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Ethernet

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Summary

Ethernet is a LAN architecture using the CSMA/CD access method. Two standards define Ethernet: Ethernet Version II (developed by DIX) and IEEE 802.3. Ethernet Version II corresponds to both layers 1 and 2 of the OSI/RM and a media speed of 10Mb. IEEE 802.3 corresponds to layer 1 and part of layer 2 of the OSI/RM and media speeds of 10Mb, 100Mb for Fast Ethernet, and 1000Mb for Gigabit Ethernet. IEEE 802.3 also defines different physical media types and generic interfaces to those physical media. MAC addresses uniquely define stations on the Ethernet segment. Bits in the MAC address define unicast, multicast, and broadcast traffic.

The CSMA/CD is the access method for the Ethernet physical media. Listening to the physical media before sending and then listening while sending to detect collisions characterize CSMA/CD. Ethernet also defines operation in a half-duplex shared-media environment and point-to-point full-duplex operation. Four different frame formats are allowed. They are specified in the router as ARPA, NOVELL-ETHER, SAP, and SNAP. Frames also have a 64-byte minimum and 1,518-byte maximum frame size. The MTU of Ethernet is 1,500 bytes. Gigabit Ethernet uses the existing frame formats from Ethernet and Fast Ethernet to allow for a clean migration path for users that need more bandwidth.

QUESTIONS AND ANSWERS:

1. What are the media speeds for Ethernet, Fast Ethernet, and Gigabit Ethernet?

A: Ethernet media speed is 10Mb. Fast Ethernet media speed is 100Mb. Gigabit Ethernet media speed is 1,000Mb or 1Gb.

2. What are the Ethernet connector types on the router?

A: AUI

10BASE-T for RJ-45 on 10Mb Ethernet

100BASE-T for RJ-45 on Fast Ethernet

MII

3. Describe the difference between unicast, multicast, and broadcast traffic.

A: Unicast traffic flows from a single source to a single destination MAC address. Multicast traffic flows from a single source MAC address to many destinations and uses a functional MAC address. Broadcast traffic is from a single source to all devices on the Ethernet segment. This is specified by a destination MAC address of all ones.

4. What field in the MAC address is used for universal administration?

A: The Organizational Unique Identifier (OUI) field is the first 24 bits of the MAC address. OUIs are assigned to individual Ethernet adapter vendors. These vendors control the second half of the MAC address to make sure the total address is globally unique.

5. Describe the operation of a CSMA/CD network.

A: Carrier Sense means that Ethernet stations listen to the Ethernet media to make sure that no other station is transmitting before they transmit. Multiple Access means that more than two stations share the same Ethernet media. This is also known as half-duplex operation or shared media. This differs from full-duplex operation on a point-to-point switched media. Carrier Detect means that the transmitting station monitors its own transmission to make sure that it can receive it without error. If an error occurs, that implies that another station was transmitting at the same time and collided with the first transmitting station.

6. What are the four different Ethernet encapsulation types?

A: From the Cisco IPX encapsulation command they are ARPA, NOVELL-ETHER, SAP, and SNAP. ARPA corresponds to Ethernet Version II, NOVELL-ETHER corresponds to IEEE 802.3 without an LLC PDU, SAP corresponds to IEEE 802.3 with an IEEE 802.2 LLC PDU, and SNAP corresponds to an IEEE 802.3 frame with an IEEE 802.2 LLC PDU and an additional SNAP header.

7. How do these encapsulations differ?

A: All three encapsulations start with destination and source MAC address fields. The Ethernet Version II or ARPA encapsulation then follows with a two-octet type field that specifies the upper-layer protocol. The NOVELL-ETHER and SAP encapsulations follow the destination and source MAC addresses with a two-octet length field. The SAP encapsulation adds an IEEE 802.2 LLC PDU. The SNAP encapsulation adds a SNAP header after the LLC PDU in IEEE 802.3 format. The destination and source SAP addresses in the LLC PDU are set to 0xAA, and there is a one-octet control field that is set to 0x03.

PRACTICE TEST

  1. The CSMA/CD access method is defined by what IEEE standard?

  2. A. 802.1

    B. 802.2

    C. 802.3

    D. 802.4

    E. 802.5

    Answer A is incorrect; 802.1 defines MAC Bridging. Answer B is incorrect; 802.2 defines LLC. Answer C is correct; 802.3 defines the CSMA/CD access method. Answer D is incorrect; 802.4 defines a token bus architecture. Answer E is incorrect; 802.5 defines a Token-Ring architecture.

  3. IEEE 802.3 defines what portions of the OSI/RM?

  4. A. All of layer 1 and none of layer 2

    B. All of layer 1 and part of layer 2

    C. All of layer 1 and all of layer 2

    D. None of layer 1 and all of layer 2

    Answer A is incorrect; this would be the OSI/RM Physical layer. Answer B is correct; IEEE 802.3 defines all of layer 1 and part of layer 2 of the OSI/RM. Answer C is incorrect; This is correct for Ethernet Version II but not for IEEE 802.3. Answer D is incorrect; this would be the Data Link layer of the OSI/RM.

