Sunday, June 25, 2017

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.


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


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.

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