Wireless Networks and Standards Guide

IT2402 MOBILE COMMUNICATION
UNIT – II
Dr.A.Kathirvel, Professor and Head, Dept of IT
Anand Institute of Higher Technology, Chennai

Unit - II
WIRELESS NETWORKS
Wireless LAN – IEEE 802.11 Standards – Architecture
– Services – Mobile Ad hoc Networks- WiFi and
WiMAX - Wireless Local Loop

LAN/WLAN World
 LANs provide connectivity for interconnecting
computing resources at the local levels of an
organization
 Wired LANs
 Limitations because of physical, hard-wired
infrastructure
 Wireless LANs provide
 Flexibility
 Portability
 Mobility
 Ease of Installation

Repeater
A repeater receives a signal, regenerates it, and passes it on.
It can regenerate and retime network signals at the bit level
to allow them to travel a longer distance on the media.
It operates at Physical Layer of OSI
The Four Repeater Rule for 10-Mbps Ethernet should be used
as a standard when extending LAN segments.
This rule states that no more than four repeaters can be used
between hosts on a LAN.
This rule is used to limit latency added to frame travel by each
repeater.

Hub
 Hubs are used to connect multiple
nodes to a single physical device,
which connects to the network.
 Hubs are actually multiport
repeaters.
 Using a hub changes the network
topology from a linear bus, to a
star.
 With hubs, data arriving over the
cables to a hub port is electrically
repeated on all the other ports
connected to the same network
segment, except for the port on
which the data was sent.

Bridge
 Bridges are used to logically separate network
segments within the same network.
 They operate at the OSI data link layer (Layer 2)
and are independent of higher-layer protocols.
 The function of the bridge is to make intelligent
decisions about whether or not to pass signals
on to the next segment of a network.
 When a bridge receives a frame on the
network, the destination MAC address is
looked up in the bridge table to determine
whether to filter, flood, or copy the frame onto
another segment
 Broadcast Packets are forwarded

Switch
Switches are Multiport Bridges.
Switches provide a unique network segment on each port,
thereby separating collision domains.
Today, network designers are replacing hubs in their wiring
closets with switches to increase their network performance and
bandwidth while protecting their existing wiring investments.
Like bridges, switches learn certain information about the data
packets that are received from various computers on the
network.
Switches use this information to build forwarding tables to
determine the destination of data being sent by one computer to
another computer on the network.

Switches: Dedicated Access
Hosts have direct connection to
switch
Full Duplex: No collisions
Switching: A-to-A’ and B-to-B’
simultaneously, no collisions
Switches can be cascaded to
expand the network
switch
A
A’
B
B’
C
C’

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Wireless networks
 Wireless PANs (Bluetooth – IEEE
802.15)
 very low range
 wireless connection to printers etc
 Wireless LANs (WiFi – IEEE
802.11)
 infrastructure as well as ad-hoc
networks possible
 home/office networking
 Multihop Ad hoc Networks
 useful when infrastructure not
available, impractical, or
expensive
 military applications, emergencies
 Wireless MANs (WiMAX-802.16)
 Similar to cellular networks
 traditional base station
infrastructure systems

802.11 Wireless LAN
Provides network connectivity over wireless media
An Access Point (AP) is installed to act as Bridge between
Wireless and Wired Network
The AP is connected to wired network and is equipped with
antennae to provide wireless connectivity
Network
connectivity
to the
legacy
wired LAN
Desktop
with PCI 802.11 LAN card
Laptop
with PCMCIA 802.11 LAN card
Access Point

802.11 Wireless LAN
Range ( Distance between Access Point and WLAN client)
depends on structural hindrances and RF gain of the antenna
at the Access Point
To service larger areas, multiple APs may be installed with a
20-30% overlap
A client is always associated with one AP and when the client
moves closer to another AP, it associates with the new AP
(Hand-Off)
Three flavors:
802.11b
802.11a
802.11g
LAN Technologies

Multiple Access with Collision Avoidance
(MACA)
Before every data transmission
Sender sends a Request to Send (RTS) frame containing
the length of the transmission
Receiver respond with a Clear to Send (CTS) frame
Sender sends data
Receiver sends an ACK; now another sender can send data
When sender doesn’t get a CTS back, it assumes collision
sender receiver
other node in
sender’s range
RTS
CTS
ACK
data
other node in
receiver’s range

WLAN : 802.11b
The most popular 802.11 standard currently in deployment.
Supports 1, 2, 5.5 and 11 Mbps data rates in the 2.4 GHz ISM
(Industrial-Scientific-Medical) band

WLAN : 802.11a
Operates in the 5 GHz UNII (Unlicensed National Information
Infrastructure) band
Incompatible with devices operating in 2.4GHz
Supports Data rates up to 54 Mbps.

