Ethernet is the most widely-installed
local area network (LAN) technology. Computers, printers and other devices use
the Ethernet protocol to communicate with each other. Originally devices using
Ethernet were connected together by coax cable in a serial, daisy-chain
fashion.
At first a data rate of 10Mbit/sec (million bits per second) was carried between PCs connected together by one or two coax cabling systems:
- 10base2 (thin coax) carried data about 185 meters before a repeater was needed.
- 10base5 (thick coax) could carry data a maximum of 500 meters without a repeater.
10baseT (twisted pair cable) appeared later and carried data 100 meters without a repeater. Today it is the most widely deployed cabling system used in LANs.
Data Collisions
Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) to maintain the orderly flow of messages on the network. In CSMA/CD, each PC listens to the line. If the line is quiet any PC can begin transmitting.
Since any PC can transmit when the line is quiet, two will inevitably start transmitting at or near the same time. The messages collide and the result is that neither message gets through.
When a collision is detected, all PCs must back off and wait a random amount of time before transmitting any new messages. If one device keeps transmitting for whatever reason, the network gets tied up and no one else can use it!
Ethernet Hub
Eventually the hub was developed. It can connect a small number of PCs together in a star formation. Think of spokes of a wheel all connected to the hub at the center. The hub monitors each PC connected to it and disconnects any PC that violates the CSMA/CD rule.
It was a great improvement over the serial, daisy-chain, coax cable system, since it could prevent one over talkative PC from taking down the network.
It also eliminated the problems that often occurred when one PC failed electronically, or was improperly disconnected from the coax cable.
If one PC failed in a certain way, it could disable the entire network.
If the coax cable at one PC was improperly connected, it split the network into two, isolated networks on either side of it. PCs in one network could no longer talk to PCs in the other network
Ethernet Switch
Switches interconnect LANs and transfer messages between them. The development of switches allowed the size of networks using Ethernet to become larger.
Without switches the size of an Ethernet system is limited. The reason for this is that computers at opposite ends of a large network are too far apart to follow the CSMA/CD protocol required for orderly traffic flow.
Collisions let each transmitting PC know that its message didn't get through. When PCs are too far apart they don't know about collisions that may be happening to their message at the far end of the network due to the very long back and forth time delays. Since they don't know their message collided and failed, they never retransmit it!
Switches allow a manageable collision domain (area) to be set up for each LAN. When a message crosses from one LAN to another through a switch, it enters a new collision domain and in effect becomes a 'new' message in that domain. The explanation of how this works is beyond the scope of this article. With the collision detection problem solved, very large networks were able to be built.
Ethernet in the Internet
The Internet is made up of millions of PCs and other equipment connected together in thousands of dispersed LANs of varying sizes. The LANs pass data to each other through banks of switches and routers (very specialized switches) that are operated by communications companies and organizations around the globe.
If you work in an office, 10baseT Ethernet is probably the way all your local PCs, printers and other equipment is connected together. The 8-pin, telephone-style plug that connects to the Network Interface Card (NIC) in the back of your PC is most likely one end of a 10baseT cable.
The speed of Ethernet keeps getting faster
- 100baseT or Fast Ethernet (100 million bits
per second) is widely deployed in LAN backbones. Backbones are high speed
data paths that are fed by many slower speed data streams, like 10baseT.
It is much like smaller roads feeding a few, slow moving cars onto a major highway (backbone), that carries more cars at higher speed. - Eventually as more and more data
comes together, 1000baseX or GB Ethernet (1 billion bits per second) is
used to carry a tremendous amount of extremely fast moving data. In the
traffic analogy, it is like an interstate highway.
This data keeps getting merged together into higher speed streams, called Optical Carriers, which use fiber optic cable to carry data. OC-192 (10 billion bits per second) is now being deployed. At this point we've left the highway and are using supersonic jets!