Information Infrastructure
Kristen96
CH14-BDC7e.pptxLecture slides prepared for “Business Data Communications”, 7/e, by William Stallings and Tom Case, Chapter 14 “Wireless LANs”.
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Chapter 14
Wireless LANs
Wireless LANs have become a significant segment of the LAN market.
Organizations are using wireless LANs as an indispensable adjunct to traditional
wired LANs, to satisfy requirements for mobility, relocation, ad hoc networking,
and coverage of locations difficult to wire.
This chapter provides a survey of wireless LANs. We begin with an overview
that looks at the motivations for using wireless LANs and summarizes the various
approaches in current use. Then, the most widely used wireless LAN schemes, IEEE
802.11, also known as Wi-Fi, is examined. Appendix H examines another popular
scheme known as Bluetooth.
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Wireless LAN Applications
Like wired LANs, such as Ethernet, wireless LANs (WLANs) provide connectivity
within a limited geographic area. One key feature of WLANs, not readily provided
by a wired LAN, is mobility: the ability to move around while remaining connected.
WLANs are a necessity in a working environment in which staff need to be mobile
but remain constantly connected to the local network. A good example of this is
the need for doctors and nurses to access patient information, hospital records, and
other medical information as they move about the hospital.
WLANs also provide outdoor connectivity. This is now found in many locations
in the form of Wi-Fi “hot spots,” which can be secured to allow employees to
remain connected in the vicinity of the building, or unsecured, such as public access
hot spots found in many municipalities.
WLANs are also useful as an adjunct to wired LANs, such as for connecting
wired LANs in adjacent building where wired connection is difficult, expensive, or
even impossible because of an intervening public area.
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Wireless LANs (WLANs)
Provide connectivity within a limited geographic area
Are a necessity in a working environment in which staff need to be mobile but remain constantly connected to the local network
Provide outdoor connectivity
Are useful as an adjunct to wired LANs, such as for connecting wired LANs in adjacent buildings where wired connections is difficult, expensive, or even impossible because of an intervening public area
Example Single Cell Wireless LAN Configuration
Figure 14.1 indicates a simple WLAN configuration that is typical of many
environments. There is a backbone wired LAN, such as Ethernet, that supports
servers, workstations, and one or more bridges or routers to link with other
networks. In addition, there is a control module (CM) that acts as an interface to
a WLAN. The control module includes either bridge or router functionality to link
the WLAN to the backbone. It includes some sort of access control logic, such as a
polling or token-passing scheme, to regulate the access from the end systems. Note
that some of the end systems are standalone devices, such as a workstation or a
server. Hubs or other user modules (UMs) that control a number of stations off a
wired LAN may also be part of the WLAN configuration.
The configuration of Figure 14.1 can be referred to as a single-cell WLAN; all
of the wireless end systems are within range of a single control module.
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Example Multiple Cell Wireless LAN Configuration
Another
common configuration, suggested by Figure 14.2, is a multiple-cell WLAN. In this
case, there are multiple control modules interconnected by a wired LAN. Each
control module supports a number of wireless end systems within its transmission
range. For example, with an infrared LAN, transmission is limited to a single room;
therefore, one cell is needed for each room in an office building that requires
wireless support.
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Wireless LAN Requirements
In addition to requirements typical of any LAN, a WLAN also has the following requirements:
Throughput
Number of nodes
Connection to backbone LAN
Service area
Battery power consumption
Transmission robustness and security
Collocated network operation
License-free operation
Handoff/roaming
Dynamic configuration
A WLAN must meet the same sort of requirements typical of any LAN, including
high capacity, ability to cover short distances, full connectivity among attached
stations, and broadcast capability. In addition, there are a number of requirements
specific to the WLAN environment. The following are among the most important
requirements for WLANs:
• Throughput: The medium access control (MAC) protocol should make as
efficient use as possible of the wireless medium to maximize capacity.
• Number of nodes: WLANs may need to support hundreds of nodes across
multiple cells.
• Connection to backbone LAN: In most cases, interconnection with stations on
a wired backbone LAN is required. For infrastructure WLANs, this is easily
accomplished through the use of control modules that connect to both types of
LANs. There may also need to be accommodation for mobile users and ad hoc
wireless networks.
• Service area: A typical coverage area for a WLAN has a diameter of 100–300 m.
• Battery power consumption: Mobile workers use battery-powered workstations
that need to have a long battery life when used with wireless adapters.
This suggests that a MAC protocol that requires mobile nodes to monitor
access points constantly or engage in frequent handshakes with a base station is
inappropriate. Typical WLAN implementations have features to reduce power
consumption while not using the network, such as a sleep mode.
• Transmission robustness and security: Unless properly designed, a WLAN
may be interference prone and easily eavesdropped. The design of a WLAN
must permit reliable transmission even in a noisy environment and should
provide some level of security from eavesdropping.
• Collocated network operation: As WLANs become more popular, it is quite
likely for two or more WLANs to operate in the same area or in some area
where interference between the LANs is possible. Such interference may
thwart the normal operation of a MAC algorithm and may allow unauthorized
access to a particular LAN.
• License-free operation: Users would prefer to buy and operate WLAN products
without having to secure a license for the frequency band used by the LAN.
• Handoff/roaming: The MAC protocol used in the WLAN should enable mobile
stations to move from one cell to another.
• Dynamic configuration: The MAC addressing and network management
aspects of the LAN should permit dynamic and automated addition, deletion,
and relocation of end systems without disruption to other users.
