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WiFi Projection
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 Wireless
LAN technology, originally developed for easy and secure communications by the
military, declassified in the mid-70’s by the University of Hawaii to
connect its island-bound campuses in the late '60's , has been adapted for use
in business enterprise environments, small businesses and more recently in the
home environment. It and the modulation scheme, Spread Spectrum (declassified in
the mid 70's), originally developed to create a secure communication medium in
the field of battle, has evolved to provide an ease of use alternative to
traditional hard-wired network installations. The wireless LAN ease of use
concept extends to address the users’ perception that connecting to network
devices is difficult and has required expert network knowledge.
Since
the initial military implementation was costly, and was not governed by
sufficient standards bodies, commercial adoption was slow to take hold.
Following the publication of several important IEEE standards, initially the
IEEE 802.3 Ethernet standard, and specifically the 802.11 standard, the wide
spread adoption of wireless technology has taken place. The IEEE 802.11b
(11 megabits per second) standard is the most widely used technology today for wireless LANs.
The Different Standards
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802.11a
As the need for bandwidth grew, 802.11a was specified in 1999 as a Physical
Layer (PHY) standard to operate at 5GHz frequency band with possible data
rates between 6-54 Mbps. Those products are slowly becoming available to a
wider public. The additional advantage of 802.11a over 802.11b is that there
is also much less interference with radio at its 5GHz frequency in
comparison to 802.11b and 802.11g. 802.11a
does increase its transmission power to 50mW, but even so, 802.11a does not
handle either longer transmit distances or obtruding objects (walls, furniture,
etc.) as well as 802.11b.
Communicating devices using 802.11b wireless technology actually require the
simultaneous use of two distinct technologies; RF radio transmission and
traditional Internet Protocol (IP) network protocols. In order to have a
complete overview of wireless fundamentals, it is necessary to understand the
concepts that originate from both disciplines. 802.11b wireless hardware is
typically used in either a simple Ad-hoc configuration or in an increasingly the
more common Infrastructure configuration. The InFocus LiteShow product utilizes
the simple Ad-hoc configuration, allowing a mobile device such as a laptop to
connect directly to a LiteShow-enabled projector to provide for cable-free
presentations.
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802.11d
As the operation, especially in the 5GHz range, may differ from country to
country (or domain to domain), the 802.11d protocol was established. It also
better defined interoperability issues.
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802.11e
With the expansion of wireless device technologies and the feature-rich
applications already in development for video and audio (voice), it was
apparent that the 802.11 PHYs were not quite optimized to full fill such
tasks. This lead to the development of 802.11e which refines a 802.11 medium
access layer (MAC) to prioritize traffic to improve quality of service (QoS)
for support of video and audio. This standard is still in progress. The non-existence of 802.11e should
not impact wireless decisions as it will affect all of the PHY layers for
802.11.
The
original 802.11 media access control protocol was designed with two modes of
communication for wireless stations. The first, Distributed Coordination
Function (DCF), is based on Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA), sometimes referred to as "listen before talk." A station
waits for a quiet period on the network and begins to transmit data and detect
collisions. DCF provides coordination, but it doesn't support any type of
priority access of the wireless medium.
An
optional second mode, Point Coordination Function (PCF), supports time-sensitive
traffic flows. Wireless access points periodically send beacon frames to
communicate network identification and management parameters specific to the
wireless network. Between the sending of beacon frames, PCF splits the time into
a contention-free period and a contention period. With PCF enabled, a station
can transmit data during contention-free polling periods. However, PCF hasn't
been implemented widely because the technology's transmission times are
unpredictable.
Because
DCF and PCF do not differentiate between traffic types or sources, the IEEE is
proposing enhancements in 802.11e to both coordination modes to facilitate QoS.
These changes would let critical service requirements be fulfilled while
maintaining backward-compatibility with current 802.11 standards.
The proposed enhancement to DCF - Enhanced Distribution Coordination Function (EDCF)
- introduces the concept of traffic categories. Each station has eight traffic
categories, or priority levels. Using EDCF, stations try to send data after
detecting the medium is idle and after waiting a period of time defined by the
corresponding traffic category called the Arbitration Interframe Space (AIFS). A
higher-priority traffic category will have a shorter AIFS than a lower-priority
traffic category. Thus stations with lower-priority traffic must wait longer
than those with high-priority traffic before trying to access the medium.
To avoid
collisions within a traffic category, the station counts down an additional
random number of time slots, known as a contention window, before attempting to
transmit data. If another station transmits before the countdown has ended, the
station waits for the next idle period, after which it continues the countdown
where it left off.
