Special Topics on Wireless Adhoc Networks

1
Special Topics on Wireless Ad-hoc Networks
Lecture 6 Wireless Local Area Networks (WLANs)

• University of Tehran
• Dept. of EE and Computer Engineering
• By
• Dr. Nasser Yazdani

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Covered topic

• How to build a small wireless local area network?
• Different current wireless technologies
• References
• Chapter 3 of the book
• Design alternative for Wireless local area
  networks,
• Many other sources for 802.11

3
Outlines

• Some basic issues
• Wireless area network standards
• 802.11 standard
• 802.11 management

4
Ideal Wireless Area network?

• Wish List
• High speed (Efficiency)
• Low cost
• No use/minimal use of the mobile equipment
  battery
• Can work in the presence of other WLANs
  (Heterogeneity)
• Easy to install and use
• Etc

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Wireless LAN Design Goals

• Wireless LAN Design Goals
• Portable product Different countries have
  different regulations concerning RF band usage.
• Low power consumption
• License free operation
• Multiple networks should co-exist

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Wireless LAN Design Alternatives

• Design Choices
• Physical Layer diffused Infrared (IR) or Radio
  Frequency (RF)?
• Radio Technology Direct-Sequence or
  Frequency-Hopping?
• Which frequency range to use?
• Which MAC protocol to use.
• Peer-Peer architecture or Base-Station approach?

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Physical Layer Alternatives

• IR
• Simple circuitry, cost-effective, no regulatory
  constraints, no Rayleigh fading (waves are
  small), also nice for micro-cellular networks...
  (multiple cells can exist in a room providing
  more bandwidth)
• RF
• more complicated circuitry, regulatory
  constraints (2.4 GHz Industrial Scientific
  Medical, ISM, bands) in the U.S.

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Physical Layer Alternatives
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Spread spectrum technology

• Problem of radio transmission frequency
  dependent fading can wipe out narrow band signals
  for duration of the interference
• Solution spread the narrow band signal into a
  broad band signal using a special code
• Side effects
• coexistence of several signals without dynamic
  coordination
• tap-proof
• Alternatives Direct Sequence, Frequency Hopping

signal
interference
spread signal
power
power
spread interference
detection at receiver
f
f
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DSSS (Direct Sequence Spread Spectrum)

• XOR of the signal with pseudo-random number
  (chipping sequence)
• generate a signal with a wider range of
  frequency spread spectrum

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FHSS (Frequency Hopping Spread Spectrum)

• Discrete changes of carrier frequency
• sequence of frequency changes determined via
  pseudo random number sequence
• Two versions
• Fast Hopping several frequencies per user bit
• Slow Hopping several user bits per frequency
• Advantages
• frequency selective fading and interference
  limited to short period
• simple implementation
• uses only small portion of spectrum at any time

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FHSS Example
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Comparison between Slow Hopping and Fast Hopping

• Slow hopping
• Pros cheaper
• Cons less immune to narrowband interference
• Fast hopping
• Pros more immune to narrowband interference
• Cons tight synchronization ? increased complexity

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Radio Technology

• Spread Spectrum Technologies
• Frequency Hopping The sender keeps changing the
  carrier wave frequency at which its sending its
  data. Receiver must be in synch with transmitter,
  and know the ordering of frequencies.
• Direct-Sequence The receiver listens to a set of
  frequencies at the same time. The subset of
  frequencies that actually contain data from the
  sender is determined by spreading code, which
  both the sender and receiver must know. This
  subset of frequencies changes during
  transmission.
• Non-Spread Spectrum requires licensing

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Frequency Hopping versus Direct Sequence

• DS advantages
• Lower cost
• FH advantages
• Higher capacity
• Interference avoidance capability If some
  frequency has interference on it, simply don't
  hop there.
• Multiple networks can co-exist Just use a
  different frequency hopping pattern.

