DCN January 26. 2007
Trying to understand how Ethernet work? What is Ethernet?
Go back in history, the island of Hawaii in the Pacific, in the late 60’s the university of Hawaii had a very clever thought, they had some offices on one side of the island and on the other side, they wanted to communicate between these offices, and their was no way for them to do this, they invented a method, which was satellite service, they developed their own little protocol to send messages to each other, they sent the messages to the satellite and it was a source address and a destination address and of course data, if add wanted to send to add four, the messages would be sent to the satellite and rebound it to the computer. Lets say SA = 1 DA=4, Three would receive the message and ignore it and would only go to its destination. Primitive and it worked. This was called Aloha net. They thought to themselves to send a single down through copper wire. There was a group in the 60’s that developed things like mice, but the most important was Ethernet, and the most was Zerox. ThreeCOM manufacturing Ethernet cards, at the same time IBM was inventing token ring, yet today token ring is gone, Ethernet survived and Ethernet is thriving, December of last year Ethernet people put the standards at running Ethernet at 10 billion bits per second, Then the standardized of 100 billion bits per second. Macaffe invented Etherenet, they put in strands of coaxial cable, DataCom cables coaxial and twisted pair cables, Coax has a thick piece of copper surrounded by a insulated matieral (plastic), surround by a weave, amazing thing about coax the copper is thick and can transfer a lot of Hrz, (hrz dead German guy, working in the 18’s with electricity), the number of wave cycles per second is called one Hrz(hertez), a million Hrz means 100 hrz per second, coax cable b.c its nice and thick can theoretical can handle 100, mill hrz. The coax in the tv can handle 600 mil hrtz, Each station takes 600mil hrtz, Back to MacAfee, we invent a little plug that attaches in the coax, in every comp a special hard that will sit in the expansion slot in each computer (THE NIC) the Network Interface Card has logic, try in to visualize the NIC is a separate device, and has simple logic burned into it, and works with your computer, The idea about the cable was SPEED, A NIC using Ethernet would take data, Operating software uses the inturp and inturrpts the cpu and sends the data, INT52 is a well known interrupt and goes through a specific area in ram grabs the data and moves it to a NIC, The Ethernet guys MacAfee used the follow rules: First the Maximum Transition Unit (MTU), 100 protocols Each has a MTU, All protocols take data, and break it in to pieces the maximum transition unit size is going to be 1500bytes, if you wanted to send 1501 bytes, it would resolve in two transmissions, that is just Ethernet, remember this and remember it NEXT SEMISTER. As the same time in the Ethernet guys IBM was inventing token ring, TOKEN RING when they designed it could send 64,000bytes (huge packets of information), we are going to figure out why the groups made these bye decision Ethernet primitive simple protocol survived, and Token ring survived. When ever a int was sent the NIC would build a frame of data, and infront of it there is a Header, Ethernet header very simple while the TCPIP complicated, HDR has DA SA TL Destination Address, Source Address, A one byte fule called either TYPE or LENGTH. Ethernet uses MAC address, All LAN addresses is how many bytes 4 bytes ip and MAC address is 6 bytes, trillion of address, IMPORTANT’ the MAC address is made up of TWO components, NO two MAC address can be the same, Network standards, the IEEE group Institute of Electronic Engineering (A GROUP), OKAY you are company 999, the first three bytes of the 6 byte address is Manufacture ID, They are identical to AHOLA net, Workstation one wants to send data to Workstation three, the NIC would build the bytes and send it to machine two and ignore the address, and send it to three. Ethernet survived and why…Ethernet was simple that’s the answer it was so basically simple that it thrived, probably for the rest of your lives, you will be working with Ethernet. Test 1 DCN386 questions TWO How does Ethernet work…here is three basic principals on how Ethernet is a base band technology meaning only ONE signal can move down the line at one time (broadband sending 100`s television channels) works CS MA CD, Carrier Sense, first logic in the NIC uses carrier sense, it uses INT52, the NIC looks for digital pulses, if it sees pulses it wont send the data, if theirs no pulses and no one is sending zero data (volts) Carrier sense, no carrier zero volts, no one is sending things, it first makes sure nobody else is using the network, the second pillar of Ethernet wisdom is called multiple access, You can easily simply find the computer on the system , the third is the most important, (CSMA CD), it’s a simplistic design, Ethernet strategy the first work station, the first NIC that detects ugly pulses has a job to do it sends out a high frequency jamming signal, the NIC would try to send it again, Ethernets simplistic approach from this collision will wait on a RANDOM number, > DATA >T L > SA > DA, Zerox ThreeCOM started Ethernet, the original ThreeCOM internet message was like DATA . TYPE . SA. DA, what do you think type stands for…it’s a two bite field that represented which higher protocol, Ethernet has never worked alone it is a delivery boy, a delivery of data. Type (a number) sends the frame to another destination reads the data and looks at the type and understands to send the data to TCP data, it can be all sorts of different TYPE, logic when sending data take note of where its from look at the data and note it should be shot of to TCP program. (HUMAN RESOURCES) IEEE our friends did few things, started making standards which allowed different types of local area networks to communicate with each other. The IEEE came up with the standard 802, this started in 1980 in Feb, they said its going to be called 802.3 and token ring 802.5, 802.11 is wireless, one of the changes was the change of TYPE and made it length, why do you need a length field, well what if you were only sending 25 bytes of data that may or might be not be a important, how does the IEEE know what type of data..they borrowed the first two bites the IEEE solved the confusion of the none 802.3. Ex: Three companies Zerox, Digital Dix, the IEEE solved the problem, today they all worktogether,, not important for dcn 286 theirs a extra thing they put on the frame theirs a preamble, its 64 bits long, 101010101010101010101010 alternating ones and zeros. Think about this you’re a NIC that cost 25$ you can send info to each other, what would happen if you were in a bad mood, and u started sending smaller bits, how does this precision they get insync, very simple very basic that works. Primitive 10baseT 10 million bits persecond using base technology over twisted pair cables, think go Cat5 Cable, Lab wed Cat5 and make the cable straight threw and crossover. Beaware of this Cat5 comes int two types UTP and STP, unshielded twisted pair, and shielded twisted pair. Cat 5 6 7 learn to love next wed. READ up to chapter three.
No comments:
Post a Comment