Computer Networks

Overview

Time: 30 Minutes

Concepts: What is a network? Types of networks. Network hardware. Cabling and wireless. Network protocols and topologies. The OSI Model. Routing and data communication.


This tutorial is designed to give a beginner some basic information about computer networks. We will be discussing how to design computer networks, the components invovled in computer networks and how data is routed across networks.

Part 1. What is a computer network?

 

A computer network is created when two or more computers connect together to share files, or hardware like printers and scanners. A network allows computers to easily share resources and files. Networks can be both wired and wireless. Wired networks use special cabling and devices  or wireless devices to allow the two computer to communicate with each other. Without a network you would have to backup your files to media such as a disk, thumb drive or external hard drive and bring it to the other computer to be opened and read or printed. The simplest form of network is the peer to peer network or (P2P).  Peer to peer network can also traverse the internet in the form of peer to peer file sharing application. 

A Basic Peer to Peer Network

A Basic Peer to Peer Network

 

In the peer to peer network there aren’t any servers involved. A server is a large computer that manages the access and file sharing on a network. Instead in a peer to peer network, each computer has local files and resources which they can share directly with each other. To connect to each other there must be some sort of wired or wireless connection made. A peer to peer network can also have a shared internet connection and other devices can also be part of this peer to peer network such as tablets and mobile devices like smartphones.

A Peer to Peer Network with Shared Internet and Wi-Fi

 

If you take this same network and add a file server to the mix, locate the files and shared resources on the server, have the server handle logon information, file security and control who has access to which resources, you then have a client server network. In the client server network, the file server would act as the gatekeeper for access to files and printers.

A Basic Client Server Network

 

Having one or more servers allows an organization to store data and backup their data in a central location. The server or servers can share files and printers and control who has access to them. Servers can also host email and websites.Some small companies may have a single server or just two or three servers. Larger companies may have 10, 20 or even a hundred servers or more depending on their needs and requirements. 


Below is a design of a more advanced network of a larger corporation. You do not have to understand this just yet, but this is just an example to show you how complicated networks can get.

A Complex Client Server Network with Multiple Servers, DMZ and Internet

 

As you can see here, this is a much larger network and it is much more complex. We have multiple servers each with their own separate job. We have networks divided by routers. Routers are devices used to connect multiple networks. They can be used to create security between networks by allowing or disallowing traffic from one point to another. We have a firewall which controls what traffic is allowed to go to the internet and what traffic from the internet is allowed into the network. We have a “Demilitarized Zone” or DMZ for short. This is where we keep servers that people can access directly from the internet. This helps protect our internal network by keeping these servers on their own network outside of the corporate network. The servers can still be inside our server room, but because of the way they are wired they are virtually on the outside of our network.


 Local Area Network or LAN 


A local area network would be your local network in your house, school or office building where everything is local to you. The computers in your area, the servers in your building and your local printers are all part of your LAN. Local Area networks usually use local physical wiring to connect.


Metropolitan Area Network or MAN 

 

A Metropolitan area network would be your branch offices in your city or nearby towns or multiple local schools connected together sharing resources. These types of offices might use a leased point to point line or wireless antennas on top of the buildings to make their connections.


Wide Area Network or WAN 


Wide area networks can connect offices and resources in other cities, states or countries.These usually require leased lines or they may connect over Virtual Private Networks which are encrypted networks that travel across the internet. The encryption from sender to receiver helps to keep the data private while it passes over the internet.



Part 2. The OSI Model

 

The OSI (Open System Interconnection) model was developed by the International Standards Organization to lay a framework for how open systems should communicate. There is nothing here you can see or touch, but it is just for people who work in the industry to get an understanding of how data is passed from device to device and processed. Understanding this model can help in systems development and in troubleshooting networking issues. There are seven layers to the OSI model.


7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical


The Application layer is the layer in which the human interfaces with the computer system. Logging in, creating and saving files as well as file transfers all take place at this level. Email applications are also a part of this layer.


The Presentation layer transforms data into the form that the application accepts. This layer formats data to be sent across a network. It is sometimes called the syntax layer.This layer works to present data between applications in a format in which the application accepts the data.


The Session Layer establishes, manages and terminates connections between Applications.

 

The Transport Layer provides for the transparent transfer of data between the source and the destination hosts.


The Network Layer is the layer provides the functionality that connects data from one node to another on different networks. This is where routing takes place. Network routers work at this level. Some (Layer 3 ) Switches also work at this level. 


The Data Link layer connects two or more nodes on a network and also provide error correction. MAC (Medium Access Control)  and LLC (Logical Link Control) both work at this level. MAC is responsible for controlling how devices in a network gain access to a medium and permission to transmit data.LLC is responsible for identifying and encapsulating network layer protocols, and controls error checking and frame synchronization.

 

The Physical Layer is the layer that cabling and hubs work at. This is your Physical end to end connections without error correction. This is just the basic electrical, physical hardware connections.



