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Class Number – CS401Class Name – Mobile and Distributed Data Access
Instructor – Sanjay Madria
Lesson Title - Location Mgmt
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Personal Communication System (PCS)Personal Communication System (PCS)
A system where wired and wireless networks are integrated for
establishing communication.
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.
PSTN
BS
VLR
HLR
EIR
AC
MSC (MTSO)MSC (MTSO)
MSMS Wireless component
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Personal Communication System (PCS)Personal Communication System (PCS)
Wireless Components
BS
MSC (MTSO)
MS Wirelesscomponent
MS
Cell
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Personal Communication System (PCS)Personal Communication System (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.
PSTNMSC
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Personal Communication System (PCS)Personal Communication System (PCS)
Problems with cellular structure
How to locate of a mobile unit in the entire coverage area?
Solution: Location management
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
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Personal Communication System (PCS)Personal Communication System (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.
Old BS New BS
MSC
Old BS New BS
MSC
MSC
Old BS New BS New BSOld BS
MSC
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Personal Communication System (PCS)Personal Communication System (PCS)
HandoffTo keep the conversation going, the Handoff
procedure should be completed while the MS (the bus) is in
the overlap region.
G
Old BS New BS
Cell overlap region
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Personal Communication System (PCS)Personal Communication System (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.
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Personal Communication System (PCS)Personal Communication System (PCS)
Roaming
Administrative constraints
Billing.
Subscription agreement.
Call transfer charges.
User profile and database sharing.
Any other policy constraints.
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Personal Communication System (PCS)Personal Communication System (PCS)
RoamingTechnical constraints
Bandwidth mismatch. For example,
European 900MHz band may not be available
in other parts of the world. 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.
Mobile station constraints.
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Personal Communication System (PCS)Personal Communication System (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.
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Personal Communication System (PCS)Personal Communication System (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).
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Personal Communication System (PCS)Personal Communication System (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.
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Personal Communication System (PCS)Personal Communication System (PCS)Registration
Two-Tier Scheme
HLR: Home Location Register
A HLR stores user profile and the geographical
location of each moving object at a pre-
specified location
VLR: Visitor Location Register
A VLR stores user profile and the current
location who is a visitor to a different cell than
its home cell.
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Personal Communication System (PCS)Personal Communication System (PCS)
Registration
Two-Tier Scheme steps. MU1 moves to cell 2.
MU1
MU1
Cell 1 Cell 2
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Personal Communication System (PCS)Personal Communication System (PCS)
Registration
Steps
1. MU1 moves to cell 2. The MSC of cell 2 launches a
registration query to its VLR 2.
2. VLR2 sends a registration message containing MU’s
identity (MIN), which can be translated to HLR address.
3. After registration, HLR sends an acknowledgment back
to VLR2.
4. HLR sends a deregistration message to VLR1 (of cell 1)
to delete the record of MU1 (obsolete). VLR1
acknowledges the cancellation.
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Personal Communication System Personal Communication System (PCS)(PCS)
Location tracking (MU2 wants to comm with MU1)
Steps
1. VLR of cell 2 is searched for MU1’s profile.
2. If it is not found, then HLR is searched.
3. Once the location of MU1 is found, then the
information is sent to the base station of cell
1.
4. Cell 1 establishes the communication.
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Introduction
• Our target is objects capable of changing their location
• We are interested in objects with identity
• We store user locations in multiple databases (DBs)
• Main questions: How do we update data when user moves? How do we locate user in DBs when it is
required?
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Locating Moving Objects• Example of moving objects
mobile devices (cars, cellular phones, palmtops, etc) mobile users (locate users independently of the device
they are currently using) mobile software (e.g., mobile agents)
• How to find their location - Two extremes Store their current location everywhere
search locally Cost of updates
Search everywhere No information is stored anywhere; search is
expensive No cost of updates
searching verses update cost
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Availability – either at all sites or at selective sites (frequently visited sites)
Currency – Stored location is always updated (it may not make sense if user moves very frequently
Precision – Exact location verses set of possible locations
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Locating Moving Objects
• What (precision), where (availability) and when (currency) to store
Av
aila
bili
ty
nowhere
at all sites
At selective sites (e.g., at frequent callers)
CurrencyNever update
Always update (at each movement)
Precision
Exact location
the whole network
Set of locations
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Overview
• Database schemas Two-tier Hierarchical
• Replication: Working set replication Replication in hierarchical schema
• Forwarding pointers: Two-tier schema Hierarchical schema
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Two Tier Scheme
• HLR – Home Location Register is associated with each mobile user, maintains current location of the user as part of the user’s profile
• HLR is located at a network location pre-specified for each user
• To locate X, X’s HLR is queried• When X is moved to new zone, X’s HLR is
contacted and updated
• Disadvantages : Global move is expensive
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Architectures of Location DBs
• Two-tier Schemes (similar to cellular phones) Home Location Register (HLR): store the
location of each moving object at a pre-specified location for the object
Visitor Location Register (VLR): also store the location of each moving object at a register at the current region
• Hierarchical Schemes Maintain multiple registries
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Two-tier Location DBs
• Search Check the VLR at your current location If object not in, contact the object’s HLR
• Update Update the old (delete) and new VLR (insert) Update the HLR
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VLRi-X|HLRi…
Two-tier Schema- Enhancement• User’s X profile is permanently stored in
Home Location Register (HLR)
• Each site maintains Visitor Location Register (VLR), stores info about users not at their home location
• During lookup at VLRi: VLRi is queried for X location HLR of X is queried upon failure
• During move from i to k: X’s HLR is updated X’s profile is deleted from VLRi
X’s profile is added to VLRk
VLR…|HLR-X
VLRk-X|HLRk…VLRi…|HLRi…
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Two-tier Schema Cont.
