# Introduction to Networking
## Definition of network

Providing an ability in a specified place and time, is called "Network". In better conditions, they provide an ability everywhere and at all times and its ideals:
* provide " everything " in " everywhere " and " every time ".
The abilities of water networks, electric networks, road networks, communication networks and treatment networks respectively are "water", "electricity", "relocation", "sound", "drug" etc. In computer science this ability in computer networks is data. The locations access to the ability in network is called a "Node".Collection of devices so that the information can be exchanged efficiently.

## What is a network?

The concept of a network is pretty simple. A couple of computers have some cables strung between them, and send data back and forth using electrical signaling over the cable. More or less the same as telephones do or, in a very rough sense, like two kids speaking into tin cans connected by a string.
But how does the data actually get from computer A to computer B? How does computer A find the physical location of computer B on the network? If they communicate with electrical signaling, so the data is traveling "at the speed of light", why does it take so long to send a big file across the network?
Gaining a scientific understanding of the answers to these questions may take years, but we shall endeavor to crack them open at the surface, and explore the definition of network in a way that makes as much sense as two kids speaking into tin cans connected by a string. In real life, networks are not simply two cans tied together by a single string, they are millions and millions of cans (technically called hosts) interconnected with many strings.
* A host is an end system connected to a network

## Servers and Clients

In networking there are always two computers in the connection. The server and the client. The server listens and waits for clients to connect to it and the client connects to the server. To use the above example the server is the computer that holds the web site and the client is your web browser. So when you type in http://www.google.com, your web browser connects to the server at http://www.google.com and they start communicating.
* A client-server protocol is one in which an always-on host (the server) listens for connections from other hosts (clients). When a connection is established, data may be transferred between the client and the host.

## Protocols

A protocol is the language that computers use to talk to each other. If two computers want to talk to each other they need to know the same language. If they don't know the same language they can't talk to each other. For example if someone who speaks Chinese and doesn't know a word of English it won't be all that easy to communicate with an English speaking person. It will be nearly impossible. The protocol used in Web Browsers and Web Servers is the HTTP protocol.
* A protocol is an established set of rules that dictate the method of communication between 2 hosts.
* Examples include HTTP, FTP, TCP, PPP

## Network of Networks

The internet is not one single being, it is a massive interconnection of hosts such as your computer, and another halfway across the world. We understand that our ISPs provide internet access to our host, but it may not necessarily provide service to the server that you are communicating with. In this case, how does information make its way from your host to the other host and vice versa?
The answer lies in the interconnection of many ISPs themselves. ISPs can be categorised into Tier 1,2 and 3 ISPs. Tier 1 ISPs are major internet service providers that usually sell access to smaller Tier 2 ISPs. Tier 2 ISPs may service entire countries or cities, but not the rest of the world. Tier 3 ISPs are also customers of Tier 2 ISPs, and usually service end users such as yourself. ISPs peer with each other to allow data from your host to reach the other host. From this, we understand that data passes through multiple ISPs in order to be delivered from one location to another.

## Getting connected

Connecting to the internet is done via a modem. However, the speed at which you connect to the internet can be influenced by the type of modem you are using, as well as the media that is used to transfer data over. This section will give you a general overview of the technologies used to send data over the internet.

### Dial Up Modem

* Quick Facts
* Transmission Media:Copper
* Maximum Theoretical Speed: 56 kb/s
A dial up modem uses a normal telephone line, and literally dials up a phone connection to the telephone providers office. Once a connection is established, credentials are exchanged and an internet connection is made available. The connection is not always on and the telephone line cannot make or receive calls while the connection is established.
Dial up modems were one of the earliest forms of access to the internet, and while it has been largely overtaken by more modern connection methods, dial-up modems are still essential in remote areas where DSL or Fiber has not been rolled out.

### DSL

* Quick Facts
* Transmission Media:Copper
* Maximum Theoretical Speed: 24 Mb/s
DSL stands for Digital Subscriber Line. Similar to Dial-Up it uses existing telephony infrastructure, however, it allows for greater speeds and the ability to use both the internet and the telephone at the same time (after installing a line filter). In households, this form of internet is best known as ADSL (Asymmetric Digital Subscriber Line). The reason behind the use of "asymmetric" is because upload speeds are not the same as download speeds.
Over the years ADSL has grown into ADSL2 and ADSL2+, bringing greater and greater speeds to end users. Your actual download speed may be influenced by how close you are to the telephone exchange. ADSL can be considered to be a form of "always-on" internet, since you can leave your modem running all day and still use the phone.

