Mobile IPv6 Types of Nodes

The Mobile IPv6 specifi cation defi nes three types of nodes. The fi rst type is the mobile node , which has the capability of moving around IPv6 networks without breaking existing connections while moving. A mobile node is assigned a permanent IPv6 address called a home address . A home address is an address assigned to the mobile node when it is attached to the home network and through which the mobile node is always reachable, regardless of its location on an IPv6 network. Because the mobile node is always assigned the home address, it is always logically connected to the home link. When a mobile node leaves its home network and attaches to another network, the node will get another address called a care-of address , which is assigned from the newly attached network. This network, which is not a home network, is called a foreign network or a visited network . A mobile node does not use a care-of address as an endpoint address when communicating with other nodes, since the address may change when the mobile node changes its point of attachment.

A second Mobile IPv6 node type is the home agent , which acts as a support node on the home network for Mobile IPv6 mobile nodes. A home agent is a router which has a proxy function for mobile nodes while they are away from home. The destination addresses of packets sent to mobile nodes are set to the home addresses of the mobile nodes. A home agent intercepts all packets which are addressed to the mobile node’s home address, and thus delivered to the home network on behalf of the mobile nodes.

This forwarding mechanism is the core feature provided by the Mobile IPv6 protocol. All IPv6 nodes which want to communicate with a mobile node can use the home address of the mobile node as a destination address, regardless of the current location of the mobile node. Those packets sent from an IPv6 node to the home address of a mobile node are delivered to the home network by the Internet routing mechanism where the home agent of the mobile node receives the packets and forwards the packets appropriately. For the reverse direction, a mobile node uses its home address as a source address when sending packets. However, a mobile cannot directly send packet nodes whose source address is a home address from its current location if it is away from home, since source addresses are not topologically correct. Sending a packet whose source address is out of the range of the network address of the sender node is a common technique when an attacker tries to hide its location when he is attacking a specific node. Such a packet may be considered as an attack. Because of this reason, the first hop router may drop such topologically incorrect packets to avoid the risk of the source spoofing attack. To solve this problem, a mobile node uses the IPv6 in IPv6 encapsulation technology. All packets sent from a mobile node while away from home are
sent to its home agent using the encapsulation mechanism. The home agent decapsulates the packets and forwards them as if the packets were sent from the home network.

A third type of Mobile IPv6 node is called the correspondent node . A correspondent node is an IPv6 node that communicates with a mobile node. A correspondent node does not have to be Mobile IPv6-capable, other than supporting the IPv6 protocol; any IPv6 node can be a correspondent node. Since the Mobile IPv6 specifi cation provides a backward compatibility to all IPv6 nodes which do not support Mobile IPv6, all IPv6 nodes can communicate with mobile nodes without any modification. However, as we have described in the previous paragraph, all packets between a mobile node and a correspondent node must be forwarded basically by the home agent of the mobile node. This process is sometimes redundant, especially when a correspondent node and a mobile node are located on topologically near networks. To solve this redundancy, Mobile IPv6 provides an optimization mechanism called the route optimization mechanism which a correspondent node may support. A mobile node can send packets directly to a correspondent node using the care-of address of the mobile node as a source address. The information of the home address of a mobile node is carried by the newly defined option for the Destination Options Header. Also, a correspondent node can send packets directly to the care-of address of a mobile node. In this case, the information of the home address is carried by the Routing Header.

A correspondent node may itself be a mobile node. In this case, two moving nodes can communicate with each other without terminating their sessions regardless of their points of attachment to the Internet.

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Mobile IPv6 Overview

Mobile IPv6 adds the mobility function to IPv6. Mobile IPv6 is specified in. An IPv6 host which supports the Mobile IPv6 function can move around the IPv6 Internet. 1 The host which supports Mobile IPv6 can change its point of attachment to the IPv6 Internet whenever it wants. If a host does not support Mobile IPv6, all the existing connections on the host are terminated when it changes its point of attachment. A connection between two nodes is maintained by the pairing of the source address and the destination address. Since the IPv6 address of an IPv6 node is assigned based on the prefix of the network, the assigned address on a given network becomes invalid when the host leaves that network and attaches itself to another network. The reason for this problem came from the nature of IP addresses. An IP address has two meanings: one is the identifier of the node and the other is the location information of the node. It would not be a big problem as long as IP nodes do not move around the Internet frequently, because, in that case, the location information would not change frequently and we could use location information as the identifier of a node. However, recent progress of communication technologies and small computers made it possible for IP nodes to move around. It is getting harder and harder to treat location information as an identifier, because the location information frequently changes.

As such the basic idea of Mobile IPv6 is to provide a second IPv6 address to an IPv6 host as an identifier in addition to the address that is usually assigned to the node from the attached network as a locator. The second address is fixed to the home position of the host and never changes even if the host moves. The fixed address is called a “ home address. ” As long as the host uses its home address as its connection information, the connection between the host and other nodes will not be terminated when the mobile host moves.

The concept of a home address provides another useful feature to a host that supports Mobile IPv6. Any IPv6 nodes on the Internet can access a host which supports Mobile IPv6 by specifying its home address, regardless of the location of the host. Such a feature will make it possible to create a roaming server. Since the home address of the roaming server never changes, we can constantly reach the server at the home address. For example, anyone could run a web server application on a notebook computer which supports Mobile IPv6 and everyone could access it without any knowledge of where the computer is located.

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Mobile IPv6

Mobile IPv6 is a mobility support protocol for IPv6 at the network layer. The specification was standardized at the IETF in June 2004. The standardization process was quite slow compared to the basic IPv6 specification. The initial working group draft of Mobile IPv6 was submitted in 1996, which compares favorably with the first IPv6 draft specification which was proposed to the IPng working group in 1995. The reason for the delay in the standardization of Mobile IPv6 was the need to solve security issues associated with the protocol. Mobile IPv6 enables IPv6 nodes to send or receive packets whose source address does not match the network prefix to which they are currently attached. That is, nodes have to use a type of source spoofing technique. In the early version of the specification, the protocol required the use of IPsec to ensure that the source address was valid. However, when we consider the real situation — a mobile node may communicate with many other nodes for which it does not have any identification information — using IPsec is almost impossible.

The IESG rejected the proposal from the Mobile IPv6 working group to standardize the protocol specification at that time, and insisted the working group propose a procedure to securely validate the source address of a mobile node. The Mobile IPv6 working group started a discussion to solve the problem in 2000 and finally developed a loose address ownership mechanism called the return routability procedure in 2002. The specification was accepted by the IESG and published as in 2004.

