Mobile Database is a database that is transportable, portable, and physically separate or detached from the corporate database server but has the capability to communicate with those servers from remote sites allowing the sharing of various kinds of data.
With mobile databases, users have access to corporate data on their laptop, PDA, or other Internet access device that is required for applications at remote sites.
The mobile discipline defines two types of mobility:
(a) terminal mobility
(b) personal mobility
Each mobility type addresses a different set of mobility problems.
TYPES OF MOBILITY
A mobile framework is composed of wired and wireless components and human users. Its wireless part implements terminal mobility and personal mobility to eliminate some of the spatial and temporal constraints from data processing activities.
Terminal Mobility:
- It allows a mobile unit (laptop, cell phone, PDA, etc.) to access desired services from any location while in motion or stationary, irrespective of who is carrying the unit. For example a cell phone can be used by its owner and it can also be borrowed by Tome one else for use.
- In terminal mobility, it is the responsibility of the wireless network to identify the communication device.
- The connection is established between two points and not between the
two persons calling each other. This type of connection in a session allows the use of communication devices to be shared among anybody.
Personal Mobility
- Here, user can use any communication device for establishing communication with the other party, Thus, a user does not have to carry any communication equipment with him.
- This facility requires an identification scheme to verify the person wishing to communicate.
In personal mobility the party is free to move, and in terminal mobility the communication unit is free to move. Voice or data communication can be supported by either types of mobility. However, to visualize a complete mobile database management system both types of mobility are essential.
Q. Explain the differences between personal mobility and terminal mobility. How do they affect the scope of wireless communication?
Location and Handoff Management
When a mobile unit crosses its cell boundary and enters a neighboring cell, the system (MSC - Mobile Switching Center) transfers the ongoing communication session to a new channel which belongs to the new base station. This migration of a mobile unit from one cell to another is managed by a process referred to as handoff.
- The process of transferring ongoing call or data connectivity from one Base Station to other Base Station.
- When a mobile moves into the different cell while the conversation is in progress then the MSC (Mobile Switching Center) transfer the call to a new channel belonging to the new Base Station.
- A handoff makes sure that the mobile unit gets the necessary channel in the new cell to continue its communication and relieves the channel it was using without any interruption.
One of the important points in a handoff is that it must be initiated and completed
before the ongoing call is lost due to reduction in signal strength.
Working: To achieve this, an optimal signal level is defined; and when this level is detected, the system initiates the handoff. This optimal level is higher than the minimum usable signal level, which is usually between -90 and -100 dBm. Thus, the difference between the handoff threshold signal level and the minimum level should not be either too large or too small. If it is too small, then calls will be dropped before handoff is complete; and if it is too large, then there will be too many handoffs.
The system has to cope with a situation called "false handoff," which occurs when
the signal strength reaches the handoff threshold due to momentary fading caused by reflection or absorption by intermediate objects. To detect that the fading is actually due to mobile unit moving away from the base station, the system measures the signal strength for a certain period of time and obtains a running average that is used to initiate the necessary handoff.
There are two types of handoff: hard handoff & soft handoff.
hard handoff
In hard handoff the mobile unit does experience momentary silence in voice communication when it relives the old channel and acquires the new channel in the new cell.
- In a hard handoff, an actual break in the connection
occurs while switching from one cell to another. - The radio links from the mobile station to the existing
cell is broken before establishing a link with the next
cell. - It is generally an inter-frequency handoff. It is a “break
before make” policy.
soft handoff.
In soft handoff, no such silence is experienced.
- In soft handoff, at least one of the links is kept when
radio links are added and removed to the mobile
station. - This ensures that during the handoff, no break occurs.
- This is generally adopted in co-located sites. It is a
“make before break” policy.
Location Management
The entire process of the mobility management component of the cellular system is responsible for two tasks:
(a) location management- that is, identification of the current geographical location or current point of attachment of a mobile unit which is required by the MSC (Mobile Switching Center) to route the call-
(b) handoff- that is, transferring (handing off) the current (active) communication session to the next base station, which seamlessly resumes the session using its own set of channels.
The entire process of location management is a kind of directory management problem where locations are current locations are maintained continuously.
One of the main objectives of efficient location management schemes is to minimize the communication overhead due to database updates (mainly HLR). The other related issue is the distribution of HLR to shorten the access path, which is similar to data distribution problem in distributed database systems.
