modul7-Ethernet Encapsulation and OSI Layer 2

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Questions and Answers

What does the Frame Check Sequence (FCS) in an Ethernet frame primarily provide?

  • Error detection for corrupted data (correct)
  • Synchronization of frame transmission
  • Protocol identification for higher layers
  • Media access control for half-duplex systems

Which statement accurately describes the MAC layer's responsibilities in Ethernet?

  • It handles error correction and retransmission of frames.
  • It manages all layer 3 and layer 4 protocols.
  • It provides addressing, framing, and error detection functionalities. (correct)
  • It governs the physical media access characteristics exclusively.

When dealing with Ethernet frames, what does a value of ≤ 1500 in the Type/Length field indicate?

  • The maximum permissible frame size.
  • The length of the Ethernet payload size. (correct)
  • The type of Ethernet frame being used.
  • The protocol stack being utilized.

What is the minimum valid frame size for Ethernet as specified?

<p>64 bytes (D)</p> Signup and view all the answers

In Ethernet, which addressing scheme is used to uniquely identify network interface cards (NICs)?

<p>48-bit logical addresses (D)</p> Signup and view all the answers

What purpose does the Preamble and Start Frame Delimiter (SFD) serve in an Ethernet frame?

<p>To assist in frame synchronization (D)</p> Signup and view all the answers

What mechanism does half-duplex Ethernet use to avoid collisions?

<p>Carrier Sense Multiple Access with Collision Detection (CSMA/CD) (B)</p> Signup and view all the answers

Which of the following is NOT a field in an Ethernet frame?

<p>Error Check Sum (A)</p> Signup and view all the answers

Flashcards

What is Ethernet?

A family of networking technologies operating at layers 2 (Data Link) and 1 (Physical) of the OSI model.

What is LLC (Logical Link Control)?

A sublayer in the Data Link layer that communicates with upper layer protocols like IPv4 and IPv6.

What is MAC (Media Access Control)?

A sublayer in the Data Link layer responsible for Ethernet framing, addressing, and error detection.

What is Data Encapsulation?

The process of putting data into a specific format before transmitting it.

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What is a MAC Address?

A unique identifier used to address network devices at the MAC layer.

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What is Error Detection?

A method used to detect and prevent data corruption during transmission.

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What is Media Access Control?

A method used by devices on a network to share the transmission medium.

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What is the minimum Ethernet frame size?

The smallest allowed size for an Ethernet frame.

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Study Notes

Ethernet Encapsulation and OSI Layer 2 Operation

  • Ethernet operates at both Layer 2 (Data Link) and Layer 1 (Physical) of the OSI model.
  • It's defined by IEEE 802.2 (LLC) and IEEE 802.3 (MAC + physical).
  • LLC (Logical Link Control): IEEE 802.2, communicates with upper-layer protocols (like IPv4, IPv6). Identifies the protocol enclosed in the frame.
  • MAC (Media Access Control): IEEE 802.3, handles Ethernet framing, addressing (source/destination MAC addresses), and error detection (FCS). Provides media access methods (e.g., CSMA/CD for half-duplex).

MAC Sublayer Functions

  • Data Encapsulation: Creates the Ethernet frame structure (header + trailer).
  • MAC Addressing: Uses unique 48-bit source and destination addresses for each network interface card (NIC).
  • Error Detection: Includes a Frame Check Sequence (FCS) to detect corrupted data using cyclic redundancy checks (CRC).
  • Media Access Control:
    • Half-Duplex: Uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection) to prevent collisions on shared media (e.g., legacy hubs).
    • Full-Duplex: Modern switches typically operate full-duplex, not using CSMA/CD.

Ethernet Frame Fields

  • Preamble & SFD (Start Frame Delimiter): 8 bytes for synchronization (not part of the frame size).
  • Destination MAC Address: 6 bytes.
  • Source MAC Address: 6 bytes.
  • Type/Length: 2 bytes. Indicates the protocol type (≥ 1536, e.g., 0x0800 for IPv4) or the payload length (≤ 1500).
  • Data (Payload): 46 to 1500 bytes.
  • Frame Check Sequence (FCS): 4 bytes for error detection (CRC).
  • Minimum Frame Size: 64 bytes (from destination MAC to FCS).
  • Maximum Frame Size: 1518 bytes (from destination MAC to FCS).
  • Smaller Frames: Frames below 64 bytes are "runt" frames and considered invalid.
  • Larger Frames: Frames above 1518 bytes are "giant" frames (unless jumbo frames are supported).

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