Wires, cables, and WiFi | Internet 101 | Computer Science | Khan Academy


my name is Tess twin lock I’m a software engineer at Google here’s a question how does it picture text message or email get sent from one device to another it isn’t magic it’s the Internet a tangible physical system that was made to move information the Internet is a lot like the postal service but the physical stuff that gets sent is a little bit different instead of like boxes and envelopes the internet ships binary information information is made of bits a bit can be described as any pair of opposites on or off yes or no we typically use a one meaning on or a zero meaning off because a bit has two possible States we call it binary code eight bits strung together makes one byte 1000 bytes all together is a kilobyte 1,000 kilobytes is a megabyte a song is typically encoded using about three to four megabytes it doesn’t matter if it’s a picture a video or a song everything on the Internet is represented and sent around as bits these are the atoms of information but it’s not like we’re physically sending ones and zeros from one place to another one person to another so what is the physical stuff that actually gets sent over the wires in the airwaves well let’s look at a small example here of how humans can physically communicate to send a single bit of information from one place to another so say we could turn on a light for a one or off for zero or our use beeps or similar sort of things of like Morse code these methods work but they’re really slow error-prone totally dependent upon humans what we really need is a machine throughout history we’ve built many systems that can actually send this binary information through different types of physical mediums today we physically send bits by electricity light and radio waves to send a bit v electricity imagine that you have two light bulbs connected by a copper wire if one device operator turns on the electricity then the light bulb lights up no electricity no light if the operators on both ends agree that light on means one and light off means zero then we have a system for sending bits of information from one person to another using electricity we have kind of a small problem if you need to send a zero like five times in a row well how can you do that in such a way that either person can actually count the number of zeros well the solution is to introduce a clock or a timer the operators can agree that the sender will send one bit per second and the receiver will sit down and record every single second and see what’s on the line to send five zeros in a row you just turn off the light wait five seconds the person on the other end of the line will write down all five seconds five ones in a row switch it on five seconds write down every second obviously we’d like to send things a little bit faster than one bit per second so we need to increase our bandwidth the maximum transmission capacity of a device bandwidth is measured by bit rate which is the number of bits that we can actually send over a given period of time usually measured in seconds a different measure of speed is the latency or the amount of time it takes for one bit to travel from one place to another from the source to the requesting device in our human analogy one bit per second was pretty fast but kind of hard for a human to keep up with so let’s say that you want to actually download a 3 megabyte song in like three seconds at 8 million bits per megabyte that means a bit rate of about 8 million bits per second obviously humans can’t send or receive 8 million bits per second but a machine could do that just fine but now there’s also a question of what sort of cable to send these messages over and how far the signals can go with an Ethernet wire the kind that you find in your home or office or school you see measurable signal loss or interference over just a few hundred feet for the internet to work all around the world we need to have an alternative method to send bits really long distances we’re talking like across oceans so what else can we use well what do we know that moves a lot faster than just like electricity through a wire like we can actually send bits as light beams from one place to another using a fiber optic cable a fiber optic cable is a thread of glass engineered to reflect light when you send a beam of light down the cable light bounces up and down the length of the cable until it is received on the other end depending on the bounce angle we can actually send multiple bits simultaneously all of them traveling at the speed of light so fiber is really really fast but more importantly the signal doesn’t really degrade over long distances this is how you can go hundreds of miles without signal loss this is why we use fiber optic cables across the ocean floors to connect one continent to another 2008 there was a cable that was actually cut near Alexandria Egypt which really interrupted the internet for most of the Middle East and India so we take this internet thing for granted but it’s really a pretty fragile physical system and fiber is awesome but it’s also really expensive and hard to work with for most purposes you’re going to find copper cable but how do we move things without wires how do we send things wirelessly Wireless bit sending machines typically use a radio signal to send bits from one place to another the machines have to actually translate the ones and zeros into radio waves of different frequencies the receiving machines reverse the process and convert it back into binary on your computer so Wireless has made our internet mobile but a radio signal doesn’t travel all that far before it completely gets garbled this is why you can’t really pick up a Los Angeles radio station in Chicago as great as Wireless is today it still relies on the wired internet if you’re in a coffee shop using Wi-Fi then the bits get sent through this wireless router and then are transferred to the physical wire to travel the really long distances of the internet the physical method for sending bits may change in the future whether it’s lasers sent between satellites or radio waves from balloons or drones but the underlying binary representation of information and the protocols for sending that in information and receiving that information have pretty much stayed the same everything on the internet whether it’s words emails images cat videos popping videos all come down to these ones and zeros being delivered by electronic pulses light beams radio waves and lots and lots of life you

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