wireless ethernet

7/29/2019 Wireless Ethernet

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Wireless Ethernet

Architecture

Physical Layer

MAC Layer


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Some of the advantages are mentioned below :

Availability of low-cost portable equipments: Due to the

technology enhancements, the equipment cost that are required for

WLAN set-up have reduced a lot. Mobility: An increasing number of LAN users are becoming mobile. These

mobile users require that they are connected to the network regardless of

where they are because they want simultaneous access to the network.

This makes the use of cables, or wired LANs, impractical if not impossible.

Wireless LAN can provide users mobility, which is likely to increaseproductivity, user convenience and various service opportunities.

Installation speed and simplicity: Wireless LANs are very

easy to install. There is no requirement for wiring every workstation and

every room. This ease of installation makes wireless LANs inherently

flexible. If a workstation must be moved, it can be done easily and withoutadditional wiring, cable drops or reconfiguration of the network.

Installation flexibility: If a company moves to a new location, the wireless

system is much easier to move than ripping up all of the cables that a

wired system would have snaked throughout the building. This also

provides portability. Wireless technology allows network to go anywherewire cannot reach.


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Reduced cost of ownership: While the initial cost ofwireless LAN can be

higher than the cost of wired LAN hardware, it is envisaged that the

overall installation expenses and life cycle costs can be significantly lower.

Long-term cost-benefits are greater in dynamic environment requiring

frequent moves and changes.

Scalability: Wireless LAN can be configured in a variety oftopologies to

meet the users need and can be easily scaled to cover a large area with

thousands of users roaming within it.


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IEEE 802.11 Architecture

IEEE has defined the specifications for a Wireless LAN, called IEEE 802.11,

which covers the physical and data link layers.

The standard defines two kinds of services :

BSS (Basic Service Set)

ESS (Extended Service Set)


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BSS(Basic Service Set )

IEEE 802.11defines the BSS as the building block of a wireless LAN.

A basic service set is made of stationary or mobile wireless stations and an

optional central base station, known as the access point (AP).

BSS without an AP is stand alone network and can not send data to other

BSSs. It is called an ad-hoc network architecture.

In this architecture, stations can form a network without the need of an

AP.

They can locate one another and agree to be part of a BSS.

A BSS with an AP is sometimes referred to as an infrastructure network.


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ESS (Extended Service Set)

An ESS is made up of two or more BSSs with APs.

In this case, the BSSs are connected through a distribution system, which is usually

a wired LAN.

The distribution system connects the APs in the BSSs.

IEEE 802.11 des not restrict the distribution system; it can be any IEEE LAN such as

Ethernet.

The extended service set uses two types of stations :

Mobile :- Mobile stations are normal stations inside a BSS.

Stationary :- Stationary stations are AP stations that are part of a wired LAN.


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ESS

When BSSs are connected, the stations within reach of one

another can communicate without the use of an AP.

However, communication between two stations in two different BSSs

usually occurs via two APs.

The idea is similar to communication in a cellular network if we considereach BSS to be cell. Each AP to be a base station.

Note that a mobile station can belong to more than one BSS at the same

time.


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IEEE 802.11, defines three types of satations based on their

mobility in a wireless Lan

No-transition A station with no-transition mobility is either

stationary or moving only inside a BSS

BSS-transition A station with BSS-transition mobility can move from

one BSS to another, but the movement is confined inside one ESS.

ESS-transition

A station with ESS-transition mobility canmove from one ESS to another.


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Physical layer specifications


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FHSS (Frequency Hopping Spread Spectrum

The idea behind spread spectrum is to spread the signal over a wider

frequency band, so as to make jamming andinterception more difficultand to minimize the effect of interference from other devices.

In FH it is done by transmitting the signal over a random sequence of

frequencies; that is, first transmitting at one frequency, then second, then

a third and so on.

The random sequence of frequencies is generated with the help of apseudorandom number generator.

As both the receiver and sender use the same algorithm to generate

random sequence, both the devices hop frequencies in a synchronous

manner and frames transmitted by the sender are received correctly by

the receiver. This is somewhat similar to sending different parts of one song over

several FM channels.

Eavesdroppers hear only unintelligible blips and any attempt to jam the

signal results in damaging a few bits only.


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DSSS(Direct Sequence Spread

Spectrum) With direct sequence spread spectrum the transmission signal is spread

over an allowed band (for example 25MHz).

A random binary string is used to modulate the transmitted signal. This

random string is called the spreading code.

The data bits are mapped to into a pattern of "chips" and mapped back

into a bit at the destination. The number of chips that represent a bit is the spreading ratio.

The higher the spreading ratio, the more the signal is resistant to

interference.

The lower the spreading ratio, the more bandwidth is available to the user.

The FCC dictates that the spreading ratio must be more than ten.

Most products have a spreading ratio of less than 20 and the new IEEE

802.11 standard requires a spreading ratio of eleven.

The transmitter and the receiver must be synchronized with the same

spreading code.


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Problems in Wireless Medium

Some of the challenges are:

The wireless LAN is prone to more interference and is less reliable.

The wireless LAN is susceptible to unwanted interception leading to

security problems.

There are so called hidden station and exposed station problems.


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The Hidden Station Problem

Consider a situation when A is transmitting to B, as in the Fig. If C senses

the media, it will not hear anything because it is out of range, and thus will

falsely conclude that no transmission is going on and will start transmit to

B. The transmission will interfere at B, wiping out the frame from

A. The problem of a station not been able to detect a potential

competitor for the medium because the competitor is too far away is

referred as Hidden Station Problem.

As in the described scenario C act as a hidden station to A, which

is also competing for the medium.


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IEEE 802.11 defines two MAC sublayers

DCF (Distributed Coordination Function)

PCF (Point Coordination Function)


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DCF (Distributed Coordination Function)

DCF uses CSMA/CA as the access method.

Wireless LANs can not implement CSMA/CD due to following

reasons :

Costly stations and increased bandwidth requirements.

Hidden station problem

Signal fading if distance is great


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CSMA/CA

flowchart


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CSMA/CA process

DIFS Distributed Inter frame Space After the station is found to be

idle, the station waits for a period of time.

RTS Request to Send after receive DIFS , then the station sends a

control frame, RTS After receiving the RTS and waiting a period of time

called the SIFS (the short inter frame space)

Then the destination station sends a control frame, called the clear to

send (CTS).


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PCF

Point Coordination Function

It is an optional access method that can be implemented in an

infrastructure network (not in an ad-hoc network).

It is implemented on top of the DCF and is used mostly for time-sensitive

transmission.

PCF has centralized, contention free polling access method.

The AP performs polling for stations that are being polled.

The stations are polled one after another, sending any data they have to

the AP.


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Frame format


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Control frames


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Subfields in FC field

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