Mobile Computing

UNIT -1ST

Mobile Computer
• A computer which you can take with you all around.
 You can do all the things which can be done with a desktop computer.
 You should be able to use same software, which you use on a desktop computer.

Mobile computer - How?
 One possibility is to have a standalone computer capable of storing large amount of software and data files,
 Processing power to support the required applications.
 Modern day laptop computer are something like this.
 Whenever you are static, connect to internet through an access point and you can do the file transfer, telnet, web browsing etc.
 While on the move, connectivity is desired for using software which requires cooperation of at least two machines.

Mobile Computing
Option for connectivity
An easy option
 Use cellular mobile phone network to connect to some Internet Service Provider and hence to internet.
 What you need - cellular phone/ corresponding modem.
 Not a good option for campus wide mobile network.
 Dependent on the mobile telephone network operator
 Cheaper wireless LAN options available

Other extreme of mobile computer
Mobile computing device
 Acts as a terminal
 Have wireless connectivity to the network
 Whatever command or application you run is executed on a remote server.
 Mobile computing device acts as remote terminal.

Issues in mobile computing networks
 Nature of medium
 Mobility
 Portability

What is Mobility

 Mobility means different things to different people. Some people are quite happy being able to get around town. Others view the world in terms of time distance-Obviously; range of motion is an important aspect of mobility.
 Another factor in mobility is ease of access. What might be considered mobile in one context is quite immobile in another.
 A more pertinent example of mobility is the ever decreasing size of cellular telephones. What was once considered a "mobile phone" had to be transported in a vehicle. This continuing decrease in size and weight of handsets has greatly increased the mobility of cellular subscribers.
 We define mobility as the ability to send and receive communications anytime anywhere. Mobility means that both source and destination devices, applications and people are free of the constraints imposed by physical location.
























WHY WIRELESS NETWORK
• Characteristics
 Mostly radio transmission, new protocols for data transmission are needed
• Advantages
 Spatial flexibility in radio reception range
 Ad hoc networks without former planning
 No problems with wiring (e.g. historical buildings, fire protection, esthetics)
 Robust against disasters like earthquake, fire – and careless users which
 Remove connectors!
• Disadvantages
 Generally very low transmission rates for higher numbers of users
 Often proprietary, more powerful approaches, standards are often restricted
 Consideration of lots of national regulations, global regulations are evolving slowly
 Restricted frequency range, interferences of frequencies

Types of Wireless Networks
1. Cellular Networks
• Base stations distributed over the area to be covered
• Each base station covers a cell
• Need of an infrastructure network connecting all base stations
• Used for mobile phone networks and data networks like Wireless LAN
2. Mobile Ad-Hoc Networks (MANETs)
• Self-configuring network of mobile nodes
• Each node serves as client and router
• No infrastructure (base stations) necessary, direct connections between any pair of nodes
• E.g. Bluetooth
3. Mesh Networks
• Enhancement of above concepts: “Ad-hoc network with infrastructure”
• Allow a whole mesh of connections between wireless nodes
• Increased fault tolerance
• E.g. used in WiMAX

Classification of Wireless Network












Wireless Personal Area Network (WPAN)











Wireless Local Area Network (WLAN)












Wireless Metropolitan Area Network (WMAN)













Wireless Wide Area Network (WWAN)












Frequencies For Communication















Basic Building Blocks











Signals











Antenna











Antenna Range













Signal Propagation














Mobility Characteristics
 Location changes
• location management - cost to locate is added to communication
 Heterogeneity in services
• bandwidth restrictions and variability
 Dynamic replication of data
• data and services follow users
 Querying data - location-based responses
 Security and authentication
 System configuration is no longer static

Wireless characteristics
 Variant Connectivity
 Low bandwidth and reliability
 Frequent disconnections
 predictable or sudden
 Asymmetric Communication
 Broadcast medium
 Monetarily expensive
 Charges per connection or per message/packet
 Connectivity is weak, intermittent and expensive

Portability Characteristics
 Resource constraints
 Mobile computers are resource poor
 Reduce program size – interpret script languages (Mobile Java?)
 Computation and communication load cannot be distributed equally
 Small screen sizes
 Asymmetry between static and mobile computers
 Battery power restrictions
 transmit/receive, disk spinning, display, CPUs, memory consume power
 Battery lifetime will see very small increase
 need energy efficient hardware (CPUs, memory) and system software
 planned disconnections - doze mode
 Power consumption vs. resource utilization













































Limitations of Mobile Environments

• Limitations of the Wireless Network
• heterogeneity of fragmented networks
• frequent disconnections
• limited communication bandwidth

• Limitations Imposed by Mobility
• lack of mobility awareness by system/applications
• route breakages

• Limitations of the Mobile Computer
• short battery lifetime
• limited capacities

Effect of Mobility on Protocol Stack

 Application
 new applications and adaptations
 Transport
 congestion and flow control
 Network
 addressing and routing
 Link
 media access and handoff
 Physical
 transmission errors and interference

Mobile Applications
 Vehicles
 transmission of news, road condition etc
 ad-hoc network with near vehicles to prevent accidents
 Emergencies
 early transmission of patient data to the hospital
 ad-hoc network in case of earthquakes, cyclones
 military ...
 Traveling salesmen
 direct access to central customer files
 consistent databases for all agents
 mobile office
 Web access
 outdoor Internet access
 intelligent travel guide with up-to-date
location dependent information
 Location aware services
 find services in the local environment, e.g. printer
 Information services
 push: e.g., stock quotes
 pull: e.g., nearest cash ATM
 Disconnected operations
 mobile agents, e.g., shopping
 Entertainment
 ad-hoc networks for multi user games

Application Adaptations for Mobility
• System-transparent, application-transparent
• the conventional, “unaware” client/server model
• System-aware, application-transparent
• the client/proxy/server model
• the disconnected operation model
• System-transparent, application-aware
• dynamic client/server model
• data broadcasting/caching
• System-aware, application-aware
• the mobile agent model

Early Wireless Communication















History Of Wireless Communication
• 1896 - Guglielmo Marconi
 First demonstration of wireless telegraphy (digital!)
 Long wave transmission, high transmission power necessary (>200kw)

• 1907 - Commercial transatlantic connections
 Huge base stations (30 100m high antennas)

• 1915 - Wireless voice transmission New York - San Francisco

• 1920 - Discovery of short waves by Marconi
 Reflection at the ionosphere
 Smaller sender and receiver, possible due to the invention of the vacuum
 tube (1906, Lee DeForest and Robert von Lieben)

• 1926 - Train-phone on the line Hamburg - Berlin
 Wires parallel to the railroad track

• 1896 - Guglielmo Marconi
 First demonstration of wireless telegraphy (digital!)
 Long wave transmission, high transmission power necessary (>200kw)

• 1907 - Commercial transatlantic connections
 Huge base stations (30 100m high antennas)

• 1915 - Wireless voice transmission New York - San Francisco

• 1920 - Discovery of short waves by Marconi
 Reflection at the ionosphere
 Smaller sender and receiver, possible due to the invention of the vacuum
 tube (1906, Lee DeForest and Robert von Lieben)

• 1926 - Train-phone on the line Hamburg - Berlin
 Wires parallel to the railroad track

• 1896 - Guglielmo Marconi
 First demonstration of wireless telegraphy (digital!)
 Long wave transmission, high transmission power necessary (>200kw)

• 1907 - Commercial transatlantic connections
 Huge base stations (30 100m high antennas)

