1 Wireless standards Path Loss Phenomena Wireless standards Path - - PDF document

1

April 2003

Wireless Networks Wireless Networks

Amnon Jonas April 2003 Amnon Jonas April 2003

May 18, 2003

• 2 -

Outline Outline

• What is wireless?

– Overview – Wireless parameters – Cellular architecture – Media multiple access

• Wireless LAN 802.11
• MANET – Mobile Ad-hoc NETworks
• Broadband fixed wireless access 802.16

April 2003

What is wireless? What is wireless?

May 18, 2003

• 4 -

Wireless Networks Wireless Networks

• Architecture

– Point to multi-point: many stations connected to the network through a single base station (Fixed W.L., 802.11, Cell. Phones, satellite) – Point to point: A wireless link between 2 points (IrDA, B.T.) – Multi-point to multi-point: Ad-hoc network (802.11)

• Services

– Internet/Network access – Voice – Video streaming

• Target market

– Residential (802.16-MMDS, cell. Phones) – Business (802.16-LMDS, satellite)

2

May 18, 2003

• 5 -

Wireless standards Wireless standards

Wired LAN High performance WLAN H2 802.11a/g HomeRF 802.11b WLAN

Mbps (PHY layer) 1 10 100 0,1

Outdoor

Fixed Walk Vehicle

Indoor

Fixed/ Desktop Walk

Mobility

UMTS Wideband Cellular WAN G S M , I S

• 9

5 , D

• A

M P S Bluetooth Fixed wireless 802.16

May 18, 2003

• 6 -

Path Loss Phenomena Path Loss Phenomena

L i n e

• f

s i g h t p a t h Reflected path

• Open space attenuation

– Signal strength ~ 1/d2

• Flat fading

– Lower signal strength due to interference of multiple signal phases

• Frequency selective fading

– Fading is depend on the frequency within the signal bandwidth – results in signal distortion

• Distance related attenuation

– Signal strength ~ 1/dγ, 2<γ<5

May 18, 2003

• 7 -

PHY parameters PHY parameters

• Bandwidth

– The width of the range of frequencies that an electronic signal occupies on a given transmission medium – Examples: voice – 3 KHz; TV – 6 MHz – There is a relation between the width of the band to its robustness

• Roll off factor
• a

lpha

– defines the sharpness of the filter in the frequency domain – The lower the alpha, the sharper the filter in the frequency domain and the higher the overshoot in the time domain – TV – 13%

• Symbol rate

Bandwidth = Symbol Rate (1 + α)

– TV – 6,000,000/1.13 = 5,309,735 Symbols/sec

May 18, 2003

• 8 -

Symbol Rate & Modulation Symbol Rate & Modulation

• Modulation

– Modulation is the addition of information to an electronic or optical signal carrier – Common modulations: AM, FM, – More complex modulations: QAM, QPSK (~QAM4) – QAM N provides log2N bits/signal – TV: QAM 64 – 256, wireless: QAM 4 - 64

• Example for channel rate calculation (6 MHz,

α=13%, QAM64):

QPSK/QAM4 QAM16

sec 32 ~ 6 sec 3 . 5 ~ 64 log 13 . 1 6

2

Mbits Msym ⋅

3

May 18, 2003

• 9 -

Error correction Error correction

• Errors caused by interference, fading and random

noise

• It works by adding redundancy to the transmitted

signal

• Mainly two types:

– Block coding (e.g. Reed Solomon) – Convolutional coding (e.g. trellis code with Viterbi decoder)

• Example: RS (188,204)

– Corrects 8 bytes from 188 bytes – Can improve 10-4 to 10-14

• FEC works best when the errors are randomly

distributed

May 18, 2003

• 10 -

Interleaving Interleaving

• Fading tends to result errors in blocks
• Interleaver randomizes the errors
• Interleavers introduce delays to the system

36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 … … 8 2 31 25 19 13 7 1 8 2 31 25 19 13 7 1