The KAME project originally used the Mobile IPv6 stack that was contributed by Ericsson. The project started to implement its own Mobile IPv6 stack in 2001 during the middle of the second term of the KAME activity. KAME implemented several versions of Mobile IPv6 to follow and validate the latest specification. The code discussed in this chapter is based on the KAME snapshot released in July 2004. At that time, the specification had already been accepted as an RFC and the code was mature.

After KAME completed the first version of their Mobile IPv6 code, they started to redesign the architecture of the mobility stack. In the new architecture most of the signal processing tasks are moved to user space, compared to the first version of Mobile IPv6 where the code was implemented in the kernel. The design is similar to the BSD Routing Socket mechanism, which separates the routing information exchange and forwarding mechanisms, with exchanging routing information in the user space and forwarding in the kernel space. There are many benefits to this design. It makes it easier to develop complicated signal processing code since developers can utilize many advanced debugging programs and techniques, while the packet processing performance is not reduced, since it is done in the kernel. Extending or replacing some of the signal processing mechanisms is also easier, which makes it possible to add support for new mobility protocols or to adapt some part of the functions to user needs. Reducing the amount of kernel modification is important when we consider merging the developed code into the original BSDs.

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The Requirements of Mobile IP

Mobile IP allows a node to change its point of attachment to the Internet without needing to change its IP address. This is not simply a configuration simplification, but can facilitate continuous application-level connectivity as the node moves from point to point.

A possible solution to this problem would be to distribute routes through the network to declare the node’s new location and to update the routing tables so that packets can be correctly dispatched. This might, at first, seem attractive, but it is a solution that scales very poorly since it would be necessary to retain host-specifi c routes for each mobile host. As the number of mobile hosts in the Internet increases (and the growth of web access from mobile devices such as cell phones and palm-tops is very rapid), it would become impractical to maintain such tables in the core of the Internet.

The solution developed by the IETF involves protocol extensions whereby packets targeted at a mobile host are sent to its home network (as if the host were not mobile) and passed to a static (nonmobile) node called the node’s home agent . The mobile host registers its real location with the home agent, which is responsible for forwarding the packets to the host.

If the mobile host is at home (attached to its home network), forwarding is just plain old IP forwarding, but if the host is roving, packets must be tunneled across the Internet to a care-of address where the host has registered its attachment to a foreign agent . At the care-of address (the end of the tunnel) the packets are forwarded to the mobile host.

Note that this tunneling process is only required in one direction. Packets sent by the mobile host may be routed through the network using the standard IP procedures. It is worth observing that although mobile IP can be used to address any IP mobility issue, its use within wireless LANs and mobile phone networks might be better served by linklayer (i.e., sub-IP) procedures such as link-layer handoff. These processes are typically built into the link-layer mechanisms and involve less overhead than mobile IP. Such processes do, however, require that the mobile host remains logically connected within the IP subnet to which its address belongs — it becomes the responsibility of the link layer to maintain connections or virtual connections into that subnet.

An alternative to tunneling in mobile IP might be to use source routing within IP. IPv4 has been enhanced with optional extensions to support source routing. However, since the source routing extensions to IPv4 are a relatively new development and are in any case optional, many (or even most) deployed IPv4 nodes do not support them. This means that they are not a lot of use for developing mobile IP services over existing IPv4 networks. They may be of more use in new networks that are being constructed for the first time since the Service Providers can insist on these extensions from their equipment vendors.

IPv6 offers some alternatives to tunneling for mobile IP by using the routing extension header. In this way the mobile node can establish communications with its home agent and then use information learned to directly route packets to the destination, bypassing the home agent. Since this feature is built into IPv6 and so supported by all IPv6 implementations, it makes IPv6 a popular option for mobile IP deployments.

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Operating System: TinyOS

TinyOS aims at supporting sensor network applications on resource-constrained hardware platforms, such as the Berkeley motes.

To ensure that an application code has an extremely small footprint, TinyOS chooses to have no file system, supports only static memory allocation, implements a simple task model, and provides minimal device and networking abstractions. Furthermore, it takes a language-based
application development approach, to be discussed later, so that only the necessary parts of the operating system are compiled with the application. To a certain extent, each TinyOS application is built into the operating system.

Like many operating systems, TinyOS organizes components into layers. Intuitively, the lower a layer is, the “ closer ” it is to the hardware; the higher a layer is, the “ closer ” it is to the application.

In addition to the layers, TinyOS has a unique component architecture and provides as a library a set of system software components. A component specification is independent of the component implementation. Although most components encapsulate software functionalities, some are just thin wrappers around hardware. An application, typically developed in the nesC language covered in the next section, wires these components together with other application specific components.

Let us consider a TinyOS application example — FieldMonitor , where all nodes in a sensor field periodically send their temperature and photosensor readings to a base station via an ad hoc routing mechanism. A diagram of the FieldMonitor application, where blocks represent TinyOS components and arrows represent function calls among them. The directions of the arrows are from callers to callees.

To explain in detail the semantics of TinyOS components, let us fi rst look at the Timer component of the FieldMonitor application. This component is designed to work with a clock, which is a software wrapper around a hardware clock that generates periodic interrupts. The method calls of the Timer component. An arrowhead pointing into the component is a method of the component that other components can call. An arrowhead pointing outward is a method that
component requires another layer component to provide. The absolute directions of the arrows, up or down, illustrate this component’s relationship with other layers. For example, the Timer depends on a lower layer HWClock component. The Timer can set the rate of the clock, and in response to each clock interrupt it toggles an internal Boolean flag, evenFlag , between true (or 1) and false (or 0). If the flag is 0, the Timer produces a timer0Fire event to trigger other components; otherwise, it produces a timer1Fire event. The Timer has an init() method that initializes its internal flag, and it can be enabled and disabled via the start and stop calls.

A program executed in TinyOS has two contexts, tasks and events , which provide two sources of concurrency. Tasks are created (also called posted ) by components to a task scheduler. The default implementation of the TinyOS scheduler maintains a task queue and invokes tasks according to the order in which they were posted. Thus tasks are deferred computation mechanisms. Tasks always run to completion without preempting or being preempted by other tasks. Thus tasks are non preemptive. The scheduler invokes a new task from the task queue only when the current task has completed. When no tasks are available in the task queue, the scheduler puts the CPU into the sleep mode to save energy.

