Wednesday, June 1, 2011

Wireless Data Communication System


There are lots of different wireless data communication systems. To help us understand them all we divide them into different categories, Local Area Networks, Metropolitan Area
Networks and Wide Area Networks. Some of the techniques used for wireless LANs and MANs are shown here, but we'll discuss those in a little more detail in the next slide.

Wide Area Networks have been divided up into two distinct groups here circuit switched and packet switched. On the circuit switched side are Satellite phones, the American mobile telephone standard AMPS, GSM and PDC. HSCSD (High-Speed Circuit-Switched Data) is a new data oriented circuit switched service for GSM.


The oldest of the packet switched services is Mobitex; a dedicated packet switched network
that has been available since 1986.

The other three systems are conceptually very similar, they all build on existing circuit switched mobile telephony infrastructures and they all add a packet switched service. GPRS provides packet data services by building on the GSM infrastructure, PPDC provides packet data services by building on PDC and CDPD provides packet data services by building on the AMPS (Advanced Mobile Phone System) system.

Packet Circuit Switched


It is extremely important to understand the difference between packet switched and circuit
switched systems, a difference, which is illustrated here.

You can compare a circuit switched data to a train and the physical medium is the train line.
Before the train (data) can use the line it has to ask and be granted access. Once this is done
the train can then proceed on the line, but it has exclusive access to the line. No other trains can use the line while our train is there. In a data communications environment,
circuit switchings main drawbacks are the call setup overhead (that is, asking for and being
granted access to the line) and the low utilization of the physical medium (only one train at a time). One advantage of circuit switching is that the call setup procedure allows users to be allocated and guaranteed a certain bandwidth. A good example of circuit switching is the analogue telephone network. Before you can talk to anyone you have to dial their number (a call request). The call is physically switched through the network until you had an end-toend circuit between your telephone and the telephone you are calling. Only when the phone is answered at the other end (a call accept) can communication begin.

What about packet switching? Well, in this case our data can be imagined as cars and the physical medium as a motorway.

Instead of sending one big chunk of data, like we did with a circuit switched train, we send smaller chunks of data, with many users on the same channel simultaneously. The advantage with this is that we don't need any call set-up procedure, we just jump in the car and go. Packet switching also gives a much higher channel utilization, because many people can use the same motorway simultaneously.

The disadvantage is that because we have no way of knowing how much data other users are
going to send there is no guarantee of bandwidth if too many people try to use the motorway at the same time it will slow down and eventually stop. The best example of a packet switched network is the global Internet.

Wireless and Mobile


" Wireless vs Mobile"


Many people use the terms wireless and mobile as synonyms. This is not strictly true. In this table we try to illustrate the differences between the terms wireless and mobile.

Those users that are not wireless and not mobile are easy to come up with. This is what youcan call "traditional data communications" and consists of conventional LAN, MAN and WAN technology, such as ethernet, FDDI or X.25.

At the other extreme, there are users that are both wireless and mobile. So here we can have
taxis using radio modems, field service engineers (recall that Mobitex's first application was for Telia's field service engineers), transport and public safety users.

What about the users that are using wireless technology but are not mobile? These are the traditional "telemetry" applications that were mentioned in chapter 13. These include parking or gas meters and coke machines.

The last set of users, those that are mobile but not wireless are a little harder to define. Imagine the following scenario, I sit in my office in Gothenburg and work on my laptop PC connected to my local network. Then I pick up my laptop, jump on a plane and fly to Stockholm. There I plug my PC into the LAN in Stockholm and carry on working as if I was on my home network. In this case I'm using wired technology - the two Local Area Networks, but in some sense I'm mobile because I move around the country. This type of access is called Nomadic Computing. An example of technology that allows this kind of movement is Mobile IP, which will be explained in more detail later.