• 1915 - Wireless voice transmission New York - San Francisco

• 1920 - Discovery of short waves by Marconi
 Reflection at the ionosphere
 Smaller sender and receiver, possible due to the invention of the vacuum
 tube (1906, Lee DeForest and Robert von Lieben)

• 1926 - Train-phone on the line Hamburg - Berlin
 Wires parallel to the railroad track

• 1896 - Guglielmo Marconi
 First demonstration of wireless telegraphy (digital!)
 Long wave transmission, high transmission power necessary (>200kw)

• 1907 - Commercial transatlantic connections
 Huge base stations (30 100m high antennas)

• 1915 - Wireless voice transmission New York - San Francisco

• 1920 - Discovery of short waves by Marconi
 Reflection at the ionosphere
 Smaller sender and receiver, possible due to the invention of the vacuum
 tube (1906, Lee DeForest and Robert von Lieben)

• 1926 - Train-phone on the line Hamburg - Berlin
 Wires parallel to the railroad track

• 1896 - Guglielmo Marconi
 First demonstration of wireless telegraphy (digital!)
 Long wave transmission, high transmission power necessary (>200kw)

• 1907 - Commercial transatlantic connections
 Huge base stations (30 100m high antennas)

• 1915 - Wireless voice transmission New York - San Francisco

• 1920 - Discovery of short waves by Marconi
 Reflection at the ionosphere
 Smaller sender and receiver, possible due to the invention of the vacuum
 tube (1906, Lee DeForest and Robert von Lieben)

• 1926 - Train-phone on the line Hamburg - Berlin
 Wires parallel to the railroad track

Cellular Concept
Cellular systems offer location-independent voice communications:
 Users can move freely while talking
 They can place calls at any time and any place
 They can be called everywhere
The cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity.
It offered very high capacity in a limited spectrum allocation without any major technological changes.
• Assuming that the cell size is kept constant and fixed spectrum per cluster:
– More cells per cluster mean:
» Fewer channels per cell
» Less system capacity
» Less co-channel interference (co-channel cells farther apart)
– Less cells per cluster mean:
» More channels per cell
» More system capacity
» More co-channel interference (co-channel cells closer together)
• Choose reuse factor N is maximize capacity per area subject to interference limitations

Fundamentals Of Cellular Systems
Fundamentals of Cellular Systems
 A BS constitutes a cell by its transmission radius
 A mobile equipment (e.g. a cellphone) always communicate with the closest base station (BS)
 100 m in cities to 35 km on the country side (GSM)
 even less for higher frequencies
 Umbrella cell: large cell that includes several smaller cells
 Avoid frequent handoffs for fast moving traffic
 Hexagonal cell shape is useful for theoretical analysis
 Practical footprint (radio coverage area) is amorphous
 The BS's are spreaded over the area to provide full coverage
 Multiple BS are aggregated in a mobile switching center (MSC)
 The MSC's are interconnected by a backbone
 The overall cellular system is granted some part of the spectrum, which is subdivided into channels
 Each BS is assigned a (sub-)set of channels to serve mobiles
 Neighboring BS's are assigned different sets of channels to avoid interference
 The same channel could be re-used by another base station having sufficient distance to avoid interference ( => frequency reuse)
 Moving mobiles will occasionally leave the transmission range of one BS to enter the range of another => handover


During a call a BS assigns a fixed portion of a slot to a mobile:
 mobiles arriving to a “full" BS will get no service
 Reducing cell size / transmission power while increasing the number of BS:
 increases the system capacity
 increases the number of handovers
Handover is initiated by the mobile, which has to constantly check the signal levels of surrounding BS
There are dierent channel assignment strategies:
 Fixed assignment: each BS is allocated a fixed set of frequencies and allocation does not change over time
 Fixed assignment with borrowing: before a call is blocked, a BS might try to ”borrow" a channel from a neighboring BS
 Dynamic assignment: MSC keeps all channels and allocates them on request to a BS

How to cope with handovers?
 Treat a handover as a new call => blocking =) connection drop=> angry users
 A guard channel concept: set aside some channels for handover calls=> wasted capacity
 Queueing off handovers: between initiation of handover and the actual event some time passes (in GSM: 1-2 seconds), this time can be used to wait for ending / leaving calls, the waiting call is then treated next
 Umbrella cells for highly mobile users

Cellular System Architecture
 Each cell is served by a base station (BS)
 Each BS is connected to a mobile switching center (MSC) through fixed links
 Each MSC is connected to other MSCs and PSTN
 Each MSC is a local switching exchange that handles
 Switching of mobile user from one base station to another
 Locating the current cell of a mobile user
• Home Location Register (HLR): database recording the current location of each mobile that belongs to the MSC
• Visitor Location Register (VLR): database recording the cell of “visiting” mobiles
 Interfacing with other MSCs
 Interfacing with PSTN (traditional telephone network) One channel in each cell is set aside for signalling information between BS and mobiles
 Mobile-to-BS: location, call setup for outgoing, response to incoming
 BS-to-Mobile: cell identity, call setup for incoming, location updating

Call Setup
 Outgoing call setup:
 User keys in the number and presses send (no dial tone)
 Mobile transmits access request on uplink signaling channel
 If network can process the call, BS sends a channel allocation message
 Network proceeds to setup the connection
 Network activity:
 MSC determines current location of target mobile using HLR, VLR and by communicating with other MSCs
 Source MSC initiates a call setup message to MSC covering target area
 Incoming call setup:
 Target MSC (covering current location of mobile) initiates a paging msg
 BSs forward the paging message on downlink channel in coverage area
 If mobile is on (monitoring the signaling channel), it responds to BS
 BS sends a channel allocation message and informs MSC
 Network activity:
 Network completes the two halves of the connection

Handoffs

Hand-off necessary when mobile moves from area of one BS into another
 BS initiated:
 BS monitors the signal level of the mobile
 Handoff occurs if signal level falls below threshold
 Increases load on BS
• Monitor signal level of each mobile
• Determine target BS for handoff
 Mobile assisted:
 Each BS periodically transmits beacon
 Mobile, on hearing stronger beacon from a new BS, sends it a greeting
• changes routing tables to make new BS its default gateway
• sends new BS identity of the old BS
 New BS acknowledges the greeting and begins to route mobile’s call
 Intersystem:
 Mobile moves across areas controlled by different MSC’s
 Handled similar to mobile assisted case with additional HLR/VLR effort
 Local call may become long-distance












Cellular Concept

The use of radio channels on the same carrier frequency to cover different areas which are separated from one another by sufficient distance so that co-channel interference is not objectionable.
Advantages of Cellular Systems with small cells
 higher capacity, higher number of users
 less transmission power needed
 more robust, decentralized
 base station deals with interference, transmission area etc. locally
Disadvantages
 Infrastructure Needed
 Handover Needed
 Frequency Planning
Problems:
 fixed network needed for the base stations
 handover (changing from one cell to another) necessary
 interference with other cells: co-channel, adjacent-channel
Important Issues:
 Cell sizing
 Frequency reuse planning
 Channel allocation strategies

HLR - VLR
What is a Home Location Register (HLR)?
 A HLR is a database of user (subscriber) information, i.e., customer profiles, used in mobile (cellular) networks. It is a key component of mobile networks such as GSM, TDMA, and CDMA networks. A HLR contains user information such as account information, account status, user preferences, features subscribed to by the user, user’s current location, etc. The data stored in HLRs for the different types of networks is similar but does differ in some details.
 HLRs are used by the Mobile Switching Centers (MSCs) to originate and deliver arriving mobile calls.

