T e c h n i c a l P a p e r
`
`IEEE 802.11b Wireless LANs
`
`Wireless Freedom at Ethernet Speeds
`
`ADVANCED MEDIA Exhibit 2034, pg. 001
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`IEEE 802.11b Wireless LANs
`Wireless Freedom at Ethernet Speeds
`
`Contents
`What’s New in Wireless LANs: The IEEE 802.11b Standard
`
`The Competitive Advantage of Going Wireless
`
`IEEE 802.11 and 802.11b Technology
`
`802.11 Operating Modes
`
`The 802.11 Physical Layer
`
`802.11b Enhancements to the PHY Layer
`
`The 802.11 Data Link Layer
`
`Association, Cellular Architectures, and Roaming
`
`Support for Time-Bounded Data
`
`Power Management
`
`Security
`
`Considerations for Choosing a Wireless LAN
`
`Ease of Setup
`
`Ease of Management
`
`Range and Throughput
`
`Mobility
`
`Power Management
`
`Safety
`
`Security
`
`Cost
`
`Conclusion
`
`2
`
`2
`
`3
`
`4
`
`4
`
`6
`
`6
`
`7
`
`9
`
`9
`
`9
`
`9
`
`9
`
`10
`
`10
`
`10
`
`11
`
`11
`
`12
`
`12
`
`12
`
`11
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`IEEE 802.11b Wireless LANs
`Wireless Freedom at Ethernet Speeds
`
`With the recent adoption of new standards for
`high-rate wireless LANs, mobile users can realize
`levels of performance, throughput, and availabil-
`ity comparable to those of traditional wired Eth-
`ernet. As a result, WLANs are on the verge of
`becoming a mainstream connectivity solution for
`a broad range of business customers.
`The most critical issue slowing WLAN
`demand until now has been limited throughput.
`This paper describes the new IEEE 802.11b
`standard for wireless transmission at rates up to
`11 Mbps, which promises to open new markets
`for WLANs. It describes 802.11 and 802.11b
`technology and discusses the key considerations
`for selecting a reliable, high-performance wireless
`LAN.
`
`What’s New in Wireless LANs: The IEEE
`802.11b Standard
`A wireless LAN (WLAN) is a data transmis-
`sion system designed to provide location-inde-
`pendent network access between computing
`devices by using radio waves rather than a
`cable infrastructure. In the corporate enter-
`prise, wireless LANs are usually implemented
`as the final link between the existing wired
`network and a group of client computers, giv-
`ing these users wireless access to the full
`resources and services of the corporate net-
`work across a building or campus setting.
`WLANs are on the verge of becoming a
`mainstream connectivity solution for a broad
`range of business customers. The wireless mar-
`ket is expanding rapidly as businesses discover
`the productivity benefits of going wire-free.
`According to Frost and Sullivan, the wireless
`LAN industry exceeded $300 million in 1998
`and will grow to $1.6 billion in 2005. To date,
`wireless LANs have been primarily imple-
`mented in vertical applications such as manu-
`facturing facilities, warehouses, and retail
`stores. The majority of future wireless LAN
`growth is expected in healthcare facilities, edu-
`cational institutions, and corporate enterprise
`office spaces. In the corporation, conference
`rooms, public areas, and branch offices are
`likely venues for WLANs.
`
`The widespread acceptance of WLANs
`depends on industry standardization to ensure
`product compatibility and reliability among
`the various manufacturers. The Institute of
`Electrical and Electronics Engineers (IEEE)
`ratified the original 802.11 specification in
`1997 as the standard for wireless LANs. That
`version of 802.11 provides for 1 Mbps and 2
`Mbps data rates and a set of fundamental sig-
`naling methods and other services.
`The most critical issue affecting WLAN
`demand has been limited throughput. The
`data rates supported by the original 802.11
`standard are too slow to support most general
`business requirements and have slowed adop-
`tion of WLANs. Recognizing the critical need
`to support higher data-transmission rates, the
`IEEE recently ratified the 802.11b standard
`(also known as 802.11 High Rate) for trans-
`missions of up to 11 Mbps. Global regulatory
`bodies and vendor alliances have endorsed this
`new high-rate standard, which promises to
`open new markets for WLANs in large enter-
`prise, small office, and home environments.
