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Mobile Services (ST 2010)Chapter 1: Introduction
Axel Küpper
Service-centric NetworkingDeutsche Telekom Laboratories, TU Berlin
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gMobile ServicesSummer Term 2010
1 Introduction1.1 The Emergence of Mobile Services
1.2 What is a Mobile Service?
1.3 Mobility Classification
1.4 Protocol Overview
1.5 Physical Layer Overview
1.6 Data Link Layer Overview
2
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g1.1 The Emergence of Mobile Services
The Fathers of the Internet
3
Creator of the first computer-
to-computer network based
on packet switching (ARPAnet)
Lawrence Roberts(*1939)
Donald Davies(*1924 †2000)
Paul Baran(*1926)
Leonard Kleinrock(*1934)
Developers of packet switching and the mathematical theory behind it
Joseph C. R. Licklider(*1915 †1990)
Formulated the earliest ideas of a global computer
network
Robert E. Kahn(*1938)
Vint Cerf(*1943)
Inventors of the Transmission Control Protocol (TCP)
Transit networksAccess networks
Local networks
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What is a Service?
4
Transit networksAccess networks
Local networks
Request
Response
ServerClient InterfaceInteraction according to protocol
Examples Email File Transfer Protocol Gopher World Wide Web
Service
Activity of a computer program performed on request for another program
Remotely accessible at well-defined interfaces and offered in a communications system
Invoked and used by protocols of the application layer
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g1.1 The Emergence of Mobile Services
The World Wide Web (I)
5
Transit networksAccess networks
Local networks
GET /hypertext/WWW/TheProject.html
?info.cern.ch
130.094.122.195
DNS
Inventor of the WWW
Tim Berners Lee(*1955)
"I just had to take the hypertext idea and connect it to the Transmission Control
Protocol and domain name system ideas and – tada! – the World Wide Web!"
Interlinked documents, accessible worldwide
Hyper Text Markup Language
Hypertext Transfer Protocol
Uniform Resource Locator
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The World Wide Web (II)
6
Transit networksAccess networks
Local networks
GET gp/product/0470092319/ref=s9_simi_gw_s3_...
Bob
Request
ResponseResponse
?amazon.com ?amazon.com
130.094.122.195130.094.122.195
Several enhancements in the last 15 years focusing on dynamic web pages, session and transaction management, search, asynchronous operation,…
Basis for many applications: content, commerce, control, communication, communities, …
Basis for realizing intra-/interorganizational supply chains → SOA
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g1.1 The Emergence of Mobile Services
Service-oriented Architectures (SoAs)
Service Composition
Interoganizational connection of services
Fixes service logic and control flows, for example, conditional, sequential, parallel, and exceptional execution
Consistent management of data exchanged between services
Service Description
Standard way to specify data and operations of a service (syntax)
Fixes a service's capabilities, behavior, and related service type (semantics)
Defines the conditions of usage (service contracts)
Service Discovery
Public directory that enables the publication of services and facilitates their discovery
Companies can publish specifications of ser-vices they provide and other enterprises can access those services using the descriptions
Software architecture that is based on the key concepts of an application frontend, a service, a service repository, and a service bus.
