last time–take home points -...

Last Time–Take Home Points

• Class Mechanics, Project, Overview

• What is Mobile Computing and Why now?

• Challenges of Mobile Computing

– Some that come from wireless networks– Some that come from mobility– Some that come from the devices/portability

• Challenges for networking: wireless propagation; frequency reuse;interference; mobility...

SUNY-Binghamton – CS527 Spring ’07 Lec. #2 1

Today

• Quick discussion of overview papers

– All but the PCS paper are general– Different perspectives on the challenges of mobile computing– Different types of people: systems people (Forman) vs. distributed

computing (Satya) vs databases (Dunham)– Different visions of the “mobile world”∗ Personal Communication Devices: think cell. phone∗ Mobile Computing: Laptops, PDAs...mobile data networks∗ Ubiquitous Computing: Pervasive computing; smart world

• Move on to Wireless Propagation and Wireless Network Organizations


Satyanarayanan’s Paper

• “What is fundamentally different about mobile computing?”

• Lists some mobility constraints; familiar to us by now

– Low resources– “Mobility is inherently hazardous”– Low and variable bandwidth– Limited battery power

• Very strong statement: “Constraints are not artifacts of currenttechnology but are intrinsic to mobility”


Satya’s Paper (cont’d)

• Flaky links – can’t be too dependent on the network – be autonomous

• Low resources – can’t be too autonomous

• He argues that it is a dynamic environment and mobile applications mustbe adaptable

– Who should adapt?∗ Application?∗ System?∗ Both?

– No specific examples, just a general layering argument


Extended Client-Server Model

• Classic scalable client-server model

– Core of trusted servers as home of data– Access to data from many client sites (untrusted)– Optimizations: caching, replication, read-ahead ...

• Extended model (Why?)

– The mobile client takes on some of the server duties (to compensatefor the intermittent links)

– Server may be asked to do some of the tasks usually done by the client(to reduce the resource demand)

• “Mobile Computing exacerbates the tension between autonomy andinterdependence that is characteristic of distributed computing”


Satya’s Paper (cont’d)

• Laundry list of accomplishments within the Coda and Odyssey projects– will delay discussion until we read more technical papers

• Second half of the paper discusses some open problems

– Not a comprehensive set– Criteria for selection: real and important problems requiring rigorous

solutions

• Will delay discussion of both until we get to Mobile Computing later inthe semester.


Mobile Computing and Databases – Anything New?

• Paper by Margaret Dunham and Sumi Helal (circa 1995 or so)

• They ask the question: are there really new database problems that areintroduced in a mobile environment?

• Specifically, are there issues beyond those that arise in distributedenvironments

• Conclusion: Many issues are the same, but the appropriate solutions aredifferent

• Short paper: Four motivating examples, then a quick discussion of“whats new”

• Assume a last hop wireless network


Example 1 – Location Dependent Queries

• Scenario: driving down the highway on vacation in Florida: need to finda restaurant, or a motel with vacancy (or maybe a doctor?)

– Queries such as: where is the closest hotel with a pool?– How do I get to the closest hospital?

• Location dependent queries

– Same question may have different answers depending on location (e.g.,where is the closest public restroom?)

• What are the research issues in this example?


Research Issues

• Mobile/Location-sensitive query languages (e.g., extensions to SQL?What would they look like?)

• Mobile Database design, especially issues related to automaticmaintenance of metadata that maps data sets to locations

• Efficient yellowpage archiving that will allow data to be dynamicallyregistered and archived

• Modeling Space?


Example 2: Travelling Salesman

• Scenario: Salesman would like to check PDA on the way over to clientto get their information (maybe including personal stuff that will helpyou sell)

• In this example, focus on caching issues: generate queries to a centralizedserver or work off of cached copy?

– Depending on state of the mobile (connectivity, battery power), thequery will be executed differently (against cache, or against server)


Research Issues

• How to specify the degree of consistency between the cache and theserver copy? (same discussion as Satya’s paper)

• Issues in data migration (essentially, the mobile node taking ownershipof the data). Do you agree that this is beneficial? What if another nodeneeds it?