`With 802.11b, WLANs will be able to achieve
`wireless performance and throughput compa-
`rable to wired Ethernet.
`Outside of the standards bodies, wireless
`industry leaders have united to form the Wire-
`less Ethernet Compatibility Alliance (WECA).
`WECA’s mission is to certify cross-vendor
`interoperability and compatibility of IEEE
`802.11b wireless networking products and to
`promote that standard for the enterprise, the
`small business, and the home. Members
`include WLAN semiconductor manufactur-
`ers, WLAN providers, computer system ven-
`dors, and software makers—such as 3Com,
`Aironet, Apple, Breezecom, Cabletron, Com-
`paq, Dell, Fujitsu, IBM, Intersil, Lucent Tech-
`nologies, No Wires Needed, Nokia, Samsung,
`Symbol Technologies, Wayport, and Zoom.
`
`The Competitive Advantage of Going
`Wireless
`Today’s business environment is characterized
`by an increasingly mobile workforce and flat-
`ter organizations. Employees are equipped
`with notebook computers and spend more of
`their time working in teams that cross func-
`
`Acronyms and
`Abbreviations
`
`AP
`access point
`
`BPSK
`Binary Phase Shift Keying
`
`BSS
`Basic Service Set
`
`CCK
`Complementary Code
`Keying
`
`CRC
`cyclic redundancy check
`
`CSMA/CA
`Carrier Sense Multiple Access
`with Collision Avoidance
`
`CSMA/CD
`Carrier Sense Multiple Access
`with Collision Detection
`
`CTS
`Clear to Send
`
`DCF
`Distribution Coordination
`Function
`
`DHCP
`Dynamic Host Configuration
`Protocol
`
`DS
`distribution system
`
`DSSS
`direct sequence spread
`spectrum
`
`ESS
`Extended Service Set
`
`ETSI
`European Telecommunica-
`tions Standards Institute
`
`FCC
`Federal Communications
`Commission (USA)
`
`22
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`

`
`tional, organizational, and geographic bound-
`aries. Much of these workers’ productivity
`occurs in meetings and away from their desks.
`Users need access to the network far beyond
`their personal desktops. WLANs fit well in
`this work environment, giving mobile workers
`much-needed freedom in their network access.
`With a wireless network, workers can access
`information from anywhere in the corpora-
`tion—a conference room, the cafeteria, or a
`remote branch office. Wireless LANs provide
`a benefit for IT managers as well, allowing
`them to design, deploy, and enhance networks
`without regard to the availability of wiring,
`saving both effort and dollars.
`Businesses of all sizes can benefit from
`deploying a WLAN system, which provides a
`powerful combination of wired network
`throughput, mobile access, and configuration
`flexibility. The economic benefits can add up
`to as much as $16,000 per user—measured in
`worker productivity, organizational efficiency,
`revenue gain, and cost savings—over wired
`alternatives.1 Specifically, WLAN advantages
`include:
`• Mobility that improves productivity with
`real-time access to information, regardless of
`worker location, for faster and more effi-
`cient decision-making
`• Cost-effective network setup for hard-to-
`wire locations such as older buildings and
`solid-wall structures
`• Reduced cost of ownership—particularly in
`dynamic environments requiring frequent
`modifications—thanks to minimal wiring
`and installation costs per device and user
`WLANs liberate users from dependence
`on hard-wired access to the network back-
`bone, giving them anytime, anywhere network
`access. This freedom to roam offers numerous
`user benefits for a variety of work environ-
`ments, such as:
`• Immediate bedside access to patient infor-
`mation for doctors and hospital staff
`• Easy, real-time network access for on-site
`consultants or auditors
`
`• Improved database access for roving super-
`visors such as production line managers,
`warehouse auditors, or construction engi-
`neers
`• Simplified network configuration with min-
`imal MIS involvement for temporary setups
`such as trade shows or conference rooms
`• Faster access to customer information for
`service vendors and retailers, resulting in
`better service and improved customer satis-
`faction
`• Location-independent access for network
`administrators, for easier on-site trou-
`bleshooting and support
`• Real-time access to study group meetings
`and research links for students
`
`IEEE 802.11 and 802.11b Technology
`As the globally recognized LAN authority, the
`IEEE 802 committee has established the stan-
`dards that have driven the LAN industry for
`the past two decades, including 802.3 Ethernet,
`802.5 Token Ring, and 802.3z 100BASE-T
`Fast Ethernet. In 1997, after seven years of
`work, the IEEE published 802.11, the first
`internationally sanctioned standard for wire-
`less LANs. In September 1999 they ratified
`the 802.11b “High Rate” amendment to the
`standard, which added two higher speeds (5.5
`and 11 Mbps) to 802.11.
