WiMax the Ultimate Broadband Solution
BASIC CONCEPT ON COMPUTER NETWORK
1.1 Network History :
Each of the past three centuries has been dominated by a single technology. The 18th centuries was the era of the great mechanical system accompanying the Industrial Revolution. The 19th century was the age of the steam engine. During the 20th century, the key information gathering, processing and distribution. Among other developments, we saw the installation of worldwide telephone networks, the invention of radio and television, the birth and unprecedented growth of the computer industry, and the launching of communication satellites.
The Internet is the largest data network on earth. The Internet consists of many large and small networks that are interconnected. Individual computers are the sources and destinations of information through the Internet. Connection to the Internet can be broken down into the physical connection, the logical connection, and applications.
Basically computer network can be separated into 2 categories based on there connectivity.
a) Wired Network.
b) Wireless Network
To discuss about the term Network a question comes at first that is,
1.1.1 “What is Network?”
A network is a set of devices connected by communication links. A mode can be a computer, printer or any other device capable of sending and receiving data generated by other nodes on the network.
Networking is the practice of linking computing devices together with hardware and software that supports data communications across these devices. The simplest kind of home network contains exactly two computers. You can use this kind of network to share files, a printer or another peripheral device, and even an Internet connection. To connect two computers for sharing network resources, consider these alternatives.
1.1.2 Distributed Processing:
Most networks use distributed processing, in which a task is divided among multiple computers. Instead of a single large being responsible for all aspects of process, separate computers handle a subset.
1.1.3 Network Criteria:
A network must be able to meet a certain number of criteria. The most important of these are performance, reliability and security.
1.1.4 Performance :
Performance can be measured in many ways, including transit time and response time. Transit time is the amount of time required for a message to travel from one device to another. Response time is the elapsed time between an inquiry and a response. The performance of a network depends on a number of factors, including the number of users, the type of transmission medium, the capabilities of the connected hardware, and the efficiency of the software.
I addition to accuracy delivery, network reliability is measured by the frequency of failure, the time it takes a link to recover from a failure, and the network’s robustness in a catastrophe.
Network security issues include protecting data from unauthorized access.
1.2 Networking Categories :
Now a days we r generally referring to four primary categories of networks. These four categories networks differ from their size, their ownership, the distance they cover, and their physical architecture.
Fig- 1.1 : Network Categories
1.3 Local Area Network(LAN) :
A local area network is privately owned and links the devices in a single office or campus. Depending on the needs of an organization and type of technology used, a LAN can be as simple as two PCs and a printer in someone’s home office or it can extend throughout a company and include audio and video peripherals.
LANs are designed to allow resources to be shared between personal computers or workstation. The resources to be shared can include hardware, software or data. A common example of a LAN, found in many business environments, links a workgroup of task-related computers for example engineering workstations or accounting PCs. One of the computers may be given a large capacity disk drive and may become a server to the other clients. Software can be stored on this central server and used as needed by licensing by the whole group.
Fig-1.2 : LAN connection among the computer accessories
1.4 Metropolitan-Area Network (MAN):
A metropolitan area network is designed to extend over an entire city. It may be a single network such a cable television network, or it may be a means of connecting a number of LANs into a larger network so that resources may be shared LAN-to-LAN as well as device-to-device. For example, a company can use a MAN to connect the LANs in all its offices throughout a city.
1.5 Wide Area Network (WAN):
A wide area network provides long distance transmission of data, voice, image, and video information over large geographic areas that may be a country, a continent or even the whole world. WANs interconnect LANs, which then provide access to computers or file servers in other locations.A WAN that is wholly owned used by single company is often referred to as an enterprise network.
Fig-1.3 : WAN connection among the computer accessories
WANs are designed to do the following:
- Operate over a large and geographically separated area
- Allow users to have real-time communication capabilities with other users
- Provide full-time remote resources connected to local services
- Provide e-mail, Internet, file transfer, and e-commerce services
1.6 WIRELESS LAN
A wireless LAN is one in which a mobile user can connect to a local area network (LAN) through a wireless (radio) connection The IEEE 802.11 group of standards specify the technologies for wireless LANs.
