Chapter 4 Review of Mobile Networks

1
Chapter 4 Review of Mobile Networks

• Networks for Pervasive Computing Systems
• Signaling and Wireless Transmission Problems
• Cellular Networks Basics From 2G, 2.5G to 3G
• Introduction to Ad hoc Networks

What are the characteristics and limitations of
each type of network? Why mobile communication is
always a problem comparing with communication in
a fixed network? What are the additional problems
that we need to deal with in mobile communication
using an integrated network?
2

• Networks for Pervasive Computing Systems
• Integration of heterogeneous networks
• with different qualities for connecting the
  various components

3
Networks in a Pervasive Computing System
Fr. Schiller
Note the variation in connection qualities
4
Networks for Pervasive Computing

• Integration of heterogeneous networks
• Network at everywhere enabling anytime-anywhere
  connectivity
• An integrated network of fixed and mobile
  networks
• Tremendous improvements in fixed network
  bandwidth
• Broadband connectivity to the home and office
  (i.e., the last mile has been solved)
• Variation in wireless connection quality and
  reliability
• How to switch from one network into another
  (vertical handoff)
• Examples of mobile communications
• Satellites communications
• WWAN (cellular digital packet data uses
  satellite, 19.2kps)
• Cellular networks GSM, GPRS, TDMA, CDMA
• Wireless LAN (IEEE 802.11a, IEEE 802.11b)
  (11-25Mps)
• Ad hoc networks (networks with dynamic
  configuration)

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Integration of heterogeneous fixed andmobile
networks with varyingtransmission characteristics
regional
Vertical Handoff From network system to another
one
metropolitan area
Horizontal Handoff from one base station to
another one
campus-based
in-car, in-house, personal area
Detection and performance of handoff operations
Why do we need handoff? Signaling problem
Fr. Schiller
6
Mobile Network Characteristics

• Variant connectivity (unstable)
• Low bandwidth and low reliability (obstacles)
• Frequent disconnection
• Predictable or unpredictable
• Location dependent
• High error rate (signaling problems)
• Error corrected coding for transmission
• Increase the message size (message overhead)
• Asymmetric communication
• Downlink bandwidth gtgt uplink bandwidth
• Downlink from base station to mobile unit
• Uplink from mobile unit to base station
• Monetarily expensive
• Charges per connection or message/packet
• What are the consequences? gt connect only if
  necessary
• Connectivity is weak and intermittent
• Pre-fetching of data under good connection

7

• Mobile Communication Basics
• Signaling and Wireless Transmission Problems
• Why are the qualities of connections of many
  mobile networks not reliable?

8
Wireless Transmission (Radio Frequency)
coax cable
optical transmission
10 km 30 kHz
100 m 3 MHz
1 m 300 MHz
10 mm 30 GHz
100 ?m 3 THz
1 ?m 300 THz
visible light
VLF
LF
MF
HF
VHF
UHF
SHF
EHF
infrared
UV
Fr. Schiller

• VLF Very Low Frequency UHF Ultra High
  Frequency
• LF Low Frequency SHF Super High Frequency
• MF Medium Frequency EHF Extra High
  Frequency
• HF High Frequency UV Ultraviolet Light
• VHF Very High Frequency
• Frequency and wave length
• ? c/f
• wave length ?, speed of light c ? 3x108m/s,
  frequency f

f increases, ? decreases Smaller the frequency,
lower the penetration power (distance)
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Frequencies and Regulations

• ITU-R holds auctions for new frequencies, manages
  frequency bands worldwide (WRC, World Radio
  Conferences)

10
Representations of Signals

• Different representations of signals, i.e.,
• Amplitude (amplitude domain)
• Frequency spectrum (frequency domain)
• Composed signals (coding) transferred into
  frequency/amplitude domain
• Receiver decode the signals

A V
A V
ts
f Hz
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Signal Propagation

• Propagation in free space always like light
  (straight line)
• No wire to determine the propagation direction
  (so in all directions)
• The receiver requires to be in the line-of-sight
  (LOS) of the sender. But radio waves can
  penetrate objects and the loss in power depends
  on the frequency. Higher, greater?
• Path loss
• Receiving power inversely proportional to the
  distance from the sender, i.e., 1/d² in vacuum
• Much more in real environments due to other
  factors resulted from the environment
• How about the situation in wired communication?
• In perfect medium, the path loss is zero in
  principles

