Chapter 2: Term-End Examination, December- 2014
MCS-022: OPERATING SYSTEM CONCEPTS AND NETWORKING MANAGEMENT
Q.1. (a) Explain the collision avoidance mechanism of CSMA/CD. Also, differentiate between CSMA/CD and token passing access methods. 8
Answer:
Collision Avoidance Mechanism of CSMA/CD (Carrier Sense Multiple Access with Collision Detection):
CSMA/CD Overview: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is a network protocol used in Ethernet networks to manage access to the shared communication medium. The primary goal of CSMA/CD is to avoid collisions and maximize the efficient use of the network.
Collision Avoidance Mechanism: The collision avoidance mechanism in CSMA/CD involves the following steps:
1. Carrier Sensing:
• Before transmitting data, a device using CSMA/CD listens to the network to determine if it is idle. It checks for the presence of a carrier signal, indicating ongoing transmissions by other devices.
2. Transmission Initiation:
• If the network is idle, the device initiates the transmission of its data. It starts sending the frame onto the network.
3. Collision Detection:
• While transmitting, the device continues to monitor the network for any collision. Collisions can occur when two or more devices attempt to transmit simultaneously, leading to a jumbled signal.
4. Jam Signal:
• If a collision is detected, the transmitting devices stop transmitting and send a jam signal to alert other devices about the collision. The jam signal ensures that all devices on the network are aware of the collision.
5. Backoff and Retransmission:
• After sending the jam signal, the devices that experienced the collision enter a random backoff period before attempting retransmission. This randomization reduces the likelihood of collisions happening repeatedly.
6. Retry Mechanism:
• The devices attempt retransmission after the backoff period. If the network remains busy, the devices may go through multiple backoff and retry cycles until successful transmission.
Difference Between CSMA/CD and Token Passing Access Methods:
CSMA/CD (Carrier Sense Multiple Access with Collision Detection):
1. Nature:
• CSMA/CD is a contention-based access method, where devices contend for access to the shared network medium.
2. Collision Handling:
• CSMA/CD uses collision detection and resolution. If a collision occurs, devices involved in the collision stop transmitting, send a jam signal, and then enter a backoff period before retransmission.
3. Topology:
• CSMA/CD is commonly used in bus and star topologies, where multiple devices share a common communication medium.
4. Efficiency:
• As the number of devices on the network increases, the probability of collisions also increases, potentially impacting the efficiency of the network.
Token Passing:
1. Nature:
• Token passing is a deterministic access method, where devices take turns accessing the network based on a token that circulates the network.
2. Collision Handling:
• Token passing inherently avoids collisions because only the device holding the token can transmit. There is no contention for access, eliminating the need for collision detection.
3. Topology:
• Token passing is commonly used in ring topologies, where devices are connected in a closed-loop, and a token circulates sequentially.
4. Efficiency:
• Token passing can be more efficient in certain scenarios as it avoids contention and collisions. However, the efficiency may decrease as the network size increases due to the time it takes for the token to circulate.
In summary, CSMA/CD and token passing represent different approaches to managing access to a shared network medium. CSMA/CD relies on contention and collision detection, while token passing uses a deterministic token circulation method to control access. The choice between these methods depends on factors such as network topology, scalability requirements, and the likelihood of collisions in the network.
(b) Describe the concept and advantages of using EFS services in Windows 2000. &nbs