31 25 19 13 8 7 2 1

Transmitter Transmitter Receiver Receiver

columns columns columns columns rows rows rows rows

May 18, 2003

• 11 -

Supercell Architecture Supercell Architecture

• Very large cell

– Large service area with a radius of up to 30 mile is covered. – High antenna at base transceiver station (BTS) ~ 80m – A high-gain rooftop directional CPE antenna – Line-of-sight (LOS) connection between transmitter and receiver

• Advantages

– Due to LOS propagation, carrier-to-noise ratio (C/N) values of around 30 dB can be sustained – Use of high-order modulation possible

• Disadvantages:

– Not scalable – Limited coverage due to the strict need for LOS links

• Attractive for:

– Initial network roll out

• Toll towers already available in many place

– Rural, low density population areas

May 18, 2003

• 12 -

Super cell

• Single cell configuration:

– Large service area - a typical radius of up to 30 mile is covered.

• Advantages

– Fast and low-cost initial deployment services a large area – All the equipment is located in a single location – simplify maintenance – Frequency reuse in angle and polarization may be possible with sectorization.

• Disadvantages:

– Not scalable. – Limited bandwidth – Limited coverage – dead areas – High power – battery lifespan, high radiation, interference to other systems, regulation limits – Expansive antennas

4

May 18, 2003

• 13 -

Supercell Architecture Supercell Architecture

• SECTOR COVERAGE

SECTOR COVERAGE

• OMNIDIRECTIONAL

OMNIDIRECTIONAL

May 18, 2003

• 14 -

Multicell Multicell Architecture Architecture

• Cellular architecture:

– Multiple cells, each cover a smaller area

• Advantages

– Frequency reuse => unlimited capacity – The architecture is scalable in both capacity and coverage

• Disadvantages:

– Infrastructure is distributed in many locations – Higher deployment and maintenance costs

May 18, 2003

• 15 -

Adaptive PHY parameters Adaptive PHY parameters

• Burst profile

– Modulation and FEC and FEC

• Dynamically assigned according to

link conditions

– Burst by burst, per subscriber station – Trade-off capacity vs. robustness in real time real time

• Roughly doubled capacity for the

same cell area

• Burst profile for downlink broadcast

channel is well

• k

n

• wn

– All other burst profiles could be configured “on the fly” – Subscriber station capabilities recognized at registration

April 2003

Multiple Access Multiple Access

5

May 18, 2003

• 17 -

Duplexing Duplexing

• Separation of uplink and downlink channels
• Frequency Division Duplexing – FDD:

– No synchronization issues. – Uplink and downlink channels may fade independently – Duplexor required to separate signals

• Time Division Duplexing – TDD:

– Simple duplex equipment – Transmission in one direction can be used to measure channel in other direction. – Flexible bandwidth allocation between uplink and downlink – Requires synchronization – Need guard bands to prevent overlap

May 18, 2003

• 18 -

Frequency Division Multiple Access Frequency Division Multiple Access

FDMA FDMA

• System bandwidth divided into channels

assigned to different users

• Advantages

– Narrowband channels – Low complexity – Allows channel estimation

• Disadvantages

– Multiple radios at base station – Dedicated channels (idle ones wasted) – Interference between adjacent channels – Difficult frequency reuse

1 1 2 2 3 3 4 4 5 5 6 6

May 18, 2003

• 19 -

Time Division Multiple Access Time Division Multiple Access

TDMA TDMA

• Time divided into slots assigned to different users
• Advantages

– Common radio equipment for all users at the base station – Overbooking can be easily supported

• Disadvantages

– Synchronization required – Wider channel exacerbate ISI – Guard band between transmissions is needed – Preamble and equalization training sequence is needed

1 2 3 1 2 3 1 1 2 3 1 2 3 1 4 5 6 4 5 6 4 4 5 6 4 5 6 4

May 18, 2003

• 20 -

Code Division Multiple Access Code Division Multiple Access

CDMA CDMA

• CDMA

– Each station can transmit over the entire spectrum all the time – Multiple simultaneous transmissions are separated using

