1. Chapter 8: Data Transmission in Mobile Communication Systems
Schiller, Voisard. Location Based Services. Morgan Kaufman
Presented by:
Fran Jarnjak
Software System Lab

2. Content Introduction Basics of Wireless Communication
Cellular-type Mobile Communication Systems
Wireless Local Area Networks
Internet-Based Mobile Communication
Ad-hoc Networking
Location-Based Service Discovery
Conclusion & QA

3. Introduction
In traditional tethered communication systems, the need for LBS was never felt.
As mobile environment was developed, LBS concept became necessary to be considered.
On top of wireless transmission, mobile communication systems can be constructed in many different ways:
Goal: provide a large number of (highly) mobile users with constant QoS at moderate data rates?
Goal: provide high data rates for few stationary users?
LBS can be built on top of many different system architectures and LBS uses them to communicate (position information).

4. Introduction
A basic of most common architectures is introduced, where the focus is on how to communicate information between two entities.
Design decisions:
Choice of transmission medium.
Infrastructure support
Type of mobility to be supported.
Two large families:
Roots in telecommunication systems.
Roots in data communication.

5. Introduction Choice of Transmission Medium
Different wireless media:
Ultrasound
Infrared light
Electromagnetic waves in radio spectrum
For LBS considered, radio wave communication is the appropriate choice:
Supports sufficiently high data rates
Acceptable distance (meters or kilometers)
However, those properties can not be simultaneously maximized due to tradeoffs between them.

6. Introduction Available infrastructure
Wireless communication has a limited range thus three different approaches can be taken to solve the problem.
Infrastructure based system:
A mobile device communicates with a fixed device (base station, access point) that is connected to a fixed, wired network. Data is transferred to another base station and transmitted wirelessly to the other mobile device.
Since, the areas around the base stations are in a cell-shape, those systems are also called: cellular systems.

7. Introduction Available infrastructure

8. Introduction Available infrastructure
Ad-Hoc/Multihop Systems:
Sometimes mobile terminals can talk directly to each other (laptops in a conference room).
Network between those terminals can be set-up in an Ad-Hoc way.
If direct connection is not available, a mobile terminal in the middle can act as an relay (multi-hop system).
Examples: Disaster relief operations (firefighters, construction sites, etc.)
Common characteristic:
Self-organizing network in set-up phase and maintenance even when the members are moving.

9. Introduction Available infrastructure

10. Introduction Available infrastructure
Hybrid systems:
A hybrid between infrastructure-based and ad-hoc/multi-hop networks.
In case a mobile terminal is far away from the base station, it can use another mobile terminal as a relay and communicate.
It’s a powerful way to widen the cell’s coverage area or increase cell’s capacity.

11. Introduction Types of Wireless Communication and Mobility
Cable replacement – to replace cables or to reduce cost of installing cables in an old office building.
Nomadic mobility – a user goes from point A to a distant point B and he can successfully connect to either infrastructure-based or ad-hoc based network at a point B, using a well-known identity irrespective of its current location.
True mobility – a user goes from point A to point B and whilst traveling, ongoing communication sessions are ongoing and not disturbed
Example: telephony calls in cellular communication systems (mobile phone)

12. Introduction Architecture Families
Wireless communication was developed by telecommunication industry and data communication industry with different system architectures.
Telecommunication industry – focus on supporting a large number of users with a QoS for phone calls, covering a large geographic area with full coverage and has mechanisms for accounting, billing, etc.
Examples: GSM, UMTS.
Data communication industry – focus on wireless counterparts of the wired networks supporting a small number of users, modest QoS, small geographic area but at high data rates and low infrastructure cost. Provides only limited mobility support.

13. Introduction Architecture Families
The primary goal is to point out what characteristics of those networks are relevant to the LBS:
What are the consequences of a particular architecture for the generation of location information ?
Where is that information available ?
How can it be accessed and how long does it take ?
What types of communication service (bandwidth, delay, dependability, cost) can a particular system offer ?
Do those communication parameters allow for certain types of location-based applications to be realized at all ?

