Transcript wlan - Computer Science, Columbia University

802.11 – Introduction, Updates and
Business cases/applications
Presented by Ashutosh Dutta
Contents
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Introduction to 802.11
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Spectrum View with respect to Unlicensed Band
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Network Topology of 802.11
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802.11 Layers Description (PHY and MAC Layers)
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Features of the standard
Update
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Standard Bodies/Forums/Alliance (3GPP, MWIF, WECA)
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Industry
Business cases and Applications
INTRODUCTION to 802.11
Spectrum View (Unlicensed Band)
Spectrum View
Unlicensed Bands
Spectrum
Typical Applications
ISM: Industry Science and Medicine
902 - 928 MHz,
234.5 MHz
Cordless Phones,
2.4 - 2.4835 GHz
83.5 MHz
Wireless LANs (WLAN)
5.725 - 5.85 GHz
125 MHz
and Wireless PBXs (WPBX)
Asynchronous:1910-1920, 2390-2400 MHz
20 MHz
WLAN
Isochronous: 1920-1930 MHz
10 MHz
WPBX
UNII (5.15-5.25 GHz)
100 MHz
Indoor applications WLAN,WPBX
UNII (5.25-5.35 GHz)
100 MHz
Short outdoor links, campus applications
UNII (5.725-5.825 GHz)
100 MHz
Long outdoor links, Point-To-Point links
Millimeter Wave (59-64 GHz)
5 GHz
Home networking applications
UPCS: Unlicensed PCS
UNII: Unlicensed National Information
Infrastructure
Unlicensed Band and Types of 802.11 Stds
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Unlicensed Band
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It is free, Huge and Nationwide Spectrum without License from FCC
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Any device can transmit in this band, with some rules like power limits
It can be used for:
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Fixed and mobile services, WLANs, Wireless Private Branch Exchange,
Spectrum Sharing, Experimentation and innovation
Types of IEEE 802.11 standard
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IEEE 802.11a:-Operate in unlicensed 5GHz band, supports 6-54 Mbps speed
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IEEE 802.11b (also called WiFi):- Operates in unlicensed 2.4GHz band,
supports up to 11Mbps (Oddly enough, the latter came before the former)
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IEEE 802.11d - LAN/MAN standard
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IEEE 802.11e - Working on QoS (quality of service) issue in LANs
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IEEE 802.11g - Objective is to double the speed of 802.11b (22 Mbps in 2.4
GHz band. Still not approved as of mid 2001.
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IEEE 802.16 - A draft Wireless LAN standard for Metro Area Networks based
on OFDM and using IEEE 802.11a as a foundation
Network Topology of 802.11 (1/2)
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Standard defines two Configurations
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Cellular and Ad-hoc
Cellular
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System is sub-divided into cells (A cell called Basic Service Set BSS)
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Each cell is controlled by a base station (called Access Point or AP)
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APs are connected through some kind of backbone called distributed
System (DS), typically Ethernet and in some cases wireless itself
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The whole interconnected wireless LAN including different cells, their
respective access points and the distribution system is called ESS.