Example 4: Insurance Adjuster Providing

Repair Estimate

• Insurance adjuster must physically examinedamaged car, and provide cost estimate for repairs

• Needs access to: (1) information about the car;(2) police reports; (3) driver’s insurance record; (4)bluebook value; and (5) physical examanation ofcar

• Traditional method (circa 1995):

1. Receive request for examanation2. Search for information from websites (bluebook

value), phone calls (insuree information), orsometimes need in person interviews (policerecords)

3. Go see the car4. File report


New Model

• Adjuster starts the transaction, which fires off subtransactions to collectthe data

– She may decide to view the data at any time– Data is getting collected automatically while she is driving to see the

car– She enters the data from the physical examanation– Claim processing is done by the time she leaves

• More efficient processing (time), and no paperwork

• But the transaction duration is now much longer


Research Issues

• Transaction is extremely long lived (hours) – must stay active even ifuser is disconnected

• Complicates transaction processing and recovery

– An asynchronous transaction model is needed– Must discriminate between voluntary/normal disconnections and

failures


So, Anything New?

• Discussion of relationship to distributed systems/databases

– One difference: Location awareness (mobile) vs. Location transparency(distributed) – example 1

– Another: Transaction Processing– Many issues are the same but appropriate solutions can be much

different because of cost/performance aspects

• Other?


Mobile Data Management Issues –

Summary

• Applications: may be location dependent and needto adapt to changes in context

• Transactions: need models to capture mobility

• Recovery:

– frequent disconnection– voluntary shutdown not a failure– transaction models that are resilient to hand-off

disconnections

• Replication:

– Different consistency constraints– New techniques for mobile caching needed

• Query Processing

– Location dependent– Costs are different

• Name Resolution: New global name strategy toaccount for mobility and disconnect


Weiser’s Monolog

• A profound technology is one that disappears

– Paper is peppered with several examples including writing and electricmotor;

• What stage is the computer at?

• Is this paper arguing for better interfaces?

– The computer on star trek movies?

• Why are we reading this paper? What does it have to do with mobilecomputing?


Ubiquitous Computing

• Paper describes initial experiences at Xerox Parc

– Tabs, Pads and boards: different in scale and intent∗ Tabs on everything, highly specific∗ Pads are like a scratch pad...a computer that is available in the

room when you need it∗ Boards for interaction

– All networked ubiquitously: combination of wired and wireless


Research Issues – A great many!

• Interfaces

– Human/Computer Interaction that is natural

• Context Awareness

• Software

– Spontaneous Interoperability...– ...Balanced Against Security and Privacy– Discovery, Interaction, Adaptation


Next Up

• Networking, bottom up

• Start with wireless links/communication

• Overview wireless network organizations


Brief History of Wireless Communication

(Part 1)

• B.C. – Use of light for “wireless communication”

• 1794 – Chappe invented the optical telegraph

• 1831 – Faraday and Henry discover electromagnetic waves and

demonstrate induction

• 1861 – Reis invented the telephone principle (Alexander Bell

commercialized it in 1876)

• 1864 – Maxwell characterizes waves completely (Maxwell’s

Equations)

• 1880s – Hertz experimentally demonstrates wave propagation

• 1895 – Marconi demonstrates wireless telegraph

– He is often credited as the “inventor of wireless comm.”

