The document discusses wireless networks and the 802.11 standard. It notes that wireless networks provide flexibility in older buildings by reducing the need for construction. While wireless networks have limited bandwidth, the limiting factor for small hotspots is likely the cost of wide area network bandwidth to infrastructure. The document also summarizes the evolution of the 802.11 standard, from the initial 1997 release with 2 Mbps speeds to later improvements offering speeds up to 54 Mbps.

1. REDES INALÁMBRICAS
Programación de dispositivos móviles.
Prof. Joel Ruiz Ibarra.
CESUES-UABJ.
Agosto/2011
Friday, August 19, 2011

2. WHY WIRELESS?
This simple example ignores the challenges of scale.
Naturally, if the new users will overload the existing
infrastructure, the infrastructure itself will need to be
beefed up. Infrastructure expansion can be expensive
and time- consuming, especially if it involves legal and
regulatory approval. However, my basic point holds:
adding a user to a wireless network can often be
reduced to a matter of configuration (moving or
changing bits) while adding a user to a fixed network
requires making physical connections (moving atoms),
and moving bits is easier than moving atoms.
Friday, August 19, 2011

3. WHY WIRELESS?
Flexibility may be particularly important in older buildings because it reduces the
need for constructions.
Once a building is declared historical, remodeling can be particularly difficult.
In addition to meeting owner requirements, historical preservation agencies must
be satisfied that new construction is not desecrating the past.
Wireless networks can be deployed extremely rapidly in such environments
because there is only a small wired network to install.
Friday, August 19, 2011

4. WIRELESS DRAWBACK.
Although wireless LANs have somewhat limited
bandwidth, the limiting factor in networking a small
hot spot is likely to be the cost of WAN bandwidth to
the supporting infrastructure.
Friday, August 19, 2011

5. WIRELESS DRAWBACK.
Friday, August 19, 2011

6. THE LIMITS.
Wireless networks do not replace fixed networks.
The main advantage of mobility is that the network
user is moving.
Friday, August 19, 2011

7. THE LIMITS.
Servers and other data center equipment must access
data, but the physical location of the server is
irrelevant.
As long as the servers do not move, they may as well
be connected to wires that do not move.
Friday, August 19, 2011

8. 802.11 were initially released in 1997. 802.11 included an infrared (IR) layer that was
never widely deployed, as well as two spread-spectrum radio layers: frequency hopping
(FH) and direct sequence (DS). (The differences between these two radio layers is
described in Chapter 10.) Initial 802.11 products were limited to 2 Mbps, which is quite
THE EVOLUTION.
slow by modern network standards. The IEEE 802.11 working group quickly began
working on faster radio layers and standardized both 802.11a and 802.11b in 1999.
Products based on 802.11b were released in 1999 and can operate at speeds of up to 11
Mbps. 802.11a uses a third radio technique called orthogonal frequency division
multiplexing (OFDM). 802.11a operates in a different frequency band entirely and
currently has regulatory approval only in the United States. As you can see from the
table, 802.11 already provides speeds faster than 10BASE-T Ethernet and is reasonably
competitive with Fast Ethernet.
Table 1-2. Comparison of 802.11 standards
IEEE Frequency
Speed Notes
standard band
1 Mbps First standard (1997). Featured both frequency-
802.11 2.4 GHz hopping and direct-sequence modulation
2 Mbps techniques.
up to 54 Second standard (1999), but products not released
802.11a 5 GHz
Mbps until late 2000.
5.5 Mbps Third standard, but second wave of products. The
802.11b 2.4 GHz most common 802.11 equipment as this book was
11 Mbps written.
up to 54
802.11g 2.4 GHz Not yet standardized.
Mbps
Friday, August 19, 2011