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Wireless LAN Technology
WLANs are categorized according to the transmission technique that is used
All current WLAN products fall into one of the following categories:
WLANs are generally categorized according to the transmission technique that is
used. All current WLAN products fall into one of the following categories:
• Spread spectrum LANs: This type of LAN makes use of spread spectrum
transmission technology. In most cases, these LANs operate in the ISM
(industrial, scientific, and medical) 2.4-GHz microwave bands so that no
Federal Communications Commission (FCC) licensing is required for their
use in the United States.
• OFDM LANs: For higher speeds, a technology known as orthogonal frequency
division multiplexing is superior to spread spectrum, and products
with this technology are now common. These LANs typically operate in either
the 2.4-GHz band or the 5-GHz band.
• Infrared (IR) LANs: An individual cell of an IR LAN is limited to a single
room, because infrared light does not penetrate opaque walls.
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Spread spectrum LANs
In most cases these LANs operate in the ISM 2.4-GHz microwave bands so that no FCC licensing is required for their use in the United States
OFDM LANs
Orthogonal frequency division multiplexing is superior to spread spectrum and typically operate in either the 2.4-GHz band or the 5-GHz band
Infrared (IR) LANs
An individual cell of an IR LAN is limited to a single room because infrared light does not penetrate opaque walls
Table 14.1 Key IEEE 802.11 Task Groups
In 1990, the IEEE 802 Committee formed a new working group, IEEE 802.11,
specifically devoted to WLANs, with a charter to develop a MAC protocol and
physical medium specification. Since that time, the demand for WLANs, at different
frequencies and data rates, has exploded. Keeping pace with this demand, the IEEE
802.11 working group has issued an ever-expanding list of standards (Table 14.1).
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Table 14.2 IEEE 802.11 Terminology
Table 14.2 briefly defines key terms used in the IEEE 802.11 standard.
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Wireless Fidelity (Wi-Fi) Alliance
Wireless Ethernet Compatibility Alliance (WECA)
Industry consortium formed in 1999 to meet the concern of whether products from different vendors will successfully interoperate
Subsequently renamed the Wi-Fi (Wireless Fidelity) Alliance
Creates a test suite to certify interoperability for 802.11 products
Term used for certified products is Wi-Fi
Concerned with a range of market areas for WLANs, including enterprise, home, and hot spots
With any networking standard, there is a concern whether products from different
vendors will successfully interoperate. To meet this concern, the Wireless
Ethernet Compatibility Alliance (WECA), an industry consortium, was formed
in 1999. This organization, subsequently renamed the Wi-Fi (Wireless Fidelity)
Alliance , creates a test suite to certify interoperability for 802.11 products. The term
used for certified products is Wi-Fi . The Wi-Fi Alliance is concerned with a range of
market areas for WLANs, including enterprise, home, and hot spots.
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IEEE 802.11 Architecture
Figure 14.3 illustrates the model developed by the 802.11 working group. The smallest
building block of a WLAN is a basic service set (BSS) , which consists of some
number of stations executing the same MAC protocol and competing for access to
the same shared wireless medium. A BSS may be isolated or it may connect to a
backbone distribution system (DS) through an access point (AP) . The AP functions
as a bridge and a relay point. In a BSS, client stations do not communicate directly
with one another. Rather, if one station in the BSS wants to communicate with
another station in the same BSS, the MAC frame is first sent from the originating
station to the AP, and then from the AP to the destination station. Similarly, a MAC
frame from a station in the BSS to a remote station is sent from the local station to
the AP and then relayed by the AP over the DS on its way to the destination station.
The BSS generally corresponds to what is referred to as a cell in the literature. The
DS can be a switch, a wired network, or a wireless network.
When all the stations in the BSS are mobile stations, with no connection to
other BSSs, the BSS is called an independent BSS (IBSS) . An IBSS is typically an
ad hoc network. In an IBSS, the stations all communicate directly, and no AP is
involved.
A simple configuration is shown in Figure 14.3, in which each station belongs
to a single BSS; that is, each station is within wireless range only of other stations
within the same BSS. It is also possible for two BSSs to overlap geographically, so
that a single station could participate in more than one BSS. Further, the association
between a station and a BSS is dynamic. Stations may turn off, come within range,
and go out of range.
An extended service set (ESS) consists of two or more BSSs interconnected
by a distribution system. Typically, the distribution system is a wired backbone
LAN but can be any communications network. The extended service set appears as
a single logical LAN to the logical link control (LLC) level.
Figure 14.3 indicates that an AP is implemented as part of a station; the AP is
the logic within a station that provides access to the DS by providing DS services in
addition to acting as a station. To integrate the IEEE 802.11 architecture with a traditional
wired LAN, a portal is used. The portal logic is implemented in a device, such
as a bridge or router, that is part of the wired LAN and that is attached to the DS.
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Distribution of Messages Within a Distribution System (DS)
The two services involved with the distribution of messages with a DS are distribution and integration
The two services involved with the distribution
of messages within a DS are distribution and integration. Distribution
is the primary service used by stations to exchange MAC frames when the frame
must traverse the DS to get from a station in one BSS to a station in another BSS.
For example, suppose a frame is to be sent from station 2 (STA 2) to STA 7 in
Figure 14.3. The frame is sent from STA 2 to STA 1, which is the AP for this BSS.
The AP gives the frame to the DS, which has the job of directing the frame to the
AP associated with STA 5 in the target BSS. STA 5 receives the frame and forwards
it to STA 7. How the message is transported through the DS is beyond the
scope of the IEEE 802.11 standard.
If the two stations that are communicating are within the same BSS, then the
distribution service logically goes through the single AP of that BSS.
