Mobile Database Systems

Mobile Database Systems

Vijay Kumar

Computer Sc. Telecommunications

University of Missouri-Kansas City

5100 Rockhill Road

Kansas City, MO 64110, USA kumar@cstp.umkc.edu


















Outline

Fully Connected Information Space

Personal Communication System (PCS)

Mobile Database Systems (MDS)

Transaction Management

Data Caching

Query Processing

Data Classification

Conclusion


Fully connected information space



 Each node of the information space has some communication capability.

 Some node can process information.

 Some node can communicate through voice channel.

 Some node can do both



Can be created and maintained by integrating legacy database systems, and wired and wireless systems (PCS, Cellular system, and GSM)


What is a Mobile Database System (MDS)?

A system with the following structural and functional properties

 Distributed system with mobile connectivity

 Full database system capability

 Complete spatial mobility

 Built on PCS/GSM platform

 Wireless and wired communication capability


What is a mobile connectivity?

A mode in which a client or a server can establish communication with each other whenever needed.

Intermittent connectivity is a special case of mobile connectivity.


What is intermittent connectivity?

A node in which only the client can establish communication whenever needed with the server but the server cannot do so.


Part 1









Architecture

Wireless communication

Bandwidth limitations

Frequency reuse

Personal Communication System

(PCS)

A system where wired and wireless networks are integrated for establishing communication.

PSTN

AC HLR

VLR

EIR

MSC (MTSO) MSC (MTSO)

MS BS MS

Wireless component

PSTN: Public Switched Network.

MSC: Mobile Switching Center. Also called MTSO

(Mobile Telephone Switching Office).

BS: Base Station.

MS: Mobile Station. Also called MU (Mobile Unit) or Mobile Host (MH).

HLR: Home Location Register.

VLR: Visitor Location Register.

EIR: Equipment Identify Register.

AC: Access Chanel.


(PCS)

PCS refers to variety of wireless access (communication) and personal mobility services provided through a small terminal at any place, and in any form. Business opportunities (E-commerce) for such services are tremendous, since every person, every organization, etc., could be equipped. Several PCS systems have been developed to meet rapid growth prompted by market demand.

Most of them are connected to Public Switched

Telephone Network (PSTN) to integrate with the wired service.

Two of the most popular PCS systems are:

 Cellular telephony

 Cordless and low-tier PCS telephony


(PCS)

Cellular telephony overview

Four popular cellular telephony networks are :

 Advanced Mobile Phone Service (AMPS)

 Global System for Mobile Communication (GSM)

 EIA/TIA IS-136 Digital Cellular System

 EIA/TIA IS-95 Digital Cellular System


(PCS)


Advanced Mobile Phone Service (AMPS)

AMPS was the first cellular system, which was developed during the 1970s by Bell Lab. From 1974 to 1978, a large scale

AMPS trial was conducted in Chicago.

Commercial AMPS service has been available since 1983.

It is based on frequency division multiple access (FDMA), AMP was designed as a high capacity system based on a frequency reuse scheme. A total of 50 MHz in the 824-849 MHz and 869-

894 MHz bands is allocated for AMPS.This spectrum is divided into 832 full-duplex channels using 1664 discrete frequencies, that is, 832 downlinks and 832 uplinks. In AMPS, the typical frequency reuse plan employs either a 12-group frequency cluster using omnidirectional antennas or a 7-group cluster using three sectors per base stations. Thus, there are about

50 channels per cell.


(PCS)


Global System for Mobile Communication (GSM)

GSM is a digital cellular system developed by Groupe Special

Mobile of Conference Europeenne des Postes et

Telecommunications (CEPT) and its successor European

Telecommunications Standard Institute (ETSI).

GSM combines time divisioin multiple access (TDMA) and FDMA.

With TDMA, the radio hardware in the base station can be shared among multiple users. In GSM the frequency carrier is divided into eight time slots where the speech coding rate is

13 Kbps.

In a GSM base station, every pair of radio transceiver-receiver supports eight voice channels, whereas an AMPS base station needs one such pair for every voice channel. The GSM development process was similar to that of

AMPS, except that no large scale trial was conducted.


(PCS)


EIA/TIA IS-136 Digital Cellular System

This system is also referred to as digital AMPS (DAMPS),

American Digital Cellular (ADC), or North American TDMA

(NA-TDMA), IS-136, the successor to IS-54, supports a TDMA air interface similar to that of GSM. IS-54 was renamed IS-136 when it reached revision C. It supports three voice channels, where the speech coding rate is 7.95 Kbps. IS-136 capacity is around three times that of AMPS. An existing AMPS system can be easily upgraded to IS-136 0n a circuit-by-circuit basis.


