Канальный уровень | Курс "Компьютерные сети"

Introduction to the Channel Layer in Telecommunications

Overview of the Channel Layer

• The lecture introduces the channel layer as part of a telecommunications course, following the physical layer.

• Key tasks of the channel layer include message transmission over communication channels and defining message boundaries within bit streams.

Functions of the Channel Layer

• The channel layer addresses two main issues: addressing (identifying target devices on a shared channel) and coordinated access (preventing data collisions when multiple devices transmit simultaneously).

• It emphasizes that while physical layers handle bit streams, the channel layer focuses on transmitting complete messages or frames.

Frame Formation at the Channel Level

Structure of Frames

• The process begins with two computers (Host 1 and Host 2), where data from a higher network level is received by the channel level.

• Each frame consists of a header, payload (network packet), and trailer, which are crucial for identifying frames during transmission.

Methods for Frame Boundary Detection

• Several methods exist to determine frame boundaries, including byte count indicators and byte stuffing techniques.

• A simple method involves prefixing each frame with its length; however, this can lead to errors if lengths are corrupted during transmission.

Advanced Techniques for Frame Delimitation

Byte Stuffing Methodology

• Byte stuffing adds special sequences at both ends of frames to indicate start and end points. This was used in older protocols like BIOS.

• Escape sequences help differentiate between control characters and actual data by prefixing them with specific escape codes.

Bit Stuffing Approach

• Modern protocols like HDLC utilize bit stuffing, adding specific patterns before or after certain bits to prevent misinterpretation as control signals.

Ethernet Technology and Frame Transmission

Ethernet Frame Structure

• In classic Ethernet technology, an 8-byte preamble indicates frame beginnings through alternating bits.

• Fast Ethernet employs redundant coding schemes that allow error detection while using non-significant symbols for signaling frame starts/ends.

Error Detection Mechanisms

Error Detection and Correction in Data Transmission

Error Detection Techniques

• Channel-level technologies utilize error detection, where erroneous frames are simply discarded without attempts to recover data.

• More complex mechanisms involve error correction, which requires additional information to identify and rectify errors using special error-correcting codes.

Error Correction Methods

• One method of error correction during data transmission is the retransmission of erroneous frames. This occurs when the sender resends data after an error is detected by the receiver.

• The process involves a sender transmitting messages, with the receiver confirming receipt. If an error occurs or confirmation fails, the sender will resend the message after a timeout period.

Retransmission Strategies

• There are two main retransmission methods:

• Stop-and-Wait: The sender transmits one frame and waits for acknowledgment before sending the next.

• Sliding Window: The sender can transmit multiple frames before needing acknowledgment for earlier ones, improving efficiency on high-speed channels.

Contextual Application of Error Handling

• Practical experience shows that simple error detection suffices for low-error environments (e.g., wired connections), while frequent errors (e.g., Wi-Fi) necessitate more robust detection and correction at the channel level.

Layered Architecture in Networking

• The OSI model was initially designed for point-to-point communication but evolved to address new challenges posed by shared media access among multiple devices.

• This led to a division of the channel layer into two sub-layers:

• Logical Link Control (LLC): Manages data transfer, framing, and error handling across various technologies.

• Media Access Control (MAC): Governs access control in shared environments.

Addressing Shared Medium Challenges

Understanding Access Control in Shared Environments

Collision and Medium Access Control

• Data cannot be read from the medium due to collisions; this is managed by the Medium Access Control (MAC) layer, which ensures that only one sender uses the communication channel at a time to prevent data loss.

Methods of Managing Access

• There are randomized methods for managing access in shared environments. For instance, if 'm' devices are connected, one device is randomly selected for data transmission with a probability of 1/n.

Token-Based Approach

• Another method involves defining rules for medium usage, such as in Token Ring technology where only the device holding the token can transmit data. After sending its data, it passes the token to another device.

Efficiency vs. Cost

• While token-based approaches can lead to more efficient bandwidth usage, they require more expensive equipment. Consequently, randomized methods have become more prevalent in practice.

Current Technologies in Use

• Currently, two main technologies dominate: Ethernet and Wi-Fi. Older technologies like Token Ring and FDDI are not studied as they are no longer widely used.

The Role of the Data Link Layer

Overview of Data Link Layer Functions

• The Data Link Layer is responsible for transmitting entire messages called frames rather than just streams of bits. It also handles error detection and correction.

Sub-layers of Data Link Layer

• The Data Link Layer consists of two sub-layers:

• Logical Link Control (LLC): Responsible for framing and error correction.

• Media Access Control (MAC): Ensures coordinated access to shared communication channels.

Technologies Covered