Basics of Computer Networking

Computer Network:
It is the interconnection of multiple devices, generally termed as Hosts connected using multiple paths for the purpose of sending/receiving data or media.
There are also multiple devices or mediums which help in the communication between two different devices which are known as Network devices. Ex: Router, Switch, Hub, Bridge.

The layout pattern using which devices are interconnected is called as network topology. Such as Bus, Star, Mesh, Ring, Daisy chain.

OSI:
OSI stands for Open Systems Interconnection. It is a reference model that specifies standards for communications protocols and also the functionalities of each layer.

Protocol:
The protocol is the set of rules or algorithms which define the way how two entities can communicate across the network and there exists different protocol defined at each layer of the OSI model. Few of such protocols are TCP, IP, UDP, ARP, DHCP, FTP and so on.

UNIQUE IDENTIFIERS OF NETWORK
Host name:
Each device in the network is associated with a unique device name known as Hostname.
Type “hostname” in the command prompt and press ‘Enter’, this displays the hostname of your machine.

IP Address (Internet Protocol Address):
Also, know as the Logical Address, is the network address of the system across the network.
To identify each device in the world-wide-web, Internet Assigned Numbers Authority (IANA) assigns IPV4 (Version 4) address as a unique identifier for each device on the Internet.
The length of the IP address is 32-bits. (Hence we have 232 IP addresses available.)
Type “ipconfig” in the command prompt and press ‘Enter’, this gives us the IP address of the device.

MAC Address (Media Access Control address):
Also known as physical address, is the unique identifier of each host and is associated with the NIC (Network Interface Card).
MAC address is assigned to the NIC at the time of manufacturing.
Length of the MAC address is : 12-digit/ 6 bytes/ 48 bits
Type “ipconfig/all” in the command prompt and press ‘Enter’, this gives us the MAC address.

Port:
Port can be referred to as a logical channel through which data can be sent/received to an application. Any host may have multiple applications running, and each of these applications is identified using the port number on which they are running.
Port number is a 16-bit integer, hence we have 216 ports available which are categorized as shown below:

Well known Ports 0–1023

Registered Ports 1024–49151

Ephemeral Ports 49152–65535

Number of ports: 65,536
Range: 0–65535
Type “netstat -a” in the command prompt and press ‘Enter’, this lists all the ports being used.

Socket:
The unique combination of IP address and Port number together are termed as Socket.

Few more concepts
DNS Server:
DNS stands for Domain Name System.
DNS is basically a server that translates web addresses or URL (ex: www.google.com) into their corresponding IP addresses. We don’t have to remember all the IP addresses of each and every website.
The command ‘nslookup’ gives you the IP address of the domain you are looking for. This also provides the information of our DNS Server.

ARP:
ARP stands for Address Resolution Protocol.
It is used to convert the IP address to its corresponding Physical Address(i.e.MAC Address).
ARP is used by the Data Link Layer to identify the MAC address of the Receiver’s machine.
RARP:
RARP stands for Reverse Address Resolution Protocol.
As the name suggests, it provides the IP address of the device given a physical address as input. But RARP has become obsolete since the time DHCP has come into the picture.

The Internet:
In simplest words, it is a global network of smaller networks interconnected using communication protocols that are standardized. The Internet standards describe a framework known as the Internet protocol suite. This model divides methods into a layered system of protocols.

These layers are as follows:

1. Application layer (highest) — concerned with the data(URL, type, etc), where HTTP, HTTPS, etc comes in.
2. Transport layer — responsible for end-to-end communication over a network.
3. Network layer — provides a data route.

The World Wide Web:
The web is a subset of the internet. It’s a system of Internet servers that support specially formatted documents. The documents are formatted in a markup language called HTML(that supports links, multimedia, etc). These documents are interlinked using hypertext links and are accessible via the Internet.

To link hypertext to the Internet, we need:

1. The markup language, i.e., HTML.
2. The transfer protocol, e.g., HTTP.
3. Uniform Resource Locator (URL), the address of the resource.

We access the web using web browsers.

Difference between Web and Internet:

URI:
URI stands for ‘Uniform Resource Identifier’, it’s like an address providing a unique global identifier to a resource on the Web. Uniform Resource Locator (URL) is the most commonly used form of a URI.

