Wireless Access Methods and Schemes
In telecommunications and computer networks, a channel access method or multiple access method allows several terminals connected to the same multi-point transmission medium to transmit over it and to share its capacity. Examples of shared physical media are wireless networks, bus networks, ring networks, hub networks and half-duplex point-to-point links.
A channel-access scheme is based on a multiplexing method, that allows several data streams or signals to share the same communication channel or physical medium. Multiplexing is in this context provided by the physical layer. Note that multiplexing also may be used in full-duplex point-to-point communication between nodes in a switched network, which should not be considered as multiple access.
A channel-access scheme is also based on a multiple access protocol and control mechanism, also known as media access control
(MAC). This protocol deals with issues such as addressing, assigning
multiplex channels to different users, and avoiding collisions. The
MAC-layer is a sub-layer in Layer 2 (Data Link Layer) of the OSI model and a component of the Link Layer of the TCP/IP model. There are two kind of physical switching techniques available to transfer data via these wireless access method schemes.
Circuit switching(Analog Method)
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel (circuit) through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the duration of the communication session. The circuit functions as if the nodes were physically connected as with an electrical circuit.The defining example of a circuit-switched network is the early analog telephone network. When a call is made from one telephone to another, switches within the telephone exchanges create a continuous wire circuit between the two telephones, for as long as the call lasts.
Packet switching(Digital Method)
Packet switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams (sequences of packets) over a shared network which allocates transmission resources as needed using statistical multiplexing or dynamic bandwidth allocation techniques. When traversing network adapters, switches, routers, and other network nodes, packets are buffered and queued, resulting in variable delay and throughput depending on the network's capacity and the traffic load on the network.
1G
Frequency Division Multiple Access (FDMA)
The frequency-division multiple access (FDMA) channel-access scheme is based on the frequency-division multiplexing (FDM) scheme, which provides different frequency bands to different data-streams. In the FDMA case, the data streams are allocated to different nodes or devices. An example of FDMA systems were the first-generation (1G) cell-phone systems, where each phone call was assigned to a specific uplink frequency channel, and another downlink frequency channel. Each message signal (each phone call) is modulated on a specific carrier frequency.
2G
2G is a digital wireless access method used by basic cellular
telephones. The name comes from the term "second generation," and
indicates that 2G is an upgrade from the outdated analog connection
method. Prior to the introduction of 2G wireless in the early 1990s,
cellphones were not digital. The radio signal of a first-generation
analog phone could be easily intercepted by anybody listening on the
same frequency. 2G digital signals are much more secure and have
significantly better sound quality. This type of wireless signal is not
designed to transfer large amounts of data, and is best suited for
standard voice calls and simple text messages. In order to allow many
different phones to share the same frequencies, CDMA and GSM standards
were developed. In basic terms, CDMA and GSM signals are sent using a
variety of digital languages, or "codes." Some of the cell phones in an
area utilize one code, while other phones communicate with a completely
different code. This prevents 2G phones from "interrupting" each other.
As of the date of publication, the 2G format is still used by many basic
cell phones, although it is slowly being replaced by newer technology.
Time division multiple access (TDMA)
The time division multiple access (TDMA) channel access scheme is based on the time-division multiplexing (TDM) scheme, which provides different time-slots to different data-streams (in the TDMA case to different transmitters) in a cyclically repetitive frame structure. For example, node 1 may use time slot 1, node 2 time slot 2, etc. until the last transmitter. Then it starts all over again, in a repetitive pattern, until a connection is ended and that slot becomes free or assigned to another node. An advanced form is Dynamic TDMA (DTDMA), where a scheduling may give different timesometimes but some times node 1 may use time slot 1 in first frame and use another time slot in next frame.
