07-11-2016, 03:09 PM
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1. How can you solve cache consistency problem (5 points)
Considering the cache consistency in mobile environment is a challenging task due to frequent disconnections and mobility of mobile users. Typical mechanisms have strong consistency. Below are some issues:
• Mobile users may be disconnected for long duration => invalidations may be lost
• Upon reconnection mobile clients will have to revalidate their cache (wastes energy and bandwidth).
Solution: Weak consistency
• Tolerate occasional inconsistencies
• Apply conflict resolution strategies subsequently
2. Compare and contrast hoarding with prefetching? (4 points)
• Hoarding is a technique to reduce the cost of cache misses during disconnection.
Prefetching is an ongoing process that transfers to the cache soon ¬to ¬be ¬needed files during periods of low network traffic. Since prefetching is continuously performed, in contrast to hoarding, keeping its overhead low is important.
• Hoarding is more critical than prefetching, since during disconnections, a cache miss cannot be serviced. Thus, hoarding tends to overestimate the client's need for data. (although, excessive estimations cannot be satisfied)
• Prefetching objective is to improve performance (throughput or response time). Hoarding Objective is to fetch all needed data into MU cache prior to disconnect, thus the goal is to facilitate disconnected operation.
• Prefetching makes use of idle time (either of the network or of the CPU) or periods of high-bandwidth connectivity to retrieve objects that an application may require in the future. Hoarding on the other hand is usually a bulk operation, done to enable mobile clients to continue operating while they are disconnected.
3. List the advantages and problems of Cellular concepts. (4 points)
Answer: The cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity. It offered very high capacity in a limited spectrum allocation without any major technological changes. Cellular concept is a system level idea to replace a single high power transmitter with many low power transmitters.
Advantages of cellular concepts:
a) Lower power handsets
b) Increases system capacity with frequency reuse
c) Higher capacity, higher number of users
• More frequent resource utilization increases the capacity
d) Less transmission power
• Reduced cell sizes, less power needed to cover the cell area
• Relaxed power amplifier specs at base stations
• Longer life-time for mobile station batteries
e) More robust, decentralized
• In case that one cell is down, overlapping of cells guarantees that a mobile is able to get connected through other base stations
f) Localized interference
• Due to smaller service areas of cells, interference is as well localized to a smaller area
g) No technological challenges in deployment
• Major problems related to minimizing the implementation and operational expenses of the system
• Technological challenges related to capacity improvement methods
Problems of cellular concepts:
a) Costs of cells
b) Handoffs between cells must be supported
c) Need to track user to route incoming call/message
d) Fixed network needed for the base stations
e) Interference with co-channel, adjacent-channel
4. List the steps of mobile phone assisted Hand-off. (4 points)
Answer: Mobile assisted handoff is a process that is used to allow a mobile phone to assist the base station in the decision to transfer the call to another base station.