Notes On Chapter Fifteen -- Wired LAN Technology (Ethernet and 802.3)

• 15.0 Study Guide
  
  
  • Know the basic structure of an Ethernet frame
  • Understand what NICs are and what they do
  • Be able to explain major features of how Thicknet, Thinnet, and Twisted-pair Ethernet work, including what hardware was used, what were the functions of the components, and how the components were assembled together to form a network.
  • Know the data rate ranges of Ethernet technologies.
  • Be familiar with the backward compatibility of forms of Ethernet
  
  
• 15.1 Introduction
  
  
  • LAN technologies
  • The basis of Ethernet
  • Original standard due to DEC/Intel/Xerox (DIX standard)
  
  
• 15.2 The Venerable Internet
  
  
  • Ethernet hardware, cabling and media have changed dramatically
  • However, all forms of Ethernet remain compatible.
  
  
• 15.3 Ethernet Frame Format
  
  
  • frame format means "the way a packet is organized"
  • See figure 15.1 on page 254 of the sixth edition of Comer.
  • Header contains source and destination addresses of 48-bits each, plus a 16-bit type field
  • data payload is variable in size (46-1500 bytes) and ends with a 4-byte CRC.
  • The NIC uses the preamble & start-frame-delimiter to detect the beginning of a frame and a 96-bit inter-frame-gap (IFG) to detect the end of a frame.
  
  
  
  
• 15.4 Ethernet Type Field and Demultiplexing
  
  
  • The type field gives an indication of what is in the data payload.
  • Often a high-level protocol utilizes Ethernet. For example, the payload of an Ethernet packet could be an IPv4 datagram, an IPv6 datagram, or an ARP message (or something else).
  • The type field of the Ethernet packet will be 0x0800 for an IPv4 datagram, 0x08DD for an IPv6 datagram, and 0x0806 for an ARP message.
  • The receiving computer relies on the contents of the type field to indicate to which module of protocol software the computer should relay the packet.
  • When things are done this way, the computers on the network are able to utilize many different network protocols simultaneously.
  
  
  
  
• 15.5 IEEE's Version of Ethernet (802.3)
  
  
  • The 802.3 Ethernet standard modifies the frame format slightly
  • The max payload is reduced to make room for a second header section.
  • The old type field becomes a packet-length field
  • There's a new 8-byte (SNAP) header that includes the packet type
  • 802.3 is backward compatible with the original frame format
  • 802.3 hasn't been used much.
  
  
  
  
• 15.6 LAN Connections and Network Interface Cards
  
  
  • A computer treats a LAN pretty much like an I/O device.
  • Like a disk, the LAN is something to which the computer can write bytes, or from which it can read bytes.
  • A disk controller has a connection to the computer's bus and a connection with the disk.
  • Likewise there are Network Interface Cards that perform a similar function for LAN's.
  • The computer bus connects to the NIC, which connects to the network medium
  • An NIC:
    • sends and receives frames (packets)
    • recognizes destination addresses in frames
    • computes and checks CRC's
  
  
• 15.7 Ethernet Evolution and Thicknet Wiring
  
  
  • Names: 10Base5, thick wire Ethernet, or Thicknet
  • An NIC attached to the computer handled digital aspects of communication
  • A transceiver tapped into "banana cable" bus handled carrier detection, and conversion between digital bit values and signals on the cable.
  • An attachment unit interface (AUI) cable formed the connection between the transceiver and the NIC.
  • Main cable and transceivers were usually remote from the computers, perhaps running along above a ceiling or beneath a floor.
  
  
  
  
• 15.8 Thinnet Ethernet Wiring
  
  
  • Names: 10Base2, Thinwire Ethernet, Thinnet
  • NIC and Transceiver combined in one device - usually internal to the computer
  • Sections of thin flexible cable connect from NIC to NIC with T-connectors.
  • Special terminators were connected to each end of the cable run
  • Advantages: low cost, easy installation
  • Disadvantages: Poor cabling quality and (other) human error can easily cause significant network downtime
  
  
  
  
• 15.9 Twisted Pair Ethernet Wiring and Hubs
  
  
  • Names: 10BaseT, Twisted Pair Ethernet
  • No coax cable. Multiple NIC's connect to a hub or switch via twisted pair wiring.
  • A hub emulates a backbone cable.
  
  
  
  
• 15.10 Physical and Logical Ethernet Topology
  
  
  • Although twisted-pair Ethernet is physically set up as a star network, logically it utilizes a bus topology.
  
  
• 15.11 Wiring an Office Building
  
  
  • Each different Ethernet wiring technology makes different demands on office wiring designs.
  • Note that twisted pair wiring can require numerous individual cables and careful labeling.
  
  
  
  
• 15.12 Variants of Twisted Pair Ethernet and Speeds
  
  
  • Ethernet cable has improved in quality and shielding, and data rates have increased:
  • 10BaseT is 10 Mbps
  • 100BaseT (Fast Ethernet) is 100 Mbps
  • 1000BaseT (Gigabit Ethernet, Gig-E) is 1000 Mbps
  • The high-speed versions use a switch instead of a hub - which simulates an isolated Ethernet segment, linked to other segments by bridges.
  • The high-speed versions will slow down if they share a segment with an NIC using a slow version.
  
  
  
  
• 15.13 Twisted Pair Connectors and Cables
  
  
  
  
  
  • Twisted pair uses RJ45 connectors, which are larger versions of the RJ11 connectors used with telephones.
  • Most people purchase ready-made patch cables
  • There are two types: straight (for computer-to-switch or computer-to-hub) and crossed (for switch-to-switch, hub-to-hub, or computer-to-computer)