Multiple Access Techniques for Wireless Communications (MAT)

Multiple Access Techniques for Wireless Communications (MAT)
In wireless communication, limited or finite numbers of radio channels are available and to share these channels simultaneously to many mobile users, Multiple Access Techniques are used. There are three major access techniques.

* Frequency Division Multiple Access (FDMA)

* Time Division Multiple Access (TDMA)

* Code Division Multiple Access (CDMA)

Frequency Division Multiple Access (FDMA)

In FDMA technique individual or unique channels are assigning to those individual users, who request for service. On demand two distant bands of frequencies are allocated for every user. One is for forward channel and other is for reverse channel. Forward channel is used to transfer information from Base Station to Mobile and Reverse channel transfer information from Mobile to Base Station. FDMA includes the following features:

* In FDMA, one phone circuit is carried by one FDMA channel at a time.

* In FDMA, if an FDMA channel is not in use, it cannot be used by other users to increase the channel capacity. It remains idle.

* FDMA is implemented in Narrowband systems, because, in FDMA, each channel supports only one circuit per carrier so the bandwidth of FDMA channels are relatively narrow (30 KHz).

* Inter symbol Interference is low in Narrowband systems because Narrowband signal has large symbol time as compare to delay spread.

* FDMA mobile systems are less complex as compare to TDMA mobile systems.

* FDMA is a continuous transmission scheme as compare to TDMA because fewer bits are needed for synchronization and framing.

* In FDMA, as unique channels are assigned to each user, so FDMA systems have higher cell site system cost as compared to TDMA system.

* In FDMA, both the transmitter and receiver operates at the same time so FDMA

* Mobile units require duplexers. These also increase the cost of FDMA subscriber units and base station.

Time Division Multiple Access (TDMA)

In TDMA, a radio spectrum is divided into time slots. These time slots are allocated for each user to transmit and receive information. The number of time slots is called a frame. Information is transferred and received in form of frame. A frame is consists a preamble, an information message and trial bits.

Preamble contains the address and synchronization information of both subscriber and Base Station to identify each other. Trial bits contain framing information.

TDMA includes the following features:

* In TDMA, by using no overlapping time slots several users shares a single carrier frequency. The number of time slots per frame depends upon several factors, such as modulation techniques and bandwidth.

* In TDMA, data transmission is not continuous and subscriber transmitter can be turned off which result in low battery consumption.

* In TDMA, handoff process is much simpler for a subscriber because of discontinuous transmission.

* In TDMA, duplexers are not required because different timeslots are used for transmission and reception.

* In TDMA, the rate of transmission is very high as compare to FDMA.

* In TDMA, synchronization is required and the time for synchronization or guard time should be minimum.

* TDMA systems require large overheads (framing bits) as compare to FDMA because of discontinuous or burst transmission.

* In TDMA, bandwidth can be utilized on demand of different users as different numbers of time slots are allocated to different users.

Code Division Multiple Access (CDMA)

In CDMA, all users transmit information simultaneously by using the same carrier frequency. Each user has its own codeword, which is orthogonal to other users. To detect the message, the receiver should know the codeword used by the transmitter. A time correlation operation is performed at receiver to detect the desired codeword and all other codeword appears as noise due to de-correlation and to minimize this noise power at receiver station power should be high. CDMA includes the following features:

* In CDMA system many users share the same frequency.

* In CDMA unlike FDMA and TDMA the number of users is not limited. It has a soft capacity. But due to large number of users its performance degrades.

* In CDMA, the spread spectrum or bandwidth is large so multipart fading is reduced.

* In CDMA, the channels data rates are very high.

* CDMA systems provides soft handoff and this handoff is performed by the MSC.

* In CDMA systems, if spreading sequences of different users are not exactly orthogonal, the Self-jamming problem is arises.

* In CDMA systems, if a desired user has less power as compare to undesired users, the near-far problem occurs.In CDMA, each user operates independently with no knowledge of the other users.