Part 3. Network Hardware and Cabling

 Now that we have an idea of what computer networks look like, let's take a closer look at the hardware components of the computer network.


Network Interface Card - NIC:  

 

A network Interface card is the device that connect the end device (Computer, Laptop, tablet or smartphone) to a network. Tablets and Smartphones usually have internal integrated wireless network interface cards that you cannot see. Laptops also have internal integrated wireless cards, but can also have wired network cards that you can plug a network cable into. Desktop Computers or Personal computers and servers usually have a wired network card in them. Wired and wireless network cards both have their advantages and disadvantages. Wired networks require you to run wire to each location where you want to put a computer. This can sometimes mean running wires through walls and ceilings. This can be time consuming and expensive, but a wired connection is extremely reliable. Wireless networks on the other hand can be easier to deploy, but can lack the reliability of a wired network. Network cards are also known as Ethernet NICs. Ethernet defines the type of standard hub or switch used in local area networks. Wireless Ethernet is also known as Wi-Fi.


Below on the left you will see a photo of a USB wireless ethernet or (Wi-Fi) adapter. On the right is a standard ethernet adapater that is connected by an ethernet patch cable.

Hub:

 

A network hub is a device that allows the connection of multiple devices on a network. These can come in many sizes ranging from 4 ports up to around 24 ports and sometimes 48 ports. Some Hubs can be stacked to make larger hubs. There is no error correction built into hubs. They provide basic physical connectivity without error correction. They are inexpensive to purchase but due to their being prone to causing network collisions (this happens when too many data packets try to cross the same path) they lack the performance of smooth operation of the network switch. Hubs and switches look very much the same but they work differently. A hub broadcasts packets to everyone on the network until it reaches its recipient. A switch is quite a bit smarter than that which is why switches perform better.


Switch:


A switch is more intelligent than a hub and uses the Media Access Control address of each computer on the network. When a packet of data is sent from one computer to another a switch acts like a traffic cop and directs traffic to the exact recipient of the data packet avoiding the network collisions and poor performance we see on a network hub. Switches can be very small to very very large. Some switches called layer 3 switches can also route. They are called a layer 3 switch because they work at the same OSI layer as a router. Switches can also create separate networks call VLANs or Virtual Local Area Networks. It assigns certain numbers to certain ports letting the switch know that these devices can all talk to each other and these other devices are separate.

Switch

A 48 port HP Procurve Ethernet Switch

Router

A Cisco 1800 Series Router

Wireless Access Point

An Aruba Wireless Access Point mounting on the ceiling

Hardware Firewall

A Cisco ASA 5505 Firewall

Bridge:


A bridge can be used to connect two different Local Area Networks together. This usually works at the Layer 2 level of the OSI model. It uses MAC addresses like a layer two switch to decide where to send the data packets. 


 Router:


A router is a device that connect 2 or more networks together. It creates a routing table to know where to send data packets destined for a particular network. This works at the layer 3 level of the OSI model. Routers can even connect to networks that use different types of data connections. It can connect an Ethernet LAN to a Broadband connection over the internet. 


Modem:


Modem stands for Modulator Demodulator. It is a network device that translates a carrier signal to a digital signal your computer can understand. In the past modems were used to send data over voice lines. This was very slow. With the invention of DSL we can now send digital signals over those same lines at much higher speeds using DSL modems.


Wireless Router:


A Wireless Ethernet router is a device that allows the user to create a local area network using wireless network cards and then connect to the internet. The router can accept both wireless and wired connections and then connect to a DSL or broadband cable network. 


Wireless Access Point:


A wireless access point looks like a wireless router but usually can only connect wireless devices to the local area network. This does not mean that you cannot connect to the wireless access point and not have internet access. It just means that the wireless access point does not connect directly to the internet but must forward your packets to another router that connects you to the internet. These are usually used in medium to large organizations that need wireless access points spread over a large area to allow many devices in many locations access to the network over a wireless or Wi-Fi connection. 


Firewall:


A firewall is a hardware device or software that controls access between the local area network and the internet. Some routers also have firewall capabilities. Firewalls usually have a little more advanced technology to help filter traffic and inspect data packets than a standard router. A Firewall can have much more control over traffic than a traditional router. They are used to stop unwanted traffic from crossing over your network. A firewall such as the one pictured earlier can also break networks up into Virtual Lans like a switch and then give those networks each a number based on how much that network is trusted. The internal network would be the trusted network, the external Internet would be untrusted and then web and email gateway servers would be on a medium trusted network called a DMZ (Demilitarized Zone). The DMZ is where you would put your servers and devices that allow connections from the Internet. This protects your internal network by keeping that untrusted traffic on a different network.

