29-09-2012, 11:43 AM
IEEE 802.11 Wireless Local Area Networks (WLANs)
Wireless Local.pdf (Size: 204.5 KB / Downloads: 38)
ABSTRACT
Within IEEE 802.11 Wireless Local Area
Networks (WLANs), client stations can move freely. But due to short
range of their Access Points (APs), they usually need to reassociate
with different APs to continue their communication. While changing
APs, a client station starts a process known as a handoff .This process
can take up to 2 seconds and that is too long for real-time applications
such as Voice over IP (VoIP).There are many solutions to change or
improve the client behavior when doing a handoff. Previously, Virtual
Access Points (VAP) were implemented on APs in which a client
station changes APs without disrupting its current communication.
Based on this new concept, there developed a solution called
Multichannel Virtual Access Points (mVAP) to take advantage of APs
operating on multiple channels. Multichannel Virtual Access Points is
implemented using PACMAP, which is a tool for packet
manipulation, and evaluated its performance. The results show that
mVAP is a new efficient technique for seamless handoffs without
performance degradation.
INTRODUCTION
IEEE 802.11 Wireless Local Area Networks
(WLANs) have become the preferred solution to extend wired
networks due to their rapid deployment and easy configuration.
Wireless networking brings the advantage of mobility allowing clients
to roam freely. In 802.11 infrastructure networks, Access Points (APs)
convey traffic between associated clients and the wired part of the
network. APs have a limited range ,so to extend coverage in a larger
area deployed multiple APs, for example one AP in every office in
the case of an enterprise Wi-Fi network. APs are interconnected
through a Distribution System (DS), generally a wired network to
enable inter-AP communications.
IEEE 802.11 STANDARDS
There are currently three IEEE 802.11 standards:
802.11a, 802.11b and 802.11g. The 802.11a standard operates in the 5
GHz ISM band, and it uses a total of 32 channels of which only 8 do
not overlap. The 802.11b and 802.11g standards both operate in the
2.4 GHz ISM band and use 11 of the 14 possible channels. Of these 11
channels, only three do not overlap. While 802.11b can operate up to a
maximum rate of 11 Mbit/sec, the 802.11g and 802.11a standards can
operate up to a maximum rate of 54 Mbit/sec. The 802.11g standard is
backwards-compatible with the 802.11b standard while the 802.11a
standard, because of the different ISM band, is not compatible with
the other two. We will focus our attention on the IEEE 802.11b
standard even though most of the concepts and notions described here
are still valid for 802.11a and 802.11g. As we said earlier, the 802.11b
operates in the 2.4 GHz ISM band. Its 14 channels are distributed over
the range from 2.402 GHz to 2.483 GHz (see figure 1), each channel
being 22 MHz wide. In US, only the first 11 channels are used. Of
these 11 channels, only channels 1, 6 and 11 do not overlap. So, in a
well conjured wireless network, all or most of the APs will operate on
channel 1, 6 and 11. Also, to avoid co-channel interference, two
adjacent APs should never be on the same channel.
IEEE 802.11 MANAGEMENT FRAMES
The IEEE 802.11 management frames enable
stations to establish and maintain communications. The following are
common IEEE 802.11 management frame subtypes.
Authentication frame: The 802.11 authentication is a process whereby
the access point either accepts or rejects the identity of a STA. The
STA begins the process by sending an authentication frame containing
its identity to the access point. With open system authentication (the
default), the STA sends only one authentication frame, and the access
point responds with an authentication frame as a response indicating
acceptance (or rejection).
Association request frame: 802.11 association enables the access point
to allocate resources for and synchronize with a STA. A STA begins
the association process by sending an association request to an access
point. This frame carries information about the STA (e.g., supported
data rates) and the SSID of the network it wishes to associate with.
After receiving the association request, the access point considers
associating with the STA, and (if accepted) reserves memory space
and establishes an association ID for the STA.
ACCESS POINTS
802.11 infrastructure networks in which Access Points
(APs) convey traffic between associated clients and the wired part of
the network. Examples of such networks are university campuses,
convention centers, airports, and corporation intranets. Because APs
have a limited range, we can extend coverage in a larger area by
deploying multiple APs, for example one AP in every office in the
case of an enterprise Wi-Fi network, thus resulting in a densely
deployed network. APs are interconnected through a Distribution
System (DS),generally a wired network to enable inter-AP
communications.
802.11 HANDOFF PROCESS
Access points (APs) provide wireless connectivity by
bridging packets from the wireless domain to an internal network. Due
to mobility, a device may move and lose the signal from its AP. In that
case, the mobile user should change to a new AP in order to maintain
its wireless connectivity. Fig.4.1 shows the main elements involved in
a layer 2 handoff: the station (STA), the old AP, the new AP, and the
distribution system (DS). It can be observed that basic service sets
(BSS1 and BSS2) must belong to the same extended service set
(ESS1). In the same way, radio channels of each cell (CHX, CHY)
shall be none mutually interfering channels. The operation to change
an association from one AP to another is known as a handover.
Original design of the IEEE 802.11 standard just considered the
handoff signaling in the wireless part.
CONCLUSIONS
I have presented Multichannel VAPs, a
new solution using the VAP scheme for multichannel WLANs, where
the client changes AP without disrupting its current communications.
The advantage of Multichannel VAPs is low latency of hand-offs,
which is required for multimedia applications such as VoIP. The
resulting solution uses an Inter-AP protocol for communication and
cooperation between APs. Thus implemented the idea in a real
environment, using a new version of framework called PACMAP, in
Atheros-based chipset wireless cards. PACMAP runs on top of the
MadWifi driver, in user space, allowing fast development of IEEE
802.11 prototypes.