• rthogonal codes
• Each station is assigned a unique m-bit code
• The signal is multiplied with the code
• The signal is recovered by multiplying with the negation code
• If the codes are orthogonal, other stations transmissions are canceled
• Otherwise, other stations transmissions are filtered out as white noise
• Advantages

– The bandwidth is shared over several users

• Disadvantages

– Requires strict time and power synchronization – Maximum rate is limited – High complexity

6

May 18, 2003

• 21 -

Orthogonal Frequency Division Multiplexing Orthogonal Frequency Division Multiplexing

OFDM OFDM

• OFDM Concept

– Transmit multiple modulated sub-carriers in parallel – Each occupies a very narrow bandwidth – Perform FFT at the transmitter and receiver

• Advantages

– Subcarriers are orthogonal => no need of guard band => spectral efficiency – OFDM combats multipath by performing FFT (Fast Fourier Transform) at the transmitter and receiver

• Disadvantages

– High peak-to-average power ratio – => transmitter power must be backed off – Relatively long blocks may cause bandwidth waste on non broadcast channels

April 2003

Wireless LAN 802.11 Wireless LAN 802.11

May 18, 2003

• 23 -

Wireless LAN 802.11 Wireless LAN 802.11

• IEEE 802.11 is extension of Ethernet standard

(IEEE 802.3) into wireless communications

• Allows roaming computers to talk to other

devices (peer-to-peer) or connect to wired network

• IEEE standard allows interoperability between
• multiple vendors products
• Two modes

– Infrastructure – Ad-hoc

May 18, 2003

• 24 -

IEEE 802.11 Features IEEE 802.11 Features

• Speeds of 1-2 (802.11) 5.5-11 (802.11a) or 6-54

(802.11b) Mb/sec

• Operating Range: 10-100m indoors, 300m
• utdoors
• Power Output Limited to 1 Watt in U.S.
• Frequency Hopping (FHSS), Direct Sequence

(DSSS), & Infrared (IrDA)

– Networks are NOT compatible with each other

• Uses unlicensed 2.4 GHz band (2.402-2.480

GHz) or 5GHz band

• Provide wireless Ethernet for wired networks

7

May 18, 2003

• 25 -

IEEE 802.11 terms IEEE 802.11 terms

• Station (STA): Any device with 802.11 MAC and PHY
• Coordination Function (CF): The logical function that determines

when a STA may transmit – PCF or DCF

• Basic Service Set (BSS): A set of stations controlled by a single

coordination function (CF)

• Distribution System (DS): A system that interconnect BSSs and

LANs to create Extended Service Set (ESS)

• Access Point (AP): An STA that provides access to the DS via

the wireless media (WM) DS DS

May 18, 2003

• 26 -

Ad Ad-

• Hoc Networks

Hoc Networks

• Collection of stations

without central coordination (no AP)

• Usually created for

temporary networks

• Also referred to as an

Independent Basic Service Set (IBSS)

• Can be routed to a wired

network

• May 18, 2003
• 27 -

Infrastructure Networks Infrastructure Networks

• Collection of stations

with central coordination

• One or more AP
• If more than 1 AP, a

portal may be needed

• APs combine to cover

large areas.

May 18, 2003

• 28 -
• Freq. Hopping Spread Spectrum
• Freq. Hopping Spread Spectrum
• Uses 79 separate 1 MHz channels from 2.402-2.480 GHz
• Hops about every 0.1 sec (22 hop pattern, 2.5 hop/sec

minimum in US)

• Immune to single frequency noise, has trouble with wideband

noise

• Many networks can be located in the same area
• Uses less power to transmit & less expensive to build than