Krafzig/Banke/Slama: "Neither a technology nor a technology standard, but instead it represents a technology-independent, high-level concept that provides architectural blueprints", for example:
7
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g1.1 The Emergence of Mobile Services
Web 2.0
88
Bob
Co-founder of Web 2.0
Tim O'Reilly (*1954)
Term that represents different concepts and paradigms of latest WWW services:
"The web as a platform" – Software applications are based on the web, not the desktop PC
Democratization – Community service and user-generated content
Mashups – new services through combination of existing services and content
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g1.1 The Emergence of Mobile Services
Peer-to-Peer Environments
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Transit networksAccess networks
Access networks
Founder and inventor of Napster
Shawn Fanning (*1980)
No dedicated roles of client and server
Overlay network of peers that share resources (bandwidth, computational power, content,…)
Peer is provider and requestor
Motivated by (illegal) mp3 sharing
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g1.1 The Emergence of Mobile Services
Cloud Computing
10
1010111010101010101010111111011010101111
01010111IaaS
SaaS
PaaS
Client(usually web
browser)
Transit networksAccess networks
Local networks
Infrastructure as a Service: virtualization of computer hardware
Platform as a Service: usage of service platforms without the cost and complexity of buying and managing underlying hardware and software layers
Software as a Service: loading software into the user's web browser on demand and executing it there; related data is stored in the cloud
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g1.1 The Emergence of Mobile Services
Integration of Mobile Networks
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Transit networksAccess/local networks
Ad hoc networks Mesh networks
Cellularnetworks
Interconnectivity with different types of wireless networks: cellular, ad-hoc, mesh,…
Requirement for many new (service) scenarios:
Mobile Internet
Fixed/Mobile Convergence and All-IP
Internet of Things and Services
Ubiquitous Computing and Context-aware Services
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g1.2 What is a Mobile Service?
Some Definitions (I)
Mobile Communications
Transmission of data to and from handheld devices
Out of two or more communicating devices, at least one handheld is mobile
Location of the device can vary either locally or globally and communication takes place through a wireless, distributed, or diversified network
Mobile Computing
Process of computation on a mobile device
Set of distributed computing systems and servers participate, connect, and synchronize through mobile communication protocols
Technology to wirelessly connect to and use centrally located information and/or application software through small, portable, and wireless computing and communication devices
Pervasive Computing
“Everywhere, everyone, at any time” paradigm
Takes into account the environment in which information and communication technology is used, for example, toys, computers, cars, homes, factories, workareas
Based on integrated processors, sensors, and actuators connected through high-speed networks and combined with new devices for viewing and display
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g1.2 What is a Mobile Service?
Some Definitions (II)
Ubiquitous Computing
Interconnecting computing devices with environmental objects
Integration of computers into practically all objects in our everyday environment, endowing them with computing abilities
Context-aware Services
Services that automatically adapt their behavior, for example, filtering or presenting information, to one or several parameters reflecting the context of a target person
Context parameter classes: personal, technical, spatial, social, and physical context
Location-based Services
Services that generate, compile, select, or filter information or perform other actions by taking into consideration the locations of one or several target persons or mobile objects
Usually process a target’s current location, her or his last known location, or locations the target has visited in the past, and present the result to the user, either on demand, automatically, or in a continuous fashion.
Location is derived from a process known as positioning, which automatically determines the geographic position of the target in real-time.
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g1.2 What is a Mobile Service?
Definition
14
What is a Mobile Service?
Activity of a computer program performed on request for another program, and at least one of them performed on a mobile device
Remotely accessible at well-defined interfaces and offered over a mobile communications system
Invoked and used by protocols of the application layer and possibly supported by enabling functions of the underlying mobile network
Follows one or several of the paradigms of mobile computing, ubiquitous and pervasive computing, and location and context-awareness and combines them with a business model
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g1.2 What is a Mobile Service?