`With 802.11b WLANs, mobile users can
`get Ethernet levels of performance, through-
`put, and availability. The standards-based
`technology allows administrators to build net-
`works that seamlessly combine more than one
`LAN technology to best fit their business and
`user needs.
`Like all IEEE 802 standards, the 802.11
`standards focus on the bottom two levels of
`the ISO model, the physical layer and data
`link layer (Figure 1 on page 4). Any LAN
`application, network operating system, or pro-
`tocol, including TCP/IP and Novell NetWare,
`will run on an 802.11-compliant WLAN as
`easily as they run over Ethernet.
`The basic architecture, features, and ser-
`vices of 802.11b are defined by the original
`
`1 “Wireless Local Area Networking: ROI/Cost-Benefit Study,” WLANA, October 1998.
`
`Acronyms and
`Abbreviations
`
`FHSS
`Frequency Hopping Spread
`Spectrum
`
`IBSS
`Independent Basic Service
`Set
`
`IEEE
`Institute of Electrical and
`Electronics Engineers
`
`IETF
`Internet Engineering Task
`Force
`
`IP
`Internet Protocol
`
`IPSec
`Internet Protocol Security
`
`ISA
`Integrated Services
`Architecture
`
`ISM
`Industry, Scientific, and
`Medical
`
`ISO
`International Organization
`for Standardization
`
`LLC
`Logical Link Control
`
`MAC
`Media Access Control
`
`MIB
`management information
`base
`
`MKK
`Radio Equipment Inspection
`and Certification Institute
`(Japan)
`
`NIC
`network interface card
`
`33
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`Application
`
`Presentation
`
`Session
`
`Transport
`
`TCP
`
`Network
`
`IP
`
`Data
`Link
`
`Logical Link Control (LLC)—802.2
`Media Access Control (MAC)—Power, security, etc.
`
`Physical
`
`FH, DS, IR, CCK(b), OFDM(a)
`
`Network
`operating
`system
`(NOS)
`
`802.11
`
`Figure 1. 802.11 and the ISO Model
`
`802.11 standard. The 802.11b specification
`affects only the physical layer, adding higher
`data rates and more robust connectivity.
`
`802.11 Operating Modes
`802.11 defines two pieces of equipment, a
`wireless station, which is usually a PC equipped
`with a wireless network interface card (NIC),
`and an access point (AP), which acts as a bridge
`between the wireless and wired networks. An
`access point usually consists of a radio, a wired
`network interface (e.g., 802.3), and bridging
`software conforming to the 802.1d bridging
`standard. The access point acts as the base sta-
`tion for the wireless network, aggregating
`access for multiple wireless stations onto the
`wired network. Wireless end stations can be
`802.11 PC Card, PCI, or ISA NICs, or
`embedded solutions in non-PC clients (such
`as an 802.11-based telephone handset).
`The 802.11 standard defines two modes:
`infrastructure mode and ad hoc mode. In infra-
`structure mode (Figure 2), the wireless network
`consists of at least one access point connected
`to the wired network infrastructure and a set
`of wireless end stations. This configuration is
`called a Basic Service Set (BSS). An Extended
`Service Set (ESS) is a set of two or more BSSs
`
`forming a single subnetwork. Since most cor-
`porate WLANs require access to the wired
`LAN for services (file servers, printers, Inter-
`net links) they will operate in infrastructure
`mode.