WLAN : 802.11g
Supports data rates as high as 54 Mbps on the 2.4 GHz band
Provides backward compatibility with 802.11b equipment

Medical Professionals
Education
Temporary Situations
Airlines
Security Staff
Emergency Centers
Wireless LAN Applications

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Wireless Local Area Networks
 The proliferation of laptop computers and other mobile
devices (PDAs and cell phones) created an obvious
application level demand for wireless local area networking.
 Companies jumped in, quickly developing incompatible
wireless products in the 1990’s.
 Industry decided to entrust standardization to IEEE
committee that dealt with wired LANS – namely, the IEEE
802 committee!!

In response to lacking standards, IEEE developed the
first internationally recognized wireless LAN standard
– IEEE 802.11
IEEE published 802.11 in 1997, after seven years of
work
Most prominent specification for WLANs
Scope of IEEE 802.11 is limited to Physical and Data
Link Layers.
IEEE 802.11 Wireless LAN Standard

Appliance Interoperability
Fast Product Development
Stable Future Migration
Price Reductions
The 802.11 standard takes into account the following
significant differences between wireless and wired
LANs:
Power Management
Security
Bandwidth
Benefits of 802.11 Standard

21
IEEE 802 Standards Working Groups
Figure 1-38. The important ones are marked with *. The ones marked with 
are hibernating. The one marked with † gave up.

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Categories of Wireless Networks
Base Station :: all communication through
an access point {note hub topology}. Other
nodes can be fixed or mobile.
Infrastructure Wireless :: base station
network is connected to the wired Internet.
Ad hoc Wireless :: wireless nodes
communicate directly with one another.
MANETs (Mobile Ad Hoc Networks) :: ad
hoc nodes are mobile.

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Wireless LANs
Figure 1-36.(a) Wireless networking with a base station. (b) Ad hoc networking.

IEEE 802 LAN Standards Family
IEEE 802.3
Carrier
Sense
IEEE 802.4
Token
Bus
IEEE 802.5
Token
Ring
IEEE 802.11
Wireless
IEEE 802.2
Logical Link Control (LLC)
PHY
OSI Layer 1
(Physical)
Mac
OSI Layer 2
(Data Link)

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The 802.11 Protocol Stack
Note – ordinary 802.11 products are no longer being manufactured.
Figure 4-25. Part of the 802.11 protocol stack.

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Wireless Physical Layer
 Physical layer conforms to OSI (five options)
1997: 802.11 infrared, FHSS, DHSS
1999: 802.11a OFDM and 802.11b HR-DSSS
2001: 802.11g OFDM
 802.11 Infrared
Two capacities 1 Mbps or 2 Mbps.
Range is 10 to 20 meters and cannot penetrate walls.
Does not work outdoors.
 802.11 FHSS (Frequence Hopping Spread Spectrum)
The main issue is multipath fading.
79 non-overlapping channels, each 1 Mhz wide at low end of
2.4 GHz ISM band.
Same pseudo-random number generator used by all stations.
Dwell time: min. time on channel before hopping (400msec).

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 802.11 DSSS (Direct Sequence Spread Spectrum)
 Spreads signal over entire spectrum using pseudo-random sequence (similar
to CDMA see Tanenbaum sec. 2.6.2).
 Each bit transmitted using an 11 chips Barker sequence, PSK at 1Mbaud.
 1 or 2 Mbps.
 802.11a OFDM (Orthogonal Frequency Divisional Multiplexing)
 Compatible with European HiperLan2.
 54Mbps in wider 5.5 GHz band  transmission range is limited.
 Uses 52 FDM channels (48 for data; 4 for synchronization).
 Encoding is complex ( PSM up to 18 Mbps and QAM above this capacity).
 E.g., at 54Mbps 216 data bits encoded into into 288-bit symbols.
 More difficulty penetrating walls.

