Bt0086 mobile computing 2

Mobile Computing-2

1. What are the features of mobile and wireless devices?

Mobile and wireless devices are usually handheld devices, and accessing

the WWW presents a more constrained computing environment compared

to desktop computers because of fundamental limitations of power and form

factor. Mass-market handheld wireless devices tend to have,

less powerful CPUs (Central Processor Units)

less memory [both ROM (Read Only Memory) and RAM (Random

Access Memory)] restricted power consumption

smaller displays

different input devices (e.g., a phone keypad, voice input, etc.).

Wireless data networks also present a more constrained communication

environment compared to wired networks. Because of fundamental

limitations of power, available spectrum, and mobility, wireless data

networks tend to have,

less bandwidth than traditional networks

more latency than traditional networks

less connection stability than other network technologies

less predictable availability.

2. What is the functionality of the WAE micro-browser environment?

The Wireless Application Environment (WAE) is a

general-purpose application environment based on the combination of

WWW and Mobile Telephony technologies. The WAE allows operators and

service providers to build applications and services that can reach wireless

platforms in an efficient and useful manner. WAE contains a micro browser

environment containing the following functionality:

Wireless Markup Language (WML): a lightweight markup language,

similar to HTML, and optimized for use in handheld mobile devices.

WMLScript: a lightweight scripting language, similar to JavaScript.

Wireless Telephony Application (WTA): telephony services and

programming interfaces.

Content formats: a set of well-defined data formats, including images,

phone book records, and calendar information.

3. What are the main elements of the WAE model? Explain.

The WAE architecture includes networking schemes, content formats,

programming languages, and shared services. Interfaces are not

standardized and are specific to a particular implementation. WAE can work

with a browser and a class of user agents used in the World Wide Web

(WWW)

.The main elements of the WAE model are WAE user agents, content

generators, standard content encoding, and WTA. WAE user agents

interpret network content referenced by a URL. Content generators are the

applications or services on origin servers, like CGI scripts, that produce

standard content formats in response to requests from user agents in Mts.

Standard content encoding allows a WAE user agent to navigate Web

content. WTA is a collection of telephony-specific extensions for call and

feature control mechanisms providing advanced Mobile Network Services.

The WAE logical model is shown in Figure 9.2. In the WAE model, the

content is transported using standard protocols in the WWW domain and an

optimized HTTP like protocol in the wireless domain. The content and

services in WAE architecture are hosted on standard Web origin servers

using proven technologies like Common Gateway Interface (CGI). The

content is located by using WWW standard URLs. WAE supports Mobile

Network Services such as Call Control and Messaging. WAE architecture

supports low bandwidth and high latency networks and considers CPU

processing constraints in MTs. WAE assumes the existence of gateway

functionality responsible for encoding and decoding data transferred from

and to the mobile client. The purpose of the encoding content delivered to

the client is to minimize the size of data sent to the client Over The Air

(OTA), and to minimize the computational energy required by the client to

process the data. The gateway functionality can be added to origin servers

or placed in dedicated gateways.

WAE is based on the architecture used for WWW proxy servers. The

situation in which a user agent, a browser, must connect through a proxy to

reach an origin server, the server that contains the desired content, is very

similar to the case of a wireless device accessing a server through a

gateway. Most connections between the browser and the gateway use WAP

Session Protocol (WSP), regardless of the protocol of the destination server.

URL refers only to the destination server’s protocol and has no bearing on

what protocols may be used in intervening connections. The gateway

performs protocol conversion by translating requests from WSP into other

protocols, and translating the responses back into WSP. Content conversion

performed by the gateway is analogous to HTML/HTTP proxies available on

the Web. In the HTTP scheme, the browser communicates with the gateway

using WSP. The gateway provides protocol conversion functions to connect

to an HTTP origin server.

WAE logical layers include user agents such as browsers, phone books,

message editors, and so on, and services and formats including common

elements and formats accessible to user agents such as Wireless Markup

Language (WML), WMLScript, image formats, vCard (electronic business

card) and vCalendar (electronic calendar and scheduling exchange)

formats, and so on. The WAE client components are shown in Figure WAE allows

the integration of domain-specific user agents with varying

architectures and environments. A WTA user agent is specified as an

extension to the WAE specification for the mobile telephony environments.