What is a Visiting Location Register (VLR)?
 A VLR is a database, similar to a HLR, which is used by the mobile network to temporarily hold profiles of roaming users (users outside their home area). This VLR data is based on the user information retrieved from a HLR. MSCs use a VLR to handle roaming users.

How are the HLR and VLR used?
Each mobile network has its own HLRs and VLRs. When a MSC detects a mobile user’s presence in the area covered by its network, it first checks a database to determine if the user is in his/her home area or is roaming, i.e., the user is a visitor.
 User in Home Area: HLR has the necessary information for initiating, terminating, or receiving a call.
 User is Roaming: VLR contacts the user’s HLR to get the necessary information to set up a temporary user profile.
The user’s location is recorded in the HLR, and in case the user roaming, it is also recorded in the VLR.
 Suppose that the user wants to make a call:
User in Home Area: MSC contacts the HLR prior to setting up the call.
 User is Roaming: MSC contacts the VLR prior to setting up the call.
Suppose that there is a call for the user (call goes to the home MSC):
 User in Home Area: Home MSC delivers the call immediately.
 User is Roaming: Home MSC contacts the VLR to determine the appropriate switch in the roaming area to handle the arriving call and then transfers the call to the roaming area MSC.


GSM
 GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity. GSM networks operate in four different frequency ranges. Most GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated.

 In the 900 MHz band the uplink frequency band is 890–915 MHz, and the downlink frequency band is 935–960 MHz. This 25 MHz bandwidth is subdivided into 124 carrier frequency channels, each spaced 200 kHz apart.

 Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel.
 There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate is 270.833 kbit/s, and the frame duration is 4.615 ms.
 The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.
 GSM has used a variety of voice codecs to squeeze 3.1 kHz audio between 5.6 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate (5.6 kbit/s) and Full Rate (13 kbit/s).
 GSM was further enhanced in 1997 with the Enhanced Full Rate (EFR) codec, a 12.2 kbit/s codec that uses a full rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.

 There are four different cell sizes in a GSM network—macro, micro, pico and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen meters; they are mainly used indoors. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.
 It is the most successful digital mobile telecommunication system in world.
 To avoid the situation for numerous co-existing analog mobile phone system running on slightly diff. carrier frequency in 2G fully digital system, the Groupe Speciale Mobile(GSM) was founded in 1982 which renamed later as global system for mobile communications (GSM).
 Four possible Handover scenarios in GSM:
1. Intra cell HO: Within a cell a narrow-band interference could make transmission at certain frequency impossible.
2. Inter-cell, Intra-BSC HO: Between cells but stays within control of the same BSC.
3. Inter-BSC, Intra-MSC HO: Ho controlled by MSC maintaining different BSC.
4. Inter MSC HO: Ho controlled by different MSCs.
 Primary goal was to provide a mobile phone system that allows users to roam throughout Europe & compatible to ISDN and PSTN systems.
 Various versions of GSM
 GSM at 900MHZ 890-915MHZ -(U) 935-960MHZ -(D)
 GSM at 1800MHZ (DCS) 1710-1785MHZ-(U) 1805-1880MHZ(D)
 GSM at 1900MHZ(PCS) 1850-1910MHZ-(U) 1930-1990MHZ-(D)
 GSM initially introduced for Rail Road as GSM Rail( 19 channels, emergency call with ack, voice group call service, voice broadcast service, priority calls, conrol of trains, switches, gates, signals).
 GSM has defined 3 different categories of services:
 Bearer services( Transparent, Non-transparent, synchronous, asynchronous data)
 Tele-services (encrypted voice transmission message services, emergency no. SMS, EMS, MMS, group 3 FAX.
 Supplementary Services: user identification, call redirection forwarding, closed user groups multiparty.
 Cell horizontal radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometers. The longest distance the GSM specification supports in practical use is 35 kilometres (22 mi). There are also several implementations of the concept of an extended cell, where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.
 Indoor coverage is also supported by GSM and may be achieved by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed when a lot of call capacity is needed indoors, for example in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of the radio signals from nearby cells.
 The modulation used in GSM is Gaussian minimum-shift keying (GMSK), a kind of continuous-phase frequency shift keying. In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (adjacent channel interference).

GSM Security
 GSM offers several security services using confidential information stored in the AuC and in the individual SIM.The security services offered by GSM are as follows:-
 Access control and Authentication: the first step includes the authentication of a valid user for the SIM. The user needs a secret PIN to access the SIM. The nest step is the subscriber authentication.
 Confidentiality: All user–related data is encrypted. After authentication, BTS and MS apply encryption to voice, data and signaling. This confidentiality exists only between MS and BTS, but doesn’t exist end-to-end or within the whole fixed GSM/ telephone network.
 Anonymity: To provide user anonymity, all data is encrypted before transmission and user identifiers are not used over the air. Instead GSM transmit a temporary identifier (TMSI) which is newly assigned by the VLR after each location update.
Algorithm A3 for authentication, A5 for encryption, A8 for generation for cipher key.

The GSM Network
 GSM provides recommendations, not requirements. The GSM specifications define the functions and interface requirements in detail but do not address the hardware. The reason for this is to limit the designers as little as possible but still to make it possible for the operators to buy equipment from different suppliers. The GSM network is divided into three major systems: the switching system (SS), the base station system (BSS), and the operation and support system (OSS). The basic GSM network elements are shown in Picture
 The Switching System The switching system (SS) is responsible for performing call processing and subscriber-related functions. The switching system includes the following functional units.

 Home Location Register (HLR) - The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the PCS operators, he or she is registered in the HLR of that operator.

 Mobile Services Switching Center (MSC) - The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others.

 Visitor Location Register (VLR) - The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.

 Authentication Center (AUC) - A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.

 Equipment Identity Register (EIR) - The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.

The GSM Network Structure
 The Base Station System (BSS)All radio-related functions are performed in the BSS, which consists of base station controllers (BSCs) and the base transceiver stations (BTSs).

 BSC - The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.

 BTS - The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC.

 The Operation and Support System :The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS). The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of OSS is to offer the customer cost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM network. An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations.

Additional Functional Elements

 Message center (MXE) - The MXE is a node that provides integrated voice, fax, and data messaging. Specifically, the MXE handles short message service, cell broadcast, voice mail, fax mail, e-mail, and notification.

 Mobile Service Node (MSN) - The MSN is the node that handles the mobile intelligent network (IN) services.

 Gateway Mobile Services Switching Center (GMSC) - A gateway is a node used to interconnect two networks. The gateway is often implemented in an MSC. The MSC is then referred to as the GMSC.

 GSM Interworking Unit (GIWU) - The GIWU consists of both hardware and software that provides an interface to various networks for data communications. Through the GIWU, users can alternate between speech and data during the same call. The GIWU hardware equipment is physically located at the MSC/VL.

GSM Channel Structure
 Depending on the kind of information transmitted (user data and control signaling), we refer to different logical channels which are mapped under physical channels (slots). Digital speech is sent on a logical channel named TCH, which during the transmission can be a allocated to a certain physical channel. In a GSM system no RF channel and no slot is dedicated to a priori to the exclusive use of anything (any RF channel can be used for number of different uses).
 Logical channels are divided into two categories :
 i) Traffic Channels (TCHs) ii)Control Channels .