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802.11b HR-DSSS (High Rate Direct Sequence Spread
Spectrum)
11a and 11b shows a split in the standards committee.
11b approved and hit the market before 11a.
Up to 11 Mbps in 2.4 GHz band using 11 million chips/sec.
Note in this bandwidth all these protocols have to deal with
interference from microwave ovens, cordless phones and
garage door openers.
Range is 7 times greater than 11a.
11b and 11a are incompatible!!

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802.11g OFDM(Orthogonal Frequency Division
Multiplexing)
An attempt to combine the best of both 802.11a and
802.11b.
Supports bandwidths up to 54 Mbps.
Uses 2.4 GHz frequency for greater range.
Is backward compatible with 802.11b.

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802.11 MAC Sublayer Protocol
In 802.11 wireless LANs, “seizing channel” does
not exist as in 802.3 wired Ethernet.
Two additional problems:
Hidden Terminal Problem
Exposed Station Problem
To deal with these two problems 802.11 supports
two modes of operation DCF (Distributed
Coordination Function) and PCF (Point
Coordination Function).
All implementations must support DCF, but PCF is
optional.

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Figure 4-26.(a)The hidden station problem. (b) The exposed station problem.

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The Hidden Terminal Problem
Wireless stations have transmission ranges and
not all stations are within radio range of each
other.
Simple CSMA will not work!
C transmits to B.
If A “senses” the channel, it will not hear C’s
transmission and falsely conclude that A can
begin a transmission to B.

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The Exposed Station Problem
This is the inverse problem.
B wants to send to C and listens to the channel.
When B hears A’s transmission, B falsely assumes
that it cannot send to C.

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Distribute Coordination Function
(DCF)
 Uses CSMA/ CA (CSMA with Collision Avoidance).
 Uses both physical and virtual carrier sensing.
 Two methods are supported:
 based on MACAW(Multiple Access with Collision
Avoidance for Wireless) with virtual carrier sensing.
 1-persistent physical carrier sensing.

 Access point (AP): A station that provides access to the DS.
 Basic service set (BSS): A set of stations controlled by a single AP.
 Distribution system (DS): A system used to interconnect a set of
BSSs to create an ESS.
DS is implementation-independent. It can be a wired 802.3
Ethernet LAN, 802.4 token bus, 802.5 token ring or another
802.11 medium.
 Extended service set (ESS):Two or more BSS interconnected by DS
 Portal: Logical entity where 802.11 network integrates with a non
802.11 network.
IEEE 802.11 Terminology

Distribution service (DS)
Used to exchange MAC frames from station in
one BSS to station in another BSS
Integration service
Transfer of data between station on IEEE 802.11
LAN and station on integrated IEEE 802.x LAN
IEEE 802.11 Services: Distribution of
Messages

Association
Establishes initial association between station and
AP
Re-association
Enables transfer of association from one AP to
another, allowing station to move from one BSS to
another
Disassociation
Association termination notice from station or AP
Association Related Services

Authentication
Establishes identity of stations to each other
De-authentication
Invoked when existing authentication is
terminated
Privacy
Prevents message contents from being read by
unintended recipient
Access and Privacy Services

IEEE 802.11 Medium Access Control
MAC layer covers three functional areas:
Reliable data delivery
Access control
Security

Reliable Data Delivery
Loss of frames due to noise, interference, and
propagation effects
Frame exchange protocol
Source station transmits data
Destination responds with acknowledgment (ACK)
If source doesn’t receive ACK, it retransmits frame
Four frame exchange for enhanced reliability
Source issues request to send (RTS)
Destination responds with clear to send (CTS)
Source transmits data
Destination responds with ACK

 Distributed Coordination Function (DCF)
Distributed access protocol
Contention-Based
Makes use of CSMA/CA rather than CSMA/CD
Suited for ad hoc network and ordinary asynchronous traffic
 Point Coordination Function (PCF)
Alternative access method on top of DCF
Centralized access protocol
Contention-Free
Works like polling
Suited for time bound services like voice or multimedia
Access Control