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Signal Propagation

• Transmission range
• Communication possible
• Low error rate
• Detection range
• Detection of the signal possible
• No communication possible
• Interference range
• Signal may not be detected
• Signal adds to the background noise

sender
transmission
distance
detection
interference
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Signal Propagation

• Receiving power additionally influenced by
  environment in propagation
• Fading (frequency dependent)
• Shadowing
• Reflection at large obstacles
• Refraction depending on the density of a medium
• Scattering at small obstacles
• Diffraction at edges

Fr. Schiller
scattering
diffraction
refraction
shadowing
reflection
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Multi-Path Propagation

• Signal can take many different paths between
  sender and receiver due to reflection,
  scattering, diffraction,
• Time dispersion a signal is dispersed over time
• Interference with neighbor symbols, Inter
  Symbol Interference (ISI)
• The signal reaches a receiver directly and phase
  shifted
• Distorted signal depending on the phases (i.e.,
  out of phase cancel each other) of the different
  parts

multipath pulses
LOS pulses
signal at sender
signal at receiver
Fr. Schiller
15
Effects of Mobility

• Channel characteristics change over time and
  location
• Signal paths change
• Different delay variations of different signal
  parts
• Different phases of signal parts
• Quick changes in the power received (short term
  fading)
• Additional changes in
• Distance to sender
• Obstacles further away
• Slow changes in the average power received (long
  term fading)
• Increase the sending power
• Thus, many factors may affect the strength of
  signals received gt No single solution for
  solving the problem except by raising the
  transmission signal strength

long term fading
power
t
Fr. Schiller
short term fading
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Multiplexing
channels ki
k2
k3
k4
k5
k6
k1

• Multiplexing in 4 dimensions
• space (si)
• time (t)
• frequency (f)
• code (c)
• Goal multiple uses of a shared medium (more
  channels)
• Maximize channel utilization
• Important guard spaces needed
• What will be the problem if the separation is
• small? Interferences and the receiver cannot
  identify the signals/noises

c
t
c
s1
t
s2
f
f
c
t
s3
f
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Frequency Multiplex

• Separation of the whole spectrum into smaller
  frequency bands (consider the whole spectrum as
  the multiple lanes of a road)
• A channel gets a certain band of the spectrum for
  the whole time
• Advantages
• Simple
• no dynamic coordination necessary
• Disadvantages
• Waste of bandwidth if the traffic is
  distributed unevenly
• Inflexible
• guard spaces (adjacent channel interference)

k2
k3
k4
k5
k6
k1
c
f
t
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Time Multiplex

• A channel gets the whole spectrum for a certain
  amount of time
• Advantages
• Only one carrier in themedium at any time
• Throughput high even for many users (RR)
• Disadvantages
• Precise synchronization necessary (timing)

k2
k3
k4
k5
k6
k1
c
f
t
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Time and Frequency Multiplex

• Combination of both methods (time frequency)
• A channel gets a certain frequency band for a
  certain amount of time
• Example GSM
• Advantages
• Better protection against tapping (more
  complicated)
• Protection against frequency selective
  interference
• Higher data rates compared tocode multiplex
• But precise coordinationrequired

k2
k3
k4
k5
k6
k1
c
f
t
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Code Multiplex
k2
k3
k4
k5
k6
k1

• Each channel has a unique code (encoding and
  decoding)
• After encoding, noise can be identified as noise
• All channels use the same spectrum at the same
  time
• Advantages
• Bandwidth efficient
• No coordination and synchronization necessary
• Good protection against interference and tapping
  (different coding schemes)
• Disadvantages
• Lower user data rates
• More complex signal regeneration
• What is the guard space? Keys for coding

c
f
t
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Breathing Cells in Code Multiplexing

• CDM systems cell size depends on current load.
  Why?
• Additional traffic appears as noise to other
  users
• If the noise level is too high, users drop out of
  cells
• How to define the cell size?

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• Cellular Networks Basics From 2G, 2.5G to 3G
• Organization of Network for Large Area
  Communication. How?
• Dividing the service areas
• into cells. Why? How?