DSSS

8

May 18, 2003

• 29 -

Direct Direct-

• Sequence Spread Spectrum

Sequence Spread Spectrum

• Signal modulated with a spreading code (11-bit Barker Sequence)
• All 802.11b compliant products use the same spreading code
• Higher data rates because of “fatter pipe” (about 11 MHz)
• Allows for some single frequency noise & higher wideband noise
• Only allows for 3 networks in same area
• Uses higher power to transmit & more expensive to build than

FHSS

May 18, 2003

• 30 -

Accessing the medium CSMA/CA Accessing the medium CSMA/CA

• Wireless LAN adapters cannot detect collisions:
• Carrier Sensing
• listen to the media to determine if it is free
• Collision Avoidance
• minimize chance for collision by

starting (random)

• back
• ff timer, when medium is sensed free, and prior to

transmission

May 18, 2003

• 31 -

CSMA/CA with MAC - level Acknowledgment - ARQ

• Collisions still can occur (interference; incapability of

sensing other carrier)

• IEEE 802.11 defines “low
• level” ACK protocol
• Provides faster error recovery
• Makes presence of high level error recovery less critical

May 18, 2003

• 32 -

“Hidden stations” - the problem

• Situation that occurs in larger cells
• Loss of performance
• Error recovery required

9

May 18, 2003

• 33 -

“Hidden stations” - the solution

• MAC level RTS/CTS protocol (Request to Send / Clear to Send)
• Can be switched off to reduce overhead (when no hidden nodes

exist)

• More robustness, and increased reliability
• No interruptions when large files are transmitted

May 18, 2003

• 34 -

Interframe Interframe space (IFS) space (IFS)

• SIFS : used by ACK, CTS, poll response
• PIFS : used by PC when issuing polls
• DIFS : used by ordinary asynchronous traffic

May 18, 2003

• 35 -

Additional 802.11 topics Additional 802.11 topics

• PCF – Point coordination function
• Time synchronization
• Power management
• Fragmentation
• WEP – Wired Equivalent Privacy
• 802.11e - QoS

April 2003

MANET MANET – – Mobile Ad Mobile Ad-

• hoc

hoc NETwork NETwork

10

May 18, 2003

• 37 -

MANET MANET

• Multi-hop Ad-hoc network
• STAs are used as gateways
• APs may be included for LAN connectivity
• Direct wireless connection is not required
• Scaleable solution with low or no infrastructure

cost

• Standardization: IETF MANET workgroup

May 18, 2003

• 38 -

Routing problems in MANET Routing problems in MANET

• Dynamic network

– Mobile stations – Users hosts serve as gateways. Gateway may be shutdown on any moment – Environment is dynamic: Fading, obstacles

• Links are not independent
• Adaptive PHY
• Battery considerations
• Limited knowledge

April 2003

Broadband Fixed Wireless Access Broadband Fixed Wireless Access 802.16 802.16

May 18, 2003

• 40 -

Fixed Wireless Access Fixed Wireless Access

• A last mile solution competitive to xDSL and Cable

technologies

– Range up to 30miles in a super-cell.

• Advantages

– Avoid distance limitation of DSL – Rapid deployment – High scalability – Granular investment to match market growth – Suitable for isolated and low density communities

• Technology challenges

– Spectrum efficiency – Self installable customer premises equipment (CPE) antenna – Non-line-of-sight (NLOS)

11

May 18, 2003

• 41 -

802.16 is based on DOCSIS & ATM 802.16 is based on DOCSIS & ATM

• Taken from DOCSIS

– Management

• Dynamic service “editing” protocol (Add/Change/Delete)
• Management message payload format

– Security: authentication and privacy – Polling categories – Initial Access

• Slightly modified allowing terminal capability negotiation
• ATM Convergence

– Support for virtual path (VP) and virtual channel (VC) switched connections – Support of end-to-end signaling of dynamically created connections: SVCs, soft PVCs. – ATM header suppression – Full QoS support

May 18, 2003

• 42 -

802.16 MAC Overview (1) 802.16 MAC Overview (1)

• Connection-oriented
• Supports difficult user environments

– High bandwidth, hundreds of users per channel – Continuous and burst traffic – Efficient use of spectrum

