Networks

1
Networks

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

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 in a Pervasive Computing System
Fr. Schiller
Note the variation in network qualities
3
Networks

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

4
Wireless Network Characteristics

• Variant connectivity
• 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)
• Connectivity is weak, intermittent and expensive
• What are the consequences?
• 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

5
Integration of heterogeneous fixed andmobile
networks with varyingtransmission characteristics
regional
vertical handoff
metropolitan area
horizontal handoff
campus-based
in-car, in-house, personal area
Detection and performance of handoff operations
Fr. Schiller
6
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

7
Frequency Division Duplex (FDD)
Forward Link
mobile
base station
Reverse Link
Fr. Schiller
Two separate frequency bands are used for forward
and reverse links
8
Frequencies for Mobile Communication

• VHF-/UHF-ranges for mobile radio (100M 3G)
• simple, small antenna for cars
• deterministic propagation characteristics,
  reliable connections
• SHF and higher for directed radio links,
  satellite communication (5G 10G)
• small antenna, beam forming
• large bandwidth available
• Wireless LANs use frequencies in UHF to SHF range
• some systems planned up to EHF
• limitations due to absorption by water and oxygen
  molecules (resonance frequencies)
• Smaller frequency gt lower the penetration power
• Weather dependent fading, signal loss caused by
  heavy rainfall etc.

9
Frequencies and Regulations

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

Fr. Schiller
10
Signals

• Different representations of signals
• amplitude (amplitude domain)
• frequency spectrum (frequency domain)
• phase state diagram (amplitude M and phase ? in
  polar coordinates)
• Composed signals transferred into frequency
  domain
• Digital signals need
• infinite frequencies for perfect transmission
• modulation with a carrier frequency for
  transmission (analog signal)

Q M sin ?
A V
A V
ts
?
I M cos ?
?
f Hz
11
Signal propagation

• Propagation in free space always like light
  (straight line)
• Receiving power proportional to 1/d² in vacuum
  much more in real environments (d distance
  between sender and receiver)
• Receiving power additionally influenced by
• 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
12
Multipath propagation

• Signal can take many different paths between
  sender and receiver due to reflection,
  scattering, diffraction
• Time dispersion 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 of
  the different parts

multipath pulses
LOS pulses
signal at sender
signal at receiver
13
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

long term fading
power
t
short term fading
Fr. Schiller
14
Multiplexing
channels ki
k2
k3
k4
k5
k6
k1

• Multiplexing in 4 dimensions
• space (si)
• time (t)
• frequency (f)
• code (c)
• Goal multiple use of a shared medium
• Important guard spaces needed
• What will be the problem if the separation is
• small?

c
t
c
s1
t
s2
f
f
c
t
s3
f
15
Frequency multiplex

• Separation of the whole spectrum into smaller
  frequency bands (consider the whole spectrum as a
  multiple lanes of 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
16
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
• Disadvantages
• precise synchronization necessary (timing)

k2
k3
k4
k5
k6
k1
c
f
t
17
Time and frequency multiplex

• Combination of both methods
• A channel gets a certain frequency band for a
  certain amount of time
• Example GSM
• Advantages
• better protection against tapping
• protection against frequency selective
  interference
• higher data rates compared tocode multiplex
• but precise coordinationrequired
• What is the guard space

k2
k3
k4
k5
k6
k1
c
f
t
18
Code multiplex
k2
k3
k4
k5
k6
k1

• Each channel has a unique code
• All channels use the same spectrum at the same
  time
• Advantages
• bandwidth efficient
• no coordination and synchronization necessary
• good protection against interference and tapping
• Disadvantages
• lower user data rates
• more complex signal regeneration
• Implemented using spread spectrum technology

c
f
t
19
Cellular Networks

• Geographic region considered as covered by a
  number of cells
• Why cellular?
• To support more channels using frequency reuses
  (space multiplexing)
• Each channel has a fixed bandwidth
• Cells modeled as polygons
• Approximating circles
• 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

20
Cell Sizes

• Cell size 0.1 30 Km (radius)
• Macro cell
• Large cell for sparsely populated area
• Micro cell
• Small cell for densely populated area
• More channels per square metre
• Lower transmitter power to reduce physical
  cluster size (cell size)
• Umbrella cell (hierarchical cells)
• 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

21
Basic System Operation

• 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

22
Basic System Operation
Source Wireless Comm Netwks
23
Basic System Operation
Source Wireless Comm Netwks
24
Changes of Cellular Networks

• From 2G to 3G
• Requires a change in the whole system
  architecture
• Mainly voice communication to voice and data
  (multimedia) communication
• What are the differences in performance
  requirements for voice communication and data
  communication?
• Delay, traffic characteristics and accuracy
  requirements???
• Change from circuit switching to multiple
  channels and then to packet switching
• Change from point-to-point to point-to-point and
  multicast (why multicast?)

