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Computer Networks

 

Hi, 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 networksalways 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-episodearc on how computer networks came into being and the fundamental principles and techniquesthat power them. INTRO The first computer networks appeared in the1950s and 60s. They were generally used within an organization– like a company or research lab – to facilitate the exchange of information betweendifferent people and computers.

This was faster and more reliable than theprevious 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 abilityto share physical resources.

For example, instead of each computer havingits own printer, everyone could share one attached to the network. It was also common on early networks to havelarge, 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 inthe 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 successful was Ethernet, developed in the early 1970sat 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 madetoday comes with its own unique MAC address for both Ethernet and WiFi. The general term for this approach is CarrierSense 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 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. The problem is, the other computer is going to try that too, and other computers on the network that have been waiting for the carrier togo 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 thecomputers use the same wait duration -- they’ll just collide again one second later. So, a random period is added: one computermight wait 1.3 seconds, while another waits 1.5 seconds. With any luck, the computer that waited 1.3seconds will wake up, find the carrier to be silent, and start transmitting.

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When the 1.5 second computer wakes up a momentlater, it’ll see the carrier is in use, and will wait for the other computer to finish. This definitely helps, but doesn’t totallysolve the problem, so an extra trick is used. As I just explained, if a computer detectsa collision while transmitting, it will wait 1 second, plus some random extra time. However, if it collides again, which suggestsnetwork 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 ExponentialBackoff, you could never have an entire university’s worth of computers on one shared ethernetcable. To reduce collisions and improve efficiency,we need to shrink the number of devices on any given shared carrier -- what’s calledthe 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, wecan 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 MACaddresses are on what side of the network. So if A wants to transmit to C, the switchdoesn’t forward the data to the other network – there’s no need. This means if E wants to transmit to F atthe same time, the network is wide open, and two transmissions can happen at once. But, if F wants to send data to A, then theswitch passes it through, and the two networks are both briefly occupied.


This is essentially how big computer networksare constructed, including the biggest one of all – The Internet – which literallyinter-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 fundamentalnetworking 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 linesrunning between Indianapolis and Missoula. If John picked up the phone wanting to callHank, in the 1910s, John would tell a human operator where he wanted to call, and they’dphysically connect John’s phone line into an unused line running to Missoula. 


For the length of the call, that line wasoccupied, and if all 5 lines were already in use, John would have to wait for one tobecome 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 inflexibleand 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 itsfull capacity, without having to share. For this reason, the military, banks and otherhigh importance operations still buy dedicated circuits to connect their data centers.


Another approach for getting data from oneplace to another is Message Switching, which is sort of like how the postal system works. Instead of dedicated route from A to B, messagesare passed through several stops. So if John writes a letter to Hank, it mightgo 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 becausethey keep a table of where to pass letters given a destination address. What’s neat about Message Switching is thatit can use different routes, making communication more reliable and fault-tolerant.


Sticking with our mail example, if there's blizzard in Minneapolis grinding things to a halt, the Chicago mail hub can decideto 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 thefastest route to Missoula is through Omaha, but Omaha thinks the fastest route is throughChicago. That's bad, because both cities are goingto look at the destination address, Missoula, and end up passing the message back and forthbetween them, endlessly.


Not only is this wasting bandwidth, but it’sa routing error that needs to get fixed! This kind of error can be detected becausethe hop count is stored with the message and updated along its journey. If you start seeing messages with high hopcounts, you can bet something has gone awry in the routing! This threshold is the Hop Limit.


A downside to Message Switching is that messagesare sometimes big. So, they can clog up the network, becausethe whole message has to be transmitted from one stop to the next before continuing onits way. While a big file is transferring, that wholelink 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 takea less efficient route.


That’s bad. The solution is to chop up big transmissionsinto many small pieces, called packets. Just like with Message Switching, each packetcontains a destination address on the network, so routers know where to forward them. This format is defined by the “InternetProtocol”, or IP for short, a standard created in the 1970s. Every computer connected to a network getsan IP Address. You’ve probably seen these as four, 8-bitnumbers written with dots in between. For example,172.217.7.238 is an IP Addressfor one of Google’s servers.


With millions of computers online, all exchangingdata, bottlenecks can appear and disappear in milliseconds. Network routers are constantly trying to balancethe load across whatever routes they know to ensure speedy and reliable delivery, whichis called congestion control. Sometimes different packets from the same message take different routes through a network.


This opens the possibility of packets arrivingat their destination out of order, which is a problem for some applications. Fortunately, there are protocols that runon 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, its 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, likethe 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!

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