14. Basics of Wireless Communication
Wireless communication has two essential problems:
1) How to communicate data between a source and a destination
2) How to organize multiple sources to send at the same time
Digital wireless communication is to transmit data between a single sender and a single receiver.

15. Basics of Wireless Communication
Sender generates a sine-shaped electromagnetic wave.
Receiver detects such a wave and reconstructs its shape.
By choosing a proper shape, digital information (0 and 1) can be transmitted.
Shaping the form of a sine wave is called modulation.
Amplitude Shift Key (ASK)
Frequency Shift Key (FSK)
Phase Shift Key (PSK)

16. Basics of Wireless Communication
ASK
Problem:
How to decide when a bit actually starts (where to look at high/low signal levels)

17. Basics of Wireless Communication
Source:
Stallings, W. [2003]
Data and Computer Communications (7th edition),
Prentice Hall, Upper Saddle River NJ
Subsequent figures in the presentation
Are taken either from the course textbook or the above reference.

18. Basics of Wireless Communication
First problem of wireless transmission is that the receiver will not see the identical shape a sender sent due to the:
Electromagnetic noise
Signal attenuation.
Electromagnetic noise arises due to random oscillations in the receiver’s circuitry.
Signal-to-noise ratio (S/N) – for a given modulation, the S/N ratio corresponds to the probability with which bits will be incorrectly received –bit error rate.
Strength of the signal depends on the power used by the sender.
When the distance between sender and receiver is doubled, arriving power is reduced to one-quarter - attenuation.
Arbitrarily increasing the power is not possible due to physical and legal limitations.
Bit error rate determines the throughput – amount of data that can be successfully transmitted per unit time.

19. Basics of Wireless Communication
Noisy signal at receiver’s end:
Left – small amount of noise
Right – large amount of noise
Attenuation

20. Basics of Wireless Communication
For a higher throughput one possibility is to use modulations that represent more than a single bit in a single time period.
Tradeoff: better S/N ratio is required which requires higher power
More than one frequency can be used to send data in parallel
Tradeoff: Frequencies are limited and not all frequencies ranges are equally suited for all application types.

21. Basics of Wireless Communication
Quadrature Amplitude Modulation (QAM)
Source: student_projects/scen167/thosguys/qam.html
Combination of ASK and PSK and uses one frequency.
For transmitting at 3600bps we transmit 3 bits at the time – hence 8 combinations are possible.
Bit value
Amplitude
Phase shift
000 1
None
001 2
010
1/4
011
100
1/2
101
110
3/4
111

22. Basics of Wireless Communication
QAM:
Transmitting:
First separate in three-bit triads:

23. Basics of Wireless Communication
A frequency is suitable for radio communication when:
1) Amount of data it can be transported (higher frequency – more data)
2) Ability to reach the receiver that do not have straight line-of-sight communication path
Ability to reach receiver has 4 properties:
Shadowing of waves due to obstacles (e.g. car).
Reflections of waves from those obstacles.
The Scattering of waves at small edges or openings of the objects.
Effect of diffraction that makes waves bend at the edges of large objects.

24. Basics of Wireless Communication
Multipath propagation is caused by those properties of wave propagation paths.

25. Basics of Wireless Communication
Those effects and properties are different for various frequencies.
Frequencies above 5 GHz hardly penetrate walls.
Mobile communication systems typically use a 800 MHz – 5GHz band, where 900 MHz – 1800 MHz typically is used for outdoor communication and 2.4 GHz – 5 GHz for indoor communication.
Further, due to multipath propagation it is not easy to decode a receiving signal since the same signal arrives over different paths and hence different times thus interfering with itself.
Measuring distance (location) by calculating signal’s attenuation is also not precise since:
Signal passing through a wall is more attenuated than passing through air.
Scattering and diffraction also cost power.