The current point of attachment or location of a subscriber (mobile unit) is expressed in terms of the cell or the base station to which it is presently connected. The mobile units (called and calling subscribers) can continue to talk and move around in their respective cells; but as soon as both or any one of the units moves to a different cell, the location management procedure is invoked to identify the new location. The unrestricted mobility of mobile units presents a complex dynamic environment, and the location management component must be able to identify the correct location of a unit without any noticeable delay.
The location management performs three fundamental tasks:
(a) location update- initiated by the mobile unit, the current location of the
unit is recorded in HLR and VLR databases
(b) location lookup- a database search to obtain the current location of the mobile unit
(c) paging- through paging the system informs the caller the location of the called unit in terms of its current base station.
These two tasks (location lookup & paging) are initiated by the MSC.
The cost of update and paging increases as cell size decreases, which becomes
quite significant for finer granularity cells such as micro- or picocell clusters. The
presence of frequent cell crossing, which is a common scenario in highly commuting zones, further adds to the cost. The system creates location areas and paging areas to minimize the cost. A number of neighboring cells are grouped together to form a location area, and the paging area is constructed in a similar way.
In some situations, remote cells may be included in these areas. It is useful to keep the same set of cells for creating location and paging areas, and in most commercial systems they are usually identical. This arrangement reduces location update frequency because location updates are not necessary when a mobile unit moves in the cells of a location area.
A mobile unit can freely move around in,
(a) active mode- In active mode the mobile actively communicates with other
subscriber, and it may continue to move within the cell or may encounter a handoff which may interrupt the communication. It is the task of the location manager to find the new location and resume the communication.
(b) doze mode- In doze mode a mobile unit does not actively communicate with other subscribers but continues to listen to the base station and monitors the signal levels around it.
c) power down mode- In power down mode the unit is not functional at all.
When it moves to a different cell in doze or power down modes, then it is neither possible nor necessary for the location manager to find the
location.
Location Lookup
A location lookup finds the location of the called party to establish the communication
session. It involves searching VLR and possibly HLR.
Step 1: The caller dials a number. To find the location of the called number (destination), the caller unit sends a location query to its base station source base station.
Step 2: The source base station sends the query to the S-LS (source location server) for location discovery.
Step 3: S-LS first looks up the VLR to find the location. If the called number is a visitor to the source base station, then the location is known and the connection is set UP.
Step 4: If VLK search fails, then the location query is sent to the HLR.
Step 5 : HLR finds the location of D-LS (destination location server).
Step 6: The search goes to D-LS.
Step 7: D-LS finds the address of D-BS (destination base station).
Step 8: Address of D-BS is sent to he HLR.
Step 9: HLR sends the address of D-BS to S-LS (source location server).
Step 10: The address of D-BS is sent to the source base station, which sets up the communication session.
Location Update
- The location update is performed when a mobile unit enters a new registration area.
- A location update is relatively expensive, especially if the HLR is distributed.
- The frequency of updates depends on the intercell movement pattern of the mobile unit such as highly commuting subscribers.
- One of the tasks of a good location management scheme is to keep such updates to a minimum.
- In the new registration area the mobile unit first registers with the base station, and the process of location update begins.
Step 1: The mobile unit moves to a new registration area which is serviced by a new
location server (New LS). The mobile unit informs the new base station about
its arrival.
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Step 2: The new base station sends the update query to New LS.
Step 3: The New LS searches the address of the HLR in its local database.
Step 4: The new location of the mobile unit is sent to HLR.
Step 5: The old location of the mobile unit is replaced by the new location.
Step 6: The HLR sends user profile and other information to New LS
Step 7: The New LS stores the information it received from HLR.
Step 8: The New LS informs the new base station that location update has been completed.
Step 9: The HLR also sends a message about this location update to the Old LS. The Old LS deletes the old location information of the mobile unit stored in its database.
Step 10: The Old LS sends a confirmation message to the HLR.
Issue:
The current location management scheme has very high search and update costs, which increase significantly in the presence of frequent cell crossing because every registration area crossing updates HLR.
Handoff Management
Degradation interval- The duration a mobile unit stays in overlap area. The objective is to complete a handoff process while the mobile unit is still in the overlap area. This implies that the handoff must not take more than the degradation interval to complete he process. If for some reason the process fails to complete in this area or within degradation interval, then the call is dropped.
Intra-system handoff- Handoff happens within a registration area, where the same MSC manages the entire process.
Inter- System handoff- Handoff occurs between two separate
registration areas where two MSCs are involved in handoff processing.