25
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 (not a
  cell)
• How to define an LA is a location management
  problem
• Mobile station (MS) subscriber identity module
  (SIM) and mobile equipment (ME)
• Base station system (BSS) base transceiver
  station (BTS) (encoding/decoding) and base
  station controller (BSC) (for channel allocation)
• Network and switching sub-system (NSS) switching
  function, subscriber profiles and mobility
  management
• Mobile switching centre (MSC), visitor location
  register (VLR) and home location register (HLR)

26
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
27
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

28
Base Station Network Subsystems

• 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
• NS provides links between cellular network and
  other public networks (PSTN, ISDN, Internet,
  etc).
• Control handovers between cells in different
  BSSs
• Authenticate MS
• Enable roaming of MS
• Consists of one (or more) MSC, GMSC, and a number
  of databases

29
Network Subsystem

• Mobile Service Switching Centre (MSC)
• Handles MS registration, authentication, location
  updating, handovers, 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

30
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

31
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
  -- TDMA

32
Short Message Services

• Short Message Service (SMS) for data transmission
  up to 160 characters
• Longer messages can be submitted by SMS
  concatenation and compression
• Two types of SMS
• Cell broadcast service Periodically deliver
  short messages to all subscribers in a given
  location area (LA)
• Point-to-Point service send short message to a
  specific user
• Multimedia Message Service (MMS)
• Build on top of SMS
• Allows users of MMS-capable phones to send
  messages combining text, images, graphics and
  sound in a single "rich" message. 
• Support GIF and JPEG, video formats such as MPEG
  4 and audio formats such as MP3

33
2.5G Enhancement of GSM

• 2.5G Enhancement of GSM
• HSCSD High Speed Circuit Switched Data
• GPRS
• 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
• Requires (only) software upgrade to network
  infrastructure
• Allow use of multiple TCHs
• Radio link protocol enhanced to support
  multi-link operation
• Some operators choose to leapfrog HSCSD
• Expensive
• Alternative technology (GPRS) around the corner

34
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 (like SMS)
• Flexible allocation of channels (1 to 8 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) 0.5 1 sec to start packet
  transmission
• Can accommodate (traffic) volume based tariffs

35
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-base infrastructure to
  packet-based infrastructure (Why??)
• 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 

36
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

37
Ad hoc Network
CMQ continuous monitoring query MSPU mobile
sensor processing unit
38
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)
• They communicate with their neighboring nodes
  using radio signals
• Limited bandwidth and may have collision if no
  coordination
• 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)
• To conserve energy some of the nodes may switch
  to doze mode of operation
• Only one of the nodes in a grid needs to be in
  active mode of operation

39
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

40
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 sequence
• 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

41
Bluetooth
42
Satellite Communications

• Traditionally (broadcast of information)
• weather satellites
• radio and TV broadcast satellites
• military satellites
• satellites for navigation and localization (e.g.,
  GPS)
• Telecommunication (point-to-point)
• global telephone connections
• backbone for global networks
• connections for communication in remote places or
  underdeveloped areas
• global mobile communication
• Satellite systems to extend cellular phone
  systems (e.g., GSM or 3G) for remote areas (why?)
• Satellites like flying routers and base stations

replaced by fiber optics
43
Three Types of Communication Links

• Mobile user link (MUL) mobile stations within
  the footprint of a satellite communicate with the
  satellite through MUL
• Gateway link (GWL) satellites communicate to
  base station (gateway) on the ground through GWL
  to other telephone systems and satellites
• Inter-satellite link (ISL) satellites may be
  able to communicate directly with each other via
  inter-satellite links without going through the
  gateway to reduce the communication delay

44
Inter Satellite Link (ISL)
Mobile User Link (MUL)
MUL
Gateway Link (GWL)
GWL
small cells (spotbeams)
base station or gateway
footprint
GSM
PSTN
ISDN
User data
PSTN Public Switched Telephone Network
Fr. Schiller
What are the routes for communication?
45
Basics

• Satellites in circular (elliptical) orbits
• attractive force Fg m g (R/r)²
• centrifugal force Fc m r ?²
• m mass of the satellite
• R radius of the earth (R 6370 km)
• r distance to the center of the earth
• g acceleration of gravity (g 9.81 m/s²)
• ? angular velocity (? 2 ? f, f rotation
  frequency)
• Stable orbit
• Fg Fc
• Distance to the earth surface depends on the
  rotation frequency
• f increases, r decreases
• When the period (1/f) 24 hours, r 35,786km
• Geostationary if it is on top of the equator

46
Satellite period and orbits
24
satellite period h
velocity x1000 km/h
20
16
12
8
4
synchronous distance 35,786 km
10
20
30
40 x106 m
radius
47
Basics

• Elliptical or circular orbits
• Complete rotation time depends on distance
  satellite-earth
• Inclination angle between the plane of satellite
  orbit and equatorial plane (inclination 0,
  implies??)
• Elevation the angle between the centre of the
  satellite beam and the plane tangential to the
  earths surface (elevation 90, implies??)
• Depending on the elevation, the signal has to
  penetrate a smaller or larger percentage of the
  atmosphere. In general, an elevation of smaller
  than 10 degrees is considered to be useless
• Footprint is the area on the earth where the
  signals of the satellite can be received
• high elevation needed, less absorption due to
  e.g. buildings
• Uplink connection base station - satellite
• Downlink connection satellite - base station