Connectivity



In most cases a mobile node wishes to talk to some remote computer. For users and applications, the way these computers communicate doesn't matter that much. Using a wired
or wireless network makes no difference as long as the packets arrive. This is why these industry standard protocols, such as TCP and IP, are used. If the applications see some standard interface then they don't need to know if they are using a wireless network or a fixed network, they just see a standard interface. These standard interfaces are extremely important to both application and network designers.

what is spyware

what is spyware

Spyware is any application that collects information about your computer activities and then sends that information to another individual or company without your knowledge or permission. Spyware often arrives bundled with freeware (free) or shareware (trial) programs, through email or instant messenger, as an Active X install, or by someone with access to your computer. Once on your drive, spyware secretly installs itself and goes to work. Spyware can be difficult to detect, and difficult (if not impossible) for the average user to remove.

Spyware can:

  • Track your online surfing habits, profile your shopping preferences, gather personal
    information (age, sex, etc, possibly credit card info, PIN numbers)
  • Send your email address to the company/person that made the spyware; that
    company/person can now send spam to your email account.
  • Decrease your connection speed/hog your internet connection by sending information
    about you and your computer to the company/person that made the spyware
  • Hijack your web browser’s start page, bombard you with pop-up advertisement boxes
  • Run in the background and slow your computer down, alter important system files,
    make your computer unstable and crash

Characteristics Wireless


Because wireless data uses an unguided medium it suffers from certain problems that do not affect its wired counterpart. One must remember that a wireless user is constrained by the network coverage. If the user moves to an area without coverage, then the user's terminal becomes useless. Although network operators try to minimize these "blackspots" they are inevitable, especially for new networks that are in the process of being deployed.

Multi-path fading, log-normal fading, inter-symbol interference and attenuation are all characteristics of the radio medium. Coupled with the limited amount of frequency spectrum available to wireless applications these attributes mean that the raw bit error rate is greater
than that found in guided mediums. This means that greater overheads are required to maintain acceptable error rates, thus reducing the amount of user data that can be sent per second.

So, wireless systems almost always offer lower bit rates than their wired alternatives.

Wireless systems also tend to have a higher latency than wired systems. For most protocols this is not a big issue, but is something that should be addressed when setting protocol parameters (e.g., timeouts should generally be longer due to the slower response times).

Information threat

The threat of information leaks

Organizations often fail to acknowledge that there is a great risk of crucial data being stolen from within the company. Various studies have shown how employees use email to send out confidential corporate information. Be it because they are disgruntled and revengeful, or because they fail to realize the potentially harmful impact of such a practice, employees use email to share sensitive data that was officially intended to remain in-house.

As the 2003 Hutton enquiry in the UK demonstrated, government officials and BBC executives were found to have used email to make disclosures that were confidential. A March 1999 PC Week article referred to a study where, out of the 800 workers surveyed, 21-31% admitted to emailing confidential information - like financial or product data - to recipients outside the company.

3G Access


Currently, third-generation mobile wireless services (3G) are coming onto the market that will allow users access the Internet, transmit data and voice, and utilize various multimedia services at speeds of 2 mbps or higher.

3G technologies are turning telephones and other devices into multimedia players, making it possible to download music and video clips. The new service is called the freedom of mobile multimedia access (FOMA) and it uses wideband code division multiple access (W-CDMA) technology to transfer data over its networks. W-CDMA sends data in a digital format over a range of frequencies, which makes the data move faster (but also uses more bandwidth than digital voice services). W-CDMA is not the only 3G technology; competing technologies include CDMAOne, which differs technically but should provide similar services.

Wireless Fidelity


Wireless broadband systems use radio signals to send and receive data and voice at speeds ranging from 128 kbps to 1.5 Mbps. However, speeds are increasing every day and many Wi-Fi systems can transmit at speedsfrom 1-12 Mbps. In general, wireless providers offer Internet access via fixed wireless technology that relies on a stationary signal base. Wireless broadband services use the same radio frequency spectrum that supports pagers, cell phones, microwave signals and more.

