IEEE 802.11b Wireless LANs. Wireless Freedom at Ethernet Speeds

14 Pages·4.154 MB·English

Technical Paper
IEEE 802.11b Wireless LANs
Wireless Freedom at Ethernet Speeds
IEEE 802.11b Wireless LANs
Wireless Freedom at Ethernet Speeds
Contents
What’s New in Wireless LANs: The IEEE 802.11b Standard 2
The Competitive Advantage of Going Wireless 2
IEEE 802.11 and 802.11b Technology 3
802.11 Operating Modes 4
The 802.11 Physical Layer 4
802.11b Enhancements to the PHY Layer 6
The 802.11 Data Link Layer 6
Association, Cellular Architectures, and Roaming 7
Support for Time-Bounded Data 9
Power Management 9
Security 9
Considerations for Choosing a Wireless LAN 9
Ease of Setup 9
Ease of Management 10
Range and Throughput 10
Mobility 10
Power Management 11
Safety 11
Security 12
Cost 12
Conclusion 12
11
IEEE 802.11b Wireless LANs The widespread acceptance of WLANs
Wireless Freedom at Ethernet Speeds depends on industry standardization to ensure
product compatibility and reliability among
Acronyms and With the recent adoption of new standards for the various manufacturers. The Institute of
Abbreviations high-rate wireless LANs, mobile users can realize Electrical and Electronics Engineers (IEEE)
levels of performance, throughput, and availabil- ratified the original 802.11 specification in
AP ity comparable to those of traditional wired Eth- 1997 as the standard for wireless LANs. That
access point ernet. As a result, WLANs are on the verge of version of 802.11 provides for 1 Mbps and 2
BPSK becoming a mainstream connectivity solution for Mbps data rates and a set of fundamental sig-
Binary Phase Shift Keying a broad range of business customers. naling methods and other services.
The most critical issue slowing WLAN The most critical issue affecting WLAN
BSS
demand until now has been limited throughput. demand has been limited throughput. The
Basic Service Set
This paper describes the new IEEE 802.11b data rates supported by the original 802.11
CCK standard for wireless transmission at rates up to standard are too slow to support most general
Complementary Code
11 Mbps, which promises to open new markets business requirements and have slowed adop-
Keying
for WLANs. It describes 802.11 and 802.11b tion of WLANs. Recognizing the critical need
CRC technology and discusses the key considerations to support higher data-transmission rates, the
cyclic redundancy check for selecting a reliable, high-performance wireless IEEE recently ratified the 802.11b standard
LAN. (also known as 802.11 High Rate) for trans-
CSMA/CA
Carrier Sense Multiple Access missions of up to 11 Mbps. Global regulatory
with Collision Avoidance What’s New in Wireless LANs: The IEEE bodies and vendor alliances have endorsed this
802.11b Standard new high-rate standard, which promises to
CSMA/CD
A wireless LAN (WLAN) is a data transmis- open new markets for WLANs in large enter-
Carrier Sense Multiple Access
with Collision Detection sion system designed to provide location-inde- prise, small office, and home environments.
pendent network access between computing With 802.11b, WLANs will be able to achieve
CTS
devices by using radio waves rather than a wireless performance and throughput compa-
Clear to Send
cable infrastructure. In the corporate enter- rable to wired Ethernet.
DCF prise, wireless LANs are usually implemented Outside of the standards bodies, wireless
Distribution Coordination as the final link between the existing wired industry leaders have united to form the Wire-
Function
network and a group of client computers, giv- less Ethernet Compatibility Alliance (WECA).
DHCP ing these users wireless access to the full WECA’s mission is to certify cross-vendor
Dynamic Host Configuration resources and services of the corporate net- interoperability and compatibility of IEEE
Protocol work across a building or campus setting. 802.11b wireless networking products and to
WLANs are on the verge of becoming a promote that standard for the enterprise, the
DS
distribution system mainstream connectivity solution for a broad small business, and the home. Members
range of business customers. The wireless mar- include WLAN semiconductor manufactur-
DSSS
ket is expanding rapidly as businesses discover ers, WLAN providers, computer system ven-
direct sequence spread
the productivity benefits of going wire-free. dors, and software makers—such as 3Com,
spectrum
According to Frost and Sullivan, the wireless Aironet, Apple, Breezecom, Cabletron, Com-
ESS
LAN industry exceeded $300 million in 1998 paq, Dell, Fujitsu, IBM, Intersil, Lucent Tech-
Extended Service Set
and will grow to $1.6 billion in 2005. To date, nologies, No Wires Needed, Nokia, Samsung,
ETSI wireless LANs have been primarily imple- Symbol Technologies, Wayport, and Zoom.
