INFORMATION TECHNOLOGY

INFORMATION TECHNOLOGY

(COMPUTER SCIENCE)

https://chsit.simdif.com

Information Technology

The study or use of systems (especially computers and telecommunications) for storing, retrieving, and sending information.

Information technology is the use of computers and other telecommunication equipments to store, retrieve, transmit, and manipulate data, or information, often in the context of a business or other enterprise. IT is considered to be a subset of information and communications technology. Wikipedia

Information technology is the study, design, development, implementation, support or management of computer-based information systems—particularly software applications and computer hardware. IT workers help ensure that computers work well for people.

COMPUTER ELECTRONIC COMPONENTS:

The Most Common Basic Electronic Components

Resistors.
Capacitors.
LEDs.
Transistors.
Inductors.
Integrated Circuits.

COMPUTATIONAL PHYSICS:

Computational physics is the subject that deals with these numerical approximations: the approximation of the solution is written as a finite (and typically large) number of simple mathematical Computer operations (algorithm), and a computer is used to perform these operations and compute an approximated solution and respective ...

Methods and algorithms

COMPUTER SOFTWARE:

Computer software, often called as software, is a set of instructions and its associated documentations that tells acomputer what to do or how to perform a task. Software includes all different software programs on a computer, such as applications and the operating system.
1. Software - Simple English Wikipedia, the free encyclopedia
2. Computer Software Definition - OpenProjects.org
3. Computer software - ScienceDaily
  
  CHS Software - HTML, CSS, & JavaScript:
  - HTML provides the basic structure of sites, which is enhanced and modified by other technologies like CSS and JavaScript.
  - CSS is used to control presentation, formatting, and layout.
  - JavaScript is used to control the behavior of different elements.
  COMPUTER HARDWARE:
  
  Computer hardware includes the physical, tangible parts or components of a computer, such as the cabinet, central processing unit, monitor, keyboard, computer data storage, graphic card, sound card, speakers and motherboard. By contrast, software is instructions that can be stored and run by hardware. Wikipedia
  
  OHM'S LAW:
  
  Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance,[1] one arrives at the usual mathematical equation that describes this relationship:[2]I=VR,{\displaystyle I={\frac {V}{R}},}
  
  where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that theR in this relation is constant, independent of the current.[3] Ohm's law is an empirical relation which accurately describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of current. However some materials do not obey Ohm's law, these are called non-ohmic.
  
  The law was named after the German physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire. Ohm explained his experimental results by a slightly more complex equation than the modern form above (see History).
  
  In physics, the term Ohm's law is also used to refer to various generalizations of the law; for example the vector form of the law used in electromagnetics and material science:
  
  J=σE,{\displaystyle \mathbf {J} =\sigma \mathbf {E} ,}
  
  where J is the current density at a given location in a resistive material, E is the electric field at that location, and σ (sigma) is a material-dependent parameter called the conductivity. This reformulation of Ohm's law is due to Gustav Kirchhoff.
  
  The IEEE Components, Packaging and Manufacturing Technology (CPMT) is aprofessional society of the IEEE that deals with the advancement ofmicrosystems packaging and manufacture.
4. IEEE Transactions on Components & Packaging Technologies
5. IEEE Transactions on Advanced Packaging
6. IEEE Transactions on Electronics Packaging Manufacturing
7. IEEE Transactions on Semiconductor Manufacturing
8. IEEE Transactions on Nanotechnology
9. Circuits and Devices Magazine

Client Server Model

Two remote application processes can communicate mainly in two different fashions:

· Peer-to-peer: Both remote processes are executing at same level and they exchange data using some shared resource.

· Client-Server: One remote process acts as a Client and requests some resource from another application process acting as Server.

In client-server model, any process can act as Server or Client. It is not the type of machine, size of the machine, or its computing power which makes it server; it is the ability of serving request that makes a machine a server.

A system can act as Server and Client simultaneously. That is, one process is acting as Server and another is acting as a client. This may also happen that both client and server processes reside on the same machine.

