Computer Networks 101

In your typical white-collar work environment, each employee has a computer at their desk. The computers are connected with each other over a LAN (local area network). If you have internet access at home, the computers in your house are most likely organized into a LAN as well. A LAN is a group of computers that are physically close to one another and that can communicate with each other over a network.  All of the computers in a LAN are said to belong to a broadcast domain, which means that if one computer sends out a broadcast message, then all the other computers can hear it.

Ethernet

One of the most common ways to join computers into a LAN is to use Ethernet. There are four properties of an Ethernet cable: EMI resistance, heat resistance, flexibility, and speed.

EMI resistance: STP (shielded twisted pair) cables are designed to protected again EMI (electromagnetic interference). A shop floor is a good example of a place where STP cables should be used because it has lots of electrical motors and other machinery. However, the vast majority of environments do not require significant protection from EMI, so they use UTP (unshielded twisted pair) cables, which are less expensive.

Heat resistance: Ethernet cabling is often run through the walls and ceilings of a building (called plenum space) in order to keep the cables out of the way. These areas of the building can get very hot. The rubbery outside of an Ethernet cable is typically made with a material called PVC. If PVC starts to melt due to high heat, it can give off poisonous fumes. A plenum-grade cable, however, will not melt in the heat because it is made out of material that is designed for heat-intensive environments. Plenum-grade cabling is much more expensive than PVC, so you should only buy it for cabling that you intend to use in plenum space.

Flexibility: You also need to think about the kind of physical wear-and-tear the cable will be getting. Will the cable sit in plenum space, untouched for most of its existence? Or will the cable spend most of its time in your office drawer, being used for various purposes around the office? Standard core cabling is made out of material that is flexible, which means you can bend it, step on it, and twist it (to a reasonable extent) without breaking it. Solid core cabling, on the other hand, is not so flexible. But its advantage is that it is a better conductor and will transfer data more effectively.

Speed rating: Every Ethernet cable has a speed rating, which defines its max data transfer speed. A cable’s speed rating is usually stamped on the outside of the cable itself. It’s sometimes referred to as a “CAT rating”, since the speed rating begins with the letters “CAT”. The maximum cable length varies between speed ratings, but for most speed ratings it is 100 meters. The ratings are measured in Mbps (megabits per second) or Gbps (gigabytes per second). To get a better feel for how fast this is, I like to divide this number by 8, which tells me how many bytes per second it supports. Note that, in order to take advantage of the full speed a cable offers, all other parts of your network infrastructure must support that speed rating, such as the network cards in the computers and the switches.

Standard	Max speed/notes
CAT 1	This is the technical name for a telephone cable! Telephone cables use a RJ-11 connector, whereas Ethernet cables use a RJ-45 connector.
CAT 3	10 Mbps, some variants support 100 Mbps
CAT 5	100 Mbps
CAT 5e	1000 Mbps
CAT 6	1000 Mbps, 10 Gbps (55 meter max cable length)
CAT 6a/e	10 Gbps
CAT 7	10 Gbps with better shielding

Switches

All the computers in an Ethernet network connect to a central device called a switch, which routes the various network data to where it needs to go. This is called a star bus topology—“star” refers to the fact that the computers connect to a central switch (instead of to each other) and “bus” refers to the central device that routes all traffic.

Note that a device called a “bus” can serve as the central device as well, but buses are much more inefficient than switches because they broadcast all messages they receive to all computers, whereas switches only send out messages to the computer that the messages are intended for. Switches used to be more expensive than buses, but not anymore.

To prevent unauthorized computers from connecting to the network, you can disable unused ports on a switch.

Structured Cabling

Larger companies have the money and talent to organize their networks using structured cabling system. The aim of such a system is to create an organized, secure (both from an information safety perspective and a physical safety perspective), and reliable way of connecting all of your company’s computers to each other.

A typical structured cabling system is organized as follows. All cabling, including Ethernet cables and telephone cables, are run from each work area (the office space that an employee occupies) to a central room called the telecommunications room. This cabling is referred to as the horizontal cabling. Each piece of horizontal cabling is referred to as a run. Vocabulary rocks!

In an ideal environment, the horizontal Ethernet cabling would run through plenum space and be of plenum-grade, solid core construction. Each work area would then contain wall outlets that connect to the horizontal cabling. It’s interesting to note that Ethernet wall outlets have CAT ratings as well! Therefore, it’s important to make sure the outlet matches the CAT rating of your horizontal cabling.

The telecommunications room is the central destination for all the horizontal cabling. It contains specially designed equipment racks which are used to store its computer equipment. All rack-mounted equipment adheres to a measurement standard, simply referred to as U, which defines the height of the equipment. 1U equals 1.75 inches. Most rack-mounted equipment is either 1U, 2U, or 4U.