The integration service enables transfer of data between a station on an IEEE
802.11 LAN and a station on an integrated IEEE 802.x LAN. The term integrated
refers to a wired LAN that is physically connected to the DS and whose stations
may be logically connected to an IEEE 802.11 LAN via the integration service. The
integration service takes care of any address translation and media conversion logic
required for the exchange of data.
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Distribution
Primary service used by stations to exchange MAC frames when the frame must traverse the DS to get from a station in one BSS to a station in another BSS
Integration
Enables transfer of data between a station on an IEEE 802.11 LAN and a station on an integrated IEEE 802.x LAN
Takes care of any address translation and media conversion logic required for the exchange of data
Association-Related Services
Transition types, based on mobility:
No transition
BSS transition
ESS transition
The DS needs to know the identity of the AP to which the message should be delivered in order for that message to reach the destination station
To meet this requirement a station must maintain an association with the AP within its current BSS
The primary purpose of the MAC layer is to transfer
MSDUs between MAC entities; this purpose is fulfilled by the distribution service.
For that service to function, it requires information about stations within the ESS
that is provided by the association-related services. Before the distribution service can
deliver data to or accept data from a station, that station must be associated . Before
looking at the concept of association, we need to describe the concept of mobility.
The standard defines three transition types, based on mobility:
• No transition: A station of this type either is stationary or moves only
within the direct communication range of the communicating stations of a
single BSS.
• BSS transition: This is defined as a station movement from one BSS to another
within the same ESS. In this case, delivery of data to the station requires that
the addressing capability be able to recognize the new location of the station.
• ESS transition: This is defined as a station movement from a BSS in one ESS
to a BSS within another ESS. This case is supported only in the sense that
the station can move. Maintenance of upper-layer connections supported by
802.11 cannot be guaranteed. In fact, disruption of service is likely to occur.
To deliver a message within a DS, the distribution service needs to know
where the destination station is located. Specifically, the DS needs to know the
identity of the AP to which the message should be delivered in order for that
message to reach the destination station. To meet this requirement, a station
must maintain an association with the AP within its current BSS. Three services
relate to this requirement:
• Association: Establishes an initial association between a station and an AP.
Before a station can transmit or receive frames on a WLAN, its identity and
address must be known. For this purpose, a station must establish an association
with an AP within a particular BSS. The AP can then communicate this
information to other APs within the ESS to facilitate routing and delivery of
addressed frames.
• Reassociation: Enables an established association to be transferred from one
AP to another, allowing a mobile station to move from one BSS to another.
• Disassociation: A notification from either a station or an AP that an existing
association is terminated. A station should give this notification before leaving
an ESS or shutting down. However, the MAC management facility protects
itself against stations that disappear without notification.
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Three services relate to this requirement
Association
Reassociation
Disassociation
Reliable Data Delivery
A WLAN using the IEEE 802.11 physical and MAC layers is subject to considerable unreliability
Even with error correction codes a number of MAC frames may not successfully be received
IEEE 802.11 includes a frame exchange protocol
When a station receives a data frame from another station it returns an acknowledgment (ACK) frame to the source station
This exchange is treated as an atomic unit not to be interrupted by a transmission from any other station
If the source does not receive an ACK within a short period of time the source retransmits the frame
As with any wireless network, a WLAN using the IEEE
802.11 physical and MAC layers is subject to considerable unreliability. Noise, interference,
and other propagation effects may result in the loss of a significant number of
frames. Even with error-correction codes, a number of MAC frames may not successfully
be received. This situation can be dealt with by reliability mechanisms at a higher
layer, such as TCP. However, timers used for retransmission at higher layers are typically
on the order of seconds. It is therefore more efficient to deal with errors at the
MAC level. For this purpose, IEEE 802.11 includes a frame exchange protocol. When
a station receives a data frame from another station, it returns an acknowledgment
(ACK) frame to the source station. This exchange is treated as an atomic unit, not to
be interrupted by a transmission from any other station. If the source does not receive
an ACK within a short period of time, either because its data frame was damaged or
because the returning ACK was damaged, the source retransmits the frame.
Thus, the basic data transfer mechanism in IEEE 802.11 involves an exchange
of two frames. To further enhance reliability, a four-frame exchange may be used.
In this scheme, a source first issues a Request to Send (RTS) frame to the destination.
The destination then responds with a Clear to Send (CTS). After receiving
the CTS, the source transmits the data frame, and the destination responds with an
ACK. The RTS alerts all stations that are within reception range of the source that
an exchange is under way; these stations refrain from transmission in order to avoid
a collision between two frames transmitted at the same time. Similarly, the CTS
alerts all stations that are within reception range of the destination that an exchange
is under way. The RTS/CTS portion of the exchange is a required function of the
MAC but may be disabled.
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Four-Frame Exchange
To further enhance reliability a four-frame exchange may be used
A source first issues a Request to Send (RTS) frame to the destination
The destination then responds with a Clear to Send (CTS)
The source transmits the data frame and the destination responds with an ACK
The RTS alerts all stations that are within reception range of the source that an exchange is under way
The CTS alerts all stations that are within reception range of the destination that an exchange is under way
The RTS/CTS portion of the exchange is a required function of the MAC but may be disabled
As with any wireless network, a WLAN using the IEEE
802.11 physical and MAC layers is subject to considerable unreliability. Noise, interference,
and other propagation effects may result in the loss of a significant number of
frames. Even with error-correction codes, a number of MAC frames may not successfully
be received. This situation can be dealt with by reliability mechanisms at a higher
layer, such as TCP. However, timers used for retransmission at higher layers are typically
on the order of seconds. It is therefore more efficient to deal with errors at the
MAC level. For this purpose, IEEE 802.11 includes a frame exchange protocol. When
a station receives a data frame from another station, it returns an acknowledgment
(ACK) frame to the source station. This exchange is treated as an atomic unit, not to
be interrupted by a transmission from any other station. If the source does not receive
an ACK within a short period of time, either because its data frame was damaged or
because the returning ACK was damaged, the source retransmits the frame.