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Cellular Networks

• Geographic region considered as covered by a
  number of connected cells
• Do you see the cell boundaries of a cellular
  system?
• How to define the boundary of a cell?
• Why cellular?
• To support more channels using frequency reuses
  (space multiplexing)
• Each channel has a fixed bandwidth (frequency
  multiplexing)
• Cells modeled as polygons
• Approximating circles (is it really a polygon?)
• Near-by cells should not use same frequency
  bands
• A frequency band can be reused after a suitable
  distance D
• D ? interference ? efficiency of reuse ?, and
  vice versa
• D chosen to balance efficiency and interference
• Usually, each cell has a fixed number of channels

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Cell Sizes

• Cell size 0.1 30 Km (radius). How to
  determine?
• Macro cell
• Large cell for sparsely populated area
• Micro cell
• Small cell for densely populated area
• More channels for the same area
• Lower transmitter power to reduce physical
  cluster size (cell size)
• Umbrella cell (hierarchical cell)
• Cover multiple micro-cells
• Used in highway to reduce number of handoffs for
  fast moving vehicles
• What are the benefits and problems for
  hierarchical cells? Handoff decision and channel
  allocation

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Umbrella cell A macro cell on top of 7 micro
cells. The mobile unit can choose to connect to
the micro cell or the macro cell
7 cells with similar size Each cell has a base
station for connecting (channel allocation) with
the mobile units within the cell
Handoff The operations to be performed by the
base stations when a mobile unit moves from one
cell into another cell. How to coordinate the
handoff operation? Need a higher level controller
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Large Cells VS. Small Cells

• How to determine the cell size? Large or small?
• Depending on the workload. Usually fixed number
  of channels per cell
• For give service area, more cells more channels,
  so better
• Benefits of small cells
• Less transmission power (proportional to cell
  size)
• Higher capacity from frequency reuses
• Local references only
• More robust as a result of distribution
• Problems of small cells
• Infrastructure needed how to divide the service
  area into cells
• How to group them for management?
• How to assign the frequencies to different cells
  to minimize interferences (frequency planning).
  Different transmitters within each interference
  range use different frequencies (FDM)
• More (horizontal) handoffs

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Cellular Concepts for Other Mobile Networks

• Can the cellular network concepts be applied to
  other mobile networks? Yes. How?
• Mobile object managements
• The service area is divided into region, i.e.,
  LAN segments and grids in ad hoc networks
• Fixed controller for managing a specific area for
  connection
• Fixed controller Vs. dynamic controller (i.e., in
  ad hoc network)
• Handoff operations
• Each base station (connection point) has limited
  communication range
• Vertical handoff and horizontal handoff
• Resource allocation and management
• Channel allocation and reservation

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Basic System Operation
Mobile station

• Base Station (BS) includes a controller and a
  number of receivers
• Mobile telecommunication switching office (MTSO)
  connects calls between mobile units
• Two types of channels available between mobile
  unit and BS
• Control channels used to exchange information
  having to do with setting up and maintaining
  calls
• Traffic channels carry voice or data connection
  between users

Base Stations
Switching Network
Public telecommunication switching network
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Basic System Operation
Source Wireless Comm Netwks
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Basic System Operation
Source Wireless Comm Netwks
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• The Development of Cellular Networks
• From voice communication to voice and data
  communication
• From circuit switching to packet switching

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Changes of Cellular Networks

• From 2G, 2.5G to 3G, then 4G???
• Requires a change in the whole system
  architecture. But it is done step by step
• Mainly voice communication (2G) to voice and data
  (multimedia) communication (3G)
• What are the differences in performance
  requirements for voice communication and data
  communication?
• Delay, traffic characteristics and accuracy
  requirements???
• Data could be highly bursty, large in volume
  for a short period of time. Encoded data are less
  affected to errors. Timing can be delayed in data
  transmission
• Change from circuit switching to multiple
  channels and then to packet switching
• Packet switching may achieve a higher bandwidth
• Change from point-to-point to point-to-point and
  multicast (why multicast?)

