Lecture 9: Data Link Layer & Ethernet Flashcards
(19 cards)
What is the primary role of the Data Link Layer in the OSI model?
The Data Link Layer enables communication between NICs on connected devices. It allows upper layers to access the physical medium, encapsulates Layer 3 packets (IPv4/IPv6) into Layer 2 frames, and performs error detection and frame rejection when corruption occurs. Its Protocol Data Unit (PDU) is the frame.
Why is learning about the Data Link Layer important?
It provides foundational knowledge for network communications.
It’s essential for careers in networking.
It is covered in the midterm exam.
It helps you understand how devices on a local network interact physically and logically.
What are the two sublayers of the Data Link Layer and their functions?
Logical Link Control (LLC): Interfaces between Layer 3 and hardware, providing flow and error control.
Media Access Control (MAC): Responsible for frame encapsulation and controlling access to the physical medium.
What are full-duplex and half-duplex communication modes?
Half-duplex: One device can either send or receive at a time. Common in legacy Ethernet hubs and WLANs.
Full-duplex: Devices can transmit and receive simultaneously. Modern Ethernet switches operate in this mode.
What are contention-based access methods?
Access where multiple nodes compete for the medium.
Examples:
* CSMA/CD (Collision Detection): Used in legacy Ethernet (wired).
* CSMA/CA (Collision Avoidance): Used in Wireless LANs (802.11).
Both work in half-duplex mode.
How does CSMA/CD work in Ethernet?
Device listens to check if medium is idle.
If idle, it sends the frame.
If a collision occurs (two devices transmit), both detect it.
Devices wait for a random backoff period and retransmit.
How does CSMA/CA work in WLANs?
Device listens to check if the medium is idle.
Before sending, includes transmission duration.
Other devices defer transmission until the medium is free.
This avoids collisions rather than detecting them.
What are controlled access methods in networks?
- Each node has pre-assigned time slots to transmit.
- Deterministic method.
- Used in older networks like Token Ring and ARCNET.
What fields are in an Ethernet frame and what do they do?
Frame Start/Stop==Delimits the beginning and end of a frame.
Addressing==Contains source and destination MACs.
Type==Identifies Layer 3 protocol (e.g., IPv4).
Control==Provides flow control features.
Data==Actual payload (Layer 3 packet).
Error Detection==Uses CRC to detect transmission errors.
How is the Ethernet MAC address structured?
- A 48-bit hardware address.
- Written in 12 hexadecimal digits (e.g., 00:1A:2B:3C:4D:5E).
- Uniquely identifies each NIC on a LAN.
How does a switch forward frames based on MAC addresses?
- If destination MAC is in MAC table ⇒ Unicast to that port.
- If not found ⇒ Flood frame to all ports except incoming.
- If address is invalid or destination is unknown ⇒ Drop frame.
What are the three Cisco IOS switching methods?
Store-and-Forward switching.
Cut-Through switching.
Fragment-Free switching.
Store-and-Forward switching
This method receives the entire frame, checks it for errors using CRC (Cyclic Redundancy Check), and only forwards it if the frame is valid. It ensures high accuracy and is used in environments where error checking and quality of service (QoS) are important.
Cut-Through switching
This method begins forwarding the frame as soon as the destination MAC address is read, without waiting for the entire frame or performing any error checks. It offers lower latency but may forward corrupt frames.
Fragment-Free switching
This is a compromise between the above two methods. It checks only the first 64 bytes of the frame (where most collisions and errors occur). If no issues are found, the frame is forwarded. This provides moderate error checking with reduced latency compared to store-and-forward.
When is Store-and-Forward preferred in networks?
When Quality of Service (QoS) is needed, e.g., Voice over IP (VoIP). It ensures only error-free frames are forwarded, preventing bandwidth waste.
What is ARP (Address Resolution Protocol)?
Maps a known IP address to an unknown MAC address.
Uses broadcast requests.
Typically causes the first ping to fail due to lookup delay.
What is ARP spoofing and why is it a security risk?
Attacker sends forged ARP replies to associate their MAC with the IP of another device (e.g., default gateway).
Can be used to intercept traffic or perform man-in-the-middle (MITM) attacks.
What are examples of data link protocols other than Ethernet?
- 802.11 (Wireless LANs)
- PPP (Point-to-Point Protocol)
- HDLC (High-Level Data Link Control)
- Frame Relay
Each has its own frame format and media access control method.
