Overview
The WLAN market is now in the early stages of what
seems to be mass adoption. In this market we see several technologies competing
each other with different operating characteristics such as modulation type,
data throughput, frequency bandwith, ...
A first, very important family of standards that
needs to be mentioned is the IEEE 802.11 group developing wireless LAN
standards, it includes task groups called 802.11b,a,e,g working on amendments.
802.11
The IEEE 802.11 specifications are wireless standards that define an
"over-the-air" interface between a wireless client and a base station or access
point, as well as among wireless clients. The 802.11 standards can be compared
to the IEEE 802.3 standard for Ethernet for wired LAN's. The existing standard
and its amendments describe WLAN PHY (Physical layer): Spread spectrum, OFDM,
Infrared and MAC layers. Work is proceeding to increase the data rate, improved
QoS-support, enhanced security, extended MAC management functionality.
The 802.11 specifications can be downloaded from
the IEEE webpage using following link: http://www.standards.ieee.org
802.11b
IEEE 802.11b, this is
the famous 11Mbps in the 2.4GHz ISM band Wi-Fi standard ratified in the second
half of 1999. This is the amendment to 802.11 that is most important for us, as
we'll see later in this text. The 11Mbps however, is purely theoretical: Wi-Fi
reaches only about 7 Mbps of throughput due to synchronization issues, ACK
overhead etc.
802.11g
IEEE 802.11g: the IEEE 802.11 Task Group G (TG g)
approved its first draft during the month of november 2001 after a long and
heavy discussion between those supporting PBCC (packet binary convolution
coding, a single carrier technique proposed by Texas Instruments) and OFDM (a
multiple carrier technique submitted by Intersil). The -g group is a natural
speed extension for the 802.11b standard. When completed, it will extend the
highly successful family of IEEE 802.11 standards, with data rates up to 54 Mbps
in the 2.4 GHz band. This draft is based on CCK, OFDM, and PBCC technologies.
The Working Group will meet in January, 2002 in order to refine the TGg draft in
preparation for publication by the second half of 2002. This task group is
working together with FCC to identify potential rule changes for this band that
can increase its utility.
802.11a
The 802.11a Task Group
(TG a) operates in the 5GHz band. Because its operating frequency is higher than
that of 802.11b, 802.11a has a smaller range. It tries to solve this distance
problem by using more power and more efficient data encoding schemes. The higher
frequency band gives the advantage of not residing in the crowded 2.4GHz region
where we see cordless phones, Bluetooth and even microwave ovens operating. The
major advantage is it's speed : the spectrum of 802.11a is divided into 8
subnetwork segments or channels of about 20 MHz each. These channels are
responsible for a number of network nodes. The channels are made up of 52
carriers of 300kHz each, and can present a maximum of 54 Mbps. This speed takes
WLAN from the first generation Ethernet (10 Mbps) to the second (Fast Ethernet,
100Mbps). [9]
The new specification is based on a OFDM modulation scheme. The RF system
operates at 5.15-5.25, 5.25-5.35 and 5.725-5.825 GHz U-NII bands. The OFDM
system provides 8 different data rates between 6 to 54 Mbit/s. It uses BPSK,
QPSK, 16-QAM and 64-QAM modulation schemes coupled with forward error correcting
coding. Important to remember: 802.11b is completely incompatible with 802.11a.
802.11e
The 802.11e Task Group (TG e) is proceeding to
build improved support for quality of service (for example for voice
transmission). The aim is to enhance the current 802.11 MAC to expand support
for LAN applications with Quality of Service requirements, to provide
improvements in security, and in the capabilities and efficiency of the
protocol. These enhancements, in combination with recent improvements in PHY
capabilities from 802.11a and 802.11b, will increase overall system performance,
and expand the application space for 802.11. Example applications include
transport of voice, audio and video over 802.11 wireless networks, video
conferencing, media stream distribution, enhanced security applications, and
mobile and nomadic access applications. This work is in the very early stages of
development. The resulting 802.11e amendment may be available by early 2002.
802.11d
The IEEE 802.11d Task Group (TG d) describes a
protocol that will allow a 802.11 device to receive the regulatory information
required to configure itself properly to operate anywhere on earth. The current
802.11 standard defines operation in only a few regulatory domains (countries).
This supplement will add the requirements and definitions necessary to allow
802.11 WLAN equipment to operate in markets not served by the current standard.
A second standard-family that needs to be mentioned
is Hiperlan. ETSI, one of the world's recognized Standards bodies, has developed
HIPERLAN, which is supported by Nokia and Ericsson. It also operates in the 5GHz
band.