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Wireless Standards
17
Distance vs. Data Rate
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Mobility vs. Data Rate
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LAN Industry

• WANs are offered as service
• Cost of infrastructure
• Coverage area
• LANs are sold as end products
• You own, no service charge
• Analogy with PSTN/PBX
• WLAN vs. WAN Cellular Networks
• Data rate (2 Mbps vs. 54 Mbps)
• Frequency band regulation (Licensing)
• Method of data delivery (Service vs. own)

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LAN standard
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Early Experiences

• IBM Switzerland,Late 1970
• Factories and manufacturing floors
• Diffused IR technology
• Could not get 1 Mbps
• HP Labs, Palo Alto, 1980
• 100 Kbps DSSS around 900 Mhz
• CSMA as MAC
• Experimental licensing from FCC
• Frequency administration was problematic, thus
  abandoned
• Motorola, 1985
• 1.73 GHz
• Abandoned after FCC difficulties

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WiFi

• Almost all wireless LANs now are IEEE 802.11
  based
• Competing technologies, e.g., HiperLAN cant
  compete on volume and cost
• 802.11 is also known as WiFi Wireless
  Fidelity
• Fidelity Compatibility between wireless
  equipment from different manufacturers
• WiFi Alliance is a non-profit organization that
  does
• the compatibility testing (WiFi.org)

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Architectures

• Distributed wireless Networks also called Ad-hoc
  networks
• Centralized wireless Networks also called last
  hop networks. They are extension to wired
  networks.

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Centralized Wlan
Ad Hoc
Laptop
Laptop
Server
DS
Pager
Laptop
PDA
Laptop
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Base-Station Approach Advantages over Peer-Peer

• No hidden terminal base station hears all mobile
  terminals, are relays their information to ever
  mobile terminal in cell.
• Higher transmission range
• Easy expansion
• Better approach to security
• Problem?
• Point of failure,
• Feasibility?

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Access Point Functions

• Access point has three components
• Wireless LAN interface to communicate with nodes
  in its service area
• Wireline interface card to connect to the
  backbone network
• MAC layer bridge to filter traffic between
  sub-networks. This function is essential to use
  the radio links efficiently

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Medium Access Control

• Wireless channel is a shared medium
• Need access control mechanism to avoid
  interference
• MAC protocol design has been an active area of
  research for many years. See Survey.

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MAC A Simple Classification
Wireless MAC
Centralized
Distributed
On Demand MACs, Polling
Guaranteed or controlled access
Random access
Our focus
SDMA, FDMA, TDMA, Polling
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Wireless LAN Architecture, Cont
Logical Link Control Layer
MAC Layer Consist of two sub layer, physical
Layer and physical convergence layer

• Physical convergence layer, shields LLC from the
  specifics of the physical medium. Together with
  LLC it constitutes equivalent of Link Layer of OSI

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Power Management

• Battery life of mobile computers/PDAs are very
  short. Need to save
• The additional usage for wireless should be
  minimal
• Wireless stations have three states
• Sleep
• Awake
• Transmit

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Power Management, Cont

• AP knows the power management of each node
• AP buffers packets to the sleeping nodes
• AP send Traffic Delivery Information Message
  (TDIM) that contains the list of nodes that will
  receive data in that frame, how much data and
  when?
• The node is awake only when it is sending data,
  receiving data or listening to TDIM.

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802.11 Features

• Power management NICs to switch to lower-power
  standby modes periodically when not transmitting,
  reducing the drain on the battery. Put to sleep,
  etc.
• Bandwidth To compress data
• Security
• Addressing destination address does not always
  correspond to location.

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IEEE 802.11 Topology

• Independent basic service set (IBSS) networks
  (Ad-hoc)
• Basic service set (BSS), associated node with an
  AP
• Extended service set (ESS) BSS networks
• Distribution system (DS) as an element that
  interconnects BSSs within the ESS via APs.

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Starting an IBSS

• One station is configured to be initiating
  station, and is given a service set ID (SSID)
• Starter sends beacons
• Other stations in the IBSS will search the medium
  for a service set with SSID that matches their
  desired SSID and act on the beacons and obtain
  the information needed to communicate
• There can be more stations configured as
  starter.

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ESS topology

• connectivity between multiple BSSs, They use a
  common DS

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Starting an ESS

• The infrastructure network is identified by its
  extended service set ID (ESSID)
• All APs will have been set according to this
  ESSID
• On power up, stations will issue probe requests
  and will locate the AP that they will associate
  with.

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802.11 Logical Architecture

• PLCP Physical Layer Convergence Procedure
• PMD Physical Medium Dependent
• MAC provides asynchronous, connectionless service
• Single MAC and one of multiple PHYs like DSSS,
  OFDM, IR
• and FHSS.