The URL consists mainly of two parts:

1. The protocol used in the transfer, e.g., HTTP.
2. The domain name.

Who governs the Internet?
The Internet is not governed, it has no single authority figure. The ultimate authority for where the Internet is going rests with the Internet Society, or ISOC.
ISOC is a voluntary membership organization whose purpose is to promote global information exchange through Internet technology.

• ISOC appoints the IAB- Internet Architecture Board. They meet regularly to review standards and allocate resources, like addresses.
• IETF- Internet Engineering Task Force. Another volunteer organization that meets regularly to discuss operational and technical problems.

Some interesting facts about computer networking are:

• The Internet was invented by Tim Berners-Lee in 1989.
• The Internet is controlled by 75 million servers.
• The backbone of the internet is made by 550, 000 miles of underwater cable.
• About one billion computer systems are connected to the internet.
• About 3.2 billion people use the internet from which 1.7 billion internet users are Asians.
• The Internet consists of five billion computing devices such as computers, phones, modems, switches, routers, etc.
• According to Google, the Internet consists of 5 million Terabytes of data.
• If the internet goes down for a day, approximately 200 billion emails and 3 billion Google searches would have to wait.
• Approximately 204 million emails per minute are sending over the Internet. 70% of them are spam.
• 269 billion emails are sent per day.
• 30, 000 websites are hacked every day.
• 50 million horsepower is required by the internet to keep running in the current state.
• Approximately 9 million adults in Britain and 1/3rd of Italians have never used the internet while China has treatment camps for internet addicts.
• Microsoft has more servers than Google. Microsoft owns over one million servers while Google has 900, 000.
• Internet bots and malware generate 61.5% or nearly two-thirds of all the website traffic.
• Approximately 1.7 trillion eurodollar worth of funds is spent online.
• Tim Berners-Lee was knighted by Queen Elizabeth.
• An email takes around 2 billion electrons to produce.
• Online dating generates approximately $1 billion dollars every year.
• Qwerty was designed to slow you down so that keys would not jam.

Computer networks mean an interconnection of autonomous (standalone) computers for information exchange. The connecting media could be a copper wire, optical fiber, microwave or satellite.

Networking Elements — The computer network includes the following networking elements:

1. At least two computers
2. Transmission medium either wired or wireless
3. Protocols or rules that govern the communication
4. Network software such as Network Operating System

Network Criteria:
The criteria that have to be met by a computer network are:

1. Performance — It is measured in terms of transit time and response time.

• Transit time is the time for a message to travel from one device to another
• Response time is the elapsed time between an inquiry and a response.

Performance is dependent on the following factors:

• The number of users
• Type of transmission medium
• A capability of connected network
• Efficiency of software

2. Reliability — It is measured in terms of

• Frequency of failure
• Recovery from failures
• Robustness during catastrophe

3. Security — It means protecting data from unauthorized access.

Goals of Computer Networks: The following are some important goals of computer networks:

1. Resource Sharing –
   Many organization has a substantial number of computers in operations, which are located apart. Ex. A group of office workers can share a common printer, fax, modem, scanner, etc.
2. High Reliability –
   If there are alternate sources of supply, all files could be replicated on two or, machines. If one of them is not available, due to hardware failure, the other copies could be used.
3. Inter-process Communication –
   Network users, located geographically apart, may converse in an interactive session through the network. In order to permit this, the network must provide almost error-free communications.
4. Flexible access –
   Files can be accessed from any computer in the network. The project can be begun on one computer and finished on another.
5. Other goals include Distribution of processing functions, Centralized management, and allocation of network resources, Compatibility of dissimilar equipment and software, Good network performance, Scalability, Saving money, Access to remote information, Person to person communication, etc.

