CS718 - Wireless Networks
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Q & A
Short Question & Answers
Q 1: Briefly note on BCH code and its working.
  • BCH … Discoverer: Bose, Chaudhuri and Hocquenghem.
  • Multiple error correcting codes
  • Generalization of Hamming Code.
  • Flexibility in choice of parameters: Block length, code rate
  • For positive pair of integers m and t, a (n, k) BCH code has parameters:
                Block length: n = 2m – 1
                Number of check bits: n – k <= mt
                Minimum distance:dmin >= 2t + 1
  • Correct combinations of t or fewer errors
  • The generator polynomial can be constructed from the factors of (X2m-1 +1)
  • Q 2: Write down the classes of A/Gb mode of GPRS.
  • 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 – e.g. Expansion cards for laptops.
  • Q 3: How MULTIMEDIA further divides in UMATS types?
      Service classes in UMTS
  • In 3G network, ME will be able to establish multiple connections simultaneously.
  • Network allows efficient cooperation between application with diverse quality of service requirements.
  • The quality can be defined by two main parameters
    Guaranteed and max bit rate possible (kbps)
    Permissible delay (ms)
      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
  • Q 4: Write down a note on HSCSD.
  • 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
  • Q 5: Write brief note on EDGE.
  • EDGE2.75G 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.
  • Q 6: Issues regarding handoff in interSystem.
    If a mobile moves from one cellular system to a different system controlled by a different MSC
      Issues to be addressed
    1. A local call becomes a long-distance call (roaming)
    2. Compatibility between two MSC must be determined
    3. Different systems have different policies and methods for managing handoff requests
    Q 7: Write about QoS profile in GPRS bearer
    QoS Profile for GPRS Bearers
      4 parameters:
    1. Service precedence:       3 classes
    2. Reliability parameter:    3 classes
    3. Delay parameters:          4 classes
    4. 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)
    Q 8: Describe about Call routing in GSM network with Detailed diagram
    Q 9: Ad-Hoc vs Cellular Networks
    Cellular Networks Ad Hoc Wireless Networks
    Fixed infrastructure-based Infrastructure less
    Guaranteed bandwidth (designed for voice traffic) Shared radio channel (more suitable for best-effort data traffic)
    Centralized routing Distributed routing
    Circuit-switched (evolving toward packet switching) Packet-switched (evolving toward emulation of circuit switching)
    Seamless connectivity (low call drops during handoffs) Frequent path breaks due to mobility
    High cost and time of deployment Quick and cost-effective deployment
    Reuse of frequency spectrum through geographical channel reuse Dynamic frequency reuse based on carrier sense mechanism
    Easier to employ bandwidth reservation Bandwidth reservation requires complex medium access control protocols
    Application domains include mainly civilian and commercial sectors Application domains include battlefields, emergency search and rescue operations, and collaborative computing
    High cost of network maintenance (backup power source, staffing, etc.) Self-organization and maintenance properties are built into the network
    Mobile hosts are of relatively low complexity Mobile hosts require more intelligence (should have a transceiver as well as routing/switching capability)
    Major goals of routing and call admission are to maximize the call acceptance ratio and minimize the call drop ratio Main aim of routing is to find paths with minimum overhead and also quick reconfiguration of broken paths
    Widely deployed and currently in the third generation of evolution Several issues are to be addressed for successful commercial deployment even
    Q 10: Why min rate of call is different from traffic density?
    The handover area is usually an intersection of two adjacent and overlapping radio cells generated by the handover hysteresis. Altering the handover hysteresis ∆HO [dB] affects the users at the borders of the two adjacent cells and results in a change of cell dwell time and handover rates. The effect is that some users in the low density radio cell delay their handover to the high density cell, whereas users in the high density cell change earlier to the low density cell. If one handover hysteresis becomes negative, an area is formed where new calls meet the handover criterion and are booked to the neighboring cell immediately. This extreme case leads to a high increase in the handover rate and could also be modeled as an increase in the new call rate in the neighboring cell.

    Course Instructor

    Dr. Ghalib A. Shah
    Ph.D Computer Engineering
    Middle East Technical University, Turkey

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