Distribution System
ESS is seen to the upper layer of
AP
AP
BSS
BSS
OSI model, as a single NW
ESS = Extended Service Set
Network Topology of 802.11 (2/2)
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Generally
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WLAN may consist of single or collection of cells
10-30 stations per AP can be supported (depends on NW traffic)
Typical coverage is 200-1000 ft (Depends on type of walls, height of
ceiling, physical obstruction
Distribution System
AP
AP
BSS
BSS
ESS = Extended Service Set
AP
BSS
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AP
BSS
Standard also defines the Concept of Portal
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It is a device that interconnects two different 802.11 LANS
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Even though standard does not requests so, typical installation will have
AP and the portal on a single physical entity
Physical Layer (1/3)
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Multiple Physical Layers supported by MAC layer
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Direct Sequence Spread Spectrum in 2.4 GHz Band
Frequency Hopping (FH) Spread Spectrum in 2.4 GHz Band
Infrared in 850-to-950nM
PHYSICAL LAYER Convergence Protocol (PLCP)
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Supports common PHY SAP
Provides Clear Channel Assessment signal (carrier sense)
802.2
Data Link
Layer
802.11 MAC
FH
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DS
Phy layer
Modulation:
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IR
DPSK Differential Phase Shift Keying
Channel Bandwidth
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Channel bandwidth is about 20 MHz for DSSS systems
Thus ISM band accommodates up to three non-overlapping channels
It is Regardless of data rate (1, 2, 5.5, or 11 Mbps)
Physical Layer (2/3)
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Principle of DSSS
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DSSS receivers employ different PN codes
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PN sequence spreads transmitted bandwidth & reduces peak power
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Upon reception, the signal is correlated with the same PN sequence to
recover the original binary data
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This technique reduces the effect of narrowband interference
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Spreading architecture of 802.11 is slightly different from cellular CDMA
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DSSS Mode Supports three data rates 1, 2 & 11 Mbps
Physical Layer (3/3)
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Principle of FHSS
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Receivers employ different FH patterns, governed by certain codes
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The frequency hops across 2.4GHz band covering 79 channels
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Each channel occupies 1Mhz of bandwidth
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Minimum hop rate for a channel is specified by regulatory bodies
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There are 22 hop patterns to choose from
For USA it is 2.5 hops per second
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Transmitter applies the same code to retrieve the signal
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FHSS supports two data rates 1 & 2 Mbps
Infrared
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Operates in the 850-to-950nM band with peak power of 2 W.
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Modulation is either 4 or 16-level pulse-positioning modulation.
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Supports two data rates; 1 and 2Mbps.
MAC Layer (Functions)
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Beyond standard functions usually performed by MAC layers, 802.11
MAC performs some typically upper layer functions I.e.
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Fragmentation
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Packet Retransmission
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Acknowledgments
MAC LAYER - Regular Functions
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Provides Basic access mechanism
Performs Encryption
MAC LAYER - Management Functions
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Performs Synchronization
Performs Power management
Supports Roaming
802.2
802.11 MAC
FH
DS
IR
Data Link
Layer
Phy layer
Each BSS has a unique 48 bit address
Each ESS has a variable length address
MAC Layer (Access Methods)
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MAC Layer defines two different access methods
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Distribution Coordination Function
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Point Coordination function
Distribution Coordination Function (DCF)
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It is A basic Access Method
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It is based on CSMA/CA (Carrier Sensed Multiple Access/Collision Avoidance)
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Note: Ethernet is based on CSMA/CD (Collision Detection)
Point Coordination function (PCF)
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It is an optional function
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It may be used to implement time bounded services like voice or video
Distinguishing Feature of PCF over DCF
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This function gives AP higher priority
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AP gains high priority because it uses Smaller IFS (inter frame space)
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By using this function, AP issues polling requests to the stations for
data transmission. Thus it controls the medium access
CSMA
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Principle of CSMA Protocol
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A station desiring to transmit senses the medium,
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Efficiency of CSMA
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It is very efficient when the medium is not heavily loaded as it allows
stations to transmit with a minimum delay
But there is always a chance of COLLISION, if two station happen to
sense simultaneously, find the medium free and transmit together
How CSMA efficiency can be improved?
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If the medium busy it defers its transmission to later time
If the medium free then it transmits
Through Collision Detection (CSMA-CD) mechanism
If collision is detected and MAC retransmits the packet by itself
(without involving upper layers) that would reduce significant delay
In Wired LAN collision can be detected by a transmitting station, using
some algorithm (Exponential Random Backoff to resolve contention)
What about CSMA-CD in WIRELESS?
Would it work?
CSMA CD and Wireless LAN
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In Wireless LAN Collision Detection can not be used because:
CD scheme is based on the assumption that::
“All the Stations Can Hear Each Other”
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but in Wireless it can not be assumed because:
If the area around the transmitting station is free, it doesn't
necessarily mean that the area around the receiver is free as well.
