Chapter 8 - Switching
Recommended viewing: TWISTED: The dramatic history of twisted-pair Ethernet
The final bit of innovation that took us from what is fundamentally fancy Serial into modern networking is called switching. Switching, more in the sense of "switching around" than "switching on". Switching is what finally enabled secure high performance networks.
Have you ever heard the story of the guy who built the first automatic telephone exchange? And the invention of the rotary phone? Back when all phones were landlines and they ran on what we called the "plain old telephone system" POTS, telephones lines were in essence just a big loop of wire that the telephone provider company kept electrified at 48v.
When you'd pick up your phone, the telephone provider had a physical board with a bunch of lights and sockets on it. Picking up the phone completes the loop of wire, making a light glow on the switch-board so the operator (a person trained in switch-board operation) could see the light, plug their own handset into the socket, and then talk to you.
They would ask you who you wanted to talk to, and then use a reference book to look up their name and find the socketthat goes to the loop of wire for their phone set. Then the operator would use an audio patch cord to jump the connection from your socket to their socket, switching around the cables to make the connection.
Now, the story goes a bit like this:
Funeral directory Almon Strowger was having trouble with his phone service. It was his belief that this was interference by the competing funerary company in town. The owner of the rival company had a wife, and that wife worked at the telephone company as a switchboard operator. The accusation from Strowger was that the rival company's owner's wife was switching phone connections to her husband's company whenever the customers were asking to talk to Strowger's company. A breach in telephone network security. Now, Bell Telephone Company denies this was ever the case but that's neither here nor there.
In response to this perceived slight, Strowger swore he'd put his rival's wife out of a job by inventing a device to take over her job. That way, nobody could interfere with connections. After years of development, he patented a device that could automatically connect two phones by a series of buttons which sent signals down the phone line. He gave each phone a number 0000 through 9999. The original design had one button per digit column, so to call number 3251, you would have to press the first button three times, the second button twice, the third button five times, and the last button once. Then the automatic switchboard would connect your call to whatever socket was assigned that number.
This was long before computers, operating by some kind of electro-mechanical device that I don't really understand. But the basic operation was set. By sending a series of signals down a telephone line, people could now have their phone connections switched up to the right destinations automatically.
Not too long after the invention and implementation of the electronic switchboard, the rotary phone was invented to simplify the dialing process for any length of phone number. Not just the original four digits.
The operation of the rotary phone is actually pretty similar to his original button design. When the user rotates the dial to a number, it causes the phone to pulse a signal down the line however many times for whatever number that you rotated it to. For example, by rotating the dial to the number 5, the rotary phone would send five pulses down the line as the dial automatically returned to its starting position by spring
So to dial that same 3251 number, it would pulse three times followed by a short pause, then you would have it pulse twice, then a short pause, then it would pulse five times, then a short pause, and then one final pulse. Then the phones would be automatically connected.
I imagine that most people don't understand why an Ethernet switch is called an Ethernet switch, but it's because an Ethernet switch works almost entirely the same way as one of these automatic telephone switchboards. An automatic telephone switch board completes an audio circuit between two telephone sets. And an Ethernet switch completes a communications circuit between two 10BASE-T Ethernet cables.
The point is, ethernet networks borrowed the idea of a switch from telephone networks. Computer systems were already taking advantage of automatic telephone switchboards by way of serial telephone modems with an auto dialer feature. By using a telephone modem with an auto dialer, a computer could already connect itself to another computer as long as that other computer had a dedicated phone number and phone line assigned to it.
An Ethernet hub is like a telephone party line where all devices receive all communications. An ethernet switch is like a telephone switch board, where the wires are switched around internall so that the two devices talking are connected directly one to the other. However instead of using phone numbers to identify each other, the computers use MAC Addresses.
The key difference here is that while a telephone switchboard switches a connection until the end of the call, an Ethernet switch only switches the connection until the single frame of data ends. A single frame as in the serial payload that has the synchronization preamble, the start bits, the destination MAC, the source MAC, the data bytes, and the hash. In just the same way that a rotary phone transmits the phone number by a series of pulses to establish the connection, the Ethernet frame's destination MAC bits are used by the Ethernet switch to establish the connection. But as soon sa the frame ends, the connection gets un-switched again. ends.
By switching these connections to deliver a frame directly from one device to another, the switch prevents frames meant for one device from ever reaching another device. This does improve the security of the network by quite a bit, but importantly it also increases the performance of the system significantly. On a switched Ethernet network, computers no longer have to filter out frames that were not addressed to them because the switch ensures they get delivered directly from the source to the destination.
At the original ethernet data rate of 10MBit/s, and to some extent even at 100MBit/s, even a slow switch did not have a huge impact on performance because it was already artificially limited by everyone having to communicate at those slow synchronized speeds. These days though, ethernet switches operate at speeds in excess of 1GBit/s and are designed to switch in hardware using ASICs (almost literally re-wiring themselves at run-time to create direct connections between devices), so that there is no degradation to system performance. Switches operate transparently almost as if they aren't there at all.
So, with the introduction of Ethernet switches we finally have something starting to resemble a modern computer network. Within one building, Ethernet can connect all of the computers efficiently and allow them to send whatever data they want wherever they want.
Across buildings, computers can utilize phone lines and modems to talk directly from one to another. But we still need a solution to allow a computer in one building to talk over Ethernet with a computer in another building, or at another university, or in another state, or in another country. All purely Ethernet based options face scaling issues that were never addressed until the introduction of Internet Protocol (IP).