The physical layer of our network stack model is the most complex of all. Its main focus is on moving ones and zeros from one end of the link to the next. The physical layer consists of devices and means of transmitting bits across computer networks. A bit is the smallest representation of data that a computer can understand. It's a one or a zero.
These ones and zeros sends across networks at the lowest level are what make up the frames and packets of data.
A standard copper network cable, once connected to devices on both ends, will carry a constant electrical charge. Ones and zeros are sent across those network cables through a process called modulation.
Modulation is way of varying the voltage of this charge moving across the cable. When used for computer networks, this kind of modulation is more specifically known as line coding. It allows devices on either end of a link to understand that an electrical charge in a certain state is a zero, and in another state is a one. Through this seemingly simple technique, modern networks are capable of moving 0 billion ones and zeros across a single network cable every second.
Twisted Pair Cabling and Duplexing
The most common type of cabling used for connecting
computing devices is known as twisted pair
It's called a twisted pair cable because it features
pairs of copper wires that are twisted together.
These pairs act as a single conduit for information,
and their twisted nature helps protect against
electromagnetic interference and crosstalk from neighboring pairs.
A standard cat six cable has eight wires
consisting of four twisted pairs inside a single jacket.
Exactly how many pairs are actually in
use depends on the transmission technology being used.
But in all modern forms of networking,
it's important to know that these cables allow for duplex communication.
Duplex communication is the concept that
information can flow in both directions across the cable.
On the flip side, a process called simplex communication is unidirectional.
Think about a baby monitor,
where the transmission of data only goes in
one direction making it a simplex communication.
A phone call on the other hand is duplex since both parties can listen and speak.
The way networking cables ensure that duplex communication is possible is
by reserving one or two pairs for communicating in one direction.
They then use the other one or two pairs for communicating in the other direction.
So, devices on either side of
a networking link can both communicate with each other at the exact same time.
This is known as full duplex.
If there's something wrong with the connection,
you might see a network link degrade and report itself as operating as half-duplex.
Half-duplex means that, while communication is possible in each direction,
only one device can be communicating at a time.
Network Ports and Patch Panels
The final steps of how the physical layer works take
place at the endpoints of our network links.
Twisted pair network cables are terminated with
a plug that takes the individual internal wires and exposes them.
The most common plug is known as an RJ45,
or Registered Jack 45.
It's one of many cable plugs specifications but by far,
the most common in Computer Networking.
A network cable with an RJ45 plug can connect to an RJ45 network port.
Network ports are generally directly
attached to the devices that make up a computer network.
Switches would have many network ports because their purpose is to connect many devices.
But servers and desktops,
usually only have one or two.
Your laptop, tablet or phone probably don't have any.
But we'll get to wireless networking in a later module.
Most network ports have two small LEDs.
One is the Link LED,
and the other is the activity LED.
The link LED will be lit when a cable is properly
connected to two devices that are both powered on.
The activity LED will flash when data is actively transmitted across the cable.
A long time ago,
the flashing activity LED corresponded directly to the ones and zeros being sent.
Today, computer networks are so fast that the activity LED
doesn't really communicate much other than if there's any traffic or not.
On switches, sometimes the same LED is used for both link and activity status.
It might even indicate other things like link speed.
You'll have to read up on a particular piece of hardware you're working with.
But there will almost always be
some troubleshooting data available to you through port lights.
Sometimes a network port isn't connected directly to a device.
Instead, there might be network ports mounted on a wall or underneath your desk.
These ports are generally connected to the network via cables,
run through the walls that eventually end at a patch panel.
A patch panel, is a device containing many network ports.
But it does no other work.
It's just a container for the endpoints of many runs of cable.
Additional cables are then generally ran from a patch panel, to switches,
or routers to provide network access,
to the computers at the other end of those links.
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