Communication

Two processes in client-server model can interact in various ways:

· Sockets

· Remote Procedure Calls (RPC)

Sockets

In this paradigm, the process acting as Server opens a socket using a well-known (or known by client) port and waits until some client request comes. The second process acting as a Client also opens a socket but instead of waiting for an incoming request, the client processes ‘requests first’.

When the request is reached to server, it is served. It can either be an information sharing or resource request.

Remote Procedure Call

This is a mechanism where one process interacts with another by means of procedure calls. One process (client) calls the procedure lying on remote host. The process on remote host is said to be Server. Both processes are allocated stubs. This communication happens in the following way:

· The client process calls the client stub. It passes all the parameters pertaining to program local to it.

· All parameters are then packed (marshalled) and a system call is made to send them to other side of the network.

· Kernel sends the data over the network and the other end receives it.

· The remote host passes data to the server stub where it is unmarshalled.

· The parameters are passed to the procedure and the procedure is then executed.

· The result is sent back to the client in the same manner.

User Datagram Protocol

The User Datagram Protocol (UDP) is simplest Transport Layer communication protocol available of the TCP/IP protocol suite. It involves minimum amount of communication mechanism. UDP is said to be an unreliable transport protocol but it uses IP services which provides best effort delivery mechanism.

In UDP, the receiver does not generate an acknowledgement of packet received and in turn, the sender does not wait for any acknowledgement of packet sent. This shortcoming makes this protocol unreliable as well as easier on processing.

Requirement of UDP

A question may arise, why do we need an unreliable protocol to transport the data? We deploy UDP where the acknowledgement packets share significant amount of bandwidth along with the actual data. For example, in case of video streaming, thousands of packets are forwarded towards its users. Acknowledging all the packets is troublesome and may contain huge amount of bandwidth wastage. The best delivery mechanism of underlying IP protocol ensures best efforts to deliver its packets, but even if some packets in video streaming get lost, the impact is not calamitous and can be ignored easily. Loss of few packets in video and voice traffic sometimes goes unnoticed.

Features

· UDP is used when acknowledgement of data does not hold any significance.

· UDP is good protocol for data flowing in one direction.

· UDP is simple and suitable for query based communications.

· UDP is not connection oriented.

· UDP does not provide congestion control mechanism.

· UDP does not guarantee ordered delivery of data.

· UDP is stateless.

· UDP is suitable protocol for streaming applications such as VoIP, multimedia streaming.

UDP Header

UDP header is as simple as its function.

UDP header contains four main parameters:

· Source Port - This 16 bits information is used to identify the source port of the packet.

· Destination Port - This 16 bits information, is used identify application level service on destination machine.

· Length - Length field specifies the entire length of UDP packet (including header). It is 16-bits field and minimum value is 8-byte, i.e. the size of UDP header itself.

· Checksum - This field stores the checksum value generated by the sender before sending. IPv4 has this field as optional so when checksum field does not contain any value it is made 0 and all its bits are set to zero.

UDP application

Here are few applications where UDP is used to transmit data:

· Domain Name Services

· Simple Network Management Protocol

· Trivial File Transfer Protocol

· Routing Information Protocol

· Kerberos

Transport Layer Introduction

Next Layer in OSI Model is recognized as Transport Layer (Layer-4). All modules and procedures pertaining to transportation of data or data stream are categorized into this layer. As all other layers, this layer communicates with its peer Transport layer of the remote host.

Transport layer offers peer-to-peer and end-to-end connection between two processes on remote hosts. Transport layer takes data from upper layer (i.e. Application layer) and then breaks it into smaller size segments, numbers each byte, and hands over to lower layer (Network Layer) for delivery.

Functions

· This Layer is the first one which breaks the information data, supplied by Application layer in to smaller units called segments. It numbers every byte in the segment and maintains their accounting.

· This layer ensures that data must be received in the same sequence in which it was sent.

· This layer provides end-to-end delivery of data between hosts which may or may not belong to the same subnet.

· All server processes intend to communicate over the network are equipped with well-known Transport Service Access Points (TSAPs) also known as port numbers.

End-to-End Communication

A process on one host identifies its peer host on remote network by means of TSAPs, also known as Port numbers. TSAPs are very well defined and a process which is trying to communicate with its peer knows this in advance.