No guarantees of service are provided, but EDCF establishes a probabilistic
priority mechanism to allocate bandwidth based on traffic categories.
Another way 802.11e aims to extend the polling mechanism of PCF is with the
Hybrid Coordination Function (HCF). A hybrid controller polls stations during a
contention-free period. The polling grants a station a specific start time and a
maximum transmit duration.
EDCF appears to be gaining more early acceptance than HCF. The 802.11e standard
isn't likely to be ratified until next spring or later. In the meantime, a group
of vendors have proposed Wireless Multimedia Enhancements (WME), much like Wi-Fi
Protected Access , to provide an interim QoS solution for 802.11 networks.
Without a standard, the risk of non-interoperable mechanisms proliferating in
the marketplace would inhibit the overall goals of the 802.11e standard. The
intention of WME is to provide a well-defined and accepted 802.11 QoS mechanism
that will prevent the spread of non-interoperable methods while waiting for the
ratification of the 802.11e standard.
The process of creating a definitive standard can be slow, but the IEEE 802.11e
standard will address existing QoS concerns
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802.11f
This protocol specification addresses the roaming need for transmission for
a user from one access point (AP) to another and ensures the continuity of
transmission; it would ultimately provide inter-access point protocol. Until
this becomes commonplace, the safest bet will be to standardize on the same
vendor for all of your access points.
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802.11g
The "g" technology specification is still in the works and is the most
recent redefinition for 802.11. Its goal is defined as extension to 20+ Mbps
rate by adding one more channel to the current three in the operation
spectrum of 2.4GHz, which would compete with 802.11a rates with
802.11b's better transmit distances, and handling of reflections and occlusions.
And because it works in the same 2.4GHz frequency, it ought to allow easier
interoperability between 802.11b and 802.11g.
802.11g also offers backward compatibility with 802.11b, so that you won't have
to toss all the 802.11b gear you've accumulated. But compatibility remains the
large question mark over 802.11b products, and it's quite likely that not all
802.11b products will talk to 802.11g access points.
802.11g
has been found to deliver better than 2X the throughput of 802.11b, although
it's well shy of matching 802.11a's performance at close range. But like both a
and b, 802.11g still lacks Quality of Service (QoS) provisions. We won't see
these until the arrival of the 802.11e spec later this year.
Even with the improved throughput, 802.11g still provides what's called best
effort service to all traffic on the wire, whether it's streaming video or
copying a large file.
802.11h
An extension of 802.11a to satisfy regulations in Europe for the
spectrum band of 5GHz by providing dynamic channel selection (DCS) and
transmit power control (TPC). If it does, it may possibly supersede 802.11a.
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802.11n
The successor to 802.11a/g aimed at consumer applications that require very
high throughput, like HDTV and streaming video. It is expected to run up to
50 times faster than the current wireless standard, 802.11b, and offer an
expanded operating distance. 802.11n isn't expected to become a standard
until late 2007.
The use of 802.11g and 802.11a technology is
rapidly growing, These two initiatives offer increased data
rates (54 megabits per second) and additional bandwidth utilization this should
allow wireless transmission of HDTV to your projector. 802.11g is a kind
of hybrid of 11a and 11b. It uses the same transmission technology found in
802.11a, which is called Orthogonal Frequency Division Multiplexing (OFDM). This
increases the amount of data transmitted in a given time slice. However, unlike
802.11a, which operates in a 5GHz band, 802.11g uses carrier frequency bands
that are around 11b's 2.4GHz primary carrier frequency.
Wireless Network
Topologies
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Ad-hoc Network Topology In this typical Ad-hoc wireless network topology; notice
that there is no requirement to have the stations connected into the Local Area
Network (LAN). In Ad-hoc mode, the stations form their own local network where
the end nodes communicate peer-to-peer without the requirement of an Access
Point. Two or more stations establish a Basic Service Set by recognizing and
communicating with each other. This type of network is therefore known as an
Independent Basic Service Set (IBSS).
Infrastructure Network Topology
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In this typical wireless Infrastructure network topology, there is a mixture of
hard-wired devices connected to the LAN that co-exist with wireless mobile
stations. In Infrastructure mode, the stations connect to an existing network
through an access point. The role of the access Point is to create a bridge
between the wireless network and the hard-wired network and therefore is
considered part of the wired network infrastructure. The mobile stations are
constrained only by the breadth of the wireless LAN that is established by the
physical location of the Access Point.
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