mobile computing seminar or presentation report
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ABSTRACT
The popularity and usefulness of the Internet seem to grow exponentially every day. As a result of this rapid growth, mobile users expect to access the Internetâ„¢s information resources and to communicate with other Internet users. However, providing efficient and reliable stream transmissions in mobile computing environments presents a significant obatacle to the development of mobile computing systems over the Intenet. this paper presents three approaches-known as Fast Retransmissions, Data Interceptions, and Packet Interceptions-that can be used to improve the performance and reliability of transmission protocols in mobile computing environments. The paper also discusses how packet interception has become the most effective approach of the three and describes its, implementation.

INTRODUCTION
One of the tióst challenging and interesting recent trends in the computer ar.d communications iniustries is the integration of mobile communications and omputing. The resulting distributed network, referred to as mobile computing system, is in, more than one way fundamentally different from conventional wired computer networks.
With the adv nt of small portable computers and technological advan in wireless communications, mobile wireless computing is likely to become vexy popti tar in the near future. Wireless links are slower and less reliable compared to wired links ar d are prone to losses of signal due to noise and fading. Furthermore, host mobility can give rise to periods of disconnection frown the fixed ietwork. The use of óxisting network protoco which were developed mainly fo the high bandwidth and faster wired links, with mobile cmputers thus give rise to unique performance problems arising from lost mobility and due to the characteristics of wireless medium.

MOBILE COMPUTING : WHEN MOBILifY MEETS COMPIT1
Wireless connectivity enhances the functionality of éomputing equipment by freeing communication from the location constraints of the wireline structure. By changing this basic characteristics, mobile computing systems, requiring researchers and users to redefine their model of networked computing.
The successful use of mobile computing faces several challenges, among them.
1. The communication link between the mobile host and the base station is unpredictabW and varies greatly due to the constantly changing location of the mobile nodes and interference of non-network entities such as buildings.
2. The topology of the network changes rapidly due to the movement and resulting connections and disconnections of the mobile nodes.
3 The available bandwidth is limited and variable, depending on location chant reüie must, therefore, be an integral part of the channel access system in order to provide service toas many potential sub as possible.
4. The power available to mobile node is limited and, as a result, the poser required for tr and receiving must be minimized.

THE EFFECTS OF MOTION IN TCP
Motion across wireless cell boundaries cause increased delays and packet losses while the network 1ea how to route data to a hostâ„¢s new location. Reliable transpot prptQcols li TCP interpret these delays and losses as signs of network congestion. They consequently throttle their transmissions, further degrading performance.
Transport-level connections will encounter types of delays and loss that are unrelated to congestion. First, communication may pause while the handoff between cells completes and. packets can again be routed to and from the mobile host. Second, packets may be lost due to futile transmissions over the wireless network when a mobile host moves out of reach of other transceivers, especially in networks with little or no overlap between cells. Third, packets may be lost due to the relatively frequent transmission errors suffered by wireless links. Some performance degradation due to these delays and losses is unavoidable.
These events also trigger congestion control procedures thai further degrade performance. In particular, TCP implementations continually measure how long acknowledgements take to return. They maintain a running average of this delay and an estimate of the expected deviation in delayfrom the average If the current delay is longer than the average by more than twice the expected deviation, TCP assumes that the packet was lost. In response, TCP retransmits the lost packet and initiates congestion control procedure to give the network a chance to recover. First, TCP drops the transmission window size toreduce the amount of data in transit through the network. Second, it activates the slow-start algorithm to
restrict the rate at which the window grows to previous levels. Third, it resets the retransmission timer to a backoff interval that doubles with each consecutive timeout.
When motion is mistaken for congestion, these procedures result in significant reductions in throughout and unacceptable interactive delays for active connections. The degradation is readily apparent, for example, to users of emerging ubiquitous computing environments.

RELIABLE STREAM TRANSMISSION
PROTOCOLS IN MOBILE COMPUTING ENVIRONMENTS
The popularity and usefulness of the Internet seem to grow exponentially eveiy day. As a result of this rapid growth, mobile us expect to access the Internet™s information resources and to communicate with other Internet users in the future. Much researàh has been done in constructing mobile networks with the Internet at the core. The purpose of that research is to make the movement of mobile hosts transparent, at least to the application layer. In other qrds, all the Internet™s resources should be made available to mobile users through existing application software.
Reliable stream transmission is a data transmission, which results in an error free and sequenced reception of the transmitted data. . And the responsible protocols for such transmissions are known as the reliable stream transmission protocols. Mobile computing environment refers to wireless transmission medium and the movement of mobile hosts (for example, high noise and interference rates, low bandwidth, weak signals, run-down battery, ard cell handoff during mobile movement).
The initial approach of providing mobile users with Internet access enhanced the functionality at the Internet rayer at allowed both the transport and application layers to remain unchanged. The enhanced Internet protocol (IP) is called the mobile W This approach, however, encountered a significant obstacle-unreasonable communication delays. To improve the performance of Internet access by mobile users, additional work beyond the mobile IP must be done to provide reliable stream transmissions in mobile computing environmeilts.
In .examing the five layers of the Internet, the transport layer with transmission control protocol (lâ„¢CP) wa identified as the bottleneck adversely affecting Internet access by mobile users. During an en-ta-end TCP connection, if a sender does not receive anexpected acknowledgement from a receiver within a certain time period (timeout), the TCP at the sender then retransmits the corresponding packet (s) and doubles the value of the timeout for possible retransmission. The assumption of this exponential timeout policy is that the data error rate at the, physical layer is extremely low. Therefore, a packet retransmission at the transport layer is considered to be caused mostly by traffic congestion at the IP layer.

However, with the addition of the wireless transmission medium (mobile cells) and the movement of mobile hosts into the Internet (called the mobile computing environ the data error rate at the physical layer in mobile cells is expected to be very high, contradicting the assumption of the TCP/IP suite. The high packet loss rate stems from physical disconnects at the physical layer due to the handoffs of the mobile lP noise and interference, run-down batteries and weak signals. Consequently, the timeout policy is no longer effective, causing unreasonable communication delays at thetransport layer. One wayof correcting this problem is to assurñe the continued existence of the high packet loss rate in the wireless transmission medium across the entire Internet. The transport layer can then be modified by allowing a different timeout pàlicy for mobile users. Fast retransmissions take a similar approach.
When analysing the performance problem of Internet access by mobile users, another solution is possible. In examining the components of the Internet, one finds that it consists of IP gateways and fixed termination hosts, which are physically linked by a wired transmission medium. The IP gateways constitute the core of the Internet, and the [ at the lntemet(top) layer controls the packets. The TCP at the transport layer in the termination hosts provides reliable stream transmission at the base of the IF. With the advent of the mobile computing environment, new components have been added to the Internet These include base stations, the wireless transmission medium (mobile cells), and mobile hosts.
In the mobile IP, a base station functions as a regular IF gateway, and it has additional modules that support the mobile IF Base stations are located at the boundary between the Internet and mobile cells, and they isolate both the mobile cells and mobile hosts from the Internet at the physical layer. The mobile IP takes advantage of this characteristic of the mobile computing environment, deploying additional modules at base stations and then hiding the movements of mobile users from both the transport and Internet layers (specifically, from the majority of the JP gateways). Two approaches-Data Interceptions and packet lnterc take advantage of this characteristic of the mobile computing environment for achieving
improved Internet access by mobile users. With these approaches, the high packet loss rates
in mobile cells ar hidden from the Internet by further enhancing the functionality of baseâ„¢
stations. Thus, optiinum perfbrmance of the Internet can be realized.

This paper reviews three approaches to reliable stream transmissions in mobile computing environment: approach 1 -Fast Retransmissions, approach 2-Data Interceptions, and approach 3-packet Interceptions.
Before progressing to the next section, note that the figure 1 illustrates a typical system architecture of the mobile IP.

In the figure, the fixed host is a wired computer sending a file destined fora mobile host across the Internet. Base stations I and 2 are two devices located in different geographical areas that function as routers on the boundaries ofthe wired Internet and mobile cells. The mobile IP executes a handoff when the mobile host moves acrosss a boundaryof the two mobile cells, denoted in the figure as mobile cell I and mobile cell 2, in the sense that the mobile JP uses base station 2 as the router instead of base station 1 after the handoff. The mobile IP embedded in the Internet can deliver a packet to the mobile host through base station I before the handoff (or through base station 2 after the handoft). The mobile IP supports data communications not only from mobile to fixed hosts but also from mobile to mobile hosts.

APPROACH I - FAST RETRANSMISSION
Focussing on communication disconnects during the haridom of mobile hosts; the Fast Retransmissions approach changes the timeout policy ifand only if handof1 in the mobile IP occur. In this approach, after the TCP software of the sender (either a fixed or mobile host) is aware of a handoff, it immediately retransmits the earliest unacknowledged packet, drops the transmission window, and initiates the slow-start algorithm.
This simple techniquereduces long delays causedby handofl of™the mobilel.R Hô it cannot resolve the performance issue raised by the high packet loss rates caused by i handoff circumstances. This approach also affects the overall performance of the Internet.
Exponential Delays
Exponential delays exist in many circumstances. In addition to handoffs of the mobile IP, which cause physical disconnects of mobile hosts from the Internet, noise and interference, weak signals, and rundown batteries also cause high packet loss rates- even when mobile iosts. are not moving. These types ofincidences cannot be dtected bythe mobile IP. The,refore, the TCP cannot synchronize itself to the fast retransmission policy from the exponential timeout policy. Consequently; this approach still exhibits unreasonable communication delays and does not fundamentally resolve the performance issue of the TCP in the mobile computing environment.
Performance Impact on the Internet
Fast retransmissjons do not take into account the high loss rates caused by non-handoff circumstances in mobile cells. Thus, the packet loss rate of a connection betwen an Internet fixed host and a mobile host average higher than that ofa connection between two fixed Internet hosts. Suppose that the average packet loss rate on the Internet is LI (5 to 15% is common across wide area networks [ ) and the average packet loss rate for mobile cells (between mobile hosts an base stations) is L2. The combined packet loss rate for connection between a fixed and mobile host then averages Ll+L2, Because L2 is significant in the sense that it is about equivalent to or even greater than LI, the overall packet loss rate, L1+L2, over the connection increases significantly. With the substantial increase in the number of mobile hosts in the future, the overall packet loss rate over the entire Internet would be significantly degraded.
In addition, the awareness of handoffs at the transport layer of the senders (either fixed or mobile hosts) requires the modification of the TCP software at both mobile hosts and fixed hosts. Further, the function of handling the nature of mobile computing environments is split into two layers-the IP layer and the transport layer-across all mobile and fixed hosts talking to mobile hosts,

* A transport layer conne between a mobile and fixed host is established as two separate connection- one over mobile cells and another over Internet. The connection over the Internet is the regular TCP in which base station pretends to be the mobile host and intercepts packets from the fixed host. The fixed host designates its peer as the mobile host and is not aware of the interception of the packets fromthe base station. The connection oyer mobile cells is a modified TCP that more effectively adapts tothe nature of the mobile computing ànvironment. The transport layers at both the mobile host and base station are aware of the
movement of the mobile host. A connection between a mobile host and a fixed host always contains a reference mdicating the first base station that initiated the connection, although the connection may already be physically moved away from the first base station
* In the base station, the socket for the connection over the Internet passes data to the t for the connection over the mobile cells.
ËœDuring a handoff of the mobile JP, both the states and the two sockets at the base i are transferred to the corresponding new base station.
APPROACH 2- DA1A INTERCEPTIONS
The nature of mobile computing environments leads to the conclusion that reliable stream ti over the Internet should be isolated from those in mobile cells. The approach discussed here, comparable to two others that were previously developed, is more effective in improving the performance of reliable stream transmissions in mobile computing environments. Because the two approaches are similar, we treat them here as asingle approach called data interceptions. The following concepts from the foundation for this approach.


Fl-I -Fixed Host
I-TCP-lndireci Transmission control Protocol
MH -Mobile H *
MSR -Mobile Support Router (base station)
TCP -Transmission Control Protocol
We call this approach Data Interceptions because it uses sockets at th base stations for incoming data. A base station can intercept a complete file from a fixed host before part of the file is transferred to a mobile host. Approach 2, or Data Interceptions, overcomes the problems inherent in approach 1, or fist retransmissions. That is, like the packet Interceptions approach discussed next, the Data Interceptions approach improves the end-to-end perfbrmance of reliable stream transmissions over rnohilc it uvii iii s™sy - It t.iW*tf$?fi™t™ italso guarantees that the pertormnan ot the Internet Is not affected ( packet loss rate is approximately max [ L2}, where LI and L2 are the average packet loss rates over thelntemet and in mObilecélls, respectivley, and where we assume that LI is about equivalent to L2).
However, this approach can cause memory exhaustion at the base stations, and it can also have gher computation over head as described in the ËœComputation Overheadâ„¢ subsection below
Memory Exhaution
The first weakness of the Data Interceptions approach is space (memoiy) over load. Memory exhaustion is also possible at the base stations. Because this approach cmploys session-oriented Data Interceptions, the space at base stations can be exhausted easily whenever the following conditions occur:
* Slow throughput in a mobile cell, which can cause memory exhaustion atthó base station. Because the throughputs on the Internet normally are much higher than those in mobile cells and the sockets at the base station accumulate as much data as they can, the speed of the memory allocations for incoming data is greater than that of the memory releases for outgoing data. Eventually, base station memory could be exhausted ifthere are too may mobile users accessing the Internet for a long period of time.
* A single connection transferring a very large volume of data (larger than the available space at the base station), which could consume all available memo ( or space) of base station if the mobile host is physically disconnected from the base station for a long period oftime. Therefore, a disconnect of a single mobile host from the base station could bring the entire mobile cel down.
Computation overhead
The following three considerations are integral parts of computation overhead, which immediately degrades communication performance:
* Two separate and relithle stream transmissions for a single connection between a mobile and fixed host are very expensive.
* The two sockets at the base station for a connection involving a mobile host must communicate locally at the base station to transfer data from one to the other.
* in the case of a handoff, the remaining data (which could be substantial) in the old base Station must be transferred to the new base through a third TCP connection between the old and new base stations.
Regarding the functional partitioning between the transport and IP layers, the Data Interceptions approach does not require any modification of the transport layer at the fixed hosts and maintains the clean functional partitioning among the five layers of the Internet. However, the transport layer at the mobile host is aware of the address of the first base station. This implies modifying the TCP software at the mobile host and splitting the function of handling the nature of the mobile computing environments among the IP and transport layers.
APPROACH 3- PACKET INTERCEPTIONS
In this section, we discuss an approach that more effectively imporves the performance of the reliable stream transmissions in mobile computing environments. Like Data Interceptions, this approach isolates the reliable stream transmissions in mobile cells from those on the Internet. The reliable stream transmissions protocol on both the Internet and the mobile hosts is the regular TCP.
The major difference when comparing the packet Interceptions and Data Interceptions approaches is in the base station where the only additional software above the mobile IP (called the mcrblle TCP [ ) acts as both an end of the regular TCP on the Internet and an end of the im dified TCP in the mobile cell. No socket is needed at the base station for packet Interc ptions.
4TCP manages a single buffer with limited size (similar to sliding windows in TCP and also called sliding windows in this paper), and it intercepts packets from both the mobile ai fixed hosts Reliable stream transmission between a mobile and fixed host is accomplished in th following manner: I
* T e mobile host initiates a TCP connection for a file transfer by using the fixed host as the peer. In addition, the fixed host takes the mobile host as its peer. The movement of the mob le host is transparent to the transport layer at both the mobile and fixed hosts.
* Tne base station intercepts tue packets from both the mobile host and the base station, and. it uses a single limited-size buffer (or sliding window) to execute the interception with minimum computational overhead for the connection.
* When a handoff of the mobie IP at the base station occurs, MTCP executes a hándoff at the transport layer, which transf the packets remaining in the sliding window to the new base station.
* In the case of a handoff, the remaining data (which could be substantial) in the old base station must be transferred to the new base through a third TCP connection between the old and new base stations.
Regarding the functional partitioning between the transport and IP layers, the Data Interceptions approach does not require any modification of the transport layer at the fixed hosts and maintains the clean functional partitioning among the five layers of the Internet. However, the transport layer at the mobile host is aware of the address of the first base station. This implies modif the TCP software at the mobile host and splitting the function of handling the nature of the mobile computing environments among the I? and transport layers.

APPROACH 3- PACKET INTERCEPTIONS
In this section, we discuss an approach that more effectively imporves the performance of the reliable stream transmissions in mobile computing environments. Like Data Interceptions, this approach isolates the reliable stream transmissions in mobile cells from those on the Internet. The reliable stream transmissions protocol on both the Internet and the mobile hosts is the regular TCP.
The major difference when comparing the packet Interceptions and Data Interceptions approaches is in the base station where the only additional software above the mobile IP (called the mcf bile TCP [ ) acts as both an end of the regular TCP on the Internet and an end of the m dified TCP in the mobile cell. No socket is needed at the base station for packet Interci :ptions.
4TCP manages a single buft with limited size (similar to sliding windows in TCP and also c ailed sliding windows in this paper), and it intercepts packets from both the mobile and fixed hosts. Reliable stream transmission between a mobile and fixed host is accomplishedin th following manner: . -
T ie mobile host initiates a TCP connection for a file transfer by using the fixed host as the peer. In addition, the fixed hcit takes the mobile host as its peer. The movement of the mob le host is transparent to the transport layer at both the mobile and fixed hosts.
Tne base station intercepts the packets from both the mobile host and the base station, anduses a single limited-size buffer (or sliding window) to execute the interception with minimum computational overl for the Coflfl
When a handoff of the mobde IP at the base station occurs, MTCP executes a hándoff at the transport layer, which transfrrs the packets remaining in the sliding window to the new base station


BS -Base Station
FH -Fixed Host
M-TCP -Mobile Transmission control Protocol
MH -Mobile Host
TCP -Transmission Control Protocol
Obviously, the packet Interception approach overcomes all the problems inherent in the previous two approaches. The following itemized summary details its advantages:
* The packet Interceptions approach is a unified approach for all the circumstances causing unreasonable communication aelays. These circumstances refer to physical disconnects of mpbile hosts at the physical layer from the Internet due to the handofi of the mobile IP, high packet loss rates due to. noise and interference, weak signals, and run-down batteries.
* Because packet Interceptions isolate the reliable stream transmissions in mobile cells from those on the Internet, packet Interceptions do not affect the overall performance of the Internet.

* Because the size of the sliding window is limited, memory exhaustion at the base stations can be prevented (assuming that the TCP connections through the base stations are also limited).
* Compared with the TCP software package, which was used at base stations implementing thedata interception approach, the MTCP software package is much less complex and minimizes computational overhead.
* The packet Interceptions approach makes movement of mobile hosts invisible tO both mobile and fixed hosts t the transport layer; it also maintains the TCP software in unmodified condition at both the fixed and mobile hosts. Although base stations are enhanced above the IF layer, the entire function of handling the nature of the mobile computing environments is accomplished solely in the base stations.

CONCLUSION
Three discrete approaches can be used to improve the end-to-end performance of reliable tréam transmissions between mobile and fixed hosts over the Inteinet: approach 1-Fast Retransmission; approach 2- Data Interceptions; and approach 3 - packet Interceptions. As the preferred approach, packet Interceptions solve all the performance problems stemming form the nature of mobile computing environments: wireless transmission media and the movement of mobile hosts (far example, high noise and interference rates, low bandwidth, weak signals, run-down batteries, and cell handoff during mobile movement). The packet Interceptions approach supports mobile compting without affecting the performance of the Internet. It prevents possible memory exhaustion and minimizes computational overhead at the base stations. Packet interceptions overcome all the problems inherent i n both the Fast Retransmissions and Data interceptions approaches.
In addition, the packet Interceptions approach maintains clean functional partitioning. The function of handling the nature of the mobile computing environments solely resides in base stations, keeping the wireless transmission medium and the movements of mobile hosts invisible to the Internet.

DEFINITION OF RELATED TERMS
Protocol:
The term protocol is used to refer to a well-known set of rules and formats to be used forcommunication between process in order to perform a given task.
A protocol is implemented by a pair of software modules located in the sending and receiving computers. For example a Transport Protocol transmits messages of any length from a sending process to a receiving process.
Protocol suite
A complete se of protocol layers is referred to as protocol suite (or protocol stack, reflecting the layered structure).

BOOKS OF REFERENCE
1. Information Super Highway, Revolution By Mi Sullinal.
2 IEEE Internet Computing.
3. OnternetE Mail From India By Kishoretarachandran.
4. IEEE Spectrum.
5. Pocket Book ofTele Communication Engineers, By Steue Winder.
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Mobile computing
Abstract

Mobile computing has been undergoing a bit of a renaissance lately. A few years ago it was a simple matter of finding a data-compatible mobile phone, a PC card modem, and a matching cable and installing it as a modem. Then people started to use PDA’s as well. Cell phones started to come with infrared ports to allow communication with laptops. Then cell phones started to come with modems built in. The connecting methods of mobile computing, its introduction, connection types, factors affecting connections, mobile applications and its limitations are explained.
Introduction Wireless networking technology has engendered a new era of computing, called mobile computing. Mobile Computing is an umbrella term used to describe technologies that enable people to access network services any place, anytime, and anywhere.
Ubiquitous computing and nomadic computing are synonymous with mobile computing. Mobile computing helps users to be productive immediately by reducing the training requirements associated with traditional automated data collection methods and provides a higher level of portability than keyboard-based systems.
Field-based users can access any information available from the system at any time to make critical business decisions. This information is available at the point of use, wherever and whenever they need it.
Portable devices like laptop and palm top computers give mobile users access to diverse sources of global information anywhere and at any time.
One of the most important and highly publicized recent developments in the PC world has been the introduction of the pen interface. By using a stylus to replace the keyboard, mobile computers are turning thousands of computer illiterate people especially those involved with field-based data collection into computer users. The market potential and breadth of application requirements for mobile computing has prompted numerous hardware and software companies to focus their efforts in providing solutions to the vertical, form-oriented marketplace.
The pen interface allows users to interact with the computer in a very natural and familiar way by entering text, numbers, and graphics in “electronic ink” directly on the screen. The pen interface also provides users with highly intuitive and efficient applications, whether tapping graphical icons to navigate through applications or selecting options from scrolling lists and checkboxes.
 Mobile computing applications can closely simulate the original paper form, providing users with a familiar look and feel. Through the use of the latest PCMCIA technology, data storage is large, fast, and more efficient with minimal power consumption and the highest level of ruggedness, while communications via modem or wireless is also tightly integrated, fulfilling the requirements of the mobile user. And standardized ports give users access to printers, barcode readers, and various other peripheral devices.
Distinction between "wireless" and "mobile."
Wireless refers to the method of transferring information between computing devices, such as a personal data assistant (PDA), and a data source, such as an agency database server, without a physical connection. Not all wireless communications technologies are mobile. For example, lasers are used in wireless data transfer between buildings, but cannot be used in mobile communications at this time.
Mobile simply describes a computing device that is not restricted to a desktop. A mobile device may be a PDA, a "smart" cell phone or Web phone, a laptop computer, or any one of numerous other devices that allow the user to complete computing tasks without being tethered, or connected, to a network. Mobile computing does not necessarily require wireless communication. In fact, it may not require communication between devices at all.
Mobile devices
Here we have seven different types of mobile devices:
Laptop computers
PDA’s and handheld PCs
Pagers
Smart phones and cellular phones
Task devices, such as bar code scanners
Blue tooth
Bridge
Laptops are typically used and supported in the same way as desktop PCs. In fact, many organizations have replaced desktops with their portable cousins, as the workforce has grown increasingly mobile.
PDA’s, however, are the least planned for and supported devices. They are undergoing rapid evolution and are being brought into organizations in the same way the earliest PCs were. That is, adventurous early adopters buy the devices for their personal use and then ask IT departments to integrate the devices into the corporate IT environment.
At present, PDA’s are most often used for storing and synchronizing personal information such as addresses, schedules, and E-mail. However, the medical industry has developed numerous applications for PDA’s. At least one Web ring a collection of Web sites with a common topic) has been created to discuss medical software that automates functions such as patient and diagnostic data entry, patient monitoring and diagnosis, and messaging. In a hospital setting, these applications may include wireless communication between staff members’ handheld devices and a base station at which patient information is stored.
Smart phones that allow users to access phone calls, two-way radio transmissions, and paging and data transmissions on one device are also finding applications in hospitals and other situations that have intense and constant need for time sensitive communications.
Pagers that support one- and two-way text messaging are also used in similar situations. Third party vendors most often provide support for these devices.
Task devices such as the parcel tracking devices used by Federal Express (FedEx) and the United Parcel Service (UPS) delivery personnel are most often bought as part of a complete system from a third-party vendor. Because they are frequently mission-critical, most corporations support task devices as rigorously as desktop computers.
Bluetooth:- A short-range wireless standard that specifies radio connections between devices within a 10-meter range of each other. Bluetooth is designed as a Personal Area Network (PAN, or WPAN for "Wireless Personal Area Network") technology with a wide variety of theoretical uses.
Bridge:- A device that connects two local-area networks (LANs), or two segments of the same LAN. Bridges simply forward packets from one segment to another without analyzing or routing messages. This allows them to connect dissimilar networks (e.g., a bridge can connect an Ethernet and Token-Ring network).

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Abstract
With the advent of the Internet and the plurality and variety of fancy applications it brought with it, the demand for more advanced services on cellular phones is increasingly
becoming urgent. Unfortunately, so far the introduction of new enabling technologies did not succeed in boosting new services. The adoption of Internet services has shown to be more difficult due to the difference between the Internet and the mobile telecommunication system. The goal of this paper is to examine the characteristics of the mobile system and to clarify the constraints that are imposed on existing mobile services. The paper will also investigate successively the enabling technologies and the improvements they brought. Most importantly, the paper will identify their limitations and capture the fundamental requirements for future mobile service architectures namely openness, separation of service logic and content, multi-domain services, personalization, Personal Area Network (PAN)-based services and collaborative services. The paper also explains the analysis of current mobile service architecture such as voice communication, supplementary services with intelligent network, enabling services on SIM with SIM application tool kit, text services with short message service, internet services with WAP and dynamic applications on mobile phones with J2ME.
Further our paper gives information on challenges of mobile computing which includes harsh communications, connections, bandwidth and heterogeneous networks. Under research issues seamless connectivity over multiple overlays, scalable mobile processing, wireless communications, mobility and portability are discussed.
1. Introduction
With digitalization the difference between telecommunication and computer net-working is fading and the same technologies are used in both fields. However, the convergence does not progress as rapidly as expected. Moving applications and ser-vices from one field to the other has proven to be very difficult or in many cases impossible. The explanation is that although the technologies in use are rather similar there are crucial differences in architecture and concepts. The paper starts with a study of how mobile services are implemented in mobile telecommunication systems and an identification of their limitations so as to meet the future needs of the future.
2. Analysis of current mobile service architectures
2.1 Voice communication

As indicated by its name, the objective of mobile telecommunications systems is to provide communication between mobile distant persons. These systems only supported direct voice communication or telephony between two participants, but supplementary services like call forwarding, barring and voice mail were added later on. The mobile telephony service is realized by components represented by grey ovals that are distributed both on the mobile phone, also called Mobile Station (MS) and on the mobile network. On the MS, there are components both on the Mobile Equipment (ME) and on the subscriber Identity Module (SIM). To establish a telephone conversation the service components on the MS are collaborating with the ones on the mobile network to allocate a channel and to maintain it throughout the session even when the MS is moving and changing base stations. The components on the mobile phone are installed by the manufacturer while the ones on the network are delivered by network suppliers.
2.2 Supplementary services with intelligent network
It does not take long time before there is a need for more advanced call control ser-vices like call forwarding, barring, voice mail, premium call, etc. As shown in Figure 3 an IN (Intelligent Network [1]) Service Control Point (SCP) is introduced in the mobile network to allow the implementation of supplementary services. It is worth mentioning that these services are derivatives centered around the voice communication service. Another restriction is that the SCP is implemented on equipment manufacturer proprietary technologies. The SCP is also located inside the telecom operator domain making third party service development difficult.
2.3 Enabling services on the SIM with SIM Application Toolkit (SAT)
The telecom operators want to have other services than telephony and its derivatives and turn to the SIM, which are their property. Unfortunately, although the SIM is a smart card having both processing and storage capabilities necessary for new services. The SIM is supposed to be the slave executing orders from its master, the ME. To remedy this, the SIM Application Tool-kit (SAT) [2] is introduced to allow applications/services residing on the SIM to control the input and output units. With SAT it is possible to develop applications on the SIM but there are many restrictions. First SAT applications should be small in size. Secondly, the installation of applications on the SIM is controlled by operators who are reluctant to open the access due to security.
2.4 Text services with Short Message Service (SMS)
SMS-C is responsible to store and forward messages to and from mobile phone (see Figure 3). In the illustration, components used for SMS are the client © in the ME advanced SMS services are implemented by perlscripets. Provisioning of SMS services requires installation of the above mentioned application on an SMS Gateway the system running the SMS Gateway to act as an SMSC itself (e.g. a PC using a radio modem through a serial port). To have direct access to an SMSC requires cooperation with the operator that owns the SMSC, which often can provide a TCP connection for sending/receiving SMS messages part of a service. The advantage of the above solution is that to receive revenue from generated traffic. The problem with access to SMS services is remembering both the service access number and the additional identifiers and parameters for a specific service (the protocol)
2.5 Internet access with WAP
Wireless Application Protocol (WAP) [5] was to provide access to the WWW on handheld terminals. A micro browser installed in the Mobile Equipment is communicating with a WAP Proxy introduced between the Internet and the mobile network to convert Internet protocols to Wireless binary protocols as shown in Figure 3. On the terminal side, a WAP browser is located in the ME and services are connected to a Web server on the network side. development of WAP services can be performed by programming experience. Most services typically consist of some static WML content together with a CGI-script as back-end that can generate dynamic content retrieved from for example other Web sites or from a DBMS. One restriction of the technology is that it is not possible to access ordinary webpages using a WAP browser.
2.6 Dynamic applications on mobile phones with J2ME (CLDC/MIDP)
Unlike a computer, the functionality of the mobile phones is defined at manufacture time and it is not possible to install new applications. With introduction of the J2ME CLDC/MIDP vast amount of sophisticated applications, called MIDLETS can be found on the Internet. With J2ME, it is possible to develop dynamic standalone applications. When it comes to SMS, there are still some restrictions in J2ME, such as access to the standard inbox for SMS messages on a handset is not allowed.
Process, the “write-once-run-anywhere” concept is not valid for this platform. through the Most Integrated Development Environments (IDE) for Java allows development of J2ME applications as well. Although J2ME is a standardised technology, performed Java Community
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04-04-2011, 01:53 PM

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.ppt   MobileComputing.ppt (Size: 219.5 KB / Downloads: 233)
What Is Mobile Computing?
What is computing?

Operation of computers (according to oxfords advance learner’s dictionary)
What is the mobile?
That someone /something can move or be moved easily and quickly from place to place
What is mobile computing?
Users with portable computers still have network connections while they move
Is using a digital camera “Mobile Computing”, or using an MP3 player or handheld computer (e.g. 3Com’s Palm Pilot or Compaq’s iPAQ 3660)?
A simple definition could be:
Mobile Computing is using a computer (of one kind or another) while on the move
Another definition could be:
Mobile Computing is when a (work) process is moved from a normal fixed position to a more dynamic position.
A third definition could be:
Mobile Computing is when a work process is carried out somewhere where it was not previously possible.
Mobile Computing is an umbrella term used to describe technologies that enable people to access network services anyplace, anytime, and anywhere.
Comparison to Wired Net.
Wired Networks
- high bandwidth
- low bandwidth variability
- can listen on wire
- high power machines
- high resource machines
- need physical access(security)
- low delay
- connected operation
Mobile Networks
- low bandwidth
- high bandwidth variability
- hidden terminal problem
- low power machines
- low resource machines
- need proximity
- higher delay
- disconnected operation
Why Go Mobile?
Enable anywhere/anytime connectivity
Bring computer communications to areas without pre-existing infrastructure
Enable mobility
Enable new applications
An exciting new research area
Types of Wireless Devices
Laptops
Palmtops
PDAs
Cell phones
Pagers
Sensors
Mobile Objects
A mobile object is some code that carries a state
A mobile object is some code that carries a state
that lives on a host
A mobile object is some code that carries a state
Lives in a host
That visits places
A mobile object is some code that carries a state
Lives in a host
That visits places
which is let in when trusted
A mobile object is some code that carries a state
Lives in a host
That visits places
which is let in when trusted
and barred when untrusted
A mobile object is some code that carries a state
Lives in a host
That visits places
which is let in when trusted
and barred when untrusted
and will refuse to go to untrustworthy places
Mobile objects can talk to their friends
Mobile Objects (Cont.)
Mobile objects can talk to their friends
but only by co-operation of the hosts
Moving Object Databases (MOD)
Deals with Mobile Objects whose geometry, position changes over time
Traditional DBMS alone is incapable for this purpose
MOD is built on top of existing DBMS to support a critical set of capabilities
Moving Object Databases (MOD) (Cont.)
DOMINO (Databases for Moving Objects Tracking) Approach
System Architecture
Moving Object Databases (MOD) (Cont.)
Omnitracs
- developed by Qualcomm
- Is a commercial system used by the transportation industry
- Provides location management by connecting vehicles, via satellites, to company DB
- Vehicles are equipped with GPS, and they they automatically and periodically report their location
Query Language for MOD
Regular query language (SQL) is nontemporal
For MOD we need Spatial and Temporal Query language
“Where is the nearest station?”
“What is the distance of the closest taxicab?”

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.ppt   0Wireless Communication basics (1).ppt (Size: 636.5 KB / Downloads: 160)
Mobile Computing
Spectrum and bandwidth
Electromagnetic signals are made up of many frequencies
Shown in the next example
Spectrum and bandwidth
The 2nd frequency is an integer multiple of the first frequency
When all of the frequency components of a signal are integer multiples of one frequency, the latter frequency is called fundamental frequency (f)
period of the resultant signal is equal to the period of the fundamental frequency
Period of s(t) is T=1/f
Fourier Analysis
Any signal is made up of components at various frequencies, in which each component is a sinusoid.
Adding enough sinusoidal signals with appropriate amplitude, frequency and phase, any electromagnetic signal can be constructed
Spectrum and bandwidth
It is the range of frequencies that a signal contains (among its components)
In the example, spectrum is from f to 3f
absolute bandwidth is the width of the spectrum
3f-f = 2f
Data Rate and bandwidth
There is a direct relationship between data rate (or signal carrying capacity) and bandwidth
Suppose we let a positive pulse represent 1 and negative pulse represent 0
Then the waveform (next slide) represents 1010..
Duration of each pulse is tbit = (1/2) (1/f)
Thus data rate is 1/ tbit = 2f bits/sec
As we add more and more frequencies the wave looks more like a square wave
Example
Looking at FIG 2(a) the bandwidth = 5f-f = 4f
If f=1MHz = 106 cycles/sec, then bandwidth = 4MHz
The period of the fundamental frequency = T = 1/f = 1 μs
So each bit takes up 0.5 μs i.e. data rate is 1/0.5 Mbps = 2 Mbps
Example
Looking at FIG 1© the bandwidth = 3f-f = 2f
If f=2MHz = 2x106 cycles/sec, then bandwidth = 4MHz
The period of the fundamental frequency = T = 1/f = 0.5 μs
So each bit takes up 0.25 μs i.e. data rate is 1/0.25 Mbps = 4 Mbps
Example
Thus a given bandwidth can support different data rate, depending on the ability of the receiver to discern the difference between 0 and 1 in the presence of noise and interference
Gain and Loss
Ratio between power levels of two signals is referred to as Gain
gain (dB) = 10 log10 (Pout/Pin)
loss (dB) = -10 log10 (Pout/Pin) = 10 log10 (Pin/Pout)
Pout is output power level and Pin is input power level
Signal of power 10mw transmitted over wireless channel, and receiver receives the signal with 2mw power:
gain (db) = 10 log10 (2/10) = -10 (0.698) = -6.98 dB
loss (db) = 6.98 dB
dBW power
dB-Watt
power in dB transmitted with respect to a base power of 1 Watt
dBW = 10 log10 P
P is power transmitted in Watt
if power transmitted is 1 Watt
dBW = 10 log10 1 = 0 dBW
1000 watt transmission is 30 dBW
dBm power
dB-milliwatt
better metric in wireless network
power in dB transmitted with respect to a base power of 1 milliwatt
dBm = 10 log10 P
P is power transmitted in milliwatt
if power transmitted is 1 milliwatt
dBm = 10 log10 1 = 0 dBm
10 milliwatt transmission is 10 dBm
802.11b can transmit at a maximum power of 100mw = 20 dBm
Channel Capacity
Four concepts :
Data Rate : rate (in bps) at which data can be communicated
Bandwidth: bandwidth of the transmitted signal as constrained by the transmitter and the medium, expressed in Hz
Noise : interfering electromagnetic signal that tend to reduce the integrity of data signal
Error rate : rate at which receiver receives bits in error i.e. it receives a 0 when actually a 1 was sent and vice-versa
Nyquist Bandwidth
Given a bandwidth of B, the highest signal rate that can be carried is 2B (when signal transmitted is binary (two voltage levels))
When M voltage levels are used, then each signal level can represent log2M bits. Hence the Nyquist bandwidth (capacity) is given by
C = 2 B log2M
Shannon’s Capacity Formula
When there is noise in the medium, capacity is given by
C <= B log2 (1 + SNR)
SNR = signal power/noise power
SNRdB = 10 log10 SNR
Bandwidth Allocation
Necessary to avoid interference between different radio devices
Microwave woven should not interfere with TV transmission
Generally a radio transmitter is limited to a certain bandwidth
802.11channel has 30MHz bandwidth
Power and placement of transmitter are regulated by authority
Consumer devices are generally limited to less than 1W power
ISM and UNII Band
Industrial, Scientific and Medical (ISM) band
902-928 MHz in the USA
433 and 868 MHz in Europe
2400 MHz – 2483.5 MHz (license-free almost everywhere)
Peak power 1W (30dBm)
but most devices operate at 100mW or less
802.11 uses the ISM band of 2.4GHz
Unlicensed National Information Infrastructure (UNII) bands
5.725 – 5.875 GHz
Antenna
An electrical conductor or system of conductors used for radiating electromagnetic energy into space or for collecting electromagnetic energy from the space
An integral part of a wireless system
Radiation Patterns
Antenna radiates power in all directions
but typically does not radiate equally in all directions
Ideal antenna is one that radiates equal power in all direction
called an isotropic antenna
all points with equal power are located on a sphere with the antenna as its center
Omnidirectional Antenna
Produces omnidirectional
radiation pattern of
equal strength in all
directions
Vector A and B are
of equal length
Directional Antenna
Radiates most power in one
axis (direction)
radiates less in other
direction
vector B is longer than
vector A : more power
radiated along B than A
directional along X
Dipole Antenna
Half-wave dipole or Hertz
antenna consists of two
straight collinear conductor
of equal length
Length of the antenna
is half the wavelength of
the signal.
Quarter-wave antenna
Quarter-wave or marconi antenna
has a veritcal conductor of
length quarter of the wavelength
of the signal
Sectorized Antenna
Several directional antenna
combined on a single pole
to provide sectorized antenna
each sector serves receivers
listening it its direction
Antenna Gain
A measure of the directionality of an antenna
Defined as the power output, in a particular direction, compared to that produced in any direction by a perfect isotropic antenna
Example: if an antenna has a gain of 3dB, the antenna is better (in that direction) than isotropic antenna by a factor of 2
Antenna Gain
Antenna gain is dependent on effective area of an antenna.
effective area is related to the physical size of the antenna and its shape
Antenna Gain is given by
where
G = antenna gain
Ae = effective area
f = carrier frequency
c = speed of light
λ = carrier wavelength
Signal Propagation
Transmission range:
receiver receives signal with an error rate low enough to be able to communicate
Detection range: transmitted power is high enough to detect the transmitter, but high error rate forbids communication
Interference range: sender interferes with other transmissions by adding to the noise
Signal Propagation
Radio waves exhibit three fundamental propagation behavior
Ground wave (< 2 MHz) : waves with low frequency follow earth’s surface
can propagate long distances
Used for submarine communication or AM radio
Sky wave (2-30 MHz) : waves reflect at the ionosphere and bounce back and forth between ionosphere and earth , travelling around the world
Used by international broadcast and amateur radio
Signal Propagation
Line of Sight (> 30 MHz) : emitted waves follow a straight line of sight
allows straight communication with satellites or microwave links on the ground
used by mobile phone system, satellite systems
Free Space loss
Transmitted signal attenuates over distance because it is spread over larger and larger area
This is known as free space loss and for isotropic antennas
Pt = power at the transmitting antenna
Pr = power at the receiving antenna
λ = carrier wavelength
d = propagation distance between the antennas
c = speed of light
Free Space loss
For other antennas
Gt = Gain of transmitting antenna
Gr = Gain of receiving antenna
At = effective area of transmitting antenna
Ar = effective area of receiving antenna
Thermal Noise
Thermal noise is introduced due to thermal agitation of electrons
Present in all transmission media and all electronic devices
a function of temperature
uniformly distributed across the frequency spectrum and hence is often referred to as white noise
amount of noise found in a bandwidth of 1 Hz is
N0 = k T
N0 = noise power density in watts per 1 Hz of bandwidth
k = Boltzman’s constant = 1.3803 x 10-23 J/K
T = temperature, in Kelvins
N = thermal noise in watts present in a bandwidth of B
= kTB where
Data rate and error rate
A parameter related to SNR that is more convenient for determining digital data rates and error rates
ratio of signal energy per bit to noise power density per Hertz, Eb/N0
R = bit rate of transmission, S= power of the signal,
Tb = time required to send 1 bit. Then R = 1/Tb
Eb = S Tb
so
Data rate and error rate
Bit error rate is a decreasing function of Eb/N0
If bit rate R is to increase, then to keep bit error rate (or Eb/N0) same, the transmitted signal power must increase, relative to noise
Eb/N0 is related to SNR as follows
B = signal bandwidth
(since N = N0 B)
Doppler’s Shift
When a client is mobile, the frequency of received signal could be less or more than that of the transmitted signal due to Doppler’s effect
If the mobile is moving towards the direction of arrival of the wave, the Doppler’s shift is positive
If the mobile is moving away from the direction of arrival of the wave, the Doppler’s shift is negative
Doppler’s Shift
where
fd =change in frequency
due to Doppler’s shift
v = constant velocity of the
mobile receiver
λ = wavelength of the transmission
Doppler’s shift
f = fc + fd
where
f = the received carrier frequency
fc = carrier frequency being transmitted
fd = Doppler’s shift as per the formula in the prev slide
Multipath Propagation
Wireless signal can arrive at the receiver through different pahs
LOS
Reflections from objects
Diffraction
Occurs at the edge of an impenetrable body that is large compared to the wavelength of the signal
Effect of Multipath Propagation
Multiple copies of the signal may arrive with different phases. If the phases add destructively, the signal level reduces relative to noise.
Inter Symbol Interference (ISI)
Multiplexing
A fundamental mechanism in communication system and networks
Enables multiple users to share a medium
For wireless communication, multiplexing can be carried out in four dimensions: space, time, frequency and code
Space division multiplexing
Channels are assigned on the basis of “space” (but operate on same frequency)
The assignment makes sure that the transmission do not interfere with each (with a guard band in between)
Space division multiplexing
Frequency Division Multiplexing
Frequency domain is subdivided into several non-overlapping frequency bands
Each channel is assigned its own frequency band (with guard spaces in between)
Frequency Division Multiplexing
Time Division Multiplexing
A channel is given the whole bandwidth for a certain amount of time
All senders use the same frequency, but at different point of time
Time Division Multiplexing
Frequency and time division multiplexing
A channel use a certain frequency for a certain amount of time and then uses a different frequency at some other time
Used in GSM systems
Frequency and time division multiplexing
Code division multiplexing
separation of channels achieved by assigning each channel its own code
guard spaces are realized by having distance in code space (e.g. orthogonal codes)
transmitter can transmit in the same frequency band at the same time, but have to use different code
Provides good protection against interference and tapping
but the receivers have relatively high complexity
has to know the code and must separate the channel with user data from the noise composed of other transmission
has to be synchronized with the transmitter
Code division multiplexing
Modulation
Process of combining input signal and a carrier frequency at the transmitter
Digital to analog modulation
necessary if the medium only carries analog signal
Analog to analog modulation
needed to have effective transmission (otherwise the antenna needed to transmit original signal could be large)
permits frequency division multiplexing
Amplitude Shift Keying (ASK)
ASK is the most simple digital modulation scheme
Two binary values, 0 and 1, are represented by two different amplitude
In wireless, a constant amplitude cannot be guaranteed, so ASK is typically not used
Amplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
The simplest form of FSK is binary FSK
assigns one frequency f1 to binary 1 and another frequency f2 binary 0
Simple way to implement is to switch between two oscillators one with f1 and the other with f2
The receiver can demodulate by having two bandpass filter

Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Uses shifts in the phase of a signal to represent data
Shifting the phase by 1800 each time data changes: called binary PSK
The receiver must synchronize in frequency and phase with the transmitter
Phase Shift Keying (PSK)
Quadrature Phase Shift Keying (Q-PSK)
Higher bit rate can be achieved for the same bandwidth by coding two bits into one phase shift.
450 for data 11
1350 for data 10
2250 for data 00
3150 for data 01
Spread Spectrum
Spreading the bandwidth needed to transmit data
Spread signal has the same energy as the original signal, but is spread over a larger frequency range
provides resistance to narrowband interference
Spread Spectrum
Direct Sequence Spread Spectrum
Takes a user bit sequence and performs an XOR with, what is known as, chipping sequence
Each user bit duration tb
chipping sequence has smaller pulses tc
If chipping sequence is generated properly it may appear as random noise
sometimes called pseudo-noise (PN)
tb/tc is known as the spreading factor
determines the bandwidth of the resultant signal
Used by 802.11b
Direct Sequence Spread Spectrum
Frequency Hopping Spread Spectrum
Total available bandwidth is split into many channels of smaller bandwidth and guard spaces
Transmitter and receiver stay on one of these channels for a certain time and then hop to another channel
Implements FDM and TDM
Pattern of channel usage : hopping sequence
Time spent on a particular channel: dwell time
Frequency Hopping Spread Spectrum
Slow hopping
Transmitter uses one frequency for several bit period
systems are cheaper, but are prone to narrow band interference
Fast hopping
Transmitter changes frequency several times in one bit period
Transmitter and receivers have to stay synchronized within smaller tolerances
Better immuned to narrow band interference as they stick to one frequency for a very short period
Receiver must know the hopping sequence and stay synchronized with the transmitter
Used by bluetooth
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