For example, when a DHCP client wants to communicate with remote DHCP server, it always requests on port number 67. When a DNS client wants to communicate with remote DNS server, it always requests on port number 53 (UDP).

The two main Transport layer protocols are:

· Transmission Control Protocol

It provides reliable communication between two hosts.

· User Datagram Protocol

It provides unreliable communication between two hosts.

Internetworking

In real world scenario, networks under same administration are generally scattered geographically. There may exist requirement of connecting two different networks of same kind as well as of different kinds. Routing between two networks is called internetworking.

Networks can be considered different based on various parameters such as, Protocol, topology, Layer-2 network and addressing scheme.

In internetworking, routers have knowledge of each other’s address and addresses beyond them. They can be statically configured go on different network or they can learn by using internetworking routing protocol.

Routing protocols which are used within an organization or administration are called Interior Gateway Protocols or IGP. RIP, OSPF are examples of IGP. Routing between different organizations or administrations may have Exterior Gateway Protocol, and there is only one EGP i.e. Border Gateway Protocol.

Tunneling

If they are two geographically separate networks, which want to communicate with each other, they may deploy a dedicated line between or they have to pass their data through intermediate networks.

Tunneling is a mechanism by which two or more same networks communicate with each other, by passing intermediate networking complexities. Tunneling is configured at both ends.

When the data enters from one end of Tunnel, it is tagged. This tagged data is then routed inside the intermediate or transit network to reach the other end of Tunnel. When data exists the Tunnel its tag is removed and delivered to the other part of the network.

Both ends seem as if they are directly connected and tagging makes data travel through transit network without any modifications.

Packet Fragmentation

Most Ethernet segments have their maximum transmission unit (MTU) fixed to 1500 bytes. A data packet can have more or less packet length depending upon the application. Devices in the transit path also have their hardware and software capabilities which tell what amount of data that device can handle and what size of packet it can process.

If the data packet size is less than or equal to the size of packet the transit network can handle, it is processed neutrally. If the packet is larger, it is broken into smaller pieces and then forwarded. This is called packet fragmentation. Each fragment contains the same destination and source address and routed through transit path easily. At the receiving end it is assembled again.

If a packet with DF (don’t fragment) bit set to 1 comes to a router which can not handle the packet because of its length, the packet is dropped.

When a packet is received by a router has its MF (more fragments) bit set to 1, the router then knows that it is a fragmented packet and parts of the original packet is on the way.

If packet is fragmented too small, the overhead is increases. If the packet is fragmented too large, intermediate router may not be able to process it and it might get dropped.

Hey, we just launched our brand new website with latest Electronics and Electrical Projects for Engineering Students! Click Here to Visit

Edgefx Kits Official Blog

Embedded Projects for Engineering Students

Electrical and Electronic Components Used in Projects

Electrical and Electronic Components

There are numerous basic electrical and electronic components are normally found in numerous circuits of peripherals like hard disks, mother boards, etc… Several circuits are designed with numerous components like resistors, capacitors, inductors, transistors, transformers, switches and fuses etc. Therefore, this article offers a quick info concerning different types of electronic and electrical components that are utilized in various electronic and electrical projects, embedded systems. Let us see each and every part in detail with diagrams. Resistors, inductors, transformers, battery, fuse.Major Electrical and Electronic Components

The major electrical and electronic components utilized in electrical and electronic projects like resistors, capacitors, fuses, transistors, integrated circuits, relays, switches, motors, circuit breakers, etc. In several circuits, these components are used to build the circuit that is classified into 2 classes such as active components and passive components.

Active and Passive Components

Active components are nothing but, a component that provides and controls energy. Passive components may be outlined as; a component that responds to the flow of voltage and either dissipates or store energy.

Resistor

Resistors

A resistor is an electrical component that restricts the flow of a current in a circuit. The electrical device may also be used to afford a particular voltage to a semiconductor. Once current flows through the resistor, the resistor absorbs the voltage and degenerates within the form of heat. Resistors might have variable or mounted resistances that may be found in thermistors, photo resistors, trimmers, potentiometers and hamsters. The current flow through a resistor is directly proportional to the voltage across the resistor terminals. The relationship is pictured by Ohm’s law. Ohm’s law states that dissipation due to the resistance and the units of resistance is ohms.