How to view system uptime in Microsoft

To view the amount of time that PC has been running with out rebooting or restarting the computer:

1) Open the Command Prompt (located under Accessories)
2) Type systeminfo
3) Once the system has finished gathering the information, scroll up and the time is displayed in the System Up Time field.

As you can see, running systeminfo also gives a lot of useful information.

Wireless Application Protocol (WAP)

In a Mobile Network, several constraints are used to deliver wireless data. For wireless, the handheld devices should be portable, have limited size and weight, which results in reduced memory capacity, less power CPU and reduced user interface. Wireless networks also have limited bandwidth, longer coverage and a degree of reliability. The wireless handheld devices can access Internet applications only if all these constraints are carefully addressed.
To address these constraints carefully, In June 1997, the Wireless Application Protocol (WAP) forum was founded. The development of this forum was important in wireless industry.

WAP forum has developed a set of wireless protocol specifications and offer these protocols to many industry groups. The result is that many mobile operators and handset manufacturers have joined the WAP-enabled devices. Before WAP-enabled devices, wireless data services were not successful. So, to improve wireless data market, WAP is introduced with limited computing and memory capacities, WAP allows a Web browser in handheld devices to access Internet.

WAP Architecture
WAP architecture consists of three operational units.
*WAP handset or handheld device
* WAP Gateway
* Origin server or Web server

The WAP handset or handheld and origin server communicates with each other through Mobile Network. WAP handset received the contents and these contents are encoded in binary format of Wireless Markup Language (WML) for efficient transmission. A WAP Gateway, which is located between Internet/server and mobile/WAP handset, receives these contents and decodes them from binary format to text format. This text format is then forward to origin server. The origin server decides what to retrieve from the text format. The Uniform Resource Locator (URL) of the server specifies the files and these files are transferred to WAP Gateway through Hypertext Transfer Protocol (HTTP). The WAP Gateway encodes the HTTP header into WML byte code and then this result is transferred to WAP handset. A Hypertext Markup Language (HTML) filter is also located between origin server and WAP Gateway. It acts like a translator and translates the HTML format into Wireless Markup Language (WML) format, which is used in wireless applications.

WAP Models

For handset devices WAP defines a user-interface model. In WAP handset devices up and down scroll keys are used for navigation. For menu option selection and context-specific functions Soft Keys are used. Some examples of WAP phone models are following.* Nokia 7110* Ericsson R320* Ericsson MC 218GSM 900/1800 dual-band supports these models as well. These models are equipped with large display, micro browser, Voice memo, full graphics, calendars, contact manager and a complete Chinese interface.

WAP Protocols

WAP Protocols are defined in terms of WAP Protocol stack.
* Wireless Datagram Protocol (WDP)
* Wireless Transport Layer Security (WTLS)
* Wireless Transaction Protocol (WTP)
* Wireless Session Protocol (WSP)
* Wireless Application Environment (WAE)

1. Wireless Datagram Protocol (WDP): The WAP transport layer is defined by the WDP. To the upper-layer protocols of the WAP, the WDP offers consistent services and with it these layers can function independently of the underlying layers.

2. Wireless Transport Layer Security (WTLS): The WAP security layer is defined by the WTLS. By using encryption and decryption techniques, the WTLS provides unchanged and uncorrupted data integrity.

3. Wireless Transaction Protocol (WTP): The WAP transaction layer is defined by the WTP. WTP is similar to Transmission Control Protocol (TCP), which controls that traffic of data and also saves processing and memory costs in WAP handset. By using retransmission and acknowledgments techniques it provides reliability.

4. Wireless Session Protocol (WSP): The WAP session layer is defined by the WSP. WSP supports client and server for exchange of contents, by establishing a session from client to server and releases the session and saves the power of WAP handset, when it is idle.

5. Wireless Application Environment (WAE): The WAP application layer is defined by the WAE. To efficiently built applications it provides an environment for mobile operators. It also defines a set of formats, images, phone book records and calendar information.