Thus, the basic data transfer mechanism in IEEE 802.11 involves an exchange
of two frames. To further enhance reliability, a four-frame exchange may be used.
In this scheme, a source first issues a Request to Send (RTS) frame to the destination.
The destination then responds with a Clear to Send (CTS). After receiving
the CTS, the source transmits the data frame, and the destination responds with an
ACK. The RTS alerts all stations that are within reception range of the source that
an exchange is under way; these stations refrain from transmission in order to avoid
a collision between two frames transmitted at the same time. Similarly, the CTS
alerts all stations that are within reception range of the destination that an exchange
is under way. The RTS/CTS portion of the exchange is a required function of the
MAC but may be disabled.
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Access Control
The 802.11 working group considered two types of proposals for
a MAC algorithm: distributed access protocols, which, like Ethernet, distribute the
decision to transmit among all the nodes using a carrier-sense mechanism; and centralized
access protocols, which involve regulation of transmission by a centralized
decision maker. A distributed access protocol makes sense for an ad hoc network
of peer workstations and may also be attractive in other WLAN configurations that
consist primarily of bursty traffic. A centralized access protocol is natural for configurations
in which a number of wireless stations are interconnected with each other
and some sort of base station that attaches to a backbone wired LAN; it is especially
useful if some of the data are time sensitive or high priority.
The end result for 802.11 is a MAC algorithm called DFWMAC (distributed
foundation wireless MAC) that provides a distributed access control mechanism
with an optional centralized control built on top of that.
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Distributed access protocol
Distributes the decision to transmit among all the nodes using a carrier-sense mechanism
Useful for an ad hoc network of peer workstations and other WLAN configurations that consist primarily of bursty traffic
Centralized access protocol
Involves regulation of transmission by a centralized decision maker
Useful for configurations in which a number of wireless stations are interconnected with each other and some sort of base station that attaches to a backbone wired LAN
DFWMAC
Distributed foundation wireless MAC
Provides a distributed access control mechanism with an optional centralized control built on top of that
IEEE 802.11 Protocol Architecture
Figure 14.4 illustrates the
architecture. The lower sublayer of the MAC layer is the distributed coordination
function (DCF). DCF uses an Ethernet-style contention algorithm to provide access
to all traffic. Ordinary asynchronous traffic directly uses DCF. The point coordination
function (PCF) is a centralized MAC algorithm used to provide contention-free
service; this is done by polling stations in turn. Higher-priority traffic, or traffic with
greater timing requirements, makes use of the PCF. PCF is built on top of DCF and
exploits features of DCF to assure access for its users. Finally, the logical link control
(LLC) layer provides an interface to higher layers and performs basic link layer
functions such as error control.
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IEEE 802.11 Physical Layer
Issued in five stages:
IEEE 802.11
Includes the MAC layer and three physical layer specifications, two in the 2.4-GHz band (ISM) and one in the infrared, all operating at 1 and 2 Mbps
IEEE 802.11a
Operates in the 5-GHz band at data rates up to 54 Mbps
IEEE 802.11b
Operates in the 2.4-GHz band at 5.5 and 11 Mbps
IEEE 802.11g
Operates in the 2.4-GHz band at data rates up to 54 Mbps
IEEE 802.11n
Operates in either the 2.4-GHz band or the 5-GHz band with data rates in the hundreds of Gbps
The physical layer for IEEE 802.11 has been issued in five stages. The first part,
simply called IEEE 802.11 , includes the MAC layer and three physical layer specifications,
two in the 2.4-GHz band (ISM) and one in the infrared, all operating at 1
and 2 Mbps. IEEE 802.11a operates in the 5-GHz band at data rates up to 54 Mbps.
IEEE 802.11b operates in the 2.4-GHz band at 5.5 and 11 Mbps. IEEE 802.11g also
operates in the 2.4-GHz band, at data rates up to 54 Mbps. Finally, IEEE 802.11n
operates in either the 2.4-GHz band or the 5-GHz band with data rates in the hundreds
of Gbps.
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Table 14.3 IEEE 802.11 Physical Layer Standards
Table 14.3 provides some details. We look at each of these in turn.
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Original IEEE 802.11
Three physical media are defined in the original 802.11 standard:
Direct-sequence spread spectrum (DSSS) operating in the 2.4-GHz ISM band, at data rates of 1 and 2 Mbps
Frequency-hopping spread spectrum (FHSS) operating in the 2.4-GHz ISM band, at data rates of 1 and 2 Mbps
Infrared at 1 and 2 Mbps, operating at a wavelength between 850 and 950 nm
Three physical media are defined in the original 802.11
standard:
• Direct-sequence spread spectrum (DSSS) operating in the 2.4-GHz ISM band,
at data rates of 1 and 2 Mbps
• Frequency-hopping spread spectrum (FHSS) operating in the 2.4-GHz ISM
band, at data rates of 1 and 2 Mbps
• Infrared at 1 and 2 Mbps, operating at a wavelength between 850 and 950 nm
The infrared option never gained market support. The other two schemes
use spread spectrum approaches. In essence, spread spectrum involves the use of
a much wider bandwidth than is actually necessary to support a given data rate.
