MOB Subject 6 - IEEE 802.11 standards Wifi operation

MOB Subject 6 - IEEE 802.11 standards Wifi operation

MOB Subject 6 - IEEE 802.11 standards

Wifi operation

1 Generality

1 One wishes to install a wireless local network in a closed environment (office) In order to cover all the desired zone, it proves necessary to install several Wifi access points (802.11b) One wishes that whatever the place or the users is, they do not have to configure differently their Wifi card? How is that possible?

A set of basic services (BSS) is the set form a acces point and cover stations in its area A system Distribution (Distribution System-DS) can connect several BSS between them to form a set of services outstretched (Extended Service Set -ESS)

2 Would you advise to use the same frequency channel or to choose different for each access point? Why?

By using frequency channels different, we reduce risks interference and we increases the bandwidth

3 What is the interest to use the RTS/CTS mechanism?

The aim is to avoid the problem of hidden terminals

If two wireless terminals do not see, they are likely to issue in the same time and I do not know that there is a collision This is the hidden terminal problem This problem was solved by the exchange of packets RTS / CTS (Request To Send / Clear To Send), which gives the control sentence a base station, which normally sees all the terminals that are in communication

her

A map will emit sends an RTS message to request authorization the base station The application

is parameter by a time value that indicates given the quantity of e sueh send The base station with a message repond CTS parameter with time value During the time indicated PERIOD, only the map that has authorization may be issued In principle, all cards see the message CTS The use of RTS / CTS signaling induced overload If there is no terminal or hidden messages to send are of small size, this overhead is not profitable 802.11b provides a threshold for the implementation of mechanically RTS / CTS If message size does not pass of the threshold, the sender tries to transmit directly without seeking authorization from RTS

4 In 802.11 standards, the flow of MAC frames is determined partly by its type The control frames such that RTS and CTS do not use systematically the maximum flow Why?

For reasons of compatibility with earlier versions, the flow of frames control should be the maximum flow rates authorized by the standards used by the Connected Equipment

5 What makes it possible to increase the flows between the different families of protocol 802.11?

The data modulation: BPSK to 802.11, CCK (Complementary Code Keying) for 802.11b and OFDM for 802.11a and 802.11g

6 An access point 802.11g sends its physical frame of coded data in OFDM.

a) What does that imply for a node provided of an 802.11b card which would be in the same zone?

It can detect the data transmitted 802.11g These data are then collected as interference The problem of hidden terminal arises

b) Propose a solution which allows solving this problem

Email RTS / CTS flow rates understandable by the nodes so that 802.11 they know that a transmission will take place between different points This solution has been adopted by standards bodies

7 Does one can make communicate together equipments provided of transmission cards 802.11a and 802.11b?

No This is not the same frequency band and not the same encoding: 5 GHz + OFDM for 802.11a, 2.4 GHz + CCK for 802.11b

8 The same question with 802.11a and 802.11g?

Not the same frequency band 5 GHz for 802.11a, 2.4 GHz for 802.11g Nevertheless, in the longer term, the IEEE 802.11g is under elaboration an important step towards the realization of dual-band 2.4 GHz / 5 GHz OFDM already being required to operate at 5 GHz, implementing 802.11g in a dual-band equipment does not add complexity "hardware" additional product finished

9 Propose 4 application domains of wireless network?

1 Extending a network (LAN): integration of a wireless LAN and a wired local network in order

to extend the coverage of local network;

2 Interconnection of local networks: two networks interconnect local radio;

3 Realization of an ad hoc network, ie a network without a central server, installed temporarily

to cope with immediate needs Realization of a network of sensors or a mesh network

4 Setting up a temporary network: lounge, conference

10 Which characteristics appear important to you at the time of the specification phase of a new wireless local network?

 The flow: The MAC protocol must use the wireless medium more efficently possible so as to maximize the capacity

 The number of users g Maple: the wireless local area networks can be brought

to manage hundreds of nodes spread over several cells

 The interconnection with the local network of the heart: in most cases, the inter-connection with terminals of the network is required In local networks without Wireless infrastructure has, this task is easily accomplished thanks to modules control that connect the two types of local networks It may also be a need ome of mobile users and wireless network ad-hocs