(PCS)


EIA/TIA IS-95 Digital Cellular System

This digital cellular system was developed by Qualcomm, and has been operating in USA since 1996.

IS-95 is based on

Code Division Multiple Access (CDMA) technology. It allows many users to share a common frequency/time channel for transmission. The channel bandwidth used by IS-95 is 1.25

MHz, which has been extended to 5 MHz in the third generation wideband CDMA proposal. The speech coding rate for IS-95 is 13 Kbps or 8 Kbps. IS-95 ’s capacity is estimated to be 10 times that of AMPS.


(PCS)

Cordless telephony technologies

Cordless Telephone, Second Generation (CT2)

Developed in Europe, and has been available since 1989. CT2 is allocated 40 FDMA channels with a 32-Kbps speech coding rate. For a user, both baseptop handset signals and handsetto-base signals are transmitted in the same frequency.

The maximum transmit power of a CT2 handset is 10 mW. In the call setup procedure, CT2 moves a call path from one radio channel to another after three seconds of handshake failure.

CT2 also supports data transmission rates of up to 2.4 Kbps through the speech code and up to 4.8 Kbps with an increased rate. CT2 does not support handoff and in a public

CT2 system, call delivery is not supported.


(PCS)

Cordless telephony technologies

Digital European Cordless Telephone (DECT)

The Digital European Cordless Telephone has been replaced by Digital Enhanced Cordless Telephone to denote global acceptance of DECT.

DECT supports high user density with a picocell design. There are 12 voice channels per frequency carrier.

Sleep mode is employed to converse handset power.

DECT also supports seamless handoff.

DECT is typically implemented as a wireless-PBX (Private Brach

Exchange) connected to PSTN.

DECT can interwork with GSM to allow user mobility.


(PCS)

Low-tier PCS telephony overview

Personal Handy Phone System (PHS)

PHS is a standard developed by the Research and

Development Center for Radio Systems (RCR), a private standardization organization in Japan.

PHS is a low-tier digital PCS system that offers telecommunication services for homes, offices, and outdoor environment, using radio access to the public telephone network or other digital networks.

PHS uses TDMA.

Sleep mode enables PHS to support five hours of talk time, or 150 hours of standby time.

PHS operates in the 1895-1918.1 MHz band.

The bandwidth is partitioned into 77 channels, each with 300 KHz bandwidth.

The band 1906.1-1918.1 MHz (40 channels) is designed for public systems, and the band 1895-1906.1 MHz (37 channels) is used for home/office applications.


(PCS)

Low-tier PCS telephony overview

Personal Access Communications Systems (PACS)

PACS is a low-power PCS system developed at

Telcordia (formerly Bellcore). TDMA is used in PACS with eight voice channels per frequency carrier. In FDD mode, the PACS uplink and downlink utilizes different

RF carriers, similar to cellular systems.


(PCS)

Cordless and low-tier PCS telephony overview

System

Cell size

High-tier Cellular Low-tier PCS Cordless

Large (0.4-22 mile) Medium (30300’) Small (3060’)

User speed High (



160 mph)

Coverage area Large/Continuous macrocell

Handset complexity

High

Medium (



60 mph)

Medium. Micro and picocell

Low

H-set power use High (100-800 mW) Low (5-10 mW)

Low (



30 mph)

Small/Zonal, picocell

Low

Low (5-10 mW)

Speech coding rate

Low (8-13 Kbps)

Delay or latency High (



600 ms)

High (32 Kpbs)

Low (



10 ms)

High (32 Kpbs)

Low (



20 ms)


(PCS)

Wireless Components

Base Station (BS): A network element that interconnects the mobile station (or Mobile unit (MU)) to the network via the air interface. Each cell in the network has a BS associated with it.

The primary function of a BS is to maintain the air interface, or medium, for communication to any mobile unit within its cell.

Other functions of BS are call processing, signaling, maintenance, and diagnostics.

The BS communicates to its mobile unit via the air interface, and to

MTSO by dedicated communication link such as T1 trunks.

Communication links on the BS to the MTSO interface are also classified into voice links and signaling link.


(PCS)


Mobile Units (MU): Also called Mobile Systems (MS) or

Mobile Hosts (MH). It consists of three components: (a) transceiver , (b) antenna , and (c) user interface .

The user interface exists only at MU, which consists of a display, a keypad for entering information, and an audio interface for speaking and hearing voice conversation.

This can be a laptop, a palmtop, or a cell phone, or any other mobile device.

A MU also stores (a) Mobile Identification Number

(MIN), (b) Electronic Serial Number (EIN), and (C) Station

Class Mark (SCM) .