The purpose of using a wider bandwidth is to minimize interference and drastically
reduce the error rate. In the case of FHSS, spread spectrum is achieved by
frequently jumping from one carrier frequency to another; thus, if there is interference
or performance degradation at a given frequency, it only affects a small
fraction of the transmission. DSSS effectively increases the data rate of a signal
by mapping each data bit into a string of bits, with one string used for binary 1
and another used for binary 0. The higher data rate uses a greater bandwidth. The
effect is to spread each bit out over time, which minimizes the effects of interference
and degradation. FHSS, which is simpler, was employed in most early 802.11
networks. Products using DSSS, which is more effective in the 802.11 scheme,
followed. However, all of the original 802.11 products were of limited utility
because of the low data rates.
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IEEE 802.11b
IEEE 802.11b is an extension of the IEEE 802.11 DSSS scheme,
providing data rates of 5.5 and 11 Mbps within the 2.4-GHz band. A higher data rate
is achieved by using a more complex modulation technique. The 802.11b specification
quickly led to product offerings, including chipsets, PC cards, access points, and
systems. Apple Computer was the first company to offer 802.11b products, with its
iBook portable computer using the AirPort wireless network option. Other companies,
including Cisco, 3Com, and Dell, have followed. Although these new products
are all based on the same standard, there is always a concern whether products from
different vendors will successfully interoperate. To meet this concern, the Wireless
Ethernet Compatibility Alliance (now called the Wi-Fi Alliance) created a test suite
to certify interoperability for 802.11b products. Interoperability tests have been
conducted, and a number of products have achieved certification.
One other concern for both the original 802.11 and the 802.11b products is
interference with other systems that operate in the 2.4-GHz band, such as Bluetooth,
HomeRF, and many other devices that use the same portion of the spectrum
(including baby monitors and garage door openers). A coexistence study group
(IEEE 802.15) is examining this issue and so far the prospects are encouraging.
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An extension of the IEEE 802.11 DSSS scheme, providing data rates of 5.5 and 11 Mbps within the 2.4-GHz band
Apple Computer was the first company to offer 802.11b products with its iBook portable computer using the AirPort wireless network option
Wi-Fi Alliance created a test suite to certify interoperability for 802.11b products
A coexistence study group (IEEE 802.15) is addressing the concern of interference with other systems that operate in the 2.4-GHz band (Bluetooth, HomeRF) and many other devices that use the same portion of the spectrum (baby monitors, garage door openers)
IEEE 802.11a
Although 802.11b achieved a certain level of success, its limited data
rate results in limited appeal. To meet the needs for a truly high-speed LAN, IEEE
802.11a was developed. IEEE 802.11a makes use of the frequency band called the
Universal Networking Information Infrastructure (UNII), which is divided into three
parts. The UNNI-1 band (5.15–5.25 GHz) is intended for indoor use; the UNNI-2
band (5.25–5.35 GHz) can be used either indoor or outdoor; and the UNNI-3 band
(5.725–5.825 GHz) is for outdoor use.
Unlike the 2.4-GHz specifications, IEEE 802.11a does not use a spread spectrum
scheme but rather uses orthogonal frequency division multiplexing (OFDM) .
OFDM, also called multicarrier modulation , uses multiple carrier signals (up to 52)
at different frequencies, sending some of the bits on each channel. The possible data
rates for IEEE 802.11a are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.
At high data rates, OFDM is particularly effective in dealing with a major
problem with wireless networks known as multipath interference . The essence of
the problem is that in a transmission from one antenna to another, multiple copies
of the signal may be received, one by a direct line of sight and other copies of the
signal by reflection off objects in the vicinity. Because these signals travel different
paths of different lengths, they arrive at slightly different times, causing interference.
The higher the data rate, the more damaging the interference. With OFDM, instead
of sending a single data stream at a high data rate over a given channel, the channel
is broken up into many subchannels and a portion or the data stream is sent on
each subchannel. Thus, roughly speaking, if there are 10 subchannels, each carries a
portion of the data stream at a data rate of only 1/10 of the original data rate. For a
more detailed discussion of multipath interference and OFDM, see Appendix I.
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Makes use of the frequency band called the Universal Networking Information Infrastructure (UNII), which is divided into three parts
The UNNI-1 band (5.15 – 5.25 GHz) is intended for indoor use
The UNNI-2 band (5.25 – 5.35 GHz) can be used either indoor or outdoor
The UNNI-3 band (5.725 – 5.825 GHz) is for outdoor use
Uses orthogonal frequency division multiplexing (OFDM)
Uses multiple carrier signals (up to 52) at different frequencies, sending some of the bits on each channel
Possible data rates are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps
Effective in dealing with multipath interference
IEEE 802.11g
Although 802.11a offered higher data rates than those by 802.11b,
acceptance of this new scheme was limited. This is because the equipment was relatively
expensive and the scheme is not compatible with either the original 802.11 or
802.11b. Instead manufacturers and customers turned to a more recent standard,
IEEE 802.11g.
IEEE 802.11g is a higher-speed extension to IEEE 802.11b, providing data rates
up to 54 Mbps, matching IEEE 802.11a. Like 802.11b, 802.11g operates in the 2.4-GHz
range and thus the two are compatible. The standard is designed so that 802.11b
devices will work when connected to an 802.11g AP, and 802.11g devices will work
when connected to an 802.11b AP, in both cases using the lower 802.11b data rate.
