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Transcript of Mobile Communication
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
LAN Components
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
18
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.
22
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.
23
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)
25
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.
26
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).
27
Wireless Physical Layer
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.
28
Wireless Physical Layer
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!!
29
Wireless Physical Layer
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.
30
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.
31
Figure 4-26.(a)The hidden station problem. (b) The exposed station problem.
32
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.
33
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.
34
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
WLAN Topology: Ad-Hoc Network
WLAN Topology: Ad-Hoc Network
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
Re-Association
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 4 Sequence
Control CRC
Frame
Body
2 2 6 6 6 6 2 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
Authentication
WEP Encryption/Decryption
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
62
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
65
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
68
MANET
Single-Hop Ad Hoc
69
MANET
Multi-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
72
Peer-to-Peer
73
Multi-hop Peer-to-Peer
74
Multi-hopping via Wireless Router
75
Hopping on the Network
76
Supports Mobility
77
Supports Non Line-of-Sight
78
Military applications
Situational Awareness (SA) and Command and Control (C2) for military.
79
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
FHP Wireless
FHP Wireless
Mesh Networks
Mesh Networks, USA
System Architecture: Mesh Networks
Networking Scenario: To Internet
SkyPilot NeighborNet
• SkyPilot Network, USA
88
WiFi
89
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 !!!
92
Context with OSI layers
93
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 , ….)
96
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
97
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)
100
WiMax
101
102
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
103
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)
104
105
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)
107
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
108
TDD Downlink subframe
109
Uplink 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
116
802.11/802.16
117
118
Wireless Local Loop (WLL)
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).
A general WLL setup
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
WLL
Basie station
Future of WLL
Depends on
economic development
existing infrastructure of a region
Offers
market competition
quick deployment
relatively reliable service at low costs
Questions ?