`Ad hoc mode (also called peer-to-peer
`mode or an Independent Basic Service Set, or
`IBSS) is simply a set of 802.11 wireless sta-
`tions that communicate directly with one
`another without using an access point or any
`connection to a wired network (Figure 3).
`This mode is useful for quickly and easily set-
`ting up a wireless network anywhere that a
`wireless infrastructure does not exist or is not
`required for services, such as a hotel room,
`convention center, or airport, or where access
`to the wired network is barred (such as for
`consultants at a client site).
`
`The 802.11 Physical Layer
`The three physical layers originally defined in
`802.11 included two spread-spectrum radio
`techniques and a diffuse infrared specification.
`The radio-based standards operate within the
`2.4 GHz ISM band. These frequency bands
`are recognized by international regulatory
`agencies, such as the FCC (USA), ETSI
`(Europe), and the MKK (Japan) for unlicensed
`
`Acronyms and
`Abbreviations
`
`NOS
`network operating system
`
`PCF
`Point Coordination Function
`
`PCI
`Peripheral Component
`Interconnect
`
`PRNG
`pseudo random number
`generator
`
`QPSK
`Quadrature Phase Shift
`Keying
`
`RC4
`Ron’s Code or Rivest’s
`Cipher
`
`RTS
`Request to Send
`
`SNMP
`Simple Network
`Management Protocol
`
`TCP/IP
`Transmission Control
`Protocol/Internet Protocol
`
`WECA
`Wireless Ethernet
`Compatibility Alliance
`
`WEP
`Wired Equivalent Privacy
`
`WLAN
`wireless local area network
`
`WLANA
`Wireless LAN Alliance
`
`44
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`
`( D S )
`
`t e m
`
`i o n s y s
`
`i b u t
`
`r
`
`t
`
`D i s
`
`s p o i n t
`c e s
`( A P )
`
`A c
`
`i o n
`
`S t a t
`
`l s
`
`i p l e c e l
`
`t
`
`( E S S ) — m u l
`
`B a s i c S e r
`
`
`v i c e S e t
`s i n g l e c e l
`
`( B S S ) —
`l
`
`v i c e S e t
`
`
`
`t e n d e d S e r
`
`E x
`
`Figure 2. Infrastructure Mode
`
`radio operations. As such, 802.11-based prod-
`ucts do not require user licensing or special
`training. Spread-spectrum techniques, in addi-
`tion to satisfying regulatory requirements,
`increase reliability, boost throughput, and
`allow many unrelated products to share the
`spectrum without explicit cooperation and
`with minimal interference.
`The original 802.11 wireless standard
`defines data rates of 1 Mbps and 2 Mbps via
`radio waves using frequency hopping spread
`spectrum (FHSS) or direct sequence spread
`spectrum (DSSS). It is important to note that
`FHSS and DSSS are fundamentally different
`signaling mechanisms and will not interoper-
`ate with one another.
`Using the frequency hopping technique,
`the 2.4 GHz band is divided into 75 1-MHz
`subchannels. The sender and receiver agree on
`
` B a s i c
`I B S S )
`(
`
`I n d e p e n d e n t
`v i c e S e t
`S e r
`
`
`
`Figure 3. Ad Hoc Mode
`
`a hopping pattern, and data is sent over a
`sequence of the subchannels. Each conversa-
`tion within the 802.11 network occurs over a
`different hopping pattern, and the patterns are
`designed to minimize the chance of two senders
`using the same subchannel simultaneously.
`FHSS techniques allow for a relatively
`simple radio design, but are limited to speeds
`of no higher than 2 Mbps. This limitation is
`driven primarily by FCC regulations that
`restrict subchannel bandwidth to 1 MHz.
`These regulations force FHSS systems to
`spread their usage across the entire 2.4 GHz
`band, meaning they must hop often, which
`leads to a high amount of hopping overhead.