CSMA/CD vs. CSMA/CA
CSMA/CD – CSMA/Collision detection
For wire communication
No control BEFORE transmission
Generates collisions
Collision Detection-How?
CSMA/CA – CSMA/Collision Avoidance
For wireless communication
Collision avoidance BEFORE transmission
Why avoidance on wireless?
Difference in energy/power for transmit & receive
Difficult to distinguish between incoming weak signals,
noise, and effects of own transmission

Interframe Space (IFS)
Defined length of time for control
SIFS - Short Inter Frame Spacing
Used for immediate response actions e.g ACK, CTS
PIFS - Point Inter Frame Spacing
Used by centralized controller in PCF scheme
DIFS - Distributed Inter Frame Spacing
Used for all ordinary asynchronous traffic
DIFS (MAX) > PIFS > SIFS (MIN)

RTS-CTS-DATA-ACK
DIFS: Distributed IFS
RTS: Request To Send
SIFS: Short IFS
CTS: Clear To Send
ACK: Acknowledgement
NAV: Network Allocation Vector
DCF: Distributed Coordination Function

MAC Frame Format
Frame
Control
Duration
ID
Addr 1 Addr 2 Addr 3 Addr 4Sequence
Control
CRC
Frame
Body
2 2 6 6 6 62 0-2312 4
802.11 MAC Header
Protocol
Version
Type SubType
To
DS
Retry
Pwr
Mgt
More
Data
WEP Order
Frame Control Field
Bits: 2 2 4 1 1 1 1 1 1 1 1
DS
From More
Frag

MAC Layer Frames
 Data Frames
 Control Frames
RTS,CTS,ACK and PS-POLL
 Management Frames
Authentication and De-Authentication
Association, Re-Association, and Disassociation
Beacon and Probe frames

IEEE 802.11 Security
 Authentication provided by open system or
shared key authentication (Authentication is used
instead of wired media physical connection)
 Privacy provided by WEP (Privacy is used to
provide the confidential aspects of closed wired
media)
 An Integrity check is performed using a 32-bit CRC

Is WLAN Secure ?
 The Parking Lot
attack
 Man in the middle
attack
 Freely available tools
like Air Snort, WEP
crack to snoop into a
WLAN

Physical Media Defined by Original
802.11 Standard
 Frequency-hopping spread spectrum
Operating in 2.4 GHz ISM band
Lower cost, power consumption
Most tolerant to signal interference
 Direct-sequence spread spectrum
Operating in 2.4 GHz ISM band
Supports higher data rates
More range than FH or IR physical layers
 Infrared
Lowest cost
Lowest range compared to spread spectrum
Doesn’t penetrate walls, so no eavesdropping

Frequency Hopping Spread
Spectrum
 Signal is broadcast over seemingly random series of radio
frequencies
 Signal hops from frequency to frequency at fixed intervals
 Receiver, hopping between frequencies in synchronization with
transmitter, picks up message
 Advantages
Efficient utilization of available bandwidth
Eavesdropper hear only unintelligible blips
Attempts to jam signal on one frequency succeed only at
knocking out a few bits

Direct Sequence Spread Spectrum
 Each bit in original signal is represented by multiple bits in
the transmitted signal
 Spreading code spreads signal across a wider frequency
band
 DSSS is the only physical layer specified for the 802.11b
specification
802.11a and 802.11b differ in use of chipping method
802.11a uses 11-bit barker chip
802.11b uses 8-bit complimentary code keying (CCK)
algorithm

IEEE 802.11a and IEEE 802.11b
IEEE 802.11a
Makes use of 5-GHz band
Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps
Uses orthogonal frequency division multiplexing (OFDM)
IEEE 802.11b
802.11b operates in 2.4 GHz band
Provides data rates of 5.5 and 11 Mbps
Complementary code keying (CCK) modulation scheme
For more information:
http://home.no.net/coverage/rapport/80211.htm

Other Standards
 Japan has introduced Millimeter Wave Wireless LAN
(MWWL).
 Europe has introduced HIPERLAN (High Performance
Radio Local Area Network)
 Features, capabilities, and technology similar to
those of IEEE 802.11 used in US
Developed by ETSI (European Telecommunications
standards institute)
Provides high speed communications (20Mbps)
Has technical advantages such as inclusion of Quality
of Service