26. Basics of Wireless Communication
Link Layer for Multiparty Wireless Communication:
When two senders send a signal at the same time, they interfere with each other and receiver will see only noise.

27. Basics of Wireless Communication
Medium Access Problem: The wireless medium must only be accessed by a single entity at a time.
1) Which entities perform such organization?
2) What are the possibilities to organize the transmissions ?
Organization can be performed by a centralized scheme (e.g. cellular networks) or by a distributed scheme where all entities involved organize the access (e.g. ad-hoc networks).
Time Division Multiple Access (TDMA) – each entitiy transmits at a specific time slot.
Frequency Division Multiple Access (FDMA) – different senders use different frequencies for communication.
Space Division Multiple Access (SDMA) - Two sender/receiver pairs that are sufficiently apart can use the same frequency due to signal attenuation, so there is no interference.
Used extensively in cellular mobile communication systems.

28. Basics of Wireless Communication
After communication is established it can happen in:
One direction (simplex)
Both directions (duplex)
Duplex can be achieved by:
Time Division Duplex (TDD) – entities take turn in sending and receiving data
Frequency Division Duplex (FDD) – entities send the entire time but using different frequencies.

29. Cellular-type Mobile Communication Systems
For mobile telephony two requirements are needed:
Full coverage.
Quality of (almost) fixed phone networks.
A first generation of analog mobile networks did not meet those requirements:
1981 Nordic Mobile Telephone (NMT) in Nordic countries and some Europe/Middle East/Asia countries
1983 Advanced Mobile Phone System (AMPS) in USA
In 1982, development of a second generation digital communication system began, later to be called Global System for Mobile Communication (GSM), which was standardized by European Telecommunication Standards Institute (ETSI).

30. Cellular-type Mobile Communication Systems
GSM design is cellular based to provide coverage by a sufficient amount of base stations.
Challenge:
Fewer base stations yields cheap cost but also larger cells that can support only limited amount of users.
Smaller cells can support larger number of users but yield a high cost.
Solution: Use smaller cell sizes in urban areas (more people) and larger cell sizes in rural areas (less people).
Membership of a mobile station within a cell is a rough indicator of the location and due to varying cell sizes location resolution is varying as well.

31. Cellular-type Mobile Communication Systems
GSM is a circuit-switched network and existing circuit-switched architecture of fixed telephony was reused.
Hand-over: A user leaves the coverage of one base station area and goes to the other area – phone call should not terminate.
First GSM mostly concerned with voice communication. Data communication was possible using a modem-like data communication with slow data rates.
Later, a desire to better support data was needed and extensions were performed – 2.5G systems that charge the user on the amount of data transferred (not time that was needed for the transfer).
Third generation (3G) is targeted to integrate data and voice communication together – Universal Mobile Telecommunications System (UMTS).

32. Cellular-type Mobile Communication Systems
The second generation (2G) – GSM
GSM organizes the network in a hierarchy to contain complexity.
Mobile stations (MS) (e.g. phone).
Base Transreceiver Station (BTS) – has radio and signal processing equipment but does not have protocol functions.
Base Station Controller (BSC) – controls several BTSes and has protocol functions.
Mobile Service Switching center (MSC)
Several BSC assigned to it
Does mobility specific tasks
It can act as a gateway to other networks (e.g. fixed network) - called Gateway MSC or GMSC

33. Cellular-type Mobile Communication Systems
GSM network also contains:
Home Location Register (HLR) database
And for each MSC: Visitor Location Register (VLR)

34. Cellular-type Mobile Communication Systems
Two types of phone calls:
Mobile-originated call (MOC): Mobile station calls a terminal in the or fixed network
Mobile-terminated call (MTC): Mobile station (or fixed network phone) calls a terminal in the mobile network.
MOC: MS contacts MSC via BTS and BSC asking for a new connection. MSC check’s MS’s eligibility for a call by consulting terminal information in its VLR and network resources. If check is succeeded, connection to a fixed phone network is completed.

