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10baseT Ethernet

Jamal Munshi, Sonoma State Univesity, 1992
All rights reserved

Layer 1 specifications
    UTP: unshielded twisted pair wiring similar to telephone
    enhanced twisting reduces em attenuation and crosstalk
    EIA "wire category" rating is specified: (3,4, or 5)
    4 pairs (8 wires) of wires are specified for each 10baset path
    4 wires are used: 2 for hub to NIC and 2 for NIC to hub (4 unused for now)
    NIC transmits on pins 1&2. Hub broadcasts on pins 3&4
    connectors and jacks are RJ45 (8 pins)
    distance limitations: NIC-hub less than 100 feet of wire (controls attenuation, distortion, interference, and crosstalk)
    specifies data transmission rate of 10 mbps = 10^7 bps (1 bit = 10^(-7) seconds = 100 nanoseconds)
    specifies baseband transmission
    specifies "manchester signal encoding"
      -2.05 volts for 50 nanoseconds followed by 0 volts for 50 nanoseconds = "1"
      0 volts for 50 nanoseconds followed by -2.05 volts for 50 nanoseconds is a "0"
      double voltage switching improves synchronization at the cost of thruput

    10baseT hubs
      a hub is a multiport repeater
      ports = rj45 connectors
      hub receives transmitted signal on pins 1&2 from sending station as received bits
      it re-writes the received bits on to pins 3&4 of all ports = "broadcast"
      all NICs connected to the hub receive the bits on pins 3&4
MAC Layer specifications
    topology is a bus: single logical wire shared by all NICs in the subnet
    media access is by CSMA/CD. (collisions may be minimized by good network design practices)
    6 octet node addressing
    MAC layer framing protocol
      8 octets of preamble and delimeter (7 octets of 10101010 and 1 octet of 10101011)
      6 octets of destination address
      6 octets of source address
      2 octets for payload length
      0-1500 octets of payload
      0-0-38 octets of padding to set minimum frame size to 64 octets
      4 octets for error check
      total overhead = 26 octets plus padding

    probability that bit error will be missed in 32-bit LRC = 1 in one billion. at 10k pps, 1 packet in 100,000 may escape error detection. errors are important when downloading programs or transmitting data for financial transactions.
    if error is detected, the packet is discarded
    carrier detection: check voltage changes on pins 3&6. if none, transmit; else wait
    collision detection: read the packet that you sent (it is being broadcast back to you by the hub) and compare what you received with what you sent. if different, a collision has occured.
    collisions occur because of transmission delay. (at 3x10^8 meters per second a signal will travel 0.3 meters or about a foot in one nanosecond. this means that in 10baseT a bit is 100 feet long.)
    multiple access: all NICs are connected to the same logical wire or physically, the same broadcasting hub. effective bandwidth delivered to each NIC depends on number of active NICs. steep degradation curve.
    overhead of packet design: depends on average packet size. if avg packet size is 1000 octets (i.e. average load = 974) then overhead = 26/1000 or 2.6%
LLC Layer specifications
    802.2 says ignore missing frames (let upper layers decide)
    normally this will be handled by OSI layer 4
10BaseT backbone networks with hubs and MAUs
    MAU = transceiver = transmitter+receiver
    10base5 using thick coax allows 500 feet trunk line between medium attachment units (MAU)
    subnet hub contains one DIX port connected to the MAU with AUI drop cable and many (normally 8, 16, 24) RJ45 10baseT ports
    MAUs and coax trunks between them form the backbone of the network
    backbone may also be formed with a 10base5 hub
    backbone limitations: max of 5 segments and 1024 nodes
    10base2 thinnet cheaper to daisy chain but less distance
    for very long runs across corporate "campus" environment: 10baseF (10 kilometers)