Advantages of WAP WAP Protocols are specially designed to reduce the amount of information, which is delivered through wireless network of limited bandwidth. To deliver the same information WAP uses 50 percent less packets as compare to HTTP/TCP.

Words by Women and Their meanings

Fine
This is the word women use to end an argument when they are right and you need to shut up.

Five Minutes
If she is getting dressed, this is half an hour.Five minutes is only five minutes if you have just been given 5 more minutes to watch the game before helping around the house.

Nothing
This is the calm before the storm. This means "something" and you should be on your toes. Arguments that begin with "nothing" usually end in "fine".

Go Ahead
This is a dare, not permission, DON'T DO IT!

Loud Sigh
Although not actually a word, the loud sigh is often misunderstood by men. A "Loud Sigh" means she thinks you are an idiot and wonders why she is wasting her time standing here and arguing with you over "Nothing".That's OkayThis is one of the most dangerous statements that woman can make to a man. "

That's Okay"
means that she wants to think long and hard before deciding how and when you will pay for your mistake.

Thanks
This is the least used of all words in the female vocabulary. If a woman is thanking you. Do not question it, just say you're welcome and back out of the room slowly.

Change information and images in System properties in MIcrosoft

Please run notepad and paste this text:

[General]

Manufacturer="You can display useful information here"Model="You can display useful information here"

[Support Information]

Line1="You can display useful information here"

Line2="You can display useful information here"

Line3="You can display useful information here"

Line4="You can display useful information here

"Line5="You can display useful information here"

Line6="You can display useful information here"

Line7="You can display useful information here"

Line8="You can display useful information here"

Line9="You can display useful information here"

Line10="You can display useful information here"

Save it as oeminfo.ini in C:\WINDOWS\System 32. You can also create a bitmap file and name it as oemlogo.bmp, save it in C:\WINDOWS\System 32. Now, try to hold down Win key while pressing Pause/break. You'll notice that the text and image you created will appear in your System properties. I hope u enjoy it!

What is Video Conferencing?

Video conferencing is a communications technology that integrates video and voice to connect remote users with each other as if they were in the same room. Each user needs a computer, webcam, microphone, and broadband internet connection for participation in video conferencing. Users see and hear each other in real-time, allowing natural conversations not possible with voice-only communications technology.Communications companies have been dabbling in video conferencing technology since as early as the late 50s, but it took the advent of broadband internet and affordable web cameras (late 90s) for video conferencing to really take off. Good bandwidth is necessary for high-fidelity streaming video and voice. Video conferencing took a serious step into mass use with the release of Microsoft Netmeeting 3.0. Now there are dozens of software vendors marketing video conferencing software and a number of investors interested in bringing video conferencing to mobile devices.Very appealing to the educational and business sectors, video conferencing allows users to save time and money on traveling and housing costs by bringing people face-to-face virtually. Many prominent universities have adopted video conferencing as an educational tool to be used in conjunction with online courses. Business leaders around the world use video conferencing to keep in touch with important contacts while on the go.Present-day applications of video conferencing technology are just the beginning. As video and voice capture technology, software, and display technologies continue to improve, the experience of video conferencing will become increasingly natural and intuitive to a wider range of users. Eventually video conferencing and similar technologies will allow the creation of "virtual cities", online spaces where people work together without the constraint of geographic proximity. This may decrease urban congestion and save the environment by making it possible for skilled workers living in the suburbs to acquire high-paying jobs without the commute.

What is Internet Fax?