35. Cellular-type Mobile Communication Systems
MTC:
BTS serving a MS first needs to be looked up
A moving terminal keeps its location in HLR and VLR approximately up-to-date
MS that is on (power on) registers with BSC that informs MSC, who updates its VLR and HLR
When gateway MSC receives a connection request for some MS it contacts HLR to see where MS was last seen and contact that other MSC.
Since MS updates its information infrequently (saving on the signaling cost) MSC has to page all of its cells to find a MS and then connection can be made.

36. Cellular-type Mobile Communication Systems
Handover:
1) User moves within to another BTS controlled by the original BSC – re-routing of the connection is needed.
2) User moves to the region of another BSC which belongs to the same MSC, re-routing is done in MSC.
3) User moves to the region of another MSC – original MSC remains the part of the call: forwards the call and remains a stable anchor point for the outside.

37. Cellular-type Mobile Communication Systems

38. Cellular-type Mobile Communication Systems
For each MS a connection to the BTS is made which corresponds to a certain time slot in the TDMA structure.
Data rate for voice-communication is 13,000 bits/s because of advanced voice coding tehniques (fixed phone: of 64Kbit/s using 8000 samples per second in 8-bit resolution)
For data communication: 9600 bit/s with high delays ( ms).
Frequently transmitting small amounts of data – useful for LBS but by the original GSM standard its either:
Not well supported (time to set up the connection)
Or Too expensive (keeping the channel open all the time)

39. Cellular-type Mobile Communication Systems
2.5G: Between Generations – GSM Extensions
Developed to support data communication via 3 main extensions.
High-speed circuit-switched data (HSCSD) – better encoding techniques for modem connections (14.4Kbits/s) and even higher data rates by combining several modem connections into one.
Changes to the existing GSM infrastructure are small but new mobile terminals are needed.
Enhanced Data Rates for GSM Evolution (EDGE) – further improving of channel coding and modulation techniques – single connection 59.2Kbits/s
Require extensive changes to both mobile and base stations
General Packet Radio Service (GPRS) – GPRS amends GSM network with a packet-switched network so a mobile stations send data when available.
Users are charged with amount of data transmitted.
Different QoS profile (data throughput, 3 reliability classes, several delay classes).
Due to the delay classes, users will feel a difference between fixed and mobile usage which needs to be considered for the LBS applications.

40. Cellular-type Mobile Communication Systems
Third Generation (3G): UMTS
In 1990s development for standardization of the follow-up to GSM began
International Telecommunications Union (ITU) issued a call for proposals for International Mobile Telecommunications 2000 (IMT-2000) where Universal Mobile Telecommunication System (UMTS) was proposed.
UMTS vision is to build a uniform system for mobile multimedia communication.

41. Cellular-type Mobile Communication Systems
UMTS is supposed to integrate various access technologies (existing GSM, WLAN, etc.) and to-be-developed radio access technologies and existing wired technologies.
Urban environments are provided with high data rates for slowly moving users (2Mbit/s) and rural areas with lower rates for highly mobile users 500 km/h).
Realized with UMTS terrestrial radio access (UTRA)
Entire fixed UMTS network is based on the Internet protocol (all-IP network).
Several service classes exist:
1) conversational (voice),
2) streaming (real-time feeds)
3) interactive (low-bandwidth terminal-type)
4) background (non-critical e.g. download).

42. Wireless Local Area Networks
Data communication via Internet is the second largest communication system used worldwide.
Internet community also went wireless with the first goal to replace cabling in the local area (building, etc.)
Requirement: medium to high data rates over short to medium distances & smooth integration
Mobility support was a secondary concern.
Resulting system uses an access point (AP) to connect wireless terminals with the fixed network.
AP acts like an Ethernet switch, and mobile terminals are finding AP by themselves.
WLAN APs tend to be installed in a more ad-hoc fashion and frequency band it uses is also open for other usages (interference may happen).
Thus, WLAN should be able to manage and organize itself.

