Wireless Local Area Networks: Recent Developments

1
Wireless Local Area Networks Recent
Developments
Raj Jain The Ohio State UniversityColumbus, OH
43210 http//www.cse.ohio-state.edu/jain/
2
Overview

• Spread Spectrum
• Wireless local area networks
• Wireless LAN standard IEEE 802.11
• Hiperlan
• Wireless ATM

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Mobile vs Wireless
Mobile
Wireless

• Mobile vs Stationary
• Wireless vs Wired
• Wireless ??media sharing issues
• Mobile ??routing, addressing issues

4
(No Transcript)
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Frequency Hopping Spread Spectrum
Frequency
Time
50 ms

• Pseudo-random frequency hopping
• Spreads the power over a wide spectrum ??Spread
  Spectrum
• Developed initially for military
• Patented by actress Hedy Lamarr (1942)
• Narrowband interference can't jam

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Spectrum
Signal
Noise
Noise
Signal
(a) Normal
(b) Frequency Hopping
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Direct-Sequence Spread Spectrum
0
1
Data
0
1
0
0
1
0
1
1
0
1
1
0
1
1
0
1
0
0
1
0
Frequency
Time
5?s

• Spreading factor Code bits/data bit, 10-100
  commercial (Min 10 by FCC), 10,000 for military
• Signal bandwidth gt10 data bandwidth
• Code sequence synchronization
• Correlation between codes ?Interference??Orthogona
  l

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DS Spectrum
Time Domain
Frequency Domain
Time
Frequency
(a) Data
Frequency
(b) Code
9
Wireless LANs

• IR ??Line of sight, short range, indoors
• RF ??Need license
• Spread-Spectrum Resistance to interference

Visible
?wave
x-rays
Infrared
Ultraviolet
1010
1011
1012
1013
1014
1015
1016
1017
1018
109
10
Wireless LANs
Infrared
Radio
Line of Sight
Diffuse
Spread Spectrum
Narrowband
PhotonicsCollaborative
InfraLAN
Motorola ALTAIR
2.4 GHz
5.7GHz
902 GHz
Proxim RangeLANNCR WaveLANTelesystems ArLAN
DS
DS
FH
FH
Windata Freeport
Proxim RangeLAN2
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Wireless LAN Products

• Alps - Radioport
• ATT - WaveLAN
• A.T. Schindler - FIRLAN
• Carrier Communications - Carriernet
• California Microwave - Radio Link
• Digital (Compaq) - RoamAbout
• IBM - Infrared wireless LAN Adapter
• Digital Ocean - Grouper
• InfraLAN Technologies - InfraLAN
• Motorola - ALTAIR Plus II
• ONeill Communications - LAWN

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WLAN Products (Cont.)

• Photonics - Collaborate Series
• Proxim- RangeLAN2
• Solectek - AirLAN
• Spectrix - SpectrixLite
• TELXON - ARLAN 600
• Travelling Software - Airshare
• WiLAN - 902-20
• Windata - FreePort
• Xerox - PARCTAB
• Xircom - NetWave

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IEEE 802.11 Features

• 1 and 2 Mbps
• Supports both Ad-hoc and base-stations
• Supports multiple priorities
• Supports time-critical and data traffic
• Power management allows a node to doze off
• Spread Spectrum Þ No licensing required.Three
  Phys Direct Sequence, Frequency Hopping,
  915-MHz, 2.4 GHz (Worldwide ISM), 5.2 GHz, and
  Diffused Infrared (850-900 nm) bands.

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Why 2.4 GHz?
IEEE 802.11
5.8 GHz (ISM)
5.2 GHz (Future?)
5.2 GHz (Future?)
2.4 GHz (ISM)
2.4 GHz (ISM)
1.9 GHz WinForum
915 MHz (ISM)
Europe
U.S.
Japan
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Hidden Node Problem
A
B
C

• C cannot hear A. It may start transmitting while
  A is also transmitting ? A and C can't detect
  collision.
• Only the receiver can help avoid collisions

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4-Way Handshake
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IEEE 802.11 MAC

• Carrier Sense Multiple Access with Collision
  Avoidance (CSMA/CA)
• Listen before you talk. If the medium is busy,
  the transmitter backs off for a random period.
• Avoids collision by sending a short message
  Ready to send (RTS)RTS contains dest. address
  and duration of message.Tells everyone to
  backoff for the duration.
• Destination sends Clear to send (CTS)
• Can not detect collision ? Each packet is acked.
• MAC level retransmission if not acked.

