Transcript Mobility Management

Mobility Management
Location Management
2. Handoff Management
1.
Mobility Management
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Traditional mobile communication applications were in two-way voice
communication, text, emails and remote file downloading.
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The emerging applications in video streaming, sensor networking, telemedicine
and surveillance are expected to dominate and shape the next generation of
mobile communication systems.
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One critical feature that enables the ubiquitous communication is the mobility
management - which is perceived to provide continuous constant quality of
service even under very harsh and unexpected conditions.
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Basic mobility management operations include location update as mobile units
move around and location lookup as mobile units are wanted.
Players
 Nokia
 Erricsson
 Cisco
 Nortel
 Siemens train system
 http://communications.siemens.com/cds/front
door/0,2241,hq_en_0_67428_rArNrNrNrN,00.
html
Mobility issues
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Radio resource management
Location info management
Security
Temporary loss of connectivity with movement
Scarce resources : Small devices, low battery power, small
CPU, less memory, light weight,….
 React to sudden change in environment due to bandwidth and
other resource changes
 ????
Location Management
 Location management schemes are based on users'
mobility and incoming call rate characteristics. The
network mobility process has to face strong
antagonism between its two basic procedures:
location update (or registration) and paging.
 The location update procedure allows the system to
keep location knowledge more or less accurately in
order to find the user. Location registration also is
used to bring the user's service profile near its
location.
 The paging process by the system sends paging
messages in all cells where the mobile terminal could
be located. A network must retain information about
the locations of endpoints in the network in order to
route traffic to the correct destinations.
Location tracking…
 Location tracking (also referred to as mobility tracking
or mobility management) is the set of mechanisms by
which location information is updated in response to
endpoint mobility. It is important to differentiate
between the identifier of an endpoint and its address
(i.e., where the endpoint is located).
 Mechanisms for location tracking provide a time-
varying mapping between the identifier and the
address of each endpoint. Most location tracking
mechanisms may be perceived as updating and
querying a distributed database (the location
database) of endpoint identifier-to-address mappings.
Two operations of LM
 The paging operation is performed by the cellular
network.
 When an incoming call arrives for a mobile station,
the cellular network will page the mobile station in all
possible cells to find out the cell in which the mobile
station is located so that the incoming call can be
routed to the corresponding base station. This
process is called paging. The number of all possible
cells to be paged is dependent on how the location
update operation is performed.
 The location update operation is performed by an
active mobile station.
Paging schemes
 Blanket paging in GSM : Paging the MS in all the cells within a
LA (location area) simultaneously. If the LA update is correct, in
the very first paging cycle, the MS will receive a paging request
& respond to it. Here the delay of the paging response is kept to
a minimum. The disadvantage is that paging has to be done in
several cells with the same LA!
 Closest-cells first: The cell where the MS was last seen is paged
first followed by subsequent rings of cells that are equidistant
from this cell in each paging cycle. Also called sequential
paging.
Location Tracking (update)
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Has two components: (1) determining when and how a change
in a location database entry should be initiated; and (2)
organizing and maintaining the location database.
 In cellular networks, endpoint mobility within a cell is transparent
to the network, and hence location tracking is only required
when an endpoint moves from one cell to another. Location
tracking typically consists of two operations: (1) updating (or
registration), the process by which a mobile endpoint initiates a
change in the location database according to its new location;
and (2) finding (or paging), the process by which the network
initiates a query for an endpoint's location (which also may result
in an update to the location database). Most location tracking
techniques use a combination of updating and finding in an
effort to select the best trade-off between update overhead and
delay incurred in finding. Specifically, updates are not usually
sent every time an endpoint enters a new cell, but rather are
sent according to a predefined strategy so that the finding
operation can be restricted to a specific area. .
Location Tracking
 Location management methods are most adapted
and widely used in current cellular networks, e.g.,
GSM, IS-54. IS-95, etc. The location management
methods are broadly classified into two groups. The
first group includes all methods based on algorithms
and network architecture, mainly on the processing
capabilities of the system. The second group
contains the methods based on learning processes,
which require the collection of statistics on
subscribers' mobility behavior, for instance.
Location update algorithms
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Two types:
Static
Dynamic
A location update scheme can be classified as either
global or local. A location update scheme is global if
all subscribers update their locations at the same set
of cells, and a scheme is local if an individual
subscriber is allowed to decide when and where to
perform location update. A local scheme is also called
individualized or per-user based.
In GSM, LM….
 Home & visiting databases are called Home Location Register
(HLR) & Visiting Location Register (VLR) respectively.
 When the MS observes a change in the LA identity, it transmits a
location update to MSC through it’s BS. The MSC contacts its
VLR with the location update. VLR does nothing if it serves the
old & the new LA.
 If the VLR has no info about the MS, it contacts the HLR of the
MS via a location registration message. The HLR authenticates
and acknowledges the location registration, updates its own
database, and sends a message to the old VLR to cancel the
registration.
Cost of the two operations
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The total location management cost is the sum of the location update cost and
the paging cost. There is a trade-off between the location update cost and the
paging cost. If a mobile station updates its location more frequently (incurring
higher location update cost), the network knows the location of the mobile
station better.
Then the paging cost will be lower when an incoming call arrives for the mobile
station. Therefore both location update and paging costs can not be minimized
at the same time. However, the total cost can be minimized or one cost can be
minimized by putting a bound on the other cost. For example, many researchers
try to minimize the location update cost subject to a constraint on the paging
cost.