IEEE 802.11g offers a wider array of data rate and modulation scheme options.
At higher data rates, 802.11g adopts the 802.11a OFDM scheme, adapted for the
2.4-GHz rate; this is referred to as ERP-OFDM, with ERP standing for extended
rate physical layer.
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A higher-speed extension to IEEE 802.11b, providing data rates up to 54 Mbps
Operates in the 2.4-GHz range
The standard is designed so that 802.11b devices will work when connected to an 802.11g AP and 802.11g devices will work when connected to an 802.11b AP, in both cases using the lower 802.11b data rate
At higher data rates 802.11g adopts the 802.11a OFDM scheme, adapted for the 2.4-GHz rate (referred to as ERP-OFDM, with ERP standing for extended rate physical layer)
Table 14.4 Estimated Distance (m) Versus Data Rate
The IEEE 802.11 standards do not include a specification of speed versus
distance objectives. Different vendors will give different values, depending on
environment. Table 14.4, based on [LAYL04], gives estimated values for a typical
office environment.
24
IEEE 802.11n
This standard is defined to operate in both the 2.4-GHz and the 5-GHz bands and can therefore be made upwardly compatible with either 802.11a or 802.11b/g
Embodies changes in three general areas:
With increasing demands being placed on WLANs, the 802.11
committee looked for ways to increase the data throughput and overall capacity
of 802.11 networks. The goal of this effort is not only to increase the bit rate of the
transmitting antennas but also to increase the effective throughput of the network.
Increasing effective throughput involves improvements to the antenna architecture
and the MAC frame structure, not simply improvements to the signal encoding
scheme. The result of these efforts is a package of improvements and enhancements
embodied in IEEE 802.11n. This standard is defined to operate in both the 2.4-GHz
and the 5-GHz bands and can therefore be made upwardly compatible with either
802.11a or 802.11b/g.
IEEE 802.11n embodies changes in three general areas: use of MIMO,
enhancements in radio transmission, and MAC enhancements.
25
Use of MIMO
Enhancements in radio transmission
MAC enhancements
Multiple-Input-Multiple-Output (MIMO)
Multiple-input-multiple-output (MIMO) antenna architecture is the most
important of the enhancements provided by 802.11n. A discussion of MIMO is
beyond our scope, so we content ourselves with a brief overview (see Figure 14.5).
In a MIMO scheme, the transmitter employs multiple antennas. The source data
stream is divided into n substreams, one for each of the n transmitting antennas.
The individual substreams are the input to the transmitting antennas (multiple
input). At the receiving end, m antennas receive the transmissions from the n source
antennas via a combination of line-of-sight transmission and multipath. The outputs
from the m receiving antennas (multiple output) are combined with the signals
from the other receive radios. With a lot of complex math, the result is a much better
receive signal than can be achieved with either a single antenna or multiple frequency
channels. The 802.11n standard defines a number of different combinations for the
number of transmitters and the number of receivers, from 2 * 1 to 4 * 4. Each
additional transmitter or receiver in the system increases the SNR (signal-to-noise
ratio). However, the incremental gains from each additional transmitter or receiver
diminish rapidly. The gain in SNR is large for each step from 2 * 1 to 2 * 2 and to
3 * 2, but the improvement with 3 * 3 and beyond is relatively small [CISC07].
In addition to MIMO, 802.11n makes a number of changes in the radio
transmission scheme to increase capacity. The most significant of these techniques,
known as channel bonding, combines two 20-MHz channels to create a 40-MHz
channel. Using OFDM, this allows for twice as many subchannels, doubling the
transmission rate.
Finally, 802.11 provides some MAC enhancements . The most significant
change is to aggregate multiple MAC frames into a single block for transmission.
Once a station acquires the medium for transmission, it can transmit long packets
without significant delays between transmissions. The receiver sends a single block
acknowledgment. Frame aggregation can result in significantly improved efficiency
in the use of the transmission capacity.
26
Average Throughput per User
Figure 14.6 gives an indication of the effectiveness of 802.11n compared to
802.11g [DEBE07]. The chart shows the average throughput per user on a shared
system. As expected, the more active users competing for the wireless capacity,
the smaller the average throughput per user. IEEE 802.11n provides a significant
improvement, especially for networks in which a small number of users are actively
competing for transmission time.
27
Gigabit WLANs
IEEE 802.11ac
Next step up for the old 802.11a Wi-Fi standard
Is in the 5-GHz band, but provides wider channels for higher data throughput
Uses a channel width of either 40 MHz or 80 MHz to deliver data
May also make use of MU-MIMO (multiuser multiple-input multiple-output)
IEEE 802.11ad
Operates in the 60-GHz band and is expected to deliver data rates of up to 6 Gbps
Downside of this millimeter band Wi-Fi is that its range will be in feet rather than yards
Several approaches to gigabit wireless LANs are in the works. IEEE 802.11, whose
standards will determine the products that become available, is developing two
standards.
The IEEE 802.11ac standard is the next step up for the old 802.11a Wi-Fi
standard. Recall that 802.11a is a 5-GHz standard with a data rate up to 54 Mbps.
Vendors were slow to get 802.11a equipment out the door. When 802.11g came along,
which works in the 2.4-GHz range, could produce the same speed, and was compatible
with the older and slower 802.11b, 802.11a became something of an orphaned
technology. Now with 802.11ac, 802.11a is seeing renewed interest. The new standard
is in the 5-Ghz band, but provides wider channels for higher data throughput.