Code division multiple access (CDMA)
The code division multiple access (CDMA) scheme is based on spread spectrum, meaning that a wider radio spectrum in Hertz is used than the data rate of each of the transferred bit streams, and several message signals are transferred simultaneously over the same carrier frequency, utilizing different spreading codes. The wide bandwidth makes it possible to send with a very poor signal-to-noise ratio of much less than 1 (less than 0 dB) according to the Shannon-Heartly formula, meaning that the transmission power can be reduced to a level below the level of the noise and co-channel interference (cross talk) from other message signals sharing the same frequency.
GSM
GSM (Global System for Mobile Communications, originally Groupe Spécial Mobile), is a standard set developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second generation (2G) digital cellular networks used by mobile phones. It became the de facto global standard for mobile communications with over 80% market share. The GSM standard was developed as a replacement for first generation (1G) analog cellular networks, and originally described a digital, circuit-switched network optimized for full duplex voice telephony. This was expanded over time to include data communications, first by circuit-switched transport, then packet data transport via GPRS (General Packet Radio Services) and EDGE (Enhanced Data rates for GSM Evolution or EGPRS).
3G
3G or "third generation" wireless access is designed to transfer data at a faster rate than the older 2G system. This speed upgrade occurred in several steps. In the year 2000, cell phone companies began to implement General Packet Radio Service, or "GPRS." This was the beginning of 3G service, and allowed wireless data speeds of up to 114 kbps. Enhanced Data Rates for Global Evolution, also called "EDGE," was implemented in 2003. This improved the transfer rate for 3G devices to 384 kbps. Several other upgrades have since raised the rate of 3G wireless to over 1 Mbps in many developed countries. These speeds allow smartphones, including the BlackBerry and iPhone, to browse the Web and send multimedia messages. 3G can also be used to wirelessly connect tablet computers and laptops to the Internet.General Packet Radio Service
General packet radio service (GPRS) is a packet oriented mobile data service on the 2G and 3G cellular communication system's global system for mobile communications (GSM). GPRS was originally standardized by European Telecommunications Standards Institute (ETSI) in response to the earlier CDPD and i-mode packet-switched cellular technologies. It is now maintained by the 3rd Generation Partnership Project (3GPP).
GPRS usage is typically charged based on volume of data transferred, contrasting with circuit switched data, which is usually billed per minute of connection time. Usage above the bundle cap is either charged per megabyte or disallowed
Enhanced Data Rates for GSM Evolution
Enhanced Data rates for GSM Evolution (EDGE) (also known as Enhanced GPRS (EGPRS), or IMT Single Carrier (IMT-SC), or Enhanced Data rates for Global Evolution) is a digital mobile phone technology that allows improved data transmission rates as a backward-compatible extension of GSM. EDGE is considered a pre-3G radio technology and is part of ITU's 3G definition.[1] EDGE was deployed on GSM networks beginning in 2003 – initially by Cingular (now AT&T) in the United States.
4G
4G is the "fourth generation" of mobile wireless technology. According
to the International Telecommunication Union, a network is considered
4G if it operates at 100 Mbps or faster. This is more than 25 times
faster than the previous 3G format. 4G wireless networks are fast enough
to replace traditional cable-based broadband Internet connections, and
can be used by both home computer and mobile smartphone users. 4G is
significantly different from 3G -- it's not just an "upgraded" version
of the older technology. A 4G network uses orthogonal frequency division
multiplexing, or "OFDM." This means that the signal is split into
several distinct sub-signals with slightly different frequencies. 4G was
first launched in the United States in 2008. Large cities were the
first to receive fourth-generation service. Major U.S. carriers plan to
extend the wireless coverage -- meaning that 4G speeds eventually will
be available throughout most of the country.
Orthogonal frequency-division multiplexing
Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, DSL broadband internet access, wireless networks, and 4G mobile communications.
OFDM is essentially identical to coded OFDM (COFDM) and discrete multi-tone modulation (DMT), and is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method. The word "coded" comes from the use of forward error correction (FEC).[1] A large number of closely spaced orthogonal sub-carrier signals are used to carry data[1] on several parallel data streams or channels. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.