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802.11 MAC Frame Format
Bytes

342346
32
6
6
2
6
6
4
2
2
6
Bytes
Encrypted to WEP
Bits
2
1
2
4
1
1
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802.11 MAC Frame Format

• Address Fields contains
• Source address
• Destination address
• AP address
• Transmitting station address
• DS Distribution System
• User Data, up to 2304 bytes long

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Special Frames ACK, RTS, CTS
bytes
2
2
6
4
Frame Control
Duration
Receiver Address
CRC

• Acknowledgement
• Request To Send
• Clear To Send

ACK
bytes
2
2
6
6
4
Frame Control
Duration
Receiver Address
Transmitter Address
CRC
RTS
bytes
2
2
6
4
Frame Control
Duration
Receiver Address
CRC
CTS
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IEEE 802.11 LLC Layer

• Provides three type of service for exchanging
  data between (mobile) devices connected to the
  same LAN
• Acknowledged connectionless
• Un-acknowledged connectionless, useful for
  broadcasting or multicasting.
• Connection oriented
• Higher layers expect error free transmission

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IEEE 802.11 LLC Layer, Cont..

• Each SAP (Service Access Point) address is 7
  bits. One bit is added to it to indicate whether
  it is order or response.
• Control has three values
• Information, carry user data
• Supervisory, for error control and flow control
• Unnumbered, other type of control packet

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IEEE 802.11 LLC lt-gt MAC Primitives

• Four types of primitives are exchanged between
  LLC and MAC Layer
• Request order to perform a function
• Confirm response to Request
• Indication inform an event
• Response inform completion of process began by
  Indication

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Reception of packets

• AP Buffer traffic to sleeping nodes
• Sleeping nodes wake up to listen to TIM (Traffic
  Indication Map) in the Beacon
• AP send a DTIM (Delivery TIM) followed by the
  data for that station.
• Beacon contains, time stamp, beacon interval,
  DTIM period, DTIM count, sync info, TIM broadcast
  indicator

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Frame type and subtypes

• Three type of frames
• Management
• Control
• Asynchronous data
• Each type has subtypes
• Control
• RTS
• CTS
• ACK

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Frame type and subtypes, Cont..

• Management
• Association request/ response
• Re-association request/ response transfer from
  AP to another.
• Probe request/ response
• privacy request/ response encrypting content
• Authentication to establish identity
• Beacon (Time stamp, beacon interval, channels
  sync info, etc.)

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Frame type and subtypes, Cont..

• Management
• TIM (Traffic Indication Map) indicates traffic to
  a dozing node
• dissociation

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802.11 Management Operations

• Scanning
• Association/Reassociation
• Time synchronization
• Power management

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Scanning in 802.11

• Goal find networks in the area
• Passive scanning
• Not require transmission
• Move to each channel, and listen for Beacon
  frames
• Active scanning
• Require transmission
• Move to each channel, and send Probe Request
  frames to solicit Probe Responses from a network

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Association in 802.11
1 Association request
2 Association response
AP
3 Data traffic
Client
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Reassociation in 802.11
1 Reassociation request
New AP
3 Reassociation response
5 Send buffered frames
2 verifypreviousassociation
Client
6 Data traffic
Old AP
4 send buffered frames
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Time Synchronization in 802.11

• Timing synchronization function (TSF)
• AP controls timing in infrastructure networks
• All stations maintain a local timer
• TSF keeps timer from all stations in sync
• Periodic Beacons convey timing
• Beacons are sent at well known intervals
• Timestamp from Beacons used to calibrate local
  clocks
• Local TSF timer mitigates loss of Beacons

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Power Management in 802.11

• A station is in one of the three states
• Transmitter on
• Receiver on
• Both transmitter and receiver off (dozing)
• AP buffers packets for dozing stations
• AP announces which stations have frames buffered
  in its Beacon frames
• Dozing stations wake up to listen to the beacons
• If there is data buffered for it, it sends a poll
  frame to get the buffered data

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Authentication

• Three levels of authentication
• Open AP does not challenge the identity of the
  node.
• Password upon association, the AP demands a
  password from the node.
• Public Key Each node has a public key. Upon
  association, the AP sends an encrypted message
  using the nodes public key. The node needs to
  respond correctly using it private key.