Ohms law V=IR from this equation, we are able to get I=V/R
Where, ‘V’ is that the voltage of the conductor, ‘I’ is the current through the conductor and ‘R’ is the resistance of the conductor.

Capacitor

A capacitor is a two terminal linear passive component that is made from 2 conductive plates with an insulator between them. The main function of a capacitor is that, it stores voltage once an electrical charge is forced onto its terminals from a power supply. It maintains the charge even when getting disconnected from the power supply. In a timing circuit, a capacitor is employed with a resistor and can also be used as a filter to permit the AC signals and block the DC signals.

The stored electrical charge is Q=CV
Where, “C” is the capacitive reactance and ‘V’ is the applied voltage.
Thus, the current flow through a capacitor is I= C dv/dt

When a DC current is applied across a capacitor, a positive (+) and negative (-) charge builds on a set of terminals. The charge stays till the capacitor is discharged. Once an AC current is applied across a capacitor, a positive and negative charge forms on a set of plates during the part of the cycle when the voltage is positive. When the voltage goes negative (-) in the second half of the cycle, then the capacitor releases before charged current, and then charges the opposite way.

Microcontroller

A microcontroller is a small computer on a single integrated circuit that assembles all the options which are found within the microprocessor. So as to serve different applications, it has a high concentration of on chip amenities like RAM, ROM, timers I/O ports, serial port, interrupt and clock circuit. These are used in numerous automatically controlled devices like remote controls, medical devices, automobile engine control systems, power tools, toys, workplace machines, and different embedded systems. Microcontrollers are used in different electronic project circuits

Inductor

An inductor or coil or a reactor is a two-terminal passive electrical component, commonly operate of an inductor is that, it stores voltage within the form of magnetic energy. Generally an inductor includes of a conductor, component wound into a coil that works on the principle of Faraday’s law of inductance. Once a current flows through the coil from the left side to the right side, the coil produces a magnetic field within the clockwise direction. The inductance of the inductor is denoted by ‘L’

L== (µ.K.N2.S)/I.

Where,

‘L’ is an inductance
‘K’ is a Nagaoka constant
‘µ’ is a Magnetic permeability
‘N’ is the number of turns of the coils
‘S’ is the Cross section area of the coil
‘I’ is the Length of the coil in axial direction

Transformer

A transformer is an electrical device and it consists of 2 coils of wire that are joined by an iron core. It offers the much required capability of changing the current and voltage levels simply. The main function of the transformer is that to increase (step-up) or decrease (step-down) AC voltages. The transformer works on the principle of Faraday’s law of electromagnetic induction, that is, mutual inductance between 2 circuits that is linked by a common magnetic flux. Transformer converts an electrical energy from one circuit to another circuit with the help of mutual induction between the 2 windings without electrical connection between them, and also converts power from one circuit to another} circuit without changing the frequency however with a different voltage level.

In a step up transformer, secondary winding contains a lot of winding than the first coil. Returning to a transformer, it has more windings in the primary than the secondary winding. These are one of the main reasons we use AC current in our homes and not DC. DC voltages can’t be modified using transformers. Transformers are out there in several sizes within the market nowadays, starting from little ones to an outsized one.

Battery

A battery is an electrical device that is employed to convert chemical energy into voltage through electrochemical discharge reactions. These are composed of one or more cells where each cell having an anode (+), the cathode (-) and the solution. The anode and the cathode are preserved to an electric circuit. Batteries are classified into 2 varieties they are primary or disposable batteries and secondary or rechargeable batteries, whereby the primary batteries don’t seem to be reversible whereas the secondary batteries are reversible.

Fuse

A fuse is a material or a bit of wire which is employed to protect the components from destruction due to excessive current flowing through them. When excessive current flows through the circuit, the wires get heated up and get broken. As a result, the current stops flowing. When the fuse melts due to overflow of current then, the part of the fuse absorbs some energy. The absorbed energy is given by I2t

Where, ‘I’ is the peak value of the current interrupted
‘t’ is the time taken to clear the fault.