Fig 1.4 : Wireless LAN
1.7 Wireless pan
A WPAN (wireless personal area network) is a personal area network – a network for interconnecting devices centered around an individual person’s workspace – in which the connections are wireless. Typically, a wireless personal area network uses some technology that permits communication within about 10 meters – in other words, a very short range. One such technology is Bluetooth.
1.8 Wireless man
A data network intended to serve an area the size of a large city. Such networks are being implemented by innovative techniques, such as running optical fibre through subway tunnels.
Fig 1.5 : Wireless LAN
1.9 Authentication,Authorization, Accounting (AAA) Technologies and Protocols
Authentication, Authorization and Accounting (AAA) is a framework for intelligently controlling access to computer network resources, enforcing policies, auditing usage, and providing the information necessary to bill for services. These combined processes are considered important for effective network management and security. The AAA is sometimes combined with auditing and accordingly becomes AAAA.
- Authentication refers to the process of validating the claimed identity of an end user or a device, such as a host, server, switch, router, and so on. Authentication is accomplished via the presentation of an identity and credentials. Examples of types of credentials are passwords, one-time tokens, digital certificates, and phone numbers (calling/called).
- Authorization refers to the act of granting access rights to a user, groups of users, system, or a process, based on their authentication, what services they are requesting, and the current system state. Authorization may be based on restrictions, for example time-of-day restrictions, or physical location restrictions, or restrictions against multiple logins by the same user. Authorization determines the nature of the service which is granted to a user. Examples of types of service include, but are not limited to: IP address filtering, address assignment, route assignment, QoS/differential services, bandwidth control/traffic management, compulsory tunneling to a specific endpoint, and encryption.
- Accounting refers to the methods to establish who, or what, performed a certain action, such as tracking user connection and logging system users. This information may be used for management, planning, billing, or other purposes. Real-time accounting refers to accounting information that is delivered concurrently with the consumption of the resources. Batch accounting refers to accounting information that is saved until it is delivered at a later time. Typical information that is gathered in accounting is the identity of the user, the nature of the service delivered, when the service began, and when it ended.
- Auditing refers to an evaluation of an organization, system, process, project or product. Audits are performed to ascertain the validity and reliability of information, and also provide an assessment of a system’s internal control.
There are many technologies and protocols defined to achieve the goals defined in the AAA (or AAAA) framework. Some of the AAA Technologies and Protocols are listed below:
- CHAP: Challenge Handshake Authentication Protocol
- DIAMETER Protocol: This protocol is designed to replace the RADIUS.
- EAP: Extensible Authentication Protocol
- MS-CHAP (MD4)
- PAP: Password Authentication Protocol
- PEAP: Protected Extensible Authentication Protocol
1.10 Wireless Roaming
Microsoft Research has developed a wireless roaming service architecture that enables personalized, seamless, and secure connectivity for mobile customers when moving across different types of wireless networks, such as cellular or wireless local-area networks (WLAN). Wireless Roaming maintains connectivity by using connection/session management when customers roam across different networks.
Wireless Roaming is compatible with MIPv6. The technology provides intelligent personalized service by supporting device and service mobility. Device mobility means that Wireless Roaming provides seamless mobility support for portable devices. For example a customer initiates a VoIP call from a SmartPhone in an office building with Wi-Fi. Leaving the building, the customer will maintain connectivity by automatically switching to GPRS or CDMA. Wireless Roaming works in a manner that is seamless and transparent to the customer. The voice conversation continues uninterrupted. Service mobility means that services like the above VoIP call survive hand-offs between different networks. Another example for improved service mobility is a customer streaming a video on a cell phone automatically experiencing improved quality when moving from a GPRS network to WLAN.
