1. 802.11 NETWORKS

2. IEEE 802.11
 The IEEE standard 802.11 specifies the most
famous family of WLANs in which many products
are available.
 As the standard‟s number indicates, this standard
belongs to the group of 802.x LAN
standards, e.g., 802.3 Ethernet or 802.5 Token
Ring.
 This means that the standard specifies the physical
and medium access layer adapted to the special
requirements of wireless LANs, but offers the same
interface as the others to higher layers to maintain
interoperability.

4. SYSTEM ARCHITECTURE
 Wireless networks can exhibit two different basic
system architectures.
 : infrastructure-based and ad-hoc.
 infrastructure-based network

5.  Several nodes, called stations (STAi), are
connected to access points (AP).
 Stations are terminals with access mechanisms to
the wireless medium and radio contact to the AP.
 The stations and the AP which are within the same
radio coverage form a basic service set (BSSi).
 The example shows two BSSs – BSS1 and BSS2
– which are connected via a distribution system.
 A distribution system connects several BSSs via the
AP to form a single network and thereby extends
the wireless coverage area.
 This network is now called an extended service
set (ESS) and has its own identifier, the ESSID.

6.  Stations within the same ESS may communicate
with each other , even though these stations may
be in different basic service areas and may even be
moving between basic service areas.

7.  The ESSID is the „name‟ of a network and is used
to separate different networks.
 Without knowing the ESSID it should not be
possible to participate in the WLAN.
 The distribution system connects the wireless
networks via the APs with a portal, which forms
the interworking unit to other LANs.
 Stations can select an AP and associate with it.
 The APs support roaming (i.e., changing access
points).
 The distribution system handles data transfer
between the different APs.
 APs provide synchronization within a BSS, support
power management, and can control medium
access to support time-bounded service.

8.  Ad-hoc network
 In addition to infrastructure-based networks, IEEE
802.11 allows the building of ad-hoc networks
between stations, thus forming one or more
independent BSSs (IBSS).
 In this case, an IBSS comprises a group of stations
using the same radio frequency.

9.  Stations STA1, STA2, and STA3 are in
IBSS1, STA4 and STA5 in IBSS2.
 This means for example that STA3 can
communicate directly with STA2 but not with STA5.
 Several IBSSs can either be formed via the
distance between the IBSSs or by using different
carrier frequencies.

10. MULTI BSS ENVIRONMENTS: “ VIRTUAL AP”
 Early 802.11 radio chips had the ability to create a
single BSS.
 An AP can connect users to only one wireless
network.
 In early developments with limited user counts , a
single logical network was sufficient.
 As wireless network grew, one network no longer
sufficed.
 Most organization get regular visitors , many of
whom have 802.11 equipment and need internet
access.
 Guests are not trusted users. One way of coping
with guest access is to create 2 extended service
sets on the same physical infrastructure.

11.  Current 802.11 chipset can create multiple
networks with the same radio.
 Using this chipsets , each AP hardware device can
create 2 BSS, one for the network named guest
and one for the network named internal.
 Wireless device see two separate networks in the
radio domain, and can connect to whatever one
suits their needs.
 This illustrates the development of virtual access
points.
 Each BSS acts like its own self-contained AP, with
its own ESSID and other relative contains.

12. THE DISTRIBUTED SYSTEM
 The DS is responsible for tracking where a station
is physically located and delivering frames
appropriately .
 When a frame is sent to a mobile station , the DS is
charged with the task of delivering it to the access
point serving the mobile station.
 Most access points operates as bridges.
 They have atleast one wireless network interface
and one ethernet network interface.
 Frames between the two network media is
controlled by a bridging engine.

13.  Frames may be sent by the bridge to the wireless
networks; any frames sent by the bridge‟s wireless
port are transmitted to all associated stations.
 The station A can send station B a frame by
relaying the frame through the bridging engine in
AP.

14. INTERACCESS POINT COMMUNICATION AS
PART OF THE DS
 This is a method to manage association.
 A wireless station is associated with only one AP at
a time. All the other Aps in the ESS need to learn
about that station.
 If a wireless station associated with AP4 sends a
frame to a station associated with AP1, the bridging
engine inside AP4 must send the frame over the
backbone ethernet to AP1 so it can be delivered to
its ultimate destination.
 Many access points on the market use an
interaccess point protocol (IAPP) over the
backbone medium

15. NETWORK BOUNDARIES
 802.11 network has fuzzy boundaries.
 As with mobile telephone networks, allowing basic
service areas to overlap increases the probability of
successful transitions between basic service areas
and offers the highest level of network coverage.

16.  Station moving from BSS2 to BSS4 is not likely to
move coverage.
 On the other hand , if ap2 fails the network is cut
into 2 disjoint parts, and station in BSS1 lose
connectivity when moving out of BSS1 into BSS4.
 Coping with coverage holes from access point
failure is a task that requires attention during the
network design phase.

17. OVERLAPPING NETWORK TYPES
 Different types of 802.11 networks may also
overlap.
 Independent BSSs may be created within the basic
within the basic service area of an access point.
 This five stations are assigned to two different
BSS, they may share same wireless medium.
 Stations may obtain access to the medium only by
using the rules specified in the 802.11 MAC; these
rules were carefully designed to enable multiple
802.11 networks to coexit in the same Spatial area.

18. NETWORK SERVICES