When the fuse blows, it offers a signal that there is some reasonably fault within the circuit. Every type of fuse is designed for an accurate quantity of current. A fuse includes of the main parts a set of contacts, metal-fuse part, connection and support body. A fuse element is made of Zn, copper, silver, aluminum or alloys to afford sure characteristics. That may be swallowed by air or material

Diode/LED (Light Emitting Diode)

LED

A diode is a device, which is made from semiconductor material that allows current to flow in one direction; it blocks the current that tries to travel against the flow in a wire. These devices are often used in several electronic circuits that convert AC to DC. An LED is the alternative of the diode, once a current is applied to an LED; it emits light at a specific frequency. LEDs are utilized in varied applications like keyboards, hard disks, TV remote controls, and these devices are largely helpful as standing indicators in computers also as battery-operated electronics Transistors

Transistors

Transistor

The transistor is a device, which consists of 3 terminals, fabricated from semiconductor material that controls the flow of voltage or current and acts as a switch for electronic signals. There are 2 varieties of transistors PNP and NPN, most circuits tend to use NPN semiconductor. Transistors are designed in numerous shapes and the 3 terminals of the semiconductor specifically, the base (B), emitter (E) and collector(C). Where,the base terminal is responsible for activating the transistor, emitter terminal is the negative lead and collector terminal is the positive lead.

Integrated Circuit

The short sort of a form of integrated circuit is IC and sometimes it is additionally known as a chip. The integrated circuit is a semiconductor device, where numerous resistors, capacitors, and transistors are fabricated. It will operate as a microprocessor, microchip, amplifier, and timer. An IC is classified as either linear or nonlinear depending on its application. Linear ICs or analog integrated circuits have continuously variable O/P which depends on the level of i/p signal. These linear ICs are used as audio frequency and radio frequency amplifiers. Digital ICs operate at only some defined states, instead of over a continuous vary of signal amplitudes. The applications of integrated circuits involve in computers, modems, computer networks and frequency counters. The basic structure of digital integrated circuits is logic gates, which work with binary knowledge.

Relays

Relays are switches, that are used to control the circuits by employing a low power signal. The relays comprise of a magnet, a coil, a series of electrical contacts and a spring. First relays were used as amplifiers in long distance telegraph circuits. There is no electrical connection between the two circuits within the relay, because that is connected through the magnetic connection. Relays are used to perform logical operations in telephone exchanges and computers etc.

Switches

Switch

A switch is an electrical device which is employed to interrupt the circuit, interrupting the current and to provide the current from one conductor to a different conductor. The switch works with ON and OFF mechanism. Switches are classified into four varieties like (SPST) single pole single throw, (SPDT) single pole double throw, (DPST) double pole single throw and (DPDT)double pole double throw.

Motors

A motor is an electrical device and the main function of the motor is that, it converts electrical energy into mechanical energy. The motor contains rotor, stator, bearings, conduit box, enclosure and eye bolt. Electrical motors are all everywhere, from common machines to our most complicated computers. These motors are competent for the tasks which they perform when compared to pneumatic or hydraulic alternatives. Motors are categorized into different types like, AC inductance motors, DC motors, stepper motor, three phase induction motor, brushlessmagnet electric motor, brushed DC motor, and switched reluctance motor.

Circuit Breakers

Circuit Breaker

A circuit breaker is a mechanical switching device which is operated automatically, which are used to defend an electric circuit from damage caused by overload or short circuit the most functions of the circuit breaker is to interrupt current flow and determine a fault condition. It includes of two contacts like moving & mounted contacts. Moving contact is employed to interrupt and create the circuit victimization stored energies in the kind of spring or compressed air. Mounted contact includes a spring that holds the moving contact when closing. These devices accommodate two coils, closing and tripping coil. Wherever closing coil is used to shut the circuit and tripping coil is employed to trip the circuit.

