Transcript PPT

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
Univ. of Tehran
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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
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Outlines




Some basic issues
Wireless area network standards
802.11 standard
802.11 management
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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 FrequencyHopping?
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
Cost
IR
<$10
Regulation
None
RF
<$20
Interference
No license on
ISM bands
Ambient Light Radiators
coverage
Performance
Spot
Moderate
Multiple
networks
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Wide Area
Depends on
Bandwidth
Possible
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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
power
interference
spread
signal
power
detection at
receiver
f

spread
interference
f
Side effects:



signal
coexistence of several signals without dynamic coordination
tap-proof
Alternatives: Direct Sequence, Frequency Hopping
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
tb
user data
0
1
XOR
tc
chipping
sequence
01101010110101
=
resulting
signal
01101011001010
tb: bit period
tc: chip period
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
FHSS: Example
tb
user data
0
1
f
0
1
1
t
td
f3
slow
hopping
(3 bits/hop)
f2
f1
f
t
td
f3
fast
hopping
(3 hops/bit)
f2
f1
t
tb: bit period
td: dwell time
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
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
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Distance vs. Data Rate
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Mobility vs. Data Rate
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LAN Industry

WANs are offered as service



LANs are sold as “end products”



Cost of infrastructure
Coverage area
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

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HP Labs, Palo Alto, 1980

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Factories and manufacturing floors
Diffused IR technology
Could not get 1 Mbps
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 can’t
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
Server
Laptop
DS
Access Point
Access Point
Pager
PDA
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Base-Station Approach
Advantages over Peer-Peer


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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
32
Preamble
34~2346
6
MPDU
PLCP
header
MAC Header
Frame Duration Addr 1 Addr 2 Addr 3 Sequence Address 4 User
Control
Control
Data
Bytes 2
2
6
6
2
6
6
CRC
4
Encrypted to WEP
Bits 2
2
Protocol
Version
4
1
1
1
Type Sub type To From
DS DS
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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

Acknowledgement
2
2
6
Frame
Receiver
Duration
Control
Address
ACK
4
CRC
bytes

Request To SendRTS
2
2
6
6
Frame
Receiver Transmitter
Duration
Control
Address Address
bytes

Clear To Send
CTS
2
2
6
Frame
Receiver
Duration
Control
Address
4
CRC
4
CRC
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..
Destination Source
SAP
SAP


Control Data
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 <-> 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
3: Data traffic
AP
Client
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Reassociation in 802.11
1: Reassociation request
3: Reassociation response
5: Send buffered frames
Client
6: Data traffic
New AP
2: verify
previous
association
Old AP
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frames
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
Busy
DIFS
PIFS
SIFS
content
window
Frame transmission
Time

Short interframe space (SIFS)


PCF interframe space (PIFS)


For highest priority frames (e.g., RTS/CTS, ACK)
Used by PCF during contention free operation
DCF interframe space (DIFS)

Minimum medium idle time for contention-based
services
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SIFS/DIFS
SIFS makes RTS/CTS/Data/ACK atomic
Example: Slot Time = 1, CW = 5, DIFS=3, PIFS=2,
SIFS=1,
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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 > SIFS

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SIFS and DIFS
DATA1
ACK1
SIFS DIFS
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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
C
D
A/B do not receive RTS/CTS from C/D.
Also do not sense D’s data transmission
B’s transmission to A at high power
interferes with reception of ACK at C
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A Plausible Solution

RTS/CTS at highest power, and DATA/ACK at smallest
necessary power level
Data sensed
A
B
C
D
Data
Interference range



RTS
Ack
A cannot sense C’s 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 Network
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

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
data link
physical
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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-andforwarding 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:
Prioritization
Contention
Phase
Phase
Cycle
Transmission Phase
1
2
3
4
Data
ACK
AP 1 2 3 4 5
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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


Hiperlan/2


http://grouper.ieee.org/groups/802/11/
http://www.etsi.org/technicalactiv/hiperlan2.htm
IETF manet (Mobile Ad-hoc Networks) working
group

http://www.ietf.org/html.charters/manet-charter.html
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