 1. Traffic Channels (TCHs) : A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined using a 26-frame multiframe, or group of 26 TDMA frames. The length of a 26-frame multiframe is 120 ms, which is how the length of a burst period is defined (120 ms divided by 26 frames divided by 8 burst periods per frame). Out of the 26 frames, 24 are used for traffic, 1 is used for the Slow Associated Control Channel (SACCH) and 1 is currently unused . TCHs for the uplink and downlink are separated in time by 3 burst periods, so that the mobile station does not have to transmit and receive simultaneously, thus simplifying the electronics
 TCHs carry either encoded speech or user data in both up and down directions in a point to point communication.
 There are two types of TCHs that are differentiated by their traffic rates.
They are:
Full Rate TCH It carries information at a gross rate of 22.82 Kbps.
Half Rate TCH It carries information with half of full rate channels(11.4kbps)

 Control Channels (CCH) : Used to control medium access, allocation of traffic channels or mobility management.
 Broadcast Control Channel (BCCH): BTs uses this channel to signal information (freq availability) to all MSs within cell
 Common Control Channel (CCCH): All info. Regarding connection setup between MS and BS is exchanged cia the CCH. Comprised of three control channels used during call origination and call paging.
Random Access Channel (RACH): A slotted Aloha channel to request access to the network
Paging Channel (PCH): Used to alert the mobile station of incoming call.
Access Grant Channel (AGCH): Used to allocate an SDCCH to a mobile for signaling, following a request on the RACH.
 Dedicated control channel (DCCH) : It is a bidirectional channel used by MS to BTs.









































GSM: Performance Characteristics
 Support for voice and data services
 Total mobility
 International access, chip‐card enables use of access points of different providers
 Worldwide connectivity
 One number, the network handles localization
 High capacity
 Better frequency efficiency, smaller cells, more customers per cell
 High transmission quality
 High audio quality and reliability for wireless, uninterrupted
 phone calls at higher speeds (e.g., from cars, trains)

Advantages of GSM
 GSM is mature; this maturity means a more stable network with robust features
 Less signal deterioration inside buildings.
 Ability to use repeaters.
 Talktime is generally higher in GSM phones due to the pulse nature of transmission.
 The availability of SIM allows users to switch networks and handsets at will, aside from a subsidy lock.
 GSM covers virtually all parts of the world so international roaming is not a problem

Disadvantages of GSM
 There is no perfect system!!
 No end-to-end encryption of user data
 No full ISDN bandwidth of 64 kbit/s to the user,
 No transparent B channel
 Security and Privacy issues
 Abuse of private data possible
 Roaming profiles accessible
 High complexity of the system
 Several incompatibilities within the GSM standards
 Safety issues
 Reduced concentration while driving
 Electromagnetic radiation

Location/Mobility Management
 Mobility Management is one of the major functions of a GSM or a UMTS network that allows mobile phones to work. The aim of mobility management is to track where the subscribers are, so that calls, SMS and other mobile phone services can be delivered to them.
 A GSM or UMTS network, like all cellular networks, is a radio network of individual cells, known as base stations. Each base station covers a small geographical area which is part of a uniquely identified location area. By integrating the coverage of each of these base stations, a cellular network provides a radio coverage over a very much wider area. A group of base stations is called a location area, or a routing area.
 The location update procedure allows a mobile device to inform the cellular network, whenever it moves from one location area to the next. Mobiles are responsible for detecting location area codes. When a mobile finds that the location area code is different from its last update, it performs another update by sending to the network, a location update request, together with its previous location, and its Temporary Mobile Subscriber Identity (TMSI).
 There are several reasons why a mobile may provide updated location information to the network. Whenever a mobile is switched on or off, the network may require it to perform an IMSI attach or IMSI detach location update procedure. Also, each mobile is required to regularly report its location at a set time interval using a periodic location update procedure. Whenever a mobile moves from one location area to the next while not on a call, a random location update is required. This is also required of a stationary mobile that reselects coverage from a cell in a different location area, because of signal fade. Thus a subscriber has reliable access to the network and may be reached with a call, while enjoying the freedom of mobility within the whole coverage area.
 When a subscriber is paged in an attempt to deliver a call or SMS and the subscriber does not reply to that page then the subscriber is marked as absent in both the MSC/VLR and the HLR (Mobile not reachable flag MNRF is set). The next time the mobile performs a location update the HLR is updated and the mobile not reachable flag is cleared.
 TMSI
The "Temporary Mobile Subscriber Identity" (TMSI) is the identity that is most commonly sent between the mobile and the network. It is a randomly allocated number that is given to the mobile, the moment it is switched on. The number is local to a location area, and so it has to be updated, each time the mobile moves to a new geographical area.
 Roaming
Roaming is one of the fundamental mobility management procedures of all cellular networks. Roaming is defined as the ability for a cellular customer to automatically make and receive voice calls, send and receive data, or access other services, including home data services, when traveling outside the geographical coverage area of the home network, by means of using a visited network. This can be done by using a communication terminal or else just by using the subscriber identity in the visited network. Roaming is technically supported by mobility management, authentication, authorization and billing procedures.

 Location Area
A "location area" is a set of base stations that are grouped together to optimise signalling. Typically, 10s or even 100s of base stations share a single Base Station Controller (BSC), the intelligence behind the base stations. The BSC handles allocation of radio channels, receives measurements from the mobile phones, controls handovers from base station to base station.
 Routing Area
A "routing area" is a subdivision of a "location area". Routing areas are used by mobiles which are GPRS-attached. GPRS ("General Packet Radio Services"), GSM’s new data transmission technology, is optimized for "bursty" data communication services, such as wireless internet/intranet, and multimedia services. It is also known as GSM-IP ("Internet Protocol") because it will connect users directly to Internet Service Providers (ISP).

GSM Services
 GSM Subscriber ServicesThere are two basic types of services offered through GSM: telephony (also referred to as teleservices) and data (also referred to as bearer services). Telephony services are mainly voice services that provide subscribers with the complete capability (including necessary terminal equipment) to communicate with other subscribers. Data services provide the capacity necessary to transmit appropriate data signals between two access points creating an interface to the network.
 Dual-tone multifrequency (DTMF)—DTMF is a tone signaling scheme often used for various control purposes via the telephone network, such as remote control of an answering machine. GSM supports full-originating DTMF.
 facsimile group III—GSM supports CCITT Group 3 facsimile. As standard fax machines are designed to be connected to a telephone using analog signals, a special fax converter connected to the exchange is used in the GSM system. This enables a GSM–connected fax to communicate with any analog fax in the network.
 short message services—A convenient facility of the GSM network is the short message service. A message consisting of a maximum of 160 alphanumeric characters can be sent to or from a mobile station.. If the subscriber's mobile unit is powered off or has left the coverage area, the message is stored and offered back to the subscriber when the mobile is powered on or has reentered the coverage area of the network. This function ensures that the message will be received.
 cell broadcast—A variation of the short message service is the cell broadcast facility. A message of a maximum of 93 characters can be broadcast to all mobile subscribers in a certain geographic area. Typical applications include traffic congestion warnings and reports on accidents.
 voice mail—This service is actually an answering machine within the network, which is controlled by the subscriber. Calls can be forwarded to the subscriber's voice-mail box and the subscriber checks for messages via a personal security code.
 fax mail—With this service, the subscriber can receive fax messages at any fax machine. The messages are stored in a service center from which they can be retrieved by the subscriber via a personal security code to the desired fax number.
 call forwarding—This service gives the subscriber the ability to forward incoming calls to another number if the called mobile unit is not reachable, if it is busy, if there is no reply, or if call forwarding is allowed unconditionally.
 barring of outgoing calls—This service makes it possible for a mobile subscriber to prevent all outgoing calls.
 barring of incoming calls—This function allows the subscriber to prevent incoming calls. The following two conditions for incoming call barring exist: baring of all incoming calls and barring of incoming calls when roaming outside the home PLMN.
 advice of charge (AoC)—The AoC service provides the mobile subscriber with an estimate of the call charges. There are two types of AoC information: one that provides the subscriber with an estimate of the bill and one that can be used for immediate charging purposes. AoC for data calls is provided on the basis of time measurements.
 call hold—This service enables the subscriber to interrupt an ongoing call and then subsequently reestablish the call. The call hold service is only applicable to normal telephony.
 call waiting—This service enables the mobile subscriber to be notified of an incoming call during a conversation. The subscriber can answer, reject, or ignore the incoming call. Call waiting is applicable to all GSM telecommunications services using a circuit-switched connection.
 multiparty service—The multiparty service enables a mobile subscriber to establish a multiparty conversation—that is, a simultaneous conversation between three and six subscribers. This service is only applicable to normal telephony.
 calling line identification presentation/restriction—These services supply the called party with the integrated services digital network (ISDN) number of the calling party. The restriction service enables the calling party to restrict the presentation. The restriction overrides the presentation.
 closed user groups (CUGs)—CUGs are generally comparable to a PBX. They are a group of subscribers who are capable of only calling themselves and certain numbers.