One piece of equipment you’re likely to find in a telecommunications room is a patch panel. A patch panel makes it easy to rearrange your network without having to mess with the horizontal cabling (which often uses fragile, solid core cables). The horizontal cabling is plugged into the back of the patch panel using a connector called a 110 punchdown block. This kind of connector connects the individual wires inside of the Ethernet cable to the patch panel. A punchdown tool is used to attach the cable in this way. Connecting the horizontal cabling to the patch panel is a time consuming process and is meant to be more or less permanent. The other side of the patch panel contains much more flexible RJ-45 ports, which are easy to plug and unplug (kind of like the telephone switches of old). Patch cables are plugged into these ports. Patch cables are short (typically 2-5 feet long), standard core, UTP Ethernet cables. You then use the patch cables to rearrange your network as you like, as often as you like.

SOHO, Bro!

SOHO environments (small office/home office) do not always have the luxury of implementing a structured cabling solution. But there are a number of technologies that you can use to form a LAN without this.

What’s the Wi-Fi password?

The most common and quickest way to create a LAN is to go wireless. Wireless networks are not as fast as wired networks, but for most purposes, they are fast enough. Various wireless standards have been released over the years, each of which varies in speed. In general, they are backwards compatible with each other. Most wireless routers support multiple standards anyway, so you don’t have to worry too much about compatibility most of the time.

Standard	Max speed	Frequency	Range
802.11b	12 Mbps	2.4Ghz	300 feet
802.11a (came out after b)	54 Mbps	5 GHz	150 feet
802.11g	54 Mbps	5 GHz	300 feet
802.11n	100+ Mbps	2.4 & 5 GHz	300+ feet
802.11ac	1 Gbps	5 GHz	300+ feet

Since Wi-Fi transmits its data over the air, securing your Wi-Fi network is of the utmost importance. The latest wireless security protocol is WPA2—all the other standards are vulnerable to security flaws, so you should never use them. Your wireless network should also be password protected, otherwise anyone can connect to it. Another way to secure your network is to configure your router to disable its SSID broadcast, which is what causes your network to appear on a device’s list of available networks. You can also enable MAC address filtering, which only gives pre-approved devices access to the network. Lastly, you should change the router’s administrator password because routers are often all configured with the same administrator password when they leave the factory.

One downside to Wi-Fi is that the wireless signal can be disrupted in many ways. Thick or metallic walls in your building can weaken or stop a wireless signal. Any devices that use the same parts of the wireless spectrum can cause interference as well, such as baby monitors and garage door openers (this is called radio frequency interference or RFI). If you have neighbors that have their own wireless networks, they can interfere with your network too. The parts of your building that get weak or no signal are called dead zones.

Ethernet over Power

If Wi-Fi isn’t an option for your particular environment, you can buy special devices that plug into your electrical outlets which allow you to create an Ethernet network using the electrical wiring of your house! This is called Ethernet over Power (not to be confused with Power over Ethernet, which supplies electrical power through an Ethernet network). This is an example of a bridge because it connects two dissimilar network technologies. Ethernet over Power only supports speeds at around 100Mbps however, so it’s not very fast.

A Primer on IP Addresses

Just like your have a home address that uniquely identifies your residence out of all the residences in the world, computers have IP addresses, which serve the same purpose.  They uniquely identify a computer in a network so that it can receive messages from other computers.

IPv4

IPv4 was created when the internet was born in 1981 and is still used today.  It is the network communication protocol that computers use to talk to each other over the internet.  An IPv4 address is a unique identifier that is used to identify an individual computer that is connected to the internet.  It is 32-bits long and is commonly represented in dotted-decimal notation.  This notation divides the bits into four, 8-bit chunks and displays each chunk as a number ranging from 0 to 255.  Each number is separated with a dot.  For example: 192.168.2.1.

At its inception, the set of all possible IPv4 addresses, called the address space, was divided into “classes”.  Each class contained a finite number of “chunks” of addresses.  The number of addresses in each chunk varied depending on the class.  The idea was that institutions, such as companies and schools, could purchase one of these chunks, and then dole out the addresses in the chunk to all the computers on their network.  Larger institutions with lots of computers could purchase a more expensive, higher class chunk that had lots of addresses, while smaller institutions that had fewer computers could purchase a cheaper, lower class chunk that had fewer sub addresses.

The classes are summarized below.  If you want to learn more about the logic behind how they were organized, I suggest you read this Wikipedia page.

Class	Number of chunks	Number of addresses in each chunk
Class A	128	16,777,216
Class B	16,384	65,536
Class C	2,097,152	256
Class D	reserved
Class E	reserved

Do you see a problem here?

The problem with this scheme was that companies were unlikely to use every address that was available to them.  The choices for the number of addresses you could have varied wildly—you could have 16,777,216, 65,536, or 256!  You couldn't have anything in between!  If a company needed, say, 1,000 addresses, they had no choice but to purchase a Class B address and put all the rest to waste.  To top it off, some of the organizations that were involved in the early development of the internet possessed Class A chunks, which they were hardly making any use of.

This started to become a pressing issue as the internet grew.  The risk that all IP addresses would be used up, called IP address exhaustion, became a real possibility.