9. Chapter 3. The 802.11 MAC
This chapter begins our exploration of the 802.11 standard in depth. Chapter 2 provided a
high-level overview of the standard and discussed some of its fundamental attributes.
MEDIUM ACCESS CONTROL
You are now at a fork in the book. Straight ahead lies a great deal of information on the
802.11 specifications. It is possible, however, to build a wired network without a
thorough and detailed understanding of the protocols, and the same is true for wireless
networks. However, there are a number of situations in which you may need a deeper
knowledge of the machinery under the hood:
• Although 802.11 has been widely and rapidly adopted, security issues have
continued to grab headlines. Network managers will undoubtedly be asked to
comment on security issues, especially in any wireless LAN proposals. To
understand and participate in these discussions, read Chapter 5. As I write this,
WEP has been fully broken and the IEEE is forging a successor to it based on
802.1x. [1] Though the final form of the new and improved security framework has
not yet become apparent, it will almost surely be based on 802.1x, which is
described in Chapter 6.
[1]
And as we go to press, 802.1x has reportedly been broken.
• Troubleshooting wireless networks is similar to troubleshooting wired networks
but can be much more complex. As always, a trusty packet sniffer can be an
invaluable aid. To take full advantage of a packet sniffer, though, you need to
understand what the packets mean to interpret your network's behavior.
• Tuning a wireless network is tied intimately to a number of parameters in the
specification. To understand the behavior of your network and what effect the
optimizations will have requires a knowledge of what those parameters really do.
• Device drivers may expose low-level knobs and dials for you to play with. Most
Friday, August 19, 2011 drivers provide good defaults for all of the parameters, but some give you

10. • Although 802.11 has been widely and rapidly adopted, security issues have
continued to grab headlines. Network managers will undoubtedly be asked to
comment on security issues, especially in any wireless LAN proposals. To
understand and participate in these discussions, read Chapter 5. As I write this,
MEDIUM ACCESS CONTROL
WEP has been fully broken and the IEEE is forging a successor to it based on
802.1x. [1] Though the final form of the new and improved security framework has
not yet become apparent, it will almost surely be based on 802.1x, which is
described in Chapter 6.
[1]
And as we go to press, 802.1x has reportedly been broken.
• Troubleshooting wireless networks is similar to troubleshooting wired networks
but can be much more complex. As always, a trusty packet sniffer can be an
invaluable aid. To take full advantage of a packet sniffer, though, you need to
understand what the packets mean to interpret your network's behavior.
• Tuning a wireless network is tied intimately to a number of parameters in the
specification. To understand the behavior of your network and what effect the
optimizations will have requires a knowledge of what those parameters really do.
• Device drivers may expose low-level knobs and dials for you to play with. Most
drivers provide good defaults for all of the parameters, but some give you
freedom to experiment. Open source software users have the source code and are
free to experiment with any and all settings.
• A number of interesting features of the standard have not been implemented by
the current products, but they may be implemented later. As these features are
rolled out, you may need to know what they are and how to use them.
As with many other things in life, the more you know, the better off you are. Ethernet is
usually trouble-free, but serious network administrators have long known that when you
do run into trouble, there is no substitute for thorough knowledge of how the network is
working. To some extent, 802.11 networks have had a "free ride" the past few years.
Friday, August 19, 2011
Because they were cool, users were forgiving when they failed; wireless connectivity was