A network is two or more devices connected through a link. A link is a communication pathway that transfer data from one device to another. Devices can be a computer, printer or any other device that is capable to send and receive data. For visualization purpose, imagine any link as a line drawn between two points.
For communication to occur, two devices must be connected in some way to the same link at the same time. There are two possible types of connections:

1. Point-to-Point Connection
2. Multipoint Connection

Point-to-Point Connection :

1. A point-to-point connection provides a dedicated link between two devices.
2. The entire capacity of the link is reserved for transmission between those two devices.
3. Most point-to-point connections use a actual length of wire or cable to connect the two end, but other options such as microwave or satellite links are also possible.
4. Point to point network topology is considered to be one of the easiest and most conventional network
   topologies.
5. It is also the simplest to establish and understand.

Example: Point-to-Point connection between a remote control and Television for changing the channels.

Multipoint Connection :

1. It is also called the Multidrop configuration. In this connection, two or more devices share a single link.
2. More than two devices share the link that is the capacity of the channel is shared now. With shared capacity, there can be two possibilities in a Multipoint Line configuration:

Spatial Sharing: If several devices can share the link simultaneously, its called Spatially shared line configuration.
Temporal (Time) Sharing: If users must take turns using the link, then its called Temporally shared or Time Shared Line configuration.

Transmission mode means transferring of data between two devices. It is also known as the communication mode. Buses and networks are designed to allow communication to occur between individual devices that are interconnected. There are three types of transmission mode:-

• Simplex Mode
• Half-Duplex Mode
• Full-Duplex Mode

Simplex Mode
In Simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit, the other can only receive. The simplex mode can use the entire capacity of the channel to send data in one direction.
Example: Keyboard and traditional monitors. The keyboard can only introduce input, the monitor can only give the output.

Half-Duplex Mode
In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa. The half-duplex mode is used in cases where there is no need for communication in both directions at the same time. The entire capacity of the channel can be utilized in each direction.
Example: Walkie- talkie in which message is sent one at a time and messages are sent in both the directions.

Full-Duplex Mode
In full-duplex mode, both stations can transmit and receive simultaneously. In full_duplex mode, signals going in one direction share the capacity of the link with signals going in other direction, this sharing can occur in two ways:

• Either the link must contain two physically separate transmission paths, one for sending and others for receiving.
• Or the capacity is divided between signals traveling in both directions.

Full-duplex mode is used when communication in both directions is required all the time. The capacity of the channel, however, must be divided between the two directions.
Example: Telephone Network in which there is communication between two persons by a telephone line, through which both can talk and listen at the same time.

In data communication terminology, a transmission medium is a physical path between the transmitter and the receiver i.e it is the channel through which data is sent from one place to another. Transmission Media is broadly classified into the following types:

1. Guided Media:
It is also referred to as Wired or Bounded transmission media. Signals being transmitted are directed and confined in a narrow pathway by using physical links.

Features:

• High Speed
• Secure
• Used for comparatively shorter distances

There are 3 major types of Guided Media:

(i) Twisted Pair Cable –
It consists of 2 separately insulated conductor wires wound about each other. Generally, several such pairs are bundled together in a protective sheath. They are the most widely used Transmission Media. Twisted Pair is of two types:

1.Unshielded Twisted Pair (UTP):
This type of cable has the ability to block interference and does not depend on a physical shield for this purpose. It is used for telephonic applications.

Advantages:

• Least expensive
• Easy to install
• High-speed capacity

Disadvantages:

• Susceptible to external interference
• Lower capacity and performance in comparison to STP
• Short distance transmission due to attenuation

2. Shielded Twisted Pair (STP):
This type of cable consists of a special jacket to block external interference. It is used in fast-data-rate Ethernet and in voice and data channels of telephone lines.

Advantages:

• Better performance at a higher data rate in comparison to UTP
• Eliminates crosstalk
• Comparatively faster

Disadvantages:

• Comparatively difficult to install and manufacture
• More expensive
• Bulky

(ii) Coaxial Cable –
It has an outer plastic covering containing 2 parallel conductors each having a separate insulated protection cover. Coaxial cable transmits information in two modes: Baseband mode(dedicated cable bandwidth) and Broadband mode(cable bandwidth is split into separate ranges). Cable TVs and analog television networks widely use Coaxial cables.