Wi-Fi
Wi-Fi is a type of wireless signal designed for local networks. A
Wireless Local Area Network, or "WLAN," typically covers an office
building or a home and does not extend beyond a relatively short range. A
Wi-Fi wireless network allows a group of users to share files and
connect to the Internet without a hard-wired network cable. There are
several varieties of Wi-Fi, each defined by the Institute of Electrical
and Electronics
Engineers (IEEE). The 802.11b Wi-Fi format operates on a radio
frequency of 2.4 GHz and has a maximum speed of 11 Mbps. 802.11g was
released in 2003 and increased the maximum Wi-Fi speed to 54 Mbps. In
2009, the 802.11n wireless standard was announced -- it significantly
upgraded the rate, to 600 Mbps. Wi-Fi networks can be used to connect
PCs, smartphones, and tablet computers to the Internet; they typically
have a range of several hundred feet.
Bluetooth
Bluetooth is a wireless networking technology intended for small,
personal networks. A short-range Bluetooth network is sometimes called a
Personal Area Network, "PAN," or "piconet." While cell phone and Wi-Fi
networks provide connectivity for many simultaneous users, a Bluetooth
connection is typically used by only one user. Small Bluetooth radios
are integrated in many electronic devices, ranging from music players to
smartphones and printers. Wireless Bluetooth can replace the tangle of
cords that these devices traditionally require. For example, a
Bluetooth-enabled smartphone can be controlled by a wireless keyboard
while also connecting to the user's headset for incoming calls.
Bluetooth devices must be "paired" or linked before use -- this prevents
unauthorized people from connecting to the personal network.
Wireless Application Protocol
Wireless
Application Protocol (WAP) is the worldwide standard for providing
Internet communications and advanced services on digital mobile devices,
such as handheld phones, pagers, and other wireless devices. This
protocol is an open, global specification that enables users of these
digital devices to securely access and interact with Internet, intranet,
and extranet applications and services.
Many phones and PDAs currently have multimedia capabilities that include:
- Retrieving e-mail
- Accessing data from company databases
- Executing stock trading
- Paying bills
- Making travel reservations
- Online requests for maps and driving directions
- Running other exciting applications
The
WAP Forum was founded in June 1997, by Ericsson, Motorola, Nokia, and
Phone.com. It has drafted a global wireless specification for all
wireless networks. The forum contributed to various industry groups,
such as wireless service providers, handset manufacturers,
infrastructure providers, and software developers.
WAP version 2 brings together open Internet standards and mobile
networking as it adds optimized support for TCP and HTTP, Extended
Hypertext Markup Language (XHTML), and Transport Layer Security.
The WAP environment
wireless devices lack the traditional desktop GUI like the Web browser. Other fundamental
limitations for handheld, wireless devices include:
- Less powerful CPUs
- Less memory
- Restricted power consumption
- Smaller displays
- Different input devices (for example, phone keypad and voice input)
Wireless
data networks present many communications challenges compared to wired
networks. For example, the wireless communication environment is
constrained because wireless links have:
- Less bandwidth
- More latency
- Less connection stability
- Less predictable availability
As a result, protocols that provide wireless applicability must be tolerant of these types of problems
Key elements of the WAP specifications
WAP defines an open standard architecture and a set of protocols to implement wireless Internet access.
- Definition of the WAP programming model
- The Extended Hypertext Markup Language Mobile Profile (XHTMLMP).
- The Wireless Markup Language (WML).
- A lightweight protocol stack
- A framework for wireless telephony
WAP Architecture
The WAP architecture illustrated in Figure
- The WAP client (the handheld device or WAP terminal)
- The application server
This
programming model is the Web programming model with some extensions and
enhancements to match the characteristics of the wireless clients. This
programming model adds two enhancements to the Web programming model:
- Push
- Wireless Telephony Support (WTA)
Feature and performance-enhancing proxies
The WAP model recommends the use of proxies to optimize the connection between the wireless clients and the Web
A variety of functions are then provided by the WAP proxy, including:
Protocol gateway
Translates
requests from a wireless protocol stack (for example, the WAP 1.x
stack—WSP, WTP, WTLS, and WDP) to the WWW protocols (HTTP and TCP/IP).
