Lan Protocol Architecture

• Three layers
  • Physical
    • We have been working on this all semester.
      • Encoding of signals
      • preamble
      • bit transmission
    • Topology we just finished.
    • Media we just briefly looked at.
  • Medium access
  • Logical Link.
• IEEE 802
  • This is a family of standards from IEEE
  • See wikipedia for a list.
  • This corresponds to the lowest levels in a protocol architecture.
  • This provides for (LLC - link)
    • Transmission of a frame with address and error detection fields
    • Reception: remove frame and check for errors
    • Govern access to the media
    • Provide an interface to perform error and flow control.
  • As well as (MAC)
    • Management of shared access medium
  • Mac frames.
  • Ethereal/wireshark
  • LLC Services
    • Unacknowledged connectionless service
      • Much like datagram
    • Connection oriented service
    • Acknowledged connectionless service.
  • LLC Protocol
    • Modeled after HDLC
    • Three different protocols as mentioned above.
    • All use the same PDU
    • 
    • LLC provides ports, much like tcp.
      • DSAP - Destination Service Access Point
      • SSAP - Source Service Access Point
      • 7 bit address
      • DSAP 1 bit indicates individual or group address
      • SSAP 1 bit indicates command or response
      • The LLC Control fileds are identical to HDLC
    • Type 1 or Unacknowledged connectionless service
      • uses an unnumbered information PDU
      • No ack
      • No error control
      • Lower level (MAC) provides error detection.
      • He describes an operation very close to ping at this level.
    • Type 2 or connection oriented
      • A connection is established via the exchange of various packets.
      • PDUs contain sequence numbers during the exchange just like HDLC
      • DISConnect PDU closes the session.
    • Type 3 or Acked connectionless
      • Each pdu is acknowledged.
      • Uses a 1 bit sequence number.
      • Only one PDU may be outstanding.
  • MAC
    • Medium Access Control protocol
    • Assumes multiple machines on a single connection (bus type)
    • Ways to address this issue
      • Centralized vs decentralized
        • Centralized affords greater control over access for providing priorities, overrides and guaranteed capacity
        • Enables the use of relative simple access logic at each station
        • Avoids problems with distributed coordination
        • Creates a single point of failure
        • May act as a bottleneck.
      • How can we do this?
        • Round Robin
          • In turn, each station is given a change to transmit.
          • For a limited time.
          • This becomes a problem as things grow.
        • Reservation
        • Contention
    • The MAC frame
      • MAC Control
      • Destination MAC address
        • 48 bit address
        • Represented as xx:xx:xx:xx:xx:xx in hex
        • Once were unique and permanently assigned to hardware
        • Distributed by vendor.
        • IEEE expects this to be exhausted by 2011
        • /sbin/arp
        • /sbin/ifconfig
      • Source MAC address
      • LLC
      • CRC
    • We will discuss CSMA/CD carrier sense multiple access collision detection in the next chapter.