### Cable

* Quick Facts
* Transmission Media:Coaxial Cables
* Maximum Theoretical Speed: 50 Mb/s
This form of connectivity does not use existing copper telephone infrastructure, rather it uses cable TV infrastructure. Internet access is provided by reserving specific channels in the Coaxial Cable for delivering data. Cable internet is shared, meaning that all connections lead in a central router which then leads into the ISP.

### Fiber to the Home

* Quick Facts
* Transmission Media:Fiber
* Maximum Theoretical Speed:1.6 Tb/s(tera bits/sec)[approx] can sometimes be even more than that.

## Switching Methods

In order to transmit data from one end to another, a common standard must be adopted for the transmission of data frames. Here we will consider Packet Switching and Circuit Switching.

### Circuit Switching

The concept of circuit switching works very much like common telephone networks today. To establish a data connection from point A to point Z, a person must work out a direct path over a number of connection routes to the destination. Once a route has been determined, the person needs to set aside resources on that line to establish his connection, after which he may start transmitting data. While resources have been allocated for that connection, no one else may use that line until the first user has disconnected his host. This raises some questions as to how people can share a circuit switched connection, 2 of methods of which are outlined below.
One of the cons of a Circuit Switched network is that it relies on a user reserving and allocating resources for himself in order to use the network. This adds to connection time and can form significant overhead in establishing connections. In the event the user decides to hop off and get a coffee without disconnecting, he will not have released the resources reserved for use by others.

#### Time Division

The circuit is divided into time slots, each of which are allocated to users wanting to use the network. In each time slot available to a user, the user has the full amount of bandwidth provided by the circuit. The more people using the circuit, the more time it takes for each frame to be passed from destination to receiver.

#### Frequency Division

The circuit is divided into a number of frequencies, each of which are allocated to a user. The user has full control of that connection, only to be limited by the bandwidth his frequency provides. Naturally the more people utilising the circuit, the less bandwidth each person has.

### Packet Switching

This form of switching works on the concept of data being sent as discrete chunks of data known as packets. When a person has data to send, a stream of packets are sent into the network, and routers forward the packets to their destination. There is no notion of reserved bandwidth, and as a result if too many packets are sent into the network, the network may become congested and packet loss will occur. This leads to the problem of congestion control.
The main idea behind packet switching networks is that nobody is using all the bandwidth of a connection at any one time, and therefore cuts out the waste of bandwidth that usually accompanies Circuit Switched networks. This results in greater efficiency, and is the main reason why packet switched networks form the backbone of today's computer networks.

## Delay

The transmission of information from one host to another is not instantaneous. The amount of time it takes for data to be transmitted from one end to another is dependent on a number of factors, namely
* Nodal Processing
* Queuing
* Transmission Delay
* Propagation Delay

### Nodal Processing

When a packet arrives at a router (the box that routes traffic to and from your computer), it doesn't get sent immediately to its destination. Many things happen within a router, which you will study in greater detail in the network and link layers. Nodal Processing is the time it takes for a router to perform error checking, forwarding table lookups and output operations. All these incur a small time cost which can add overhead to the transmission of data.

### Queuing

Once a packet has been processed, it needs to be queued and sent. Packets are sent depending on several factors, including the level of congestion in the network and flow control

### Transmission Delay

Transmission delay is time it takes for a data packet to be sent, given its size and the bandwidth of the link over which it is being sent.

### Propagation Delay

While negligible over very short distance, the physical propagation of data over cables can contribute to the overall delay in data transmission. For example the speed of transmission over a particular medium could be 2x10$^8$ m/s. While this appears to be a very fast speed, it is not instantaneous. Over very long distances (inter-continental), the effect of propagation delay can be seen. Note that we haven't considered the possibility of minor line faults or interference which may cause bottlenecks in the transmission medium.

## See Also

* Network Fundamentals
* Computer Networks

## External Resources
### Websites

* Basic Networking Concepts on About.com
* Networking guids on comptechdoc.org

### Books

How Networks Work
Custodial note: Please help expand this list.