CS718 Wireless Networks midterm past questions

CS718 Wireless Networks midterm past questions

Question No.1. GPRS modes A/Gb and lu? 

Answer:

A/Gb mode

  • Class A: MS can operate simultaneous packet switched and circuit switched services
  • Class B: MS can operate either one at one time
    • Most common for handsets today
  • Class C: MS can operate only packet switched services
    • g. expansion cards for laptops

Iu mode

  • CS/PS mode: Same as Class A in A/Gb mode
  • PS mode: MS can only operate packet switched services
  • CS mode: MS can only operate circuit switched services

 Question No.2. Note of EDGE? 

Answer:

EDGE (2.75G)

  • More advanced upgrade to 2G that requires addition of new hardware and software
  • Developed as a path to become eventual 3G high speed data access
  • New modulation 8-PSK in addition to GSM standard GMSK.
  • Allows nine different formats known as Multiple modulation and Coding Scheme (MCS)
  • Each MCS state may either use GMSK (low rate) or 8-PSK (high rate).
  • A family of MCS for each GSM slot and users can adaptively determine best MCS setting
  • User start first with max error protection and max data rate until the link has unacceptable outage or delay
  • By combining different channels (multi-carrier trans), EDGE provides upto several megabits per second data throughput.

Question No.3. HLR and VLR description? 

Answer:

  • Home Location Register (HLR)
    • A database used for storage and management of subscriptions.
    • data about subscribers, including a subscriber’s service profile, location information, and activity status.
    • When an individual buys a subscription, he or she is registered in the HLR of that operator.
  • Visitor Location Register (VLR)
    • A database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers.
    • The VLR is always integrated with the MSC.
    • For roaming user, VLR connected to that MSC will request data about the mobile station from the HLR through MSC.

Question No.4. Trunking System concept and description? 

Answer:

  • Trunking
    • Allows a large number of users to share a small number of channels
    • Channel allocated per call basis from a pool of available channels
    • Relies on statistical behavior of users so that a fixed number of channels (circuits) may accommodate a large random user community
    • Trunking theory is used to determine number of channels for particular area (users)
    • Tradeoff between the number of available channels and likelihood of call blocking during peak calling hours
  • Trunking Theory
    • Developed by Erlang, Danish Mathematician, how a large population can be accommodated by a limited number of servers, in late 19th century
    • Today, used to measure traffic intensity
    • 1 Erlang represents the amount of traffic intensity carried by a completely occupied channel
      • e. one call-hour per hour or one call-minute per minute
      • 5 Erlang: Radio channel occupied 30 minutes during 1 hour

Question No.5. QoS of GPRS BEARER

Answer:

  • 4 parameters:
  1. Service precedence
  • 3 classes
  1. Reliability parameter
  • 3 classes
  1. Delay parameters
  • 4 classes
  1. Throughput parameter
  • Maximum and mean bit rates

QoS profile is included in Packet Data Protocol (PDP) context

Negotiation managed through PDP procedures (activation, modification and deactivation)

Question No.6 What is ZigBee Protocol? Explain the diagram of routing in zigbee?

Answer:

  • IEEE 802.15.4 / ZigBee
    • Low power with range of 100m
    • Low rate about 20 kbps

 Question No.6. QOS multimedia UMTS

Answer:

  • Based on the QoS criteria, multimedia services has been further classified
    • Conversational
      • The most delay sensitive, e.g. applications video telephony, VoIP
    • Streaming
      • Flow which is steady and continuous, it is server to user
    • Interactive
      • Web browsing is an example. A user may request timetables of buses, trains or flight schedule
    • Background
      • Short messages, file transfer, email that has least stringent requirements of QoS

 Question No.7 Analog vs digital signal.

  • Analog signal – signal intensity varies in a smooth fashion over time.No breaks or discontinuities in the signal
  • Digital signal – signal intensity maintains a constant level for some period of time and then changes to another constant level

 Question No.8 cell splitting vs cell clustering.