HIPERLAN-reference model
Medium Access Control
(MAC) Sublayer
Channel Access Control
(CAC) Sublayer
Physical (PHY) Layer
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
higher layer protocols
OSI
Reference Model
HIPERLAN
Reference Model

Future of WLAN
 WLANs move to maturity
Higher Speeds
Improved Security
Seamless end-to-end protocols
Better Error control
Long distances
New vendors
Better interoperability
Global networking
Anywhere, anytime, any-form connectivity…

Mobile Ad Hoc Networks
(MANETs)

Why we need ad-hoc networks?
Ease & Speed of deployment.
Do not need backbone infrastructure support
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What is ad-hoc networks?
Decentralized multi-hop relaying network, where each
node performs routing.
When we need ad-hoc networks?
In many scenarios where deployment of a wired
network is impractical or impossible
4 W’s for Ad-hoc Networks
Where we need ad-hoc networks?
Military Applications
Emergency Operations
Meeting rooms

Mobile Ad-hoc NETworks (MANET)
 Self-configuring infrastructureless network of
mobile devices.
 Each device is free to move independently in
any direction, and change its links to other
devices frequently.
 The primary challenge is equipping each
device to continuously maintain the
information required to properly route traffic.
63
Single hop – Nodes communicate directly Multi hop – Traffic has to be forwarded
Ad-hoc networks - Classifications
a
b
c
d
a
b
c
d
a b

64
MANET
Two types of wireless networks:
Infrastructure network
base stations are the bridges
a mobile host will communicate with the
nearest base station
handoff is taken when a host roams from one
base to another

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MANET
Ad hoc network:
infrastructure less: no fixed base stations
without the assistance of base stations for
communication
Due to transmission range constraint,
two MHs need multi-hop routing for
communication
quickly and unpredictably changing topology

66
Cell Phone Networks
 Infrastructure
Network

67
MANET
 MANET = Mobile Ad Hoc Networks
a set of mobile hosts, each with a transceiver
no base stations; no fixed network infrastructure
multi-hop communication
needs a routing protocol which can handle changing
topology

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MANET
 Single-Hop Ad Hoc

70
MANET
 Single-hop Vs. Multi-hop systems

71
Ad Hoc typical applications
 Personal area networking
cell phone, laptop, ear phone, wrist watch
 Military environments
soldiers, tanks, planes
 Civilian environments
car network
 meeting rooms
sports stadiums
boats, small aircraft
 Emergency operations
search-and-rescue
policing and fire fighting

74
Multi-hopping via Wireless Router

78
Military applications
Situational Awareness (SA) and Command and Control (C2) for
military.

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Nokia Roof Top Wireless Routing
A wireless broadband solution for residential markets,
based on a multi hop Ad-Hoc (mesh) networking.

Nokia Roof Top
 Roof Top solution (Nokia, Finland)
Wireless router
a radio frequency (RF) modem
a digital Internet protocol (IP) router

FHP
 FHP Wireless, USA
 ad hoc network in a campus

Mesh Networks
 Mesh Networks, USA

System Architecture: Mesh Networks

Networking Scenario: To Internet

SkyPilot NeighborNet
• SkyPilot Network, USA

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What is the goal of 802.11 standard ?
To develop a Medium Access Control (MAC) and
Physical Layer (PHY) specification for wireless
connectivity for fixed, portable and moving stations
within a local area.

90
802.11 sub-standards
 802.11 MAC (Media Access Control) ratified 1999
 802.11b PHY 2.4 GHz (max 11 Mbps) ratified 1999
 802.11a PHY 5.0 GHz (max 54 Mbps) ratified 1999
 802.11g PHY 2.0 GHz (max 54 Mbps) ratified 2003
 802.11i Security draft number XXX
 802.11e QoS, Multimedia draft number XXX
 802.11h European regulations for 5GHz draft number XXX
 802.11h Japan regulations for 5GHz draft number XXX

91
Do I need any license to use 802.11
device ?
No , 2.4 GHz and 5.0 GHz are public available
frequency !!!

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Logical Link Control Services

94
Standard 802.11 frame format

95
Frames types and subtypes
Three types of frames:
Control
(ACK,RTS,CTS ,Power Save …)
Management
(Beacon,Probe Request ,Probe Response,
Association request , Association response …)
Data
(Data, Null Data, Data_CF_Ack , ….)