Value-added services such as video entertainment, instant messaging or internet browsing are enhanced by enabling dynamic federation/collaboration among different types of networks and devices. Wireless Roaming guarantees a trustworthy environment following the IPSec standards for secure communication and encryption.
OSI Reference Model
2.1. What is OSI Reference Model ?
The International Organization for Standardization (ISO) began developing the Open Systems Interconnection (OSI) reference model in 1977. It was created to standardize the rules of networking in order for all systems to be able to communicate. In order for communication to occur on a networking using different device drivers and protocol stacks, the rules for communication must be explicitly defined. The OSI model deals with the following issues;
- How a device on a network sends it’s data, and how it knows when are where to send it
- How a device on a network receives it’s data, and how to know where to look for it.
- How devices using different languages communicate with each other.
- How devices on a network are physically connected to each other.
- How protocols work with devices on a network to arrange data.
The OSI model is broken down into 7 layers. Although the first layer is #1, it is always shown at the bottom of the model. We’ll explain why later. Here are the seven layers.
1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer
2.1. Protocol Stacks
In order for each layer of the model to communicate with the levels above and below it, certain rules were developed. These rules are called Protocols, and each protocol provides a specific layer of the model with a specific set of tasks or services. Each layer of the model has it’s own set of protocols associated with it. When you have a set of protocols that create a complete OSI model, it is called a Protocol Stack. An example of a protocol stack is TCP/IP, the standard for communication over the internet, or AppleTalk for Macintosh computers.
As stated before, protocols define how layers communicate with each other. Protocols specifically work with ONLY the layer above and below them. They receive services from the protocol below, and provide services for the protocol above them. This order maintains a standard that is common to ALL forms of networking.
In order for two devices on a network to communicate, they must both be using the same protocol stack. Each protocol in a stack on one device must communicate with it’s equivalent stack, or peer, on the other device. This allows computers running different operating systems to communicate with each other easily, such as having Macintosh computers on a Windows NT network.
2.2 Communications Between Stacks
When a message is sent from one machine to another, it travels down the protocol stack or layers of the model, and then up the layers of the stack on the other machine. As the data travels down the stack, it picks up headers from each layer (Except the physical layer). Headers contain information that is read by the peer layer on the stack of the other computer. As the data travels up the levels of the peer computer, each header is removed by it’s equivalent protocol. These headers contain different information depending on the layer they receive the header from, but tell the peer layer important information, including packet size, frames, and datagrams. Each layer’s header and data are called data packages, or service data units. Although it may seem confusing, each layer has a different name for it’s service data unit. Here are the common names for service data units at each level of the OSI model
|Application||Messages and Packets|
|Transport||Datagrams, Segments, and Packets|
|Network||Datagrams and Packets|
|Data Link||Frames and Packets|
|Physical||Bits and Packets|
3. The Physical Layer
The lowest layer on the OSI model, and probably the easiest to understand is the physical layer. This layer deals with the physical, electrical, and cable issues involved with making a network connection. It associates with any part of the network structure that doesn’t process information in any way.
The physical layer is responsible for sending the bits across the network media. It does not define what a bit is or how it is used merely how it’s sent. The physical layer is responsible for transmitting and receiving the data. It defines pin assignments for serial connections, determines data synchronization, and defines the entire network’s timing base.
Items defined by the physical layer include hubs, simple active hubs, terminators, couplers, cables and cabling, connectors, repeaters, multiplexers, transmitters, receivers, and transceivers. Any item that does not process information but is required for the sending and receiving of data is defined by this layer.
Figure 1.2 : Task of physical layer is transmitting stream of bytes through the transmission medium.
There are several items addresses by this layer. They are;
· Network connections types, including multi-point and point-to-point networks.
· Network Topologies, including ring, star, bus, and mesh networks.
· Analog or Digital signaling.
· Bit Synchronization (When to send data and when to listen for it).
· Baseband Vs. Broadband transmissions.
· Multiplexing (Combining multiple streams of data into one channel).
· Termination, to give better signal clarity and for node segmentation.
