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Lesson 03 - Physical Layer


In this lesson we'll look at the physical layer of the TCP/IP model.

Conductive Media vs Non Conductive Media

Last chapter we learned about encoding, the process of converting 1's and 0's to the medium's required format for transport.  There are two categories of mediums used in the industry, conductive and non conductive.  Conductive media requires direct contact to transmit data.  These are the cables that connect your devices to the network.  They include both copper and fiber cables.

Non conductive media doesn't require a physical connection to operate.  These are your wireless standards, like WiFi, or Bluetooth.

Encoding the data works differently depending on what type of media you're using.  
  • Copper: Electronic voltages
  • Fiber: Light wavelengths
  • Wireless: Radio frequencies
Time is a factor when encoding.  If both sides agree that a +5 voltage represents a 1 that won't be enough to have successful communication.  If one device tries to send two 1's in a row how does the receiving computer know when the first 1's ends and the second 1 starts?  Time is used to break up the data.  If the encoding standard calls for 5 seconds of time per bit then ten seconds of 5 volts would represent two 1's.  As long as both sides use the same standard, or common language, this will work.

Conductive Media

Conductive media includes both copper and fiber connections.  Copper cables are categorized based on their abilities.  Below is a table that shows the characteristics of the different cable types.  Today, categories 1 - 4 aren't used.  

 Type Twists  Frequency  Max Distance  Max Speed
 Category 1  N/A  1 MHz  N/A  1 Mbps
 Category 2  2 or 3 per foot  10 MHz  45 meters  4 Mbps
 Category 3  3 per foot  10 MHz  100 meters  10 Mbps
 Category 4  5 or 6 per foot  20 MHz  100 meters  16 Mbps
 Category 5  3 or 4 per inch  100 MHz  100 meters  100 Mbps
 Category 5e  3 or 4 per inch  100 MHz  100 meters  1,000 Mbps
 Category 6  5 or 6 per inch  250 MHz  100 meters  1,000 Mbps
 Category 6a  5 or 6 per inch  500 MHz  100 meters  1,000 Mbps

Copper cable is susceptible to electrical interference.  Most of the time this isn't a problem, but if you're in an environment where it's a problem you may want to purchase shielded cable.  Normally the cable is unshielded known as Unshielded Twisted Pair (UTP), but when needed you can purchase Shielded Twisted Pair (STP).

Let's take a look at some of these and other types of cables.

Phone Cable - This is a flat (non-twisted) four wire cable.  Christmas Trees and Bumblebees. RJ-11 - Used for phone cable.  RJ = Registered Jack Category 5 Cable - Twisted cable with four pair, Unshielded Twisted Pair (UTP)
Category 5 Cable - This one is Shielded Twisted Pair (STP) Category 6 Cable - Twisted cabled with four pair, notice the plastic divider to keep the four pair separated. RJ-45 - Used for category 3 - 6 cable as the end that allows you to plug it into network port. RJ = Registered Jack    
RJ-45 - With a molded boot. RJ-45 - Without a molded boot. (Notice the color pattern - EIA/TIA 568b) Keystone Jack - Notice this is color coded for the EIA/TIA 568b pattern.
Fiber - The top is an CS type end, and the lower is an ST type end.
Category 5 vs 5e vs 6  

EIA/TIA 568a and 568b Patterns
In the images above the EIA/TIA 568b standard was mentioned a couple times.  The standard refers to the order in which the cables are used.  The cables we find in a modern network installations contain eight strands.  The eight strands are separated into four pair which are twisted together.  Each pair is referred to by their color, and contain one solid color, and one stripped.  For example, with the orange pair the two cables are Orange and White Orange.  The orange cable is the solid cable, and the white orange is a white cable with an orange stripe.

The purpose of the two 568 standards is to make sure the strands carry data to the correct spot.  The sending device will send out pins 1 and 2.  The receiving device will listen on pins 3 and 6.  A cable wired with the 568a standard on one side and the 568b standard on the other side will make sure the signal goes to the correct spot.  Looking at the diagram below you can see the white stipe cable takes the date sent out pin 1 and brings it to pin 3 on the white green strand.  You can also see pin 2 brings the data to pin 6 on the green cable.  A cable with a 568a pattern on one end and a 568b on the other is referred to as a crossover cable.