Application Areas and Components
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Application areas
Paradigms
Mechanisms
3/4G networksAd-Hoc networksWLAN, PAN,…
Mobile phonesSmartphonesBadgesSensors,…
Networks and devices
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g1.3 Mobility Classification
Mobility Concepts for Supporting the Mobile User (I)
16
Terminal Mobility
Refers to the seamless mobility of mobile devices
Requires wireless connection between mobile device and base stations (in case of infrastructure-based networks) or between several mobile devices (in case of ad-hoc communications)
Registration, call, and connection between mobile device and network are kept while in motion
Key functions: wireless communications, handover, location management, roaming
Fixedassignment
Wiredconnection
No mobility support
Fixedassignment Wireless
connection
Terminal mobility
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g1.3 Mobility Classification
Mobility Concepts for Supporting the Mobile User (II)
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Personal Mobility
Means that users can access their services according to their subscription from different devices and device types
User is available via her/his telephone number or email address
User profiles and services are available across device boundaries
Key functions: authentication mechanism
Network A
Network B
Temporaryassignment
Wiredconnection
Personal mobility
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g1.3 Mobility Classification
Mobility Concepts for Supporting the Mobile User (III)
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Service mobility: home-centric service control for roamers
Visited Network
Home Network
Server
Service request
Service response
Client
Service mobility: visited-centric service control for roamers
Visited Network
Home Network
Server
Service request
Service response
Client MobilityControler
Inquiry
Permission
Service Mobility
Enables usage of tailored and personalized services even if the user is roaming to foreign networks
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g1.3 Mobility Classification
Mobility Concepts for Supporting the Mobile User (IV)
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Session Mobility
Allows to interrupt sessions and to resume them later possibly from another terminal or another network
Server
ServerClient
Session mobility between networks
Server
Client
Client
Session mobility between terminals
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g1.3 Mobility Classification
Mobility with regard to the Network Topology
20
Micro Mobility
Mobility support within a radio cell or between different cells, but within a single access network
Mobility management at data link layer
Macro Mobility
Mobility support between different access networks connected to a common core network or within a certain geographic region
Mobility management at network layer
Global Mobility
Mobility support between different core network and/or network operators
Mobility management at network layer and/or at application layer
Locatio
n-u
pd
ate frequ
ency
Dis
tan
ce
Far
High
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g1.4 Protocol Overview
Protocol Stack (I)
21
Radio CorePhysical
Data link
Network
Transport
Application
Physical
Data link
Network
Transport
Application
Physical
Data link
Network
Physical
Data link
Protocol stack: layered architecture of protocols for managing the data exchange between peers (end systems and intermediate systems)
Protocol: set of rules prescribing the format and meaning of messages, frames, and packets exchanged between the peer entities within a layer
Service: set of operations that a layer provides to the next higher layer
End-systems need a full protocol stack comprising all layers
Intermediate systems do not necessarily need all layers
Entities at the same level communicate with each other
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g1.4 Protocol Overview
Protocol Stack (II)
Physical layer
Sender: transformation of a bit stream into signals for transmission
Receiver: transformation of signals into a bit stream
Tasks: frequency selection, generation of carrier frequency, signal detection, modulation of data onto a carrier frequency, and encryption
Data link layer
Point-to-point or point-to-multipoint connection between sender and receiver(s)
Medium access, multiplexing, detection and correction of transmission errors, and detection of data frames (synchronization)
Network Layer
Connection between two entities over many intermediate systems
Routing of packets through a network
Addressing, routing, localization, handover between different networks
Example: Internet Protocol (IP)
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Radio CorePhysical
Data link
Network
Transport
Application
Physical
Data link
Network
Transport
Application
Physical
Data link
Network
Physical
Data link
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g1.4 Protocol Overview
Protocol Stack (III)