The WTA extensions allow for accessing and interacting with mobile

telephone features, like call control, and other applications assumed on the

telephones, such as phone books and calendar applications. The features

and capabilities of a user agent are decided by those who implement them.

The main elements of the WAE model are WAE user agents, content

generators, standard content encoding, and WTA. WAE user agents

interpret network content referenced by a URL. Content generators are the

applications or services on origin servers, like CGI scripts, that produce

standard content formats in response to requests from user agents in Mts.

Standard content encoding allows a WAE user agent to navigate Web

content. WTA is a collection of telephony-specific extensions for call and

feature control mechanisms providing advanced Mobile Network Services.

4. What is the role of the repository in the WTA services?

The repository is a persistent storage module within the MT that may be

used to eliminate the need for network access when loading and executing

frequently used WTA services. The repository also addresses the issue of

how a WTA service developer ensures that time-critical WTA events are

handled in a timely manner. The repository addresses the issues of how the

WTA services developer preprogram the device with content, and how the

WTA services developer improves the response time for a WTA service.

The repository can be accessed by a service using one of the following

methods:

A WTA event associated with a channel is detected, and the user agent invokes a

URL as specified by the associated channel;

The end user accesses services stored in the repository through an

implementation dependent representation (for instance, a menu

containing the labels of the channels) of the allowed services (channels

explicitly specified as user accessible by the channel definition) in the

repository;

The content of URL retrieved from the repository may be given to the

user agent by providing the URL in content or delivering it by Service

Indication (SI).

The WTA applications, that is, content loaded or otherwise received from

the WTA server, may access the repository.

5. What are the WLAN’s operating speeds?

A WLAN is a Wireless Local Area Network — a network of one or more computers and related peripherals that is, or can be, wireless. A WLAN may combine both wired and wireless connections. Wireless networks are not as speedy as some users would like, and there are several factors that are known to affect WLAN speed.One factor that affects WLAN speed is the wireless standard used by the network devices. The standard is called 802.11, but there are different versions available: 802.11a, 802.11b, 802.11g, and 802.11n. 802.11n, the most recent standard, operates in the 2.4GHz or 5 GHz range, has speeds from 108 Mbps (Megabits per second) to 600 Mbps, and is backwards compatible with 802.11a, b, and g. 802.11g also operates at 2.4 GHz and has a speed of 54 Mbps. 802.11b operate in the 2.4 GHz band at 11 Mbps, while 802.11a operates in the 5 GHz range with a maximum data rate of 54 Mbps.If all the devices are connected to the router are capable of the same 802.11 standard, then the router should be set for that standard, rather than mixed-mode. This is because allowing earlier standards slows down the data. If all devices work on 802.11n standard, this will increase WLAN speed.

Routers come with one or more built-in antennas, but they are often inexpensive and not as good as they might be. They may or may not be made to be removed and upgraded. If they are removable, replacing them with an appropriate antenna for the network can increase WLAN speed.

If a 2.4 GHz network is being used for the WLAN, it is possible to speed it up by avoiding the devices that typically interfere with 2.4 GHz wireless transmissions.

These include 2.4 GHz cordless phones, Bluetooth® adapters, baby monitors, and microwave devices. Cordless phones that will not interfere include those operating at 900 MHz, 1.9 GHz, and 5.8 GHz.

6. What is the radio frequency (RF) band in which the LANs operate?

In a wireless LAN (WLAN), the

connection between the client and user exists through the use of a wireless

medium such as Radio Frequency (RF) or Infrared (IR) communications

.This allows the mobile user to stay connected to the network. The wireless

connection is usually accomplished by the user having a handheld terminal

or a laptop computer that has an RF interface card installed inside the

terminal or through the PC Card slot of the laptop. The client connection

from the wired LAN to the user is made through an Access Point (AP) that

can support multiple users simultaneously. The AP can reside at any node

on the wired network and performs as a gateway for wireless users’ data to

be routed onto the wired network.

The network

communications use a part of the radio spectrum that is designated as

license-free. In this band, of 2.4 to 2.5 GHz, the users can operate without a

license when they use equipment that has been approved for use in this

license-free band

The 2.4- GHz band has been designated as license-free by the

International Telecommunications Union (ITU) and is available for use,

license-free in most countries in the world. The rules of operation are

different in almost every country but they are simil ar enough so that the

products can be programmed for use in every country without changing the

hardware component.