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Infrastructure Model includes:
 Stations (STA)
any wireless device
 Access Point (AP)
connects BSS to DS
controls access by STA’s
 Basic Service Set (BSS)
a region controlled by an AP
mobility is supported within a single
BSS
 Extended Service Set (ESS)
a set of BSS’s forming a virtual BSS
mobility is supported between BSS’s
in an ESS
 Distribution Service (DS)
connection between BSS’s
802.11 MAC –Infrastructure model
DS
BSS1
BSS2
BSS3
STA1
STA2
STA3
ESS1
AP1
AP2
AP3

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802.11 MAC supports infrastructure
and ad hoc network models
Ad Hoc Model includes:
Stations (STA)
any wireless device
act as distributed AP
Independent Basic Service
Set (IBSS)
BSS forming a self
contained network
no AP and no connection
to the DS
IBSS
STA1
STA2
STA3

98
Two types of access to air
 DCF (distributed coordination function )
 means everybody can speak and try
 to get air : 100% on the market
 PCF (point coordination function)
 means ONE point coordinator (BOSS)
 who will allowed you to speak
 (like in bluetooth)

99
Summary of required features and
difficulties vs 802.11 features
 Features
 High speed operation (PHY only)
 Fair access (DCF, PCF)
 Time-bounded access (PCF)
 Flexible configuration (BSS, IBSS)
 Security (WEP)
 Mobility support (ESS)
 Low power (PS)
 Difficulties
Hidden terminals (RTS/CTS)
Capture (CSMA/CA, ACK)
Noise and interference (ACK, frag)
Limited spectrum (licencing, PHYs)

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WiMAX
 Goal: Provide high-speed Internet access to home and
business subscribers, without wires.
 Base stations (BS) and subscriber stations (SS)
 Centralized access control to prevents collisions
 Supports applications with different QoS requirements
 WiMAX is a subset of IEEE 802.16 standard

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IEEE 802.16 standards
 802.16.1 (10-66 GHz, line-of-sight, up to 134Mbit/s)
 802.16.2 (minimizing interference between coexisting
WMANs)
 802.16a (2-11 Ghz, Mesh, non-line-of-sight)
 802.16b (5-6 Ghz)
 802.16c (detailed system profiles)
 P802.16e (Mobile Wireless MAN)

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Physical layer
 Allows use of directional antennas
 Allows use of two different duplexing schemes:
Frequency Division Duplexing (FDD)
Time Division Duplexing (TDD)
 Support for both full and half duplex stations
 Adaptive Data Burst profiles
Transmission parameters (e.g. Modulation, FEC) can be
modified on a frame-by-frame basis for each SS
Profiles are identified by ”Interval Usage Code”

106
Time Division Duplexing (TDD)

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Media Acces Control (MAC)
 Connection oriented
 Connection ID (CID), Service Flows
 Channel access: decided by BS
 UL-MAP
 Defines uplink channel access
 Defines uplink data burst profiles
 DL-MAP
 Defines downlink data burst profiles
 UL-MAP and DL-MAP are both transmitted in the
beginning of each downlink subframe

110
Uplink periods
 Initial Maintenance opportunities
 Ranging - to determine network delay and to request power or
profile changes
 Collisions may occur in this interval
 Request opportunities
 SSs request bandwith in response to polling from BS
 Collisions may occur in this interval
 Data grants period
 SSs transmit data bursts in the intervals granted by the BS
 Transition gaps between data intervals for synchronization

111
Bandwidth request
 SSs may request bandwidth in 3 ways:
Use the ”contention request opportunities” interval upon
being polled by the BS
Send a standalone MAC message called ”BW request” in
an allready granted slot
Piggyback a BW request message on a data packet

112
Bandwidth allocation
 BS grants/allocates bandwidth in one of two modes:
 Grant Per Subscriber Station (GPSS)
 Grant Per Connection (GPC)
 Decision based on requested bandwidth and QoS
requirements vs available resources
 Grants are notified through the UL-MAP