Online Sites for Electrical and Electronic Components

Online Stores

Here is the list of the online stores where you can place an order to buy these components and project kits. These online stores provide completely different choice to place associate order like money on delivery and debit cards or Mastercard payments. Additionally to the current, these online stores deliver the elements and kits by traveler services like communicating, DTD, FedEx etc.

LOS ANGELES GLOBAL IT PRIVATE MANAGEMENT

Application Protocols

There are several protocols which work for users in Application Layer. Application layer protocols can be broadly divided into two categories:

· Protocols which are used by users.For email for example, eMail.

· Protocols which help and support protocols used by users.For example DNS.

Few of Application layer protocols are described below:

Domain Name System

The Domain Name System (DNS) works on Client Server model. It uses UDP protocol for transport layer communication. DNS uses hierarchical domain based naming scheme. The DNS server is configured with Fully Qualified Domain Names (FQDN) and email addresses mapped with their respective Internet Protocol addresses.

A DNS server is requested with FQDN and it responds back with the IP address mapped with it. DNS uses UDP port 53.

Simple Mail Transfer Protocol

The Simple Mail Transfer Protocol (SMTP) is used to transfer electronic mail from one user to another. This task is done by means of email client software (User Agents) the user is using. User Agents help the user to type and format the email and store it until internet is available. When an email is submitted to send, the sending process is handled by Message Transfer Agent which is normally comes inbuilt in email client software.

Message Transfer Agent uses SMTP to forward the email to another Message Transfer Agent (Server side). While SMTP is used by end user to only send the emails, the Servers normally use SMTP to send as well as receive emails. SMTP uses TCP port number 25 and 587.

Client software uses Internet Message Access Protocol (IMAP) or POP protocols to receive emails.

File Transfer Protocol

The File Transfer Protocol (FTP) is the most widely used protocol for file transfer over the network. FTP uses TCP/IP for communication and it works on TCP port 21. FTP works on Client/Server Model where a client requests file from Server and server sends requested resource back to the client.

FTP uses out-of-band controlling i.e. FTP uses TCP port 20 for exchanging controlling information and the actual data is sent over TCP port 21.

The client requests the server for a file. When the server receives a request for a file, it opens a TCP connection for the client and transfers the file. After the transfer is complete, the server closes the connection. For a second file, client requests again and the server reopens a new TCP connection.

Post Office Protocol (POP)

The Post Office Protocol version 3 (POP 3) is a simple mail retrieval protocol used by User Agents (client email software) to retrieve mails from mail server.

When a client needs to retrieve mails from server, it opens a connection with the server on TCP port 110. User can then access his mails and download them to the local computer. POP3 works in two modes. The most common mode the delete mode, is to delete the emails from remote server after they are downloaded to local machines. The second mode, the keep mode, does not delete the email from mail server and gives the user an option to access mails later on mail server.

Hyper Text Transfer Protocol (HTTP)

The Hyper Text Transfer Protocol (HTTP) is the foundation of World Wide Web. Hypertext is well organized documentation system which uses hyperlinks to link the pages in the text documents. HTTP works on client server model. When a user wants to access any HTTP page on the internet, the client machine at user end initiates a TCP connection to server on port 80. When the server accepts the client request, the client is authorized to access web pages.

To access the web pages, a client normally uses web browsers, who are responsible for initiating, maintaining, and closing TCP connections. HTTP is a stateless protocol, which means the Server maintains no information about earlier requests by clients.

HTTP versions

· HTTP 1.0 uses non persistent HTTP. At most one object can be sent over a single TCP connection.

· HTTP 1.1 uses persistent HTTP. In this version, multiple objects can be sent over a single TCP connection.

Application Layer Introduction

Application layer is the top most layer in OSI and TCP/IP layered model. This layer exists in both layered Models because of its significance, of interacting with user and user applications. This layer is for applications which are involved in communication system.

A user may or may not directly interacts with the applications. Application layer is where the actual communication is initiated and reflects. Because this layer is on the top of the layer stack, it does not serve any other layers. Application layer takes the help of Transport and all layers below it to communicate or transfer its data to the remote host.

When an application layer protocol wants to communicate with its peer application layer protocol on remote host, it hands over the data or information to the Transport layer. The transport layer does the rest with the help of all the layers below it.