4. The Data Link Layer
The Data Link Layer is responsible for the flow of data over the network from one device to another. It accepts data from the Network Layer, packages that data into frames, and sends them to the Physical Layer for distribution. In the same way, it receives frames from the physical layer of a receiving computer, and changes them into packets before sending them to the Network Layer.
The Data link Layer is also involved in error detection and avoidance using a Cyclic Redundancy Check (CRC) added to the frame that the receiving computer analyses. This second also checks for lost frames and sends requests for re-transmissions of frames that are missing or corrupted at this level.
The most important aspect of the Data Link Layer is in Broadcast networks, where this layer establishes which computer on a network receives the information and which computers relay or ignore the information. It does so by using a Media Access Control (MAC) address, which uniquely identifies each Network Interface Card (NIC).
Bridges, Intelligent Hubs, And NICs are all associated with the Data Link Layer.
The Data Link Layer is sub-divided into two layers. This is done because of the two distinct functions that each sub-division provides.
Logical Link Control – Generates and maintains links between network devices
Media Access Control – Defines how multiple devices share a media channel
The Logical Link Control provides Service Access Points (Saps) for other computers to make reference to when transporting data the to upper layers of the OSI Model.
Media Access Control gives every NIC a unique 12 digit hexadecimal address. These addresses are used by the Logical Link Control to set up connections between NICs. Every MAC address must be unique or they will cause identity crashes on the network. The MAC address is normally set at the factory, and conflicts are rare. But in the case of a conflict, the MAC address is user set-able.
5. The Network Layer
The third layer of the OSI model is the Network layer. This layer is responsible for making routing decisions and forwards packets that are farther then one link away. By making the network layer responsible for this function, every other layer of the OSI model can send packets without dealing with where exactly the system happens to be on the network, whether it be 1 hop or 10 hops away.
In order to provide it’s services to the data link layer, it must convert the logical network address into physical machine addresses, and vice versa on the receiving computer. This is done so that no relaying, routing, or networking information must be processed by a level higher in the model then this level. Essentially, any function that doesn’t provide an environment for executing user programs falls under this layer or lower.
Because of this restriction, all systems that have packets routed through their systems must provide the bottom three layers’ services to all packets traveling through their systems. Thus, any routed packet must travel up the first three layers and then down those same three layers before being sent farther down the network. Routers and gateways are the principal users of this layer, and must fully comply with the network layer in order to complete routing duties.
The network layer is also responsible for determining routing and message priority. By having this single layer responsible for prioritization, the other layers of the OSI model remain separated from routing decisions.
This layer is also responsible for breaking large packets into smaller chucks when the original packet is bigger then the Data Link is set. Similarly, it re-assembles the packet on the receiving computer into the original-sized packet. There are several items addresses by this layer. They are;
· Addressing for logical network and service addresses.
· Circuit message and packet switching
· Route discovery and selection
· Connection services, including layer flow control and packet sequence control.
· Gateway Services
6. Transport Layer
The transport layer’s main duty is to unsure that packets are send error-free to the receiving computer in proper sequence with no loss of data or duplication. This is accomplished by the protocol stack sending acknowledgements of data being send and received, and proper checksum/parity/synchronization of data being maintained.
The transport layer is also responsible for breaking large messages into smaller packets for the network layer, and for re-assembling the packets when they are received from the network layer for processing by the session layer.
7. Session Layer
The session layer is the section of the OSI model that performs the setup functions to create the communication sessions between computers. It is responsible for much of the security and name look-up features of the protocol stack, and maintains the communications between the sending and receiving computers through the entire transfer process. Using the services provided by the transport layer, the session layer ensures only lost or damaged data packets are re-sent, using methods referred to as data synchronization and checkpointing. This ensures that excess traffic is not created on the network in the event of a failure in the communications.
The session layer also determines who can send data and who can receive data at every point in the communication. Without the dialogue between the two session layers, neither computer would know when to start sending data and when to look for it in the network traffic.
