Network Layer - successful delivery of individual packets across network.
    Routing of individual data packets.
     
    Logical (virtual) addressing such as Internet Protocol.

    Transferring of packets across dissimilar node or hop.  
      Framing of packet to traverse a particular hop.
   
    Control of congestion on network.

    Still working at node to node level.

    Not concerned with getting packets to final destination.
      just to next best guess node along way.
  
    IP, ICMP*

  Data Link Layer - successful transmission of bits.
    Electronic/logical side of the connection. zeros and ones.

    Breaking data stream up into check-able frames of bits.
  
    Handling speed differences at different nodes of network.

    Deals with individual packets or frames.

    Data destination addressable only to other nodes on particular network.
      * Specialized node called gateway router provides access on/off local
        network. This selection most likely occurs at presentation layer(?),
        but may involve other layers.

    CSMA/CD, CSMA/CA mostly handled at this level.

    Hardware address of interface. MAC#
      Media access control
        Source and destination pair define a connection between 2 nodes on
        local network.
        IEEE 802 protocol 48-bit MAC
          24 bit (3-byte) Organizationally Unique Identifier OUI
            Obtained from IEEE. Network card manufacturer has at least 1.
              Public - can be used to ID manufacturer.
          24 bit (3-byte) Network Interface Controller NIC. 
            Each card receives a unique NIC
         * On Unix systems, /usr/sbin/arp, will give a list of mac# discovered
           on the local network. 
   
  Physical Layer - sending of bits.
    Equivalent of a physical bus.

    Mechanical, electrical, and physical transmission medium.

    Voltage and timing, Establishment and termination of connections, 
      uni or bidirectional, connector pin count.

    Also interface design - hubs, repeaters, etc. 
    
  In practice, the Ethernet protocol combines physical and data link layers.
    And delegates some aspects to the network layer and takes over some others
    from the network layer.