These are transmitted upon power on, cell initiated sampling, and cell origination.


(PCS)


MSC (MTSO)

MS

BS

Cell

MS

Wireless component


(PCS)

Wireless channels are limited

Item

Mobile

Phones

Europe (MHz)

NMT : 453-457, 463-467

GSM : 890-915, 935-960,

1710-1785, 1805-1880

Cordless

Phones

CT1+ : 885-887, 930-932

CT2 : 864-868

DECT : 1880-1900

US (MHz)

AMPS, TDMA, CDMA

824-849, 869-894

GSM, TDMA, CDMA

1850-1910, 1930-1990

PACS

1850-1910,1930-1990;

PACS-UB : 1910-1930

Japan (MHz)

PDC : 810-826

940-956,

1429-1465,

1477-1513.

PHS

1895-1918;

JCT : 254-380

NMT:

PDC:

PACS:

PHS:

Nordic Mobile Telephone

Pacific Digital Cellular

Personal Access Communications System

Personal Handyphone System

PACS-UB: PACS Unlicensed Band

JCT: Japanese Cordless Telephone

(Taken from Mobile Communications by Jochen Schiller)


Limited channels must be utilized efficiently. It is done so by (a) Frequency reuse and (b) Mobile cell

Frequency reuse

The goal of every mobile service provider is to manage as many simultaneous calls as possible.

In USA each cellular provider is allocated 25 MHz of spectrum, 12.5

MHz for transmitting (downstream) and 12.5 MHz for receiving (upstream). Cellular system is duplex because transmitting and receiving are allocated their own frequencies.

A person on a mobile call only needs the allocated frequency of the cell, thus there is no reason somebody else on the other end of the town cannot be using the same frequency in a different cell. The concept of multiple users using the same frequency at the same time for communication is called frequency reuse.


Frequency reuse (continued)

For frequency reuse to work correctly it is imperative that each base station has just sufficient power to reach its cell boundary. If it puts out too much power, then it will not only reach the intended cell site, it will reach unintended cell sites, which others may be using at the same frequency for a totally different conversation. This limitation on transmitted power, however, is also an advantage because the cellular phone’s battery will last longer.


Mobile cell

Within the cellular allocation the USA is divided into

Metropolitan Statistical Areas (MSAs) and Rural Statistical Areas

(RSAs) .

There are six PCS service providers authorized to provide mobile service in each of these areas.

Within their geographical region, each service provider divides their area into smaller segments called cells. Each of this cell has a Base

Station.

Ideally, the system has a large number of very small hexagons (cell). The greater the number of hexagons, the more simultaneous calls the system can handle.

However, larger number of hexagons increases the cost of implementation.

Thus, cell coverage is a dynamic activity, which is constantly changing in response to increases in demand.


(PCS)

Mobile cells

Metropolitan area Metropolitan area

Base Station

Coverage area in one cell

BS

BS BS

Coverage area in three cells

Large cells.

Low density

Small cells.

High density

Smaller cells.

Higher density


(PCS)

Mobile cells

The entire coverage area is a group of a number of cells. The size of cell depends upon the power of the base stations.

MSC PST N


(PCS)

Frequency reuse

A

A

2

2

7

6

1

5

3

4

7

6

1

5

3

4

D

7

6

2

1

5

3

4

A

A

A

A

A

D



R

3 N

D = distance between cells using the same frequency

R = cell radius

N = reuse pattern (the cluster size, which is 7).

Thus, for a 7-cell group with cell radius R = 3 miles, the frequency reuse distance D is 13.74 miles.


Problems with cellular structure

 How to maintain continuous communication between two parties in the presence of mobility?

Solution: Handoff

 How to maintain continuous communication between two parties in the presence of mobility?

Solution: Roaming

 How to locate of a mobile unit in the entire coverage area?

Solution: Location management


(PCS)

Handoff

A process, which allows users to remain in touch, even while breaking the connection with one BS and establishing connection with another BS.

MSC

MSC

New BS

Old BS

New BS Old BS

MSC

MSC

New BS

Old BS New BS Old BS


(PCS)

Handoff

To keep the conversation going, the Handoff procedure should be completed while the MS (the bus) is in the overlap region .

Cell overlap region

G

Old BS New BS


(PCS)

Handoff issues

 Handoff detection

 Channel assignment

 Radio link transfer


(PCS)

Handoff detection strategies







Mobile-Controlled handoff (MCHO)

Network-Controlled handoff (NCHO)

Mobile-Assisted handoff (MAHO)


(PCS)

Mobile-Controlled Handoff (MCHO)

In this strategy, the MS continuously monitors the radio signal strength and quality of the surrounding

BSs. When predefined criteria are met, then the MS checks for the best candidate BS for an available traffic channel and requests the handoff to occur.

