CS331 · Chapter 4 · Annotated SlidesLocal Area Networks: Ethernet
Original
Overview

Local Area Networks: Ethernet

Ethernet from top to bottom: IEEE 802 and its LLC/MAC split, every field of the frame and its size, the 48-bit MAC address and how to tell unicast, multicast and broadcast apart, the four generations from 10 Mbps to 10 Gbps, and why full-duplex switched Ethernet no longer needs CSMA/CD. (On the Slides page this deck is listed as Data Link Layer Part 3.)

How to read this page

Every original slide is shown unchanged. Yellow marks on a slide point to a numbered note: hover or tap a mark to jump to its note, and hover a note to see its marks. Questions tagged Past exam appeared on a real paper; Prediction questions are likely angles, not confirmed. Pick an answer to each question: a wrong pick tells you why it is wrong, so you can try again.

All 25 slides
  1. 1 Local Area Network: LANs
  2. 2 Chapter 4: Outline
  3. 3 IEEE standards
  4. 4 Ethernet
  5. 5 802.3 MAC frame
  6. 6 IEEE Ethernet
  7. 7 IEEE standard for LANs
  8. 8 Ethernet Frame
  9. 9 Ethernet address
  10. 10 Ethernet address of a desktop PC Ethernet card
  11. 11 Note: unicast or multicast?
  12. 12 Note: broadcast
  13. 13 Unicast and multicast addresses
  14. 14 Example
  15. 15 Ethernet frame
  16. 16 Note: frame length
  17. 17 Ethernet evolution through four generations
  18. 18 Ethernet Standards
  19. 19 10Base-T implementation
  20. 20 10Base-F implementation
  21. 21 Fast Ethernet implementations
  22. 22 Gigabit Ethernet implementations
  23. 23 Full Duplex Operation
  24. 24 Full-duplex switched Ethernet
  25. 25 Switched Ethernet
Slide 1 / 25

Local Area Network: LANs

Slide 1: Local Area Network: LANs

Notes

  1. IEEE 802 → frame format → MAC addresses → Ethernet generations → full duplex and switching.

  2. Tip Frame fields and sizes: preamble 7 + SFD 1 (physical-layer header) · destination 6 · source 6 · type/length 2 · data 46–1500 · CRC 4.

Slide 2 / 25

Chapter 4: Outline

Slide 2: Chapter 4: Outline

Notes

  1. One section: 4.1 Ethernet.

Slide 3 / 25

IEEE standards

Slide 3: IEEE standards

Notes

  1. Seen again Ch 3 Part 2, slide 17: How CSMA/CD works

  2. Seen again Ch 1 Part 1, slide 32: IEEE as a standards body

Exam angle

PredictionQ1

Which access method does classic Ethernet use?

PredictionQ2

DIX Ethernet was developed by:

Slide 4 / 25

Ethernet

Slide 4: Ethernet

Notes

  1. The only difference: DIX has a TYPE field, IEEE 802.3 has a LENGTH field (and the 8th preamble byte becomes the SFD).

  2. Trap DIX → Type. 802.3 → Length. Same 2-byte position.

Exam angle

PredictionQ1

The 2-byte field after the source address in an IEEE 802.3 frame is:

Slide 5 / 25

802.3 MAC frame

Slide 5: 802.3 MAC frame

Notes

  1. Tip Frame fields and sizes: preamble 7 + SFD 1 (physical-layer header) · destination 6 · source 6 · type/length 2 · data 46–1500 · CRC 4.

Exam angle

PredictionQ1

How long is the CRC field of an Ethernet frame?

PredictionQ2

How long is each Ethernet address field?

Slide 6 / 25

IEEE Ethernet

Slide 6: IEEE Ethernet

Notes

  1. Memory trick 802.3 = MAC for Ethernet. 802.2 = LLC on top, the same for every LAN.

  2. Seen again Ch 3 Part 2, slide 5: The two data link sublayers

Exam angle

PredictionQ1

Which IEEE standard is the LLC sublayer?

PredictionQ2

Which sublayer is specific to each LAN technology?

Slide 7 / 25

IEEE standard for LANs

Slide 7: IEEE standard for LANs

Notes

  1. One LLC, many MACs, each MAC with its own physical layers.

Slide 8 / 25

Ethernet Frame

Slide 8: Ethernet Frame

Notes

  1. Preamble: 10101010 to sync clocks; in 802.3 the 8th byte is the SFD 10101011.

  2. Type (DIX) says which network-layer protocol is inside (0800 = IP, 0806 = ARP). Values start at 0x0600 = 1536.

  3. Memory trick The two-byte field: ≤ 1500 → length (802.3); ≥ 1536 → type (DIX).

Exam angle

PredictionQ1

The type value 0x0800 in an Ethernet frame means the payload is:

PredictionQ2

If the data is only 20 bytes, how many pad bytes are added?

PredictionQ3

An Ethernet receiver corrects a frame whose CRC check fails.