What is Multiple Access
 Multiple users want to communicate in a common geographic area
 Cellular Example: Many people want to talk on their cell phones. Each phone must communicate with a base station.
 Imagine if only one person could talk on their cell phone at a time!
 Problem: How should we share our resources so that as many users as possible can communicate simultaneously
 The concept behind multiple access is to permit a number of users to share a common channel. The two traditional ways of multiple access are Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA).

















Access Scheme

FDMA
 In Frequency Division Multiple Access, the frequency band is divided in slots. Each user gets one frequency slot assigned that is used at will. It could be compared to AM or FM broadcasting radio where each station has a frequency assigned. FDMA demands good filtering.

TDMA
 In Time Division Multiple Access, the frequency band is not partitioned but users are allowed to use it only in predefined intervals of time, one at a time. Thus, TDMA demands synchronization among the users

Access Scheme (CDMA)

CDMA
 CDMA, for Code Division Multiple Access, is different than those traditional ways in that it does not allocate frequency or time in user slots but gives the right to use both to all users simultaneously. To do this, it uses a technique known as Spread Spectrum. In effect, each user is assigned a code which spreads its signal bandwidth in such a way that only the same code can recover it at the receiver end. This method has the property that the unwanted signals with different codes get spread even more by the process, making them like noise to the receiver.
 CDMA (Code-Division Multiple Access) refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.
 CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. There are trillions of possible frequency-sequencing codes, which enhance privacy and makes cloning difficult.
 The CDMA channel is nominally 1.23 MHz wide. CDMA networks use a scheme called soft handoff, which minimizes signal breakup as a handset passes from one cell to another. The combination of digital and spread-spectrum modes supports several times as many signals per unit bandwidth as analog modes. CDMA is compatible with other cellular technologies; this allows for nationwide roaming.
 The original CDMA standard, also known as CDMA One and still common in cellular telephones in the U.S., offers a transmission speed of only up to 14.4 Kbps in its single channel form and up to 115 Kbps in an eight-channel form. CDMA2000 and wideband CDMA deliver data many times faster.

Advantages of CDMA
• Frequency diversity
• multiple path resistance
• privacy –security
• graceful degradation

Disadvantages of CDMA
• Self jamming
• near-far problem
• soft hand off

GSM vs. CDMA
• GSM
o GSM is a widely spread standard
o GSM provided by BSNL, AIRTEL, ESCOTEL etc
o GSM users are almost 8 times in number than CDMA users worldwide
o GSM is far better than CDMA in voice quality
o GSM base stations consumes more power than CDMA and also covers a less distance
o Cell size in GSM is small compared to GSM.
o It covers a large area of more than 25 Kms.
o GSM offers slower data download
o On a GSM phone your account information along with your contact list and other personal data are stored on a SIM card (Subscriber Identity Module)
o Maximum download speed of 384kbps (around 140kbps in practice).
o Europe, South Africa, Australia, and many Middle and Far East countries have chosen to adopt GSM
o It uses TDMA.
o It is 2nd generation
o Its year of first use was 1991
o Roaming is worldwide
o Battery life is very good due to simple protocol, good coverage and mature, power efficient chipsets
o Hard Handoff

• CDMA
o CDMA is a patented technology
o CDMA provided by Reliance
o CDMA users are almost 8 times less in number than GSM users worldwide
o CDMA is poor than GSM in voice quality
o CDMA base stations consumes less power than GSM and also covers a large distance
o Cell size in CDMA is larger compared to GSM
o user cannot go beyond a short distance charging area (SDCA) - which is roughly a radius of 25 km
o CDMA offers faster data download
o On a CDMA phone, your account information is programmed into your cellular phone
o Maximum download speed of about 2mb/s (about 700kbps in practice)
o CDMA is mostly used in America and some parts of Asia
o It uses CDMA
o It is 3rd generation
o Its year of first use was 2000
o Roaming is limited
o Battery life lower due to high demands of CDMA power control and young chipsets
o Soft Handoff

Handoff
 In cellular telecommunications, the term handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another. In satellite communications it is the process of transferring satellite control responsibility from one earth station to another without loss or interruption of service. The British English term for transferring a cellular call is handover, which is the terminology standardised by 3GPP within such European originated technologies as GSM and UMTS.
 In telecommunications there may be different reasons why a handoff (handover) might be conducted:
 when the phone is moving away from the area covered by one cell and entering the area covered by another cell the call is transferred to the second cell in order to avoid call termination when the phone gets outside the range of the first cell;
 when the capacity for connecting new calls of a given cell is used up and an existing or new call from a phone, which is located in an area overlapped by another cell, is transferred to that cell in order to free-up some capacity in the first cell for other users, who can only be connected to that cell.
 Handoff Management Phases:
 Initiation Phase ( With hysteresis, without hysteresis )
 Execution Phase ( allocation of new radio resources, exchange of control messages)
Handoff Strategies: Depending on the information used and the action taken to initiate the handoff, the methods for handoff can be
1. Mobile Controlled Handoff (MCHO) e.g. DECT
2. Network Controlled Handoff NCHO) eg. AMPS, CT-2 Plus
3. Mobile Assisted Handoff (MAHO) e.g. GSM, IS-95 CDMA
 In non-CDMA networks when the channel used by the phone becomes interfered by another phone using the same channel in a different cell, the call is transferred to a different channel in the same cell or to a different channel in another cell in order to avoid the interference.

 In non-CDMA networks when the user behavior changes, e.g. when a fast-traveling user, connected to a large, umbrella-type of cell, stops then the call may be transferred to a smaller macro cell or even to a micro cell in order to free capacity on the umbrella cell for other fast-traveling users and to reduce the potential interference to other cells or users (this works in reverse too, when a user is detected to be moving faster than a certain threshold, the call can be transferred to a larger umbrella-type of cell in order to minimize the frequency of the handoffs due to this movement)

 in CDMA networks a soft handoff (see further down) may be induced in order to reduce the interference to a smaller neighboring cell due to the "near-far" effect even when the phone still has an excellent connection to its current cell;

Handoff Features
 The Effective handoff scheme should have the following features.
1. Fast & Lossless
2. Minimal no. of Control signal exchanges
3. Scalable with network size
4. Capable of recovering from link features , such as abrupt loss of radio link.

