Chapter 1 Basic concepts of wireless data networks (cont'd.)

Chapter 1 Basic concepts of
wireless data networks (cont’d.)
Part 4: Wireless network operations
Oct 6 2004 1
Mobility management
• Consists of location management and handoff
management
• Location management refers to the activities
a wireless network should perform in order to
keep track of the MS
• Handoff management moves the ongoing
connection from one AP to another without
disruption
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Location management
• Functionality of location management:
• Track the location of the MS
• Determine the status of the MS
• Location management consists of three parts:
• Location update: MS sends messages regarding its
changing points of access to the network
• Paging: for the network to locate the MS to a
particular cell
• Location information dissemination: store and
distribute the location information related to the
MSs serviced by the network
• Basic issue: tradeoff between update and
paging
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Location update algorithms
• Two types:
• Static: network topology decides when updates
are to be sent
• Dynamic: mobility, call patterns are used for
initiating location updates
• Location area: several cells grouped into an
LA with the same LA id
• Each BS broadcasts this LA id
• Location update is sent when MS receives a new
LA id
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Static location update
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Location update in GSM
• Several cells controlled by the same
BSC form an LA
• Three cases for MS to update location:
• Upon powering up, receives a different LA
id with stored one
• Crosses a boundary of LA
• After a period of time predetermined by
network
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Location area with ping-pong effect
• Problem: MS frequently crosses the boundary of two
LAs
• Solution: dwell timer employed, update only when
the MS has stayed in the new LA long enough
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Dynamic location update
• State-based location update
• Update decision is based on current state
information
• Time elapsed, distance traveled, no. of LAs crossed, no.
of calls received, etc.
• User-profile-based update
• A sequential list of LAs the MS may move to is
maintained
• Location update needed only when the MS moves
to a new LA not on the list
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Paging schemes
• Paging: broadcasting a message in a cell or a
group of cells to elicit a response from the MS
• Paging only the cell where the MS is located,
reduce the cost of paging: very difficult
• Blanket paging in GSM
• Paging the MS in all cells within an LA
• Advantage: delay is kept to be minimum
• Disadvantage: paging is done in several cells
• “Closest-cells first” paging: the cell where the MS
was last seen is paged first, followed by
subsequent rings of cells
• Timeout is used to declare the MS as unreachable
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Location information dissemination
• Anchor is used for storing MS location
information
• Several anchors employed for decreasing loading
and improving reliability
• Each MS is associated with a home database
• User profile like mobile id, location, accounting,
etc. stored in home database
• User location is maintained in terms of visiting
network which is currently serving the MS and
visiting database
• Disadvantage: paging is done in several cells
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Location information dissemination in
GSM
• Home and visiting database are called home
location register (HLR) and VLR
• When MS observes a change on LA, location
update sent to MSC
• MSC contacts its VLR
• If VLR serves both old and new LA, nothing is
done
• If the VLR has no information about this MS, it
contacts its HLR for registration
• The HLR updates the new location of this MS
and cancels the registration in old VLR
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Handoff management
• Handles issues related to an MS with an
ongoing connection moving from one
coverage area to another coverage area
• Consists of two steps:
• Determine if a handoff is required
• The rest of the network is made aware of this and
the connection is restructured
• There are two types of issues:
• Architectural issue
• Handoff decision time algorithm
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Two basic actions in handoff
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Important issues in handoff
HANDOFF
Handoff architectures
Decision time algorithms
Handoff procedures
Handoff algorithms
Handoff
control
NCHO
MCHO
MAHO
Association
Re-association
dissociation
registration
Handoff
methodology
Hard HO
Seamless HO
Soft HO
Handoff
metrics
RSS,
Path loss
Cell ranking
Traditional,
Neural nets
etc
Handoff
Performance
measures
Call blocking
HO blocking
HO rate
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Architectural issues in handoff
• Dissociation: informing the old serving point of
attachment about the handoff
• Re-association: re-associate itself to the new point
of access to the fixed network
• Handoff types:
• hard handoff: break down the old connection before
connecting to the new BS
• Seamless handoff: keep the old connection until new
connection is formed
• Soft handoff: keep two connections simultaneously
• Network controlled handoff: handoff decision is made by
network entity
• Mobile controlled handoff: decision is made by mobile
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Handoff decision time algorithms
• Traditional algorithms: employ
thresholds to compare the values of
metrics from different points of
attachment and then decide when to
handoff
• RSS, path loss, CIR, SIR, block error rate
• Avoid ping-pong effect: hysteresis margin,
dwell timers
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Some traditional algorithms
• RSS: P new > P old, choose new BS
• RSS + threshold: P new > P old and P old < T
• RSS + hysteresis: P new > P old + H
• RSS+hysteresis+threshold: P new > P old + H
and P old < T
• Algorithm with dwell timer: a timer is started
with the condition is met. If the condition
remains true until this timer expires, handoff
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Handoff algorithms with RSS
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Measured RSS
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Generic handoff management process
1. Handoff decision is made and initiated
2. MS registers with the new visiting database
3. The new visiting DB communicates with
home DB for profile and authentication
4. Home DB responds. Two DBs updated.
5. Home DB sends dissociation to old visiting
DB and flush packets
6. Old visiting DB flushes packets and updates
DB
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Generic handoff process
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Power management
• Power management is needed because:
• Signals from MSs and BSs will cause interference
to other BSs and MSs: transmission power should
be properly controlled to maintain required SIR
• Correctly controlling transmit powers can enhance
the battery life
• Coverage area and hence handoff are also affected
• There is a need for wireless networks to keep
track of the radio resource, signal strength, and
associated information related to communication
between MSs and several BSs.