European Telecommunica- mented in vertical applications such as manu-
tions Standards Institute
facturing facilities, warehouses, and retail The Competitive Advantage of Going
FCC stores. The majority of future wireless LAN Wireless
Federal Communications growth is expected in healthcare facilities, edu- Today’s business environment is characterized
Commission (USA) cational institutions, and corporate enterprise by an increasingly mobile workforce and flat-
office spaces. In the corporation, conference ter organizations. Employees are equipped
rooms, public areas, and branch offices are with notebook computers and spend more of
likely venues for WLANs. their time working in teams that cross func-
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tional, organizational, and geographic bound- • Improved database access for roving super-
aries. Much of these workers’ productivity visors such as production line managers,
occurs in meetings and away from their desks. warehouse auditors, or construction engi-
Users need access to the network far beyond neers Acronyms and
Abbreviations
their personal desktops. WLANs fit well in • Simplified network configuration with min-
this work environment, giving mobile workers imal MIS involvement for temporary setups
FHSS
much-needed freedom in their network access. such as trade shows or conference rooms
Frequency Hopping Spread
With a wireless network, workers can access • Faster access to customer information for
Spectrum
information from anywhere in the corpora- service vendors and retailers, resulting in
tion—a conference room, the cafeteria, or a better service and improved customer satis- IBSS
Independent Basic Service
remote branch office. Wireless LANs provide faction
Set
a benefit for IT managers as well, allowing • Location-independent access for network
them to design, deploy, and enhance networks administrators, for easier on-site trou- IEEE
without regard to the availability of wiring, bleshooting and support Institute of Electrical and
Electronics Engineers
saving both effort and dollars. • Real-time access to study group meetings
Businesses of all sizes can benefit from and research links for students IETF
deploying a WLAN system, which provides a Internet Engineering Task
powerful combination of wired network IEEE 802.11 and 802.11b Technology Force
throughput, mobile access, and configuration As the globally recognized LAN authority, the
IP
flexibility. The economic benefits can add up IEEE 802 committee has established the stan- Internet Protocol
to as much as $16,000 per user—measured in dards that have driven the LAN industry for
IPSec
worker productivity, organizational efficiency, the past two decades, including 802.3 Ethernet,
Internet Protocol Security
revenue gain, and cost savings—over wired 802.5 Token Ring, and 802.3z 100BASE-T
alternatives.1Specifically, WLAN advantages Fast Ethernet. In 1997, after seven years of ISA
Integrated Services
include: work, the IEEE published 802.11, the first
Architecture
• Mobility that improves productivity with internationally sanctioned standard for wire-
real-time access to information, regardless of less LANs. In September 1999 they ratified ISM
worker location, for faster and more effi- the 802.11b “High Rate” amendment to the Industry, Scientific, and
Medical
cient decision-making standard, which added two higher speeds (5.5
• Cost-effective network setup for hard-to- and 11 Mbps) to 802.11. ISO
wire locations such as older buildings and With 802.11b WLANs, mobile users can International Organization
solid-wall structures get Ethernet levels of performance, through- for Standardization
• Reduced cost of ownership—particularly in put, and availability. The standards-based
LLC
dynamic environments requiring frequent technology allows administrators to build net- Logical Link Control
modifications—thanks to minimal wiring works that seamlessly combine more than one
MAC
and installation costs per device and user LAN technology to best fit their business and
Media Access Control
WLANs liberate users from dependence user needs.
