Question

In a network based on the bus topology, the bus is a nonshareable resource for which the machines must compete in order to transmit messages. How is deadlock (see the optional Section 3.4) controlled in this context?

Short answer

Deadlock is controlled using CSMA with collision detection and backoff mechanisms.

Step-by-step solution

Understanding the Bus Topology

In bus topology, all devices share a common communication line or bus. Each device listens on the bus for data packets and has the capability to transmit messages onto the bus. However, only one device can use the bus at any given time. Thus, controlling access to this resource is crucial to avoid a bottleneck.

Identifying Potential Deadlock

Deadlock can occur if two or more machines simultaneously attempt to transmit on the bus. This situation can result in a standstill where no message is successfully sent, and each machine waits indefinitely for the bus to become free.

Implementing Carrier Sense Multiple Access (CSMA)

One common method to control deadlock in a bus topology is by using Carrier Sense Multiple Access (CSMA). In this method, a device wishing to transmit senses the bus for any ongoing traffic. If the bus is free, the device proceeds to transmit its message; if not, it waits for the bus to become free to avoid collision.

Using Collision Detection and Backoff Algorithm

To further control deadlock, a collision detection mechanism can be incorporated, known as CSMA/CD (Collision Detection). If two devices transmit simultaneously, causing a collision, each device stops sending data, waits for a random backoff time, and then attempts to retransmit. This reduces the chance of continuous collisions and promotes fairness.

Key concepts

Bus Topology

Bus topology is a network configuration where all devices are connected to a single communication line, or "bus." It means that all the data that devices want to send flows via this bus. Imagine it like a single highway where every car (or device) must travel. This setup is simple and cost-effective, which is why it's often used in smaller networks.

In a bus topology, each device checks the bus for data packets. Only one device can send a message at a time, necessitating a control mechanism to prevent data collisions. If two devices attempt to send messages simultaneously, they could collide, resulting in data loss or a network slowdown.

Benefits of bus topology include its ease of implementation and low cost. However, its disadvantages involve limited cable length and slow performance with heavy network traffic as all devices are sharing the same communication line.

Deadlock Prevention

Deadlock refers to a scenario where different devices are waiting endlessly for a resource—in this case, the bus. In bus topology, deadlocks can occur if two or more devices attempt to transmit data simultaneously. To break the deadlock, network protocols must ensure that transmissions are coordinated efficiently.

Preventing deadlock is crucial in maintaining a smooth network operation. Here are some mechanisms used:

• Careful monitoring of the network traffic to ensure that data packets do not collide.
• Protocols that prioritize messages, allowing important data to be transmitted first.
• Implementing wait strategies, like backoff algorithms, to manage data flow.

By applying these strategies, networks can avoid the standstill caused by deadlock, preserving data integrity and communication efficiency.

Carrier Sense Multiple Access

Carrier Sense Multiple Access, or CSMA, is a method used in network communication to avoid collisions. In CSMA, before a device sends a message, it "listens" to the bus for any existing traffic. If the bus is silent, indicating no other device is transmitting, the device proceeds with its communication.

This system greatly enhances the bus topology's efficiency by ensuring that devices do not transmit messages simultaneously. However, it relies heavily on each device's ability to detect whether the channel is clear. There are variations of CSMA, such as CSMA/CD (Collision Detection) and CSMA/CA (Collision Avoidance), each designed to handle network traffic and collision issues differently.

Collision Detection and Backoff Algorithm

The collision detection and backoff algorithm is an extension of CSMA, specifically known as CSMA/CD. This mechanism helps manage situations where, despite the presence of CSMA, collisions may still occur if two devices begin transmission simultaneously.

In collision detection, each device on the network can sense if a collision has occurred while transmitting data. Once a collision is detected, the device stops transmitting and waits for a random "backoff" period before attempting to resend the message. This random wait helps reduce the probability of the same devices colliding again when they resend data.

The backoff algorithm promotes fairness by ensuring that all devices have an equal chance to access the communication line after a collision. It ensures that no single node gets preferential treatment, contributing to organized and efficient network data management.