7. What are the fundamental components of a GSM network? Explain.

The fundamental components of a GSM network are shown in Figure

A user carries a Mobile Station (MS), which can communicate over the air

with a base station, called Base Tranceiver Station (BTS) in GSM. The BTS

contains transmitter and receiver equipment, such as antennas and

amplifiers, as well as a few components for signal and protocol processing.

For example, error protection coding is performed in the BTS, and the link-

level protocol for signaling on the radio path is terminated here. In order to

keep the base stations small, the essential control and protocol intelligence

resides in the Base Station Controller (BSC). It contains, for example,

protocol functions for radio channel allocation, channel setup and

management of handovers. Typically, several BTSs are controlled by one

BSC.

In practice, the BTS and BSC are connected by fixed lines or point-to-point

radio links. BTS and BSC together form the radio access network. The

combined traffic of the users is routed through a switch, called the Mobile

Switching Center (MSC). It performs all the switching functions of a

switching node in a fixed telephone network, e.g., in an Integrated Services

Digital Network (ISDN). This includes path search, data forwarding and

service feature processing. The main difference between an ISDN switch

and an MSC is that the MSC also has to consider the allocation and

administration of radio resources and the mobility of the users. The MSC,

therefore, has to provide additional functions for location registration of

users and for the handover of a connection in the case of changing from cell

to cell.

A cellular network can have several MSCs with each being responsible for a

part of the network (e.g., a city or metropolitan area). Calls originating from

or terminating in the fixed network are handled by a dedicated Gateway

MSC (GMSC). The interworking of a cellular network and a fixed network

(e.g., PSTN, ISDN) is performed by the Interworking Function (IWF). It is

needed to map the protocols of the cellular network onto those of the

respective fixed network. Connections to other mob ile or international

networks are typically routed over the International Switching Center (ISC)

of the respective country.

A GSM network also contains several types of databases. The Home

Location Register (HLR) and the Visited Location Register (VLR) store the

current location of a mobile user. This is needed since the network must

know the current cell of a user to establish a call to the correct base station.

In addition, these registers store the profiles of users, which are required for

charging and billing and other administrative issues. Two further databases

perform security functions: the Authentication Center (AUC) stores security-

related data such as keys used for authentication and encryption; the

Equipment Identity Register (EIR) registers equipment data rather than

subscriber data.

The network management is organized from a central place, the Operation

and Maintenance Center (OMC). Its functions include the administration of

subscribers, terminals, charging data, network configuration, operation,

performance monitoring and network maintenance. The operation and

maintenance functions are based on the concept of the Telecommunication

Management Network (TMN) which is standardized in the ITU-T series

M.30.

8. Give the classification of Logical channels.

On Layer 1 of the OSI Reference Model, GSM defines a series of logical

channels, which are made available either in an unassigned random access

mode or in a dedicated mode assigned to a specific user. Logical channels

are divided into two categories as shown in table

The Traffic Channels (TCHs) are used for the transmission of user

payload

data (speech, data). They do not carry any control information of Layer 3.

Communication over a TCH can be circuit-switched or packet-switched. In

the circuit-switched case, the TCH provides a transparent data connection

or a connection that is specially treated according to the carried service (e.g.

telephony). For the packet-switched mode, the TCH carries user data of OSI

Layers 2 and 3 according to the recommendations of the X.25 standard or

similar standard packet protocols.

A TCH may either be fully used (full-rate TCH, TCH/F) or be split into two

half-rate channels (half-rate TCH, TCH/H), which can be allocated to

different subscribers. Following ISDN terminology, the GSM traffic channels

are also designated as Bm channel (mobile B channel) or Lm channel

(lower-rate mobile channel, with half the bit rate). A Bm channel is a TCH for

the transmission of bit streams of either 13 kbit/s of digitally coded speech

or of data streams at 14.5, 12, 6 or 3.6 kbit/s. Lm channels are TCH

channels with less transmission bandwidth than Bm channels and transport

speech signals of half the bit rate (TCH/H) or bit streams for data services

with 6 or 3.6 kbit/s.