A major competitor of Wi-Fi is HomeRF 2.0,
operating at 10Mbps by using Wide Band Frequency Hopping (WBFH). It is supported
by Siemens, Compaq, Motorola and Intel (which has recently decided to take the
802.11b-way).
Dominant standards
Of the WLAN technologies mentioned above, a few are
expected to be dominant over the next five years: 802.11b,g,a.
With the ratification of the 11 Mbps Wi-Fi standard in the second half of 1999,
guaranteed interchangeability and declining costs became a reality and
traditional networking players such as Cisco, Lucent, 3COM, Intel and Texas
Instruments entered the WLAN market. Another major advantage of 802.11b is the
existence of the Wireless Ethernet Compatibility Alliance (WECA), where wireless
industry leaders have united outside the standards bodies.The WECA webpages can
be found at: http://www.wirelessethernet.org/
As outlined by the 802.11b specification, chip sets
use a modulation scheme known as Complementary Code Keying (CCK) to transmit
data signals at 11 Mbps through an unlicensed portion of the spectrum found at
2.4GHz. Considered revolutionary at the time 802.11b gave way to a new
generation of products that allowed an Ethernet connection to finally break free
of wires but its speed was still only one-tenth that of its wired equivalent.
In order to enhance the standard, the IEEE's
overall Working Group that oversaw the development of 802.11 assigned individual
tasks to several specialty groups. The mission of 802.11g was to boost the data
transmission to rates of 54 Mbps while still maintaining interoperability to
earlier specs.Very recently (November 16, 2001) the 802.11g extension passed a
serious battle between OFDM and PBCC defenders. After much political infighting,
this enhanced 802.11b/Wi-Fi standard got ratified.
When the original 802.11b specification was approved in 1999, the IEEE
concurrently approved the specs for 802.11a. These chip sets are designed to use
the OFDM scheme to transmit data at 54 Mbps through a separate portion of the
spectrum (located in the 5GHz range). 802.11a is only licensed for usage in
North America as opposed to 802.11b which is accepted throughout Europe and Asia
as well. A great problem is that 802.11b and 802.11a were never meant to
interoperate. Still, several vendors from start-ups like Sunnyvale, Calif.-based
Atheros Communications to household names like Intel and 3Com have already
announced their support of 802.11a. 802.11a will not be meaningful until late
2002 : it is a very new technology which at this moment is not already
attracting mass enterprises.
IEEE 802.11
Purpose and general description of the 802.11 standard
The aim of the 802.11 standard was to develop a MAC
and PHY layer for wireless connectivity for fixed, portable and moving stations
within a local area. The higher OSI-layers are the same as in any other 802.X
standard, this means that at this level there is no difference percebtible
between wired and wireless media.
The 802.11 standard describes the functions and services required by a compliant
device to operate within ad hoc and infrastructure networks as well as the
aspects of station mobility.The difference between ad hoc and infrastructure
networks will be explained further in the text. The standard defines the MAC
procedures to support the asynchronous MAC service data unit (MSDU) delivery
services; several PHY signalling techniques and interface functions that are
controlled by the IEEE802.11 MAC. The standard permits the operation of an IEEE
802.11 conformant device within a WLAN that may coexist with multiple
overlapping IEEE 802.11 WLANs and describes the requirements and procedures to
provide privacy of user information being transferred over the wireless medium
and authentication of IEEE 802.11 conformant devices. [2]
When talking about WLAN, a very critical aspect is the limited throughput. This
was a problem of 802.11: it only provided 1Mbps and 2Mbps rates which of course
are too slow to support common requirements and explains the not that fast
starting process of WLAN.
The 802.11 operation modes
There are two operation modes defined in IEEE
802.11: Infrastructure Mode and Ad Hoc Mode.(figure1)
Infrastructure mode
In infrastructure mode,
the wireless network consists of at least one access point (AP) connected to the
wired network infrastructure and a set of wireless end stations. An access point
controls encryption on the network and may bridge or route the wireless traffic
to a wired ethernet network (or the Internet). Access points that act as routers
can also assign an IP address to your PC's using DHCP services. AP's can be
compared with a basestation used in cellular networks.
This configuration is called a Basic Service Set (BSS). An Extended Service Set
(ESS) consists of two or more BSSs forming a single subnetwork. Traffic is
forwarded from one BSS to another to facilitate movement of wireless stations
between BSSs. Almost always the distribution system which connects this networks
is an Ethernet LAN. Since most corporate WLANs require access to the wired LAN
for services (file servers, printers, Internet links) they will operate in
infrastructure mode. This also is the case in our exercise: Wireless LAN access
points in auditoria.
