Information that is to be sent over an Ethernet connection is collected into a package called an Ethernet Frame. The frame includes the original data plus added information. The structure of the frame is similar to a sector of a hard disk. The organization of the frame can be broken down to its individual components.

   |   |   |   |   |                  |   |
   | 1 | 2 | 3 | 4 |        5         | 6 |
   |   |   |   |   |                  |   |

  1. The first field consists of eight bytes (64 bits) and is called the preamble. This field synchronizes the receiver to the sender, and lets other nodes (computers) on the network know that a transmission is in progress. The last two bits of the preamble are always 11.

  2. The second field, consisting of six bytes (48 bits), is the destination address of the frame. This address can be for a single node (my computer), a group of related nodes (all computers in the accounting department only), or all nodes (also called broadcasting).

  3. The third field is the origination address. This field is also six bytes in length. The node that created the frame has its address encoded here so any replies can be sent back to the originating node.

  4. The fourth item is the type field. This is a two byte (16 bit) field that identifies the protocol of the frame.

  5. Field 5 is the actual data sent. This field is variable in length. The minimum is 46 bytes, the maximum is 1,500 bytes. Anything larger than 1500 bytes will be broken up by the originating node into several frames and transmitted separately.

  6. The last field is the frame check sequence, and is 4 bytes (32 bits) long. This field contains error detection and correction data. This field is similar to a CRC check. The receiving station generates an error code from the combined destination, origination, type and data fields. It then compares it to the frame check sequence. If they do not match, the receiving node sends a retransmit request back to the originating node.

As you can see, downloading a 9Meg MP3 file is quite an undertaking. The 9Meg file must be broken up into small parts, enclosed in an Ethernet frame, then sent to your computer. Note that the Ethernet frame adds a lot of extra information to the package, so you are actually receiving almost 12Megs of information.

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