MACHO is used in DECT and PACS.


(PCS)

Network-Controlled

Handoff

(NCHO)

In this strategy, the surrounding BSs, the MSC or both monitor the radio signal.

When the signal’s strength and quality deteriorate below a predefined threshold, the network arranges for a handoff to another channel. NCHO is used in CT-2 Plus and

AMPS.


(PCS)

Mobile-Assisted Handoff (MAHO)

It is a variant of NCHO strategy. In this strategy, the network directs the MS to measure the signal from the surrounding BSs and to report those measurements back to the network. The network then uses these measurements to determine where a handoff is required and to which channel.

MACHO is used in GSM and IS-95 CDMA.


(PCS)

Handoff types with reference to the network

 Intra-system handoff or Inter-BS handoff

The new and the old BSs are connected to the same MSC.

MSC

Old BS New BS


(PCS)

Intra-system handoff or Inter-BS handoff

Steps

1.

The MU (MS) momentarily suspends conversation and initiates the handoff procedure by signaling on an idle (currently free) channel in the new BS. Then it resumes the conversation on the old BS.

M SC

Old BS New BS


(PCS)


2.

Upon receipt of the signal, the MSC transfers the encryption information to the selected idle channel of the new BS and sets up the new conversation path to the MS through that channel. The switch bridges the new path with the old path and informs the MS to transfer from the old channel to the new channel.

MSC

Old BS New BS


(PCS)


3.

After the MS has been transferred to the new BS, it signals the network and resumes conversation using the new channel.

MSC

Old BS New BS


(PCS)


4.

Upon the receipt of the handoff completion signal, the network removes the bridge from the path and releases resources associated with the old channel.

MSC

Old BS New BS


(PCS)

Handoff types with reference to the network

 Intersystem handoff or Inter-MSC handoff

The new and the old BSs are connected to different MSCs.

PSTN

BS1 BS2

BS1 BS2

MSC A

Trunk

MSC B MSC A

Trunk

MSC B


(PCS)

Handoff types with reference to link transfer

 Hard handoff

The MS connects with only one BS at a time, and there is usually some interruption in the conversation during the link transition.

 Soft handoff

The two BSs are briefly simultaneously connected to the MU while crossing the cell boundary. As soon as the mobile's link with the new BS is acceptable, the initial BS disengages from the MU.


(PCS)


Hard handoff

1.

MU temporarily suspends the voice conversation by sending a link suspend message to the old BS.

2.

3.

4.

MU sends a handoff request message through an idle time slot of the new BS to the network.

The new BS sends a handoff ack message and marks the slot busy.

The MU returns the old assigned channel by sending a link resume message to the old BS.


(PCS)


Hard handoff

5.

6.

7.

8.

MU continues voice communication while the network prepares for the handoff.

Upon receipt of a handoff request message, the new BS sends a handoff ack message and reconfigures itself to effect the handoff.

The MSC inserts a bridge into the conversation path and bridges the new BS.

Finally, the network informs the MU to execute the handoff via both the new and old BSs by sending the handoff execute message.


(PCS)


Hard handoff

9.

MU releases the old channel by sending an access release message to the old BS.

10.

Once the MU has made the transfer to the new BS, it sends the network a handoff complete message through the new channel, and resumes the voice communication. The network removes the bridge from the path and frees up the resources associated with the old channel.


(PCS)


Soft handoff

1.

2.

3.

MU sends a pilot strength measurement message to the old BS, indicating the new BS to be added.

The old BS sends a handoff request message to the MSC. If the MSC accepts the handoff request , it sends a handoff request message to the new

BS.

The BS sends a null traffic message to the MU to prepare the establishment of the communication link.


(PCS)


Soft handoff

4.

5.

The new BS sends a join request message to the

MSC.

The MSC bridges the connection for the two BSs, so that the handoff can be processed without breaking the connection.

The new BS sends a handoff ack message to the old BS via the MSC. The old BS instructs the MU to add a link to the new BS by exchanging the handoff command and handoff complete messages.


(PCS)


Soft handoff

6.

The old BS and the MSC conclude this procedure by exchanging the required handoff information.

The quality of the new link is guaranteed by the exchange of the pilot measurement request and the pilot strength measurement message pair between the MU and the new BS.


(PCS)

Roaming

Roaming is a facility, which allows a subscriber to enjoy uninterrupted communication from anywhere in the entire coverage space.

A mobile network coverage space may be managed by a number of different service providers.