 The coverage area of a cell: the diameter of a typical cell of a network WLAN varies between 100 and 300 m

 The energy consumption and battery life: Mobile Users use terminals fed by batteries Good autonomy is often required to work over in wireless environments A pro-MAC protocol which requires a constant scan of the access point or induces frequent exchanges with a base station is therefore not appropriate for this view

 The typical implementations of wireless local area networks are in implements features that can reduce energy consumption when the system is not used as standby

 The robustness of transmission and security: if not properly designed, a network Wireless LAN will be subject to interference and can easily be listened to Design

a wireless LAN must provide a robust transmission including

a very noisy environment and must provide a certain level of protection against the tapping The wireless local area networks becoming increasingly popular, it is likely that two or more networks functioning in the same area or in an area or interferences between local networks are possible Such interference can interfere with the normal operation of a MAC algorithm and allow the wrong access has one of these local networks

 The ability to operate without a license: users prefer to purchase and operate equipment for wireless LAN without having to reserve a license on the tape

of frequencies used by the local network

 The management of mobility: the MAC protocol as used in wireless LAN should enable mobile devices to connect to another cell (han-do/roaming)

 The con guration dynamic MAC addressing and managing the local network must allow the dynamic addition and automates terminals, and their removal displacement seamless service to other users

11 Which difference is there between a mono-cellular and multi-cellular wireless local network?

Wireless LAN single cell: all terminals are within reach of a similar access point

Wireless LAN multi-cell: several access points are interconnected to

work-to a local wired network, each access point manages a number of terminals

located within reach

12 What are the advantages and the disadvantages of wireless local network using infrared technologies?

Advantages:

 The infrared frequency band is very large (virtually unlimited), which o

re the possibility of achieving extremely high flow rates

 The infrared band is regulated in any country, the beaches of microwave frequencies wave are

 The infrared light is reflected diffusely by small colored objects; thus, it is possible to use Refexions ceiling to cover entirely in one room

 The infrared light does not penetrate walls and opaque objects This has two advantages: first, the infrared communication can be more easily

secured against the listening as microwave communications, and secondly,

a separate infrared system can be operated in each piece of building without interference, which allows to build very large local networks infrared

 Another advantage of the infrared: the equipment is relatively inexpensive and simple

Disadvantages:

 Many equipment an indoor infrared radiation suffer interference intense enough from the sunlight and the lighting These rays appear as ambient noise for an infrared receiver, which requires the use of a higher power level than would otherwise be necessary and which also limits the scope

 Increasing the power emitted is limited by the constraints of safety (eye protection) and reasonable power consumption

13 Which difference is there between an access point and a gate within the meaning of 802.11 standards?

For the purposes of the standard, a point of access for connecting several wireless local area networks 802.11 but does not access a local network of other technology

A portal provides him an interconnection point between a wireless LAN 802.11 a local wired network

In practice, the access points sold in stores are also equipped with a based portal

14 List some services offered by the IEEE 802.11 standard

 Authentication: used by the terminals to identify each other Each terminal proves his knowledge of a password

 Association: establishment of an association between a terminal and an access point Exchange of information on the possibilities of transmission of access point and

terminals

 Reassociation: to transfer an association established an access point to another, allowing the roaming terminal in a BSS to another

 Dissociation: hypnotics by a terminal or a point of acc are an association has ended A terminal must send this hypnotics before leaving the lattice or extinction of

 Desauthentication: This service is invoked when an authentication ends

 Distribution: used by the terminals to share them when MAC frames the frame must pass through the distribution system (Distribution System (DS)) to pass a terminal attached to

a set of basic services (Basic Service Set (BSS)) has a terminal attached to another BSS

 Integration: enables the transfer of data between a terminal of a local network 802.11 and

a terminal of a local network integrated 802.x (MSDU delivery: MAC Service Data Unit delivery)

 Security and encryption: avoids reading the contents of messages by other persons as the intended recipient