`In contrast, the direct sequence signaling
`technique divides the 2.4 GHz band into 14
`22-MHz channels. Adjacent channels overlap
`one another partially, with three of the 14
`being completely non-overlapping. Data is
`sent across one of these 22 MHz channels
`without hopping to other channels. To com-
`pensate for noise on a given channel, a tech-
`nique called “chipping” is used. Each bit of
`user data is converted into a series of redun-
`dant bit patterns called “chips.” The inherent
`redundancy of each chip combined with
`spreading the signal across the 22 MHz
`channel provides for a form of error checking
`and correction; even if part of the signal is
`
`55
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`
`damaged, it can still be recovered in many
`cases, minimizing the need for retransmissions.
`
`802.11b Enhancements to the PHY Layer
`The key contribution of the 802.11b addition
`to the wireless LAN standard was to standard-
`ize the physical layer support of two new speeds,
`5.5 Mbps and 11 Mbps. To accomplish this,
`DSSS had to be selected as the sole physical
`layer technique for the standard since, as
`noted above, frequency hopping cannot sup-
`port the higher speeds without violating cur-
`rent FCC regulations. The implication is that
`802.11b systems will interoperate with 1 Mbps
`and 2 Mbps 802.11 DSSS systems, but will
`not work with 1 Mbps and 2 Mbps 802.11
`FHSS systems.
`The original 802.11 DSSS standard
`specifies an 11-bit chipping—called a Barker
`sequence—to encode all data sent over the air.
`Each 11-chip sequence represents a single data
`bit (1 or 0), and is converted to a waveform,
`called a symbol, that can be sent over the air.
`These symbols are transmitted at a 1 MSps (1
`million symbols per second) symbol rate using
`a technique called Binary Phase Shift Keying
`(BPSK). In the case of 2 Mbps, a more sophis-
`ticated implementation called Quadrature
`Phase Shift Keying (QPSK) is used; it doubles
`the data rate available in BPSK, via improved
`efficiency in the use of the radio bandwidth.
`To increase the data rate in the 802.11b
`standard, advanced coding techniques are
`employed. Rather than the two 11-bit Barker
`sequences, 802.11b specifies Complementary
`Code Keying (CCK), which consists of a set of
`64 8-bit code words. As a set, these code
`words have unique mathematical properties
`that allow them to be correctly distinguished
`
`from one another by a receiver even in the
`presence of substantial noise and multipath
`interference (e.g., interference caused by
`receiving multiple radio reflections within a
`building). The 5.5 Mbps rate uses CCK to
`encode 4 bits per carrier, while the 11 Mbps
`rate encodes 8 bits per carrier. Both speeds use
`QPSK as the modulation technique and signal
`at 1.375 MSps. This is how the higher data
`rates are obtained. Table 1 shows the differences.
`To support very noisy environments as
`well as extended range, 802.11b WLANs use
`dynamic rate shifting, allowing data rates to be
`automatically adjusted to compensate for the
`changing nature of the radio channel. Ideally,
`users connect at the full 11 Mbps rate. How-
`ever when devices move beyond the optimal
`range for 11 Mbps operation, or if substantial
`interference is present, 802.11b devices will
`transmit at lower speeds, falling back to 5.5,
`2, and 1 Mbps. Likewise, if the device moves
`back within the range of a higher-speed trans-
`mission, the connection will automatically
`speed up again. Rate shifting is a physical-
`layer mechanism transparent to the user and
`the upper layers of the protocol stack.
`
`The 802.11 Data Link Layer
`The data link layer within 802.11 consists of
`two sublayers: Logical Link Control (LLC)
`and Media Access Control (MAC). 802.11
`uses the same 802.2 LLC and 48-bit address-
`ing as other 802 LANs, allowing for very sim-
`ple bridging from wireless to IEEE wired
`networks, but the MAC is unique to WLANs.