The ultimate sources of triggered execution are events from hardware: clock, digital inputs, or other kinds of interrupts. The execution of an interrupt handler is called an event context . The processing of events also runs to completion, but it preempts tasks and can be preempted by other events. Because there is no preemption mechanism among tasks and because events always preempt tasks, programmers are required to chop their code, especially the code in the
event contexts, into small execution pieces, so that it will not block other tasks for too long.

Another trade-off between nonpreemptive task execution and program reactiveness is the design of split-phase operations in TinyOS. Similar to the notion of asynchronous method calls in distributed computing, a split-phase operation separates the initiation of a method call from the return of the call. A call to a split-phase operation returns immediately, without actually performing the body of the operation. The true execution of the operation is scheduled later; when the execution of the body finishes, the operation notifies the original caller through a separate method call. An example of a split-phase operation is the packet send method in the Active Messages (AM) component. Sending a packet is a long operation, involving converting the packets to bytes, then to bits, and ultimately driving the RF circuits to send the bits one by one. Without a split-phase execution, sending a packet will block the entire system from reacting to new events for a significant period of time. In the TinyOS implementation, the send() command in the AM component returns immediately. However, it is the caller’s responsibility to remember that the packet has not yet been sent. When the packet is indeed sent, the AM component will notify its caller by a sendDone() method call. Only at this time is the AM component ready to accept another packet.

In TinyOS, resource contention is typically handled through explicit rejection of concurrent requests. All split-phase operations return Boolean values indicating whether a request to perform the operation is accepted. In the above example, a call of send() , when the AM component is still sending the fi rst packet, will result in an error signaled by the AM component. To avoid such an error, the caller of the AM component typically implements a pending lock to remember not to request further sendings until the sendDone() method is called. To avoid loss of packets, a queue should be incorporated by the caller if necessary.

In summary, many design decisions in TinyOS are made to ensure that it is extremely lightweight. Using a component architecture that contains all variables inside the components and disallowing dynamic memory allocation reduces the memory management overhead and makes the data memory usage statically analyzable. The simple concurrency model allows high concurrency with low thread maintenance overhead. As a consequence, the entire FieldMonitor system takes only 3 KB of space for code and 226 bytes for data. However, the advantage of being lightweight is not without cost. Many hardware idiosyncrasies and complexities of concurrency management are left for the application programmers to handle. Several tools have been developed to give programmers language level support for improving programming productivity and code robustness. We introduce in the next two sections two special-purpose languages for programming sensor network nodes. Although both languages are designed on top of TinyOS, the principles they represent may apply to other platforms.

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Node-Level Software Platforms

Most design methodologies for sensor network software are node-centric, where programmers think in terms of how a node should behave in the environment. A node-level platform can be a node-centric operating system, which provides hardware and networking abstractions of a sensor node to programmers, or it can be a language platform, which provides a library of components to programmers.

A typical operating system abstracts the hardware platform by providing a set of services for applications, including fi le management, memory allocation, task scheduling, peripheral device drivers, and networking. For embedded systems, due to their highly specialized applications and limited resources, their operating systems make different trade-offs when providing these services. For example, if there is no file management requirement then a file system is obviously not needed. If there is no dynamic memory allocation then memory management can be simplified. If prioritization among tasks is critical then a more elaborate priority scheduling mechanism may be added.

TinyOS and TinyGALS are two representative examples of node-level programming tools that we will cover in detail in this section. Other related software platforms include Mat é, a virtual machine for the Berkeley motes. Observing that operations such as polling sensors and accessing internal states are common to all sensor network application, Mat é defines virtual machine instructions to abstract those operations. When a new hardware platform is introduced with support for the virtual machine, software written in the Mat é instruction set does not have to be rewritten.

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Sensor Network Programming Challenges

Traditional programming technologies rely on operating systems to provide abstraction for processing, I/O, networking, and user interaction hardware. When applying such a model to programming networked embedded systems, such as sensor networks, the application programmers need to explicitly deal with message passing, event synchronization, interrupt handing, and sensor reading. As a result, an application is typically implemented as a finite state machine (FSM) that covers all extreme cases: unreliable communication channels, long delays, irregular arrival of messages, simultaneous events, and so on. In a target tracking application implemented on a Linux operating system and with directed diffusion routing, roughly 40% of the code implements the FSM and the glue logic of interfacing computation and communication.

For resource-constrained embedded systems with real-time requirements, several mechanisms are used in embedded operating systems to reduce code size, improve response time, and reduce energy consumption. Microkernel technologies modularize the operating system so that only the necessary parts are deployed with the application. Real-time scheduling allocates resources to more urgent tasks so that they can be finished early. Event-driven execution allows the system to fall into low-power sleep mode when no interesting events need to be processed. At the extreme, embedded operating systems tend to expose more hardware controls to the programmers, who now have to directly face device drivers and scheduling algorithms, and optimize code at the assembly level. Although these techniques may work well for small, stand-alone embedded systems, they do not scale up for the programming of sensor networks for two reasons.

● Sensor networks are large-scale distributed systems, where global properties are derivable from program execution in a massive number of distributed nodes. Distributed algorithms themselves are hard to implement, especially when infrastructure support is limited due to the ad hoc formation of the system and constrained power, memory, and bandwidth resources.

● As sensor nodes deeply embed into the physical world, a sensor network should be able to respond to multiple concurrent stimuli at the speed of changes of the physical phenomena of interest.

In the rest of the chapter, we give several examples of sensor network software design platforms. We discuss them in terms of both design methodologies and design platforms . A design methodology implies a conceptual model for programmers, with associated techniques for problem decomposition for the software designers. For example, does the programmer think in terms of events, message passing, and synchronization, or does he/she focus more on information architecture and data semantics? A design platform supports a design methodology by providing design-time (precompile time) language constructs and restrictions, and run-time (postcompile time) execution services.

There is no single universal design methodology for all applications. Depending on the specifi c tasks of a sensor network and the way the sensor nodes are organized, certain methodologies and platforms may be better choices than others. For example, if the network is used for monitoring a small set of phenomena and the sensor nodes are organized in a simple star topology, then a client – server software model would be sufficient. If the network is used for monitoring a large area from a single access point (i.e., the base station), and if user queries can be decoupled into aggregations of sensor readings from a subset of sensor nodes, then a tree structure that is rooted at the base station is a better choice. However, if the phenomena to be monitored are moving targets, then neither the simple client – server model nor the tree organization is optimal. More sophisticated design methodologies and platforms are required.