With a fi xed wireless system, the provider installs a small antenna (dish) at the user’s premises which is wired to a modem on the user’s computer. Radio waves sent from the user’s antenna to the provider’s antenna connect the computer to the Internet. For optimal usage, the user’s
antenna should have a clear line-of-sight to the wireless broadband provider’s antenna. Bad weather, rugged topography and line-of-sight obstructions can impede service.

tooth are not interoperable. Bluetooth is a computing and telecommunications industry specifi cation that describes how mobile phones, computers and PDAs can easily interconnect with each other as well as telephones and computers using a wireless connection over very short distances. Bluetooth and Wi-Fi are different in a number of ways and should not be viewed as competitors. The biggest difference between the technologies is that Wi-Fi boasts faster data transfer speeds and range, making it a good replacement for Ethernet systems, while Bluetooth requires less power and is prominent in small systems such as personal digital assistants (PDAs).

Advantage and Disadvantage of Bus Network

A network layout where one trunk cable provides the main path of communications. Each node is then connected to this common cable. This common cable also known as “Backbone”

In this type of arrangement the ends of the cables must have terminators installed to complete the circuit and devises are attached to the Drop lines or taps. This type of topology is very poor at handling cable problems as all of the stations beyond the affected area will be brought down.

Bus Network Advantages:
  • Ease of installation
  • Low cabling requirements

Bus Network Disadvantages:

  • Difficult to reconfigure nodes.
  • Fault isolation is difficult.
  • Too many connections to the backbone can seriously degrade performance.
  • A fault in the bus cable causes the whole network to be disabled.

Benefits of Wireless

Originally employed to free network elements to roam in manufacturing and warehouse facilities, wireless LANs (WLANs) are now deployed throughout the enterprise as a cost-saving measure. They can be used to replace wired LANs, or as an extension of a wired infrastructure.

It costs far less to deploy a wireless LAN than to deploy a wired version. The savings isn’t just in the falling cost of wireless components; it’s in the installation of the infrastructure as well. A major cost of installing and modifying a wired network is the expense to run network and power cables, all in accordance with local building codes. For this reason, WLANs are being installed even when the network is comprised only of non-roaming desktops. Examples of additional applications where the decision to deploy WLANs results in large cost savings include:

  • Additions, moves, and changes within an organization
  • Installation of temporary networks
  • Installation of hard-to-wire locations
  • Elimination of costly leased lines
Wireless LANs give the enterprise more mobility and flexibility by allowing workers to stay connected to the Internet and to the network as they roam from one coverage area to another. This increases efficiency by allowing data to be entered and accessed on site.

Besides being very simple to install, WLANs are easy to understand and use. With few exceptions, everything to do with wired LANs also applies to wireless LANs. They function like, and are commonly connected to, wired Ethernet networks. They not only free the networked laptop to roam, but dramatically simplify and lower the cost of network deployment.

WiFi Network

Each component of a WLAN requires a radio transceiver and antenna. Components are either stations or access points. Stations (STAs) are wireless LAN client radios. They can be incorporated into a LAN card installed in a desktop, a USB adapter, a PCMCIA or PC card, or can be integrated into the notebook or handheld device itself. Access Points (APs) form a bridge between wireless and wired LANs.

A Basic Service Set (BSS) is formed when two or more stations have recognized each other and established a network. The network can be configured in two basic ways:

  • Peer-to-peer (ad hoc mode) – This configuration is identical to its wired counterpart, except without the wires. Two or more STAs can talk to each other without an AP. When two or more stations form an ad hoc network, this is referred to as an Independent Basic Service Set (IBSS).
  • Client/Server (infrastructure networking) – This configuration consists of multiple stations connected to an AP, which acts as a bridge to a wired network. A BSS in this configuration is referred to as being in infrastructure mode.
An Extended Service Set (ESS) is formed when multiple overlapping BSSs (each containing an AP) are connected together by means of a distribution system, usually a wired Ethernet LAN. BSSs whose ranges overlap must transmit on different channels to avoid interference.

Range between STAs and APs is up to 100 m (depending on data rate), but the overall range of an ESS is limited only by the range of the wired distribution system. Also, ESSs can be further extended with wireless links up to several miles by the use of directional range extender antennas.

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