Advantages:

• High Bandwidth
• Better noise Immunity
• Easy to install and expand
• Inexpensive

Disadvantages:

• Single cable failure can disrupt the entire network

(iii) Optical Fibre Cable –
It uses the concept of reflection of light through a core made up of glass or plastic. The core is surrounded by a less dense glass or plastic covering called the cladding. It is used for transmission of large volumes of data.

Advantages:

• Increased capacity and bandwidth
• Lightweight
• Less signal attenuation

Disadvantages:

• Difficult to install and maintain
• High cost
• Fragile

2. Unguided Media:
It is also referred to as Wireless or Unbounded transmission media.No physical medium is required for the transmission of electromagnetic signals.

Features:

• A signal is broadcasted through the air
• Less Secure
• Used for larger distances

There are 3 major types of Unguided Media:

(i) Radiowaves –
These are easy to generate and can penetrate through buildings. The sending and receiving antennas need not be aligned. Frequency Range:3KHz — 1GHz. AM and FM radios and cordless phones use Radiowaves for transmission.

Further Categorized as (i) Terrestrial and (ii) Satellite.

(ii) Microwaves –
It is a line of sight transmission i.e. the sending and receiving antennas need to be properly aligned with each other. The distance covered by the signal is directly proportional to the height of the antenna. Frequency Range:1GHz — 300GHz. These are majorly used for mobile phone communication and television distribution.

(iii) Infrared –
Infrared waves are used for very short distance communication. They cannot penetrate through obstacles. This prevents interference between systems. Frequency Range:300GHz — 400THz. It is used in TV remotes, wireless mouse, keyboard, printer, etc.

The cast term here signifies some data(stream of packets) is being transmitted to the recipient(s) from the client(s) side over the communication channel that helps them to communicate. Let’s see some of the “cast” concepts that are prevailing in the computer networks field.

1. Unicast –

This type of information transfer is useful when there is a participation of a single sender and a single recipient. So, in short, you can term it as a one-to-one transmission. For example, a device having IP address 10.1.2.0 in a network wants to send the traffic stream(data packets) to the device with IP address 20.12.4.2 in the other network, then unicast comes into the picture. This is the most common form of data transfer over the networks.

2. Broadcast –

Broadcasting transfer (one-to-all) techniques can be classified into two types :

• Direct Broadcasting –
  This is useful when a device in one network wants to transfer packet stream to all the devices over the other network. This is achieved by translating all the Host ID part bits of the destination address to 1, referred to as Direct Broadcast Address in the datagram header for information transfer.

This mode is mainly utilized by television networks for video and audio distribution.
One important protocol of this class in Computer Networks is Address Resolution Protocol (ARP) that is used for resolving the IP address into a physical address which is necessary for underlying communication.

3. Multicast –

In multicasting, one/more senders and one/more recipients participate in data transfer traffic. In this method traffic recline between the boundaries of unicast (one-to-one) and broadcast (one-to-all). Multicast lets the server’s direct single copies of data streams that are then simulated and routed to hosts that request it. IP multicast requires the support of some other protocols like IGMP (Internet Group Management Protocol), Multicast routing for its working. Also in Classful IP addressing Class D is reserved for multicast groups.

The arrangement of a network which comprises of nodes and connecting lines via the sender and receiver is referred as network topology. The various network topologies are :

a) Mesh Topology :

In a mesh topology, every device is connected to another device via the particular channel.

Figure 1: Every device is connected with another via dedicated channels. These channels are known as links.

• If suppose, N number of devices are connected with each other in mesh topology, then total number of ports that is required by each device is ​ N-1. In the Figure 1, there are 5 devices connected to each other, hence total number of ports required is 4.
• If suppose, N number of devices are connected with each other in mesh topology, then total number of dedicated links required to connect them is NC2 i.e. N(N-1)/2. In the Figure 1, there are 5 devices connected to each other, hence total number of links required is 5*4/2 = 10.

Advantages of this topology :

• It is robust.
• A fault is diagnosed easily. Data is reliable because data is transferred among the devices through dedicated channels or links.
• Provides security and privacy.

