CSMA/CD

CSMA/CD is Carrier Sense Multiple Access, Collission Detection.

This is a medium access protocol.

The nodes/stations contending for the medium first listen for an on-going talk over the medium. When somebody is already talking, there will be a carrier signal on the medium which carries the message. If the listening station senses a carrier, that means, the medium is already owned by somebody else. So it backs out for a random period of time and try again. 

The principle is, talk when nobody else is NOT talking.

But there is a chance that more than one stations detect NO CARRIER on the medium at the same time and start talking. There occures a collission.

On each medium there is a mechanism to detect collission.

When the talker detects a collission, he backs out and sends a JAM signal, for the other station, in case, he didnt detect the collission. Eventually both parties backs out.

They come back after a random period of time.

What is LAN

LAN is Local Area Network.

It is a computer network, the distance between computers is considerably less. This distance factor is now not important, technologies exist, by which I can physically spread my LAN across continents.

The LAN technology "was" used to network local machines.

The media used to be "shared" - means, there are many NODES, which CONTENT for the MEDIA.

These CAPITALIZED phrases are important.

NODES - computers or other devices or hosts which need to participate in the network

CONTENT - since the media is shared, a node has to "own" the media before it can "talk". Multiple nodes will compete with each other to "own" the media and there should be some logic to gain ownership

MEDIA - is used by the nodes to send/receive data

The most popular LAN technolgy today is ETHERNET. In the beginning, ETHERNET used coaxial cable as the media. There will be one coaxial cable running from one end of the floor to the other. Computers needing connection to the network will TAP the cable using a T connector.



Ethernet uses CSMA/CD to ACCESS the medium.

What is Wireless LAN

Wireless LAN is a Local Area Network using wireless technology instead of wires. The media used for exchanging data is usually radio waves in the ISM band.

Wireless LAN is standardized by 802.11 standards.

The phrase "usually radio waves" is important - the standard accommodates for IR also. Almost all laptops and hand held devices has IR built in, it is very inexpensive. But I am not sure, if there is any serious implementation of Wireless LAN over IR available today.