Sunday, August 26, 2012

Telephone Generation


Telephones have gone through three distinct generations, with different technologies:
1. Analog voice.
2. Digital voice.
3. Digital voice and data (Internet, e-mail, etc.).
First-Generation Mobile Phones: Analog Voice:
Enough about the politics and marketing aspects of mobile phones. Now let us look at the technology, starting with the earliest system. Mobile radio telephones were used sporadically for maritime and military communication during the early decades of the 20th century. In 1946, the first system for car-based telephones was set up in St. Louis. This system used a single large transmitter on top of a tall building and had a single channel, used for both sending and receiving.
To talk, the user had to push a button that enabled the transmitter and disabled the receiver. Such systems, known as push-to-talk systems, were installed in several cities beginning in the late 1950s. CB-radio, taxis, and police cars on television programs often use this technology.
In the 1960s, IMTS (Improved Mobile Telephone System) was installed. It, too, used a high-powered (200-watt) transmitter, on top of a hill, but now had two frequencies, one for sending and one for receiving, so the push-to-talk button was no longer needed.

Second-Generation Mobile Phones (2G): Digital Voice:
The first generation of mobile phones was analog; the second generation was digital. Just as there was no worldwide standardization during the first generation, there was also no standardization during the second, either. Four systems are in use now: D-AMPS, GSM, CDMA, and PDC. PDC is used only in Japan and is basically D-AMPS modified for backward compatibility with the first-generation Japanese analog system. The name PCS (Personal Communications Services) is sometimes used in the marketing literature to indicate a second-generation (i.e., digital) system. Originally it meant a mobile phone using the 1900 MHz band, but that distinction is rarely made now.

Third-Generation Mobile Phones (3G): Digital Voice and Data
What is the future of mobile telephony? Let us take a quick look. A number of factors are driving the industry.
First, data traffic already exceeds voice traffic on the fixed network and is growing exponentially, whereas voice traffic is essentially flat. Many industry experts expect data traffic to dominate voice on mobile devices as well soon.
Second, the telephone, entertainment, and computer industries have all gone digital and are rapidly converging. Many people are drooling over a lightweight, portable device that acts as a telephone, CD player, DVD player, e-mail terminal, Web interface, gaming machine, word processor, and more, all with worldwide wireless connectivity to the Internet at high bandwidth.
More realistic is 2 Mbps for stationary indoor users (which will compete head-on with ADSL), 384 kbps for people walking, and 144 kbps for connections in cars. Nevertheless, the whole area of 3G, as it is called, is one great cauldron of activity. The third generation may be a bit less than originally hoped for and a bit late, but it will surely happen.
The basic services that the IMT-2000 network is supposed to provide to its users are:
1. High-quality voice transmission.
2. Messaging (replacing e-mail, fax, SMS, chat, etc.).
3. Multimedia (playing music, viewing videos, films, television, etc.).
4. Internet access (Web surfing, including pages with audio and video).
Additional services might be video conferencing, telepresence, group game playing, and m-commerce (waving your telephone at the cashier to pay in a store). Furthermore, all these services are supposed to be available worldwide (with automatic connection via a satellite when no terrestrial network can be located), instantly (always on), and with quality-of-service guarantees.

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Telephone Digital Services


[II]DIGITAL SERVICES:
The Digital services are more advantageous than the analog services in the sense that they are less sensitive to noise and other forms of interference. The cost of installation is also very low as compared to analog services as it needs only to differentiate between two or three levels of voltage instead of a continuous range of values.
Digital systems are further subdivided into-
a) Switched/56 service,
b) DS and c) DDS.

a) Switched/56 service: It is a digitalised version of analog switched line that allows data rate up to 56Kbps. To communicate through this service, both parties must subscribe. A caller with normal telephone service cannot connect to a telephone or computer with switched/56 even by using a modem. Subscribers don’t need modems to transmit digital data, but instead they need another device called the DSU (Digital service unit) which changes the rate of digital data created by subscriber’s device to 56Kbps and encodes it into a format used by service provider.
DSU is more expensive than modems, but still preferred as it has advantages like better speed, better quality, less susceptibility to noise and can allow bandwidth on demand service (which facilitates video conferencing, multimedia etc.).