Problems with this topology :

• Installation and configuration are difficult.
• Cost of cables is high as bulk wiring is required, hence suitable for less number of devices.
• Cost of maintenance is high.

b) Star Topology :

​ In the star topology, all the devices are connected to a single hub through a cable. This hub is the central node and all others nodes are connected to the central node. The hub can be passive ​in nature i.e., not intelligent hub such as broadcasting devices, at the same time the hub can be intelligent known as active ​hubs. Active hubs have repeaters in them.

Figure 2: A star topology having four systems connected to the single point of connection i.e. hub.

Advantages of this topology :

• If N devices are connected to each other in the star topology, then the number of cables required to connect them is N. So, it is easy to set up.
• Each device requires only 1 port i.e. to connect to the hub.

Problems with this topology :

• If the concentrator (hub) on which the whole topology relies fails, the whole system will crash down.
• Cost of installation is high.
• Performance is based on the single concentrator i.e. hub.

c) Bus Topology :

​ Bus topology is a network type in which every computer and network device is connected to a single cable. It transmits the data from one end to another in the single direction. No bi-directional feature is in the bus topology.

Figure 3: A bus topology with a shared backbone cable. The nodes are connected to the channel via drop lines.

Advantages of this topology :

• If N devices are connected to each other in a bus topology, then the number of cables required to connect them is 1 ​which is known as backbone cable and N drop lines are required.
• Cost of the cable is less as compared to other topology, but it is used to build small networks.

Problems with this topology :

• If the common cable fails, then the whole system will crash down.
• If the network traffic is heavy, it increases collisions in the network. To avoid this, various protocols are used in the MAC layer known as Pure Aloha, Slotted Aloha, CSMA/CD etc.

d) Ring Topology :

​ In this topology, it forms a ring connecting devices with its exactly two neighboring devices.

Figure 4: A ring topology comprises 4 stations connected with each forming a ring.

The following operations take place in ring topology are :

1. One station is known as a monitor station which takes all the responsibility to perform the operations.
2. To transmit the data, a station has to hold the token. After the transmission is done, the token is to be released for other stations to use.
3. When no station is transmitting the data, then the token will circulate in the ring.
4. There are two types of token release techniques: Early token release releases the token just after the transmitting the data and Delay token release releases the token after the acknowledgment is received from the receiver.

Advantages of this topology :

• The possibility of collision is minimum in this type of topology.
• Cheap to install and expand.

Problems with this topology :

• Troubleshooting is difficult in this topology.
• Addition of stations in between or removal of stations can disturb the whole topology.

e) Hybrid Topology :

​This topology is a collection of two or more topologies which are described above. This is a scalable topology which can be expanded easily. It is a reliable one but at the same, it is a costly topology.

Figure 5: A hybrid topology which is a combination of ring and star topology.

The Network allows computers to connect and communicate with different computers via any medium. LAN, MAN, and WAN are the three major types of the network designed to operate over the area they cover. There are some similarities and dissimilarities between them. One of the major differences is the geographical area they cover, i.e. LAN covers the smallest area; MAN covers an area larger than LAN and WAN comprises the largest of all.
There are other types of Computer Networks also, like :

• PAN (Personal Area Network)
• SAN (Storage Area Network)
• EPN (Enterprise Private Network)
• VPN (Virtual Private Network)

Local Area Network (LAN) –
LAN or Local Area Network connects network devices in such a way that personal computer and workstations can share data, tools, and programs. The group of computers and devices are connected together by a switch, or stack of switches, using a private addressing scheme as defined by the TCP/IP protocol. Private addresses are unique in relation to other computers on the local network. Routers are found at the boundary of a LAN, connecting them to the larger WAN.

Data transmits at a very fast rate as the number of computers linked are limited. By definition, the connections must be high speed and relatively inexpensive hardware (Such as hubs, network adapters, and Ethernet cables). LANs cover smaller geographical area (Size is limited to a few kilometers) and are privately owned. One can use it for an office building, home, hospital, schools, etc. LAN is easy to design and maintain. A Communication medium used for LAN has twisted pair cables and coaxial cables. It covers a short distance, and so the error and noise are minimized.