IEEE 802.11a uses channels with a width of 20 MHz; 802.11ac uses a channel width of
either 40 MHz or 80 MHz or perhaps even 160 MHz to deliver data. 802.11ac may also
make use of MU-MIMO (multiuser multiple-input multiple-output). In MU-MIMOs
simultaneous streams will be transmitted to different users on the same channels.
As of this writing, not all of these details have been worked out, but already products
have been announced that support data rates up to 1 Gbps.
The IEEE 802.11ad standard operates in the 60-GHz band and is expected to
deliver data rates of up to 6 Gbps. The downside of this millimeter band Wi-Fi is
that its range will be in feet rather than yards. 802.11ad will be able to cover a room,
but not much more. As with 802.11ac, products have been announced and will soon
appear, as of the time of this writing.
28
Li-Fi
In October of 2011, a number of companies and industry groups formed the Li-Fi
Consortium, whose objective is to promote high-speed optical wireless systems. The
motivation for optical WLANs is that optical systems can help to deal with a looming
capacity problem. As radio-based wireless becomes ever more prevalent, more
and more devices are using the WLAN frequencies to transmit large volumes of
data. But there is only a limited amount of radio spectrum available. Using light
waves offers the potential of overcoming this problem by exploiting an entirely separate
part of the electromagnetic spectrum, one that is already ubiquitous because
it is used for illumination.
The basic technical approach is to vary the intensity of the light from a light
source to encode binary data. The flickering is so slight that it is imperceptible to
the human eye. Incandescent light bulbs and fluorescent tubes are not suitable for
the rapid modulation required. However, LEDs, which are replacing these older
technologies at a rapid rate, are well-suited to high-speed modulation. Some are
already equipped with photosensors (to be able to turn on at night) and adding
photosensors to existing products is not a big technical challenge. Speeds of 10 Gbps
have been achieved and speeds of up to 100 Gbps are contemplated.
One limitation of Li-Fi is that it requires line-of-sight between transmitter and
receiver, either direct or by reflection from walls and ceilings. Yet for secure applications,
this could be an asset. Further, with the use of strategically placed switches
and routers, the line-of-sight limitation is manageable.
29
In October 2011 a number of companies and industry groups formed the Li-Fi Consortium
Objective is to promote high-speed optical wireless systems
Using light waves offers the potential of overcoming the problem of a limited amount of the radio spectrum available
The basic technical approach is to vary the intensity of the light from a light source to encode binary data
LEDs are well-suited to high-speed modulation
Speeds of 10 Gbps have been achieved and speeds of up to 100 Gbps are contemplated
One limitation of Li-Fi is that it requires line-of-sight between transmitter and receiver, either direct or by reflection from walls and ceiling
IEEE 802.11 Security Considerations
Access and privacy services:
There are two characteristics of a wired LAN that are not inherent in a WLAN.
1. In order to transmit over a wired LAN, a station must be physically connected
to the LAN. On the other hand, with a WLAN, any station within radio range
of the other devices on the LAN can transmit. In a sense, there is a form of
authentication with a wired LAN, in that it requires some positive and presumably
observable action to connect a station to a wired LAN.
2. Similarly, in order to receive a transmission from a station that is part of a
wired LAN, the receiving station must also be attached to the wired LAN.
On the other hand, with a WLAN, any station within radio range can receive.
Thus, a wired LAN provides a degree of privacy, limiting reception of data to
stations connected to the LAN.
Access and Privacy Services
IEEE 802.11 defines three services that provide a WLAN with the two features just
mentioned:
• Authentication: Used to establish the identity of stations to each other. In a
wired LAN, it is generally assumed that access to a physical connection conveys
authority to connect to the LAN. This is not a valid assumption for a WLAN,
in which connectivity is achieved simply by having an attached antenna that is
properly tuned. The authentication service is used by stations to establish their
identity with stations they wish to communicate with. IEEE 802.11 supports
several authentication schemes and allows for expansion of the functionality of
these schemes. The standard does not mandate any particular authentication
scheme, which could range from relatively unsecure handshaking to public-key
encryption schemes. However, IEEE 802.11 requires mutually acceptable, successful
authentication before a station can establish an association with an AP.
• Deauthentication: This service is invoked whenever an existing authentication
is to be terminated.
• Privacy: Used to prevent the contents of messages from being read by other
than the intended recipient. The standard provides for the optional use of
encryption to assure privacy.
30
Authentication
Used to establish the identity of stations to each other
Mutually acceptable successful authentication is required before a station can establish an association with an AP
Deauthentication
This service is invoked whenever an existing authentication is to be terminated
Privacy
Used to prevent the contents of messages from being read by other than the intended recipient
Provides for the optional use of encryption to assure privacy
Wireless LAN Security Standards
Wired Equivalent Privacy (WEP)
Defined by 802.11 for privacy
Contained major weaknesses
Wi-Fi Protected Access (WPA)
A set of security mechanisms that eliminates most 802.11 security issues
Was based on the current state of the 802.11i standard
As 802.11i evolves, WPA will evolve to maintain compatibility
The original 802.11 specification included a set of security features for privacy
and authentication that, unfortunately, were quite weak. For privacy , 802.11 defined
the Wired Equivalent Privacy (WEP) algorithm. The privacy portion of the
802.11 standard contained major weaknesses. Subsequent to the development of
WEP, the 802.11i task group has developed a set of capabilities to address the
WLAN security issues. In order to accelerate the introduction of strong security
into WLANs, the Wi-Fi Alliance promulgated Wi-Fi Protected Access (WPA)
as a Wi-Fi standard. WPA is a set of security mechanisms that eliminates most
802.11 security issues and was based on the current state of the 802.11i standard.