To use an Internet fax service, a subscriber signs up for a small monthly fee and is assigned a fax number. When a fax is sent to the subscriber's number, the service accepts the fax, turning it into a file that is forwarded to the subscriber's existing email address.
To send an Internet fax, the subscriber creates an email. The fax content can be written in the body of the email, and attachments are normally permitted. The destination fax number is entered in the "To" field, along with "@" and the Internet fax service name. For example, if the fax number were 1-550-234-5678, the email would be addressed to 15502345678@site.com. In the "Subject" field the user enters the name of the recipient, "Attn: Mr. Jam" for example.
Discover the advantages of technology nowadays! Read further on the best Email to Fax Service reviews!
A small toolbar, available for download from the Internet fax service, is used as a plug-in inside the email program. Clicking on the "Send" button on the toolbar sends the email to the Internet fax service. Upon receipt, an automated program converts the email to a fax format, then faxes to the number supplied in the "To" header. The recipient receives the fax normally, through his or her fax machine.
This service can be very convenient. Of the many advantages, your assigned fax number is active 24 hours a day, so that your account can accept faxes around the clock. Contrast this with using a fax machine, which relies on your phone system being free to accept a fax. For those that use one phone line for fax, telephone, and dial-up, an Internet fax service is especially handy. Internet fax services also save paper and fax toner, and do not rely on a serviceable fax machine. All faxes are sent and received as files. They can be printed by choice, but it isn't necessary.
A further advantage of an Internet fax service is that the subscriber does not need to be familiar with faxing software or fax machines. The convenience of sending and receiving faxes by using the familiar email interface appeals many people.
One of the best features of Internet fax services is that many offer toll-free fax numbers for the U.S. and Canada, so that your U.S. and Canadian clients can fax you for free. Sending faxes is also free of phone charges for the subscriber, as he or she is only sending emails.
When looking into Internet fax services, consider a service that offers a secure website. This provides a mobile "portal" to send/receive faxes while you're on the road or away from home. Confirmation of sent faxes should be standard, so you can be sure your client got your communication. The ability to send to more than one fax number at a time can also be valuable. Check for a page capacity, as some services may limit the amount of pages the subscriber can send/receive in a month.

http://www.gigasize.com/get.php?d=m001xz84glc

Removing of Password from a "pst" File in Outlook

Author: Raji Subramanian
Microsoft produced a utility called "pst19upg.exe" which was used to upgrade old versions of "pst" files to the later version. One side effect of this was that it removed any passwords from the "pst" file.
Therefore if you find that you cannot access a "pst" file because the client has forgotten their password or has left the company, you can use this utility to get access to the data.
NOTE: This process does NOT work for PST files that have been created in the new UNICODE format in Outlook 2003 and higher that allows PST files to exceed 2gb.
Before You Start
Before using the utility, make certain you have at lease three times the size of the "pst" file in temporary free the disk space.
For example:
Original "pst" file is 150mb
Copy of "pst" file is 150mb
"psx" file (created during process) is 150mb
New "pst" file 150mb
Therefore an additional 450mb of temporary free space required for process to complete
Requirements
You will need two utilities:
1. "pst19upg.exe" - You will need to find this from a third party web site as Microsoft have removed it from theirs. It is not distributed from here due to licensing concerns. Try this site: http://www.computerhope.com/download/updates.htm or search on Google.
2. "scanpst.exe" - This can be found on all systems with Outlook installed. Search for it using the built in tools if it isn't on your start menu.
How to strip a password from a "pst" file using the "pst19upg.exe"
1. Calculate the necessary free space ("pst" file size x 3) to run the utility.
2. Find that amount of temporary space in which to perform the recovery.
3. Copy the utility into the "recovery" free space.
4. Copy the target PST file into the "recovery" free space. There is the possibility this utility may corrupt the contents of a "pst" so always use a copy of the file. Remember that Outlook needs to be closed completely before you can copy the file.
5. Run "scanpst.exe" (without any LOG or Backup options enabled) against the copy of "pst" and "Repair", if prompted.
6. Open a Command Prompt and change into drive/directory of the "recovery" free space.
7. At the command prompt, type "pst19upg.exe -x filename.pst" and hit enter (where filename.pst is the name of the "pst" file you are working on). A progress bar will appear. This utility will create afile from the target "pst" called "filename.psx" (where filename is the same name as the original "pst" file).
8. When complete, rename the original pst file to something else (for example filename-old.pst).
9. At the command prompt, type "pst19upg.exe -i filename.psx" and hit enter. Again a progress bar willappear.This utility now creates a password free "pst" file from the "psx" file.
10. Run "scanpst.exe" (again without LOG or Backup options enabled) against recovered "pst" and "Repair", if prompted.
11. Test access to file using Outlook, then return it to the client.