Answer:

CELL SPLITTING

  • Achieve capacity improvement by decreasing R and keeping D/R (cell reuse ratio) unchanged
  • Divide the congested cells into smaller cells
    • Smaller cells are called micro cells
  • If radius of cell is cut to half, approximately four cells would be required
    • Increased number of cells would increase the number of clusters, which in turn increase the capacity
  • Allows a system to grow by replacing larger cells with smaller cells without upsetting the allocation scheme

CELL SECTORING

  • Keep cell radius unchanged and decrease D/R
  • Increases SIR so that cluster size may be reduced
    • SIR is improved using directional antennas
    • Hence increasing frequency reuse without changing transmission power
  • Cell is partitioned  into 3 120o sectors or 6 60o sectors as shown in Fig

MICROCELL ZONES

  • A cell is divided into zones with a single BS sharing the same radio equipment
  • Zones are connected through coaxial cable, fiber optics or microwave links to the BS
  • Superior to sectoring since antennas are placed at outer edges of the cells and any channel may be assigned to any zone by BS
  • As mobile travels from one zone to other, it retains same channel, BS simply switches the channel to a different zone.
  • Co-channel interference is minimized becuase
    • Large BS is replaced by several low powered tx
    • Improves S/I

Question No.9 HSCSD

Answer:

  • Works in circuit switch mode.
  • Speed increased by allowing single user to use consecutive time slots in GSM standard
  • Relaxes error control coding algorithms specified in GSM increasing data rate from 9.600 to 14.400 Kbps
  • By using 4 slots, raw data rate of up to 57.6 kbps to individual user.
  • Ideal for dedicated streaming or real-time interactive web sessions

 Question No.10 write brief note on EDGE 

Answer:

  • GPRS data rates still fall short compared to that promised by 3G.
  • The delay in deployment of 3G technology led to the emergence of  EDGE
  • Phase 1 (Release’99 & 2002 deployment) supports best effort packet data at speeds up to about 384 kbps
  • Phase 2 (Release’2000 & 2003 deployment) will add Voice over IP capability
  • GMSK is used in GPRS, only one bit per symbol is used
  • In EDGE, Octogonal PSK (8-PSK) is used which enables a threefold higher data rate of 59.2 kbps per radio time slot.
    • Achieved by transmitting 3 bits per symbol.
  • GMSK has constant amplitude modulation while 8-PSK has variations in amplitude.
  • This changes the radio frequency characteristics requiring changes in BS.
  • minor changes in hardware and software in existing systems, leads to major changes in network performance.
  • Radio network Planning
    • Coding Scheme: nine modulation and coding schemes (MCS) that provide different throughput as shown in table

Question No.11 Issues regarding handoff in interSystem 

Answer:

    • If a mobile moves from one cellular system to a different system controlled by a different MSC
    • Issues to be addressed
      • A local call becomes a long-distance call (roaming)
      • Compatibility between two MSC must be determined
      • Different systems have different policies and methods for managing handoff requests

Question No.12 What are the reasons of call dropping

Answer:

    • Excessive delay by MSC due to high traffic load
    • ∆ is set too small for handoff time
    • No channels are available on any of nearby BS

 

Question No.13.List GPRS Channels and Describe Logical Channels

Answer:

Physical Channels

  • Defined by timeslot (0-7) and radio frequency channel
  • Shared Basic Physical Sub Channel

Shared among several users (up to 8)

  • Dedicated Basic Physical Sub Channel

One user

  • Packet Data Channel (PDCH)

Dedicated to packet data traffic from logical channels (next slide)

o Control

o User data

Logical Channels

  • Mapped by the MAC to physical channels
  • Control channels for control, synchronization and signaling

Common

Broadcast

Dedicated

  • Packet Traffic channels

Encoded speech

Encoded data

Question No.14 WRITE A BRIEF NOTE ON ENHANCED DATA RATE FOR GSM EVOLUTION (EDGE) 

Answer:

  • More advanced upgrade to 2G that requires addition of new hardware and software
  • Developed as a path to become eventual 3G high speed data access
  • New modulation 8-PSK in addition to GSM standard GMSK.
  • Allows nine different formats known as Multiple modulation and Coding Scheme (MCS)
  • Each MCS state may either use GMSK (low rate) or 8-PSK (high rate).
  • A family of MCS for each GSM slot and users can adaptively determine best MCS setting
  • User start first with max error protection and max data rate until the link has unacceptable outage or delay
  • By combining different channels (multi-carrier trans), EDGE provides upto several megabits per second data throughput.