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Global System for Mobile Communication

• A 2G cellular network
• Circuit switching for voice/data transmission
• Establish a communication path
• Point-to-point communication
• Cells are grouped into location area (LA) for
  mobility management
• Otherwise, many handoffs
• Location is updated when crossing an LA (more
  than more than a cell)
• How to define an LA is a location management
  problem
• Components
• Mobile station (MS)
• Base station system (BSS)
• Network and switching sub-system (NSS)

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GSM Components

• GSM PLMN (Public Land Mobile Network)
• PSTN
• Base Station Subsystem Network Subsystem
• SIM subscriber Identity Module BSC Base
  Station Controller MSC Mobile Service
• ME Mobile Equipment HLR Home Location
  Register Switching Centre
• BTS Base Transceiver Station VLR Visitor
  Location Register EIR Equipment Identity
• AuC Authentication Centre GMSC gateway
  MSC Register

HLR
VLR
MSC
GMSC
SIM
BTS
BSC
ME
EIR
AuC
BTS
Mobile station MS
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Mobile Station

• Subscriber Identity Module (SIM)
• Smart card carrying users identity IMSI
  (International Mobile Subscriber Identity) and a
  secret key for authentication
• Based on the users identity, system can retrieve
  subscriber service data (e.g., subscribe to call
  forwarding, SMS, etc)
• Can be protected by a PIN
• Optionally store other user data (e.g. phone
  book)
• ME (Mobile Equipment)
• Phone or Mobile devices capable of taking on a
  SIM
• Uniquely identified by an IMEI (International
  Mobile Equipment Identifier)
• Implement the air (radio) interface to the BTS
  and protocols for interfacing to the BSC

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Base Station

• Base Transceiver Station (BTS)
• Implement the radio channels (transmitter and
  receiver) in the cell covered (defined) by the
  BTS.
• Link to BSC
• Base Station Controller (BSC)
• Manages radio resources for one or more BTSs.
• Handles radio channel set-up, frequency hopping,
  handovers among its cells

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Network Subsystem

• Mobile Service Switching Centre (MSC)
• Handles MS registration, authentication, location
  updating, handoffs, call routing
• Connects to the PSTN and other networks
• Home Location Register (HLR)
• Store each subscribers subscription data and
  current location of the MS
• Visitor Location Register (VLR)
• Keep entry for each MS currently located in the
  geographic area controlled by the VLR
• Subscription information (is requested from the
  HLR of the subscriber) is also stored to support
  call processing

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Network Subsystem

• Equipment Identity Register (EIR)
• Contains the IMEIs (International Mobile
  Equipment Identity) of all valid mobile equipment
  on the network
• An IMEI can be marked - if stolen, or not
  approved
• Authentication Centre (AuC)
• Store a copy of the secret key in each SIM card
• The key is used for authentication and encryption
• AuC is a protected database

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Air Interface

• A (full) GSM networks allocation
• 890 - 915 MHz (25 MHz bandwidth) for uplink
• 935 - 960 MHz (25 MHz bandwidth) for downlink
• Frequency Division Duplex (FDD)
• Related PCS network operates at 1800 MHz (1900
  MHz in US)
• Combined FDMA and TDMA for channel definition
• FDMA Frequency division multiple access
• TDMA Time division multiple access
• The 25 MHz bandwidth is divided into 124 carrier
  frequencies (bands) each of 200 KHz -- FDMA
• One of more bands are assigned to each base
  station
• Each carrier band is time divided into time-slots
  (called burst periods)
• 8 time-slots group into a frame
• A GSM physical channel is composed of
  corresponding time-slots in consecutive frames

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2.5G Enhancement of GSM

• 2.5G Enhancement of GSM as a transition for
  supporting better data services
• HSCSD - High Speed Circuit Switched Data
• GPRS General Packet Radio Service
• EDGE - Enhanced Data Rates for Global Evolution
• HSCSD (High Speed Circuit Switched data)
• Combined use of multiple TCHs
• 19.6 - over 100 kbps (practical maximum 56 kbps)
• Circuit switch Connection-oriented service
• Not good for bursty and asymmetric data traffic
• Requires (only) software upgrade to network
  infrastructure
• Allow use of multiple TCHs
• Radio link protocol enhanced to support
  multi-link operation

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GPRS (General Packet Radio Service)

