Ethernet

Ethernet, developed in 1976, is the most widely-installed LAN technology, and typically uses coaxial or UTP cable. The most commonly installed Ethernet systems are called 10BASE-T, which provides transmission speeds up to 10 Mbps. 'Fast Ethernet' or 100BASE-T provides transmission speeds up to 100 megabits per second and is typically used for servers, LAN backbone systems and in workstations with high-bandwidth needs. Gigabit Ethernet provides an even faster level of backbone support at 1000 megabits per second (1 gigabit or 1 billion bits per second).

Devices are connected to the network cable and compete for attention using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol.

CS = Carrier sense:  the problem with having a bunch of devices all connected to the same wire is that they may all want to communicate at once, but only one message at a time can travel across the cable. Carrier sense is used when a computer wants to transmit over the wire. It "listens" - and waits until there is no activity on the cable. When it sees its chance, it transmits.

MA = Multiple Access: When one Ethernet station transmits, all the stations on the medium hear the transmission

CD = Collision Detection: Carrier sense does not guarantee that two devices will not sense the same silence and both transmit simultaneously, and cause a 'collision'. Collision Detection detects this event. Each node involved in the collision will wait a random number of milliseconds, then repeat the transmission attempt. The random waiting time is important: if two conflicting computers both waited the same amount of time before retransmitting, they would collide again - and again - and again.

When a node wants to communicate to another node, it transmits its packet (also known as a frame), a short message containing key data such as a destination address to say where the message should go, and a source address saying which node sent it.

The frame travels to every node on the segment. Each node inspects the frame to see if it is addressed to him. If not, the node ignores it. If the frame is addressed to the node, the node opens the frame and reads its contents.

Ethernet is the most widely-installed local area network (LAN) technology, and was developed in 1976. Specified in a standard, IEEE 802.3, an Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps. Devices are connected to the cable and compete for access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol.

Fast Ethernet or 100BASE-T provides transmission speeds up to 100 megabits per second and is typically used for LAN backbone systems, supporting workstations with 10BASE-T cards. Gigabit Ethernet provides an even higher level of backbone support at 1000 megabits per second (1 gigabit or 1 billion bits per second).

How Ethernet works

When a node wants to communicate to another node, it transmits its frame. The frame travels to every node on the segment. Each node inspects the frame to see if it is addressed to him. If the frame is not addressed to the node, the node ignores it. If the frame is addressed to the node, the node opens the frame and reads its contents.

The exception is a broadcast address, which is a special message intended to be read by every node (like a message on the P.A. as opposed to a comment from one person to another).

Token Ring, the main alternative to Ethernet, uses a different strategy to avoid computers talking at the same time.

Some Ethernet vocabulary:

Medium - the cable or wireless connection that the devices on the network connect to: coaxial copper cable, a twisted pair (like CAT5) or fiber optic cabling.

Segment - a network rarely has a single main wire that computers connect to. Each "branch" of the network is a segment.

Node - devices that attach to the medium are called stations or nodes (e.g. computers, printers)

Frames - short messages made up of variably sized chunks of information that travel across the network Frames must be constructed following strict rules so the receiver of the frame can understand its contents. These rules describe: minimum and maximum lengths for frames, what required pieces of information must appear in the frame (e.g. a destination address to say where the message should go, and a source address saying which node sent it).

Multiple Access: When one Ethernet station transmits, all the stations on the medium hear the transmission

Carrier sense: the problem with having a bunch of devices all connected to the same wire is that they may all want to communicate at once, but only one message at a time can travel across the cable.Imagine a room full of students all wanting to say something at the same time. Carrier sense is used when a computer wants to transmit over the wire. It "listens" - and waits until there is no activity on the cable. When it sees its chance, it grabs it and transmits.

Collision Detection: But this method does not guarantee that two devices do not sense the same silence and both transmit simultaneously: this often happens in conversation when two people start speaking at once. While the humans would look at each other and one would say "Go on...", the computers can't do this. They use Collision Detection: if they detect they have transmitted over the top of another computer, they wait a random amount of time (in milliseconds), then listen again for the next chance to transmit. The random waiting time is important: if the two conflicting computers both waited the same amount of time before retransmitting, they would collide again - and again - and again.

Repeaters: an Ethernet segment has a limited length. After a certain distance, the electrical signal of the frame fades and dies. To get around the length restrictions, a repeater can be attached to a segment. The repeater listens to the incoming frame and passes it along (amplifying it in the process). It's like a line of people passing a message over a long distance by spreading themselves out and shouting the message to the next person in turn.

Bridges: As you can imagine, if you have hundreds of devices on the same segment, the message "traffic" on the segment would get congested: so many devices would be trying to transmit that they would be waiting a long time to get their chance.

From "How Stuff Works" - if station A wanted to transmit to station B, the bridge would hear the message but would not pass it along to segment 2 because it knows the message is irrelevant to segment 2.