Source of Information : Elsevier Wireless Networking Complete 2010

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Traditional WAP Networking Environment

WAP allows the presentation and delivery of information and services to wireless devices such as mobile telephone or hand-held computer. The major players in WAP space are the wireless service provider (WSP) and the enterprise. The WSP is the wireless equivalent of an ISP. The role of the WSP is to provide access to back-end resources for wireless users. The WSP provides additional service because wireless users must transition from a wireless to a wired environment (unlike an Internet environment where the user is already “ on ” on the Internet). The WSP’s space contains a modem bank, remote access service (RAS) server, router, and potentially a WAP gateway. The environment is similar to the wired environment, where all connection type services are provided by the WSP. The WSP handles the processing associated with incoming WAP communications, including the translation of wireless communication from the WAP device through the transmission towers to a modem bank and RAS and on to the WAP gateway. The modem bank receives incoming phone calls from the user’s mobile device, the RAS server translates the incoming calls from a wireless packet format to a wired packet format, and the router routes these packets to correct destinations. The WAP gateway is used to translate the WAP into traditional Internet protocol (TCP/IP). The WAP gateway is based on proxy technology. Typical WAP gateways provide the following functionality:

● Domain name server (DNS) service (e.g., to resolve domain names used in URLs).
● A control point for management of fraud and service utilization.
● Act as a proxy, translating the WAP protocol stack to the Internet protocol stack.

Many gateways also include a transcoding function that will translate an HTML page into a WML page that is suited to a particular device type. The enterprise space contains the backed web and application servers that provide the enterprises ’ transactions. Generally, the WSP maintains and manages the WAP gateway, but there are circumstances under which this is not desirable. This is due to the presence of an encryption gap caused by the ending of the WTLS session at the gateway. The data are temporarily in clear text on the gateway until they are reencrypted under the SSL session established with the enterprise’s web server. In such cases, the WAP gateway should be maintained at the enterprise. The problem with this solution is the absence of the DNS client at the mobile device, which would require the storage of profiles for every target on the mobile device. This also requires that the enterprise set up a relationship with service provider whereby all incoming packets destined for the enterprise (identified by the IP address) are immediately routed by the WSP directly to the enterprise and are never sent to the WSP’s gateway.

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Optimal WAP Bearers

The WAP is designed to operate over a variety of different services, including SMS, circuitswitched data (CSD), and packet-switched data (PSD). The bearers offer differing levels of QoS with respect to throughput, error rate, and delays. The WAP is designed to compensate for or tolerate these varying levels of service:

● SMS: Given its limited length of 160 characters per short message, the overhead of the WAP that would be required to be transmitted in an SMS message would mean that even for the simplest of transactions several SMS messages may have to be sent.

● CSD: Most of the trial-based services use CSD as the underlying bearer. CSD lacks immediacy — a dial-up connection taking about 10 sec is required to connect the WAP client to the WAP gateway; and this is the best case scenario when there is a complete end-to-end digital call.

● USSD: It is a means of transmitting information or instructions over a GSM network. In USSD a session is established and the radio connection stays open until the user, application, or time-out releases it. USSD text messages can be up to 182 characters in length. USSD can be an ideal bearer of WAP on GSM networks. USSD is preferable due to the following advantages:
1. Turnaround response times for interactive applications are shorter for USSD.
2. Users need not access any particular phone menu to access services with USSD but they can enter the command directly from the initial mobile phone screen.

● GPRS: GPRS is a new bearer because it is immediate, relatively fast, and supports virtual connectivity, allowing relevant information to be sent from the network as and when it is generated. There are two effi cient means of delivering proactively sending (pushing) content to a mobile phone: by SMS (which is, of course, one of the WAP bearers) or by the user maintaining more or less a permanent GPRS session with the content server. WAP incorporates two different connection modes — WSP connection mode or WSP connection protocol. This is similar to the two GPRS point-to-point services — connection-oriented and connectionless. For the interactive menu-based information exchanges that WAP anticipates, GPRS and WAP can be ideal bearers for each other.

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Wireless Datagram Protocol

Wireless datagram protocol (WDP) offers a consistent service to the upper layer protocols of WAP and communicates transparently over one of the available bearer services. The services offered by WDP include application addressing by port numbers, optional segmentation and reassembly, and optional error detection.

WDP supports several simultaneous communication instances from a higher layer over a single underlying WDP bearer service. The port number identifies the higher layer entity above the WDP. Reusing the elements of the underlying bearers and supporting multiple
bearers, WDP can be optimized for efficient operation within the limited resources of a mobile device.

The WDP adaptation layer is the layer of the WDP that maps the WDP functions directly onto a specific bearer. The adaptation layer is different for each bearer and deals with the specific capabilities and characteristics of that particular bearer service. At the gateway, the adaptation layer terminates and passes the WDP packets on to a WAP proxy/server via a tunneling protocol.

If WAP is used over a bearer UDP, the WDP layer is not needed. On other bearers, such as GSM SMS, the datagram functionality is provided by WDP. This means that WAP uses a datagram service, which hides the characteristics of different bearers and provides port number functionality.

Processing errors can occur when the WDP datagrams are sent from one WDP provider to another. For example, a wireless data gateway is unable to send a datagram to the WAP gateway, or the receiver does not have enough buffer space to receive large messages. The wireless control message protocol (WCMP) provides an efficient error-handling mechanism for WDP.

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Wireless Transport Layer Security

The purpose of wireless transport layer security (WTLS) is to provide transport layer security between a WAP client and the WAP Gateway/Proxy. WTLS is a security protocol based on the industry standard transport layer security (TLS) protocol with new features such as datagram support, optimized handshake, and dynamic key refreshing. The WTLS layer is modular and depends on the required security level of the given application, or characteristics of the underlying network, whether it is used or not. WTLS is optional and can be used with both the connectionless and the connection mode WAP stack confi guration. In addition, WTLS provides an interface for managing secure connections. The primary goal of WTLS is to provide the following features between two communicating applications:

● Data integrity: WTLS contains facilities to ensure that data sent between the terminal and an application server are unchanged and not corrupted.

● Privacy: WTLS contains facilities to ensure that data transmitted between the terminal and an application server is private and cannot be understood by any intermediate parties that may have intercepted the data stream.

● Authentication: WTLS contains facilities to establish the authenticity of the terminal and application server.

● Denial-of-service protection: WTLS contains facilities for detecting and rejecting data that are replayed or not successfully verifi ed. WTLS makes many typical denialof service attacks harder to accomplish and protects the upper protocol layers.