• GSM circuit switched data service
• Not well suited to some common applications
  (e.g., web traffic)
• SMS is too restricted
• Store-and-forward (non-realtime, short messages
  only)
• Packet data traffic channels (PDTCHs)
• Transmit data packets
• Flexible allocation of channels for data
  transmission
• Use overlaying packet switching on existing
  circuit switched GSM network
• Radio resources can be shared dynamically between
  speech and data services
• Always on connectivity and suitable for bursty
  traffic
• Bit rates from 9kps to 170kps per user
• Fast response time (no connection set-up/release
  overheads)
• Can accommodate (traffic) volume based tariffs
• Update in system for routing and forwarding

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3G Networks

• ITU (International Telecommunication Union)
  started specification process (International
  Mobile Telecommunication 2000 (IMT-2000))
• Higher frequency band (2 GHz and beyond) with
  larger bandwidth
• Shift from voice traffic to data traffic and
  mixed traffic
• Change from circuit-based infrastructure to
  packet-based infrastructure
• Support 144Kbps (high-speed movement), 384Kbps
  (pedestrian) and 2Mbps (stationary)
• Hope to converge towards one international
  standard for 3G
• This is unlikely to be fulfilled because of
  vendors' self interests, existing infrastructure
  dependencies and migration steps like 2.5G
  GSM/GPRS, CDMA and Edge 

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Next wireless network

• 3G or wireless LAN or both (4G)
• The role of satellite communication may become
  more important
• Integration with other networks providing
  integrated services
• Provide an option for choosing amongst the set of
  available connections
• Efficient management of workload within a cell or
  a service area
• Handoff detection and management
• Efficient support of data services especially
  real-time data services (QoS)
• Location management remains an important issue
  and needs to be integrated with other
  location-dependent services
• Mobile phone operators may provide location
  information to other applications for supporting
  location-dependent services

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• Other Networks for Pervasive Computing
• Ad Hoc Networks and Satellite Communications

45
Ad hoc Network
MSPU mobile sensor processing unit
Grid why rectangular not polygon
46
Ad hoc Network

• Communication by radio frequency
• Provide point-to-point, multicast and broadcast
• A large number of mobile (fixed) nodes
• No fixed configuration (moving, mobile ad hoc
  network (MANET))
• They communicate with their neighboring nodes
  using radio signals
• Limited bandwidth and may have collision if no
  coordination (medium access control protocol)
• The neighboring nodes should not be far from it
• If a node (source node) wants to communicate with
  another node (destination node), it may rely on
  relay nodes to forward the message to the
  destination node
• Since the bandwidth is very limited, it is
  important to find the best route with the
  smallest number of relay nodes to the destination
• Minimize the number of hop counts (energy and
  bandwidth)
• The service area may be divided into grids based
  on the communication range of the node R (R Vs.
  grid size)
• Only one of the nodes in a grid needs to be in
  active mode of operation
• To conserve energy, some of the nodes may switch
  to doze mode of operation gt changing
  communication path

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BlueTooth

• Integrate voice/data ad hoc network
• Originally developed by Ericsson in 1998
• A radio network operating in 2.4-2.483GHz
• Does not require line-of-sight positioning of
  connected units
• Each bluetooth unit has a unique ID (48-bit
  address from the IEEE 802 standard)
• The maximum range is 10 meters but can be
  extended to 100 meters by increasing the power
• Bluetooth devices are protected from radio
  interference (noisy environment) by changing
  their frequencies arbitrarily upto a maximum of
  1600 times a second, a technique known as
  frequency hopping
• Low energy consumption (lt0.1W) and can switch to
  power saving mode
• The radio chip consumers only 0.3mA in standby
  mode, which is less than 3 of the power used by
  a standard mobile phone

48
BlueTooth

• Bluetooth units can be connected to form a
  piconet (or called personal area network)
• The connection can be point-to-point and
  multi-point (up to 7)
• A bluetooth device can be a part of more than one
  piconet by suitably sharing the time
• Each piconet is identified by a different
  frequency hopping sequences
• When establishing a piconet, one unit will act as
  a master and the other(s) as slave(s) for the
  duration of the piconet connection
• The master units clock and hopping sequence are
  used to synchronize all other devices in the
  piconet
• The bluetooth baseband protocol is a combination
  of circuit and packet switching
• Each packet is transmitted in a different hop
  frequency

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Bluetooth
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References

• Schiller, Mobile Communications, sections 2.4,
  2.5 and 2.8