Transport layer
Establishment of an end-to-end connection
Quality-of-service (QoS), flow and congestion control
Example: Transmission Control Protocol (TCP)
Application layer
Protocols designed for fulfilling communication needs of an application
Example: Hypertext Transfer Protocol (HTTP)
23
Radio CorePhysical
Data link
Network
Transport
Application
Physical
Data link
Network
Transport
Application
Physical
Data link
Network
Physical
Data link
What is a gateway? Intermediate system containing peers up to the transport or application layer Used in mobile networks…
… for converting between different application layer protocols… for converting between different transport layer protocols… for improving performance of transport/application layer protocols… for supporting mobility
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g1.5 Physical Layer Overview
The “Wireless Transmission Chain”
Receiving Station
Transmitting Station
Modulation
Source decoding(e.g. speech
processing)
Multiplexing
Channel decoding(Block decoding,
convolutional decoding,
Deinterleaving)
Encryption
Decryption
Channel encoding(Block encoding,
convolutional encoding,
interleaving)
Demultiplexing
Source encoding(e.g. speech
processing)
Demodulation
The “Air Interface”
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g1.5 Physical Layer Overview
Signals
Data is transmitted via radio signals in wireless networks
Radio signal: electromagnetic wave…
…generated by a transmitter in dependence on the data to be transferred (modulation*),
…emitted by the antenna of the transmitter,
…caught by the antenna of the receiver, and
…sampled by the receiver to recover the data bits (de-modulation)
Carrier frequency/carrier: radio signal of a constant frequency generated by the receiver for modulation
Carrier frequency can be described by a sine wave (defined by three parameters)
Each parameter can be used for the modulation of data
Amplitude Shift Keying
Frequency Shift Keying
Phase Shift Keying
ttt tfAtg 2sin
Amplitude Frequency Phase
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g1.5 Physical Layer Overview
Modulation (I)
Modulation: modification of a carrier’s parameters (amplitude, frequency, phase) according to the pattern provided by another signal
Shift keying: digital modulation, i.e., the modulating signal is a digital signal derived from a sequence of symbols
1 0 0 1 0 1 1 0Data
Carrier
Modulated
signal
Pulses
Phase Shift Keying (PSK)
1 0 0 1 0 1 1 0Data
Carrier
Modulated
signal
Pulses
Frequency Shift Keying (FSK)
1 0 0 1 0 1 1 0Data
Carrier
Modulated
signal
Pulses
Amplitude Shift Keying (ASK)
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g1.5 Physical Layer Overview
Modulation (II)
Quadrature Amplitude Modulation
(QAM) Combination of ASK and QPSK
Eight signal states → three bits are transmitted in a single step
Used as one alternative for IEEE 802.11
I
Q
000 001
010
100
110
111
011Data 010 101 000 111 011 110 001 100
101
Source: U. Reimers: DVB- The Family of International Standards for Digital Video Broadcasting, Springer-Verlag 27
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g1.5 Physical Layer Overview
Error Sources (I)
Error sources
Because of phenomena like multipath propagation, path loss, and Doppler shift wireless transmission is susceptible to comparatively high error rates
Bit error rates
Bit error rate (BER): probability of any given bit being in error
Wireless links: BER=10-1-10-5
Electrical wired link: BER=10-9
Optical wired link: BER=10-12
Path Loss
Signals attenuate over distance when travelingfrom the transmitting to the receiving antenna
Degree of attenuation is referred to as path loss
Path loss is influenced by
Distance between transmitting andreceiving antenna
Terrain and obstacles in-between
Frequency components of the signals
Isotropic antenna
Distance d
Surface of a sphere
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g1.5 Physical Layer Overview
Error Sources (II)
Multipath Propagation
Due to reflection, scattering, and diffraction signal reaches the receiver over various paths
Signals traveling along different paths with different lengths arrive at the receiver at different times
Copies of a signals may arrive at different phases - if these phases add destructively, the signal level relative to noise declines
Cumulative signal is smeared or spread (delay spread) and, as a consequence, intersymbol interference occurs
As the mobile antenna moves, the location of various obstacles change, hence the number, magnitude, and timing of the secondary pulse change
Doppler Shift
Change in the frequency of a wave resulting from the relative velocity of a transmitter with respect to a receiver
Frequency increases when moving towards the base station
Frequency decreases when moving away from the base station Moving source29
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g1.6 Data Link Layer Overview
Multiple “Users” of the Air Interface
The “Air Interface”Cellular
networks
Local and personal area networksMilitary
networks
Aviation and seafaring
...