The design goals of an effective Handoff scheme include
1. Loss Handoff delay
2. Low cell loss
3. Small buffer required
4. Efficient use of resources.

Types of Handoff
 The most basic form of handoff (handover) is when a phone call in progress is redirected from its current cell (called source) and its used channel in that cell to a new cell (called target) and a new channel. In terrestrial networks the source and the target cells may be served from two different cell sites or from one and the same cell site (in the latter case the two cells are usually referred to as two sectors on that cell site). Such a handoff, in which the source and the target are different cells (even if they are on the same cell site) is called inter-cell handoff. The purpose of inter-cell handoff is to maintain the call as the subscriber is moving out of the area covered by the source cell and entering the area of the target cell.
 A special case is possible, in which the source and the target are one and the same cell and only the used channel is changed during the handoff. Such a handoff, in which the cell is not changed, is called intra-cell handoff. The purpose of intra-cell handoff is to change one channel, which may be interfered or fading with a new clearer or less fading channel.
 A hard handoff is one in which the channel in the source cell is released and only then the channel in the target cell is engaged. Thus the connection to the source is broken before the connection to the target is made -- for this reason such handoffs are also known as break-before-make. Hard handoffs are intended to be instantaneous in order to minimise the disruption to the call. A hard handoff is perceived by network engineers as an event during the call.
 A soft handoff is one in which the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. In this case the connection to the target is established before the connection to the source is broken, hence this handoff is called make-before-break. The interval, during which the two connections are used in parallel, may be brief or substantial. For this reason the soft handoff is perceived by network engineers as a state of the call, rather than a brief event. A soft handoff may involve using connections to more than two cells, e.g. connections to three, four or more cells can be maintained by one phone at the same time. When a call is in a state of soft handoff the signal of the best of all used channels can be utilised for the call at a given moment or all the signals can be combined to produce a clearer copy of the signal. The latter is more advantageous, and when such combining is performed both in the downlink (forward link) and the uplink (reverse link) the handoff is termed as softer. Softer handoffs are possible when the cells involved in the handoff have a single cell site.


GPRS
 General Packet Radio Service (GPRS) is a Mobile Data Service available to users of Global System for Mobile Communications (GSM) and IS-136 mobile phones. It provides data rates from 56 up to 114 Kbps.
 GPRS data transfer is typically charged per kilobyte of transferred data, while data communication via traditional circuit switching is billed per minute of connection time, independent of whether the user has actually transferred data or has been in an idle state. GPRS can be used for services such as Wireless Application Protocol (WAP) access, Short Message Service (SMS), Multimedia Messaging Service (MMS), and for Internet communication services such as email and World Wide Web access.
 2G cellular systems combined with GPRS is often described as "2.5G", that is, a technology between the second (2G) and third (3G) generations of mobile telephony. It provides moderate speed data transfer, by using unused Time division multiple access (TDMA) channels in, GPRS is integrated into GSM Release 97 and newer releases.
 It was originally standardized by European Telecommunications Standards Institute (ETSI), but now by the 3rd Generation Partnership Project (3GPP).
 The General Packet Radio Service (GPRS) is a new nonvoice value added service that allows information to be sent and received across a mobile telephone network. It supplements today's Circuit Switched Data and Short Message Service. GPRS is NOT related to GPS (the Global Positioning System), a similar acronym that is often used in mobile contexts.


 GPRS Handset Classes: There are three different classes of devices.
1. Class A handsets can do both voice and data at the same time (simultaneously). If you were to receive a voice call will using the Internet, say, the connection would be placed on busy while you answer the call, rather than have it disconnected.
2. Class B handsets are voice and packet data capable, but not at the same time. It can only support either a voice or data service at a time. But like in Class A above, a voice call would put the data call on hold, and vice versa.
3. Class C handsets can handle only non-simultaneous data and voice calls. The user must manually select the service they wish to connect to. (SMS is also optional for Class C terminals).

 Classes of GPRS Services: Mobile devices can request different types of traffic to be priortised in a attempt to give the user their desired level of connectivity. There are 4 types of classes.
1. Precedence Class: An application can be assigned precedence classes 1,2, or 3. If an application has higher precedence(1) than another(3) then its traffic will be given a higher priority.
2. Delay classes: Applications can request predictive delay classes which guarantee an average and 95% delay.
3. Reliability class: application can request differing levels of reliability for its data depending on its tolerance of data loss.
4. Throughput class: Applications can choose different profiles for throughput.

GPRS Network System Architechture
 A GSM network mainly consists of four components.
 Mobile Station (MS) carried by the subscriber .
 Base Station Subsystem (BSS) controls radio link with mobile station .
 Mobile Switching Center (MSC) is the central component of the NSS. Operates all switching functions for the mobiles within its jurisdiction. Interface between mobile and other (including fixed) network.
 Network Databases : Home Location Register and Visitor Location Register together with MSC provides the call routing and roaming capabilities of GSM.
 In order to integrate GPRS into the existing GSM network, two major new core network elements are introduced: the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support node (GGSN).
 Serving GPRS Support Node (SGSN): An SGSN is responsible for the delivery of data packets from and to the mobile stations within its service area. SGSNs send queries to Home Location Registers (HLRs) to obtain profile data of GPRS subscribers. SGSNs detect new GPRS mobile stations in a given service area; and, finally, SGSNs process registration of new mobile subscribers and keep a record of their location inside a given service area.
 Gateway GPRS Support Node (GGSN) : GGSNs are used as interfaces between the GPRS backbone network and the external Public Packet Data Networks. GGSNs maintain routing information that is necessary to tunnel the Protocol Data Units (e.g IP) to the SGSNs that service particular mobile stations. Other functions include network and subscriber screening and address mapping. One or more GGSNs may support multiple SGSNs.
 In addition to the new GPRS components, following existing GSM network elements must also be enhanced in order to support GPRS.
 Base Station System (BSS): must be enhanced to recognize and send user data to the SGSN that is serving the area.
 Home Location Register (HLR): must be enhanced to register GPRS user profiles and respond to queries originating from SGSNs regarding these profiles.


As can be seen, there are a number of new standardized network interfaces introduced:
• Gb Frame relay connection between the SGSN and the PCU within the BSS. This transports both user data and signaling messages to/from the SGSN.
• Gn The GPRS backbone network implemented using IP LAN/WAN technology. Used to provide virtual connections between the SGSN and GGSN.
• Gi The point of connection between GPRS and the external networks, each referenced by the Access Point Name. This will normally be implemented using IP WAN technology.
• Gr Interface between the HLR and SGSN that allows access to customer subscription information. This has been implemented using enhancements to the existing GSM C7 MAP interface.

















To use GPRS, users specifically need:

 A mobile phone or terminal that supports GPRS (existing GSM phones do NOT support GPRS)

 A subscription to a mobile telephone network that supports GPRS;

 Use of GPRS must be enabled for that user. Automatic access to the GPRS may be allowed by some mobile network operators, others will require a specific opt-in;

 Knowledge of how to send and/or receive GPRS information using their specific model of mobile phone, including software and hardware configuration (this creates a customer service requirement);

 A destination to send or receive information through GPRS. Whereas with SMS this was often another mobile phone, in the case of GPRS, it is likely to be an Internet address, since GPRS is designed to make the Internet fully available to mobile users for the first time. From day one, GPRS users can access any web page or other Internet applications- providing an immediate critical mass of uses.


