Wireless LAN Basics IEE802.11

Wireless LAN Basics  IEE802.11

IEEE802.11 Overview
                       The fundamental building block of the 802.11 architec is the cell, known as the basic service set (BSS) in 802.11 parlance. A BSS typically contains one or more wireless stations and a central base station, known as an access point (AP) in 802.11terminology.
            This is on IEEE 802.11 WiFi training is designed for people who work on development, testing and verification of IEEE 802.11 protocol. Students/Developer  will learn the internals of IEEE 802.11 protocol that helps them with a much better understanding of their current IEEE 802.11 work.

       

 OSI LAYERS:

The Open Systems Interconnection model (OSI model) is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology.
Upper layers:

• 7. application
• 6. presentation
• 5. session

Lower layers:

• 4. transport
• 3. network
• 2. data link
• 1. physical

What is the difference between WLAN and WiMAX?

                        Answer-1.WLAN is used as wireless local area network for providing connectivity between WLAN compliant devices. WiMAX is used as wide area network for providing access between various wireless devices. WLAN standards are evolving including 11a, 11b, 11g, 11n, 11ac, 11ad and more. WiMAX follows IEEE standards viz. 16d and 16e

WLAN Network Basics
WLAN network topology
– Infrastructure Mode
– Repeater Mode
– Bridge Mode
– Ad-hoc Mode
Channel scanning and synchronization
– Passive Scan
– Active Scan
Authentication and association
– Open Authentication
– Shared Key Authentication

Explain the WLAN physical layer frame as per 802.11a?

         WLAN has physical layer frame formats different in 11a, 11b, 11g, 11n and 11ac? They all have three generic fields i.e. preamble, header and data payload (PSDU). Preamble carries STF (short training field) and LTF (long training field) used for front end synchronization. Front end synchronization includes time offset estimation and correction; frequency offset estimation and correction and channel estimation and equalization. Header carries two important parameters rate and length. Rate specify modulation-code rate of the data payload part and length field specify length of data payload in units of OFDM symbol

         What is the difference between 802.11a, 11b, 11g and 802.11n? Answer-3.The difference between the 11a, 11b, 11g and 11n lies in terms of data rate, frequency of operation, distance coverage and more

Physical Layer Technologies
Radio channels and frequencies
– Frequency Channel Allocation for 802.11a/b/g
Modulation technologies
– Direct Sequence Spread Spectrum
– Orthogonal Frequency Division Multiplexing (OFDM)
PHY data rates used
– 802.11b
– 802 11a
– 802 11g
– 802 11n
Improving data transfer:diversity and polarization
– Antenna Diversity
– Polarization

What are the messages exchange between STA and AP in WLAN?

         There are various messages exchanged between Station (STA) and Access Point (AP) in a WLAN network for various purpose such as establishing connection, data transfer, terminate the connection and more. Access points are devices which help extend wired network with wireless capabilities

What is the change in WLAN-11ac with respect to previous versions of WLAN

               WLAN-11ac has been introduced after previous version of WLAN which include 802.11a, 11b, 11g and 11n. 80MHz channel bandwidth is added

Wireless LAN Standards
802.11a – 54 Mbps standard, 5 GHz signaling (ratified 1999)
802.11b – 11 Mbps standard, 2.4 GHz signaling (1999) 11 Mbps standard, 2.4 GHz signaling (1999)
802.11c – operation of bridge connections (moved to 802.1)
802.11d – worldwide compliance with regulations for use of wireless signal
spectrum (2001)
802 11e – Quality of Service (QoS) support (2005)
802.11f – Inter access point protocol to support roaming clients (2003)
802.11g – 54 Mbps standard, 2.4 GHz signaling (2003)
802.11h – Enhanced version of 802.11a to support European regulatory
requirements (2003)
802.11i – Security improvements for the 802.11 family (2004)
802.11j – Enhancements to 5 GHz signaling to support Japan regulatory
requirements (2004) requirements (2004)
802.11k – WLAN system management (in progress)
802.11l – Skipped to avoid confusion with 802.11i

What is the difference between WiFi and Bluetooth?

         WiFi fall under WLAN category while Bluetooth fall under WPAN category. WLAN specifications are published under IEEE 802.11 and Bluetooth under IEEE 802.15 standards. Bluetooth is the standard for wireless personal area networks or WPAN. It allows high speed transmission of data over very short distances.
802.11m – Maintenance of 802.11 family documentation
802.11n – Future 100+ Mbps standard (in progress)

Protocol Architecture
The Physical Medium Dependent Layer
The Data Link Layer
The Network Layer
The Transport Layer

Explain OSI stack and differentiate with TCPIP protocol stack?

        OSI protocol stack is the generic stack developed to make it easy for different manufacturers devices inter-operate without any interfacing issues. TCP IP stack is specifically designed for internet applications. 

802.11 MAC Protocol
Framing data to be transmitted
– Management Frames
– Control Frames
– Data Frames
Spacing between frames
– Inter Frame Spaces (IFS)
Avoiding collisions: carrier sensing
– Physical Carrier Sensing
– Virtual Carrier Sensing

What is the difference between ad-hoc and infrastructure mode in IEEE 802.11? \

          In ad-hoc mode WLAN mobile and stationary terminals referred as STAs (stations) communicate directly. In the infrastructure mode STAs communicate via entity called as AP (Access Point). It is similar to mesh and star topologies used in other wireless networks. Infrastructure mode used to connect with wired network.