  5. IEEE 802.3 specifies what media speeds? (Choose all that apply.)

  6. A. 10Mb

    B. 16Mb

    C. 100Mb

    D. 155Mb

    E. 1,000Mb

    Answer A is correct; 10Mb is an IEEE 802.3 media speed. Answer B is incorrect; 16Mb is an IEEE 802.5 media speed. Answer C is correct; 100Mb is an IEEE 802.3 media speed for Fast Ethernet. Answer D is incorrect; 155Mb or OC-3 is commonly used in ATM networks. Answer E is correct; 1,000Mb is 1Gb is defined as an IEEE 802.3 media speed.

  7. Which traffic types are defined by the MAC address? (Choose all that apply.)

  8. A. Unicast

    B. Half-duplex

    C. Multicast

    D. Full-duplex

    E. Broadcast

    Answer A is correct; I/G Bit equals zero defines unicast traffic. Answer B is incorrect; duplex is a Physical layer characteristic. Answer C is correct; I/G Bit equals one and all bits not one define multicast traffic. Answer D is incorrect; duplex is a Physical layer characteristic. Answer E is correct; a MAC address of all ones defines broadcast traffic.

  9. Which statements apply to CSMA/CD half-duplex operation? (Choose all that apply.)

  10. A. Stations sense the media before transmission.

    B. Stations transmit while other stations are transmitting.

    C. Stations wait to transmit until other stations are not transmitting.

    D. Stations monitor the media for collisions.

    E. Stations retransmit after collisions at a fixed backoff time interval.

    Answer A is correct; this is called carrier sense. Answer B is incorrect; this applies to full-duplex operation. Answer C is correct; this is also part of carrier sense. Answer D is correct; this is collision detection. Answer E is incorrect; if the backoff time is fixed in the network, collisions will recur. Random backoff times are used.

  11. What is the MTU for Ethernet?

  12. A. 64

    B. 512

    C. 1,500

    D. 1,518

    E. 2,048

    Answer A is incorrect; this is the minimum frame size. Answer B is incorrect; this is a valid transmission unit size but not the maximum. Answer C is correct; the MTU for Ethernet is 1,500. Answer D is incorrect; this is the maximum frame size. Answer E is incorrect; this values exceeds the maximum frame size.

  13. The Ethernet Version II frame format contains which fields? (Choose all that apply.)

  14. A. preamble

    B. SOF

    C. destination/source MAC address

    D. length

    E. LLC PDU

    F. SNAP header

    Answer A is correct; the preamble is a valid field. Answer B is incorrect; this only applies to IEEE 802.3. Answer C is correct; the destination/source MAC addresses are required. Answer D is incorrect; Ethernet Version II uses a type field instead of a length field. Answer E is incorrect; this is valid only for IEEE 802.3. Answer F is incorrect; this is valid only for the SNAP encapsulation type.

  15. The IEEE 802.3 frame format contains which fields? (Choose all that apply.)

  16. A. preamble

    B. SOF

    C. destination/source MAC address

    D. length

    E. LLC PDU

    F. SNAP header

    Answer A is correct; the preamble is a valid field. Answer B is correct; the SOF is a valid field. Answer C is correct; the destination/source MAC address are valid fields. Answer D is correct; IEEE 802.3 uses the length field. Answer E is correct; the LLC PDU is after the length field. Answer F is incorrect; this is valid only in the SNAP encapsulation type.

  17. The SNAP encapsulation type contains which fields? (Choose all that apply.)

  18. A. preamble

    B. SOF

    C. destination/source MAC address

    D. length

    E. LLC PDU

    F. SNAP header

    Answer A is correct; the preamble is a valid field. Answer B is correct; the SOF is a valid field. Answer C is correct; the destination/source MAC address are valid fields. Answer D is correct; IEEE 802.3 uses the length field. Answer E is correct; the LLC PDU is after the length field. Answer F is correct; this is valid only in the SNAP encapsulation type.

  19. Which of the following statements are true about Gigabit Ethernet? (Choose all that apply.)

  20. A. Supports 1,000Mb media speed

    B. Allows for an optional IEEE 802.3x flow control

    C. Supports half- and full-duplex operation

    D. Uses extended super frame format

    E. Supports long-wave laser over single- and multimode fiber

    Answer A is correct; 1,000Mb is 1Gb. Answer B is correct; this is not true for Fast Ethernet. Answer C is correct; Gigabit Ethernet supports half- and full-duplex operation. Answer D is incorrect; Gigabit Ethernet uses existing Ethernet frame formats.

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