48
Inclination
plane of satellite orbit
satellite orbit
perigee
d
inclination d
equatorial plane
49
Elevation
Elevation angle e between center of satellite
beam and surface
minimal elevation elevation needed at least to
communicate with the satellite
e
footprint
50
Atmospheric attenuation
Attenuation of the signal in
Example satellite systems at 4-6 GHz
50
40
rain absorption
30
fog absorption
e
20
10
atmospheric absorption
5
10
20
30
40
50
elevation of the satellite
51
Four types of Satellites

• Four different types of satellite orbits can be
  identified depending on the shape and diameter of
  the orbit
• GEO geostationary orbit 36000 km above earth
  surface
• LEO (Low Earth Orbit) 500 - 1500 km
• MEO (Medium Earth Orbit) or ICO (Intermediate
  Circular Orbit) 6000 - 20000 km
• HEO (Highly Elliptical Orbit) elliptical orbits

52
Orbits II
GEO (Inmarsat)
HEO
MEO (ICO)
LEO (Globalstar,Irdium)
inner and outer Van Allen belts
earth
1000
10000
Van-Allen-Belts ionized particles 2000 - 6000 km
and 15000 - 30000 km above earth surface
35768
km
Fr. Schiller
53
Geostationary satellites (GEO)

• Orbit 35,786 km distance to the earth surface,
  orbit in equatorial plane (inclination 0)
• ? complete rotation exactly one day, satellite
  is synchronous to earth rotation
• fixed antenna positions, no adjusting necessary
• satellites typically have a large footprint (up
  to 34 of earth surface), therefore difficult to
  reuse frequencies
• bad elevations in areas with latitude above 60
  due to fixed position above the equator
• high transmit power needed (why?)
• high latency due to long distance (0.25s for one
  way)
• ? not useful for global coverage for small
  mobile phones and data transmission, typically
  used for radio and TV transmission

54
LEO systems

• Orbit 500 - 1500 km above earth surface with a
  rotational period of 90-120 min
• high elevation to ensure the quality of the link
• visibility of a satellite 10 - 40 minutes
• latency comparable with terrestrial long distance
  connections, 5 - 10 ms
• LEOs can be used for voice and data communication
  
• smaller footprints, better frequency reuse
• but handover is necessary from one satellite to
  another (a LEO is a like a moving base station)
• many satellites necessary for global coverage
• more complex systems due to moving satellites
  (handover and localization problems)
• Short life time (3-5 years) due to atmospheric
  drag and radiation from the inner Van Allen belt
• Additional energy consumption for packet routing

55
MEO systems

• Orbit 5000 - 12000 km above earth surface
  (between GEO LEO)
• comparison with LEO systems
• slower moving satellites
• less satellites needed
• simpler system design
• for many connections no hand-over needed
• higher latency, 70 - 80 ms
• higher sending power needed
• special antennas for small footprints needed

56
Routing

• One solution inter satellite links (ISL)
• reduced number of gateways needed
• forward connections or data packets within the
  satellite network as long as possible
• only one uplink and one downlink per direction
  needed for the connection of two mobile phones
• Problems
• more complex focusing of antennas between
  satellites
• high system complexity due to moving routers
• higher fuel consumption
• thus shorter lifetime
• Systems may use gateways and additionally
  terrestrial networks

57
Localization of mobile stations

• Gateways maintain registers with user data
• HLR (Home Location Register) static user data
• VLR (Visitor Location Register) (last known)
  location of the mobile station
• SUMR (Satellite User Mapping Register)
• satellite assigned to a mobile station
• current positions of all satellites
• Registration of mobile stations
• mobile station broadcasts a signal
• satellites receiving the signal report to the
  gateway
• gateway determines the location of the mobile
  station via the location of the satellites
• requesting user data from HLR
• updating VLR and SUMR

58
Localization of mobile stations

• Calling a mobile station
• localization using HLR/VLR similar to GSM
• with the information in SUMR, connection setup
  using the appropriate satellite

59
Handoff in Satellite systems

• Several additional situations for handover in
  satellite systems compared to cellular
  terrestrial mobile phone networks caused by the
  movement of the satellites
• Intra satellite handover
• handover from one spot beam to another (different
  spot beams may have different frequencies for
  communication)
• mobile station still in the footprint of the
  satellite, but in another cell
• Inter satellite handover
• handover from one satellite to another satellite
• mobile station leaves the footprint of one
  satellite
• Quality of communication Vs. handoff frequency

60
Handoff in Satellite systems

• Gateway handoff
• Handover from one gateway to another
• mobile station still in the footprint of a
  satellite, but gateway leaves the footprint
• Inter system handoff
• Handover from the satellite network to a
  terrestrial cellular network
• mobile station can reach a terrestrial network
  again which might be cheaper, has a lower latency
  etc.

61
References

• Schiller 2.4, 2.5, 2.8 and 5
• GSM and GPRS
• Bluetooth