802.3 and 802.11 Frame - jibingl/CCNA-CCNP GitHub Wiki

IEEE 802.3 (Ethernet) Frame

The size of a Ethernet frame (Eth-Header + Data + Eth-Trailer) must between 64 - 1518 bytes. If smaller, padding 0s to DATA portion; if larger, fragmented.

                          |<----- Ehternet-Header (14B) ------->|<-------Data-------->|<-Trailer->|
 7 bytes          1 byte      6 bytes     6 bytes      2 bytes       46-1500 bytes      4 bytes
+----------------+--------+-------------------------------------+---------------------+-----------+
|   Preamble     |   SFD  |  Dest-MAC  |  Src-MAC   |Type/Length|         DATA        | FCS(CRC)  |
| 10101010 * 7   |10101011|            |            |           |                     |           |  
+----------------+--------+-------------------------------------+---------------------+-----------+

Note: Preamble and SFD are not calculated into the Ethernet frame length.

  • Preamble: Synchronize clock

  • SFD - Start Frame Delimeter

  • Type/Legnth: Type is when value >= 1536 (IPv4_0x0800; IPv6_0x86DD). Length is when value =< 1500.

  • FCS - Frame Check Sequence, detect corrupted data by running CRC (Cyclic Redundancy Check) algorithm on data.

IEEE 802.11 (Wireless) Frame

 2 bytes  2 bytes    6 bytes      6 bytes      6 bytes    2 bytes    6 bytes   2 bytes  4 bytes    various bytes     4 bytes
+-------+--------+------------+------------+------------+--------+------------+-------+---------+------------------+----------+
| Frame |Duration|   Address  |   Address  |   Address  |Sequence|   Address  | QoS   |    HT   |      DATA        | FCS(CRC) | 
|Control|  /ID   |     1      |      2     |      3     | Control|      4     |Control| Control |                  |          |
+-------+--------+------------+------------+------------+--------+------------+-------+---------+------------------+----------+
  • Frame Control: Provide info about type and subtype. There are three types:
    • Management: Beacon, Probe request & response, Authentication, and Association request & response. Those msg used for connection process between wireless devices and APs.
    • Control: Controll access to the medium (radio frequency). Like RTS (Request to Send), CTS (Clear to Send), and ACK.
    • Data: Actual data packets.
  • Duration/ID: Depending on message type, it can be the time dedicatedly used by the channel to transmit the frame, or identifier for the association.
  • Addresses: Up to 4 addresses can be present.
    • Destination Address (DA): Final recipient
    • Source Address (SA): Original sender
    • Receiver Address (RA): Immediate recipient
    • Transmitter Address (TA): Immediate sender
  • Sequence Control: To reassemble fragments and eliminate duplicate frames.
  • HT (High Throughput) Control: Added in 802.11n to enable High Throughput operations.
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