1. The document discusses the features of mobile and wireless devices, including less powerful CPUs, less memory, smaller displays, and different input devices compared to desktop computers. It also discusses the constrained communication environment of wireless networks, including less bandwidth, more latency, less connection stability, and less predictable availability compared to wired networks. 2. It describes the Wireless Application Environment (WAE) micro-browser environment, which includes the Wireless Markup Language (WML), WMLScript scripting language, Wireless Telephony Application (WTA) for telephony services, and content formats like images and phone book records. 3. The main elements of the WAE model are WAE user agents that interpret network content

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Mobile Computing
BT0086 Part-2
By Milan K Antony

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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.

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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

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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

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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

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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.

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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;

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 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.

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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

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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

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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

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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

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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.

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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

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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.

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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

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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

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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 nodes
are 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

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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.
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