`The 802.11 MAC is very similar in con-
`cept to 802.3, in that it is designed to support
`multiple users on a shared medium by having
`the sender sense the medium before accessing
`
`Table 1. 802.11b Data Rate Specifications
`
`Data Rate
`
`Code Length
`
`Modulation
`
`Symbol Rate
`
`Bits/Symbol
`
`1 Mbps
`
`2 Mbps
`
`5.5 Mbps
`
`11 Mbps
`
`11 (Barker Sequence)
`
`11 (Barker Sequence)
`
`8 (CCK)
`
`8 (CCK)
`
`BPSK
`
`QPSK
`
`QPSK
`
`QPSK
`
`1 MSps
`
`1 MSps
`
`1.375 MSps
`
`1.375 MSps
`
`1
`
`2
`
`4
`
`8
`
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`
`it. For 802.3 Ethernet LANs, the Carrier Sense
`Multiple Access with Collision Detection
`(CSMA/CD) protocol regulates how Ethernet
`stations establish access to the wire and how
`they detect and handle collisions that occur
`when two or more devices try to simultane-
`ously communicate over the LAN. In an
`802.11 WLAN, collision detection is not pos-
`sible due to what is known as the “near/far”
`problem: to detect a collision, a station must
`be able to transmit and listen at the same
`time, but in radio systems the transmission
`drowns out the ability of the station to “hear”
`a collision.
`To account for this difference, 802.11
`uses a slightly modified protocol known as
`Carrier Sense Multiple Access with Collision
`Avoidance (CSMA/CA) or the Distributed
`Coordination Function (DCF). CSMA/CA
`attempts to avoid collisions by using explicit
`packet acknowledgment (ACK), which means
`an ACK packet is sent by the receiving station
`to confirm that the data packet arrived intact.
`CSMA/CA works as follows. A station
`wishing to transmit senses the air, and, if no
`activity is detected, the station waits an addi-
`tional, randomly selected period of time and
`then transmits if the medium is still free. If
`the packet is received intact, the receiving sta-
`tion issues an ACK frame that, once success-
`fully received by the sender, completes the
`process. If the ACK frame is not detected by
`the sending station, either because the original
`data packet was not received intact or the
`ACK was not received intact, a collision is
`assumed to have occurred and the data packet
`is transmitted again after waiting another ran-
`dom amount of time.
`CSMA/CA thus provides a way of sharing
`access over the air. This explicit ACK mecha-
`nism also handles interference and other radio-
`related problems very effectively. However, it
`does add some overhead to 802.11 that 802.3
`does not have, so that an 802.11 LAN will
`always have slower performance than an
`equivalent Ethernet LAN.
`Another MAC-layer problem specific to
`wireless is the “hidden node” issue, in which
`two stations on opposite sides of an access
`point can both “hear” activity from an access
`
`point, but not from each other, usually due to
`distance or an obstruction. To solve this prob-
`lem, 802.11 specifies an optional Request to
`Send/Clear to Send (RTS/CTS) protocol at
`the MAC layer. When this feature is in use, a
`sending station transmits an RTS and waits
`for the access point to reply with a CTS. Since
`all stations in the network can hear the access
`point, the CTS causes them to delay any
`intended transmissions, allowing the sending
`station to transmit and receive a packet
`acknowledgment without any chance of colli-
`sion. Since RTS/CTS adds additional over-
`head to the network by temporarily reserving
`the medium, it is typically used only on the
`largest-sized packets, for which retransmission
`would be expensive from a bandwidth stand-
`point.
`Finally, the 802.11 MAC layer provides
`for two other robustness features: CRC check-
`sum and packet fragmentation. Each packet
`has a CRC checksum calculated and attached
`to ensure that the data was not corrupted in
`transit. This is different from Ethernet, where
`higher-level protocols such as TCP handle
`error checking. Packet fragmentation allows
`large packets to be broken into smaller units
`when sent over the air, which is useful in very
`congested environments or when interference
`is a factor, since larger packets have a better
`chance of being corrupted. This technique
`reduces the need for retransmission in many
`cases and thus improves overall wireless net-
`work performance. The MAC layer is respon-
`sible for reassembling fragments received,
`rendering the process transparent to higher-
`level protocols.
