(SEM VIII) THEORY EXAMINATION 2017-18 MOBILE COMPUTING

SECTION A (Brief Explanations)

Roaming
Roaming is the ability of a mobile user to continue communication services while moving from one network area to another. It allows a subscriber to use services outside the home network through agreements between operators.

Collisions on PHY and MAC Layer
At the physical layer, collisions occur when two signals overlap and interfere during transmission. At the MAC layer, collisions are related to medium access control when multiple devices attempt to transmit at the same time, causing data corruption and retransmission.

Directional Antenna in Mobile Phones
Directional antennas are generally not suitable for mobile phones because users move frequently and signals must be received from all directions. Mobile phones use omnidirectional antennas to maintain connectivity while moving.

Advantages of IPv6 for Mobility
IPv6 improves mobility by providing larger address space and built-in support for Mobile IP. It reduces the need for NAT and simplifies routing, making mobile communication more efficient.

Relation between GSM and GPRS
GSM is a circuit-switched network mainly used for voice communication, while GPRS is an extension of GSM that provides packet-switched data services. GPRS enables internet access over GSM networks.

Mobile IP
Mobile IP is a protocol that allows a mobile device to maintain its IP address while moving between different networks. It ensures continuous internet connectivity without changing the permanent IP address.

Disadvantages of M-TCP
M-TCP suffers from higher delay due to frequent disconnections and handoffs. It also increases overhead because of additional control mechanisms and may reduce throughput in unstable wireless links.

Fisheye State Routing
Fisheye State Routing is a routing protocol where detailed routing information is maintained for nearby nodes, while less detailed information is kept for distant nodes. This reduces routing overhead in large networks.

Hidden Node and Exposed Node Problems
The hidden node problem occurs when two nodes cannot hear each other but transmit to the same receiver, causing collision. The exposed node problem occurs when a node unnecessarily stops transmission because it senses another transmission, even though it would not cause interference.

HLR and VLR in GSM
The Home Location Register stores permanent subscriber information, while the Visitor Location Register temporarily stores data of users currently in a specific area. Together they manage user mobility and authentication.

SECTION B (Brief but Clear)
 

Generations of Wireless Systems
1G systems were analog and mainly supported voice calls. 2G introduced digital communication and better voice quality along with SMS services. 3G enabled high-speed data services like video calling and internet access. Each generation improved speed, capacity, and service quality.

Effect of Mobility on Data Replication
Mobility creates challenges in maintaining consistent data copies across devices. Frequent disconnections may cause outdated data or synchronization issues. Special replication strategies are used to maintain data consistency in mobile platforms.

Why CSMA/CD is Not Suitable for Wireless LAN
CSMA/CD is not suitable because wireless nodes cannot detect collisions effectively while transmitting. Due to signal fading and hidden node problems, collision detection becomes difficult, so CSMA/CA is used instead.

Security Threats in Mobile Communication
Mobile communication faces threats such as eavesdropping, impersonation, data modification, and denial of service. Authentication ensures only authorized users access services, while encryption maintains privacy during data transfer.

DSDV vs AODV
DSDV is a proactive routing protocol that maintains routing tables continuously, while AODV is reactive and discovers routes only when needed. Proactive protocols reduce delay but increase overhead, whereas reactive protocols reduce overhead but may cause initial delay.
 

SECTION C (Brief Explanation)
 

SDMA, TDMA, FDMA, CDMA
These are multiple access techniques used in wireless communication. FDMA separates users by frequency bands, TDMA separates by time slots, CDMA separates by unique codes, and SDMA separates users based on spatial location. Each technique improves capacity in different ways.

Characteristics of MANET and DSR
A MANET is a self-configuring network without fixed infrastructure. Nodes act as routers and move freely. In DSR, path discovery finds routes only when needed, and path maintenance ensures broken links are repaired dynamically.

Frequency Reuse
Frequency reuse allows the same frequency band to be used in different cells separated by sufficient distance. This increases system capacity without increasing bandwidth and improves spectrum efficiency.

Agent Advertisement in Mobile IP
Agent advertisement informs mobile nodes about available foreign agents. It contains network information and care-of addresses that help mobile nodes register in foreign networks.

Ad Hoc vs Other Networks
Ad hoc networks do not rely on fixed infrastructure and allow direct communication between nodes. They are flexible and useful in disaster or military operations but face challenges like limited bandwidth and security risks.

Mobile IP Entities and Data Transfer
Mobile IP includes a mobile node, home agent, and foreign agent. When a mobile node moves, the home agent forwards packets to the foreign agent, which delivers them to the mobile node. This ensures continuous communication.

WAP Security Layer
WAP defines its own security layer because traditional internet security protocols are heavy for mobile devices. However, it may cause end-to-end security gaps where data is decrypted at intermediate gateways.

Fast Retransmit and Fast Recovery in Mobile TCP
Fast retransmit resends lost packets quickly without waiting for timeout. Fast recovery avoids slow start and maintains transmission efficiency after packet loss, improving performance in wireless environments.