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Ad-Hoc vs Infrastructure
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Peer-to-Peer or Base Stations?

• Ad-hoc (Autonomous) Group
• Two stations can communicate
• All stations have the same logic
• No infrastructure, Suitable for small area
• Infrastructure Based Access points (base units)
• Stations can be simpler than bases.
• Base provide connection for off-network traffic
• Base provides location tracking, directory,
  authentication Þ Scalable to large networks
• IEEE 802.11 provides both.

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IEEE 802.11 Architecture
Server
Ad-hocStation
AccessPoint
AccessPoint
Station
Ad-hoc Station
Station
Station
Station
2nd BSS
Basic Service Set
Ad-hocnetwork
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IEEE 802.11 Priorities
DIFS
Contention Window
PIFS
Random Backoff
Busy
SIFS
Frame
Time
Carrier Sensed

• Initial interframe space (IFS)
• Highest priority frames, e.g., Acks, use short
  IFS (SIFS)
• Medium priority time-critical frames use Point
  Coordination Function IFS (PIFS)
• Asynchronous data frames use Distributed
  coordination function IFS (DIFS)

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Time Critical Services
CFP Repetition Interval
Contention-Free Period
Contention Period
DCF Access
PCF Access
Time
Beacon

• Timer critical services use Point Coordination
  Function
• The point coordinator allows only one station to
  access
• Coordinator sends a beacon frame to all
  stations.Then uses a polling frame to allow a
  particular station to have contention-free access
• Contention Free Period (CFP) varies with the load.

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Power Management

• A station can be in one of three states
• Transmitter on
• Receiver only on
• Dozing Both transmitter and receivers off.
• Access point (AP) buffers traffic for dozing
  stations.
• AP announces which stations have frames
  buffered.Traffic indication map included in each
  beacon.All multicasts/broadcasts are buffered.
• Dozing stations wake up to listen to the
  beacon.If there is data waiting for it, the
  station sends a poll frame to get the data.

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IEEE 802.11 Security

• Authentication
• New nodes issue a "request for authentication"
• Network sends a block of random text.
• The node encrypts it with network password and
  returns.
• Currently, one shared secret key (password) per
  net.
• The same encryption algorithm is used for
  privacy.Wired Equivalency Privacy (WEP)
  Algorithm is based on RC4 PRNT algorithm
  developed by RSA Data Security, Inc.

25
Status and Future

• 802.11 including both MAC and PHY approved June
  1997.
• More bandwidth in future by
• 1. Better encoding Multilevel modulation ? 8
  Mbps
• 2. Fewer channels with more bandwidth ? 4 MHz
  channels. Or Entire ISM band for one channel.
• 3. Find another band. May get 150 MHz band in
  5-GHz band. Fifteen 10-MHz channels with 15-20
  Mb/s.

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HIPERLAN

• High Performance Radio LAN
• European Telecom Standards Institute (ETSI)'s
  subtechnical committee RES10.
• 5.12-5.30 GHz and 17.1-17.3 GHz bands
• Phy 23.5 Mbps on 23.5 MHz, non-spread spectrum
  (GMSK)
• MAC CSMA/CA but different from IEEE 802.11
• Peer-to-peer only.
• Power management Nodes announce their wakeup
  cycle. Other nodes send according to the cycle. A
  low-bit rate header allows nodes to keep most
  ckts off.