Ad-Hoc Mode
Ad-Hoc mode is a set of
802.11 wireless stations that communicate directly with each other without using
an access point or any connection to a wired network. This basic topology is
useful in order to quickly and easily set up a wireless network anywhere a
wireless infrastructure does not exist such as a hotel room, a convention
center, our airport.
Ad-Hoc Mode is also called peer-to-peer mode or an Independent Basic Service Set
(IBSS)
Figure 1: 802.11 modes[3]
Important to notice when talking about ad-hoc
networks is the capacity of this configuration. As examined in [6], the capacity
of wireless ad-hoc networks can be very low, due to the requirement that nodes
forward each others' packets. Capacity is the limiting factor: a large mobility
causes a high volume of routing queries and updates which brings along high
congestion, which leads to packet losses.
The 802.11 physical layer
Two main technologies are used for wireless
communications: Radio Frequency and InfraRed. RF in this case is located in the
2.4GHz ISM-band. RF is capable of being used for 'not line of sight' and longer
distance situations. IR is not a useful technology for use in a WLAN system
since it is used for short distance communications: there is a standard for such
products called IrDA.
There are two methods of spread spectrum modulation used within the unlicensed
2.4-GHz frequency band: frequency hopping spread spectrum (FHSS) and direct
sequence spread spectrum(DSSS). Spread spectrum is ideal for data communications
because it is less susceptible to radio noise and creates little interference,
it is used to comply with the regulations for use in the ISM band.
Using frequency hopping, the 2.4GHz band is divided into 75 1-MHz-channels. FHSS
allows for a less complex radio design than DSSS but FHSS is limited to a 2-Mbps
data transfer rate, the reason for this are the FCC regulations that restrict
subchannel bandwith to 1 MHz, causing many hops which means a high amount of
hopping overhead. For wireless LAN applications, DSSS is a better choice. DSSS
divides the 2.4GHz band into 14 channels (in the US only 11 channels are
available). Channels used at the same location should be seperated 25 MHz from
each other to avoid interference. This means that only 3 channels can exist at
the same location (figure 2). FHSS and DSSS are fundamentally different
signalling mechanisms and are not capable of interoperating with each other.
Figure2: DSSS channels [3]
The Physical Layer is further subdivided in two sublayers : a Physical Layer
Convergence Procedure (PLCP) sublayer and aPhysical Media Dependent (PMD)
sublayer.
PLCP adapts the capabilities of the physical medium
dependent system to the Physical Layer service. It presents an interface for the
MAC sublayer to write to and provides carrier sense and Clear Channel Assessment
(CCA).
PMD defines the method of transmitting and
receiving data through a wireless medium between two or more stations each using
the same modulation system. It takes care of the wireless encoding.
The 802.11 datalink layer
A 802.11 datalink layer is divided in two
sublayers: Logical Link Control (LLC) and Media Access Control (MAC). The LLC
sublayer is the same in 802.11 and other 802 LANs and can easily be plugged in
into a wired LAN, but 802.11 defines a different MAC protocol.
For Ethernet LANs, the CSMA/CD protocol regulates the access of the stations. In
a WLAN collision detection is not possible.
The 802.11 standard defines the protocol and compatible interconnection of data
communication equipment via the air, radio or infrared, in a local area network
(LAN) using the CSMA/CA medium sharing mechanism. This basic access method for
802.11 is called Distributed Coordination Function (DCF) and its mandatory for
all stations.
A second media access control method, the Point
Coordination Function (PCF), is an optional extension to DCF. PCF provides a
time division duplexing capability to allow the access point to deal with
timebounded, connection-oriented services. Using this method, one AP controls
the access through a polling system.
CSMA/CA (figure3) needs each station to listen to other users. If the channel is
idle
the station is allowed to transmit. If it is busy, each station waits until
transmission stops, then enters into a random back off procedure. This prevents
multiple stations from owning the medium immediately after completion of the
preceding transmission. Packet reception in DCF requires acknowledgements (ACK).
The period between completion of packet transmission and start of the ACK frame
is one Short Inter Frame Space (SIFS). ACK frames have a higher priority than
other traffic. Fast acknowledgement is one of the features of the 802.11
standard, because it requires ACKs to be handled at the MAC sublayer.