on hard-wired access to the network back- Like all IEEE 802 standards, the 802.11 MIB
bone, giving them anytime, anywhere network standards focus on the bottom two levels of management information
base
access. This freedom to roam offers numerous the ISO model, the physical layer and data
user benefits for a variety of work environ- link layer (Figure 1 on page 4). Any LAN MKK
ments, such as: application, network operating system, or pro- Radio Equipment Inspection
• Immediate bedside access to patient infor- tocol, including TCP/IP and Novell NetWare, and Certification Institute
(Japan)
mation for doctors and hospital staff will run on an 802.11-compliant WLAN as
• Easy, real-time network access for on-site easily as they run over Ethernet. NIC
consultants or auditors The basic architecture, features, and ser- network interface card
vices of 802.11b are defined by the original
1“Wireless Local Area Networking: ROI/Cost-Benefit Study,” WLANA, October 1998.
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Application
Presentation
Acronyms and Session
Abbreviations
Network
NOS operating
Transport TCP
network operating system system
(NOS)
PCF
Point Coordination Function Network IP
PCI
Peripheral Component Data Logical Link Control (LLC)—802.2
Interconnect Link Media Access Control (MAC)—Power, security, etc.
802.11
PRNG
pseudo random number Physical FH, DS, IR, CCK(b), OFDM(a)
generator
QPSK Figure 1. 802.11 and the ISO Model
Quadrature Phase Shift
Keying
802.11 standard. The 802.11b specification forming a single subnetwork. Since most cor-
RC4
affects only the physical layer, adding higher porate WLANs require access to the wired
Ron’s Code or Rivest’s
data rates and more robust connectivity. LAN for services (file servers, printers, Inter-
Cipher
net links) they will operate in infrastructure
RTS 802.11 Operating Modes mode.
Request to Send
802.11 defines two pieces of equipment, a Ad hoc mode (also called peer-to-peer
SNMP wireless station, which is usually a PC equipped mode or an Independent Basic Service Set, or
Simple Network with a wireless network interface card (NIC), IBSS) is simply a set of 802.11 wireless sta-
Management Protocol and an access point (AP), which acts as a bridge tions that communicate directly with one
between the wireless and wired networks. An another without using an access point or any
TCP/IP
Transmission Control access point usually consists of a radio, a wired connection to a wired network (Figure 3).
Protocol/Internet Protocol network interface (e.g., 802.3), and bridging This mode is useful for quickly and easily set-
software conforming to the 802.1d bridging ting up a wireless network anywhere that a
WECA
standard. The access point acts as the base sta- wireless infrastructuredoes not exist or is not
Wireless Ethernet
Compatibility Alliance tion for the wireless network, aggregating required for services, such as a hotel room,
access for multiple wireless stations onto the convention center, or airport, or where access
WEP
wired network. Wireless end stations can be to the wired network is barred (such as for
Wired Equivalent Privacy
802.11 PC Card, PCI, or ISA NICs, or consultants at a client site).
WLAN embedded solutions in non-PC clients (such
wireless local area network as an 802.11-based telephone handset). The 802.11 Physical Layer
The 802.11 standard defines two modes: The three physical layers originally defined in
WLANA
Wireless LAN Alliance infrastructure mode and ad hoc mode. In infra- 802.11 included two spread-spectrum radio
structure mode (Figure 2), the wireless network techniques and a diffuse infrared specification.
consists of at least one access point connected The radio-based standards operate within the
to the wired network infrastructure and a set 2.4 GHz ISM band. These frequency bands
of wireless end stations. This configuration is are recognized by international regulatory
called a Basic Service Set (BSS). An Extended agencies, such as the FCC (USA), ETSI
Service Set (ESS)is a set of two or more BSSs (Europe), and the MKK (Japan) for unlicensed
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Distribution system (DS)
Acces(As Pp)oint
Station
Basic Sersvinicgel eS ceeEt lx(lBteSnSd)e—d Service Set (ESS)—multiple cells
Figure 2. Infrastructure Mode
radio operations. As such, 802.11-based prod- a hopping pattern, and data is sent over a
ucts do not require user licensing or special sequence of the subchannels. Each conversa-
training. Spread-spectrum techniques, in addi- tion within the 802.11 network occurs over a
tion to satisfying regulatory requirements, different hopping pattern, and the patterns are
increase reliability, boost throughput, and designed to minimize the chance of two senders
allow many unrelated products to share the using the same subchannel simultaneously.