WTLS protocol is optimized for low-bandwidth bearer networks with relatively long latency. These features make it possible to certify that the sent data have not been manipulated by a third party, that privacy is guaranteed, that an author of a message can be identified, and that both parties cannot falsely deny having sent their messages.

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WAP Architecture - Wireless Transaction Protocol

Wireless transaction protocol (WTP) does not have security mechanisms. WTP has been defined as a light-weight transaction-oriented protocol that is suitable for implementation in “ thin ” clients and operates efficiently over wireless datagram networks. Reliability is improved through the use of unique transaction identifi ers, acknowledgments, duplicate removal, and retransmissions. There is an optional user-to-user reliability function in which WTP user can confirm every received message. The last acknowledgment of the transaction, which may contain out-of-band information related to the transaction, is also optional. WTP has no explicit connection set-up or tear-down phases. This improves efficiency over connection-oriented services. The protocol provides mechanisms to minimize the number of transactions being replayed as a result of duplicate packets. WTP is designed for services oriented toward transactions, such as browsing. The basic unit of interchange is an entire message and not a stream of bytes. Concatenation may be used,
where applicable, to convey multiple packet data units (PDUs) in one service data unit (SDU) of the datagram transport. WTP allows asynchronous transactions. There are three classes of transaction service:

● Class 0: Unreliable “ send ” with no result message. No retransmission if the sent message is lost.

● Class 1: Reliable “ send ” with no result message. The recipient acknowledges the sent message; otherwise the message is resent.

● Class 2: Reliable “ send ” with exactly one reliable result message. A data request is sent and a result is received which is fi nally acknowledged by the initiating part.

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WAP Architecture - Wireless Session Protocol

Wireless session protocol (WSP) provides a means for the organized exchange of content between cooperating client/server applications. Its functions are to:

● Establish a reliable session from the client to the server and release the session in an orderly manner.

● Agree on a common level of protocol functionality using capability negotiation.

● Exchange content between client and server using compact encoding.

● Suspend and resume the session.

● Provide HTTP 1.1 functionality.

● Exchange client and server session headers.

● Interrupt transactions in process.

● Push content from server to client in an unsynchronized manner.

● Negotiate support for multiple, simultaneous asynchronous transactions.

The core of the WSP design is a binary form of HTTP. Consequently, all methods defined by HTTP 1.1 are supported. In addition, capability negotiation can be used to agree on a set of extended request methods, so that full compatibility to HTTP applications can be retained. HTTP content headers are used to define content type, character set encoding, language, etc., in an extensible manner. However, compact binary encoding is defined for the well-known headers to reduce protocol overhead.

The life cycle of a WSP is not tied to the underlying transport protocol. A session can be
suspended while the session is idle to free up network resources or save battery power. A lightweight session re-establishment protocol allows the session to be resumed without the overload of full-blown session establishment. A session may be resumed over a different bearer network.

WSP allows extended capabilities to be negotiated between peers (as an example this allows for both high-performance, feature-full implementation as well as simple, basic, and small implementations). WSP provides an optimal mechanism for attaching header information to the acknowledgment of a transaction. It also optionally supports asynchronous requests so that a client can submit multiple requests to the server simultaneously. This improves utilization of air time and latency as the result of each request can be sent to the client when it becomes available.

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WAP Architecture - Wireless Telephony Application

The wireless telephony application (WTA) environment provides a means to create telephony services using WAP. WTA utilizes a user-agent, which is based on the WML user-agent, but extends its functionality that meets special requirements for telephony services. This functionality includes.

● WTAI : An interface toward a set of telephony-related functions in a mobile phone that can be invoked from WML and/or WMLScript. These functions include call management, handling of text messages, and phone book control. WTAI enables access to functions that are not suitable for allowing common access to them, e.g., setting up calls and manipulating the phone book without user acknowledgment.

● Repository: Since it is not feasible to retrieve content from a server every now and then, repository makes it possible to store WTA services in the device in order to enable access to them without accessing the network.

● Event handling: Typical events in a mobile network are incoming calls, call disconnect, and call answered. The event handling within WTA enables WTA services stored in the repository to be started in response to such events.

● WTA service indication: It is a content type that allows a user to be notifi ed about events of different kinds (e.g., new voice mails) and be given the possibility to start the appropriate service to handle the event. In the most basic form, the WTA service indication makes it possible to send a URL and a message to a wireless device.

The WTA framework relies on a dedicated WTA user-agent to carry out these functions. Only trusted content providers should be able to make content available to the WTA user-agent. Thus, it must be possible to distinguish between servers that are allowed to supply the user agent with services containing these functions and those who are not. To accomplish this, the WTA user-agent retrieves its services from the WTA domain, which, in contrast to the Internet, is controlled by the network operator. WTA services and other services are separated from each other using WTA access control based on port numbers.

Source of Information : Elsevier Wireless Networking Complete 2010

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WAP Architecture - Wireless Application Environment

The uppermost layer in the WAP stack, the wireless application environment (WAE), is a general-purpose application environment based on a combination of WWW and mobile telephony technologies. The primary objective of the WAE is to establish an interoperable environment that allows operators and service providers to build applications and services that can reach a wide variety of different wireless platforms in an effi cient and useful manner. Various components of the WAE are:

● Addressing model: WAP uses the same addressing model as the one used on the Internet (i.e., URL). A URL uniquely identifi es a resource on a server that can be retrieved using well-known protocols. WAP also uses Uniform Resource Identifiers (URI). A URI is used for addressing resources that are not necessarily accessed using known protocols. An example of using a URI is local access to a wireless device’s telephony functions.

● WML: It is WAP’s analogy to HTML and is based on XML. It is WAP’s answer to problems such as creating services that fi t on small hand-held devices, low bandwidth wireless bearers, etc. WML uses a deck/card metaphor to specify a service. A card
is typically a unit of interaction with the user (i.e., either presentation of information or request for information from the user). A collection of cards is called a deck, which usually constitutes a service. This approach ensures that a suitable amount of information is displayed to the user simultaneously since interpage navigation can be avoided to the maximum possible extent. Key features of WML include variables, text formatting features, support for images, support for soft-buttons, navigation control, control of browser history, support for event handling (e.g., telephony services) and different types of user interactions (e.g., selection lists and input fields). One of the key advantages of WML is that it can be binary encoded by the WAP Gateway/Proxy in order to save bandwidth in the wireless domain.