CIDR

As shown, the way the class system divided up its chunks of addresses was very coarse-grained, which resulted in lots of wasted addresses.  To combat this, the class system was done away with in 1993 and replaced with a system called CIDR (Classless Inter-Domain Routing).  This system gives organizations many more choices regarding how many addresses they are assigned, which results in less wasted addresses.

CIDR uses something called variable-length subnet masking (VLSM), which allows the address's subnet mask (the part that identifies which organization an address belongs to) to be of any size.  The class system, on the other hand, only permitted the subnet mask to be 8 bits (Class A), 16 bits (Class B), or 24 bits (Class C) long.  With CIDR, if your company only needed 1,000 addresses, you could purchase a 1,024 chunk (22-bit subnet mask, leaving 10-bits for the address, 2^10=1,024).

CIDR notation consists of an IP address, followed by the number of bits the address uses for its subnet mask.  For example, 192.168.100.14/22 represents the IP address 192.168.100.14 with the first 22 bits of that address being the subnet mask.

But CIDR is only a stop-gap measure.  The IPv4 address space consists of about 4.3 billion addresses, which seems like a lot.  But on a global scale, it is not.  If the internet continues to grow, the IPv4 address space will soon run out.  A more permanent solution would be to increase the length of the IP address.  Enter IPv6.

IPv6

Created in 1998, IPv6 addresses are a whopping 128 bits long, resulting in an incredibly large address space of 3.4 x 10^38 (the number of grains of sand on Earth...or something?).

IPv6 addresses are represented as eight, four character, hexadecimal strings separated by colons.

FEDC:0000:0000:0000:00CF:0000:BA98:1234

Because they are so long, there are tricks you can employ to make them shorter.  If a segment contains all zeroes, you can replace the segment with a single zero:

FEDC:0:0:0:00CF:0:BA98:1234

If an address contains consecutive segments which consist of all zeroes, you can replace them with a double colon (but you can only use this trick once):

FEDC::00CF:0:BA98:1234

And if a segment begins with zeroes, you can leave the zeroes out (unless the segment contains all zeroes, in which case you must leave one zero in):

FEDC::CF:0:BA98:1234

IPv6 and IPv4 are not compatible with each other, which complicates the migration process.  While it is likely that the network card in your computer supports both IPv4 and IPv6, the infrastructure around the globe that makes the internet work cannot switch over so easily.  It will be a long and piecemeal process.  But if all goes well, you won't even know it happened.

The Laser Printing Process

There are many different kinds of printers on the market. In office environments, laser printers are by far the most numerous. Not only do they produce good quality printouts, but they are fast, which is important when you have people to please and deadlines to meet. Therefore, computer technicians have to be very familiar with how laser printers work so breakdowns can be fixed and Janet can get her TPS reports on time.

A laser printer follows a specific process when printing a sheet of paper. The process can be divided into seven steps.

1. Processing

In order to start printing, the printer has to first receive print data from a computer. The program the user is printing from (say, a word processor) has to first convert the document to some kind of format the printer understands. Many Windows applications use a system called GDI (graphical device interface), which is used in conjunction with the specific printer driver, to generate this print data.

The application then sends the print data to the print spooler, which is responsible for queuing up print jobs and sending them one at a time to the printer. Once the print job has been completely sent to the printer, it disappears from the print spooler (whether the printer is done printing it or not).

Note that, while it is possible to cancel a print job from the print spooler, this only stops the flow of information from the computer to the printer. For example, if the spooler sends half of the print job before you cancel it, the printer will print exactly that, even if the job is canceled before any pages came out of the printer. Therefore, you should also press the “stop” button on the printer itself to be sure the printing truly stops (do not pull out the paper tray, as this could jam the printer).

2. Charging

The rest of this process is centered around an important part of the printer called the drum. The drum is a cylinder shaped component which is used to transfer images onto the sheets of paper.  It does this using positive and negative electrical charges.

In the Charging step, the primary corona wire (or primary charge roller) gives the drum's surface a uniform negative electrical charge.

3. Exposing

A laser draws a positively charged image into the drum (hence the name, “laser printer”).

4. Developing

Negatively-charged toner particles attach themselves to the positively-charged parts of the drum the laser drew from the last step. "Toner" is the stuff that makes up the image on the piece of paper (it is a laser printer's “ink”).

5. Transferring

Here is where the actual piece of paper comes into play. The transfer corona (or transfer roller) applies a positive charge to a sheet of paper. Then, the negatively-charged toner particles on the drum attach themselves to the positively-charged paper. Voila! The toner has been “transferred” to the page.

6. Fusing

At this point, the toner is simply resting on top of the page like a layer of dust. In the Fusing step, the toner is melted onto the page using a heating element called the fuser (toner is mostly made of plastic). Hot Pockets! The page is now done!

7. Cleaning

Now that the page is done, the printer has to be “reset” for the next page. Notably, the drum must be cleaned. First, any residual toner is scraped off using a rubber cleaning blade. Then, erase lamps give the drum a neutral charge. Go back to step 2.