8. The Presentation and Application Layers
The presentation layer is responsible for protocol conversation, data translation, compression, encryption, character set conversion, and graphical command interpretation between the computer and the network.
The main working units in the presentation are the network redirectors, which make server files visible on client computers. The Network redirector is also responsible for making remote printers appear as if they were local.
The application layer provides services that support user applications, such as database access, e-mail services, and file transfers. The application layer also allows Remote Access Servers to work, so that applications appear local on remotely hosted servers.
Wireless Broadband Access
3.1 Defining Wireless Broadband:
The term wireless broadband generally refers to high-speed hundred kilobits data transmission occurring within an infrastructure of more or less fixed points, including both stationary subscriber terminals and service provider base station. This is distinct from mobile data transmission where the subscriber can expect to access the network while in transmit and where only the network operator’s base stations occupy fixed locations. We can expect that this distinction will become somewhat blurred in the future inasmuch as several manufacturers are developing very high-speed wireless networking equipment that will support mobility or stationary usage almost equally emphasis of high-speed wireless service providers serving stationary subscribers will remain. Broadband wireless, as it’s today, is properly a competitor to optical fiber, hybrid fiber coax, DSL, and to a much lesser extent, broadband satellite.
Third-generation (3G) and 2.5G cellular telephone networks, which have special provision fro delivering medium-speed packet data services, are not, in most instance, directly competitive with broadband wireless services. They share a radio frequency air link and, in some cases, core technologies, but they serve a different type of customer and present different type of customer and present different types of services offerings.
3.2 Introducing IEEE 802.16 Standards
The IEEE 802.16 Working Group on Broadband Wireless Access Standards, which was established by IEEE Standards Board in 1999, aims to prepare formal specifications for the global deployment of broadband Wireless Metropolitan Area Networks. The Workgroup is a unit of the IEEE 802 LAN/MAN Standards Committee. A related future technology Mobile Broadband Wireless Access (MBWA) is under development in IEEE 802.20.
Forum is to promote and certify compatibility and interoperability of broadband wireless products.
3.2.1 802.16 Standards:
The first 802.16 standard was approved in December 2001. It delivered a standard for point to multipoint Broadband Wireless transmission in the 10-66 GHz band, with only a line-of-sight (LOS) capability. It uses a single carrier (SC) physical (PHY) standard.
802.16a was an amendment to 802.16 and delivered a point to multipoint capability in the 2-11 GHz band. For this to be of use, it also required a non-line-of-sight (NLOS) capability, and the PHY standard was therefore extended to include Orthogonal Frequency Division Multiplex (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA). 802.16a was ratified in January 2003 and was intended to provide “last mile” fixed broadband access.
802.16c, a further amendment to 802.16, delivered a system profile for the 10-66 GHz 802.16 standard.
In September 2003, a revision project called 802.16d commenced aiming to align the standard with aspects of the European Telecommunications Standards Institute (ETSI) HIPERMAN standard as well as lay down conformance and test specifications. This project concluded in 2004 with the release of 802.16-2004 which superseded the earlier 802.16 documents, including the a/b/c amendments.
An amendment to 802.16-2004, IEEE 802.16e-2005 (formerly known as IEEE 802.16e), addressing mobility, was concluded in 2005. This implemented a number of enhancements to 802.16-2004, including better support for Quality of Service and the use of Scalable OFDMA, and is sometimes called “Mobile WiMAX”, after the WIMAX forum for interoperability.
Amendments in progress
802.16e-2005 — Mobile 802.16
802.16f-2005 — Management Information Base
802.16g-2007 — Management Plane Procedures and Services
802.16k-2007 — Bridging of 802.16 (an amendment to 802.1D)
Amendments under development:
802.16h — Improved Coexistence Mechanisms for License-Exempt Operation
802.16i — Mobile Management Information Base
802.16j — Multihop Relay Specification
802.16Rev2 — Consolidate 802.16-2004, 802.16e, 802.16f, 802.16g and possibly 802.16i into a new document.
