The Physical Layer
Table of Contents
Moving Bits Across the Wire
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 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(demo below). 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 techniques, modern networks are capable of moving 10 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.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 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.
References:
https://www.coursera.org/learn/computer-networking/lecture/Nihjd/moving-bits-across-the-wire
https://www.coursera.org/learn/computer-networking/lecture/BtiAw/twisted-pair-cabling-and-duplexing
https://www.coursera.org/learn/computer-networking/lecture/3b1Hi/network-ports-and-patch-panels
网友评论