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The most common forms of copper twisted pair cables used in networking are Cat5, Cat5e, and Cat6 (categories) cables. Cat5e cables have mostly replaced those older Cat5 cables because their internals reduce crosstalk. Crosstalk is when an electrical pulse on one wire is accidentally detected on another wire. So, the receiving end isn't able to understand the data causing a network error. The second primary form of networking cable is known as Fiber, short for fiber optic cables. Fiber cables contain individual optical fibers which are tiny tubes made out of glass about the width of a human hair. These tubes of glass can transport beams of light. Unlike copper, which uses electrical voltages, fiber cables use pulses of light to represent the ones and zeros of the underlying data. Fiber is even sometimes used specifically in environments where there's a lot of electromagnetic interference from outside sources because this can impact data being sent across copper wires.
A hub is a physical layer device that allows for connections from many computers at once. This causes noise on the network and causes a collision domain. A collision domain is a network segment where only one device can communicate at a time. If multiple systems try sending data at the same time, the electrical pulses sent across the cable can interfere with each other. This causes these systems to have to wait for a quiet period before they try sending their data again. A switch is very similar to a hub, since you can connect many devices to it so they can communicate. The difference is that while a hub is a layer 1 or physical layer device, a switch is a level 2 or data link device. This means that a switch can actually inspect the contents of the Ethernet protocol data being sent around the network, determine which system the data is intended for and then only send that data to that one system.
A router is a device that knows how to forward data between independent networks. A router operates at layer three, a network layer. Routers store internal tables containing information about how to route traffic between lots of different networks all over the world. The most common type of router you'll see is one for a home network, or a small office. These devices generally don't have very detailed routing tables. The purpose of these routers is mainly just to take traffic originating from inside the home, or office LAN, and to forward it along to the ISP. A core router usually has many different connections to many other routers. Routers share data with each other via a protocol known as Border Gateway Protocol (BGP). That lets them learn about the most optimal paths to forward traffic. When you open a web browser and load a webpage, the traffic between computers and the web servers could have traveled over dozens of different routers. The Internet is incredibly large and complicated. And routers are global guides for getting traffic to the right places.
The simplest way to think of a server is as something that provides data to something requesting that data. The thing receiving the data is referred to as a client.
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 that we'll learn about when we cover the other layers. Ones and zeros are sent across those network cables through a process called modulation. Modulation is a 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.
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. 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.
The final steps of how the physical layer works take place at the end points 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 RJ-45 or Registered Jack 45.