• Protocol-Independent core (ATM, IP, Ethernet, …)
• Balances between stability of contentionless and

efficiency of contention-based operation

• Fragmentation
• Packing (concatenation)

– Can save up to 10% of the bandwidth

May 18, 2003

• 43 -

MAC overview (2) MAC overview (2)

• DOCSIS management:

– Dynamic services protocol – Management message format

• DOCSIS security: authentication & privacy
• DOCSIS initial access – modified for capability negotiation
• ATM convergence:

– Support for Virtual Path (VP) and Virtual channel (VC) switched connections – Support for end-to-end signaling for dynamic services: SVCs and soft PVCs

• ATM header suppression
• ATM QoS
• Supports multiple 802.16 PHYs (TDD & FDD)

April 2003

TCP/IP and Wireless Access TCP/IP and Wireless Access

12

May 18, 2003

• 45 -

Problem overview Problem overview

• Wireless network may produce

– Packet loss due to BER – Delay due to interleaving, ARQ

• TCP congestion detection

– Timeout indicates packet or ack was lost – Duplicate acks may indicate packet reordering

• Acks up through last successful in-order packet received
• Called a “cumulative” ack
• After three duplicate acks, assume packet loss, not

reordering

• Receipt of duplicate acks means some data is still

flowing

May 18, 2003

• 46 -

Response to congestion Response to congestion

• Basic timeout and retransmission

– If sender receives no ack for data sent, timeout and retransmit – Exponential back-off – Timeout value is sum of smoothed RT delay and 4 X mean deviation – (Timeout value based on mean and variance of RTT)

• Congestion “avoidance” (really congestion control)

– Uses congestion window (cwnd) for more flow control – Cwnd set to 1/2 of its value when congestion loss occurred – Sender can send up to minimum of advertised window and cwnd – Use additive increase of cwnd (at most 1 segment each RT) – Careful way to approach limit of network

May 18, 2003

• 47 -

Responses to congestion (2) Responses to congestion (2)

• Slow start – used to initiate a connection

– In slow start, set cwnd to 1 segment – With each ack, increase cwnd by a segment (exponential increase) – Aggressive way of building up bandwidth for flow – Also do this after a timeout – aggressive drop in offered load – Switch to regular congestion control once cwnd is one half of what it was when congestion occurred

• Fast retransmit and fast recovery

– After three duplicate acks, assume packet loss, data still flowing – Sender resends missing segment – Set cwnd to ½ of current cwnd plus 3 segments – For each duplicate ack, increment cwnd by 1 (keep flow going) – When new data acked, do regular congestion avoidance

May 18, 2003

• 48 -

Interaction with TCP Interaction with TCP

• Packet loss due to noise or hand
• ffs

– Enter congestion control – Slow increase of cwnd

• Bursts of packet loss and hand
• ffs

– Timeout – Enter slow start (very painful!)

• Cumulative ack scheme not good with bursty losses

– Missing data detected one segment at a time – Duplicate acks take a while to cause retransmission – TCP Reno may suffer coarse time-out and enter slow start!

• Partial ack still causes it to leave fast recovery

– TCP New Reno still only retransmits one packet per RTT

• Stay in fast recovery until all losses acked

13

May 18, 2003

• 49 -

Multiple losses in window Multiple losses in window

• Assume cwnd of 10
• 2nd and 5th packets lost
• 3rd duplicate ack causes retransmit of

2nd packet

• Also sets cwnd to 5 + 3 = 8
• Further duplicate acks increment

cwnd by 1

• Ack of retransmit is partial ack since

packet 5 lost

• In TCP Reno this causes us to leave

fast retransmit

• Deflate congestion window to 5, but

we’ve sent 11!

ack1 ack1 ack1 ack1 1 6 7 9 10 ack4 2 ack4 ack4 2 3 4 5 8 ack1 11

Cwnd=8 Cwnd=9 Cwnd=5 Cwnd=10