Amendments at pre-draft stage:
802.16m — Advanced Air Interface. Data rates of 100 Mbit/s for mobile applications and 1 Gbit/s for fixed applications, cellular, macro and micro cell coverage, with currently no restrictions on the RF bandwidth (which is expected to be 20 MHz or higher).The proposed work plan would allow completion of the standard by December 2009 for approval by March 2010
3.2.2 802.16e-2005 Technology
The 802.16 standard essentially standardizes 2 aspects of the air interface – the physical layer (PHY) and the Media Access Control layer (MAC). This section provides an overview of the technology employed in these 2 layers in the current version of the 802.16 specification (which is strictly 802.16-2004 as amended by 802.16e-2005, but which will be referred to as 802.16e for brevity).
802.16e uses Scalable OFDMA to carry data, supporting channel bandwidths of between 1.25 MHz and 20 MHz, with up to 2048 sub-carriers. It supports adaptive modulation and coding, so that in conditions of good signal, a highly efficient 64 QAM coding scheme is used, whereas where the signal is poorer, a more robust BPSK coding mechanism is used. In intermediate conditions, 16 QAM and QPSK can also be employed. Other PHY features include support for Multiple-in Multiple-out (MIMO) antennas in order to provide good NLOS (Non-line-of-sight) characteristics (or higher bandwidth) and Hybrid automatic repeat request (HARQ) for good error correction performance.
The 802.16 MAC describes a number of Convergence Sublayers which describe how wireline technologies such as Ethernet, ATM and IP are encapsulated on the air interface, and how data is classified, etc. It also describes how secure communications are delivered, by using secure key exchange during authentication, and encryption using AES or DES (as the encryption mechanism) during data transfer. Further features of the MAC layer include power saving mechanisms (using Sleep Mode and Idle Mode) and handover mechanisms.
A key feature of 802.16 is that it is a connection oriented technology. The subscriber station (SS) cannot transmit data until it has been allocated a channel by the Base Station (BS). This allows 802.16e to provide strong support for Quality of Service (QoS).
IEEE 802.16e-2005 improves upon IEEE 802.16-2004 by:
1. Adding support for mobility (soft and hard handover between base stations). This is seen as one of the most important aspects of 802.16e-2005, and is the very basis of ‘Mobile WiMAX’.
2. Scaling of the Fast Fourier Transform (FFT) to the channel bandwidth in order to keep the carrier spacing constant across different channel bandwidths (typically 1.25 MHz, 5 MHz, 10 MHz or 20 MHz). Constant carrier spacing results in a higher spectrum efficiency in wide channels, and a cost reduction in narrow channels. Also known as Scalable OFDMA (SOFDMA). Other bands not multiples of 1.25 MHz are defined in the standard, but because the allowed FFT subcarrier numbers are only 128, 512, 1024 and 2048, other frequency bands will not have exactly the same carrier spacing, which might not be optimal for implementations.
3. Improving NLOS coverage by utilizing advanced antenna diversity schemes, and hybrid-Automatic Retransmission Request (HARQ)
4. Improving capacity and coverage by introducing Adaptive Antenna Systems (AAS) and Multiple Input Multiple Output (MIMO) technology
5. Increasing system gain by use of denser sub-channelization, thereby improving indoor penetration
6. Introducing high-performance coding techniques such as Turbo Coding and Low-Density Parity Check (LDPC), enhancing security and NLOS performance
7. Introducing downlink sub-channelization, allowing administrators to trade coverage for capacity or vice versa
8. Enhanced Fast Fourier Transform algorithm can tolerate larger delay spreads, increasing resistance to multipath interference
9. Adding an extra QoS class (enhanced real-time Polling Service) more appropriate for VoIP applications.
Because the IEEE only sets specifications but does not test equipment for compliance with them, the WiMAX Forum runs a certification program wherein members pay for certification. WiMAX certification by this group is intended to guarantee compliance with the standard and interoperability with equipment from other manufacturers. The mission of the Forum is to promote and certify compatibility and interoperability of broadband wireless products.
