Computer Networking: Introduction

Networks must be able to speak the same language to each other. Humans follow a series of rules, and computers have to do the same and this thing that we follow is called a protocol. Computer networking is the full scope of how computers communicate with each other, ensuring that computers can hear each other, speak protocols other computers can understand, and repeat messages not fully delivered. There are lots of models, but I want to discuss the TCP/IP 5-layer models or the 7 layer OSI models. It's super important to know these models, and the protocols at each layer carry the data of the layer above them in order to get data from one layer to the next.  Both of these protocols are required to work at the same time for the internet and business networks to work. 

To really understand networking, we need to understand all of the components involved and there are many models, and one course is the TCP/IP 5 layer model. The Physical Layer represents the physical devices that interconnect computers. This includes the specifications and connectors. The DataLink Layer, or the network layer, is responsible for defining a common way to communicate signals. Ethernet standards also define protocol responsible for getting data to nodes on the same network or link. The third layer is the network layer, which allows different networks to communicate with each other through routers. The most common thing to do this is called IP, or internet protocol. Network software is divided into client and server, with the client initializing a request for data, and the server software answering the client request. A single node might be running multiple client and server applications. The next layer is the transport layer, which sorts out which client and server programs are supposed to get the data. The datatype used in this layer is the Transport layer, the Transmission Control Protocol, often known as TCP. UDP is User Datagram Protocol, TCP provides mechanisms that data is reliably delivered, while UDP does not.  The transport layer makes sure that data gets to the right applications running on those nodes. The fifth layer is the application layer, which are protocols that allow you to browse the web, etc., and interact with them directly. 

The physical layer is the delivery truck. The data link layer is how the delivery goes from one intersection to the next. The network layer gets from address A to address B. The transport layer is when the delivery driver knocks to say that your package has arrived. The application layer is the contents of the package itself. 

Lots of different network devices can be used to allow computers to properly communicate with each other. 

Cables connect different devices, allowing data to communicate with them. Sending device communicates binary data across copper wires by changing the voltage between 2 ranges, and the system at the receiving end can interpret these as binary 1's and 0's and the most common forms of copper twisted-pair cables are Cat5, Cat5e, and Cat6 cables. Categories have different numbers of twists in wires, etc. Cat5 is older, but from the outside, they look about the same. The important thing to know is the different turns can determine how resistant these cables are to outside interference. Crosstalk is when an electrical pulse on one wire is accidentally detected on another wire, which can lead to errors. Higher quality specifications make it less likely that data needs to be retransmitted.  Cat6 is more strict, and can transfer data more reliable, but have a shorter maximum distance when used at a higher speed. Fiber cables have optical fibers that have tubes of glass, which use pulses of light to represent 1s and 0s and are used in environments with electromagnetic interference. Fiber can transport data over much longer distances than copper can. 

Cables allow you to form point-to-point network connections, with 2 devices. A hub is physical layer devices that allow connections from many computers at once, which will talk to many devices at once, and this creates a collision domain, a network segment where only one device can communicate at a time, which means systems have to wait for a quiet period before communicating and this is the primary reason why hubs are fairly rare. A much more common way is a network switch, which is very similar to a Hub, but the Switch is a Data Link Layer Device which it can actually inspect the contents of the ethernet cable, and determine which system the data is intended for and only send that data to that one system. This allows for higher overall throughput. They connect computers to a single network, known as a LAN, or Local Area Network.

A  router is a device that knows how to forward data between interdependent networks. A router operates at layer 3, the network layer. A router can inspect IP data to determine where to send things. The most common type of router is one for a home office or a smart box. It takes traffic from the home and forwards it to the ISP, or the internet service provider. A way more sophisticated router takes over once traffic is at the isp. Core ISP routers deal with much more complexities in making decisions on where to share traffic. Core routers have many different connections to many other routers, and routers share data with a Border Gateway Protocol or BGP, letting them learn about the most optimal paths to forward traffic. When you open a web browser, the traffic between computers and the servers can travel over many routers. Routers are responsible for getting traffic to the right places. 

A server is something that provides data to something requesting that data, and the thing receiving that data is referred to as a client. Almost all nodes are servers or clients at one time. In most networking topologies, all the nodes are primarily either a server or a client. The client's reason is to get the data from the servers so that the user at the computer can do their work. The server is anything that can provide data to a client, but we use the word provide data for primary nodes on the network.