The control and management of a cellular network demands a very high

signaling effort. Even when there is no active connection, signaling

information (for example, location update information) is permanently

transmitted over the air interface. The G SM signaling channels offer a

continuous, packet-oriented signaling service to MSs in order to enable

them to send and receive messages at any time over the air interface to the

BTS. Following ISDN terminology, the GSM signaling channels are also

called Dm channels (mobile D channel).They are further divided into

Broadcast Channel (BCH), Common Control Channel (CCCH) and

Dedicated Control Channel (DCCH)

The unidirectional BCHs are used by the BSS to broadcast the same

information to all MSs in a cell. The group of BCHs consists of three

channels.

Broadcast Control Channel (BCCH): On this channel, a series of information

elements is broadcast to the MSs, which characterize the organization of the

radio network, such as radio channel configurations (of the currently used

cell as well as of the neighboring cells), synchronization information

(frequencies as well as frame numbering) and registration identifiers (LAI,

CI, BSIC). In particular, this includes information about the structural

organization (formats) of the CCCH of the local BTS. The BCCH is

broadcast on the first frequency assigned to the cell (the so-called BCCH

carrier).

Frequency Correction Channel (FCCH): On the FCCH, information about

correction of the transmission frequency is broadcast to the Mss.

Synchronization Channel (SCH): The SCH broadcasts information to identify

a BTS, i.e. BSIC. The SCH also broadcasts data for the frame

synchronization of a MS, i.e. Reduced Frame Number (RFN) of the TDMA

frame.

FCCH and SCH are only visible within protocol Layer 1, since they are only

needed for the operation of the radio subsystem. There is no access to them

from Layer 2. In spite of this fact, the SCH messages contain data, which

are needed by Layer 3 for the administr ation of radio resources. These two

channels are always broadcast together with the BCCH.

9. Explain with block diagram the basic elements of the GSM

transmission chain.

! A schematic overview of the basic elements of the GSM

transmission chain. The stream of sampled speech data is fed into a source

encoder, which compresses the data by removing unnecessary redundancy.

The resulting information bit sequence is passed to the channel encoder. Its

purpose is to add, in a controlled manner, some redundancy to the

information sequence. This redundancy serves to protect the data against

the negative effects of noise and interference encountered in the

transmission through the radio channel. On the receiver side, the introduced

redundancy allows the channel decoder to detect and correct transmission

errors. GSM uses a combination of block and convolutional coding.

Moreover, an interleaving scheme is used to deal with burst errors that

occur over multipath and fading channels. Next, the encoded and

interleaved data are encrypted to guarantee secure and confident data

transmission. The encrypted data are subsequently mapped to bursts which

are then multiplexed as explained in previous sections. Finally, the stream of

bits is differential coded and modulated.

After transmission, the demodulator processes the signal, which was

corrupted by the noisy channel. It attempts to recover the actual signal from

the received signal. The next steps are demultiplexing and decryption. The

channel decoder attempts to reconstruct the original information sequence

and, as a final step, the source decoder tries to reconstruct the original

source signal.

10. What is an ad hoc network? Explain.

In ad hoc networks all nodesare mobile and can be connected dynamically in an arbitrary manner. All nodes of these networks behave as routers and take part in discovery and maintenance of routes to other nodes in the network. Ad hoc networks are very useful in emergency search-and-rescue operations, meetings, or conventions in which persons wish to quickly share information and data acquisition operations in inhospitable terrain. An ad hoc network is a collection of mobile nodes forming a temporary network without the aid of any centralized administration or standard support services regularly available in conventional networks. We assume that the mobile hosts use wireless radio frequency transceivers as their network interface, although many of the same principles will apply to infrared and wire-based networks. Some form of routing protocol is necessary in these ad hoc networks since two hosts wishing to exchange packets may not be

able to communicate directly. The routing protocols meant for wired networks cannot be used for mobile ad hoc networks because of the mobility of networks. The ad hoc routing protocols can be divided into two classes: table-driven and on-demand routing, on the basis of when and how the routes are discovered. In table- driven routing protocols, consistent and up-to-date routing information to all nodes is maintained at each node, whereas in on-demand routing the routes are created only when desired by the source host. We discuss a few of the current table-driven protocols as well as on-demand protocols.