● WMLScript: It is used for enhancing services written in WML. WMLScript can be used for validation of user input. Since WML does not provide any mechanisms for achieving this, a round trip to the server would be needed in order to determine whether user input is valid if scripting was not available. Access to local functions in a wireless device is another area in which WMLScript is used (e.g., access to telephony related functions). WMLScript libraries contain functions that extend the basic WMLScript functionality. This provides a means for future expansion of functions without having to change the core of WMLScript. Just as with WML, WMLScript can be binary encoded by the WAP Gateway/Proxy in order to minimize the amount of data sent over the air.

Source of Information : Elsevier Wireless Networking Complete 2010

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Introduction to Wireless Application Protocol

WAP has become the de facto global industry standard for providing data to wireless hand-held mobile devices [4 – 10] . WAP takes a client/server approach and incorporates
a relatively simple microbrowser into the mobile phone, requiring only limited resources
on mobile phones. WAP puts the intelligence in the WAP Gateways while adding just
a microbrowser to the mobile phones themselves. Microbrowser-based services and
applications reside temporarily on servers, not permanently in phones. The WAP is aimed
at turning mass-market phones into a network-based smart phone. The philosophy behind
WAP’s approach is to use as few resources as possible on the hand-held device and compensate for the constraints of the device by enriching the functionality of the network.
WAP specifi es a thin-client microbrowser using a new standard called wireless markup
language ( WML ) that is optimized for wireless hand-held mobile devices. WML is a stripped down version of HTML.

WAP specifies a proxy server that acts as a gateway between the wireless network and the wire line Internet, providing protocol translation and optimizing data transfer for the wireless handset. WAP also specifies a computer-telephony integration application programming interface ( API ), called wireless telephony application interface ( WTAI ), between data and voice. This enables applications to take full advantage of the fact that this wireless mobile device is most often a phone and a mobile user’s constant companion. On-board memory on a WAP phone can be used for off-line content, enhanced address books, bookmarks, and text input methods.

The importance of WAP can be found in the fact that it provides an evolutionary path for application developers and network operators to offer their services on different network
types, bearers, and terminal capabilities. The design of the WAP standard separates the
application elements from the bearer being used. This helps in the migration of some
applications from short message service (SMS) or circuit-switched (CS) data to general
packet radio service (GPRS), for example. WAP 1.0 was optimized for early WAP-phones.

The WAP cascading style sheet (WAP CSS) is the mobile version of a cascading style sheet. It is a subset of CSS2 (the cascading style sheet language of the WWW) plus some
WAP-specific extensions. CSS2 features and properties that are not useful for mobile Internet applications are not included in WAP CSS. WAP CSS is the companion of XHTML Mobile Profi le (XHTML MP). Both of them are defined in the WAP 2.0 specification, which was created by the WAP forum. XHTML MP is a subset of XHTML, which is the combination of HTML and XML. There are many WAP 2.0-enabled cell phones on the market currently. Before creating WAP 2.0, developers used WML to build WAP sites and HTML/XHTML/CSS to build web sites. Now with WAP 2.0 they can make use of the same technologies to create both web sites and WAP sites. Documents written in XHTML MP/WAP CSS are viewable on ordinary PC web browsers, since XHTML MP and WAP CSS are just the subsets of XHTML and CSS.

The following are the goals of WAP:
● Independent of wireless network standards;

● Interoperability: Terminals from different manufacturers must be able to communicate with services in the mobile network;

● Adaptation to bounds of wireless networks: Low bandwidth, high latency, less connection stability;

● Adaptation to bounds of wireless devices: Small display, limited input facilities, limited memory and CPU, limited battery power;

● Efficient: Provide quality of service (QoS) suitable to the behavior and characteristics of the mobile world;

● Reliable: Provide a consistent and predictable platform for deploying services;

● Secure: Enable services to be extended over potentially unprotected mobile networks while preserving the integrity of data;

● Applications scale across transport options;

● Applications scale across device types;

● Extensible over time to new networks and transport. WAP is envisaged as a comprehensive and scalable protocol designed for use with:

● Any mobile device from those with a one-line display to a smart phone;

● Any existing or planned wireless service such as the SMS, CS data, unstructured
supplementary services data (USSD) and GPRS;

● Any mobile network standard such as code division multiple access (CDMA), global system of mobile communications (GSM), or universal mobile telephone system (UMTS); WAP has been designed to work with all cellular standards and is supported by major worldwide wireless leaders such as AT & T wireless and NTT DoCoMo;

● Multiple input terminals such as keypads, keyboards, touch-screens, etc.

Source of Information : Elsevier Wireless Networking Complete 2010

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Wireless Signal Modulation

Signal modulation is a technique used to combine a signal being transmitted with a carrier signal for transmission. The receiver demodulates the transmitted signal and regenerates the original signal. Normally the carrier signal is a sine wave of a high frequency. The input signal could be digital (digital modulation) or analog (analog modulation). In either case, the three basic characteristics of a signal are utilized for modulation. The device that performs this modulation and demodulation is the modem. Modulation is often referred to as signal encoding. Analog signals can be modulated by the following methods.


Amplitude Modulation
For AM signals, the output signal is a multiplication of the input signal with a carrier wave. The amplitude of the carrier wave is determined by the input analog signal. The frequency of the resulting output signal is centered at the frequency of the carrier. As its name implies, AM radio that operates in the frequency band of 520 to 1605.5 KHz uses AM.


Frequency Modulation
Rather than vary the amplitude of the carrier wave, FM alters the transient frequency of the carrier according to the input signal. Again, as its name implies, FM radio that operates within the frequency band of 87.5 to 108 MHz uses FM.


Phase Modulation
In phase modulation (PM), the phase of the carrier signal is used to encode the input signal. Like AM, FM and PM shift the frequency of the input signal to a band centered at the carrier frequency. Both FM and PM require higher bandwidths. Analog modulation is necessary for transmitting a wireless analog signal such as voice over a long distance. Directly transmitting the signal itself to the receiver without applying modulation would require a large antenna to be effective, as the frequency of voice signals falls into the range of 30 to 3000 Hz. For digital data, if the medium only facilitates analog transmission (e.g., air), some digital modulation techniques will have to be employed. A carrier wave is also used to carry binary streams being transmitted, according to some keying schemes in digital modulation. Below is a list of digital modulation schemes:

» Amplitude-shift keying (ASK): ASK uses presence of a carrier wave to represent a binary one and its absence to indicate a binary zero. While ASK is simple to implement, it is highly susceptible to noise and multipath propagation effects. Because of that, ASK is primarily used in wired networks, especially in optical networks where the bit error rate (BER) is considerably lower than that of wireless environments.