– Long wave transmission – 20kW transmission power

– 1901 – First transatlantic wireless transmission followed

• 1906 – First Radio broadcast

• 1907 – Commercial transatlantic wireless service (huge towers

with lots of transmission power)

– 300ft antennae with huge power

• 1920 – First commercial Radio Station starts operation in

Pittsburgh


Brief History of Wireless Communication

(Part 2)

• 1910s–20s “mobile” transmitters and receivers start to appear

after the invention of vacuum tubes (1906)

• 1920 – Marconi discovers short waves

– short waves bounce off the ionosphere

• 1928 – Baird experiments with TV broadcasting

– Color TV transmitted; First TV station (WGY in NY)

• 1933 – Frequency modulation invented; dramatically improves

quality of transmission

• 1946 – AT&T develops cellular concepts, introduces mobile

telephone service

– line of sight analog FM transmission; 50 miles from base

– 120kHz per voice channel

• 1950s and 60s – German A-Netz and B-Netz wireless phone

network; other European projects after that

– No support for roaming

• Mid-60s, AT&T Improved Mobile Telephone Service (IMTS)

• 1979 – Advanced Mobile Phone System (AMPS) standard;

started in the US in 1983


More Recent History

• 1980s, total 6 incompatible analog cellular systems in Europe;

motivate 2nd generation

• Early 1990s – Fully Digital Systems

– DECT in Europe

– GSM in Europe; JDC/PDC in Japan

• In the US, several competing and incompatible technologies

– AMPS (analog)

– IS-136 (D-AMPS, TDMA based digital) – AT&T, Cingular

(acquired from Bell South and others)

– IS-95 (CDMA based digital) – Verizon, Sprint PCS

– IDen (TDMA based) – NexTel

– GSM1900 – Cingular

• 1996: AT&T and Sprint offer PCS

– Cellular vs. PCS; macro-cell vs. micro-cell

– Smaller/lighter phones, cheaper access points


More recent history

• Data communication systems (WLANs)

– 1996/1997 HIPERLAN standard from Europe

– 1997– IEEE 802.11 appears to be the winner

– More recently: Newer 802.11’s, Bluetooth, Ultra Wide

Band, WiMAX, ...

• 1998 – Motorola’s Iridium Sattelite network finally operational

• 1998 – proposals for 3G cellular start

– Emerging technologies, all CDMA based that combine

cellular telephony with IP/data networking

– Mid-2000s: cdma2000, UMTS, GPRS, WCDMA, and

EDGE

• 2010+: 4th generation? What does it look like?

• Mobile Ad hoc networks (MANETs), Vehicular (VANETs),

Mesh Networks...


Wireless Personal Communication – What is it?

• Why read this paper?

– Overview of wireless personal communication technologies as of 1995– A bridge towards starting the discussion of our first real topic

• Authors (correctly) note that among the wireless PCS hype, people aretalking about different things

– Paper provides a survey of the technologies, and makes informedprojections about future directions

– Much of this stuff is outdated; we will not discuss closely


What is out there?

• Driver: Mobility in Communications

– Freedom from wires is good – look at cellularphones and cordless phones; amazing growth

• Pdersonal Communication: What is it?

– Cellular telephony– Cordless phones– Paging and messages– Wide Area mobile data networks (data over

cellular)– Wireless local area networks

• Voice/Data dichotomy (familiar to us in the wirednetwork arena)

– Packet oriented vs. circuit oriented service– Attempts to integrate them using the same

infrastructure (why?)


Other Service Differentiators

• Portability and Mobility

– Within buildings (cordless phone, WLANs)– Within campus, city (cellular radio, WLANs, wide-area)– Throughout a country or a state (cellular, Wide Area wireless, paging,

sattelite)– World-wide (cellular? Sattelite?)

• Multimedia communication

– Two way voice; data; messaging; video? ...


Summary/Discussion

• Any commercial service is driven by user demands balanced againsttechnological restrictions

– User demand is for portability and mobility– User demand for different types of services– Commercial race: who can supply the best service at the most

competitive price∗ ...and using what technology?