They must cooperate with each other to provide roaming facility.

Roaming can be provided only if some administrative and technical constraints are met.


(PCS)

Roaming

Administrative constraints











Billing.

Subscription agreement.

Call transfer charges.

User profile and database sharing.

Any other policy constraints.


(PCS)

Roaming

Technical constraints







Bandwidth mismatch. For example, European

900MHz band may not be available in other parts of the world. This may preclude some mobile equipment for roaming.

Service providers must be able to communicate with each other.

Needs some standard.

Mobile station constraints.


(PCS)

Roaming

Technical constraints

 Integration of a new service provider into the network. A roaming subscriber must be able to detect this new provider.





Service providers must be able to communicate with each other.

Needs some standard.

Quick MU response to a service provider’s availability.

 Limited battery life.


(PCS)

Roaming

Two basic operations in roaming management are

 Registration (Location update): The process of informing the presence or arrival of a MU to a cell.

 Location tracking: the process of locating the desired MU.


(PCS)

Roaming

Registration (Location update): There are six different types of registration.

 Power-down registration .

Done by the MU when it intends to switch itself off .

 Power-up registration .

Opposite to power-down registration. When an MU is switched on, it registers.

 Deregistration . A MU decides to acquire control channel service on a different type of network (public, private, or residential).


(PCS)

Roaming

Registration (Location update): There are six different types of registration.

 New system/Location area registration : when the location area of the MU changes, it sends a registration message.

 Periodic registration : A MU may be instructed to periodically register with the network.

 Forced registration : A network may, under certain circumstances, force all MUs to register.


(PCS)

Registration

Two-Tier Scheme

HLR: Home Location Register

A HLR stores user profile and the geographical location.

VLR: Visitor Location Register

A VLR stores user profile and the current location who is a visitor to a different cell that its home cell.


(PCS)

Registration

Two-Tier Scheme steps. MU1 moves to cell 2.

Cell 1 Cell 2

MU1

MU1


(PCS)

Registration

Steps

1.

2.

3.

4.

MU1 moves to cell 2. The MSC of cell 2 launches a registration query to its VLR 2.

VLR2 sends a registration message containing MU’s identity (MIN), which can be translated to HLR address.

After registration, HLR sends an acknowledgment back to VLR2.

HLR sends a deregistration message to VLR1 (of cell

1) to delete the record of MU1 (obsolete).

VLR1 acknowledges the cancellation.


(PCS)

Location tracking

Steps

1.

2.

3.

4.

VLR of cell 2 is searched for MU1 ’s profile.

If it is not found, then HLR is searched.

Once the location of MU1 is found, then the information is sent to the base station of cell 1.

Cell 1 establishes the communication.


(PCS)


Two-Tier Scheme steps location search

Id HLS

Dest Dest-HLS

- 4

3

Source ls

2 10

Source-mss

1

9

5

Dest

HLS

8

Id LS

Dest Dest-ls

- -

6

Id MSS

Dest Dest-mss

- -

7 Dest ls

Src

Dest


(PCS)


Two-Tier Scheme steps location update

5

Id LS

MU New-ls

- -

Old-ls

10

9

HLS

4

3

Id HLS

MU HLS

- -

6

Id MSS

MU New-mss

- -

7

New-ls

2 8

New-mss

1

MU


Part 2











Architecture

Data categorization

Data management

Transaction management

Recovery


A Reference Architecture (Client-Server model)

PSTN

DB DB

DBS DBS

HLR

MSC

BSC

Fixed host

Fixed host

MU

BS

MU

BS

MU

BS

MU

MU

VLR

MSC

BSC


MDS Applications













Insurance companies

Emergencies services (Police, medical, etc.)

Traffic control

Taxi dispatch

E-commerce

Etc.


MDS Limitations













Limited wireless bandwidth

Wireless communication speed

Limited energy source (battery power)

Less secured

Vulnerable to physical activities

Hard to make theft proof.


MDS capabilities













Can physically move around without affecting data availability

Can reach to the place data is stored

Can process special types of data efficiently

Not subjected to connection restrictions

Very high reachability

Highly portable


Objective

To build a truly ubiquitous information processing system by overcoming the inherent limitations of wireless architecture.


MDS Issues

 Data Management

 Data Caching

 Data Broadcast (Broadcast disk)

 Data Classification

 Transaction Management

 Query processing

 Transaction processing

 Concurrency control

 Database recovery


MDS Data Management Issues

How to improve data availability to user queries using limited bandwidth?

Possible schemes

 Semantic data caching: The cache contents is decided by the results of earlier transactions or by semantic data set.