There’is an ambiguity in understanding Application Layer and its protocol. Not every user application can be put into Application Layer. except those applications which interact with the communication system. For example, designing software or text-editor cannot be considered as application layer programs.

On the other hand, when we use a Web Browser, which is actually using Hyper Text Transfer Protocol (HTTP) to interact with the network. HTTP is Application Layer protocol.

Another example is File Transfer Protocol, which helps a user to transfer text based or binary files across the network. A user can use this protocol in either GUI based software like FileZilla or CuteFTP and the same user can use FTP in Command Line mode.

Hence, irrespective of which software you use, it is the protocol which is considered at Application Layer used by that software. DNS is a protocol which helps user application protocols such as HTTP to accomplish its work.

Transmission Control Protocol

The transmission Control Protocol (TCP) is one of the most important protocols of Internet Protocols suite. It is most widely used protocol for data transmission in communication network such as internet.

Features

· TCP is reliable protocol. That is, the receiver always sends either positive or negative acknowledgement about the data packet to the sender, so that the sender always has bright clue about whether the data packet is reached the destination or it needs to resend it.

· TCP ensures that the data reaches intended destination in the same order it was sent.

· TCP is connection oriented. TCP requires that connection between two remote points be established before sending actual data.

· TCP provides error-checking and recovery mechanism.

· TCP provides end-to-end communication.

· TCP provides flow control and quality of service.

· TCP operates in Client/Server point-to-point mode.

· TCP provides full duplex server, i.e. it can perform roles of both receiver and sender.

Header

The length of TCP header is minimum 20 bytes long and maximum 60 bytes.

· Source Port (16-bits) - It identifies source port of the application process on the sending device.

· Destination Port (16-bits) - It identifies destination port of the application process on the receiving device.

· Sequence Number (32-bits) - Sequence number of data bytes of a segment in a session.

· Acknowledgement Number (32-bits) - When ACK flag is set, this number contains the next sequence number of the data byte expected and works as acknowledgement of the previous data received.

· Data Offset (4-bits) - This field implies both, the size of TCP header (32-bit words) and the offset of data in current packet in the whole TCP segment.

· Reserved (3-bits) - Reserved for future use and all are set zero by default.

· Flags (1-bit each)

o NS - Nonce Sum bit is used by Explicit Congestion Notification signaling process.

o CWR - When a host receives packet with ECE bit set, it sets Congestion Windows Reduced to acknowledge that ECE received.

o ECE -It has two meanings:

§ If SYN bit is clear to 0, then ECE means that the IP packet has its CE (congestion experience) bit set.

§ If SYN bit is set to 1, ECE means that the device is ECT capable.

o URG - It indicates that Urgent Pointer field has significant data and should be processed.

o ACK - It indicates that Acknowledgement field has significance. If ACK is cleared to 0, it indicates that packet does not contain any acknowledgement.

o PSH - When set, it is a request to the receiving station to PUSH data (as soon as it comes) to the receiving application without buffering it.

o RST - Reset flag has the following features:

§ It is used to refuse an incoming connection.

§ It is used to reject a segment.

§ It is used to restart a connection.

o SYN - This flag is used to set up a connection between hosts.

o FIN - This flag is used to release a connection and no more data is exchanged thereafter. Because packets with SYN and FIN flags have sequence numbers, they are processed in correct order.

· Windows Size - This field is used for flow control between two stations and indicates the amount of buffer (in bytes) the receiver has allocated for a segment, i.e. how much data is the receiver expecting.

· Checksum - This field contains the checksum of Header, Data and Pseudo Headers.

· Urgent Pointer - It points to the urgent data byte if URG flag is set to 1.

· Options - It facilitates additional options which are not covered by the regular header. Option field is always described in 32-bit words. If this field contains data less than 32-bit, padding is used to cover the remaining bits to reach 32-bit boundary.

Addressing

TCP communication between two remote hosts is done by means of port numbers (TSAPs). Ports numbers can range from 0 – 65535 which are divided as:

System Ports (0 – 1023)
User Ports ( 1024 – 49151)
Private/Dynamic Ports (49152 – 65535)

Connection Management

TCP communication works in Server/Client model. The client initiates the connection and the server either accepts or rejects it. Three-way handshaking is used for connection management.