spectrum without explicit cooperation and FHSS techniques allow for a relatively
with minimal interference. simple radio design, but are limited to speeds
The original 802.11 wireless standard of no higher than 2 Mbps. This limitation is
defines data rates of 1 Mbps and 2 Mbps via driven primarily by FCC regulations that
radio waves using frequency hopping spread restrict subchannel bandwidth to 1 MHz.
spectrum (FHSS) or direct sequence spread These regulations force FHSS systems to
spectrum (DSSS). It is important to note that spread their usage across the entire 2.4 GHz
FHSS and DSSS are fundamentally different band, meaning they must hop often, which
signaling mechanisms and will not interoper- leads to a high amount of hopping overhead.
ate with one another. In contrast, the direct sequence signaling
Using the frequency hopping technique, technique divides the 2.4 GHz band into 14
the 2.4 GHz band is divided into 75 1-MHz 22-MHz channels. Adjacent channels overlap
subchannels. The sender and receiver agree on 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
InSdeerpviecned Seentt (BIBaSsiSc) scphraenandeinl gp rtohvei dsiegsn faolr aac rfoosrsm t hoef 2er2r oMr Hchzecking
Figure 3.Ad Hoc Mode and correction; even if part of the signal is
55
damaged, it can still be recovered in many from one another by a receiver even in the
cases, minimizing the need for retransmissions. presence of substantial noise and multipath
interference (e.g., interference caused by
802.11b Enhancements to the PHY Layer receiving multiple radio reflections within a
The key contribution of the 802.11b addition building). The 5.5 Mbps rate uses CCK to
to the wireless LAN standard was to standard- encode 4 bits per carrier, while the 11 Mbps
ize the physical layer support of two new speeds, rate encodes 8 bits per carrier. Both speeds use
5.5 Mbps and 11 Mbps. To accomplish this, QPSK as the modulation technique and signal
DSSS had to be selected as the sole physical at 1.375 MSps. This is how the higher data
layer technique for the standard since, as rates are obtained. Table 1 shows the differences.
noted above, frequency hopping cannot sup- To support very noisy environments as
port the higher speeds without violating cur- well as extended range, 802.11b WLANs use
rent FCC regulations. The implication is that dynamic rate shifting, allowing data rates to be
802.11b systems will interoperate with 1 Mbps automatically adjusted to compensate for the
and 2 Mbps 802.11 DSSS systems, but will changing nature of the radio channel. Ideally,
not work with 1 Mbps and 2 Mbps 802.11 users connect at the full 11 Mbps rate. How-
FHSS systems. ever when devices move beyond the optimal
The original 802.11 DSSS standard range for 11 Mbps operation, or if substantial
specifies an 11-bit chipping—called a Barker interference is present, 802.11b devices will
sequence—to encode all data sent over the air. transmit at lower speeds, falling back to 5.5,
Each 11-chip sequence represents a single data 2, and 1 Mbps. Likewise, if the device moves
bit (1 or 0), and is converted to a waveform, back within the range of a higher-speed trans-
called a symbol, that can be sent over the air. mission, the connection will automatically
These symbols are transmitted at a 1 MSps (1 speed up again. Rate shifting is a physical-
million symbols per second) symbol rateusing layer mechanism transparent to the user and
a technique called Binary Phase Shift Keying the upper layers of the protocol stack.
(BPSK). In the case of 2 Mbps, a more sophis-
ticated implementation called Quadrature The 802.11 Data Link Layer
Phase Shift Keying (QPSK)is used; it doubles The data link layer within 802.11 consists of
the data rate available in BPSK, via improved two sublayers: Logical Link Control (LLC)
efficiency in the use of the radio bandwidth. and Media Access Control (MAC). 802.11
To increase the data rate in the 802.11b uses the same 802.2 LLC and 48-bit address-
standard, advanced coding techniques are ing as other 802 LANs, allowing for very sim-
employed. Rather than the two 11-bit Barker ple bridging from wireless to IEEE wired
sequences, 802.11b specifies Complementary networks, but the MAC is unique to WLANs.