11. drivers provide good defaults for all of the parameters, but some give you
freedom to experiment. Open source software users have the source code and are
free to experiment with any and all settings.
• A number of interesting features of the standard have not been implemented by
the current products, but they may be implemented later. As these features are
MEDIUM ACCESS CONTROL
rolled out, you may need to know what they are and how to use them.
As with many other things in life, the more you know, the better off you are. Ethernet is
usually trouble-free, but serious network administrators have long known that when you
do run into trouble, there is no substitute for thorough knowledge of how the network is
working. To some extent, 802.11 networks have had a "free ride" the past few years.
Because they were cool, users were forgiving when they failed; wireless connectivity was
a privilege, not a right. And since there were relatively few networks and relatively few
users on those networks, the networks were rarely subjected to severe stresses. An
Ethernet that has only a half dozen nodes is not likely to be a source of problems;
problems occur when you add a few high-capacity servers, a few hundred users, and the
associated bridges and routers to glue everything together. There is no reason to believe
that wireless will be any different. A couple of access points serving a half dozen users
will not reveal any problems. But when the user community grows to a few dozen, and
you have several overlapping wireless networks, each with its own set of access points,
you can expect to see the effects of stress.
That is why you should read this chapter. Now on to the details.
The key to the 802.11 specification is the MAC. It rides on every physical layer and
controls the transmission of user data into the air. It provides the core framing operations
and the interaction with a wired network backbone. Different physical layers may provide
different transmission speeds, all of which are supposed to interoperate.
Friday, August 19, 2011

12. MEDIUM ACCESS CONTROL
that wireless will be any different. A couple of access points serving a half dozen users
will not reveal any problems. But when the user community grows to a few dozen, and
you have several overlapping wireless networks, each with its own set of access points,
you can expect to see the effects of stress.
That is why you should read this chapter. Now on to the details.
The key to the 802.11 specification is the MAC. It rides on every physical layer and
controls the transmission of user data into the air. It provides the core framing operations
and the interaction with a wired network backbone. Different physical layers may provide
different transmission speeds, all of which are supposed to interoperate.
802.11 does not depart from the previous IEEE 802 standards in any radical way. The
standard successfully adapts Ethernet-style networking to radio links. Like Ethernet,
802.11 uses a carrier sense multiple access (CSMA) scheme to control access to the
transmission medium. However, collisions waste valuable transmission capacity, so
rather than the collision detection (CSMA/CD) employed by Ethernet, 802.11 uses
collision avoidance (CSMA/CA). Also like Ethernet, 802.11 uses a distributed access
scheme with no centralized controller. Each 802.11 station uses the same method to gain
access to the medium. The major differences between 802.11 and Ethernet stem from the
differences in the underlying medium.
This chapter provides some insight into the motivations of the MAC designers by
describing some challenges they needed to overcome and describes the rules used for
Friday, August 19, 2011

13. that wireless will be any different. A couple of access points serving a half dozen users
will not reveal any problems. But when the user community grows to a few dozen, and
you have several overlapping wireless networks, each with its own set of access points,
you can expect to see the effects of stress.
MEDIUM chapter. Now on to theCONTROL
That is why you should read this
ACCESS details.
The key to the 802.11 specification is the MAC. It rides on every physical layer and
controls the transmission of user data into the air. It provides the core framing operations
and the interaction with a wired network backbone. Different physical layers may provide
different transmission speeds, all of which are supposed to interoperate.
802.11 does not depart from the previous IEEE 802 standards in any radical way. The
standard successfully adapts Ethernet-style networking to radio links. Like Ethernet,
802.11 uses a carrier sense multiple access (CSMA) scheme to control access to the
transmission medium. However, collisions waste valuable transmission capacity, so
rather than the collision detection (CSMA/CD) employed by Ethernet, 802.11 uses
collision avoidance (CSMA/CA). Also like Ethernet, 802.11 uses a distributed access
scheme with no centralized controller. Each 802.11 station uses the same method to gain
access to the medium. The major differences between 802.11 and Ethernet stem from the
differences in the underlying medium.
This chapter provides some insight into the motivations of the MAC designers by
describing some challenges they needed to overcome and describes the rules used for
access to the medium, as well as the basic frame structure. If you simply want to
understand the basic frame sequences that you will see on an 802.11 network, skip ahead
to the end of this chapter. For further information on the MAC, consult its formal
specification in Clause 9 of the 802.11 standard; detailed MAC state diagrams are in
Annex C.
Friday, August 19, 2011

14. REFERENCES.
802.11 Wireless Networks - Definitive guide. Mattew
Gast. Editorial O’reilly. (2002).
Friday, August 19, 2011