 Data Broadcast on wireless channels



How to improve data availability to user queries using limited bandwidth?

Semantic caching

 Client maintains a semantic description of the data in its cache instead of maintaining a list of pages or tuples.

 The server processes simple predicates on the database and the results are cached at the client.


Data Broadcast (Broadcast disk)

A set of most frequently accessed data is made available by continuously broadcasting it on some fixed radio frequency. Mobile Units can tune to this frequency and download the desired data from the broadcast to their local cache.

A broadcast ( file on the air ) is similar to a disk file but located on the air.


Data Broadcast (Broadcast disk)

The contents of the broadcast reflects the data demands of mobile units. This can be achieved through data access history, which can be fed to the data broadcasting system.

For efficient access the broadcast file use index or some other method.



How MDS looks at the database data?

Data classification

 Location Dependent Data (LDD)

 Location Independent Data (LID)



Location Dependent Data (LDD)

The class of data whose value is functionally dependent on location.

Thus, the value of the location determines the correct value of the data.

Location Data value

Examples : City tax, City area, etc.



Location Independent Data (LID)

The class of data whose value is functionally independent of location. Thus, the value of the location does not determine the value of the data.

Example : Person name, account number, etc.

The person name remains the same irrespective of place the person is residing at the time of enquiry.




Example: Hotel Taj has many branches in India.

However, the room rent of this hotel will depend upon the place it is located. Any change in the room rate of one branch would not affect any other branch.

Schema: It remains the same only multiple correct values exists in the database.




LDD must be processed under the location constraints. Thus, the tax data of Pune can be processed correctly only under Pune’s finance rule.

Needs location binding or location mapping function.




Location binding or location mapping can be achieved through database schema or through a location mapping table.



Location Dependent Data (LDD) Distribution

MDS could be a federated or a multidatabase system. The database distribution (replication, partition, etc.) must take into consideration

LDD.

One approach is to represent a city in terms of a number of mobile cells, which is referred to as

“ Data region ”. Thus, Pune can be represented in terms of N cells and the LDD of Pune can be replicated at these individual cells.



Concept Hierarchy in LDD

In a data region the entire LDD of that location can be represented in a hierarchical fashion.

City data

County 1 data County 2 data County n data

Subdivision 1 data Subdivision data Subdivision m data


MDS Query processing

Query types







Location dependent query

Location aware query

Location independent query




A query whose result depends on the geographical location of the origin of the query.

Example

What is the distance of Pune railway station from here?

The result of this query is correct only for “here” .




Situation: Person traveling in the car desires to know his progress and continuously asks the same question. However, every time the answer is different but correct.

Requirements: Continuous monitoring of the longitude and latitude of the origin of the query.

GPS can do this.


MDS Transaction Management

Transaction properties : ACID

Consistency, Isolation, and Durability).

(Atomicity,

Too rigid for MDS.

Flexibility can be introduced using workflow concept.

Thus, a part of the transaction can be executed and committed independent to its other parts.



Transaction fragments for distribution.

PST N

DB DB HLR

DBS DBS MSC

BSC

Fixed host

Fixed host

BS

M U

M U

M U

BS

M U

BS

M U

VLR

MSC

BSC



Transaction fragments for distributed execution

Execution scenario: User issues transactions from his/her MU and the final results comes back to the same MU.

The user transaction may not be completely executed at the MU so it is fragmented and distributed among database servers for execution.

This creates a Distributed mobile execution.



A mobile transaction (MT) can be defined as

T i is a triple <F, L, FLM>; where

F = {e

1

, e

2

, …, e n

} is a set of execution fragments,

L = {l

1

, l

2

, …, l n

} is a set of locations, and

FLM = {flm

1

, flm

2

, …, flm n

} is a set of fragment location mapping where



j, flm i

(e i

) = l i



An execution fragment e ij where is a partial order e ij

= {

 j

,

 j

}



 i

= OS j



{N i

} where OS j

=

 k

O jk

, O jk



{read, write},

 and N j

{Abort

L

, Commit

L

}.

For any O jk and O jl where O jk data object x, then either O jk

 j

= R(x) and O jl

O jl or O jl

 j

= W(x) for

O jk

.



Mobile Transaction Models

Kangaroo Transaction : It is requested at a MU but processed at DBMS on the fixed network. The management of the transaction moves with MU. Each transaction is divided into subtransactions.

Two types of processing modes are allowed, one ensuring overall atomicity by requiring compensating transactions at the subtransaction level.




Reporting and Co-Transactions : The parent transaction (workflow) is represented in terms of reporting and co-transactions which can execute anywhere.

A reporting transaction can share its partial results with the parent transaction anytime and can commit independently.