Transmissions other than ACKs must wait at least one DCF inter frame space
(DIFS)
before transmitting data. If a transmitter senses a busy medium, it determines a
random back-off period by setting an internal timer to an integer number of slot
times. Upon expiration of a DIFS, the timer begins to decrement. If the timer
reaches zero, the station may begin transmission. If the channel is seized by
another station before the timer reaches zero, the timer setting is retained at
the decremented value for subsequent transmission.
Figure 3 : CSMA/CA algorithm [3]
The method described above relies on the underlying assumption that every
station can hear all other stations. This is not always the case: this problem
is known as the Hidden-Node Problem.
The hidden node problem arises when a station is able to succesfully receive
frames from two other transmitters but the two transmitters can not receive
signals from each other. In this case a transmitter may sense the medium as
being idle even if the other one is transmitting. This results in a collision at
the receiving station.
To provide a solution for this problem, another mechanism is present: the use of
RTS/CTS frames (figure 4). A Request To Send (RTS) frame is sent by a potential
transmitter to the receiver and a Clear To Send (CTS) frame issent from the
receiver in response to the received RTS frame. If the CTS frame is not received
within a certain time interval the RTS frame is retransmitted by executing a
backoff algorithm. After a succesful exchange of the RTS and CTS frames the data
frame can be sent by the transmitter after waiting for a SIFS. RTS and CTS
include a duration field that specifies the time interval necessary to transmit
the data frame and the ACK. This information is used by stations which can hear
the transmitter or the receiver to update their Net Allocation Vector (NAV), a
timer which is always decremented.
The drawback of using RTS/CTS is an increased overhead which may be very
important for short data frames, the efficiency of RTS/CTS depends upon the
length of the packets. RTS/CTS is typically used for large-size packets, for
which retransmissions would be expensive from a bandwith viewpoint.
Figure 4: RTS/CTS [3]
Two other robustness features of the 802.11 MAC layer are the CRC checksum and
packet fragmentation. Each packet has a CRC attached to ensure its correctness.
This is different from Ethernet, where higher-level protocols such as TCP handle
error checking. Packet fragmentation is very useful in congested or high
interference environments since larger packets have a better chance to get
corrupted. The MAC layer is responsible for reassembling the received fragments,
this makes the process transparent to higher-level protocols.
IEEE 802.11b
In 2000, 802.11b became the
standard wireless ethernet networking technology for both business and home.
That year, wireless networking took a giant leap with the release of 11 Mbps
products, based on this 802.11b standard (commonly known as Wi-Fi).
First generation of wireless adapters supported 1 or 2 Mbps. This is very low
compared to wired ethernets, defined by the Institute of Electrical and
Electronics Engineers (IEEE) in the 802.3 standard, which are able to operate at
10 Mbps, 100Mbps, or even 1000Mbps
802.11b transmits at 2.4 GHz, the same spectrum as microwave ovens. The cards
use less power than a mobile phone. Cisco warns that their PCMCIA card should be
more than 4 cm from your body, and the access point's antenna should be at least
15 cm away from anyone.
Enhancements to the physical layer
One of the major aims of IEEE 802.11 task group b
was to develop a high-speed physical layer in the 2.4 GHz ISM band, compatible
with the earlier existing 802.11 products.
IEEE 802.11b physical layer is an extension to IEEE 802.11 physical layer which
supports 1 and 2 Mbps. IEEE 802.11b also can support higher data rates of 5.5
and 11 Mbps by using CCK (proposed by Lucent Technologies and Harris
Semiconductor) with QPSK modulation and DSSS technology. DSSS technology is
chosen because FHSS can not support this higher speeds without violating the
100mW rule as defined by FCC regulations. In DSSS systems each info-bit is
XOR-ed with a longer PRN sequence. The result is a high speed digital stream
which is then modulated onto a carrier frequency using DPSK.
When receiving a DSSS signal, a matched filter correlator is used. This
correlator removes the PN sequence in order to recover the original data stream.
At high data rates (5.5 or 11 Mbps) DSSS receivers use different PN codes and a
bank of correlators to recover the data. The high rate modulation is called
Complementary Code Keying (CCK).
To support noisy environments and an extended
range, 802.11b uses dynamic rate shifting. Data rates are adjusted, for example
in very bad situations they will be shifted from 11Mbps to 5.5, 2 and finally
1Mbps.
Regardless the data rate, the channel bandwith is about 20 MHz for DSSS systems.
This is the reason why the ISM band will accomodate maximum 3 non-overlapping
channels.This rate-shifting also implies interopating possibilities with 802.11
DSSS, but not with 802.11 FHSS.