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Inter Frame Spacing

• SIFS Short inter frame space dependent on PHY
• PIFS point coordination function (PCF) inter
  frame space SIFS slot time
• DIFS distributed coordination function (DCF)
  inter frame space PIFS slot time
• The back-off timer is expressed in terms of
  number of time slots.

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802.11 Frame Priorities

• Short interframe space (SIFS)
• For highest priority frames (e.g., RTS/CTS, ACK)
• PCF interframe space (PIFS)
• Used by PCF during contention free operation
• DCF interframe space (DIFS)
• Minimum medium idle time for contention-based
  services

DIFS
PIFS
contentwindow
Frame transmission
Busy
SIFS
Time
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SIFS/DIFS

• SIFS makes RTS/CTS/Data/ACK atomic
• Example Slot Time 1, CW 5, DIFS3, PIFS2,
  SIFS1,

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Priorities in 802.11

• CTS and ACK have priority over RTS
• After channel becomes idle
• If a node wants to send CTS/ACK, it transmits
  SIFS duration after channel goes idle
• If a node wants to send RTS, it waits for DIFS gt
  SIFS

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SIFS and DIFS
DATA1
ACK1
backoff
RTS
DIFS
SIFS
SIFS
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Energy Conservation

• Since many mobile hosts are operated by
  batteries, MAC protocols which conserve energy
  are of interest
• Two approaches to reduce energy consumption
• Power save Turn off wireless interface when
  desirable
• Power control Reduce transmit power

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Power Control with 802.11

• Transmit RTS/CTS/DATA/ACK at least power level
  needed to communicate with the receiver
• A/B do not receive RTS/CTS from C/D. Also do not
  sense Ds data transmission
• Bs transmission to A at high power interferes
  with reception of ACK at C

B
C
D
A
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A Plausible Solution

• RTS/CTS at highest power, and DATA/ACK at
  smallest necessary power level
• A cannot sense Cs data transmission, and may
  transmit DATA to some other host
• This DATA will interfere at C
• This situation unlikely if DATA transmitted at
  highest power level
• Interference range sensing range

Data sensed
B
C
D
A
Data
RTS
Ack
Interference range
63

• Transmitting RTS at the highest power level also
  reduces spatial reuse
• Nodes receiving RTS/CTS have to defer
  transmissions

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Bridge Functions

• Speed conversion between different devices,
  results in buffering.
• Frame format adaptation between different
  incompatible LANs
• Adding or deleting fields in the frame to convert
  between different LAN standards

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02.11 Activities IEEE

• 802.11c Bridge Operation (Completed. Added to
  IEEE 802.1D)
• 802.11d Global Harmonization (PHYs for other
  countries. Published as IEEE Std 802.11d-2001)
• 802.11e Quality of Service. IEEE Std
  802.11e-2005
• 802.11f Inter-Access Point Protocol (Published
  as IEEE Std Std 802.11F-2003)
• 802.11h Dynamic Frequency Selection and transmit
  power control to satisfy 5GHz band operation in
  Europe. Published as IEEE Std 802.11h-2003
• 802.11i MAC Enhancements for Enhanced Security.
  Published as IEEE Std 802.11i-2004
• 802.11j 4.9-5 GHz operation in Japan. IEEE Std
  802.11j-2004
• 802.11k Radio Resource Measurement interface to
  higher layers. Active.

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02.11 Activities IEEE

• 802.11m Maintenance. Correct editorial and
  technical issues in 802.11a/b/d/g/h. Active.
• 802.11n Enhancements for higher throughput (100
  Mbps). Active.
• 802.11p Inter-vehicle and vehicle-road side
  communication at 5.8GHz. Active.
• 802.11r Fast Roaming. Started July 2003.
  Active.
• 802.11s ESS Mesh Networks. Active.
• 802.11T Wireless Performance Metrics. Active.
• 802.11u Inter-working with External Networks.
  Active.
• 802.11v Wireless Network Management enhancements
  for interface to upper layers. Extension to
  80211.k. Active.
• Study Group ADS Management frame security.
  Active
• Standing Committee Wireless Next Generation WNG
  Globalization jointly w ETSI-BRAN and MMAC.
  Active.