Network Cabling


Ethernet cables - Cat 5, Cat 5E, Cat 6 and Cat 6 A: 


Ethernet cable is made up of twisted pairs of copper wire covered by shielding. In side the cable are 4 pairs of wire twisted together which is why ethernet cable is also known as twisted pair. There are various classes of cabling based on the data speed you need it to handle. Ethernet cable can only be run in lengths of around 100 meters or about 328 feet before it needs to be terminated or connected to another device. You can plug the cable into a switch or a hub and then run another cable for another 100 meters. This is important to know when you are planning your network. Ethernet cabling like we stated earlier comes in different classes known as categories or cat for short. Cat 5 cabling can handle network speeds of up to 100 megabits per second. Cat 5 E (the e means Enhanced) can handle up to 1 gigabit per second. Cat 6 and Cat 6 A can handle up to 10 gigabits per second so it is ten times faster than Cat 5 E. Cat 6 A (the A is for augmented) is designed to handle interference better than standard Cat 6. The reason that we need to keep the cable length to 100 meters or less is because of something called Attenuation. As the data moves over the wire the signal degrades after a certain distance. In order to maintain optimal performance you want to make sure you do not run your ethernet cables longer than the recommended distance. Also be careful to keep your wires away from other devices or electronics that may create interference which can also degrade your signal. 


Fiber Optic Cabling:


Fiber optic cabling is designed to carry beams of light at high speed over long distances. It is much more expensive than ethernet. Like ethernet there are different grades of cable for different uses. Two types of fiber cables are called single mode and multi-mode fiber. Single mode fiber uses very thin glass strands and a laser to generate light while multi-mode fibers use LEDs. Different fiber cables are rated at 10 gigabits per second, 40 gigabits per second and 100 gigabits per second.Data can also travel much farther over a fiber optic cable than ethernet because light does not degrade the way and electrical signal does. Some fiber can be run up to 80 or 100 kilometers. That is about 50 to 60 miles. On each end of the fiber cable is a fiber transceiver that takes the electrical signal and converts it to light pulses and when data it received it take the light pulses and converts it back to an electrical signal. 

 

When buying and installing fiber optic cabling it is important to check with your vendor that you get cable and transceivers with the correct specifications so that they work together.



.

A standard ethernet patch cable

This is what a standard ethernet patch cable looks like.

Inside an ethernet patch cable

 Inside of an ethernet cable you can see the strands of twisted pairs. There are 4 pairs of wire, 8 wires in all.

RJ 45 ethernet connector

 At the end of an ethernet patch cable is a connector called an RJ45. This looks like the end of a telephone cable but it is much larger. A phone cable uses and RJ11 or RJ12 connector which is much smaller. 

Fiber Optic Cable

 A standard fiber optic patch cable 

Fiber Optic Connectors

The ends on a fiber cable are much different than an ethernet cable.  

Fiber Tranceivers

 

Here are two different types of fiber transceivers. The one on the left is designed to convert fiber over to ethernet. As you can see the fiber data ports are covered with a protective cover and you can clearly see the ethernet port. The reason fiber ports are always covered is because finer optics and fiber components being made of stands of glass are very fragile.The transceiver on the right is another kind of transceiver designed to plug into a special port on a switch with special fiber optic ports. There is no need for an ethernet port because it plugs directly into the switch.

Part 4. Network Topology

If customers can’t find it, it doesn’t exist. Clearly list and describe the services you offer. Also, be sure to showcase a premium service.


Network topology it the layout of the network. Some are more common than others. Some were used in the early days of computer networking when coaxial cable was used. It is very rare to see coaxial cable being used for anything other than your cable box connection to your cable company. These days almost all wiring is either ethernet or fiber.


Here are some standard topologies.


Point to point Topology


A point to point topology is just a simple network with a cable run from one point to another.There are no other connections except for the two end point. This is commonly used when two networks are connected together between two branch offices. 



Bus Topology

A bus topology is a single strand of cable known as the backbone and off of that cable are many devices that all connect to the backbone.

Star Topology

 A star topology is very common especially in ethernet networks. Think of the hub or switch in the center and the cables run out to each endpoint.

Ring Topology

A ring topology is like a closed loop and packets circle around the ring.In some rings traffic can pass in both directions in the event the ring is broken in one place.  

Mesh Topology

In a mesh topology there are many redundant connections and data can travel over any path. This allows the data to reach its destination even when there is a break in one or more data connections. 

Part 5. Network Protocols

A network protocol is basically an agreement between two end devices on how they are going to communicate with each other. Both ends need to be configured to use the same protocol. When two people talk to each other in the same language such as English, both parties understand each other. If one party responds in Portuguese and the other part does not know Portuguese then there is a breakdown in communication and data is not transferred.


TCP/IP is probably the most common protocol.It stands for Transmission Control Protocol / Internet Protocol. TCP/IP is a routable protocol. This means that a router can use this protocol to send data from one network to another over routed connections. This is why it is used over the internet. The internet is basically just a bunch of different networks all connected together using routers. 


NetBeui (pronounced net-boo-ee) is a non routable protocol. It can be used on a local area network only. It was designed to be a quick easy way to get computers on a LAN to see and communicate with each other.


We will learn more about TCP/IP in our TCP/IP tutorial. We will learn about other network protocols, routing, switching and firewalls in our more advanced networking class. 

See our class Schedule!