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NOV/DEC-'09/IT1402-Answer Key

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Nov/Dec 2009

Part-A

1. What are the main problems of signal propagation?

Ans:Power additionally influenced by

o fading (frequency dependent)o shadowingo Reflection at large obstacleso Refraction depending on the density of a mediumo scattering at small obstacleso Diffraction at edges

2. Give the use of SDMA.

Ans: Space Division Multiple Access (SDMA) is used for allocating separatedspaces to users in wireless networks. The basis for the SDMA algorithm is

formed by cells and sectorized antennas which constitute the infrastructure

implementing space division multiplexing (SDM)

3. Define GPRS.

Ans: The General Packet Radio Service provides packet mode transfer forapplications that exhibit traffic patterns such as frequent transmission of small

volumes.

4. What do you mean by DAMA?Ans:Demand Assigned Multiple Access Channel efficiency only 18% for Aloha,36% for Slotted Aloha (assuming Poisson distribution for packet arrival andpacket length).Reservation can increase efficiency to 80%

5. What are the various versions of a physical layer defined in IEEE 802.11

standards?

Ans: IEEE 802.11-83.5 MHz

IEEE 802.11a -300 MHz

IEEE 802.11b 83.5 MHzIEEE 802.11g - 83.5 MHz

6. List the three low power states to save battery power in a Bluetooth device.

Ans: PARK state

HOLD state

SNIFF state7. What are the two kinds of entities in mobile IP?

Ans: Two kinds of entities

Foreign network

Home network

8. Define Generic Routing Encapsulation.

Ans: GRE allows the encapsulation packets of one protocol suite into the payload

portion of another protocol suite.

www.Vidyarthiplus.com


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9. What are the key elements of the WAP specification?

Ans:

Networks and Network Bearers

TCP/IP as Transport Protocol

Processors

10.What are the goals of WTLS layer?

Ans: It provides the upper-level layer of WAP with a secure transport service

interface that preserves the transport service interface below it. In addition, WTLS

provides an interface for managing (e.g., creating and terminating) secureconnections. It provides functionality similar to TLS 1.0 and incorporates

additional features such as datagram support, optimized handshake and dynamic

key refreshing.

Part-B

11.a) i) Discuss the advantage and disadvantage of cellular system with small

cells.[6 Marks]

Ans: Cellular Network Organization

Areas divided into cells

Each served by its own antenna(s)

Band of frequencies allocated

Neighbors are equidistant

Architecture

PSTN

MTSO

Base Station and AntennaCellular Concept

Limited number of frequencies => limited channels

Single high power antenna => limited number of users

Smaller cells => frequency reuse possible => more number of

users

Base stations (BS): implement space division multiplex

Each BS covers a certain transmission area (cell)

Each BS is allocated a portion of the total number of channels

available

Cluster: group of nearby BSs that together use all available

channels

Mobile stations communicate only via the base station

FDMA, TDMA, CDMA may be used within a cell

As demand increases (more channels are needed)

Number of base stations is increased

Transmitter power is decreased correspondingly to avoidinterference



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Advantages:

Higher capacity, higher number of users

Less transmission power needed

More robust, decentralized

Base station deals with interference, transmission area etc. locally

Disadvantages:Fixed network needed for the base stationsHandover (changing from one cell to another) necessaryInterference with other cells: co-channel, adjacent-channelCellular Architecture

Each tower serves one cell Different frequencies

Handoff calls

Each cell is served by a base station (BS)

Each BS is connected to a mobile switching center (MSC)through fixed links

Each MSC is connected to other MSCs and PSTN



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11.a) ii) Explain briefly the types of spread spectrum techniques. [10 Marks]

Ans:

Types of spread spectrum

DSSS (Direct Sequence Spread Spectrum) IFHSS (Frequency Hopping Spread Spectrum) I



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11.b) i) Compare TDMA, FDMA and CDMA. [6 Marks]

Ans:



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11.b) ii) Explain the different types of phase shift key modulation. [10 Marks]

Ans:



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12)a) Describe the system architecture of a GSM. [16 Marks]

Ans:



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12)b) i) Write a note on LEO satellite system. [8 Marks]

Ans: Leo circulate on a lower orbit

It exhibit a much shorter period

It will only be visible from the earth for around ten minutes.Used for low bandwidth services.