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Wireless ATM Terminology
Switch
WirelessTerminalAdapter
AccessPoint
Terminal
MobilitySupport

• Wireless Terminal Adapter Allows a terminal
  communicate via wireless
• Wireless Access Point Allows a switch to
  communicate via wireless
• Mobility Support Adapter Allows a switch to
  maintain VCCs with Mobile terminals and switches

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Reference Configurations

• 1. Fixed Wireless Access 2. Mobile End-Users,
• 3. Mobile Networks 4. Ad Hoc Networks
• 5. PCS Access 6. PCS Interworking

Fixed
WiredATMNetwork
Mobile
Fixed
Mobile
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ATM Interfaces
PublicUNI
PrivateUNI
PrivateSwitch
Computer
Carriers
PrivateNNI
PublicNNI
PublicSwitch
PublicSwitch
PrivateUNI
PrivateSwitch
Computer
B-ICI

• UNI User Network Interface
• NNI Network-node interface
• B-ICI Broadband Inter-carrier interface

PublicSwitch
Carrier
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WATM Protocol Stacks

• Wireless Access Layer (WAL) includesPHY, MAC,
  and LLC layers.
• M Mobility enhanced Handoff, Location, QoS
• PNNI, UNI, BICI support transport of mobility
  info

User Plane
Control Planes
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Summary

• Spread spectrum Frequency hopping or direct
  sequence
• Proprietary LANs Photonics, RangeLan, ALTAIR
• LAN Standards IEEE 802.11, Hiperlan
• Wireless ATM work is just beginning

32
Wireless Key References

• For a detailed list of references see
  http//www.cis.ohio-state.edu/jain/refs/wir_refs
  .htm
• E. Prem, Wireless Local Area Networks, Aug 97,
  http//www.cis.ohio-state.edu/jain/cis788-97/wire
  less_lans
• X. Cong, Wireless ATM - An Overview, Aug 97,
  http//www.cis.ohio-state.edu/jain/cis788-97/wire
  less_atm
• Baseline Text for Wireless ATM specifications,
  ATM Forum/btd-watm-01.06.txt, February 1998.

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• I. Brodsky, Wireless Computing, Van Nostrand
  Reinhold, 1997.
• R. A. Dayem, Mobile Data Wireless LAN
  Technologies, Prentice-Hall, 1997
• J. Ahmadi, et al, Design Issues in Wireless
  LANs, J. of High Speed Networks, 1996, pp
  87-104
• R. LaMaire, et al, "Wireless LANs and Mobile
  Networking Standards and Future Directions,"
  IEEE Communications Magazine, August 1996, pp.
  86-94, http//www.comsoc.org/pubs/ci/comsoc/

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Thank You!
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1. Fixed Wireless Access

• No mobility support

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2. Mobile End-Users
ATMSwitch
WirelessATM Terminal
ATMSwitch

• Issues Location, handover, rerouting, ...

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3. Mobile Networks
MobileATMNetwork
ATMSwitch
MobilitySupportingATMNetwork

• Examples Air-planes, Low Earth Orbit Satellites
  with on-board switches

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4. Ad Hoc Networks
ForwardingNode
WirelessATM Terminal

• Forwarding nodes Forward from one radio
  frequency to another

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5. PCS Access
Inter-workingFunction
BaseStation
MobilitySupportingATM Network
PCSTerminal
BaseStation
Inter-workingFunction

• End devices are PCS terminals, e.g., digital
  cellular, digital cordless, etc.

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6. PCS Interworking
PCSNetwork
PCS/WATMTerminal
Mobility SupportingATM Network
ATMSwitch

• End devices are dual mode PCS/WATM

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Status

• Group officially began in August 96
• Settled Reference Configurations
• Current Issues
• Handover Terminal changing access point
• Location Management Using the same address
  regardless of location, keeping track of location
• Future
• Routing, Addressing
• Traffic Management
• Access Point Control Protocol
• Network Management

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Architecture

• Basic Service Area (BSA) Cell
• Each BSA may have several wireless LANs
• Extended Service Area (ESA) Multiple BSAs
  interconnected via Access Points (AP)
• Basic Service Set (BSS) Set of stations
  associated with an AP
• Extended Service Set (ESS) Set of stations in
  an ESA
• Ad-hoc networks coexist and interoperate with
  infrastructure-based networks.

