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Ethernet: Generations, MAC Frames, and Implementations

Learn about the three generations of Ethernet, the structure of MAC frames, and different Ethernet implementations such as 10Base5, 10Base2, 10Base-T, and 100Base-TX.

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Ethernet: Generations, MAC Frames, and Implementations

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  1. Chapter 14 Local Area Networks:Ethernet

  2. Figure 14.1Three generations of Ethernet

  3. PLS sublayer: encodes and decodes data [using manchester encoding]. AUI: Medium independent interface between PLS and MAU. MAU (Transceiver): Medium-dependent. It’s a transmitter and receiver; it can detect collisions; Can be internal or external. MDI (Medium Dependent Interface): Used to connect the transceiver to the medium. Just a connector like jack or tap.

  4. Data link layer is divided into logical link control (LLC) sublayer and medium access control (MAC) sublayer. MAC Sublayer Access Method: CSMA/CD Frame contains destination and source physical address. No Acknowledging procedure and thus known as unreliable. Preamble: Alternating 0s and 1s; used for synchronizing; 7bytes (56 bits). Start Frame Delimiter (SFD): 10101011 indicates the start of the frame. Last two bits alerts that the next field is destination address. Length/Type: if less than 1518, it indicates the length of data field. If greater than 1536, it indicates the type of PDU. Data: 46 to 1500 bytes; CRC: CRC-32 Figure 14.2802.3 MAC frame

  5. Figure 14.3Minimum and maximum length • Minimum length restriction because: • Collision must be before a physical layer sends a frame out of the station. • If the entire frame is sent out before a collision is detected, it is too late. The MAC layer has already discarded the frame, thinking that the frame has reached the destination. • Maximum length restriction is historical.

  6. Each station has a network interface card (NIC) Physical address: 6-byte [48 bits] It is written in hexadecimal notation using a hyphen to separate bytes from each other. Figure 14.4Ethernet addresses in hexadecimal notation • Source address is always a unicast address – frame from only on station. • Destination address can be unicast [one to one] or multicast [a group of people] or broadcast [all members of the network].

  7. Transceiver (Medium attachment Unit): Medium-independent. It creates the appropriate signal for each particular medium. There is a MAU for each type of medium used in 10-Mbps Ethernet. Transceiver is a transmitter and receiver. It transmits signals over the medium; it receives signals over the medium; it also detects collisions. 10Base5 is called as Thick Ethernet or Thicknet; Uses coaxial cable. Uses Bus topology. Transceiver cable is called as Attachment unit interface (AUI) cable. Figure 14.11Connection of a station to the medium using 10Base5

  8. Thin Ethernet or Cheapernet. Uses Bus topology with an internal transceiver or a point-to-point connection via an external transceiver. Internal transceiver does not need AUI cable. Figure 14.12Connection of stations to the medium using 10Base2

  9. Twisted-pair Ethernet. Physical star topology Stations are connected to a hub with an internal transceiver or an external transceiver. Figure 14.13Connection of stations to the medium using 10Base-T

  10. Fiber Link Ethernet. Uses star topology to connect stations to a hub Normally an external transceiver called fiber-optic MAU is used. Transceiver is connected to the hub by using two pairs of fiber-optic cables. Figure 14.14Connection of stations to the medium using 10Base-FL

  11. Without bridges, all the stations share the bandwidth of the network. Bridges divide the network into two. Bandwithwise, each network is independent. Figure 14.15Sharing bandwidth • With bridges, 10 Mbps network is shared only by 6 [actually 7 as bridge acts as one station]stations.

  12. Using bridges, collision domain becomes much smaller and the probability of collision is reduced tremendously. Figure 14.17Collision domains in a nonbridged and bridged network

  13. A layer 2 switch is an N-port bridge with additional sophistication that allows faster handling of the packets. Figure 14.18Switched Ethernet

  14. As there are two links, one each for sending and receiving, we don’t need CSMA/CD here. No flow or error control here. Flow and error control is provided by a new sublayer, called the MAC control, which is added between the LLC and MAC sublayer. Figure 14.19Full-duplex switched Ethernet

  15. Autonegotiation: Allows two devices to negotiate the mode or data rate of operation. Transceiver [PHY sublayer] does the job of encoding and decoding. RS looks at passing data as 4-bit nibbles to MII. MII = AUI; Supports both 10 and 100 Mbps; Has 4 bits parallel path; Management functions are added. Figure 14.20Fast Ethernet physical layer

  16. Figure 14.22Fast Ethernet implementations • Two wire or four wire. • Two wire: 100Base-X: With twisted pair (100Base-TX) or Fiber optic (100Base-FX) • Four wire: Twisted pair (100BaseT4)

  17. Internal or external transceiver. Uses 4B/5B for synchronization. Figure 14.23100Base-TX implementation

  18. Uses two pairs of fiber-optic cables in a physical star topology. Figure 14.25100Base-FX implementation

  19. 100Base-TX Can provide data rate of 100Mbps, but it requires the use of category 5 UTP or STP cable. 100Base-T4 was designed to use CAT-3 [voice-grade twisted pair] or higher UTP. Implementation uses four pairs of UTP for transmitting 100 Mbps. Figure 14.27100Base-T4 implementation

  20. Figure 14.28Using four wires in 100Base-T4 • To cut down the number of pairs to four, two pairs are designed for unidirectional transmission and the other two for bidirectional transmission. • The two unidirectional pairs are always free in one direction to carry collision signals.

  21. RS sends 8-bit parallel data to PHY via GMII. GMII is a logical interface and not physical. Operates at 1000 Mbps, Has Management functions. There is no GMII cable or connector. PHY: There is no external transceiver. MDI: Connects transceiver to the medium. For Gigabit Ethernet, only the RJ-45 and fiber-optic connectors are defined. Figure 14.29Physical layer in Gigabit Ethernet

  22. Access: Half-duplex using CSMA/CD or Full-duplex with no need for CSMA/CD 1000Base-X: Two wire implementation Short wave optical fiber (1000Base-SX) Long wave optical fiber (1000Base-LX) Short copper jumpers (1000Base-CX) using STP. 1000Base-T: Four-wire version using twisted-pair cable [UTP]. Figure 14.30Gigabit Ethernet implementations

  23. Both 1000Base-SX and 1000Base-LX use two fiber-optic cables. Transceiver in all implementations are internal Uses 8B/10B for synchronization. Figure 14.311000Base-X implementation

  24. Figure 14.331000Base-T implementation • Designed to use Category 5 UTP. • Four twisted pairs achieve a transmission rate of 1 Gbps. • To send 1.25Gbps over four pairs of UTP, 1000Base-T uses an encoding scheme called 4D-PAM5 (4-dimensional, 5-level pulse amplitude modulation). • Five levels of pulse amplitude modulation are used.

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