Also can perform DNS lookups for the URLs requested by the client.
Content encoders and decoders
Translates WAP content for better utilization of the underlying link, reducing the bandwidth use by different compression techniques.
User Agent Profile Manager
Used mainly to communicate the client’s device capabilities and personal preferences to the server
applications.
Caching proxy
Caching frequently accessed resources, a caching proxy can improve network utilization
Overall Architecture
Wireless Application Protocol
Here's what happens when you access a Web site using a WAP-enabled device:- You turn on the device and open the minibrowser.
- The device sends out a radio signal, searching for service.
- A connection is made with your service provider.
- You select a Web site that you wish to view.
- A request is sent to a gateway server using WAP.
- The gateway server retrieves the information via HTTP from the Web site.
- The gateway server encodes the HTTP data as WML.
- The WML-encoded data is sent to your device.
- You see the wireless Internet version of the Web page you selected.
What happens between the gateway and the client relies on features of different parts of the WAP protocol stack. Let's take a look at each part of the stack:
- WAE - The Wireless Application Environment holds the tools that wireless Internet content developers use. These include WML and WMLScript, which is a scripting language used in conjunction with WML. It functions much like Javascript.
- WSP - The Wireless Session Protocol determines whether a session between the device and the network will be connection-oriented or connectionless.
What this is basically talking about is whether or not the device needs
to talk back and forth with the network during a session. In a
connection-oriented session, data is passed both ways between the device
and the network; WSP then sends the packet to the Wireless Transaction
Protocol layer (see below). If the session is connectionless, commonly
used when information is being broadcast or streamed from the network to the device, then WSP redirects the packet to the Wireless
Datagram Protocol layer.
- WTP - The Wireless Transaction Protocol acts like a traffic cop, keeping the data flowing in a logical and smooth manner. It also determines how to classify each transaction request: Reliable two-way Reliable one-way Unreliable one-way The WSP and WTP layers correspond to Hypertext Transfer Protocol (HTTP) in the TCP/IP protocol suite.
- WTLS - Wireless Transport Layer Security provides many of the same security features found in the Transport Layer Security (TLS) part of TCP/IP. It checks data integrity, provides encryption and performs client and server authentication.
- WDP - The Wireless Datagram Protocol works in conjunction with the network carrier layer (see below). WDP makes it easy to adapt WAP to a variety of bearers because all that needs to change is the information maintained at this level.
- Network carriers - Also called bearers, these can be any of the existing technologies that wireless providers use, as long as information is provided at the WDP level to interface WAP with the bearer.
Once the information is received by the WAP client, it is passed to the minibrowser.
This is a tiny application built into the wireless device that provides
the interface between the user and the wireless Internet.
Wireless(WAP) Application Development
Wireless Markup Language
WAP uses Wireless Markup Language (WML), which includes the Handheld Device Markup Language (HDML) developed by Phone.com.WML can also trace its roots to eXtensible Markup Language (XML). A markup language is a way of adding information to your content that tells the device receiving the content what to do with it. The best known markup language is Hypertext Markup Language (HTML). Unlike HTML, WML is considered a meta language. Basically, this means that in addition to providing predefined tags, WML lets you design your own markup language components. WAP also allows the use of standard Internet protocols such as UDP, IP and XML.
There are three main reasons why wireless Internet needs the Wireless Application Protocol:
- Transfer speed
- Size and readability
- Navigation
The relatively small size of the LCD on a cell phone or PDA presents another challenge. Most Web pages are designed for a resolution of 640x480 pixels, which is fine if you are reading on a desktop or a laptop. The page simply does not fit on a wireless device's display, which might be 150x150 pixels. Also, the majority of wireless devices use monochrome screens. Pages are harder to read when font and background colors become similar shades of gray.
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