`
`Association, Cellular Architectures, and Roaming
`The 802.11 MAC layer is responsible for how
`a client associates with an access point. When
`an 802.11 client enters the range of one or
`more APs, it chooses an access point to associ-
`ate with (also called joining a Basic Service
`Set), based on signal strength and observed
`packet error rates. Once accepted by the access
`point, the client tunes to the radio channel to
`which the access point is set. Periodically it
`surveys all 802.11 channels in order to assess
`whether a different access point would provide
`
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`B a c k b o n e n e t w o r k
`
`s p o i n t
`c e s
`( A P )
`
`A c
`
`r o a m i n g
`l
`c e l
`I n t e r
`f
`a n d h a n d o f
`
`-
`
`• Coverage easily expanded
`• Load balancing
`• Scalability and incremental growth
`• Transparent to the user
`
`Figure 4. Access Point Roaming
`
`it with better performance characteristics. If it
`determines that this is the case, it reassociates
`with the new access point, tuning to the radio
`channel to which that access point is set
`(Figure 4).
`Reassociation usually occurs because the
`wireless station has physically moved away
`from the original access point, causing the sig-
`nal to weaken. In other cases, reassociation
`occurs due to a change in radio characteristics
`in the building, or due simply to high network
`traffic on the original access point. In the latter
`case this function is known as “load balanc-
`ing,” since its primary function is to distribute
`
`the total WLAN load most efficiently across
`the available wireless infrastructure.
`This process of dynamically associating
`and reassociating with APs allows network
`managers to set up WLANs with very broad
`coverage by creating a series of overlapping
`802.11b cells throughout a building or across
`a campus. To be successful, the IT manager
`ideally will employ “channel reuse,” taking
`care to set up each access point on an 802.11
`DSSS channel that does not overlap with a
`channel used by a neighboring access point
`(Figure 5). As noted above, while there are 14
`partially overlapping channels specified in
`
`1
`
`11
`
`6
`
`1
`
`1
`
`11
`
`Figure 5. Unlimited Roaming
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`802.11 DSSS, there are only three channels
`that do not overlap at all, and these are the
`best to use for multi-cell coverage. If two APs
`are in range of one another and are set to the
`same or partially overlapping channels, they
`may cause some interference for one another,
`thus lowering the total available bandwidth in
`the area of overlap.
`
`Support for Time-Bounded Data
`Time-bounded data such as voice and video is
`supported in the 802.11 MAC specification
`through the Point Coordination Function (PCF).
`As opposed to the DCF, where control is dis-
`tributed to all stations, in PCF mode a single
`access point controls access to the media. If a
`BSS is set up with PCF enabled, time is
`spliced between the system being in PCF
`mode and in DCF (CSMA/CA) mode. Dur-
`ing the periods when the system is in PCF
`mode, the access point will poll each station
`for data, and after a given time move on to the
`next station. No station is allowed to transmit
`unless it is polled, and stations receive data
`from the access point only when they are
`polled. Since PCF gives every station a turn to
`transmit in a predetermined fashion, a maxi-
`mum latency is guaranteed. A downside to
`PCF is that it is not particularly scalable, in
`that a single point needs to have control of
`media access and must poll all stations, which
`can be ineffective in large networks.
`
`Power Management
`In addition to controlling media access, the
`802.11 HR MAC supports power conservation
`to extend the battery life of portable devices.
`The standard supports two power-utilization
`modes, called Continuous Aware Mode and
`Power Save Polling Mode. In the former, the
`radio is always on and drawing power, whereas
`in the latter, the radio is “dozing” with the
`access point queuing any data for it. The
`client radio will wake up periodically in time
`to receive regular beacon signals from the
`access point. The beacon includes information
`regarding which stations have traffic waiting
`for them, and the client can thus awake upon
`beacon notification and receive its data,
`returning to sleep afterward.
`
`Security
`802.11 provides for both MAC layer (OSI
`Layer 2) access control and encryption mecha-
`nisms, which are known as Wired Equivalent
`Privacy (WEP), with the objective of provid-
`ing wireless LANs with security equivalent to
`their wired counterparts. For the access control,
`the ESSID (also known as a WLAN Service
`Area ID) is programmed into each access point
`and is required knowledge in order for a wire-
`less client to associate with an access point. In
`addition, there is provision for a table of MAC
`addresses called an Access Control List to be
`included in the access point, restricting access
`to clients whose MAC addresses are on the list.