GPRS Services
 Multimedia Messaging Service (MMS)
 Push to talk over Cellular PoC / PTT
 Instant Messaging and Presence -- Wireless Village
 Internet Applications for Smart Devices through Wireless Application Protocol (WAP)
 Point-to-point (PTP) service: internetworking with the Internet (IP protocols)
 Short Message Service (SMS)
 Future enhancements: flexible to add new functions, such as more capacity, more users, new accesses, new protocols, new radio networks.
 USB GPRS modem:USB GPRS modems use a terminal-like interface USB 2.0 and later, data formats V.42bis, and RFC 1144 and external antennas. Modems can be add in cards (for laptop) or external USB devices which are similar in shape and size to a computer mouse.
 GPRS can be used as the bearer of SMS. If SMS over GPRS is used, an SMS transmission speed of about 30 SMS messages per minute may be achieved. This is much faster than using the ordinary SMS over GSM, whose SMS transmission speed is about 6 to 10 SMS messages per minute



Limitations Of GPRS
 GPRS does impact a network's existing cell capacity.
 Only limited resources.
 Use for one purpose precludes simultaneous use for another.
 Maximum speed of 171.2 kbps only theoretically.
 Single user would need all 8 time slots.
 Network operator would never allow that.
 Bandwidth limited.
 Limited cell capacity for all users
 Speeds much lower in reality

Features of GPRS
 Faster data transfer rates
GPRS currently supports an average data rate of 115 Kbps, but this speed is only achieved by dedicating all eight time slots to GPRS. Instead, carriers and terminal devices will typically be configured to handle a specific number of time slots for upstream and downstream data. The aggregate cell site bandwidth is shared by voice and data traffic. GPRS operators will vary in how they allocate the bandwidth. Typically, they will configure the networks to give precedence to voice traffic; some may dedicate time slots to data traffic to ensure a minimum level of service during busy voice traffic periods. Unused voice capacity may be dynamically reallocated to data traffic.

 Always-on connection
An “always-on” connection eliminates the lengthy delays required to reconnect to the network to send and receive data. Information can also be pushed to the end user in real time.

 Robust connectivity
GPRS improves data transmission integrity with a number of mechanisms. First, user data is encoded with redundancies that improve its resistance to adverse radio conditions. The amount of coding redundancy can be varied, depending on radio conditions. GPRS has defined four coding schemes CS1 through CS4. Initially, only CS1 and CS2 will be supported, which allows approximately 9 and 13 Kbps in each time slot. If an error is detected in a frame received in the BSS, the frame may be repeatedly retransmitted until properly received before passing it on to the GPRS core network.

 Broad application support
Like the Internet, GPRS is based on packet-switched data. This means that all native IP applications, such as email, Web access, instant messaging, and file transfers can run over GPRS. In addition, its faster data transfer rates enable GPRS to accommodate higher-bandwidth applications (such as multimedia Web content) not suited to slower GSM dial-up connections. GPRS is particularly well suited for applications based on the Wireless Application Protocol (WAP).

 Security support
GPRS builds on the proven authentication and security model used by GSM. At session initiation, a user is authenticated using secret information contained on a smart card called a Subscriber Identity Module (SIM). Authentication data is exchanged and validated with records stored in the HLR network node. GPRS enables additional authentication using protocols such as RADIUS before the subscriber is allowed access to the Internet or corporate data networks.

INTERWORKING WITH THE EXTERNAL INTERNET
 Before a GPRS mobile station can use GPRS services it must obtain an address used in the packet data network (a PDP address) and create a PDP context. The context describes the characteristics of the connection to the packet data network (PDP type, PDP address, service precedence, reliability, delay, throughput and GGSN). With an active PDP context, packets from mobile station will be sent to its current SGSN first, then this SGSN encapsulates the IP packets, examines the PDP context, and routes them to appropriate GGSN. The GGSN encapsulates the packets and sends them out on the IP network. Similarly packets from the external packet data network will be routed to the GGSN first, which then queries the HLR and obtains the information where the MS is currently located in. It encapsulates the incoming packets and tunnels them to the current SGSN of the mobile user. The SGSN recalculates the packets and delivers them to MS. Each GGSN has an IP address and each mobile station has been assigned an IP address by its GGSN. Thus the MS's IP address has the same network prefix as the IP address of its GGSN.
 In GPRS network, user's current locations are managed in two levels: Micro mobility management tracks the current routing area or cell of the mobile station. It is performed by the SGSN. Macro mobility management keeps track of the mobile station's current SGSN and stores it in the HLR, VLR, and GGSN.

GPRS Transmission Plane Protocol Reference Model



















 All data within the GPRS backbone, i.e. between the GSNs (SGSN-GGSN), is transferred using the GTP (GPRS tunneling protocol). GTP can use two different transport protocols, either reliable TCP for X.25 packets or the non-reliable UDP used for IP packets.
 To adapt to the different characteristics of the underlying networks, the Subnetwork Dependent Convergence Protocol (SNDCP) is used between an SGSN and the MS
 On top of SNDCP and GTP user packet data is tunneled from the MS to the GGSN and vice versa.
 To achieve high reliability of packet transfer between SGSN and MS, a special LLC is used, which comprises ARQ and FEC mechanisms.
 A Base Station Subsystem GPRS Protocol (BSSGP) is used to convey routing and QoS -related information between the BSS and SGSN. BSSGP doesn’t perform error correction and works on top of Frame relay (FR) network.
 Radio link dependent protocols are needed to transfer data over the Um interface. The Radio Link Protocol (RLC) provides a reliable link.
 The MAC controls access with signaling procedures for the radio channel and their maping of LLC frames onto the GSM physical channels.
 The radio interface at Um needed for GPRS doesn’t require fundamental changes compared to standard GSM.

GPRS: air interface
Radio Link Control (RLC)
 Segmentation of the LLC-Frames in RLC blocks
 Block size dependent on short-term channel conditions
 Backward error correction and data flow control by Automatic Repeat Request (ARQ) protocol
 repeating not repairable RLC blocks selectively
Medium Access Control ( MAC)
 Channel reservation contains:
- one/several time slots (Packet Data Channels PDCH) of one
frequency
 one uplink status flag (USF) per Packet Data Channel (PDCH), channel partition of up to 8 ms
 Medium Access Control ( MAC)
 Reservation in the uplink (MS to BSS):

 MS sends reservation request on a Random Access Channel (Slotted ALOHA)
 BTS allocates a (split) channel and sends packet assignment
 MS sends data depending on the current priority (USF flag)

 Reservation in the Downlink (BSS to MS):

 BTS displays transmitting request and informs about the reserved channel
 MS supervises the reserved channel and receives
 Physical Link Control
 adaptive forward error correction (FEC) dependent on short-term channel conditions
 temporal scrambling (Interleaving) of the bursts and Mapping on reserved PDCH (Packet Data Channel)
 procedure to recognize overbooking situations on the physical channel

GPRS Channel Encoding











GPRS Applications
 Chat
 Textual and visual information
 Still & moving images
 Web browsing
 Document sharing/Collaborate working
 Audio
 Email, File Transfer…

GSM Vs. GPRS

It is circuit switched. It is packet switched.
It is not ‘Always-on’. It is ‘Always-on’.
You’re charged for the time the channel is reserved. - You’re charged for the amount of data that’s being transported, not for the time that the unit is online.
The System uses the same TDMA (Time Division Multiple Access) link with one out of seven time slots. The GPRS connection in the t610 can use as many as 4+1 time slots.
Circuit switching provides the customer with a dedicated channel all the way to the destination. The customer has exclusive use of the circuit for the duration of the call, With packet switching, the operator assigns one or more dedicated channels specifically for shared use. These channels are up and running 24 hours a day, and when you need to transfer data, you access a channel and transmit your data.
The standard data rate of a GSM channel is 22.8 kbps It provides data rates from 56 up to 114 Kbps.