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One way to overcome the problem could be:
To use full duplex radio capable of transmitting & receiving at once.
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But it would increase the price significantly
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How to overcome these problem?
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802.11 overcomes these problems by a mechanism called:
COLLISION AVOIDANCE (CA) with POSITIVE ACKNOWLEDGMENT
CSMA-CA has two steps
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Step-1:-Physical Carrier Sensing (PCS)
Step-2:- Virtual Carrier Sensing (VCS)
CSMA/CA Mechanism With PCS
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Transmitting Station Senses the Medium.
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If the medium busy, it defers
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If the medium free, for a Specified Time, it transmits.
(Specified time is called DIFS, Distributed Inter Frame Space, in the standard),
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Receiving Station Receives the packet,
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Checks its CRC, &
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Sends an Acknowledgment to the Transmitting Station
Transmitting Station waits for the Acknowledgement:
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If it is receives ACK, it means no collision occurred,
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If Not, it keeps on retransmitting until
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Either it gets the ACK
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Or thrown away after a given number of trials
But in this process, how the probability of collisions is avoided?
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Lets look at Step-2: Virtual Carrier Sensing (VCS) mechanism
CSMA/CA Mechanism With VCS
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VCS is based on the fact that wireless stations can’t hear each other,
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Transmitting/Source Station
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First transmits a short control packet RTS Request To Send
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RTS includes source, destination & duration of the following transaction
Receiving/Destination station
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If medium free, responds with response control packet CTS Clear To Send
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CTS includes the same Duration information
All other Stations receiving either RTS or CTS will
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Set their VCS indicator “NAV”, Network Allocation Vector, for a given
duration & Use this information (NAV) together with Physical carrier sense
when sensing medium
SIFS:- Short Inter frame Space
Source
Dest
others
G3 RTS
G1
G1
G1
G3
Data
CTS
Defer Access
PIFS:- Point coordinated IFS
DIFS:- Distributed IFS
EIFS:- Extended IFS
Ack
NAV (RTS)
NAV (CTS)
G1= SIFS
G3= DIFS
Next MPDU
Backoff after Defer
CSMA/CA Mechanism
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This mechanism reduces probability of collision
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On Receiver Area
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On Transmitter Area
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because it will hear CTS & reserve the medium as busy until the end
of the transaction
because of the the contained duration information
One more advantage
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RTS/CTS being short frames also reduce the overhead of collisions
since these are recognized faster (compared to big data packet)
NOTE:Standard allows transmission of packets smaller than RTS, without RTS/CTS
transaction. It is controlled per station by a parameter RTS Threshold.
Fragmentation and Reassembly
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Wired LANS handle packets of several hundreds of bytes
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WLANs do not prefer to handle long packets because of:
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(e.g. Ethernet longest packet could be 1518 bytes long)
Higher Bit Error Rate of radio link is responsible for packet corruption
Packet corruption probability increases with packet size
In case of Packet corruption (either because of noise or collision), the
smallest the packet, the less overhead it causes to retransmit it.
Thus fragmentation and reassembly is performed
This is done at MAC layer
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It is considered as a better solution to deal big packets
MSDU
MAC
HDR Frame body CRC
MAC
HDR Frame body CRC
MAC
HDR Frame body CRC
Steps to Access the AP
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Access: Station Can Access BSS
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When it Powers Up
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When it awakes from the Sleep Mode
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When it Just enters the BSS area
Synchronization: Station can synchronize to AP either.
