Wireless Networking




Дата канвертавання27.04.2016
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Wireless Networking



Time-Division Multiple Access (TDMA)

What is TDMA? TDMA (time division multiple access) is a technology used in digital cellular telephone communication to divide each cellular channel into three time slots in order to increase the amount of data that can be carried.

How it Works? TDMA works by time-division multiplexing: sending multiple signals (each of which has its own time slot) simultaneously on a single carrier in the form of a complex signal, and then recovering the separate signals at the receiving end. For TDMA, the carrier is divided into three time slots, each of which serves one subscriber. The information is broken into tiny data packets, which are transmitted in timed bursts in the 30-megahertz range. At the receiving end, the separate information streams are recovered. See also FDMA (frequency division multiple access) and CDMA (code-division multiple access).

TDMA was developed in response to the basic wireless network problem:

large numbers of users and limited frequency allotments. TDMA increases network efficiency by enabling single connections to carry multiple data channels, offering a three-fold increase in capacity over Advanced Mobile Phone Service (AMPS) networks.

Flexible and scalable, TDMA facilitates step-by-step migration to digital operation. TDMA can be implemented seamlessly across both 800- and 1900-MHz networks. Its hierarchical cell structure allows service providers to increase capacity where demand is greatest, in high-use areas.

TDMA is applied in Digital-American Mobile Phone Service, Global System for Mobile communications, and Personal Digital Cellular (PDC). However, each of these systems implements TDMA in a somewhat different and incompatible way. TDMA was first specified as a standard in EIA/TIA Interim Standard 54 (IS-54). IS-136, an evolved version of IS-54, is the United States standard for TDMA for both the cellular (850 MHz) and personal communications services (1.9 GHz) spectrums. TDMA is also used for Digital Enhanced Cordless Telecommunications.
Code Division Multiple Access (CDMA)

The term CDMA refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications. As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.

CDMA employs analog-to-digital conversion (ADC) in combination with spread spectrum technology. Audio input is first digitized into binary elements. The frequency of the transmitted signal is then made to vary according to a defined pattern (code), so it can be intercepted only by a receiver whose frequency response is programmed with the same code, so it follows exactly along with the transmitter frequency. There are trillions of possible frequency-sequencing codes; this enhances privacy and makes cloning difficult.

The CDMA channel is nominally 1.23 MHz wide. CDMA networks use a scheme called soft handoff, which minimizes signal breakup as a handset passes from one cell to another. The combination of digital and spread-spectrum modes supports several times as many signals per unit bandwidth as analog modes. CDMA is compatible with other cellular technologies; this allows for nationwide roaming.

The original CDMA standard, also known as CDMA One and still common in cellular telephones in the U.S., offers a transmission speed of only up to 14.4 Kbps in its single channel form and up to 115 Kbps in an eight-channel form. CDMA2000 and wideband CDMA deliver data many times faster.

Global System for Mobile communication


GSM (Global System for Mobile communication) is a digital mobile telephone system that is widely used in Europe and other parts of the world. GSM uses a variation of time division multiple access (Time Division Multiple Access) and is the most widely used of the three digital wireless telephone technologies (TDMA, GSM, and CDMA). GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either the 900 MHz or 1800 MHz frequency band.

GSM is the de facto wireless telephone standard in Europe. GSM has over 120 million users worldwide and is available in 120 countries, according to the GSM MoU Association. Since many GSM network operators have roaming agreements with foreign operators, users can often continue to use their mobile phones when they travel to other countries.

American Personal Communications (APC), a subsidiary of Sprint, is using GSM as the technology for a broadband personal communications service (personal communications services). The service will ultimately have more than 400 base stations for the palm-sized handsets that are being made by Ericsson, Motorola, and Nokia. The handsets include a phone, a text pager, and an answering machine.

GSM together with other technologies is part of an evolution of wireless mobile telemmunication that includes High-Speed Circuit-Switched Data (High-Speed Circuit-Switched Data), General Packet Radio System (General Packet Radio Services), Enhanced Data GSM Environment (Enhanced Data GSM Environment), and Universal Mobile Telecommunications Service (Universal Mobile Telecommunications System).



Roaming service


Roaming service is the ability to get access to the Internet when away from home at the price of a local call or at a charge considerably less than the regular long-distance charges. For example, if you normally get access to the Internet from an access provider in Brooklyn, New York and are travelling to Hong Kong, you can call a designated access provider in Hong Kong. Instead of paying long distance charges to your local provider in Brooklyn, you pay the local phone connection charge in Hong Kong and possibly a modest additional charge for the service.

Roaming service is made possible through Internet service providers (Internet service provider) who have cooperative agreements to grant each others' customers local access to the Internet. Special software allows cooperating ISPs to keep track of and calculate prearranged payments for usage differences. Here's how it works for the user:



  1. The Internet user must already subscribe to an ISP that offers roaming service arrangements.

  2. Assuming the ISP does, the user can determine a cooperating ISP in a city to which the user is travelling.

  3. In the travel location, the user can call the local ISP's designated phone number through the computer modem, entering information during login that will identify the user's home ISP.

  4. The "foreign" ISP will contact the ISP and determine that the user is a valid user.

  5. The "foreign" ISP will grant the user access to the Internet. The user will be able to access e-mail from the home mail server.

  6. The user will be charged at local phone rates. In addition, depending on the particular service arrangement, the home ISP may levy an additional hourly usage charge of several dollars an hour or a monthly charge in case the service is used during that month.

A similar roaming service is provided by some cooperating cellular telephone or personal digital assistant (personal digital assistant) service providers.

If you are travelling and simply need to be able to exchange e-mail, you can consider getting a freemail membership (usually free) from HotMail, Rocketmail, or other freemail providers. Hotmail also offers POP3 server accounts for access to up to four e-mail accounts you may already have, assuming you remember the POP3 server name and your user IDs and passwords.

If you subscribe to a somewhat global service such as AT&T's WorldNet or the IBM Global Network, you may already be able to access your account in certain cities through your provider's local point-of-presence (POP) on the Internet without having to pay for a long-distance call.


WAP


WAP (Wireless Application Protocol) is a specification for a set of communication protocol to standardize the way that wireless devices, such as cellular telephones and radio transceivers, can be used for Internet access, including e-mail, the World Wide Web, newsgroups, and Internet Relay Chat (Internet Relay Chat). While Internet access has been possible in the past, different manufacturers have used different technologies. In the future, devices and service systems that use WAP will be able to interoperate.

The WAP layers are:

Wireless Application Environment (WAE)

Wireless Session Layer (WSL)

Wireless Transport Layer Security (WTLS)

Wireless Transport Layer (WTP)

The WAP was conceived by four companies: Ericsson, Motorola, Nokia, and Unwired Planet (which is now Phone.com).
Wireless Markup Language

WML (Wireless Markup Language), formerly called HDML (Handheld Devices Markup Language), is a language that allows the text portions of Web pages to be presented on cellular telephone and personal digital assistants (personal digital assistant) via wireless access. WML is part of the Wireless Application Protocol (WAP) that is being proposed by several vendors to standards bodies. The Wireless Application Protocol works on top of standard data link protocols, such as Global System for Mobile communication, code-division multiple access, and Time Division Multiple Access, and provides a complete set of network communication programs comparable to and supportive of the Internet set of protocols.



WML is an open language offered royalty-free. Specifications are available at Phone.com's Web site. According to Phone.com, any programmer with working knowledge of HTML, common gateway interface, and Structured Query Language should be able to write a presentation layer using WML. A filter program can be written or may be available from a vendor that will translate HTML pages into WML pages.




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