Hi, I’m Carrie Anne, and welcome to CrashCourse
Computer Science! The internet is amazing. In just a few keystrokes, we can stream videos
on Youtube -- Hello! -- read articles on Wikipedia, order supplies on amazon, video chat with
friends, and tweet about the weather. Without a doubt, the ability for computers,
and their users, to send and receive information over a global
telecommunications network forever changed the world. 150 years ago, sending a letter from London
to California would have taken two to three weeks, and that’s if you paid for express
mail. Today, that email takes a fraction of a second. This million fold improvement in latency,
that’s the time it takes for a message to transfer, juiced up the global economy helping
the modern world to move at the speed of light on fiber optic cables spanning the globe. You might think that computers and networks
always went hand in hand, but actually most computers pre-1970 were humming away all alone. However, as big computers began popping up
everywhere, and low cost machines started to show up on people’s desks, it became
increasingly useful to share data and resources, and the first networks of computers appeared. Today, we’re going to start a three-episode
arc on how computer networks came into being and the fundamental principles and techniques
that power them. INTRO The first computer networks appeared in the
1950s and 60s. They were generally used within an organization
– like a company or research lab – to facilitate the exchange of information between
different people and computers. This was faster and more reliable than the
previous method of having someone walk a pile of punch cards, or a reel of magnetic tape,
to a computer on the other side of a building ‒ which was later dubbed a sneakernet. A second benefit of networks was the ability
to share physical resources. For example, instead of each computer having
its own printer, everyone could share one attached to the network. It was also common on early networks to have
large, shared, storage drives, ones too expensive to have attached to every machine. These relatively small networks of close-by
computers are called Local Area Networks, or LANs. A LAN could be as small as two machines in
the same room, or as large as a university campus with thousands of computers. Although many LAN technologies were developed
and deployed, the most famous and succesful was Ethernet, developed in the early 1970s
at Xerox PARC, and still widely used today. In its simplest form, a series of computers
are connected to a single, common ethernet cable. When a computer wants to transmit data to
another computer, it writes the data, as an electrical signal, onto the cable. Of course, because the cable is shared, every
computer plugged into the network sees the transmission, but doesn’t know if data is
intended for them or another computer. To solve this problem, Ethernet requires that
each computer has a unique Media Access Control address, or MAC address. This unique address is put into a header that
prefixes any data sent over the network. So, computers simply listen to the ethernet
cable, and only process data when they see their address in the header. This works really well; every computer made
today comes with its own unique MAC address for both Ethernet and WiFi. The general term for this approach is Carrier
Sense Multiple Access, or CSMA for short. The “carrier”, in this case, is any shared
transmission medium that carries data – copper wire in the case of ethernet, and the air
carrying radio waves for WiFi. Many computers can simultaneously sense the
carrier, hence the “Sense” and “Multiple Access”, and the rate at which a carrier
can transmit data is called its Bandwidth. Unfortunately, using a shared carrier has
one big drawback. When network traffic is light, computers can
simply wait for silence on the carrier, and then transmit their data. But, as network traffic increases, the probability
that two computers will attempt to write data at the same time also increases. This is called a collision, and the data gets
all garbled up, like two people trying to talk on the phone at the same time. Fortunately, computers can detect these collisions
by listening to the signal on the wire. The most obvious solution is for computers
to stop transmitting, wait for silence, then try again. Problem is, the other computer is going to
try that too, and other computers on the network that have been waiting for the carrier to
go silent will try to jump in during any pause. This just leads to more and more collisions. Soon, everyone is talking over one another
and has a backlog of things they need to say, like breaking up with a boyfriend over a family
holiday dinner. Terrible idea! Ethernet had a surprisingly simple and effective
fix. When transmitting computers detect a collision,
they wait for a brief period before attempting to re-transmit. As an example, let’s say 1 second. Of course, this doesn’t work if all the
computers use the same wait duration -- they’ll just collide again one second later. So, a random period is added: one computer
might wait 1.3 seconds, while another waits 1.5 seconds. With any luck, the computer that waited 1.3
seconds will wake up, find the carrier to be silent, and start transmitting. When the 1.5 second computer wakes up a moment
later, it’ll see the carrier is in use, and will wait for the other computer to finish. This definitely helps, but doesn’t totally
solve the problem, so an extra trick is used. As I just explained, if a computer detects
a collision while transmitting, it will wait 1 second, plus some random extra time. However, if it collides again, which suggests
network congestion, instead of waiting another 1 second, this time it will wait 2 seconds. If it collides again, it’ll wait 4 seconds,
and then 8, and then 16, and so on, until it’s successful. With computers backing off, the rate of collisions
goes down, and data starts moving again, freeing up the network. Family dinner saved! This “backing off” behavior using an exponentially
growing wait time is called Exponential Backoff. Both Ethernet and WiFi use it, and so do many
transmission protocols. But even with clever tricks like Exponential
Backoff, you could never have an entire university’s worth of computers on one shared ethernet
cable. To reduce collisions and improve efficiency,
we need to shrink the number of devices on any given shared carrier -- what’s called
the Collision Domain. Let go back to our earlier Ethernet example,
where we had six computers on one shared cable, a.k.a. one collision domain. To reduce the likelihood of collisions, we
can break this network into two collision domains by using a Network Switch. It sits between our two smaller networks,
and only passes data between them if necessary. It does this by keeping a list of what MAC
