Infra-Red
Physical Layer
One infrared standard is supported which operates in the
850-to-950nM band with peak power of 2 W. The modulation for infrared
is accomplished using either 4 or 16-level pulse-positioning modulation.
The physical layer supports two data rates; 1 and 2Mbps.
Direct
Sequencing Spread Spectrum (DSSS) Physical Layer
The DSSS physical layer uses an 11-bit Barker Sequence to
spread the data before it is transmitted. Each bit transmitted is modulated
by the 11-bit sequence. This process spreads the RF energy across a
wider bandwidth than would be required to transmit the raw data. The
processing gain of the system is defined as 10x the log of the ratio
of spreading rate (also know as the chip rate) to the data. The receiver
despreads the RF input to recover the original data. The advantage of
this technique is that it reduces the effect of narrowband sources of
interference. This sequence provides 10.4dB of processing gain which
meets the minimum requirements for the rules set forth by the FCC. The
spreading architecture used in the direct sequence physical layer is
not to be confused with CDMA. All 802.11 compliant products utilize
the same PN code and therefore do not have a set of codes available
as is required for CDMA operation.
Frequency
Hopping Spread Spectrum (FHSS) Physical Layer
The FHSS physical layer has 22 hop patterns to choose from.
The frequency hop physical layer is required to hop across the 2.4GHz
ISM band covering 79 channels. Each channel occupies 1Mhz of bandwidth
and must hop at the minimum rate specified by the regulatory bodies
of the intended country. A minimum hop rate of 2.5 hops per second is
specified for the United States.
Each of
the physical layers use their own unique header to synchronize the receiver
and to determine signal modulation format and data packet length. The
physical layer headers are always transmitted at 1Mbps. Predefined fields
in the headers provide the option to increase the data rate to 2 Mbps
for the actual data packet.
The
MAC Layer
The
MAC layer specification for 802.11 has similarities to the 802.3 Ethernet
wired line standard. The protocol for 802.11 uses a protocol scheme
know as carrier-sense, multiple access, collision avoidance (CSMA/CA).
This protocol avoids collisions instead of detecting a collision like
the algorithm used in 802.3. It is difficult to detect collisions in
an RF transmission network and it is for this reason that collision
avoidance is used. The MAC layer operates together with the physical
layer by sampling the energy over the medium transmitting data. The
physical layer uses a clear channel assessment (CCA) algorithm to determine
if the channel is clear. This is accomplished by measuring the RF energy
at the antenna and determining the strength of the received signal.
This measured signal is commonly known as RSSI. If the received signal
strength is below a specified threshold the channel is declared clear
and the MAC layer is given the clear channel status for data transmission.
If the RF energy is above the threshold, data transmissions are deferred
in accordance with the protocol rules. The standard provides another
option for CCA that can be alone or with the RSSI measurement. Carrier
sense can be used to determine if the channel is available. This technique
is more selective sense since it verifies that the signal is the same
carrier type as 802.11 transmitters. The best method to use depends
upon the levels of interference in the operating environment. The CSMA/CA
protocol allows for options the can minimize collisions by using request
to send (RTS), clear-to-send (CTS), data and acknowledge (ACK) transmission
frames, in a sequential fashion. Communications is established when
one of the wireless nodes sends a short message RTS frame. The RTS frame
includes the destination and the length of message. The message duration
is known as the network allocation vector (NAV). The NAV alerts all
others in the medium, to back off for the duration of the transmission.
The receiving station issues a CTS frame which echoes the senders address
and the NAV. If the CTS frame is not received, it is assumed that a
collision occurred and the RTS process starts over. After the data frame
is received, an ACK frame is sent back verifying a successful data transmission.
A common limitation with wireless LAN systems is the "hidden node" problem.
This can disrupt 40% or more of the communications in a highly loaded
LAN environment. It occurs when there is a station in a service set
that cannot detect the transmission of another station to detect that
the media is busy. In figure 1 stations A and B can communicate. However
an obstruction prevents station C from receiving station A and it cannot
determine when the channel is busy. Therefore both stations A and C
could try to transmit at the same time to station B. The use of RTS,
CTS, Data and ACK sequences helps the prevent the disruptions caused
by this problem.
Figure 1
Security
provisions are addressed in the standard as an optional feature for
those concerned about eaves dropping. The data security is accomplished
by a complex encryption technique know as the Wired Equivalent Privacy
Algorithm (WEP). WEP is based on protecting the transmitted data over
the RF medium using a 64-bit seed key and the RC4 encryption algorithm.
WEP, when enabled, only protects the data packet information and does
not protect the physical layer header so that other stations on the
network can listen to the control data needed to manage the network.
However, the other stations cannot decrypt the data portions of the
packet.
Power
management is supported at the MAC level for those applications requiring
mobility under battery operation. Provisions are made in the protocol
for the portable stations to go to low power "sleep" mode during a time
interval defined by the base station.
What
the future holds
The
IEEE 802.11 WLAN standard will be one of the first generations of standardization
for wireless LAN networks. This standard will set the pace for the next
generation standard, addressing the demands for higher performance higher
data rates and higher frequency bands. Interoperability between WLAN
products from different equipment manufacturers will be important to
the success of the standard. These products will be implemented on ISA,
or PCMCIA cards for use in handheld personal computers, PDAs, laptops
or desktop applications. Wireless LAN applications are currently mostly
in vertical markets. It is expected that many horizontal applications
will follow as 802.11 network infrastructure is installed. Over time
the increase in demand for 802.11 products is expected to increase competition
and to make wireless LANs more competitive and economical for virtually
all applications requiring wireless connectivity. On the horizon is
the need for higher data rates, for applications requiring wireless
connectivity at 10Mbps and higher. This will allow WLANs to match the
data rate of the majority of wired LANs. There is no current definition
of the characteristics for the higher data rate signal. However, for
many of the options available to achieve it there is a clear upgrade
path for to maintain interoperability with 1 and 2 Mbps systems while
providing the higher data rate as well.
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