Ethernet. Метод CSMA/CD | Курс "Компьютерные сети"

Ethernet Access Method: CSMA-CD

Introduction to Ethernet

• The lecture is presented by Andrey Sazykhin, focusing on Ethernet and the CSMA-CD access method.

• Ethernet is currently the dominant technology for wired local area networks (LANs), alongside wireless technologies like Wi-Fi.

• The discussion centers on classic Ethernet, which utilizes a shared medium for data transmission.

Network Topology

• The network topology discussed is a bus topology, which can be either a physical bus cable or a star configuration using a hub.

• In this setup, any data sent into the shared Ethernet medium is visible to all connected devices.

Collision Management in Shared Medium

• Only one computer should transmit data at any given time; simultaneous transmissions lead to collisions and data loss.

• A collision occurs when signals from multiple computers overlap, making it impossible to read any of the messages.

CSMA-CD Explained

• To prevent collisions, an access control method called Carrier Sense Multiple Access with Collision Detection (CSMA-CD) is employed.

• The name consists of three parts:

• Multiple Access: Several computers share the same medium.

• Carrier Sensing: Computers check if the medium is free before transmitting.

• Collision Detection: Mechanism to identify if two computers transmitted simultaneously.

Detailed Breakdown of CSMA-CD Components

Multiple Access

• This part emphasizes that multiple computers utilize a shared environment for communication.

Carrier Sensing

• Computers only transmit when they detect that the medium is free; they listen for carrier signals indicating ongoing transmissions.

• A carrier signal represents the fundamental frequency used for data transmission; changes in this signal indicate active communication from another device.

Collision Detection

• If two devices transmit simultaneously, they will detect a collision by comparing sent and received signals.

• Upon detecting a collision, devices stop transmitting and send out a "jam" signal to inform others about the collision occurrence.

Operational Phases of CSMA-CD

Transmission Period

• During this phase, one computer successfully accesses the shared medium and transmits its data.

Idle Period

• This period occurs when no devices are transmitting; it indicates that the medium is free.

Contention Period

• This phase arises when multiple computers attempt to transmit simultaneously leading to potential collisions until one gains exclusive access again.

Data Frame Transmission Process in Classic Ethernet

Frame Structure and Synchronization

• Before sending actual data frames, computers first send a preamble consisting of alternating bits for synchronization between sender and receiver.

Understanding Ethernet Frame Transmission and Collision Management

Ethernet Frame Processing

• All computers in a shared environment read the frame and store data in their buffer. They first check the first 6 bytes of the frame for the recipient's MAC address.

• A special mode exists where network adapters accept all frames regardless of their intended MAC address, useful for network diagnostics.

• After transmission, a gap (inter-frame interval) is maintained to prevent monopolization of the channel by one computer.

Collision Detection and Handling

• When multiple computers attempt to transmit simultaneously, a competition period occurs, leading to potential collisions.

• Upon detecting a collision, computers pause and retry transmission after waiting for a random time to avoid repeated collisions.

Exponential Backoff Algorithm

• The wait time is determined by multiplying a random number l by 512-bit intervals; this interval varies based on network technology.

• The range from which l is chosen increases exponentially with each failed attempt at transmission, starting from 0 up to 2^n - 1 , where n is the attempt number.

Limitations of Classic Ethernet

• As more attempts are made due to high traffic, delays increase exponentially, significantly reducing data transfer speeds.

• Classic Ethernet struggles with scalability; it operates effectively only with around 30 active computers generating less than 30% load on shared media.

Security Concerns and Delivery Variability

• Security risks arise as all data in shared environments can be intercepted by any connected computer.

• Variable frame delivery times complicate real-time traffic management (e.g., voice or video streaming).

Conclusion on CSMA/CD Methodology

• The classic Ethernet uses CSMA/CD for medium access control but suffers from significant drawbacks like poor scalability and low security.

Introduction to Switched Ethernet Technology

Overview of Switched Ethernet

• To address existing network issues, developers introduced a new technology called "Switched Ethernet," which eliminates the shared medium used in traditional Ethernet.

• In Switched Ethernet, point-to-point connections are utilized instead of a shared environment, enhancing data transmission efficiency.

• The key device that enables this approach is known as a "switch," which plays a crucial role in managing network traffic.