QoS in 802.16e is supported by allocating each connection between the SS and the BS (called a service flow in 802.16 terminology) to a specific QoS class. In 802.16e, there are 5 QoS classes:
|802.16e-2005 QoS classes|
|Unsolicited Grant Service||UGS||Real-time data streams comprising fixed-size data packets issued at periodic intervals||T1/E1 transport|
|Extended Real-time Polling Service||ertPS||Real-time service flows that generate variable-sized data packets on a periodic basis||VoIP|
|Real-time Polling Service||rtPS||Real-time data streams comprising variable-sized data packets that are issued at periodic intervals||MPEG Video|
|Non-real-time Polling Service||nrtPS||Delay-tolerant data streams comprising variable-sized data packets for which minimum data rate is required||FTP with guaranteed minimum throughput|
|Best Effort||BE||Data streams for which no minimum service level is required and therefore may be handled on a space-available basis||HTTP|
The BS and the SS use a service flow with an appropriate QoS class (plus other parameters, such as bandwidth and delay) to ensure that application data receives QoS treatment appropriate to the application.
3.3 Introducing Wireless Broadband Technologies
In this section we need to discuss with the several of wireless network technologies provided all over world. This discussed technologies are basically based on the wireless network unit. So we are going discuss about it in below one by one.
3.3.1 CDMA(Code Division Multiple Access)
Code division multiple access (CDMA) is a channel access method utilized by various radio communication technologies.
One of the basic concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a bandwidth of frequencies. This concept is called multiplexing.
CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel.
By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of “spread-spectrum” signaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.
An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different directions (spatial division). In CDMA, they would speak different languages. People speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.
4. CDMA has been used in the OmniTRACS satellite system for transportation logistics
CDMA is a spread spectrum multiple access technique. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is simply logically XOR (exclusive OR) added with the faster code. The figure shows how spread spectrum signal is generated. The data signal with pulse duration of Tb is XOR added with the code signal with pulse duration of Tc. (Note: bandwidth is proportional to 1/T where T = bit time) Therefore, the bandwidth of the data signal is 1/Tb and the bandwidth of the spread spectrum signal is 1/Tc. Since Tc is much smaller than Tb, the bandwidth of the spread spectrum signal is much larger than the bandwidth of the original signal.
Fig 3.1: CDMA signal Processing
Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. The best performance will occur when there is good separation between the signal of a desired user and the signals of other users. The separation of the signals is made by correlating the received signal with the locally generated code of the desired user. If the signal matches the desired user’s code then the correlation function will be high and the system can extract that signal. If the desired user’s code has nothing in common with the signal the correlation should be as close to zero as possible (thus eliminating the signal); this is referred to as cross correlation. If the code is correlated with the signal at any time offset other than zero, the correlation should be as close to zero as possible. This is referred to as auto-correlation and is used to reject multi-path interference.
3.3.2 GSM (Global System for Mobile)
Global System for Mobile communications (GSM) is the most popular standard for mobile phones in the world. Its promoter, the GSM Association, estimates that 82% of the global mobile market uses the standard.GSM is used by over 3 billion people across more than 212 countries and territories. Its ubiquity makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signalling and speech channels are digital, and thus is considered a second generation (2G) mobile phone system. This has also meant that data communication was easy to build into the system.
The ubiquity of the GSM standard has been an advantage to both consumers and also to network operators.GSM also pioneered a low-cost, to the network carrier, alternative to voice calls, the Short message service, which is now supported on other mobile standards as well. Another advantage is that the standard includes one worldwide Emergency telephone number, 112. This makes it easier for international travellers to connect to emergency services without knowing the local emergency number.
GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using a pre-shared key and challenge-response. Communications between the subscriber and the base station can be encrypted. The development of UMTS introduces an optional USIM, that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user – whereas GSM only authenticated the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation. GSM uses several cryptographic algorithms for security. The A5/1 and A5/2 stream ciphers are used for ensuring over-the-air voice privacy. A5/1 was developed first and is a stronger algorithm used within Europe and the United States; A5/2 is weaker and used in other countries. Serious weaknesses have been found in both algorithms: it is possible to break A5/2 in real-time with a ciphertext-only attack, and in February 2008, Pico Computing, Inc revealed its ability and plans to commercialize FPGAs that allow A5/1 to be broken with a rainbow table attack . The system supports multiple algorithms so operators
Subscriber Identity Module
One of the key features of GSM is the Subscriber Identity Module (SIM), commonly known as a SIM card. The SIM is a detachable smart card containing the user’s subscription information and phonebook. This allows the user to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking, and is illegal in some countries.
GSM Network structure
The network behind the GSM system seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.
Fig 3.2: The structure of a GSM network
3.3.3 GPRS (General Packet Radio Service)
General Packet Radio Service (GPRS) is a packet oriented Mobile Data Service available to users of Global System for Mobile Communications (GSM) and IS-136 mobile phones. It provides data rates from 56 up to 114 kbit/s.
GPRS can be used for services such as Wireless Application Protocol (WAP) access, Short Message Service (SMS), Multimedia Messaging Service (MMS), and for Internet communication services such as email and World Wide Web access. GPRS data transfer is typically charged per megabyte of traffic transferred, while data communication via traditional circuit switching is billed per minute of connection time, independent of whether the user actually is using the capacity or is in an idle state. GPRS is a best-effort packet switched service, as opposed to circuit switching, where a certain Quality of Service (QoS) is guaranteed during the connection for non-mobile users.
Services and hardware
GPRS upgrades GSM data services providing:
- Multimedia Messaging Service (MMS)
- Push to talk over Cellular PoC / PTT
- Instant Messaging and Presence — Wireless Village
- Internet Applications for Smart Devices through Wireless Application Protocol (WAP)
- Point-to-point (PTP) service: internetworking with the Internet (IP protocols)
- Short Message Service (SMS)
- Future enhancements: flexible to add new functions, such as more capacity, more users, new accesses, new protocols, new radio networks.
USB GPRS modem
USB GPRS modems use a terminal-like interface USB 2.0 and later, data formats V.42bis, and RFC 1144 and external antennas. Modems can be add in cards (for laptop) or external USB devices which are similar in shape and size to a computer mouse.GPRS can be used as the bearer of SMS. If SMS over GPRS is used, an SMS transmission speed of about 30 SMS messages per minute may be achieved. This is much faster than using the ordinary SMS over GSM, whose SMS transmission speed is about 6 to 10 SMS messages per minute
3.3.4 EDGE (Enhanced Data Rates for GSM Evolution) & EGPRS(Enhanced GPRS)
Enhanced Data rates for GSM Evolution (EDGE), Enhanced GPRS (EGPRS), is a digital mobile phone technology that allows increased data transmission rates and improved data transmission reliability. EDGE is generally classified as 2.75G, although it is part of ITU‘s 3G definition.EDGE has been introduced into GSM networks around the world since 2003, initially by Cingular (now AT&T) in the United States.
EDGE can be used for any packet switched application, such as an Internet connection. High-speed data applications such as video services and other multimedia benefit from EGPRS’ increased data capacity. EDGE Circuit Switched is a possible future development.
In addition to Gaussian minimum-shift keying (GMSK), EDGE uses <href=”#Higher-order_PSK.2F8_phase_shift_keying” title=”Phase shift keying”>higher-order PSK/8 phase shift keying (8PSK) for the upper five of its nine modulation and coding schemes. EDGE produces a 3-bit word for every change in carrier phase. This effectively triples the gross data rate offered by GSM. EDGE, like GPRS, uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. It introduces a new technology not found in GPRS, Incremental Redundancy, which, instead of retransmitting disturbed packets, sends more redundancy information to be combined in the receiver. This increases the probability of correct decoding.