Terrestrial television
Terrestrial
radio
Satellite
communications
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g1.6 Data Link Layer Overview
Multiplexing (I)
Motivation
Subdivides a communication medium into different channels, each of which to be used for data transmission independent from the other channels
Fixes a communication channel by separation of space, time, frequency, and code or a combination thereof
Communication channel refers to an association of sender(s) and receiver(s) that want to exchange data
Medium Access Control
Dynamically assigns communication channel resulting from multiplexing to users
Analogy
Highways with several lanes
many users: car drivers
medium: highway
interference: accidents
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Space division multiplex
Time division multiplex
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g1.6 Data Link Layer Overview
Multiplexing (II)
Space Division Multiplexing (SDM)
Separation of channels by mapping each channel onto a dedicated space
Non-overlapping interference ranges
Enabled by sector and directional antennas
Usually used in combination with frequency, time, or code division multiplexing
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g1.6 Data Link Layer Overview
Multiplexing (III)
Frequency Division Multiplexing (FDM)
Subdivision of the frequency dimension into several non-overlapping frequency bands, each continuously carrying one channel
Guard spaces between frequency bands to avoid overlapping (adjacent channel interference)
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g1.6 Data Link Layer Overview
Multiplexing (IV)
Time Division Multiplexing (TDM)
All senders alternately use the same frequency at different points in time
Avoidance of transmission overlaps (co-channel interference) by time gaps (guard spaces)
Requires precise synchronization between senders (either by a precise clock or by a dedicated synchronization signal accessible for all senders)
Flexible, as senders with heavy load can be assigned more sending time and senders with light load less sending time
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g1.6 Data Link Layer Overview
Multiplexing (V)
FDM and TDM combined
Channel can use a certain frequency band for a certain amount of time
Guard spaces in the time and in the frequency dimension
Robust against small-scale fading by using frequency hopping (fast change of frequency bands)
Deployed in GSM and DECT
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g1.6 Data Link Layer Overview
Multiplexing (VI)
Code Division Multiplexing (CDM)
All channels use the same frequency band at the same time
Separation by codes, guard spaces corresponds to the distance between codes (orthogonal codes)
Good protection against interference and tapping (i.e., signals are spread on a broad frequency band, and interpretation of a signal is only possible with matching code)
High complexity of the receivers
Precise synchronization between sender and receiver
Initially used in military application
Designated multiplexing technique for UMTS/IMT-2000
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g1.6 Data Link Layer Overview
Medium Access (I)
Medium Access
Access methods in wireless systems are responsible to coordinate the successful operation of multiple terminals over the wireless medium (air)
Access methods were originally developed for wired media and later on adopted to the wireless medium
Differences in wired and wireless media requires modifications to make access methods suitable for the wireless medium
Main differences of wired and wireless media
Availability of bandwidth
The “air” is shared between all wireless systems and cannot be multiplied by laying additional cables as done in wired transmission
Wireless access methods must cope for the efficient use of the one and only medium “air”
Reliability of transmission
Phenomena like multipath propagation and doppler shift cause high error rates and make wireless transmission less reliable in comparison to wired media
Wireless networks often use different packet sizes and modified access methods to optimize the performance to the specifics of the unreliable wireless medium
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g1.6 Data Link Layer Overview
Medium Access (II)
Fixed-assignment access
Originates from the telecommunications domain
Make efficient use of communication resources when each user has a steady flow of information to be transmitted (e.g., digitized voice traffic, data file transfer, facsimile transmission)
Access is coordinated by dedicated signaling channels that exchange short messages to obtain resources (e.g., links, switches,...) in the telephone network at the beginning of the conversation and to release resources at the end of the call
Access methods assign a slot of time, a portion of frequency, or a specific code to the user (preferably for the entire length of the conversation)
Random Access
Originates from the data communications domain
Provides a more flexible and efficient way of managing channel access for transmitting short, bursty messages (e.g., Web browsing)
Each packet of the data burst carries some signaling information related to the address of the destination and the source
Provides the user with varying degrees of freedom in gaining access to the network whenever information is to be sent
Randomness of user access may cause contention among the users for access to a channel, resulting in collisions of contenting transmissions
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g1.6 Data Link Layer Overview
Medium Access (III)
Fixed Assignment Methods
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
FDMA/TDMA
Code Division Multiple Access (CDMA)
Random Assignment Methods
ALOHA
Slotted ALOHA
Carrier Sense Multiple Access (CSMA) - Slotted vs. Unslotted | non-persistent vs. p-persistent
Multiple Access with Collision Avoidance (MACA)
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For detailedinformation see:
