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Master of Computer Science 1 - MOB Mobile Internet and Surrounding
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
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
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
The data modulation: BPSK to 802.11, CCK (Complementary Code Keying) for 802.11b and
OFDM for 802.11a and 802.11g.
Fladenmuller Baey - ”IEEE 802.11standards” 1/8
Master of Computer Science 1 - MOB Mobile Internet and Surrounding
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
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
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.
Fladenmuller Baey - ”IEEE 802.11standards” 2/8
Master of Computer Science 1 - MOB Mobile Internet and Surrounding
 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
 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
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
 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
Fladenmuller Baey - ”IEEE 802.11standards” 3/8
Master of Computer Science 1 - MOB Mobile Internet and Surrounding
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
 Another advantage of the infrared: the equipment is relatively inexpensive and simple.
 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
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
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
 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
 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.
Fladenmuller Baey - ”IEEE 802.11standards” 4/8
Master of Computer Science 1 - MOB Mobile Internet and Surrounding
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
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
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.
Fladenmuller Baey - ”IEEE 802.11standards” 5/8
Master of Computer Science 1 - MOB Mobile Internet and Surrounding
a station wants Wait until the
to transmit media is free
Waiting DIFS
Listening medium
the load timer No already
No timer backoff
medium is calculated?
decremented by one timeslot
Waiting DIFS and listens media
the No medium is
medium is free
transmit (data) No timer
receipt of No limit receipt of
an ACK retransmissions No an ACK
Yes Yes
data transmission
successful data transmission data transmission
failed successful
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