`For data encryption, the standard pro-
`vides for optional encryption using a 40-bit
`shared-key RC4 PRNG algorithm from RSA
`Data Security. All data sent and received while
`the end station and access point are associated
`can be encrypted using this key. In addition,
`when encryption is in use, the access point
`will issue an encrypted challenge packet to any
`client attempting to associate with it. The
`client must use its key to encrypt the correct
`response in order to authenticate itself and
`gain network access.
`Beyond Layer 2, 802.11 HR WLANs
`support the same security standards supported
`by other 802 LANs for access control (such as
`network operating system logins) and encryp-
`tion (such as IPSec or application-level
`encryption). These higher-layer technologies
`can be used to create end-to-end secure net-
`works encompassing both wired LAN and
`WLAN components, with the wireless piece
`of the network gaining unique additional
`security from the 802.11 feature set.
`
`Considerations for Choosing a Wireless LAN
`While the bulk of this paper has described
`how 802.11b wireless LANs are alike, there
`are still many ways for wireless LAN vendors
`to differentiate themselves in the marketplace
`that will affect a customer’s purchasing deci-
`sion. We cover some of these areas below.
`
`Ease of Setup
`To install a wireless LAN one must install and
`configure APs and PC Cards. The most
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`important piece of this effort is proper place-
`ment of the APs. Access point placement is
`what ensures the coverage and performance
`required by the network design. There are sev-
`eral features that provide assistance in the
`installation process:
`• Site survey. For complete wireless LANs
`employing a cellular architecture, proper
`placement of APs is best determined by per-
`forming a site survey, in which the person
`installing the WLAN can place APs and
`record signal strength and quality informa-
`tion while moving about the intended cov-
`erage area. While most vendors provide a
`site survey tool, these utilities vary in the
`amount and quality of information they
`provide, as well as in their logging and
`reporting capabilities.
`• Power over Ethernet. Some vendors ship
`APs that can be powered over the Ethernet
`cable that connects the access point to the
`wired network. This is usually implemented
`by a piece of equipment in the wiring closet
`that takes in AC power and the data con-
`nection from the wired switch, and then
`outputs DC power over unused wire pairs
`in the networking cable that runs between
`the module and the access point. This fea-
`ture eliminates the need to run an AC
`power cable out to the access point (usually
`located on the wall or ceiling), making
`installation quicker and more affordable.
`• Easy-to-use NIC and access point configu-
`ration tools. Once the APs are installed,
`both APs and NICs must be configured for
`use. As with any technical product, the
`quality of the user interface determines the
`amount of time required to configure the
`network for operation. In addition, some
`vendors supply tools for bulk configuration
`of access points on the same network, greatly
`easing network setup. Finally, having a vari-
`ety of methods to access the access point is
`helpful to ensure simple setup. Configura-
`tion options include telnet; Web-based; or
`SNMP-based over the Ethernet cable, from
`a wireless station, or via a serial port built
`into the access point.
`
`Ease of Management
`Since an 802.11 wireless LAN differs from
`standard 802.3 and 802.5 wired LANs only at
`OSI Layers 1 and 2, one should expect at least
`the same level of manageability from these
`products as one finds for wired networking
`products. At a minimum, the products should
`come with SNMP 2 support so that they can
`be automatically discovered and managed
`using the same tools employed for wired LAN
`equipment. And one should assess carefully
`what can be controlled via the SNMP MIB.
`Some products measure and control a number
`of Ethernet and radio variables in the access
`point, while others provide only a basic Ether-
`net MIB.
`Beyond SNMP, it is useful to be able to
`configure and probe APs via an easy-to-use
`interface like a Web browser. Some vendors
`have built Web servers into their APs for this
`reason. Finally, the ability to manage, config-
`ure, and upgrade APs in groups simplifies
`WLAN administration.
`
`Range and Throughput
`802.11b WLANs communicate