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By Passive Scanning
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OR By Active Scanning
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Station Sends Probe Request Frames and waits for Probe Response
Either can be chosen according to Performance & Power Consumption tradeoff
Beacon Frames
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Station waits to receive a Beacon Frame from AP. It is a periodic frame sent by
the AP with synchronization information
These frames contain value of AP’s clock on the moment of transmission. The
Receiving stations check the value of their clock and correct it accordingly
Stations need to keep Synchronization
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To keep hopping synchronized
and to
Save power during sleep mode
Security
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Security Holds paramount importance to prevent intruders from
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Accessing the network resources (Authentication)
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Hijack the WLAN traffic (Eavesdropping)
Authentication Process (performed after synchronization)
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Each (TX & RX) Proves the knowledge of a given password
WEP (Wired Equivalent Privacy) Security
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Data security is accomplished by a complex encryption technique called WEP
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WEP shared key is based on RC4 algorithm
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It protects transmitted data over the air using 64-bit seed key & RC4 algorithm.
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WEP only protects the data packet information and not the physical layer
header so that other stations on the network can listen to the control data
needed to manage the network. However, the other stations cannot decrypt the
data portions of the packet
Association Process (performed after authentication)
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Each communicates their capabilities
Power Saving – Sleep Mode
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Stations
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In this mode a station sleeps to save its power
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Stations inform AP before entering sleep mode
Access Point
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Maintains the updated record of the sleeping terminals
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Buffers the packet addressed to them
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Sends periodically (as apart of its Beacon Frames) indication about
waiting messages
Stations
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Also Wake up periodically to check these Frames
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If they find any indication of waiting /buffered frame, then they stay
awake and send a pole message to AP
Multicast and broadcasts are stored by the AP and transmitted at pre
known time (DTIM), where all sleeping station (who wish to receive
this kind of information) should be awake.
Roaming
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Moving from one cell to another without loosing the connection
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WLAN roaming is easier than Cellular roaming because:
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WLAN Roaming is more difficult than Cellular roaming because:
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On voice a temporary disconnection is not noticeable, but in WLAN
Performance is effected as upper layer protocols initiate retransmission
802.11doesn’t define roaming but basic tools/message formats only I.e.
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Transition from cell to cell may be performed during packet
transmission (In cellular it is during the conversation)
1. Active Scanning, 2. Passive Scanning and 3. Re-association Process
Everything else is left up to network vendors
Inter-Access Point Protocol (IAPP)
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IAPP was jointly developed when everything else was left to vendors.
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IAPP was developed by Aironet, Lucent & Digital Ocean.
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IAPP extends multi-vendor interoperability to the roaming function.
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It addresses roaming within a single ESS, between 2/or more ESSs
Ad-Hoc Networking
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What is Ad-hoc Network?
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How does it work?
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It is a LAN without an Access Point
Part of AP’s functionality is performed by the end-user stations
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Like Beacon Generation
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Synchronization
Functions supported by 802.11 ad-hoc Mode of operation?
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File transfer between two notebooks users
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Coworkers meeting outside the office
Functions not supported by 802.11 ad-hoc Mode of operation?
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Frame-relaying between two stations not in range
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Power Saving features etc
UPDATES
WECA (Wireless Ethernet Compatibility Alliance)
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Mission
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To provide certification of compliance with the IEEE 802.11 Standard
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To ensure cross vendor interoperability
WECA & Security
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The most recent concern about WLAN are related to its SECURITY
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Several security solutions including 802.1x, VPNs and RADIUS have
been examined. (Report commissioned by IBM 19th October ‘01)
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NextComm also announced key hopping technique using MD-5
algorithm (Sept ’01)
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Companies like ORiNOCO and Agere have deployed some new
security techniques using DIAMETER, VPN and RADIUS
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A latest survey about 802.11 adoption in corporations
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40% have already implemented & 30% will do so within 18 months
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BT usage is negligible today, expected to raise to 17 % in 18 months
3GPP
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Evolution Workshop (17-18 October 2001)
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3GPP Plenary Meeting (September 2001)
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Telia, Nortel, DoCoMo, AT&T Wireless, and Nokia, presented different
proposals
Nortel Presented the Stand alone data cell concept based on 802.11
NOKIA said that make AN fully independent to make complementary
access systems (802.11, HiperLAN 2, BT, ADSL) as a part of 3GPP
Other companies stressed on the interworking of UMTS and WLANS
Decision was that 3GPP will focus on completion of existing releases.