IPV4 & IPV6 Features

Author By: Raji Subramanian
IPv4 Features
1. Invented to support the Pentagon and select Universities
2. Ensures address uniqueness on the Internet and on the Intranet
3. Permits address sharing using Network Address Translation (NAT) via IPv4 Multiplexing. Note: IPv4 at layer-4 support 65536 ports per protocol:
a. TCP - 0-65535
b. UDP - 0-65535
4. Supports 2^32 or approximately 4.3 billion addresses
5. IPv4 is a 32-bit addressing system
Note: Computers are binary or base-2 systems. i.e. (0 or 1)
6. Represents addresses using 4-bytes (32-bits) or 4-Octets
Note: 1-Octet = 8-bits = 1-byte
7. Uses Classful (A-E)
8. Uses Classless (CIDR) notation - i.e. /24 = 24-bits or 255.255.255.0
9. Standardized in September, 1981 via RFC-791
10. IPv4 consists of a 20-to-60-byte variable length header
11. Fragmentation is performed by routers
12. Options are embedded in the IP packet, resulting in variable lenghts
13. Subnet mask is used to distinguish between networks and hosts
14. IPv4 uses broadcasts heavily - i.e. ARP
15. Configuration depends heavily on manual(Static) or DHCP(Dynamic)
16. Provides ICMP for debugging. i.e. PING, traceroute, mtr, etc.
17. Multicast is optional
IPv6 Features:
1. Increased address space to 128-bits vs. 32-bits
2. /48 or 48-bits are assigned to the organization and is globally unique
3. /16 is used for organizational subnetting - 16-bits
4. /64 to define unique hosts per subnet
5. Minimum 1280-byte MTU compared to 576-byte MTU for IPv4
6. Automatic Configuration is now a reality:
a. Link-local address is auto-configured
b. Router Solicitation is performed by IPv6 hosts
c. DHCPv6 Solicitation is performed by IPv6 hosts
7. 2-Types of Address Configurations:
a. Stateless - without DHCPv6 server and/or router - uses multicast
b. Stateful - with DHCPv6 server and/or router
8. Routers pass along prefixes to be auto-configured on IPv6 hosts
9. DHCPv6 servers pass along prefixes and/or additional settings
10. IPv6 obviates the need for NAT because ALL hosts/devices are assigned a Globally Uniqued ID (GUID)
11. Supports 3-Types of addresses:
a. Unicast - unique address for a device (host/router/phone/etc.)
b. Multicast address - groups common systems (routers/hosts/etc.)
c. Anycast address - represents closest address, especially in multihomed environments
12. IPv6 hosts uses Neighbor Discovery (ND) over multicast to find hosts
13. ND replaces ARP and other broadcast-based techniques
14. Facilitates hierarchial routing from Internet backbone to the farthest regions of an organization's networks
INTERNET BACKBONE
-ISP
-Organization
-Organization's Subnets
15. IPv6 uses Hexadecimal to represent the 128-bit addresses because HEX is base-16, and requires less space to note addresses
16. IPv6 supports JumboGrams - packets larger than 64k
17. IPv6 segments 128-bit address into 16, 8-bit fields or 16-Octets
18. IPSEC - provides data integrity and encryption services
19. Each HEX value represents a nibble (4-bits) or half byte
inet6 addr: fe80::211:43ff:fe5a:bce5/64 Scope:Link
inet addr:192.168.75.15 Bcast:192.168.75.255 Mask:255.255.255.0

Raji Subramanian

This is my first day in which i have launched this blog hoping that it will be helpful for most of them to improve the technical skill and knowledge sharing.Let we walk together to share our knowledge.

Raji Subramanian