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802.11n

• Trend HDTV and flat screens are taking off Media
  Center Extenders from Linksys and other vendors
• Application HDTV and streaming video (over
  longer distances than permitted by 802.15.3
  WPANs)
• 11n Next Generation of 802.11
• At least 100 Mbps at MAC user layer ? 200 Mbps
  at PHY ? 4x to 5x faster than 11a/g
• (802.11a/g have 54 Mbps over the air and 25 Mbps
  to user)
• Pre-11n products already available
• Task Group n (TGn) setup Sept 2003
• Expected Completion March 2007

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802.11n

• Uses multiple input multiple output antenna
  (MIMO)
• Data rate and range are enhanced by using spatial
• multiplexing (N antenna pairs) plus antenna
  diversity occupies one WLAN channel, and in
  compliance with 802.11
• Backwards compatible with 802.11 a,b,g
• One access point supports both standard WLAN and
  MIMO devices

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HIPERLAN

• 1995 ETSI technical group RES 10 (Radio Equipment
  and Systems) developed HIPERLAN/1 wireless LAN
  standards using 5 channels in 5.15-5.3 GHz
  frequency range
• Technical group BRAN (Broadband Radio Access
  Network) is standardizing HIPERLAN/2 for wireless
  ATM
• ETSI URL for Hiperlan information
  http//www.etsi.org/frameset/home.htm?
  /technicalactiv/Hiperlan/hiperlan2.htm

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HIPERLAN Characteristics

• HIPERLANs with same radio frequencies might
  overlap
• Stations have unique node identifiers (NID)
• Stations belonging to same HIPERLAN share a
  common HIPERLAN identifier (HID)
• Stations of different HIPERLANs using same
  frequencies cause interference and reduce data
  transmission capacity of each HIPERLAN
• Packets with different HIDs are rejected to avoid
  confusion of data

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HIPERLAN Protocol Layers

• Data link layer logical link control (LLC) sub
  layer MAC sub layer channel access control
  (CAC) sub layer

network
LLC
data link
MAC
physical
CAC
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HIPERLAN Protocol Layers, Cont..

• MAC sub layer
• Keeps track of HIPERLAN addresses (HID NID) in
  overlapping HIPERLANs
• Provides lookup service between network names and
  HIDs
• Converts IEEE-style MAC addresses to HIPERLAN
  addresses
• Provides encryption of data for security

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HIPERLAN Protocol Layers, Cont..

• MAC sub layer
• Provides multi hop routing certain stations
  can perform store-and-forwarding of frames
• Recognizes user priority indication (for
  time-sensitive frames)

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HIPERLAN Protocol Layers, Cont..

• CAC sub layer
• Non-preemptive priority multiple access (NPMA)
  gives high priority traffic preference over low
  priority
• Stations gain access to channel through channel
  access cycles consisting of 3 phases

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HIPERLAN CAC Protocol

• CAC sub layer

Cycle
Prioritization Phase
Transmission Phase
Contention Phase
1
2
3
4
Data
ACK
AP
1
2
3
4
5
Time
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HIPERLAN Protocol Layers, Cont

• CAC is designed to give each station (of same
  priority) equal chance to access the channel
• First stations with highest priority data are
  chosen. The rest will back off until all higher
  priority data is transmitted.
• Stations with the same priority level data,
  compete according to a given rule to choose
  survivors
• Survivors wait a random number of time slots and
  then listen to see if the channel is idle

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HIPERLAN Protocol Layers, Cont

• If the channel is idle then it starts
  transmitting.
• Those who could not transmit wait until next
  period

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HIPERLAN/2

• To support QoS, Handoff and integrate WLAN with
  next generation Cellular sys.
• Supporting IP ATM at 54Mbps
• Use TDMA as MAC
• DLC (Data Link Control) layer constitutes a
  logical link Between AP and MT to ensure a
  connection oriented Communication.

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Related Standards Activities

• IEEE 802.11
• http//grouper.ieee.org/groups/802/11/
• Hiperlan/2
• http//www.etsi.org/technicalactiv/hiperlan2.htm
• IETF manet (Mobile Ad-hoc Networks) working group
• http//www.ietf.org/html.charters/manet-charter.ht
  ml