System Evolution – Cordless Phones

• Accepted Compromises (Assumptions)

– Few users per MHz– Few users per BS (1 to 1?)– Large number of BS’s per area– Short transmission range

• Common characteristics

– 32kbps speech encoding– Small xmit power (10mWatt)– Cheap low complexity DSP– Low transmission delay– Dynamic channel allocation– Time Division Duplex


System Evolution – Cellular Phones

• Accepted Compromises

– Max users per MHz– Max users per cell site– High transmitter power– High user-set complexity– Low circuit (voice) quality– High network complexity

• Common Characteristics

– Low bit-rate speech coding (8-13kbps)– Makes use of speech inactivity– High transmission delay (200ms)– High compelxity DSP– Fixed channel allocation– Frequency Division Duplex– Power control


System Evolution – Wide Area Data Networks

• Accepted Compromises

– High Mobility (vehicular/pedestrian)– Low data rate


Wireless Local Area Networks

• Low mobility high speed data communication

• Operates within confined regions

• Two Alternative Organizations

– Centralized Control/Base Station arbitrates medium– Self Organization (or Ad hoc networks)

• Currently operate in the ISM bands (3 bands, 902–928MHz, 2400–2483MHz, and 5725–5850MHz)

• Data PCS (1910–1930 MHz)

• There are rules governing usage in each band (etiquette in the case ofPCS)


Evolution – Other Technologies

• Paging/Messaging

– Alphanumerical pagers– 1-way and 2-way paging– Local/Regional/nationwide systems

• Sattelite-based Mobile systems

– Wide area, expensive infrastructure– Large regional coverage but outside buildings– Low capacity cells– Large delays– LEOs/MEOs/GEOs


The Future (or present?)

• Grand unification of the technologies (why?)

– Universal Mobile Telecom. Systems (UMTS): universal access devicethat accesses a single network

• Low-tier vs. high-tier PCS

– Authors argue for low-tier from a capacity/cost perspective


How out of date is this paper?

• March 1995 auctions

– Three main players (PCS, Sprint Telecom, AT&T)

• Many of the projected trends are happening

• IEEE 802.11 and other standards for WLANs.

• Hardware is significantly better

• Battery life improves much more slowly (2x every 5–10 years) thanprocessor appetite (doubles every 18 months)

• Mobile IP standard

• ad hoc networks; sensor networks; ubiquitous computing; augmentedreality; ...


On to our first real topic – Physical Layer

• Wireless Propagation and Medium Access Overview

– We do not need to know the details of theelectrical engineering

– But, any good researcher/engineer has to knowthe characteristics of the underlying physicalstructure

– Great tutorial by Randy Katz

• Then we will move on to the Medium AccessProblem

– In cellular: channel assignment, call blocking,variable rate

– Distributed Medium Access (Busy Tone, DualBusy Tone, CSMA, MACA/MACAW, Clustering)

– Protocols: IEEE 802.11, Bluetooth, HomeRF,HiperLAN


Modulation

• Sinusoidal waves are the least bandwidth demanding

• Sinusoidal wave represented as: A · cos(ωt + φ)

– A is the amplitude, ω is the frequency, and φ isthe phase

• Idea: Use modulation to encode data by changingone of the three parameters of a carrier sinusoidalsignal

• Picture above: Amplitude Modulation, sameprinciple used AM radio

– 1, amplitude is A; 0, amplitude is 0


Modulation (cont’d)

• Frequency modulation: change ω; e.g., transmit 495KHz sinusoid if 0,505KHz if 1

• How can multiple FM stations share the same “airwaves”? What doesyour radio receiver do?


Phase Shift Modulation

• Phase shift modulation: change φ

– PSM is widely used; usually multiple bits areencoded together (e.g., QPSK, 2-bits togetherwith 90 degree shifts in phase)

• Which modulation technique to use?


Modulation

• Hybrid techniques:

– QAM (Quadrature Amplitude Modulation):change phase and amplitude

– 4 phases, 2 amplitude levels can encode 8 values

• Shannon’s Limit still rules

• More aggressive encoding means less noise immunity

– Important idea: change the encoding rate basedon the channel conditions. E.g., Satteliteencoding schemes usually less aggressive thanwireline encoding

• Additional reliability can be achieved using ForwardError Correction (sometimes at the physical layer –Viterbi’s convolution codes) or ARQ

– Again, apply error control mechanisms to meetthe channel demands


last time–take home points -...

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