43. Wireless Local Area Networks

44. Wireless Local Area Networks
IEEE family: a b
Concerned about physical and link layers of the ISO/OSI model
802.11b uses direct sequence spread spectrum and a orthogonal frequency division multiplexing in the physical level.
11MBit/s b and 54MBit/s for a but in practice numbers are less
Further data rate is adapted depending on the distance: Closer – higher, far away from AP – slower.
systems offer two different link layers:
Point coordination function (PCF) – AP is in control when a certain terminal can access the medium. QoS properties can be controled.
Distributed coordination function (DCF) – solves medium access problem in a distributed fashion – every terminal listens if the medium is free before transmitting for some random time.
In current implementations of the DCF is used.

45. Wireless Local Area Networks
IEEE systems are naturally packet-switched and are a natural fit with Internet applications.
Their short range allows for deriving a rough estimate of terminal position from an AP.
Major shortcoming: Limited mobility support.
Moving from one AP to another in the same sub-net is good but:
How to handle long-distance mobility in an IP context ?

46. Internet-based Mobile Communication
Internet address:
Uniquely identifies a terminal
Used to route the data in the fixed network
In the mobile network, network topology changes as the mobile terminal moves and location information must be topologically correct so that the data is sent to the correct part of the IP network.
Mobile IP approach solves this problem by using two different addresses for a mobile terminal.

47. Internet-based Mobile Communication
Home address: A terminal is known to others using this address and it never changes.
Care-of address: temporary address that is topologically correct and can be used to route the data.
Mapping between two are done by the home agent (proxy).
In Mobile IP, data is sent to the home address and home agent redirects data to the temporary care-of address.
Security and QoS support for Mobile IP are still active research areas.

48. Internet-based Mobile Communication

49. Ad-Hoc Networking
Ad-hoc networks can solve communication needs within a group of terminals without the help of some infrastructure.
They typically use multi-hop radio communication to communicate and medium access control needs to be collaboratively solved.
Most research is concentrated on the routing problem:
All terminal together have to ensure the data reaches its destination
Environment is dynamic: terminals are mobile, can fail or simply be turned off.
A specific case are sensor networks: numerous, cheap nodes, with simple processing, communication and sensing capabilities distributed randomly.
Data rates are modest but the scaling requirement is extreme.
To determine location information, some nodes location needs to be known and then the idea is to iteratively spread such information to the neighbors. Location information can be used to solve routing problem.

50. Ad-Hoc Networking
Geographic routing: Forward the data to the node that minimizes the distance to the target.
Location information is more difficult to obtain and potentially more useful and is still an area of much active research.

51. Location-Based Services Discovery
Users location is sometimes not relevant but rather a context is more important.
Example: A user wants to print out something on the close-by printer but he doesn’t know how to access such a printer.
In PAN, MP3 players wants to find earphones, etc.
Service Location Protocols: Providing information about what services exists and where and how they can be accessed.
A global solution is not possible (DB containing all world’s printers) so a localizing the search for such services is needed.
Localization fits well with ad-hoc networks and to a certain degree with WLAN’s.

52. Location-Based Services Discovery
Three main approaches can be distinguished:
1) Request for services are flooded through the entire network. This approach is possible for a small number of devices – like PAN networks.
2) Introducing a dedicated service directory node which is well-know to other participants.
Challenge is how to provide identity of such a node – DHCP to provide node’s address, multi-cast requests to a well-known multicast address, or directory node periodically broadcasting its address.
3) Distributed service discovery – implementing on top of current sensor-networks primitives (Pub/Sub) is currently an active research area.

53. Conclusion and Q&A
The fundamental tradeoffs between distance, transmission power/interference and data rate determine many design decisions for mobile communication systems.
Two different families evolved:
1) cellular, wide area systems
2) short-to-mid range WLAN
When designing LBS for mobile communication systems one needs to keep in mind about the limited frequency a mobile terminal can update its location information (delay, cost, data rates particularly for fast-moving mobile terminals).