addresses are on what side of the network. So if A wants to transmit to C, the switch
doesn’t forward the data to the other network – there’s no need. This means if E wants to transmit to F at
the same time, the network is wide open, and two transmissions can happen at once. But, if F wants to send data to A, then the
switch passes it through, and the two networks are both briefly occupied. This is essentially how big computer networks
are constructed, including the biggest one of all – The Internet – which literally
inter-connects a bunch of smaller networks, allowing inter-network communication. What’s interesting about these big networks,
is that there’s often multiple paths to get data from one location to another. And this brings us to another fundamental
networking topic, routing. The simplest way to connect two distant computers,
or networks, is by allocating a communication line for their exclusive use. This is how early telephone systems worked. For example, there might be 5 telephone lines
running between Indianapolis and Missoula. If John picked up the phone wanting to call
Hank, in the 1910s, John would tell a human operator where he wanted to call, and they’d
physically connect John’s phone line into an unused line running to Missoula. For the length of the call, that line was
occupied, and if all 5 lines were already in use, John would have to wait for one to
become free. This approach is called Circuit Switching,
because you’re literally switching whole circuits to route traffic to the correct destination. It works fine, but it’s relatively inflexible
and expensive, because there’s often unused capacity. On the upside, once you have a line to yourself
– or if you have the money to buy one for your private use – you can use it to its
full capacity, without having to share. For this reason, the military, banks and other
high importance operations still buy dedicated circuits to connect their data centers. Another approach for getting data from one
place to another is Message Switching, which is sort of like how the postal system works. Instead of dedicated route from A to B, messages
are passed through several stops. So if John writes a letter to Hank, it might
go from Indianapolis to Chicago, and then hop to Minneapolis, then Billings, and then
finally make it to Missoula. Each stop knows where to send it next because
they keep a table of where to pass letters given a destination address. What’s neat about Message Switching is that
it can use different routes, making communication more reliable and fault-tolerant. Sticking with our mail example, if there’s
a blizzard in Minneapolis grinding things to a halt, the Chicago mail hub can decide
to route the letter through Omaha instead. In our example, cities are acting like network
routers. The number of hops a message takes along a
route is called the hop count. Keeping track of the hop count is useful because
it can help identify routing problems. For example, let’s say Chicago thinks the
fastest route to Missoula is through Omaha, but Omaha thinks the fastest route is through
Chicago. That's bad, because both cities are going
to look at the destination address, Missoula, and end up passing the message back and forth
between them, endlessly. Not only is this wasting bandwidth, but it’s
a routing error that needs to get fixed! This kind of error can be detected because
the hop count is stored with the message and updated along its journey. If you start seeing messages with high hop
counts, you can bet something has gone awry in the routing! This threshold is the Hop Limit. A downside to Message Switching is that messages
are sometimes big. So, they can clog up the network, because
the whole message has to be transmitted from one stop to the next before continuing on
its way. While a big file is transferring, that whole
link is tied up. Even if you have a tiny, one kilobyte email
trying to get through, it either has to wait for the big file transfer to finish or take
a less efficient route. That’s bad. The solution is to chop up big transmissions
into many small pieces, called packets. Just like with Message Switching, each packet
contains a destination address on the network, so routers know where to forward them. This format is defined by the “Internet
Protocol”, or IP for short, a standard created in the 1970s. Every computer connected to a network gets
an IP Address. You’ve probably seen these as four, 8-bit
numbers written with dots in between. For example,172.217.7.238 is an IP Address
for one of Google’s servers. With millions of computers online, all exchanging
data, bottlenecks can appear and disappear in milliseconds. Network routers are constantly trying to balance
the load across whatever routes they know to ensure speedy and reliable delivery, which
is called congestion control. Sometimes different packets from the same
message take different routes through a network. This opens the possibility of packets arriving
at their destination out of order, which is a problem for some applications. Fortunately, there are protocols that run
on top of IP, like TCP/IP, that handle this issue. We’ll talk more about that next week. Chopping up data into small packets, and passing
these along flexible routes with spare capacity, is so efficient and fault-tolerant, it’s
what the whole internet runs on today. This routing approach is called Packet Switching. It also has the nice property of being decentralized,
with no central authority or single point of failure. In fact, the threat of nuclear attack is why
packet switching was developed during the cold war! Today, routers all over the globe work cooperatively
to find efficient routings, exchanging information with each other using special protocols, like
the Internet Control Message Protocol (ICMP) and the Border Gateway Protocol (BGP). The world's first packet-switched network,
and the ancestor to the modern internet, was the ARPANET, named after the US agency that
funded it, the Advanced Research Projects Agency. Here’s what the entire ARPANET looked like
in 1974. Each smaller circle is a location, like a
university or research lab, that operated a router. They also plugged in one or more computers
– you can see PDP-1’s, IBM System 360s, and even an ATLAS in London connected over
a satellite link. Obviously the internet has grown by leaps
and bounds in the decades since. Today, instead of a few dozen computers online,
it’s estimated to be nearing 10 billion. And it continues to grow rapidly, especially
with the advent of wifi-connected refrigerators and other smart appliances, forming an “internet
of things”. So that’s part one – an overview of computer
networks. Is it a series of tubes? Well, sort of. Next week we’ll tackle some higher-level
transmission protocols, slowly working our way up to the World Wide Web. I’ll see you then!