A co-transaction is a special class of reporting transaction, which can be forced to wait by other transaction.




Clustering : A mobile transaction is decomposed into a set of weak and strict transactions.

The decomposition is done based on the consistency requirement. The read and write operations are also classified as weak and strict.




Semantics Based : The model assumes a mobile transaction to be a long lived task and splits large and complex objects into smaller manageable fragments. These fragments are put together again by the merge operation at the server. If the fragments can be recombined in any order then the objects are termed reorderable objects.



Mobile Transaction execution.

DBS1

DBS2

T

2

(e

4

, e

5

)

MU3

MU1 T

1

(e

1

, e

2

, e

3

)

MU2

DBS4

DBS3



Serialization of concurrent execution.





Two-phase locking based (commonly used)





Timestamping

 Optimistic

Reasons these methods may not work satisfactorily

Wired and wireless message overhead.

Hard to efficiently support disconnected operations.

 Hard to manage operations.

locking and unlocking



Serialization of concurrent execution.

New schemes based on timeout, multiversion, etc., may work. A scheme, which uses minimum number of messages, especially wireless messages is required.



Database update to maintain global consistency.

Database update problem arises when mobile units are also allowed to modify the database. To maintain global consistency an efficient database update scheme is necessary.



Transaction commit.

In MDS a transaction may be fragmented and may run at more than one nodes (MU and DBSs). An efficient commit protocol is necessary.

2-phase commit (2PC) or 3-phase commit (3PC) is no good because of their generous messaging requirement.

A scheme which uses very few messages, especially wireless, is desirable.




One possible scheme is “ timeout ” based protocol.

Concept: MU and DBSs guarantee to complete the execution of their fragments of a mobile transaction within their predefined timeouts.

Thus, during processing no communication is required.

At the end of timeout, each node commit their fragment independently.




Protocol: TCOT-Transaction Commit On Timeout

Requirements

Coordinator: Coordinates transaction commit

Home MU: Mobile Transaction (MT) originates here

Commit set: Nodes that process MT (MU + DBSs)

Timeout: Time period for executing a fragment



Protocol: TCOT-Transaction Commit On Timeout













MT arrives at Home MU.

MU extract its fragment, estimates timeout, and send rest of MT to the coordinator.

Coordinator further fragments the MT and distributes them to members of commit set.

MU processes and commits its fragment and sends the updates to the coordinator for DBS.

DBSs process their fragments and inform the coordinator.

Coordinators commits or aborts MT.



Transaction and database recovery.

Complex for the following reasons

 Some of the processing nodes are mobile

 Less resilient to physical use/abuse

 Limited wireless channels

 Limited power supply

 Disconnected processing capability




Desirable recovery features

 Independent recovery capability

 Efficient logging and checkpointing facility

 Log duplication facility










Independent recovery capability reduces communication overhead.

Thus, MUs can recover without any help from DBS

Efficient logging and checkpointing facility conserve battery power

Log duplication facility improves reliability of recovery scheme




Possible approaches

 Partial recovery capability

 Use of mobile agent technology




Possible MU logging approaches

 Logging at the processing node (e.g., MU)

 Logging at a centralized location (e.g., at a designated DBS)

 Logging at the place of registration (e.g., BS)

 Saving log on Zip drive or floppies.


Mobile Agent Technology

A mobile agent is an independent software module capable of







Migrating to any node on the network

Capable of spawning and eliminating itself

Capable of recording its own history



A mobile agent can be used for the following activities, which are essential for recovery.









Centralized and distributed logging

Log carrier. A Mobile unit may need to carry its log with it for independent recovery

Log processing for database recovery

Transaction commit or abort



Possible approaches







Agent broadcast on a dedicated wireless channel

Pool of agents at every processing node

Agent migration to a required node.


Mobile E-commerce

What is E-commerce?

Mapping of business activity on the network.

The network may be mobile of ad-hoc in which case the scope of business activities significantly increases.



Why mobile E-commerce?

To make business activity free from spatial constraints. This allows tremendous flexibility to customers as well as to vendors.

Important gain: Making information available at the right time, at the right location, and in a right format.



Requirements for a mobile E-system











Security

Reliability

Efficient

Customer trust

Quality of service



These requirements are difficulty and complex to achieve

Security

Conventional key approaches needs revision.

Reliability

Hard to provide mainly because of the unreliability and limitations of resources.




Efficient

This capability can be easily improved mainly because of the elimination of spatial constraints.

Customer trust

A time consuming activity. Customer do not easily trust electronic communication and always wants to see a reliable backup service.




Quality of service

Mobility and web provides ample scope for improving the quality of service.

