosh computers and is best suited to small networks of Macs. It allows for linear bus, star, or tree topologies using twisted pair cable and can connect up to 32 devices (computers, printers, and file server). Local talk uses the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) process for transmitting data. A single network can be up to 300 meters. Its major disadvantage is that it is slow, transmits data at only 230 Kbps.



FDDI (Fiber Distributed Data Interface)


FDDI uses fiber optics for speeds of up to 100 Mbps. The system is used primarily to interconnect two or more LANs, often over long distances. It employs the token ring passing process to transmit data; but use a dual counter-rotating ring topology, meaning there are two rings of cable with two tokens circulating in opposite directions.
Wireless

Not all networks are connected with cabling; some networks are wireless. The term 'wireless network' refers to two or more computers communicating using standard network rules or protocols, but without the use of cabling to connect the computers together. Wireless LANs use high frequency radio signals, infrared light beams, or lasers to communicate between the workstations and the file server or hubs. Each workstation and file server on a wireless network has some sort of transceiver/antenna to send and receive the data. Information is relayed between transceivers as if they were physically connected. For longer distance, wireless communications can also take place through cellular telephone technology, microwave transmission, or by satellite.

Fig 7b: Desktop PC Wireless LAN card Fig 7c: Laptop PC Wireless LAN card
A wireless local area network (WLAN) consists of two key components: an access point (also called a base station) and a wireless card. Information can be transmitted between these two components as long as they are fairly close together (up to 100 metres indoors or 350 metres outdoors). The types of infrared communications that we could use in schools are line-of-sight and scattered broadcast. Line-of-sight communication means that there must be an unblocked direct line between the workstation and the transceiver. If a person walks within the line-of-sight while there is a transmission, the information would need to be sent again. This kind of obstruction can slow down the wireless network.
Scattered infrared communication is a broadcast of infrared transmissions sent out in multiple directions that bounces off walls and ceilings until it eventually hits the receiver. Networking communications with laser are virtually the same as line-of-sight infrared networks.
Wireless LANs have several disadvantages. They provide poor security, and are susceptible to interference from lights and electronic devices. They are also slower than LANs using cabling.



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