A crossover cable is needed when you hook two devices together that are similar in type.  In most situations you aren't hooking to devices directly together.  You have a device in the middle that will take the data from pin 1 and send it to pin 2, and pin 3 to pin 6.  When you have a device in the middle providing this function you can wire the cable with the same pattern on both side.  A cable wired on both ends with the same patten is referred to as a straight through cable.

Structured Wiring

At the center of our network we have a device that connects everything together called a switch.  We'll learn more about switches in a later lesson.  For the purposes of this lesson we'll see it as a device that connects all our network devices together.  Below you can see a simple representation of this idea.  All our devices connect directly to the switch.

The above network may work for us in our smaller home networks, but in a large corporate environment it won't.  Having all the cables run directly from the devices to the switch would create a big mess.  We solve this by using structured wiring.  Structured wiring is where the cable is terminated with female ports at both ends containing no electronic devices in the middle.  We use keystone jacks in the wall jacks to create our female ports at the remote ends.  A patch panel is used near the switch to organize all the cables coming in and provide female ports ready to patch into the switch.  On the back of both the patch panel and the keystone jacks you punch down the colored strands of the cable creating the physical connection.  In the image below you can see how the patch panel and keystone jack fit into the network.  If all the computers and switch were removed we would be left with a structured network ready to plug in new equipment.

Connecting to the Network

In the next few images we will follow a request from a computer to the fiber network.

The request comes out of the network port of the computer to the RJ-45 which is connected to a category 5 patch cable.

The category 5 patch cable runs to the wall where the cable's other RJ-45 plugs into the female port of the keystone jake. The category 5 cables coming from the keystone jack run through the walls and ceilings towards your wiring closet.
In the wiring closet all the cables from the nearby room converge and are terminated into the back of the patch panel.

In the front of the patch panel we find female ports.  Plugged into those ports are category 5 patch cables.  The category 5 patch cables go from the  patch panel to the switch.  The switch  connects all the devices together.
If the destination is connected to another wiring closet then we have to take the fiber to that closet.  The switch has a built in transceiver providing us with fiber ports. The fiber patch cables go from the switch to a fiber patch cable.  The back of the patch cable contains the fiber coming from the other closet.  The thicker orange cable terminates into the back of a fiber patch panel in the destination closet.

The trip from one computer to another computer will travel through multiple things along the way.  The structured wiring is there to make the network more organized and be independent of any electronic device.  Both the copper and fiber patch cables provide the needed ports to start plugging in your devices. The diagram below shows an overview of where all these pieces fit in the puzzle.

Non Conductive Media

Non conductive media refers to our wireless network.  A wireless network uses radio waves to transmit data.  A wireless device can run in one of two modes.
  • Infrastructure - This mode all devices talk to each other through a centralized Wireless Access Point (AP).  This is a point to multipoint configuration, the AP acts like a switch.
  • Ad Hoc - In this mode two devices are talking directly to each other.  This is a point to point connection.
The 802.11 standards represent what we know as WiFi.  Over the years the specs has been changing and improving.  In the table below you will see the different standards available and the differences between them.
Standard Speed Frequency Range
 802.11a  54 Mbps  5 GHz   100 ~ 400 ft
 802.11b  11 Mbps  2.4 GHz  100 ~ 450 ft
 802.11g  54 Mbps  2.4 GHz  100 ~ 450 ft
 802.11n  600 Mbps  2.4 & 5 GHz  200 ~ 800 ft
 802.11ac  1,300 Mbps  5 GHz  100 ft
 Bluetooth  24 Mbps  2.4 GHz  33 ft

When setting up your wireless network you can purchase equipment that will let you connect different types of antennas.  When looking at antennas there are two different types to consider.
  • Omni - This is the type of antenna built into many of our AP's.  This is a point to multipoint antenna that creates a circle of coverage with a semi fixed radius.
  • Yagi - This type is used for point to point connections, it's a more focused antenna with less coverage, but a higher distance.


The term wasn't mentioned yet in this lesson.  But the chunk of data at the physical network is referred to as bits.  The diagram from the last chapter has been updated with this new term.

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