» Frequency-shift keying (FSK): Similar to FM, FSK uses two or more frequencies of a carrier wave to represent digital data. Binary FSK (BFSK), which employs two carrier frequencies for 0 and 1, is the most commonly used FSK. The resulting signal can be mathematically defined as the sum of two amplitude-modulated signals of different carrier frequencies. If more than two carrier frequencies are used for modulation, each frequency may represent more than one bit, thereby providing a higher bandwidth than ASK.

» Phase-shift keying (PSK): PSK uses the phase of a carrier wave to encode digital data. Binary PSK simply reverses phase when the data bits change. Multilevel PSKs use more evenly distributed phases in the phase domain, with each phase representing two or more bits. One of most commonly used PSK schemes is quadrature PSK, in which the four phases of 0, π /2, π , and 3 π /2 are used to encode two digits. PSK can be implemented in two ways. The first is to produce a reference signal at the receiver side and then compare it with the received signal to decide the phase shift. This method somewhat complicates things at the receiver end, as the transmitter and the receiver must be synchronized periodically to ensure that the reference signal is being generated correctly. Another method is differential PSK (DPSK). In DPSK, the reference signal is not a separated signal but is the one preceding the current wave in question. One of the second generation cellular systems, Digital-Advanced Mobile Phone Service (DAMPS), uses DPSK.

ASK and PSK can be combined to offer more variations of phase shifts on the phase domain. Quadrature amplitude modulation (QAM) is such a scheme in which multiple levels of amplitudes coupled with several phases provide far more unique symbol shifts over the same bandwidth than used by PSK over the same bandwidth. QAM is widely used in today’s modems.

Apart from analog and digital modulation, another category of modulation that should be discussed for wireless communication is analog-to-digital data modulation, a procedure sometimes referred to as digitization. Two major digitization schemes are pulse-code modulation (PCM) and delta modulation. PCM samples an input analog signal in short intervals, and each sample is converted into a symbol representing a code. To reconstruct the original input signal from samples, the sampling rate must be higher than twice the highest frequency of the input signal. In other words, given a sample rate offs, a frequency higher than 2 fs in the input signal will not be recovered in the reconstruction. Delta modulation uses a staircase-like sample function to approximate the input signal. The resulting digital data comprise a series of 1’s and 0’s indicating the ups and downs, respectively, of the staircase function.

In the wireless world, signals transmitted through the air are primarily high-frequency analog signals. In wireless voice communication, the user’s voice is digitalized into digital data and then modulated to analog-based band signals (digital modulation), which are finally modulated with a carrier wave for transmission. For wireless data transmission, the first step of this procedure is not necessary. In either case, the receiver takes the reverse order of these steps to recover the transmitted data or voice.

Source of Information :  Elsevier Wireless Networking Complete 2010

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Wireless Signal Multiplexing

Modulations of analog signals or digital data are concerned with a single input signal to be converted efficiently into other forms. In contrast, multiplexing is a collection of schemes that addresses the issue of transmitting multiple signals simultaneously in a wireless system in the hopes of maximizing the capacity of the system. The devices for multiplexing and demultiplexing are multiplexers and demultiplexers, respectively. If signals of the same frequency are spatially separated from each other such that no frequency overlapping occurs at any given place, then multiple signals of different frequencies can be transmitted and received without a problem. Radio stations are an excellent example of this spatial division multiplexing : AM and FM radio signals only cover the area in which the radio stations are located, and they cannot interfere with other radio signals on the same frequency in adjacent areas. Apart from spatial division multiplexing, three prominent schemes of multiplexing have been devised.


Frequency-Division Multiplexing
In frequency-division multiplexing (FDM), signals from a transmitter are modulated to a fixed frequency band centered at a carrier frequency (i.e., a channel). To avoid inference, these channels have to be separated by a sufficiently large gap (i.e., a guard band) in the frequency domain; hence, transmission and reception of signals in multiple channels can be performed simultaneously but independently. Analog cellular systems use FDM; in these systems, calls are separated by frequency.


Time-Division Multiplexing
Time -division multiplexing (TDM) allows multiple channels to occupy the same frequency band but in small alternating slices of time following a sequence known to both the transmitter and the receiver. Each channel makes full use of the bandwidth of the medium but only contributes a portion of the overall data rate. Coordination among the transmitters is necessary to prevent conflicting use of the frequency band. When applied to digital signals, TDM can be done on bit level, byte level, block level, or levels of larger quantities. GSM and D-AMPS both use TDM but in different ways. TDM and FDM can be combined to increase the robustness of the system. In this case, signals from a transmitter are modulated onto different carriers for a certain amount of time and jump to another carrier, effectively creating a “frequency-hopping” phenomenon.


Code-Division Multiplexing
Code -division multiplexing (CDM) makes better use of a frequency band than FDM and TDM. Signals from different transmitters are transmitted on the same frequency band at the same time but each has a code to uniquely identify itself. The orthogonal codes mathematically ensure that signals cannot interfere with each other at the receiver. CDM effectively converts the problem of limited frequency space into ample code space but adds the overhead of implementation complexity. The transmitter and receiver must be synchronized such that individual signals can be correctly received and decoded. Compared to FDM, CDM provides greater security against signal tapping because transmitted signals appear as noise if the receiver does not know the code. CDM is the underlying multiplexing scheme of orthogonal frequency-division multiplexing (OFDM). CDMA cellular systems use similar CDM schemes to provide multiple wireless communication channels access to the same frequency band. Another multiplexing scheme, wavelength-division multiplexing (WDM), is very common in optical networks using fiber as the transmission medium. It is actually FDM for fiber, which offers an extremely high bandwidth. In WDM, a fiber can be divided into a number of wavelengths (nanometers), each of which can be assigned to a transmission channel. Dense wavelength-division multiplexing (DWDM) systems support eight or more wavelengths. Because of their high data rate, WDM and DWDM are the predominant multiplexing schemes used by optical networks in the wired Internet backbone.

Source of Information :  Elsevier Wireless Networking Complete 2010

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Wireless Signal Propagation

A radio signal can be described in three domains: time domain, frequency domain, and phase domain. In the time domain, the amplitude of the signal varies with time; in the frequency domain, the amplitude of the signal varies with frequency; and in the phase domain, the amplitude and phase of the signal are shown on polar coordinates. According to Fourier’s theorem, any periodic signal is composed of a superposition of a series of pure sine waves and cosine waves whose frequencies are harmonics (multiples) of the fundamental frequency of the signal; therefore, any periodic signal, no matter how it was originally produced, can be reproduced using a sufficient number of pure waves.