Code Keying (CCK), which consists of a set of The 802.11 MAC is very similar in con-
64 8-bit code words. As a set, these code cept to 802.3, in that it is designed to support
words have unique mathematical properties multiple users on a shared medium by having
that allow them to be correctly distinguished 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 11 (Barker Sequence) BPSK 1 MSps 1
2 Mbps 11 (Barker Sequence) QPSK 1 MSps 2
5.5 Mbps 8 (CCK) QPSK 1.375 MSps 4
11 Mbps 8 (CCK) QPSK 1.375 MSps 8
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it. For 802.3 Ethernet LANs, the Carrier Sense point, but not from each other, usually due to
Multiple Access with Collision Detection distance or an obstruction. To solve this prob-
(CSMA/CD) protocol regulates how Ethernet lem, 802.11 specifies an optional Request to
stations establish access to the wire and how Send/Clear to Send (RTS/CTS) protocol at
they detect and handle collisions that occur the MAC layer. When this feature is in use, a
when two or more devices try to simultane- sending station transmits an RTS and waits
ously communicate over the LAN. In an for the access point to reply with a CTS. Since
802.11 WLAN, collision detection is not pos- all stations in the network can hear the access
sible due to what is known as the “near/far” point, the CTS causes them to delay any
problem: to detect a collision, a station must intended transmissions, allowing the sending
be able to transmit and listen at the same station to transmit and receive a packet
time, but in radio systems the transmission acknowledgment without any chance of colli-
drowns out the ability of the station to “hear” sion. Since RTS/CTS adds additional over-
a collision. head to the network by temporarily reserving
To account for this difference, 802.11 the medium, it is typically used only on the
uses a slightly modified protocol known as largest-sized packets, for which retransmission
Carrier Sense Multiple Access with Collision would be expensive from a bandwidth stand-
Avoidance (CSMA/CA) or the Distributed point.
Coordination Function (DCF). CSMA/CA Finally, the 802.11 MAC layer provides
attempts to avoid collisions by using explicit for two other robustness features: CRC check-
packet acknowledgment (ACK), which means sum and packet fragmentation. Each packet
an ACK packet is sent by the receiving station has a CRC checksum calculated and attached
to confirm that the data packet arrived intact. to ensure that the data was not corrupted in
CSMA/CA works as follows. A station transit. This is different from Ethernet, where
wishing to transmit senses the air, and, if no higher-level protocols such as TCP handle
activity is detected, the station waits an addi- error checking. Packet fragmentation allows
tional, randomly selected period of time and large packets to be broken into smaller units
then transmits if the medium is still free. If when sent over the air, which is useful in very
the packet is received intact, the receiving sta- congested environments or when interference
tion issues an ACK frame that, once success- is a factor, since larger packets have a better
fully received by the sender, completes the chance of being corrupted. This technique
process. If the ACK frame is not detected by reduces the need for retransmission in many
the sending station, either because the original cases and thus improves overall wireless net-
data packet was not received intact or the work performance. The MAC layer is respon-
ACK was not received intact, a collision is sible for reassembling fragments received,
assumed to have occurred and the data packet rendering the process transparent to higher-
is transmitted again after waiting another ran- level protocols.
dom amount of time.
CSMA/CA thus provides a way of sharing Association, Cellular Architectures, and Roaming
access over the air. This explicit ACK mecha- The 802.11 MAC layer is responsible for how
nism also handles interference and other radio- a client associates with an access point. When
related problems very effectively. However, it an 802.11 client enters the range of one or
does add some overhead to 802.11 that 802.3 more APs, it chooses an access point to associ-
does not have, so that an 802.11 LAN will ate with (also called joining a Basic Service
always have slower performance than an Set), based on signal strength and observed
equivalent Ethernet LAN. packet error rates. Once accepted by the access
Another MAC-layer problem specific to point, the client tunes to the radio channel to
wireless is the “hidden node” issue, in which which the access point is set. Periodically it
two stations on opposite sides of an access surveys all 802.11 channels in order to assess
point can both “hear” activity from an access whether a different access point would provide
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Backbone network Acces(As Pp)oint
Intear-ncde lhl aronadmofifng
•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 the total WLAN load most efficiently across
determines that this is the case, it reassociates the available wireless infrastructure.