As 802.11i evolves, WPA will evolve to maintain compatibility. WPA is examined
in Chapter 19.
31
Summary
Wireless LAN applications
Wireless LAN requirements
Wireless LAN technology
Wi-Fi architecture and services
IEEE 802.11 architecture
IEEE 802.11 services
Gigabit WLANs
Gigabit Wi-Fi
Li-Fi
Chapter 14: Wireless LANs
IEEE 802.11 medium access control
IEEE 802.11 physical layer
IEEE 802.11 security considerations
Access and privacy services
Wireless LAN security standards
Chapter 14 summary.
32
UM
UM = user module CM = control module
CM
Server
Ethernet switch
Bridge or Router
Figure 14.1 Example Single-Cell Wireless LAN Configuration
CM
UM
Figure 14.2 Example Multiple-Cell Wireless LAN Configuration
UM UM UM
CM
UM
UM
UM
UM
Frequency 1
Frequency 2
CM
UM
UM
UM
UM
Frequency 3
100-Mbps Ethernet Switch
Bridge or Router
Standard Scope
IEEE 802.11a
Physical layer: 5-GHz OFDM at rates from 6 to 54 Mbps
IEEE 802.11b
Physical layer: 2.4-GHz DSSS at 5.5 and 11 Mbps
IEEE 802.11c
Bridge operation at 802.11 MAC layer
IEEE 802.11d
Physical layer: Extend operation of 802.11 WLANs to new regulatory domains (countries)
IEEE 802.11e
MAC: Enhance to improve quality of service and enhance security mechanisms
IEEE 802.11g
Physical layer: Extend 802.11b to data rates >20 Mbps
IEEE 802.11i
MAC: Enhance security and authentication mechanisms
IEEE 802.11n
Physical/MAC: Enhancements to enable higher throughput
IEEE 802.11T
Recommended practice for the evaluation of 802.11 wireless performance
IEEE 802.11ac
Physical/MAC: Enhancements to support 0.5–1 Gbps in 5-GHz band
IEEE 802.11ad
Physical/MAC: Enhancements to support ≥ 1 Gbps in the 60- GHz band
Access point (AP) Any entity that has station functionality and provides access to the distribution system via the wireless medium for associated stations
Basic service set (BSS)
A set of stations controlled by a single coordination function
Coordination function The logical function that determines when a station operating within a BSS is permitted to transmit and may be able to receive PDUs
Distribution system (DS)
A system used to interconnect a set of BSSs and integrated LANs to create an ESS
Extended service set (ESS)
A set of one or more interconnected BSSs and integrated LANs that appear as a single BSS to the LLC layer at any station associated with one of these BSSs
MAC protocol data unit (MPDU)
The unit of data exchanged between two peer MAC entites using the services of the physical layer
MAC service data unit (MSDU)
Information that is delivered as a unit between MAC users
Station Any device that contains an IEEE 802.11 conformant MAC and physical layer
Access point (AP)/STA1
Basic service Set (BSS) Basic service
Set (BSS)
IEEE 802.3 LAN
Access point (AP)/STA5
STA2
STA3
STA4
STA6
Portal
Ethernet switch
STA7
Figure 14.3 Example WLAN Extended Service Set
Distribution system (DS)
Point Coordination
Function (PCF)
Contention-free service
Contention service
Figure 14.4 IEEE 802.11 Protocol Architecture
MAC Layer
Distributed Coordination Function (DCF)
Logical Link Control
2.4-Ghz direct sequence spread
spectrum 5.5 Mbps 11 Mbps
2.4-Ghz DS-SS 6, 9, 12,
18, 24, 36, 48, 54 Mbps
2.4-Ghz 5-GHz OFDM, MIMO
IEEE 802.11a IEEE 802.11b
5-Ghz orthogonal FDM
6, 9, 12, 18, 24, 36,
48, 54 Mbps
IEEE 802.11g IEEE 802.11n
802.11a 802.11b 802.11g 802.11n Peak data
throughput* 23 Mbps 6 Mbps 23 Mbps 60 Mbps (20 MHz
channel) 90 Mbps (40 MHz
channel) Peak signalling
rate 54 Mbps 11 Mbps 54 Mbps 124 Mbps (20
MHz channel) 248 Mbps (40 MHz channel)
RF band 5 GHz 2.4 GHz 2.4 GHz 2.4 GHz or 5 GHz
Channel width 20 MHz 20 MHz 20 Mhz 20 MHz or 40 MHz
Number of spatial streams
1 1 1 1, 2, 3, or 4
*This is the actual data throughput you get with real equipment under ideal conditions. Real- world performance is lower than this due to noise. Capacity is also shared among wireless clients. When two devices use the same access point, the capacity is typically divided in two, though it’s possible some clients will use more of the capacity than others. Source: [OU07].
Data Rate (Mbps) 802.11b 802.11a 802.11g
1 90+ — 90+
2 75 — 75
5.5(b)/6(a/g) 60 60+ 65
9 — 50 55
11(b)/12(a/g) 50 45 50
18 — 40 50
24 — 30 45
36 — 25 35
48 — 15 25
54 — 10 20
Transmitter Receiver
Antenna
Object
Figure 14.5 MIMO Scheme
MIMO
signal
processing
MIMO
signal
processing
0
5
10
15
20
25
25201510
Figure 14.6 Average Throughput per User
Simultaneous users/AP 802.11n 802.11g
T hr
ou gh
pu t (
M bp
s)
5