Electronic signals for wireless communication must be converted into electromagnetic waves by an antenna for transmission. Conversely, an antenna at the receiver side is responsible for converting electromagnetic waves into electronic signals. An antenna can be omnidirectional or directional, depending on specific usage scenarios. For an antenna to be effective, it must be of a size consistent with the wavelength of the signals being transmitted or received. Antennas used in cell phones are omnidirectional and can be a short rod on the handset or hidden within the handset. A recent advancement in antenna technology is the multiple-in, multiple out (MIMO) antenna, or smart antenna, which combines spatially, separated small antennas to provide high bandwidth without consuming more power or spectrum. To take advantage of multipath propagation, these small antennas must be separated by at least half of the wavelength of the signal being transmitted or received.

A signal emitted by an antenna travels in the air following three types of propagation modes: ground-wave propagation, sky-wave propagation, and line-of-sight (LOS) propagation. AM radio is a kind of ground-wave propagation, where signals follow the contour of the Earth to reach a receiver. SW radio and HAM amateur radio are examples of sky-wave propagation, where radio signals are reflected by ionosphere and the ground along the way. Beyond 30 MHz, LOS propagation dominates, meaning that signal waves propagate on a direct, straight path in the air. It is noteworthy that radio signals of LOS propagation can also penetrate objects, especially signals of large wavelength (and thus low frequency). Satellite links, infrared light, and communication between base stations of a cellular network are examples of LOS propagation.



Attenuation
The strength or power of wireless signals decreases when they propagate in the air, just as visible light does. As soon as radio waves leave the transmitter’s antenna, some amount of energy will be lost as the electromagnetic field propagates. The effect will become more evident over a long distance as the signal disperses in space; therefore, the received power of the signal is invariably less than the signal power at the transmitting antenna. In the most ideal circumstances (i.e., in vacuum), signal power attenuation is proportional to d 2 , where d denotes the distance between the transmitter and the receiver. This effect is sometimes referred to as free space loss. In reality, beside free space loss, a number of other factors have to be considered to determine signal attenuation, such as weather conditions, atmospheric absorption, and space rays. In addition, signal attenuation is more severe at high frequencies than at low frequencies, resulting in signal distortion.

When it encounters obstacles along the path, a signal may experience more complex attenuation than power reduction. For example, for visible light we are well aware of the following effects: shadowing, reflection, and refraction. Likewise, for high-frequency wireless signals, such effects also exist. Shadowing and reflection occur when a signal encounters an object that is much larger than its wavelength. Though the reflected signal and the shadowed signal are comparatively weak, they in effect help to propagate the signal to spaces where LOS is impossible. For example, when reflection and shadowing are caused by buildings in an urban area, signals from an antenna of a base station may be able to reach cell phone users within a building in the area, although it might be a good idea for the user to walk close to the window for better signal strength (perceived as a number of “bars” displayed on the cell phone screen). Refraction (bending) occurs when a wave passes across the boundary of two media. Moreover, wireless signals are also subject to scattering and diffraction. Specifically, when the size of an obstacle is on the order of the signal wavelength or less, the signal will be scattered into a number of weaker pieces. Diffraction occurs when a signal hits the edge of an obstacle and is deflected into a number of directions.


Noise
The receiver of a wireless communication system must be able to detect transmitted (most likely attenuated and distorted) signals from unwanted noises. Common types of noise are thermal noise (white noise) produced by any electronic circuitry; intermodulation noise, which occurs when two frequencies of signals are modulated and transmitted over the same medium; crosstalk between two channels; and impulse noise generated by instantaneous electromagnetic changes. To cope with noises in received signals, a wireless system has to ensure that the transmitted signals are sufficiently stronger than the noises. Another approach is to employ spread spectrum schemes (explained below) that convert a signal over a wide range of frequencies of low power density as random noise. Wireless signals are subject to various impairments or distortion along the way from the transmitter to the receiver. To quantify these effects, the signal-to-noise ratio (SNR) is used to represent the ratio of the power in a signal to the power of the noise. SNR is usually computed in decibels as the product of 10 and the logarithm of the raw power ratio.


Multipath Propagation
The receiver of a wireless system is exposed to all radio waves in its surrounding environment; therefore, it may receive indirect signals from different paths, such as reflected signals, shadowed signals, and refracted signals, as well as signals generated by other means of propagation, all carrying the same signal with different levels of attenuation and distortion. These signals may impose some negative effect on the direct signal to a great extent. The most severe effect of multipath propagation is intersymbol interference (ISI). ISI is caused by overlapping of delayed multipath pulses (of a primary pulse) and subsequent primary LOS pulses, where one or multiple pulses represent a bit. The degree of attenuation of these pulses may vary from time to time due to path changes or environmental disturbances, making it more difficult to recover the transmitted bits. To prevent ISI from occurring, the first primary pulse and the second pulse have to be separated by a sufficient time difference such that the delayed multipath pulses of the first can be differentiated from the second LOS pulse. This implies that the symbol rate of the signal and bandwidth of the radio channel are limited by multipath propagation.

Source of Information :  Elsevier Wireless Networking Complete 2010

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Other Fixed or Mobile Wireless Communications Frequency

» Digital cordless phone: The Digital Enhanced Cordless Telecommunications (DECT) standard in Europe defi nes the use of the frequency band 1880 – 1990 MHz for digital cordless phone communication. In the United States, cordless phones use three frequency bands: 900 MHz, 2.4 GHz, and 5.8 GHz, each of which is also intensively used by other short-range wireless communication technologies.

» Global positioning system (GPS): GPS satellites use the frequency bands 1575.42 MHz (referred to as L1) and 1227.60 MHz (L2) to transmit signals.

» Meteorological satellite services: The UHF band from 1530 to 1650 MHz (the L band) is commonly used by meteorological satellites, as well as some global environmental monitoring satellites. Part of the UHF and SHF bands are used for military satellite communication.

» Radio-frequency remote control, such as remote keyless entry systems and garage door openers. These short-range wireless systems, commonly used for automobiles, operate at 27, 128, 418, 433, and 868 MHz in the United States; 315 and 915 MHz in Europe; and 426 and 868 MHz in Japan.

Source of Information : Elsevier Wireless Networking Complete 2010  

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