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Three issues of power management
• Power control: algorithms, protocols, and techniques
employed to dynamically adjust the transmit power
• Power-saving mechanism: employed to save battery
life of a mobile terminal by explicitly making the MS
enter a suspended or semi-suspended mode of
operation with limited capabilities
• Energy-efficient design: saving battery life via changes
in protocol design, coding and modulation, and
software
• Radio resource management: control signaling and
associated protocols employed to keep track of
relationships between signal strength, available radio
channels, and so on in a system so as to enable an
MS or the network to optimally select the best radio
resource for communications.
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Power control
• To minimize co-channel interference and hence
maximize network capacity, MSs and BSs are forced to
operate at the lowest power
• Higher transmitted power, better SIR in current cell, but
causes more interference to other cells
• An example of power control:
• AMPS with N=7
• Channels 1, 8, 15 assigned to cell A
• Shaded A transmits power 6 times as large as the other BS.
Consider the mobile user, we have
PR
−4
= t
r
−4
4
5PD
+ 6PD
−
t L t L
S
=
1
11
⎛ DL
⎜
⎝ R
4
⎞
⎟
⎠
• For N=7, the SIR is 16dB, 2dB less.
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Effect of large transmit power
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Open and closed loop power control
• Open loop control:
• Used for reverse link (MS to BS)
• Decision based on measurement for forward link
• Measured value above the threshold, reduce power
• Otherwise, increase
• Not an ideal mechanism:
• No direct relationship between reverse and forward links
• Time lag in implementing power control
• Closed loop control:
• Feedback between BS and MS is set up
• Receiver indicates the received signal quality
• Transmitter adjusts power accordingly
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Examples
• Open loop control in CDMA
• MS closer to BS should transmit at lower power
• MS adjusts its power based on the total received
power from all BSs
• Closed loop power control in GSM
• MS measures the RSS and signal quality of
neighboring BSs and reports back to BS
• BS also measures RSS, signal quality and distance
to each MS
• BS determines required minimum power and
informs MS
• Step: 2dB
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Centralized and distributed power control
• Goal: to uniformly render the same SIR, the
maximum possible SIR in the system, to all
users
• Two methods:
• Centralized power control: a central controller has
the knowledge of all radio links (transmit powers,
received powers, SIRs, BERs). An optimization
algorithm is implemented to maximize the minimum
SIR and minimize the maximum SIR
• Distributed power control: mobile terminals adjust
power in discrete steps. The power adjustments
made by the MSs result in the transmit powers
iteratively converging to the optimal solution
• In GSM, step size is 2 dB
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Power saving mechanisms
• Different consumed power in different states:
• Transmission, highest
• Receiving, second highest
• Standby, least
• Lucent’s WaveLAN;
• 1.825W in transmit mode
• 1.8W in receive mode
• 0.18W in standby mode
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Discontinuous transmission and repetition
• Do not send unnecessary information
• No data or voice to send
• Stop transmission: discontinuous
• Repeat data at a far lower signal power: repetition
• Discontinuous transmission in GSM
• GSM transmits a comfort noise signal when no
speech
• MS enters hangover state if no speech activity
• After hangover elapses, silence identifier frame
transmitted at larger intervals
• Receiver inserts comfort noise when silence identifier
detected
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Sleep modes
• When there is no activity, entirely shut off RF
hardware
• Sleep mode in IS-136
• Enter sleep mode for long periods on standby
• MS is required to monitor only a few time slots to
determine whether there is a call or not
• Monitor neighboring channels for handoff and
broadcast
• Sleep mode in IEEE 802.11
• MS can enter sleep mode and notifies AP
• AP buffers packets to MS in sleep mode
• Beacons sent indicating buffered packets
• MS wakes up when it receives the beacon
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Energy efficient designs
• Hardware design
• Lower power digital CMOS, mobile CPU
• Link layer and MAC design
• MAC design:
• Reduce collision
• Better protocols for sleep mode and broadcast
• Link layer
• ARQ and FEC
• interleaving
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An example: Adaptive energy efficient goback-N
ARQ protocol
• Classical go-back-N:
• Up to M packets may be sent
• If packet N lost, NACK received or time-out
• Retransmit packets N to M
• Is channel is still bad, all these may be lost
• Modified scheme:
• When NACK received or time out, a small probe
packet is sent
• Is probe packet is correctly acknowledged, resume
normal go-back-N ARQ
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Energy efficient designs (cont’d)
• Transport layer design
• By tuning the parameters of TCP or
modifying TCP, energy efficiency can be
improved
• Software approaches
• Several levels of power consumption
provided
• OS should decide when and what mode of
operation to enter
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Summary
• Mobility management:
• Location management
• Location update
• Paging
• Location information dissemination
• Handoff
• Architectural issues
• Handoff time decision algorithm
• Power management
• Power control
• Power saving mechanism
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