Advantage:

Using advanced compression scheme.For voice communication.

Using low transmit power in the range of 1W

Smaller footprint for better frequency reuse.Disadvantage:

Need for many satellites if global coverage is to be used.

More satellites in orbit.Short life time (5 8 years)

12)b) ii) Explain the transport mechanism of DAB [8 Marks].

Ans:

Digital Audio Broadcasting:

Introduction (2)

Two basic transport mechanism (2)

Main service channel

Fast information channelProgram associated data (1)

DAB frame structure (2)Each frames are divided into three frames (3)

Synchronization channelFast information channel

Main service channelComponents of a DAB senderMultimedia object transfer protocol



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13 a) i) Explain the basic architecture of an IEEE 802.11 MAC data frame.

[8 Marks]



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13)a) ii) Discuss briefly the HyperLAN2. [8 Marks]

Ans:



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13)b) Explain about the IEEE 802.11b standard. [16 Marks]

Ans:



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14)a) i) Discuss about the IP in IP encapsulation. [8 Marks]

Ans: IP in IP Encapsulation:Encapsulation format (4)Fields are, (4)

Version

Internet Header LengthLength

IP Identification

Fragment offset

Time to Live

IP checksum

IP address of HA

Care of Address of COA

IP address of CN

IP address of MN

14 a) ii) Explain the Dynamic Source Routing. [8 Marks]Ans: Split routing into discovering a path and maintaining a path

Discover a path:

Only if path for packets to a certain destination is needed and no path

available

maintaining a path:Only while path is in use one has to make sure that it can be used

continuously

No periodic updates needed!

Path discovery:

Broadcast a packet with destination address and unique ID

If a station receives a broadcast packet If the station is the receiver (i.e., has the correct destination

address) then return the packet to the sender (path was collected in

the packet)

Ifthe packet has already been received earlier (identified via ID)then discard the packet Otherwise, append own address and

broadcast packet Sender receives packet with the current path

(address list)

Optimizations:

Limit broadcasting if maximum diameter of the network is known

Caching of address lists (i.e. paths) with help of passing packets:

Stations can use the cached information for path discovery (ownpaths or paths for other hosts)

Maintaining paths:

After sending a packet:

Wait for a layer 2 acknowledgement (if applicable)

Listen into the medium to detect if other stations forward the

packet (if possible)

Request an explicit acknowledgement



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If a station encounters problems it can inform the sender of apacket or look-up a new path locally

14 b) Explain the Dynamic Host Configuration Protocol. [16 Marks]

Ans:



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15)a) i) Explain the concept of Snooping TCP. [8 Marks]

Ans:

15 a) ii) Explain the WAE logical model. [8 Marks]

Ans: Goals

Device and network independent application environment for low-bandwidth, wireless devices

considerations of slow links, limited memory, low computing power,

small display, simple user interface (compared to desktops)



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Integrated Internet/WWW programming modelHigh interoperability

WAE Logical mode:

15 b) Explain the architecture of WAP 2.0 [16 Marks]

Ans: Wireless Application Protocol (WAP)_ Empowers mobile users with wireless devices

_ A standard created by wireless and Internet companies

wapforum.org:

Co-founded by Ericsson, Motorola, Nokia, Phone.com

Goals

Deliver Internet services to mobile devices

Enable applications to scale across a variety of transportoptions and device types

Independence from wireless network standards

GSM, CDMA IS-95, TDMA IS-136, 3G systems (UMTS,WCDMA)



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IT1402 Nov Dec 2009 Answer