113
Bandwidth Request-Grant Protocol
BS
SS1
SS2
1
2.1
2.2
1. BS allocates bandwidth to SSs for
transmitting bandwidth request.
2.1 SS1 transmits bandwidth requests.
2.2 SS2 transmits bandwidth requests.
4. BS allocates bandwidth to SSs for
transmitting data based on their
bandwidth requests. Bandwidth is
also allocated for requesting more
bandwidth.
5.1 SS1 transmits data and bandwidth
requests.
5.2 SS2 transmits data and bandwidth
requests.
4
5.1
5.2

114
Scheduling services
 Unsolicited Grant Service (UGS)
 Real-time, periodic fixed size packets (e.g. VoIP)
 No periodic bandwith requests required
 Real-Time Polling Service (rtPS)
 Real-time, periodic variable sizes packets (e.g MPEG)
 BS issues periodic unicast polls
 Non-Real-Time Polling Service (nrtPS)
 Variable sized packets with loose delay requirements (FTP)
 BS issues unicast polls regularly (not necessarily periodic)
 Can also use contention requests and piggybacking
 Best Effort Service
 Never polled individually
 Can use contention requests and piggybacking

115
Example
Total Uplink Bytes = 100
2 SS and 1 BS
SS1
Demands:
UGS = 20
rtPS = 12
nrtPS = 15
BE = 30
SS2
Demands:
UGS = 10
rtPS = 10
nrtPS = 15
BE = 20
Total Demand Per
Flow:
UGS = 30
rtPS = 22
nrtPS = 30
BE = 50
Flows: UGS rtPS nrtPS BE
1st Round 40 30 20 10
30 22 20 10
Excess Bytes = 18
2nd Round 30 22 20+12 10+6
30 22 32 16
Excess Bytes = 2
3rd Round 30 22 30 16+2
30 22 30 18
SS1 Allocation = 20 +12 + 15 + 9 = 56
SS2 Allocation = 10 +10 + 15 + 9 = 44

Definition
 What is WLL?
WLL is a system that connects subscribers to the local
telephone station wirelessly.
 Systems WLL is based on:
Cellular
Satellite (specific and adjunct)
Microcellular
 Other names
Radio In The Loop (RITL)
Fixed-Radio Access (FRA).

WLL services
 Desirable:
Wireless feature should be transparent
Wireline Custom features
 Other:
Business related
Hunt groups,
Call transfers
Conference calling
Calling cards, coin phones
V.29 (9600bps)
ISDN (64kbps)

WLL should provide…
Toll-quality service
Expand from a central office to about 5 miles
Low license cost
Subscriber costs equivalent or better than
copper

Ideas for U.S. market
 Supplement Copper Lines
Easier third telephone line
Data service
 Fixed Mobile Users
Take phone wherever you want / charged on 2 levels
“home” could mean neighborhood
Charged regular mobile rate if you’re on the road

Cost Considerations
 Wireless cost is constant over distance for WLL
 Wireline depends on distance AND terrain

Situations “made” for WLL
 Environments where 3rd line is degraded might be cheaper to
go wireless
 Where it’s impossible to lay copper (3rd world, small islands)
 Business parks, industrial areas
 Speedy deployment, stop gap application till wireline is in
90-120 days for activation

Developed vs Developing
 Developed: Wireline service
Firmly established, cellular penetration is relatively high
Incumbent operator would use it to install 2nd, 3rd lines,
coverage to rural areas
2nd or 3rd competitive operator deploy it for fast & cost
effective deployment
Quick way to establish market presence
cellular complement to their offerings

Developed vs Developing
 Developing
Quick & easy to deploy in countries with little copper line
service, so as to accommodate people on enormous
waiting lists for basic service
Low maintenance costs
Allows more competition in provider market

Examples
 UK
150 PTOs have licenses for wireless
Focus on regional networks
WLL Commercial services
Ionica, Atlantic Telecom, Scottish Telecom
 Poland
Most exciting market in eastern Europe
Local loop is the bottleneck
150,000 WLL lines since 1996 (15% of new)
Ericsson, Motorola contracts