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Frame Format
FrameControl
Duration/ID
Address 1
Address 2
2B
2B
6B
6B
Address 3
SequenceControl
Info
CRC-32
Address 4
2B
6B
0-2034B
4B
6B

• Frame control Protocol version and frame type
  management, data, control
• Duration Network Allocation Vector (NAV)
• Info 0-2304 bytes long

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Frame Control
2b
2b
4b
1b
1b
ProtocolVersion
Type
Sub Type
ToDS
FromDS
1b
1b
1b
1b
1b
MoreFrag
Retry
MoreData
WEP
Rsvd
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802.11 Address Fields

• Address 1 All stations filter on this addr.
• Address 2 Transmitter
• Address 3 Depends upon to/from
• Address 4 Original source

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IEEE 802.11 Phy

• Three Phys specified
• Direct Seq. Spread Spectrum (DSSS)
• Frequency Hopping Spread Spectrum (FHSS)
• Diffused Infrared (DFIR) Wide angle
• DSSS and FHSS operate in 2.4-2.4835 GHz
  Industrial, Scientific, and Medical (ISM) band
  (International)Some early systems use 902-928
  MHz band. Different PHY specifications for
  915-MHz, 2.4-, 5.2 GHz, and Infrared (850-900 nm)
  bands.
• SS at 1 or 2 Mbps. DFIR at 1 Mbps.

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FHSS Phy

• 2.4 GHz ISM Band.
• 1 and 2 Mbps
• Three sets of frequency hopping patterns. Each
  set has 22 hopping sequences (22 Channels).
  Total 66 channels. 12 in Japan.
• Consecutive frequencies in each sequence are at
  least 6 MHz apart to avoid a narrowband
  interferer.
• Adjacent or overlapping cells use different
  patterns.
• Many channels ? FH systems better than DS in
  dense (overlapping cells) environment.

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DSSS Phy

• 2.4 GHz band
• 11 chip spreading factor
• 11 DS center frequencies (11 Channels)
• Only 3 channels without overlap.
• 10 mW to 100 mW transmitted power
• 1 and 2 Mbps
• DBPSK for 1 Mbps. DQPSK for 2 Mbps.

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Infrared Phy

• Baseband transmission
• 850 to 950 nm range of IR
• 1 Mbps or 2 Mbps
• Diffuse IR
• Up to 10 m in typical officesCould be 20 m with
  better receivers.
• For 1 Mbps, 4-bits are mapped to 16 ppm symbol
• For 2 Mbps, 2 bits are mapped to 4 ppm symbol

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HIPERLAN MAC

• CSMA/CA but different from IEEE 802.11
• Immediate transmission if medium idle for more
  than 1700 bit times
• Otherwise three phases Prioritization,
  elimination, yield
• Prioritization Only nodes of highest priority
  contend further.This phase has 1-5 slots of 256
  bits.Node with packet of priority p transmits in
  slot p1 if no higher priority burst. At the end
  of first burst, phase ends.

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• Elimination Nodes of the same priority
  contend.Each node transmits for a geometrically
  distributed number of slots and listens in the
  next slot. Stops if another burst is heard. Node
  with longest burst wins.
• Yield Each surviving node transmits for a
  geometrically distributed number of slots while
  listening. Stop if they hear any transmission.
• After yield phase, the collision probability is
  less than 3.

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• The packet time limit and priority (high or
  normal) determines channel access priority. Five
  channel access priorities.
• Packet life time and residual life time are
  transmitted along with the packet.
• Packets are discarded if their life time expires.

53
HIPERLAN Routing

• Nodes can optionally route packets for other
  nodes.
• Mechanisms for update and maintenance of routing
  information are defined.

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HIPERLAN Power Mgmt

• p-saver A node can announce that it listens
  periodically with a short duty cycle. The node
  can power down most of its circuits at all other
  times. Other nodes (p-supporters) send only when
  p-saver is expected to be up
• Nodes that relay multicasts announce their
  schedule.Nodes wanting to hear the multicast
  wake up accordingly.
• LBR Header Each packet has a short low-bit rate
  (LBR) 1.4706 Mb/s header. Nodes can decide
  whether to listen to the rest of the packet.

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• Even when the nodes are listening, they can keep
  the error correction, equalization, and other
  circuits off most of the time.