In each of these cases the handoff processing is completed in three steps:
1. Handoff detection: The system detects when a handoff process needs to be initiated.
2. Assignment of channels: During handoff processing the system identifies new channels to be assigned for continuous connectivity.
3. Transfer of radio link: The identified channels are allocated to the mobile unit.
1. Handoff detection
There are three approaches for detecting handoff effectively and accurately.
Mobile-Assisted Handoff (MAHO)
In this approach, every mobile unit continuously measures the signal strength from surrounding base stations and notifies the 1strength data to the serving base station. The strength of these signals are analyzed,
and a handoff is initiated when the strength of a neighboring base station exceeds the strength of the serving base station
Mobile-Controlled Handoff (MCHO)
In this scheme the Mobile Unit (MU) is responsible for detecting a handoff. The MU continuously monitors the signal strength from neighboring base stations and identifies if a handoff is necessary. If it finds the situation for more than one handoff, then it selects the base station with strongest signal for initiating a handoff.
Network-Controlled Handoff (NCHO)
In this scheme, Mobile Unit (MU) does not play any role in handoff detection. The BS monitors the signal strength used by MUs and if it falls below a threshold value, the BS initiates a handoff. In this scheme also BS and MSC are involved in handoff detection.
2. Assignment of Channels
Objectives
- To achieve a high degree of channel utilization
- Minimize chances of dropping connection due to unavailability of channel
Such failure is always possible in a high traffic area. If a channel is not available, then the call may be blocked (blocked calls); and if a channel could not be assigned, then call is terminated (forced termination). The objective of a channel allocation scheme is to minimize forced termination. A few schemes are
Nonprioritized Scheme: In this scheme the base station does not make any distinction between the channel request from a new call or from a handoff process. If a free channel is not available then the call is blocked and may subsequently be terminated.
Reserved Channel Scheme: In this scheme a set of channels are reserved for allocating to handoff request. If a normal channel is available, then the system assigns it to a handoff request; otherwise the reserved channel is looked for. If no channels are available in either set, the call is blocked and could be dropped.
Queuing Priority Scheme: In this scheme a channel is assigned based on some
priority. If a channel is available, then the handoff request is process immediately;
otherwise the request is rejected and the call is dropped. There is a waiting queue
where requests are queued. When a channel becomes available, then one of the
requests from the waiting queue is selected for processing. The queuing policy may be First in First Out (FIFO) or it may be measured-based or some other scheme. In the measured-based approach the request which is close to the end of its degradation interval is assigned a channel first. In the absence of any free channel the call is terminated.
Intracell handoff: Link or channel transfer occurs for only one BS. In this handoff a MU only switches channel.
Intercell or Inter-BS handoff: The link transfer takes place between two BSs which are connected to the same BSC.
Inter-BSC handoff: The link transfer takes place between two BSs which are connected to two different BSCs and the BSC is connected to one MSC .
Intersystem or Inter-MSC handoff: The link transfer takes place between two BSs which are connected to two different BSCs. These two BSCs are connected to two different MSCs
There are two ways to achieve link transfer. One way is referred to as Hard Handoff and the other as Soft Handoff.
In the presence of multiple wireless service providers the continuous connectivity is provided through Roaming. Thus when a mobile moves from one GSM to another system PCS or GSP or some other, the location of MU must be informed by the new service provider to the old service provider. This facility is called roaming facility.
EFFECT OF MOBILITY ON THE MANAGEMENT OF DATA
Data Categorization
The data distribution in conventional distributed database systems can be done in three ways:
(a) partitioned
(b) partial replication
(c) full replication.
The presence of processor mobility adds another dimension to conventional data distribution. It introduces the concept of Location-Dependent Data (LDD).
Location-Dependent Data (LDD): It is a class of data where datavalues are tightly linked to specific geographical location.
Location Dependent Query: LDD gives rise to Location-Dependent Query and Location-Aware Query. A location-dependent query needs LDD for computing the result.
Location-Aware Query: This type of query includes reference to a particular location either by name or by suitable geographical coordinates.
Some of the important reasons were: the presence of handoff, which is unpredictable;
the presence of doze mode, disconnected mode, and forced disconnection; lack of
necessary resources such as memory, and wireless channels; presence of location-
dependent data; etc.
each approach, a number of schemes were developed and each approach addressed some specific issues.
The basic ACID transaction model was unable to handle mobility aspect and location dependent processing, which are now quite common in transactional requests.
There are basically two ways to handle transactional requests on MDS:
