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Multiplexing is a term used to describe how a signal can be divided among multiple users. This spectrum sharing allows wireless operators to maximize the use of their spectrum to accommodate a large number of users over fewer channels. For digital systems, three main multiplexing techniques are being used for wide area networks: frequency division, time division, and code division. A fourth method, called orthogonal frequency division, is the most complex of all of these methods. It is commonly used in high-speed local area networks, as discussed earlier in this chapter, but is starting to grow in popularity for wide area networks as well.

1. Frequency-division multiplexing (FDM). Numerous signals are combined on a single channel. Each signal on the channel is assigned unique frequency for communication. The caller and the receiver tune to the same frequency to communicate. This is similar to how radio stations work. Each has its own frequency band over which it broadcasts. To listen to a particular channel, you tune the receiver to that particular frequency. For person-to-person communication, this is a very inefficient use of spectrum, hence is only used by analog wireless networks.
2. Time-division multiplexing (TDM). As with FDM, numerous signals are combined on a single channel, but with TDM they are divided into separate time slots. The time segments are assigned to an individual user and are rotated at regular intervals. The receiver interprets the appropriate time slot (channel) to receive the information. This technique allows for variation in the number of signals sent along the line, and constantly adjusts the time intervals to maximize bandwidth. Many of the current second-generation wireless systems are based on time-division multiplexing as it provides efficient use of spectrum with minimal interference.
3. Code-division multiplexing (CDM). Rather than dividing the signal using frequency or time, CDM attaches a code to each signal, and sends them all over the same broad spectrum. This results in very high spectrum efficiency and low levels of interference by other signals. Even though all of the signals are being broadcast at once, a receiver will only accept the signals with the right code. This technique is used in several second-generation wireless networks and is the basis for nearly all third-generation networks.

Advantages

The following are the key advantages of wireless Internet (thin client) architecture:

1.  Minimal to zero software deployment. This allows applications to be deployed without any additional client-side configuration. Updates to these applications are also straightforward since only the server has to be updated.
2.  Extends Internet computing model. Many corporate applications are based on the Internet model. Wireless Internet is a natural extension to these applications.
3.  Familiar user interface. Many users are familiar with a browser interface to their applications. Providing a similar interface on mobile devices allows them to be productive immediately; there is no learning curve.
4.  Enterprise integration. If an existing Internet application is being extended, the application logic and enterprise integration layers may already be taken care of. This is a tremendous benefit, as enterprise integration often proves to be the most resource-intensive part of a mobile application.
5.  Security. All of the data is stored on the server behind corporate firewalls. No data is stored on the client.

Disadvantages

Wireless Internet architectures have some disadvantages as well, namely:

1.  Wireless connectivity. To access any data, all of which resides on the server, you need wireless connectivity. This can be problematic when users are moving between multiple locations. The exception is when browsers have content-caching capabilities. That said, even when caching is available, there is still a very limited amount of data and logic available to perform transactions.
2.  Simple user interface. Many microbrowsers have limited capabilities for graphics or other “rich” components. Graphics are also often avoided to minimize the amount of data being downloaded over potentially slow wireless networks.
3.  Application performance. For each request being transferred over a wireless network, performance can be an issue. This is due partially to network throughput and partially to network latency.
4.  Application testing. Controlling, predicting, and testing the behavior of the application is difficult on the full range of microbrowsers. When emulation software is used to simulate devices, it is not always an accurate representation of the end-user experience since it is not executing over a wireless network.
5.  Availability. If a server-side problem occurs, all users will be brought to a halt.
6.  Security. Total control of the environment is not available in most cases, because a wireless gateway exists that may lead to security concerns.
7.  Cost. Wireless airtime fees can become an issue if the mobile user has to constantly be connected to use the application. On circuit-switched networks, where fees are charged based on the time connected, not the data transferred, charges are incurred even when a user is reading Web content or filling in a form.

The GPRS provides a set of GSM services for data transmission in packet mode within a PLMN. In packet-switched mode, no permanent connection is established between the mobile and the external network during data transfer. Instead, in circuit-switched mode, a connection is established during the transfer duration between the calling entity and the called entity. In packet-switched mode, data is transferred in data blocks, called packets. When the transmission of packets is needed, a channel is allocated, but it is released immediately after. This method increases the network capacity. Indeed, several users can share a given channel, since it is not allocated to a single user during an entire call period.

One of the main purposes of GPRS is to facilitate the interconnection between a mobile and the other packet-switched networks, which opens the doors to the world of the Internet. With the introduction of packet mode, mobile telephony and Internet converge to become mobile Internet technology. This technology introduced in mobile phones allows users to have access to new value-added services, including:

1. Client-server services, which enable access to data stored in databases. The most famous example of this is access to the World Wide Web (WWW) through a browser.
2. Messaging services, intended for user-to-user communication between individual users via storage servers for message handling. Multimedia Messaging Service (MMS) is an example of a well-known messaging application.
3. Real-time conversational services, which provide bidirectional communication in real-time. A number of Internet and multimedia applications require this scheme such as voice over IP and video conferencing.
4. Tele-action services, which are characterized by short transactions and are required for services such as SMS, electronic monitoring, surveillance systems, and lottery transactions.

BREW is an application development platform created by Qualcomm for mobile phones. It was originally developed for CDMA handsets, but has since been ported to other air interfaces including GSM/GPRS. BREW is a software platform that can download and run small programs for playing games, sending messages, sharing photos, etc. The main advantage of BREW platforms is that the application developers can easily port their applications between all Qualcomm devices. BREW runs between the application and the wireless device’s chip operating system so as to enable a programmer to develop applications without needing to code for system interface or understand wireless applications

You’re on your way to a pivotal meeting but the flight’s been delayed. You need to notify your client, make some last minute adjustments to your presentation, and figure out the fastest route from the airport to their office. In short, you really need to have your office with you. The answer is MOBILIZE YOURSELF! Explains how to maximize the mobility of the technology you have today—and it provides smart answers about the mobile technologies and services you might be considering.
The need to display content on mobile devices continues to burgeon — especially with the seamless extension of enterprise data onto the small screen. As a result, solutions for shortening the content-to-mobile implementation cycle are on the rise. If you want to take your content mobile, and do it quickly and efficiently, have a look at mobile frameworks.
The purpose of a mobile framework is to utilize a standards-based architecture that leverages existing IT infrastructure and maximizes return on investment. Such a framework is relatively easily deployed and extends enterprise content to mobile users with both good quality and user experience. The purpose of a mobile framework is somewhat different from actual mobile development tools, which are specifically used to develop mobile applications. Mobile frameworks extend existing applications (such as Customer Resource Management (CRM) to mobile workforces and their devices. Just make sure the mobile framework is truly a mobile framework.

Mobile framework Requirements/Features

This list will help you identify a mobile framework. Usually, a mobile framework will include the following:
* A graphical development tool that visually maps complex server logic and data to device resident applications.
* Flexibility for customization and the ability to incorporate new code (MMAPI, .NET, Insignia)
* Secure access and user identity
* An API extension to let other applications “hook” in
* Enabling of media (video, audio, images) transformation (Java MMAPI, Insignia Mobile Media Framework)
* Ability to provide lightweight technology for mobile agent-based distributed computing (Taco Framework)
* Ability to provide for content syndication (Jetspeed)
* Ability to provide content transformation
* Agents for device detection
* Asynchronous transactional messaging for guaranteed data delivery and integrity
* Allowance of true mobile client applications with seamless disconnected and real-time operation
Mobility is still a new approach to the workplace. Reaching business systems puts special demands and requirements on access, security, and collaboration infrastructure. This can all be reassured with mobile frameworks.
The name MOBILE is derived from the first letter in each of the six categories that make up the framework.
The six categories are:
M the need for mobility
O the need to improve operations
B the need to break business barriers
I the need to improve information quality
L the need to decrease transaction lag
E the need to improve efficiency

So whats important in Mobile Framework is to get the technologies that take you farther and  to:
• Learn how to choose, customize, and synchronize notebook computers, Pocket PCs, and other mobile devices

• Evaluate your connectivity options—infrared, wired and wireless LANs, cyber cafés, and more

• Make your e-mail, files, corporate data, and favorite Microsoft Office applications road-ready—and secure

• Do more with less gear—get driving directions, enjoy music and video, even manage your stocks and expenses on mobile devices

• Help your company stay agile with sales automation, ERP, and other applications for mobile employees

• Know how to buy accessories and gadgets that aren’t just cool, but solve real business problems
Thus Mobile Framework has a large spectrum of different applications, requirements and features.
But we Specalites are concentrating on small part of that spectrum and that is “sales automation, ERP, and other applications for mobile employees”