b) Digital Data Service (DDS): It is the digitalised version of analog leased line with a maximum data rate of 64Kbps. Like Switched/56, DDS also requires the use of a DSU. The DSU in this service is however cheaper than the one used in Switched/56. It does not use a dial-pad like the Switched/56.

c) Digital Signal (DS) Service: DS is an advanced version of the Switched/56 and the DDS. It incorporates a hierarchy of digital services like that of an analog service. Some of its hierarchical services are:
·         DS0: Resembles DDS, Single digital channel of 64Kbps.
·         DS1: 1.54Mbps service. It can carry other desired multiplexed channels also.
·         DS2: 6.31Mbps service. It can be used as a single or multiplexed channel.
·         DS3: 44.37Mbps service.
·         DS4: 274.17Mbps service.






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Telephone Systems


THE TELEPHONE SYSTEM:
The traditional telephone system (even if it someday gets multi gigabit end to-end fiber) will still not be able to satisfy a growing group of users. People now expect to make phone calls from airplanes, cars, swimming pools, and while jogging in the park. Within a few years they will also expect to send e-mail and surf the Web from all these locations and more. Consequently, there is a tremendous amount of interest in wireless telephony.
[The telephone system is the most basic application of the multiplexing. It includes common carriers that offer local and long distance service to subscribers. These carriers also include local companies. The line connecting a subscriber to that of a network is called a Service line.]

Wireless telephones come in two basic varieties: cordless phones and mobile phones (sometimes called cell phones). Cordless phones are devices consisting of a base station and a handset sold as a set for use within the home. These are never used for networking, so we will not examine them further. Instead we will concentrate on the mobile system, which is used for wide area voice and data communication.

Telephone services are basically categorised in two classes:
1: Analog services.
2: Digital services.
[I] ANALOG SERVICES:

Analog services are further subdivided into –
 Analog switched services and Analog leased services.

(a) Analog switched services: It is the most familiar dial up service generally used in a home telephone. It uses two wire or four wire twisted pair cables to connect the subscriber’s handset to the network through an exchange. The connection is called Local loop. (Sometimes referred to as ‘Public switched telephone network’ or PSTN). The bandwidth of the signal is usually between 0 to 4000 Hz. When the caller dials a number, the call is conveyed to a switch or a series of switches at the exchange. The appropriate switches are then activated to link the caller’s line to that of the person being called. The switch connects the two lines during the duration of the call.

(b) Analog leased services: It offers customers the opportunity to lease a line, sometimes called a dedicated line that is permanently connected to another customer. Even though the connection always passes through the switches in the telephone network, subscribers experience it as a single line because the switch is always closed, i.e. no dialling is needed.

Telephone carriers also offer a service called the Conditioning which means improving the quality of a line by lessening attenuation, signal distortion or delay distortion. Conditioned lines are analog, but their quality makes them usable for digital data communication if they are connected to modems.

 [2] DIGITAL SERVICES

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Thursday, August 23, 2012

Railway Enquiry Websites List

Railway Enquiry Websites List

 Now a days that every contents like data, information, music, videos, news everything is available on Internet.So all users can find any data, news, videos, information by using Internet.

 Some people have to travel in Railway for going outside somewhere.They goes to Railway Station to Book  Tickets and they want to find the information about Railway Routes, Train Availability, Ticket Fare, Seat Availability, PNR Status, Train Current Running Status etc.But The users sometime for shyness they don't ask all the questions from the Railway Enquiry Officer or sometime Railway Enquiry Officers are too busy and they don't provide all the information on Railway that user needs.

 So, for the users sometime It's going to be big problem and they can't their Destination on the time.For the immidiate users to travel it's too hard to get all information on Railway.Sometimes the Station distance is too far or users don't have time to get information on Railway.That's why users need all the information on Railway immidiately.

 Now, that's not a problem for the Inernet users.Users can get all the information On Railway Routes, Train Availability, Ticket Fare,Seat Availability, PNR Status, Train Current Running Status by using following list of websites.





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