Establishment

Client initiates the connection and sends the segment with a Sequence number. Server acknowledges it back with its own Sequence number and ACK of client’s segment which is one more than client’s Sequence number. Client after receiving ACK of its segment sends an acknowledgement of Server’s response.

Release

Either of server and client can send TCP segment with FIN flag set to 1. When the receiving end responds it back by ACKnowledging FIN, that direction of TCP communication is closed and connection is released.

Bandwidth Management

TCP uses the concept of window size to accommodate the need of Bandwidth management. Window size tells the sender at the remote end, the number of data byte segments the receiver at this end can receive. TCP uses slow start phase by using window size 1 and increases the window size exponentially after each successful communication.

For example, the client uses windows size 2 and sends 2 bytes of data. When the acknowledgement of this segment received the windows size is doubled to 4 and next sent the segment sent will be 4 data bytes long. When the acknowledgement of 4-byte data segment is received, the client sets windows size to 8 and so on.

If an acknowledgement is missed, i.e. data lost in transit network or it received NACK, then the window size is reduced to half and slow start phase starts again.

Error Control &and Flow Control

TCP uses port numbers to know what application process it needs to handover the data segment. Along with that, it uses sequence numbers to synchronize itself with the remote host. All data segments are sent and received with sequence numbers. The Sender knows which last data segment was received by the Receiver when it gets ACK. The Receiver knows about the last segment sent by the Sender by referring to the sequence number of recently received packet.

If the sequence number of a segment recently received does not match with the sequence number the receiver was expecting, then it is discarded and NACK is sent back. If two segments arrive with the same sequence number, the TCP timestamp value is compared to make a decision.

Multiplexing

The technique to combine two or more data streams in one session is called Multiplexing. When a TCP client initializes a connection with Server, it always refers to a well-defined port number which indicates the application process. The client itself uses a randomly generated port number from private port number pools.

Using TCP Multiplexing, a client can communicate with a number of different application process in a single session. For example, a client requests a web page which in turn contains different types of data (HTTP, SMTP, FTP etc.) the TCP session timeout is increased and the session is kept open for longer time so that the three-way handshake overhead can be avoided.

This enables the client system to receive multiple connection over single virtual connection. These virtual connections are not good for Servers if the timeout is too long.

Congestion Control

When large amount of data is fed to system which is not capable of handling it, congestion occurs. TCP controls congestion by means of Window mechanism. TCP sets a window size telling the other end how much data segment to send. TCP may use three algorithms for congestion control:

· Additive increase, Multiplicative Decrease

· Slow Start

· Timeout React

Timer Management

TCP uses different types of timer to control and management various tasks:

Keep-alive timer:

· This timer is used to check the integrity and validity of a connection.

· When keep-alive time expires, the host sends a probe to check if the connection still exists.

Retransmission timer:

· This timer maintains stateful session of data sent.

· If the acknowledgement of sent data does not receive within the Retransmission time, the data segment is sent again.

Persist timer:

· TCP session can be paused by either host by sending Window Size 0.

· To resume the session a host needs to send Window Size with some larger value.

· If this segment never reaches the other end, both ends may wait for each other for infinite time.

· When the Persist timer expires, the host re-sends its window size to let the other end know.

· Persist Timer helps avoid deadlocks in communication.

Timed-Wait:

· After releasing a connection, either of the hosts waits for a Timed-Wait time to terminate the connection completely.

· This is in order to make sure that the other end has received the acknowledgement of its connection termination request.

· Timed-out can be a maximum of 240 seconds (4 minutes).

Crash Recovery

TCP is very reliable protocol. It provides sequence number to each of byte sent in segment. It provides the feedback mechanism i.e. when a host receives a packet, it is bound to ACK that packet having the next sequence number expected (if it is not the last segment).

When a TCP Server crashes mid-way communication and re-starts its process it sends TPDU broadcast to all its hosts. The hosts can then send the last data segment which was never unacknowledged and carry onwards.