IEEE 802.3 standard (Ethernet)


           
         Fields in Ethernet Frame Format
           
            Preamble à It contains 7 bytes of alternating 0s and 1s that alert the receiving system to the coming frame and enable it to synchronize its input timing.
            Start frame delimiter à This field tells the receiver that everything that follows is data, starting with the addresses.
            Destination address à This field is allotted six bytes and contains the physical address of the packet’s next destination.
            Source address à This field is allotted six bytes and contains the physical address of the last device to forward the packet.
            Length/type of PDU à This field is allotted 2 bytes, indicates the number of bytes in the coming PDU.
            802.2 frame (PDU) à This field contains the entire 802.2 frame as a modular, removable unit.
            CRC à This field contains error detection information.

IEEE 802.5 standard (Token Ring)
                  IEEE 802.5 standard is token ring.  In token ring, each station may transmit only during its turn and may send only one frame during each turn.  The mechanism that coordinates this rotation is called token passing
Token Ring frame format

Data / Command frame
SD – Start Delimiter:
      The first field of the data/command frame, SD, is one byte long and is used to alert the receiving station to the arrival of a frame as well as to allow it to synchronize its retrieval timing.
AC – Access control:
      The AC field is one byte long and includes four subfields.  The first three bits are the priority field.  The fourth bit is called the token bit.
Frame Control (FC):          
The FC field is one byte long and contains two fields.  The first is a one-bit field used to indicate the type of information contained in the PDU.
     Destination Address:
            The two-to-six byte DA field contains the physical address of the frame’s next destination.  If its ultimate destination is another network, the DA is the address of the router to the next LAN on its path.
     Source Address:
            The SA field is also two to six bytes long and contains the physical address of the sending station.  If the ultimate destination of the packet is a station on the same network as the originating station, the SA is that of the originating station.
     Data:
            The sixth field, data, is allotted 4500 bytes and contains the PDU.  A token ring frame does not include a PDU length or type field.
     CRC:
            The CRC field is four bytes long and contains a CRC-32 error detection sequence.
     ED – End delimiter:
            The ED is a second flag field of one byte and indicates the end of the sender’s data and control information.
     Frame status (FS):
            The last byte of the frame is the FS field.  It can be set by the receiver to indicate that the frame has been read, or by the monitor to indicate that the frame has already been around the ring.
     Token frame
           Token frame is a reservation frame, it has three fields: SD, AC, and ED.  The SD indicates that a frame is coming.  The AC indicates that the frame is a token and includes the priority and reservation fields.  The ED indicates the end of the frame.
     Abort frame
            An abort frame carries no information.  It has only two fields: SD, and ED.  It can be generated by the sender to stop its own transmission.