Connection Setup
PSTN
Switch
function
WLL
Controller
AM
HLR
Transceiver WASU
Trunk
Air
Interface
UWLL
TWLL
 Wireless Access Network Unit(WANU)
 Interface between underlying telephone
network and wireless link
 consists of
Base Station Transceivers (BTS)
Radio Controller(RPCU)
Access Manager(AM)
Home Location Register(HLR)
WANU
 Wireless Access Subscriber Unit
(WASU)
 located at the subscriber
 translates wireless link into a
traditional telephone
connection

Important Results of Fixed to Fixed
Propagation in WLLs
 Signal channel is not a Rayleigh fading channel:
Power control algorithms are simpler and can be utilized
more effectively
 Channel Randomness is lost:
Makes analysis difficult
 Pathloss exponent is considerably smaller (Why?):
20dB/dec compared to 40dB/dec
Decreases cell capacity
Allows for larger coverage area

Fixed to Fixed Propagation(cont’d)
 No handoffs necessary:
Decreases hardware costs and system complexity
Increases quality of service through accurate traffic
predictions
 Allows usage of directional antennas:
Can greatly reduce interference and increase cell capacity
-30dB
30dB
0o 60o
-40dB
10dB
0o 120o 180o
BS antenna Subscriber antenna

In-Cell Interference (CDMA)
I = (Nh – 1)aS  NhaS
a = voice activity factor
 Nh = total # of houses
S = power received at cell site from every house

Out-of-Cell Interference
Pathloss: 20dB/dec as opposed to 40dB/dec
 need to take in account more tiers
Only from houses whose antennas are directed at
the center cell base station

Interference from Another Cell
• Blue area is region of
interferers for C
• It is Not a perfect pie shape
• If w = (1/2)*(antenna width)
(in radians)
• W = w+2sin-1((R/D)sin(w/2))
• If w<<1 and R<<D:
W = w (1+(R/D))
is the “pie” arc length

Per-Tier Interference
 Integration over W and all the cells at tier n yields:
 In = [aNhSw/(3sqrt(3))][1/n]
 for n>4
 Interference is proportional to antenna width w and inversely
proportional to the tier number.
 Decreasing the antenna width can greatly reduce
interference.
 As the number of tiers approaches infinity, so does the total
interference. Therefore, system capacity is a function of the
total number of tiers in the system.

Capacity comparison
for 5 MHz spectrum allocation
Detail IS-95 CDMA IS-136 TDMA ETSI (GSM)
Mobile WLL Mobile WLL Mobile WLL
Chan. BW
(kHz)
1250 1250 30 30 200 200
# channels 4 4 167 167 25 25
Eb/N0 7 dB 6dB 18dB 14dB 12dB 12dB
Freq. Reuse 1 1 7 4 3 3
Effective Chan.
Per sect.
4 4 7.95 13.92 2.78 2.78
Erlangs per cell
Per MHz
38.3 48.7 9.84 19.6 9.12 9.12

Comparison
WLL Mobile Wireless Wireline
Good LOS component Mainly diffuse components No diffuse components
Rician fading Rayleigh fading No fading
Narrowbeam directed
antennas
Omnidirectional antennas Expensive wires
High Channel reuse Less Channel reuse Reuse Limited by wiring
Simple design, constant
channel
Expensive DSPs, power
control
Expensive to build and
maintain
Low in-premises mobility
only, easy access
High mobility allowed,
easy access
Low in-premises mobility,
wiring of distant areas
cumbersome
Weather conditions effects Not very reliable Very reliable

Examples of services provided
 Marconi WipLL (wireless IP local loop)
Based on Frequency hopping CDMA
Internet Protocol 64kbps to 2.4Mbps rates Committed
Information Rate or best effort service
 Lucent WSS (wireless subscriber system)
800 to 5000 subscribers per switch
Uses FDMA/FDD 12 Km to 40Km coverage
 GoodWin WLL
DECT standards
9.6 kbps rate
Specified conditions -5°С...+55°С, 20...75% humidity

Future of WLL
 Depends on
economic development
existing infrastructure of a region
 Offers
market competition
quick deployment
relatively reliable service at low costs

Wireless Networks and Standards Guide

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