Approved a work item on interworking of UMTS and Hiperlan, and
agreed to send a liaison to IEEE 802.11 and Home link for the same
HiperLAN (European version of 802.11) developed by ETSI
The general features
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High-speed transmission (54 Mbit/s) through OFDM
Automatic frequency allocation
Quality-of-Service (QoS) support, Security support, Mobility support
Network & application independence
Power saving features
3GPP2
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3GPP2
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It seems that no activity is going-on on this issue.
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However 3GPP2 All IP requirement document includes support for
inter-technology hand-off (3G-WLAN)
MWIF
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WG-4 will develop architectural requirements to support 802.11,
cdma2000 and WCDMA interoperability
Industry
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Nokia achieved WLAN roaming for Sonera (Finnish GSM operator)
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No GSM infrastructure is used as a part of the wireless access
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Carrier will setup hotspot in a public areas with std WLAN equipment
802.11b APs & gateways
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Users would use their SIM card in WLAN card to access hotspot
Lucent has also announced the solution for 802.11 interworking with
cellular systems cdma2000, WCDMA, GPRS
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The architecture introduces a protocol gateway
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It will translate various protocols into a single common protocol
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Thus various networks will maintain/use single subscriber profile
Wayport successfully tested Windows XP in conjunction with 802.11
BUSINESS CASES & APPLICATIONS
WLAN Applications/Business Cases
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802.11 will be the 1st generation of standard for WLANs
It will set the pace for the next generation standard, addressing
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Interoperability between products is another important factor
These products will be implemented on ISA, or PCMCIA cards for use
in handheld PCs, PDAs, laptops or desktop applications.
WLAN applications
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The demands for higher performance
The demand for higher data rates
The demand for higher frequency bands
Are currently mostly in vertical markets
Many horizontal applications will follow as 802.11 infrastructure is
installed
It will become more competitive & economical for virtually all
applications requiring wireless connectivity
On the horizon, is a need for wireless connectivity at 10Mbps & higher
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High speed & interoperability will give birth to numerous applications
WLAN Applications/Business Cases
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General Applications
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Cost Saving
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Can be provided in Hospitals, Factories, Restaurants, Goods
Distribution in warehouses, Stock Exchanges, Retail Business
Tracking
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Cost saving in wiring offices, factories, educational campuses
Mobile computing solutions
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Streaming video, rapid images transfer, Internet/intranet terminals,
customer self-scanning services, presentations, high-bandwidth,
immediate communications for data-intensive applications, and
Various applications providing Mobility to the users.
Monitoring Cards carried by students and Scanner at the bus door can
allow parent to track school buses on Web site to see if their child
headed home from school. Student identification number, scanned
could be transmitted to some cite via WLAN
Fleet Management
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If used for Public Buses, passengers can find the location/waiting time
of the bus on their handheld
WLAN Applications/Business Cases
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Cost Saving in T1-Connection
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WLAN bridging to connect a school, a medical building and a
distributor to an ISP, bypassing T-1 connection
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In the same way Networking Schools for Distance Learning
Graphic from Aironet site
Can Bluetooth compete with WLAN
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BT is a Cable replacement technology not network technology.
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It Holds limited ability to do handoffs.
(Some vendors are trying to achieve this ability)
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It faces interoperability issues on PHY and application layer.
(only few profiles agreed upon between some vendors)
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Communication flow could potentially go directly through the laptop without
going through AP & corporate firewall, necessitating an increased need for
"personal firewalls" residing on Laptop.
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Its Security protocols only authenticate the device, not the user. Thus
additional application level security is needed
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It gives a max speed of 721kbps for data, compared to 11Mbps for 802.11b
predict a move to 5GHz by 2004 and further higher. Thus not desirable for
streaming video, or downloading big files from the Internet to a laptop or PDA
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From these arguments it can be inferred that it can not compete with WLAN