15 How is the association concept linked to the mobility?

The mobility makes reference to the types of transition can be a mobile terminal in an

environment 802.11 no transition, roaming from one BSS to another within a same ESS roaming

in an ESS has a another

The association is a service that allows a terminal that has made a transition to identify the access point in a BSS, which then allow him to participate in exchanges of data with other terminals

16 In what has the IEEE 802.11a standard a better potential than the IEEE 802.11b or IEEE 802.11g standard?

The IEEE 802.11a greater potential than other standards in the frequency band of 2.4 GHz because it has a much wider bandwidth In terms of frequency, 802.11a has 8 disjoint bands in the U.S (200 MHz) and 13 in Europe, against 3 disjoint bands for 802.11b (83 MHz) and three bands separated for 802.11g (83 MHz)

The IEEE 802.11a, however, difficult to win because its installation is more complex and 802.11g makes it an important competition because of its compatibility

with 802.11b and its equivalent flow rates

17 Why is the effective flow lower than its theoretical flow?

On the one hand, the flow varies depending on the quality of the link, the scope and interference with the environment On the other hand, the use of time intervals (Inter Frames Spaces (IFS) in English) used to manage access to waste a lot of time for the actual transmission

18 Wifi telephony terminals are already on sale Why this solution is not viable in the long term?

In wireless networks, there is no QoS The voice packets are coming into competition with other transmissions without having a particular priority The voice packets are then strongly

retarded.For this to work to the reasonable, must be the only wireless device connects to the access point The IEEE 802.11 is expected to improve this by assigning a high priority to sensitive packet

2 Access Method

1 Represent graphically, with assistance of a diagram of floods, the algorithm follow-up by a transmitter to send data

a station wants

to transmit

(data)

Listeningmedium

the

medium is

free

Waiting DIFS

No

Yes

the

medium is

free

Yes

No

transmit (data)

receipt of

Yes

data transmission

successful

limit retransmissions

data transmission failed

Yes

Wait until the media is free Waiting DIFS

Timer already calculated?

load timer timer backoff No

decremented by one timeslot and listens media

the medium is free

Yes

timer expires No

Yes

receipt of

an ACK No

data transmission successful Yes

2 Supplement the figure 1 by indicating the moments when the frames will be sent according

to the access method define by standard 802.11 The moments when a terminal wants to emit are marks by an arrow One will take the duration of each data frame into account as well as the value of the withdrawal timer specific to each terminal Is there had collision?

See Fig below

Yes, there was a collision between the issuance of Terminal 2 and the terminal 5

Fig.1 – Emission of data packets according to the DFWMAC-DCF protocol

3 When a terminal wishes to emit but notes that the support is not free, it waits for duration equal to the value of a withdrawal timer before trying new emission The value of this withdrawal timer corresponds with a time slots, take out randomly in an interval included

between 0 and W This interval is called contention window The value of W varies according to the collisions which occur Initially, W = 7 Each time that one collision occurs,

W is double W is however limited the maximum value of 255 When a transmission effects

successfully, W is reinitialized In your opinion, why has one chosen this proceed?

If the window is small, the risk of collision are increased If it is large, there is less good use of the network because the terminals are inactive longer on average without delivering In case of low load, it is preferable that W is small

4 Does the CSMA/CA access method appear to you that it is more or less efficacy than the CSMA/CD of the Ethernet network?

In CSMA / CD, each terminal shall issue the media is free The collision detection is immediate There is no mechanism for payment The withdrawal mechanism is implemented only in a collision

In CSMA / CA, when a terminal wishes to emit but notes that the media is not free, it expects outset for a period equal to the value of a timer for withdrawal before attempting to transmit again An additional period is also introduced to the acquittal This affects the utilization of support and therefore effective throughput of the system but can reduce the risk of collision

The resolution of the problem of hidden terminal mechanism requires the use of RTS / CTS, which also leads to further delay

The actual throughput of CSMA / CA is lower than that of an Ethernet network

5 RTS/CTS Mechanism

a) Does the use of RTS/CTS packets allow avoiding collisions?

No

b) What are the exchanges of RTS/CTS packets used for?

To avoid the problem of hidden terminal

To reduce the risk of collision because the period of restraint corresponds to the transmission of short messages of control rather than data frames