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Thin Ethernet (10B2 / IEEE 802.3a)

Thin Ethernet (10B2 / IEEE 802.3a). Segment length < 185m and > 0.5 m Up to 30 attached nodes Cable flexible and cheap Integrated or external transceiver connected via a BNC 'T' connector Used mainly for workgroups Difficult to manage (i.e. breaks in cable difficult to locate).

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Thin Ethernet (10B2 / IEEE 802.3a)

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  1. Thin Ethernet (10B2 / IEEE 802.3a) • Segment length < 185m and > 0.5 m • Up to 30 attached nodes • Cable flexible and cheap • Integrated or external transceiver connected via a BNC 'T' connector • Used mainly for workgroups • Difficult to manage (i.e. breaks in cable difficult to locate)

  2. 10BT/802.3i • Connects the computer directly (i.e. using a point to point link) to a wiring hub. • Segment length 100 m • Cable flexible and very cheap • Standard RJ-45 connector used (also can be used for telephone and other networks) • Used mainly for work groups (requires a hub to connect to the LAN) • Easy to manage (also can be used for telephone and other networks)

  3. Ethernet Repeaters and Hub • operate at the Physical Layer • connect together one or more Ethernet cable segments to provide signal amplification and regeneration • A network of repeaters and hubs is therefore called a "Shared Ethernet" or a "Collision Domain". • Only one host is allowed to transmission within a Collision Domain An 3Com Office Hub (8 ports) • No (or little) processing (memory) unit • all ports operate as one Ethernet LAN • all connected NICs operate in the half-duplex (CSMA/CD) mode at the same transmission speed Stack of several hubs to wire a building

  4. Ethernet Bridges & Switches • bridge has to forward frames from one LAN to another • Operates at the data link layer • Bridges Must learn which addresses belong to the computers connected via each port. • Separate Collision Domain

  5. Ethernet Switches • Solution to put 300 hosts in one Ethernet (in one building) • Separate the Collision domain • Contain a high-speed backplane • Store-and-forward (inboard memory for each line) • ranging from the simplest low cost devices • to expensive high performance switches 3Com LinkSwitch 1000 Cisco Catalyst 5000 Switch

  6. Frame Format • A sent at t0 • B send around t0+ • B realize collision and send noise burst • A hear the noise burst at around t0+ 2 • A must not conclude transmission success before t0+2 • Preamble • Dest Addr: 6-byte MAC address • Source Addr: 6-byte MAC address • Length: 1500 bytes maximum • Data • Pad: minimum 64 bytes • To distinguish valid fames and garbage • Transmission should last longer than round-trip delay. • Checksum

  7. Round-trip Delay and Transmission Time • With maximum length 2500 meters • Four repeaters • Round-trip time: 50 usec • At 10 Mbps, one bit last 100 nsec • Thus 500 bits are needed to last 50 usec • Which is roughly 64 bytes

  8. Binary Exponential Backoff • How stations randomly wait after a collision • One slot is 51.2 usec (round-trip delay) • At first collision, each station waits 0 or 1 slots • After the second collision, each station waits either 0, 1, 2, or 3 slots randomly. • If a third collision occurs, wait 0 to 7 slots. • So on and so forth • Maximum slots to wait is 1023 after 16 straight collisions

  9. Fast Ethernet and Gigabit Ethernet • 100 Base-T4 • 100 Base-TX • All gigabit Ethernet is point-to-point • Full-duplex mode • Must use switch (not hub!) • No contention • No need for media sensing • Half-Duplex mode • Connected with Hub • Collision is possible

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