with the new access point, tuning to the radio This process of dynamically associating
channel to which that access point is set and reassociating with APs allows network
(Figure 4). managers to set up WLANs with very broad
Reassociation usually occurs because the coverage by creating a series of overlapping
wireless station has physically moved away 802.11b cells throughout a building or across
from the original access point, causing the sig- a campus. To be successful, the IT manager
nal to weaken. In other cases, reassociation ideally will employ “channel reuse,” taking
occurs due to a change in radio characteristics care to set up each access point on an 802.11
in the building, or due simply to high network DSSS channel that does not overlap with a
traffic on the original access point. In the latter channel used by a neighboring access point
case this function is known as “load balanc- (Figure 5). As noted above, while there are 14
ing,” since its primary function is to distribute partially overlapping channels specified in
1
1
6
11 11
1
Figure 5.Unlimited Roaming
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802.11 DSSS, there are only three channels Security
that do not overlap at all, and these are the 802.11 provides for both MAC layer (OSI
best to use for multi-cell coverage. If two APs Layer 2) access control and encryption mecha-
are in range of one another and are set to the nisms, which are known as Wired Equivalent
same or partially overlapping channels, they Privacy (WEP), with the objective of provid-
may cause some interference for one another, ing wireless LANs with security equivalent to
thus lowering the total available bandwidth in their wired counterparts. For the access control,
the area of overlap. the ESSID (also known as a WLAN Service
Area ID) is programmed into each access point
Support for Time-Bounded Data and is required knowledge in order for a wire-
Time-bounded data such as voice and video is less client to associate with an access point. In
supported in the 802.11 MAC specification addition, there is provision for a table of MAC
through the Point Coordination Function (PCF). addresses called an Access Control List to be
As opposed to the DCF, where control is dis- included in the access point, restricting access
tributed to all stations, in PCF mode a single to clients whose MAC addresses are on the list.
access point controls access to the media. If a For data encryption, the standard pro-
BSS is set up with PCF enabled, time is vides for optional encryption using a 40-bit
spliced between the system being in PCF shared-key RC4 PRNG algorithm from RSA
mode and in DCF (CSMA/CA) mode. Dur- Data Security. All data sent and received while
ing the periods when the system is in PCF the end station and access point are associated
mode, the access point will poll each station can be encrypted using this key. In addition,
for data, and after a given time move on to the when encryption is in use, the access point
next station. No station is allowed to transmit will issue an encrypted challenge packet to any
unless it is polled, and stations receive data client attempting to associate with it. The
from the access point only when they are client must use its key to encrypt the correct
polled. Since PCF gives every station a turn to response in order to authenticate itself and
transmit in a predetermined fashion, a maxi- gain network access.
mum latency is guaranteed. A downside to Beyond Layer 2, 802.11 HR WLANs
PCF is that it is not particularly scalable, in support the same security standards supported
that a single point needs to have control of by other 802 LANs for access control (such as
media access and must poll all stations, which network operating system logins) and encryp-
can be ineffective in large networks. tion (such as IPSec or application-level
encryption). These higher-layer technologies
Power Management can be used to create end-to-end secure net-
In addition to controlling media access, the works encompassing both wired LAN and
802.11 HR MAC supports power conservation WLAN components, with the wireless piece
to extend the battery life of portable devices. of the network gaining unique additional
The standard supports two power-utilization security from the 802.11 feature set.
modes, called Continuous Aware Mode and
Power Save Polling Mode. In the former, the Considerations for Choosing a Wireless LAN
radio is always on and drawing power, whereas While the bulk of this paper has described
in the latter, the radio is “dozing” with the how 802.11b wireless LANs are alike, there
access point queuing any data for it. The are still many ways for wireless LAN vendors
client radio will wake up periodically in time to differentiate themselves in the marketplace
to receive regular beacon signals from the that will affect a customer’s purchasing deci-
access point. The beacon includes information sion. We cover some of these areas below.
regarding which stations have traffic waiting
for them, and the client can thus awake upon Ease of Setup
beacon notification and receive its data, To install a wireless LAN one must install and
returning to sleep afterward. configure APs and PC Cards. The most
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