Ex: If there is no delay constraint, the cells can paged sequentially in order of
decreasing probability, which will result in the minimal paging cost. If all cells are
paged simultaneously, the paging cost reaches the maximum while the paging
delay is the minimum.
Emerging issues in LM…
 Database architecture for 3G & 4G..(access to db
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and management of queries to reduce delay)
Reduce load on a centralized db (such as HLR), local
caches of the MS can be maintained…..used by
Mobile IP.
Alternate location update strategies & paging
algorithms are investigated.
Traffic modeling to investigate the performance..
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What is Handoff???
 Involves entire gamut of actions required to handle
an ongoing connection when the mobile terminal
moves from one point of access to another.
 Handoff is v important cos of the cellular architecture
used for spectrum utilization.
 Reference:
http://media.wiley.com/product_data/excerpt/28/0471
4190/0471419028.pdf
Handoff effects
 In cellular n/w involving voice, you hear a audible
click when handoff takes place (changing point of
access from one BS to another BS)
 In WLAN, packets are lost when handoff changes the
point of access from one AP to another. Additional
congestion control mechanisms required.
 Causes ping-pong effect due to several handoffs
between two BS back & forth: has toll on user quality
reception & network load.
Types of handoffs
 Hard handoff (further has: Intra and Inter cell
handoffs): A firm decision is made when to handoff &
has no simultaneous connection between two or
more stations.
 Soft handoff (further has: multiway soft handoffs &
softer handoffs): A conditional decision is made
whether to handoff or not. Depending on the pilot
signal from 2 or more BSs, eventually a hard decision
is made. In the interim period user has simultaneous
traffic with all candidate BSs.
Hard handoff
A hard handoff ….
 In a hard handoff, the link to the prior BS is
terminated before or as the user is transferred to the
new cell’s BS; the MS is linked to no more than one
BS at any given time.
 Hard handoff is primarily used in FDMA and TDMA
,where different frequency ranges are used in
adjacent channels in order to minimize channel
interference. So when the MS moves from one BS to
another BS, it becomes impossible for it to
communicate with both BSs (since different
frequencies are used).
When handoff should be initiated?
Handoff initiation-Performance evaluation
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 It is assumed that the signal is averaged over time, so that rapid
fluctuations due to the multipath nature of the radio environment can be
eliminated.
 Figure shows a MS moving from one BS (BS1) to another (BS2). The
mean signal strength of BS1 decreases as the MS moves away from it.
Similarly, the mean signal strength of BS2 increases as the MS
approaches it.
 Relative Signal Strength: This method selects the strongest received
BS at all times. The decision is based on a mean measurement of the
received signal. This method is observed to provoke too many
unnecessary handoffs, even when the signal of the current BS is still at
an acceptable level.
Relative Signal Strength with Threshold:
 its relative value is compared to the signal strengths of the two BSs at
the point at which they are equal.
 If the threshold is higher than this value, say T1 in Figure, this scheme
performs exactly like the relative signal strength scheme, so the
handoff occurs at position A.
 If the threshold is lower than this value, say T2 in Figure, the MS would
delay handoff until the current signal level crosses the threshold at
position B.
 In the case of T3, the delay may be so long that the MS drifts too far
into the new cell. This reduces the quality of the communication link
from BS1 and may result in a dropped call. Also causes co-channel
interference to users. The scheme may create overlapping cell
coverage areas. A threshold is not used alone in actual practice
because its effectiveness depends on prior knowledge of the crossover
signal strength between the current and candidate BSs.
Handoff decision
 Network-Controlled Handoff
 Mobile-Assisted Handoff
 Mobile-Controlled Handoff
Network-Controlled Handoff
 The network makes a handoff decision based on the
measurements of the MSs at a number of BSs. In
general, the handoff process (including data
transmission, channel switching, and network
switching) takes 100–200 ms. Information about the
signal quality for all users is available at a single point
in the network that facilitates appropriate resource
allocation. Network-controlled handoff is used in firstgeneration analog systems such as AMPS (advanced
mobile phone system), TACS
(total access communication system), and NMT
(advanced mobile phone system).
Mobile-Assisted Handoff
 In a mobile-assisted handoff process, the MS makes
measurements and the network makes the decision.
 In the circuit-switched GSM (global system mobile),
the BS controller (BSC) is in charge of the radio
interface management…. means allocation and
release of radio channels and handoff management.
 The handoff time between handoff decision and
execution in such a circuit-switched GSM is
approximately 1 second.
Mobile-Controlled Handoff
 In mobile-controlled handoff, each MS is completely
in control of the handoff process.
 This type of handoff has a short reaction time (on the
order of 0.1 second). MS measures the signal
strengths from surrounding BSs and interference
levels on all channels.
 A handoff can be initiated if the signal strength of the
serving BS is lower than that of another BS by a
certain threshold.
Handoff schemes
 Based on traffic models
Handoff Schemes in Single Traffic
Systems
 In this scheme: we have non-priority, priority, and queuing handoff
schemes for a single traffic system such as either a voice or a data
system.
 we assume that a system has many cells, with each having S
channels. The channel holding time has an exponential distribution
with mean rate . Both originating and handoff calls are generated in
a cell according to Poisson processes, with mean rates O and H,
respectively.
 We assume here a system with a homogeneous cell. The focus is
on a single cell (called the marked cell). Newly generated calls in the
marked cell are labeled originating calls (or new calls). A handoff
request is generated in the marked cell when a channel holding MS
approaches the marked cell from a neighboring cell with a signal
strength below the handoff threshold.