Slide 9 / 25

Ethernet address

Slide 9: Ethernet address

Notes

  1. Seen again Ch 1 Part 2, slide 28: MAC = physical = NIC address

Exam angle

PredictionQ1

An Ethernet MAC address is:

PredictionQ2

A source MAC address can be a multicast address.

Slide 10 / 25

Ethernet address of a desktop PC Ethernet card

Slide 10: Ethernet address of a desktop PC Ethernet card

Notes

  1. The MAC is printed on the card and stored in its ROM. 00-60-97 is the vendor part.

Slide 11 / 25

Note: unicast or multicast?

Slide 11: Note: unicast or multicast?

Notes

  1. It is the least significant bit of the FIRST byte: 0 → unicast, 1 → multicast. That bit lives in the second hex digit, which is why “even/odd” works.

  2. Trap “Second digit” means the second HEX digit (part of byte 1), not the second byte.

Slide 12 / 25

Note: broadcast

Slide 12: Note: broadcast

Exam angle

PredictionQ1

The broadcast address is a special case of a multicast address.

Slide 13 / 25

Unicast and multicast addresses

Slide 13: Unicast and multicast addresses

Notes

  1. The marked bit is the last bit of byte 1.

Slide 14 / 25

Example

Slide 14: Example

Notes

  1. Only the second hex digit matters.

Exam angle

PredictionQ1

The destination address 01:00:5E:00:00:01 is:

PredictionQ2

The destination address 3C:52:82:11:AA:07 is:

PredictionQ3

The destination address 4B:12:00:9A:10:F0 is:

Slide 15 / 25

Ethernet frame

Slide 15: Ethernet frame

Notes

  1. Frame length counts destination → CRC (not the preamble/SFD): 6 + 6 + 2 + data + 4.

Slide 16 / 25

Note: frame length

Slide 16: Note: frame length

Notes

  1. 64 = 18 header/trailer bytes + 46 minimum data. 1518 = 18 + 1500 maximum data.

  2. Memory trick 18 bytes of overhead: 6 + 6 + 2 + 4. Add 46 → 64. Add 1500 → 1518.

Exam angle

PredictionQ1

The minimum Ethernet frame length is:

PredictionQ2

The maximum Ethernet frame length (without preamble) is:

Slide 17 / 25

Ethernet evolution through four generations

Slide 17: Ethernet evolution through four generations

Exam angle

PredictionQ1

Fast Ethernet runs at:

Slide 18 / 25

Ethernet Standards

Slide 18: Ethernet Standards

Exam angle

PredictionQ1

In the slides, 10 Gigabit Ethernet uses only fiber-optic cable.

Slide 19 / 25

10Base-T implementation

Slide 19: 10Base-T implementation

Notes

  1. 10Base-T: 10 = 10 Mbps, Base = baseband (digital), T = twisted pair. Two pairs: one to send, one to receive.

  2. Memory trick Name = speed + Base(band) + medium: T = twisted pair, F = fiber, X/LX/SX = fiber kinds, CX = copper.

Exam angle

PredictionQ1

In “10Base-T”, the T stands for:

Slide 20 / 25

10Base-F implementation

Slide 20: 10Base-F implementation
Slide 21 / 25

Fast Ethernet implementations

Slide 21: Fast Ethernet implementations

Notes

  1. 100Base-TX (2 pairs Cat 5), 100Base-FX (fiber), 100Base-T4 (4 pairs Cat 3).

Slide 22 / 25

Gigabit Ethernet implementations

Slide 22: Gigabit Ethernet implementations

Exam angle

PredictionQ1

Which Gigabit Ethernet standard uses long-wave fiber?

Slide 23 / 25

Full Duplex Operation

Slide 23: Full Duplex Operation

Notes

  1. Four requirements: full-duplex NIC, two wire pairs, a SWITCH (not a hub), dedicated point-to-point links.

  2. Trap A hub can’t do full duplex: it repeats to everyone, so collisions are still possible.

Exam angle

PredictionQ1

100 Mbps Ethernet in full-duplex mode has a theoretical rate of:

PredictionQ2

Full-duplex Ethernet can use a hub as the central device.

PredictionQ3

List the requirements for Ethernet to run in full-duplex mode, and say what happens to CSMA/CD.

Compare with the model answer

Full-duplex NICs; two wire pairs (one each direction); a switch, not a hub, as the central device; and a dedicated point-to-point link from each station to the switch. Each station becomes its own collision domain, so there are no collisions and CSMA/CD is no longer needed.

Slide 24 / 25

Full-duplex switched Ethernet

Slide 24: Full-duplex switched Ethernet

Notes

  1. Separate transmit and receive paths per station.

  2. Seen again Ch 6, slide 9: How a link-layer switch filters frames

Slide 25 / 25

Switched Ethernet

Slide 25: Switched Ethernet

Notes

  1. Switch = one collision domain per port. (A hub = one big shared collision domain.)

Exam angle

PredictionQ1

An office replaces its 24-port hub with a 24-port switch. How many collision domains are there before and after?

Compare with the model answer

Before: 1, since a hub repeats every frame to all ports so all 24 stations share one collision domain. After: 24, one per switch port.