– Does not support locality search in nearby locations
impossible, always need to reg. with HA
possible distant HLR is always contacted upon move
– Home Location register is permanent resettlement is not supported Does not scale well; Home
location is always contacted
+ Relatively simplemax 2 operations for lookup3 operations for update
VLRi-X|HLRi…
VLR…|HLR-X
VLRk-X|HLRk…VLRi…|HLRi…
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Standards
• EIA/TIA (Electronic Indus Asso.) and GSM(Global System for mobile Comm.) – Use HLR and VLR
• Mobile-IP - Two IP addresses – Home address and Care-of –Address (Current point of attachment)
• Care-of-address is either the address of FA or IP address acquired by the node in the current network
• Mobile node registers its care of address with its home address
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Hierarchical Schema• A hierarchy of location
databases is maintained• Internal node maintains
information about user registered in the set of zones in its subtree
• Leaf node contains actual location of objects in its coverage
• Intermediate node contains location information for all objects covered by its children in a form of:
Pointers to lower level DBs
or Actual location of each object
18 19 20
7
15 16 17
6
12 13 14
5
8 9 10
4
2 3
1X
X
X
X 12 Y 16
Y 16
Y 16
Y 16
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Hierarchical Scheme• LCA(I,j) – least common ancestor of nodes I and j• Parameter that affect the performance of the most location
management scheme Relative frequency of the move Call operations of each user
• Call to mobility ratio (CMRi) = Ci/Ui
Where Ci is the expected number of calls to user Pi over a period T and Ui the number of moves made by Pi over a period T
• LCMRij (local call to mobility ratio involves origin of calls)
= Cij/Ui
• For hierarchical scheme, LCMRij = LCMRik where k is a child of j; the call to mobility ratio for a user Pi and an internal node j is the ratio of the number of calls to Pi originated from any zone at J’s subtree to the number of moves made by Pi
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Lookup in Hierarchical Schema• Whenever lookup for object X at
ii is initiated at jj:• The tree is traversed from jj
upwards to LCA(i,j)
Then• Pointers are traversed
downwards from LCA(i,j) to ii • Location of X is found at ii
(pointer case)
OR• Location of X is found in
LCA(i,j)
(actual location case)• LCA(i,j) – Least Common
Ancestor of ii and jj• Example: (j,ij,i)) = {(8,12), (19,16)}
18 19 20
7
15 16 17
6
12 13 14
5
8 9 10
4
2 3
1X
X
X
X 12 Y 16
Y 16
Y 16
Y 16
LCA(8,12)LCA(19,16)
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Moving in Hierarchical Schema• Whenever object X moves from
jj to ii:• Pointers on the path jj,
…,LCA(j,i),…ii are altered
OR• All databases on paths root,…,jj
and root,…,ii are updated with new location of X
18 19 20
7
15 16 17
6
12 13 14
5
8 9 10
4
2 3
1X
X
X
Y 16
Y 16
Y 16
X Y
14
14
14
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updates
• Pointer case - When user x moves from 15 to 18, the entries at 18,7, 6, 3, 15 are updated ; x is deleted from the database at 15 and 6, at 3 it is updated and are added at 18 and 7
• Actual location case – entries are updated at 0, 7,6,3,18,15
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Evaluation of Hierarchical Schema
+ Mobile object is not bound to HLR
+ Advantage of locality moves and lookups is taken
– Increased number of operations DB operations
Communication messages
– Increased load and storage requirements for DBsIntermediate DBs store location information for all objects covered by its children
Root DB stores location information for ALL objects
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Partitions
• Avoid maintaining locations at all levels, reduce search cost
• Partitions grouping zones for each user among which it moves frequently
• Partition exploits locality• Information whether a user is currently in the partition is
maintained at the LCA of all nodes in partition called representative of the partition
• Representative does not know exact location• Reduces overall search cost, increases update cost when
a user crosses partition, both representatives must be informed
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Overview
• Database schemas Two-tier Hierarchical
• Replication: Working set replication Replication in hierarchical schema
• Forwarding pointers: Two-tier schema Hierarchical schema
• Other topics• Relation to KDE3 project• Evaluation
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Locating Moving Objects
• Caching cache the callee’s location at the caller side
(large Call to Mobility Ratio)• Replication
replicate the location of a moving object at its frequent callers (large CMR)
• Forwarding Pointers do not update the VLR and the HLR, leave a forwarding
pointer from the old to the new VLR (small CMR) When and how forwarding pointers are purged?
