Token based networking.

A 'token' is passed from node to node.

If a node needs to initiate communication, 
  it is allowed control of communication for a limited time.

  Communication can be between chosen node and one or more other
    nodes in either direction.

  Once time expired, 'token' is passwd to next node.

If a node does not need to communicate, 
  'token' is immediately passed to next node.


Token based networks guarentee each node gets a fair share of
  bandwidth.

Physical - physical linked in a ring.
  IEEE 802.5
  Good security.
  Physical location IDs next node.
  Each node acts as a repeater.

  IBM Token ring (obsolete)
    Proprietary.
    Expensive.


Logical - nodes assigned sequenced IDs.
  IEEE 802.4 - token bus.

  Physically can use any of the topologies, tree, bus, star, ring.  

  Requires additional logic to handle inserting or deleting nodes 
    in the virtual ring.

  FDDI (Fiber distributed data inteface)
    Double ring. Self healing.
    Expensive.
    Designed for nodes separated by miles.

  Arcnet


Token Ring (IBM) Algorithm

  When a system connects to the ring, it listens for a while. 

  If it hears nothing, a single 3-byte token is generated.
    A special system is set up to guard against multiple tokens.

  While systems are linked into token ring.
    The "empty" token is passed from device to device around the ring.

    If a system wants to transmit 
      It converts token to data or command frame.

        Access control - priority field, flags empty or filled token, other.
          Bits 0-2 - priority, 3 - Token type, 4 - monitor, 5-7 - reserved.

        Frame control - type of frame (data or control) and control info.
          0-1 - type of info (control or data)
          2-7 - if control, control code.          

        DA - Destination Address (Media access control, MAC - hardware address)
        SA - Source Address. (SA and DA are 6 bytes)
        * If you are communicating with a system outside the local network
           then the IP number of the destination is carried in the data 
           portion of the frame.

        Data frame up to 4500 bytes of data.

        Frame Check Sequence - CRC error check. 

        Ending delimiter - End of main frame. Complement ot Starting delimiter.

        FS - frame control. Sender sets bit, when appropriate receiver sees 
          packet, it clears bit and packet continues back around to sender.

      Once the frame has travelled the 'ring' and returned to sender
        Sender "removes" the data thus re-creating
        the default "empty" token. The token must return to the original sender
        before being available for the next packet in a communication session 
        or to the next station desiring to transmit.
        
        Generally, a node will desire to send several packets that make up 
          a 'session'. 
   
      Optional 
        Other stations may request next access to token. 
        And stations can be assigned priority.

  Abort Frame - special purpose frame generated by sender to abort session.

Virtual token rings require additional protocol that allows 
   discovery that 
   a node has been inserted into the network.
     or
   a node has been removed from the network.

   Both may require re-assigning node sequence IDs.
  
 
Polled networks. 
  Most commonly found in local system buses.
  
  Where a master polls each connected node in some predetermined order
    of priority. Like virtual token without the actual token.

  USB uses polling for interrupt handing.

  Controller uses device's ID as its node @