Security Protocols in WLANs
Common WLAN Attacks
– Passive Attacks: eavesdropping
– PHY Layer attacks: RF Jamming
– Active Attacks: hacking
– Man in the Middle Attack
WLAN Security Solutions
– WEP-SharedKey
– WPA-PSK
– WPA-Open
– Server Based Authentication
– Server-based security: 802.1x / 802.11i

Newton’s laws of motion-Physics

Newton’s laws of motion

Newton’s laws of motion:

Relations between the forces acting on a body and the motion of the body, first formulated by English physicist and mathematician Sir Isaac Newton.

Newton’s first law:

 It states that, if a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed unless it is acted upon by a force. This postulate is known as the law of inertia. The law of inertia was first formulated by Galileo Galilei for horizontal motion on Earth and was later generalized by René Descartes. 

Before Galileo it had been thought that all horizontal motion required a direct cause, but Galileo deduced from his experiments that a body in motion would remain in motion unless a force (such as friction) caused it to come to rest. 

Newton’s second law:

It is a quantitative description of the changes that a force can produce on the motion of a body. It states that the time rate of change of the momentum of a body is equal in both magnitude and direction to the force imposed on it. The momentum of a body is equal to the product of its mass and its velocity. Momentum, like velocity, is a vector quantity, having both magnitude and direction. 

A force applied to a body can change the magnitude of the momentum, or its direction, or both. Newton’s second law is one of the most important in all of physics.

 For a body whose mass m is constant, it can be written in the form F = ma, where F (force) and a (acceleration) are both vector quantities. If a body has a net force acting on it, it is accelerated in accordance with the equation. Conversely, if a body is not accelerated, there is no net force acting on it.

          A lesson proving immovable objects and unstoppable forces are one and the same.

Newton’s third law states:

That when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction. The third law is also known as the law of action and reaction. This law is important in analyzing problems of static equilibrium, where all forces are balanced, but it also applies to bodies in uniform or accelerated motion. The forces it describes are real ones, not mere bookkeeping devices. 

For example, a book resting on a table applies a downward force equal to its weight on the table. According to the third law, the table applies an equal and opposite force to the book. This force occurs because the weight of the book causes the table to deform slightly so that it pushes back on the book like a coiled spring. 

            In the 20th century Newton’s laws were replaced by quantum mechanics and relativity as the most fundamental laws of physics. Nevertheless, Newton’s laws continue to give an accurate account of nature, except for very small bodies such as electrons or for bodies moving close to the speed of light. Quantum mechanics and relativity reduce to Newton’s laws for larger bodies or for bodies moving more slowly.

mechanics:

   Newton’s laws of motion and equilibrium

                    science concerned with the motion of bodies under the action of forces, including the special case in which a body remains at rest. Of first concern in the problem of motion are the forces that bodies exert on one another. This leads to the study of such topics as gravitation, electricity.

evolution: 

 Genetic equilibrium: the Hardy-Weinberg law

     They were, evolution could not occur. Why, then, is the law significant if its assumptions do not hold true in nature? The answer is that it plays in evolutionary studies a role similar to that of Newton’s first law of motion in mechanics. Newton’s first law says that a body not acted upon by a net external force remains at rest or maintains a constant velocity.

Physics:Dimensions and Units

Physics

Mechanics: Dimensions and Units

                    There is a difference between dimensions and units. A dimension is a measure of a physical variable (without numerical values), while a unit is a way to assign a number or measurement to that dimension.

 For example, length is a dimension, but it is measured in units of feet (ft) or meters (m).

               There is one most common primary unit systems, The International System of Units (SI units, from Le System International d’Unites, more commonly simply called metric units)

                 In total, there are seven primary dimensions. Primary (sometimes called basic) dimensions are defined as independent or fundamental dimensions, from which other dimensions can be obtained.

                 The primary dimensions are: mass, length, time, temperature, electric current, amount of light, and amount of matter. For most mechanical and thermal science analyses, however, only the first four of these are required. The others will not be of concern to most mechanical engineering analyses.

            SI derived units include the hertz , the newton , the pascal (unit of pressure or stress) , the ohm , the farad , the joule , the coulomb , the tesla , the lumen , the becquerel , the siemen , the volt , and the watt .

SI base units:
              

  The meter (abbreviation, m) is the SI unit of displacement or length. One meter is the distance traveled by a ray of electromagnetic (EM) energy through a vacuum in 1/299,792,458 (3.33564095 x 10 ^-9 ) second. 

  The kilogram (abbreviation, kg) is the SI unit of mass. It is defined as the mass of a particular international prototype made of platinum-iridium and kept at the International Bureau of Weights and Measures. It was originally defined as the mass of one liter (10 ^-3cubic meter) of pure water.

  The second (abbreviation, s or sec) is the SI unit of time. One second is the time that elapses during 9.192631770 x 10 ⁹ cycles of the radiation produced by the transition between two levels of Cesium 133. 

  The kelvin (abbreviation K), also called the degree Kelvin (abbreviation, ^o K), is the SI unit of temperature. One Kelvin is 1/273.16 (3.6609 x 10 ^-3 ) 

  The ampere (abbreviation, A) is the SI unit of electric current. One ampere is the current that would produce a force of 0.0000002 (2 x 10 ^-7 ) newton between two straight, parallel, perfectly conducting wires having infinite length and zero diameter, separated by one meter in a vacuum.

 The candela (abbreviation, cd) is the SI unit of luminous intensity. It is the electromagnetic radiation, in a specified direction, that has an intensity of 1/683 (1.46 x 10 ^-3 ) watt per steradian.