Early LAN’s had data rates in the 4 to 16 Mbps range. Today, speeds are normally 100 or 1000 Mbps. Propagation delay is very short in a LAN. The smallest LAN may only use two computers, while larger LANs can accommodate thousands of computers. A LAN typically relies mostly on wired connections for increased speed and security, but wireless connections can also be part of a LAN. The fault tolerance of a LAN is more and there is less congestion in this network. For example A bunch of students playing Counter-Strike in the same room (without internet).

Metropolitan Area Network (MAN) –
MAN or Metropolitan area Network covers a larger area than that of a LAN and smaller area as compared to WAN. It connects two or more computers that are apart but resides in the same or different cities. It covers a large geographical area and may serve as an ISP (Internet Service Provider). MAN is designed for customers who need high-speed connectivity. Speeds of MAN range in terms of Mbps. It’s hard to design and maintain a Metropolitan Area Network.

The fault tolerance of a MAN is less and also there is more congestion in the network. It is costly and may or may not be owned by a single organization. The data transfer rate and the propagation delay of MAN are moderate. Devices used for transmission of data through MAN are Modem and Wire/Cable. Examples of a MAN are the part of the telephone company network that can provide a high-speed DSL line to the customer or the cable TV network in a city.

Wide Area Network (WAN) –
WAN or Wide Area Network is a computer network that extends over a large geographical area, although it might be confined within the bounds of a state or country. A WAN could be a connection of LAN connecting to other LAN’s via telephone lines and radio waves and may be limited to an enterprise (a corporation or an organization) or accessible to the public. The technology is high speed and relatively expensive.

There are two types of WAN: Switched WAN and Point-to-Point WAN. WAN is difficult to design and maintain. Similar to a MAN, the fault tolerance of a WAN is less and there is more congestion in the network. A Communication medium used for WAN is PSTN or Satellite Link. Due to long distance transmission, the noise and error tend to be more in WAN.

WAN’s data rate is slow about a 10th LAN’s speed since it involves increased distance and increased number of servers and terminals etc. Speeds of WAN ranges from few kilobits per second (Kbps) to megabits per second (Mbps). Propagation delay is one of the biggest problems faced here. Devices used for transmission of data through WAN are Optic wires, Microwaves, and Satellites. Example of a Switched WAN is the asynchronous transfer mode (ATM) network and Point-to-Point WAN is the dial-up line that connects a home computer to the Internet.

Conclusion –
There are many advantages of LAN over MAN and WAN, such as LAN’s provide excellent reliability, high data transmission rate, they can easily be managed and shares peripheral devices too. Local Area Network cannot cover cities or towns and for that Metropolitan Area Network is needed, which can connect city or a group of cities together. Further, for connecting Country or a group of Countries, one requires Wide Area Network.

An access network is a type of network which physically connects an end system to the immediate router (also known as the “edge router”) on a path from the end system to any other distant end system. Examples of access networks are ISP, home networks, enterprise networks, ADSL, mobile network, FITH etc.

Types of access networks:

• Ethernet –
  It is the most commonly installed wired LAN technology and it provides services on the Physical and Data Link Layer of OSI reference model. Ethernet LAN typically uses coaxial cable or twisted pair wires.
• DSL –
  DSL stands for Digital Subscriber Line and DSL brings a connection into your home through telephone lines and a DSL line can carry both data and voice signals and the data part of the line is continuously connected. In DSL you are able to use the Internet and make phone calls simultaneously. DSL modem uses the telephone lines to exchange data with digital subscriber line access multiplexer (DSLAMs). In DSL we get 24 Mbps downstream and 2.5 Mbps upstream.
• FTTH –
  Fiber to the home (FTTH) uses optical fiber from a Central Office (CO) directly to individual buildings and it provides high-speed Internet access among all access networks. It ensures high initial investment but lesser future investment and it is the most expensive and most future-proof option amongst all these access networks.
• Wireless LANs –
  It links two or more devices using wireless communication within a range. It uses high-frequency radio waves and often includes an access point for connecting to the Internet.
• 3G and LTE –
  It uses cellular telephony to send or receive packets through a nearby base station operated by the cellular network provider. The term “3G internet” refers to the third generation of mobile phone standards as set by the International Telecommunications Union (ITU). Long Term Evolution (LTE) offers high-speed wireless communication for mobile devices and increased network capacity.

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