Question No.15 WHICH ISSUES ARE IDENTIFIED WHEN INTERSYSTEM HANDOFF OCCURS? 

Answer:

  • If a mobile moves from one cellular system to a different system controlled by a different MSC
  • Issues to be addressed
    • A local call becomes a long-distance call (roaming)
    • Compatibility between two MSC must be determined
    • Different systems have different policies and methods for managing handoff requests

 

Question No.16 SPECIFY THE CLASSES OF A/Gb MODE IN GPRS? 

Answer:

A/Gb mode

  • Class A: MS can operate simultaneous packet switched and circuit switched services
  • Class B: MS can operate either one at one time
    • Most common for handsets today
  • Class C: MS can operate only packet switched services
    • g. expansion cards for laptops

Question No.17 Write two drawbacks of ALOHA?

Answer:

user accesses medium as soon as it has a packet ready to transmit

after transmission, user waits a length of time > round-trip delay in the network, for

an ACK from the receiver

if no ACK arrives, user waits a random interval of time (to avoid repeated collision)

and retransmits

Advantages:

simple, no synchronization among users required

Disadvantages:

low throughput under heavy load conditions

probability of collision increases as number of users increases

max throughput = 18% of channel capacity

 

Question No.18 Explain IS-95 reverse link?

Answer:

IS-95 or cdmaOne

Supports up to 64 users that are orthogonally coded

Channel bandwidth is 1.25 MHz

Widely deployed in N. America, Korea, Japan, China, S. America, Australia

Channel data rate is 1.2288 Mchips/s (Mega Chips)

Question No.19 CLASSIFY THE MULTIMEDIA SERVICES BASED ON THE QoS CRITERIA IN UMTS ? 

Answer:

MULTIMEDIA – Video Services

  • 4G wireless systems are expected to deliver efficient multimedia services at very high data rates.
  • Basically there are two types of video services: bursting and streaming video services.
  • Streaming: is performed when a user requires real-time video services, in which the server delivers data continuously at a playback rate.
  • Bursting: is basically file downloading using a buffer and this is done at the highest data rate taking advantage of the whole available bandwidth.

 

Few more questions are:

Q1. Write note on gian signal.
Q2. Write note on DCCH (data control channel)
Q3. Difference between HLR and VLR?
Q4. Write two drawbacks of ALOHA?
Q5. Draw diagram of PDP context activation.

Q4. Why multiplexing is effective?

Q5. what is near-Far effect?

Q6. what are first generation standards?

Q7. what are link adaptation and incremental link. Write benefits?

Q8. write walsher?
a) 2×2
b) 4×4

Q9. Difference between 802.11a vs 802.11b.

Q10. Ad-hoc vs. Cellular network.

Q11. S-MAC drawback in WSN

Q12. Overview of WiMax

Q13. Traffic Data access in WCDMA

Q14. characteristic of ad-hoc and future challenges

Q15. Factors involving in Sensor network designing

Q16. Overview of WIMAX

Q17. WPAN support Contention based and contention free access, Write a Algorithm for both.

Q18. Write a brief note on High Speed Circuit Switch Data (HSSD)?
Q19. WHy the mean rate of Calls in different from the traffic intensity of the cellular network?
Q20. Write a brief note on Supervisory signals generated in Base Station?
Q21. Give 5 Limitation of the 3G Network.
Q22. Give the system Overview of AMPS. How many kinds of channels include in Base Station