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Caching – Two Tier Scheme
• Current location of the callee may be reused by subsequent calls originated from same region
• Every time a user x is called, its location is cached at the VLR in the caller’s zone so it can be reused
• Caching Is useful for those users who frequently receives calls relative to the rate at which they relocate
• To locate a user , the cache at the VLR of the caller’s zone is queries first, if found then query is launched to the indicated zone without contacting the user’s HLR else HLR is queried
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Caching – Hierarchical scheme• When a call is made from zone i to user x located at zone j, the search
traverse from i to LCA (i,j) and then down to j• Ack is returned back to i from j• Forward and Reverse bypass pointers
Forward bypass is an entry at an ancestor of i, say s, that points to ancestor of j, say, t
Reverse is from t to s• During the next call from zone i to user x, search will travel until s then
follow t via LCA (i,j) or via a shorter path• In case of simple caching, s and t can be at the leaf while in level
caching, s and t belongs to any level and possibly to a different one.• Placing a bypass at higher level node s makes this entry available to
all calls made in s’s subtree, but will travel long path to reach s• Placing high node t will increase lookup cost, cache entry remains
valid as long as the user move’s inside t’s subtree
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Cache Invalidation• Eager caching – Every time a user is moved new location,
all cache entries for this user’s location are updated • Location of the cache entries must be centrally known,
failure of central location can cause problem• Lazy caching – a move operation does not mean updating
cache• When look up, either the user is still in the indicated
location or it has moved out (cache miss) Cache miss- HLR is contacted and then cache is updated Cache update only on cache miss Overhead – cache location must be visited first
• Saving of lazy caching over eager caching if hit ratio threshold for a user in a zone must exceed the (cost of lookup when there is a hit) / (the cost of lookup in non-caching scheme). Depends on querying HLR and VLR
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Replication
• Location (profile) of selected users is replicated at selected sites:
Enhances lookup response time Reduces network load during lookup Creates overhead during updates
• Replication is judicious if following holds:
number of lookups for object i from location j during T number of updates for i during T - savings per lookup; - cost per update
• Additional parameters: service capacity of DBs, etc.• Replication is possible at both caller and receiver locations
(1)jiC ,
iU
iji UC ,
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Working Set Replication• Applicable to two-tier schema• Replicas are kept at frequent callers of X
– working set of X• Equation holds for every member j of the
set:
• Every time a call to X is made: From a member of a set –
no updates required
From nonmember k of a set –
if (1) holds for k, k is added to the set
• Every time X moves: (1) is evaluated for each member of X
working set If (1) does not hold for member k, it is
removed from the set
(1)xjx UC ,
1
3
4
5
2
8
7
9
X1
5
6(1)? 66
X
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Replication in Hierarchical Schema• Takes advantage of locality in
movement• Local Call to Mobility Ratio is
used to determine feasibility
• and are thresholds for determining replication of nodes
• If , j is assigned a replica
• If , j is not used for replication
• If , database depends on topology of network
k U
CLCMR
i
ki
ji
,
,
maxS minS
max, SLCMR ji
min, SLCMR ji
maxmin , SLCMRS ji 18 19 20
7
15 16 17
6
12 13 14
5
8 9 10
4
2 3
1
…
+ +
+
+
Ci 8, Ci 14,i
U
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Overview
• Database schemas Two-tier Hierarchical
• Replication: Working set replication Replication in hierarchical schema
• Forwarding pointers: Two-tier schema Hierarchical schema
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VLRi-X|HLRi…
VLR…|HLR-X
VLRk-X|HLRk…VLRi…|HLRi…
VLRi-X |HLRi…
VLR…|HLR-X
VLRk-X |HLRk…
Forwarding Pointers (two-tier schema)• Pointers could be used to reduce
communication overhead and query load at the HLR
• When X moves from ii to jj a pointer from VLR at ii to VLR at jj is added
• During lookup if no information on X is found at current VLR, HLR of X is queried and pointers are followed
• Chain of pointers is managed not to exceed length K
• Useful for users receiving calls infrequently and reallocate often
• It has been showed that cost savings for K<5, are 20%-60%
5.0, iki UCVLRn-X|HLRn…
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Forwarding Pointers (hierarchical schema)• Updating and lookup in
hierarchical schema lack efficiency when LCA(i,j) is at higher levels
• Consider schema with entries as pointers to lower level DBs
• In simple forwarding a forwarding pointer connects two leaf nodes
Allows cheaper updates
• In level forwarding a forwarding pointer connects two intermediate nodes
Allows cheaper lookups18 19 20
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15 16 17
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12 13 14
5
8 9 10
4
2 3
1X
X
X
X
X
X
LCA(12,15)
18 19 20
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15 16 17
6
12 13 14
5
8 9 10
4
2 3
1X
X
X
X
Simple forwarding
18 19 20
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15 16 17
6
12 13 14
5
8 9 10
4
2 3
1X
X
X
X
Level forwarding