Operating System Structures

Operating System Structures

Ø In this part of the course, we will briefly look at the operating systems from its functional point of view; that is the services which are provided by the operating system. A detailed discussion will follow in the subsequent lectures.

Common System Components

§ Due to the complex nature of the modern operating systems, it is partitioned into smaller component. Each component performs a well-defined function w

§ ith well-defined inputs and outputs.

§ Many modern operating systems have the following components.

- Process Management

- Main Memory Management

- File Management

- I/O System Management

- Secondary Management

- Networking

- Protection System

- Command-Interpreter System

Basic OS Organization

Process Management

§ A process is a program in execution. For example

- A batch job is a process

- A time-shared user program is a process

- A system task (e.g. spooling output to printer) is a process.

§ Remember a program itself is not a process rather it is a passive entity.

§ A process needs certain resources, including CPU time, memory, files, and I/O devices, to accomplish its task. These resources are either given to the process when it is created or when it is running. When the process completes, the OS reclaims all the resources.

§ The operating system is responsible for the following activities in connection with process management.

- Process creation and deletion.

- Process suspension and resumption.

- Provision of mechanisms for:

o Process synchronization

o Process communication

Main

Memory Management

§ Memory is a large array of words or bytes, each with its own address. It is a repository of quickly accessible data shared by the CPU and I/O devices.

§ Main memory is a volatile storage device. It loses its contents in the case of system failure.

§ The operating system is responsible for the following activities in connections with memory management:

- Keep track of which parts of memory are currently being used and by whom.

- Decide which processes to load when memory space becomes available.

- Allocate and deallocate memory space as needed.

File Management

§ Most visible component of OS. Computers can store information on several different types of physical media (e.g. magnetic tap,

§ magnetic disk, CD etc).

§ For convenient use of the computer system, the OS provides a uniform logical view of information storage.

§ A file a logical storage unit, which abstract away the physical properties of its storage device.

§ A file is a collection of related information defined by its creator. Commonly, files represent programs (both source and object forms) and data.

§ The operating system is responsible for the following activities in connections with file management:

- File creation and deletion.

- Directory creation and deletion.

- Support of primitives for manipulating files and directories.

- Mapping files onto secondary storage.

- File backup on stable (nonvolatile) storage media.

I/O System Management

§ The I/O system consists of:

- A buffer-caching system

- A general device-driver interface

- Drivers for specific hardware devices

Secondary Storage management

§ Since main memory (primary storage) is volatile and too small to accommodate all data and programs permanently, the computer system must provide secondary storage to back up main memory.

§ Most modern computer systems use disks as the principle on-line storage medium, for both programs and data.

§ The operating system is responsible for the following activities in connection with disk management:

- Free space management

- Storage allocation

- Disk scheduling

Networking (Distributed Systems)

§ A distributed system is a collection processors that do not share memory or a clock. Each processor has its own local memory.

§ The processors in the system are connected through a communication network.

§ Communication takes place using a protocol.

§ A distributed system provides user access to various system resources.

§ Access to a shared resource allows:

- Computation speed-up

- Increased data availability

- Enhanced reliability

Protection System

§ Protection refers to a mechanism for controlling access by programs, processes, or users to both system and user resources.

§ The protection mechanism must:

- Distinguish between authorized and unauthorized usage.

- Specify the controls to be imposed.

- Provide a means of enforcement.

Command-Interpreter System

§ Many commands are given to the operating system by control statements which deal with:

- Process creation and management

- I/O handling

- Secondary-storage management

- Main-memory management

- File-system access

- Protection

- Networking

§ The program that reads and interprets control statements is called variously:

- Command-line interpreter

- Shell (in UNIX)

§ Its function is to get and execute the next command statement.

Operating System Services

§ Program execution – system capability to load a program into memory and to run it.

§ I/O operations – since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/O.

§ File-system manipulation – program capability to read, write, create, and delete files.

§ Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or message passing.

§ Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs.

Ø Additional functions exist not for helping the user, but rather for ensuring efficient system operations.

§ Resource allocation – allocating resources to multiple users or multiple jobs running at the same time.

§ Accounting – keep track of and record which users use how much and what kinds of computer resources for account billing or for accumulating usage statistics.

§ Protection – ensuring that all access to system resources is controlled

Requesting Services from OS

§ System Call

- Process traps to OS Interrupt Handler

- Supervisor mode set

- Desired function executed

- Returns to application

§ Message Passing

- User process constructs message indicating function (service needed)

- Invokes send to pass message to OS

- Process blocks

- ……

- OS receives message

- OS initiates function execution

- Upon function completion, OS returns “OK”

- Process unblock…

System Calls

§ System calls provide the interface between a process and the operating system. These calls are generally available as assembly language instructions

§ . Some systems also allow to make system calls from a high level language, such as C.

§ Three general methods are used to pass parameters between a running program and the operating system.

- Pass parameters in registers.

- Store the parameters in a table in memory, and the table address is passed as a parameter in a register.

- Push (store) the parameters onto the stack by the program, and pop off the stack by operating system.

Types of System Calls

§ Process control – load, execute, abort, end, create process, allocate and free memory, wait event etc.

§ File management – Create file, delete file, open, close, read, write, get file attribute etc.

§ Device management – Request device, release device, read, write, logically attach or detach device etc.

§ Information maintenance – Get time and date, set time and date, get process attribute etc.

§ Communications – create, close communication connection, send, receive messages, etc.

System Programs

§ System programs provide a convenient environment for program development and execution. The can be divided into:

- File manipulation – create, delete, copy, rename, print, list etc.

- Status information – Display date, time, disk space, memory size, etc.

- File modification – Create and modify files using text editors.

- Programming language support – Compilers, assemblers, and interpreters.

- Program loading and execution – Loaders, linkers.

- Communications – remote login, send and receive messages.

§ Most users’ view of the operation system is defined by system programs, not the actual system calls.

System Structure

§ Modern OS should be developed carefully due to their size and complexity.

§ A common approach is to divide the systems into small components.

MS-DOS System Structure

§ MS-DOS – written to provide the most functionality in the least space

- Not divided into modules

- Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

Unix System Structure

§ UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts.

o Systems programs – use kernel supported system calls to provide useful functions such as compilation and file manipulation.

o The kernel

- Consists of everything below the system-call interface and above the physical hardware

- Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level.

Layered Approach

§ The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.

§ An OS layer is an implementation of an abstract object that is the encapsulation of data and operations that can manipulate those data. These operations (routines) can be invoked by higher-level layers. The layer itself can invoke operations on lower-level layers.

§ Layered approach provides modularity. With modularity, layers are selected such that each layer uses functions (operations) and services of only lower-level layers.

§ Each layer is implemented by using only those operations that are provided lower level layers.

§ The major difficulty is appropriate definition of various layers.

Microkernel System Structure

§ Moves as much from the kernel into “user” space.

§ Communication takes place between user modules using message passing.

§ Benefits:

- Easier to extend a microkernel

- Easier to port the operating system to new architectures

- More reliable (less code is running in kernel mode)

- More secure