An integration of mobility, web, data warehousing and workflow offers tremendous growth potential and a very controlled way of managing business activities.


Conclusions and summary

Wireless network is becoming a commonly used communication platform. It provides a cheaper way to get connected and in some cases this is the only way to reach people. However, it has a number of easy and difficult problems and they must be solved before MDS can be built.

This tutorial discussed some of these problems and identified a number of possible approaches.


Conclusions and summary

The emerging trend is to make all service providing disciplines, such as web, E-commerce, workflow systems, etc., fully mobile so that any service can be provided from any place. Customer can surf the information space from any location at any time and do their shopping, make flight reservation, open bank account, attend lectures, and so on. This is what the wireless technology driving us to.


References

1.

2.

Acharya, S., Alonso, R., Franklin, M., and

Zdonik, S. Broadcast Disks: Data management for Asymmetric Communication Environments.

Proc. ACM SIGMOD Conf., San Jose, May,

1995.

Alonso, R., and Korth, H.

Database Systems

Issues in Nomadic Computing. Proc. ACM

SIGMOD International Conf. on management of

Data, May 1993.


References

3.

4.

Barbara, D., and Imielinski, T.

Sleepers and

Workaholics: Caching Strategies in Mobile

Environments.

Proc.

ACM SIGMOD Conf.,

Minneapolis, May, 1994.

Chrysanthis, P. K., Transaction Processing in

Mobile Computing Environment, in IEEE

Workshop on Advances in Parallel and

Distributed Systems, October 1993.


References

5.

6.

7.

Dhawan, C.

Mobile Computing.

McGraw-Hill,

1997.

Dunham, M. H., Helal, A., and Balakrishnan, S., A

Mobile Transaction Model That Captures Both the Data and Movement Behavior, ACM/Baltzer

Journal on Special Topics in Mobile Networks and Applications, 1997.

Forman, H. George and Zahorjan, J.

The

Challenges of Mobile Computing, IEEE

Computers, Vol. 27, No. 4, April 1994.


References

8.

9.

Pitoura, E.

and Bhargava, B., Maintaining

Consistency of Data in Mobile Distributed

Environments. Proceedings of 15th International

Conference on Distributed Computing Systems.,

1995.

Pitoura, E.

and Bhargava, B., Building

Information Systems for Mobile Environments,

Proc.

3rd.

Int.

conf.

on Information and

Knowledge Management, Washington, DC, No.

1994.


References

10.

11.

Vijay Kumar, “Timeout-based Mobile

Transaction Commit Protocol”, 2000 ADBIS-

DASFAA Symposium on Advances in Databases and Information Systems, Prague, Sep. 5-8,

2000.

Shaul Dar, Michael Franklin, Bjorn T. Johnsson,

Divesh Srivastava, and Michael Tan, “Semantic

Data Caching and Replacement”, Proc. Of the

22 nd VLDB Conference, Mumbai, India, 1996.


References

12.

13.

14.

E. Pitoura and G. Samaras, “Data Management for Mobile Computing”, Kluwer Academic

Publishers, 1998.

E. Turban, at. el., “Electronic Commerce: A

Managerial Perspective”, Prentice Hall, 2000.

L.

Loeb, “Secure Electronic Transactions”,

Artech House, 1998.

Mobile Database Systems - University of Missouri

Outline

What is a Mobile Database System (MDS)?

What is a mobile connectivity?

What is intermittent connectivity?

Part 1

Architecture

Wireless communication

Bandwidth limitations

Frequency reuse

Low-tier PCS telephony overview

Low-tier PCS telephony overview

Cordless and low-tier PCS telephony overview

Wireless Components

Wireless Components

Wireless Components

MSC (MTSO)

MS

BS

MS

Wireless component

Wireless channels are limited

Mobile cell

Mobile cells

Mobile cells

Problems with cellular structure

Handoff

Handoff issues

Handoff detection strategies

Mobile-Controlled Handoff (MCHO)

Handoff

Mobile-Assisted Handoff (MAHO)

Handoff types with reference to the network

Handoff types with reference to the network

Handoff types with reference to link transfer

Handoff types with reference to link transfer

Handoff types with reference to link transfer

Handoff types with reference to link transfer

Handoff types with reference to link transfer

Handoff types with reference to link transfer

Handoff types with reference to link transfer

Registration

Registration

Part 2

Architecture

Data categorization

Data management

Transaction management

Recovery

A Reference Architecture (Client-Server model)

MDS Applications

MDS Limitations

MDS capabilities

Objective

MDS Issues

Related documents

Add this document to collection(s)

Add this document to saved

Suggest us how to improve StudyLib