The very first standard Ethernet frame was the DIX standard.
I show an illustration of this frame below in Figure 1.
Figure 1, Illustration of the DIX (Ethernet) Frame.
We call this frame the “DIX” frame because DEC-Intel and Xerox Corporations (DIX) invented this standard.
With some very small changes to this Frame, the Institute for Electrical and Electronic Engineers (IEEE) developed its standard for an Ethernet frame. It referred to this particular frame structure as the IEEE 802.3 (Basic) Ethernet frame.
In another blog post, I will discuss the IEEE 802.3 (Basic) Ethernet frame in greater detail.
The DIX (Ethernet) frame can be as short as 64 bytes and as long as 1518 bytes (not including the Preamble).
A couple of things that differentiate this frame from the IEEE 802.3 (Basic) frame are:
- The Preamble, and
- The Type Field
I will discuss each of these items below.
The Preamble
I identify the location of the Preamble within the DIX frame below in Figure 2.
Figure 2, The Location of the Preamble within the DIX Frame.
The main difference between the Preamble, within the DIX Frame, and that within the IEEE 802.3 (Basic) Frame is how they are defined.
I also show the byte format of the Preamble within the DIX frame below in Figure 3.
Figure 3, The Byte-format of the Preamble within the DIX Frame.
I will list the characteristics/features of the Preamble below.
- Consists of 8-bytes
- The first seven (7) bytes contain the value of 10101010
- The 8th byte contains the value: 10101011
- The last two bits (of the Preamble) are set to “[1, 1,]” to denote the end of the Preamble and the start of the Destination Address field.
The Type Field
I show the location of the Type field (within the DIX Frame), below in Figure 4.
Figure 4, Illustration of the Location of the Type-Field within the DIX Frame.
In the DIX Standard, the Type field is a 16-bit field containing an identifier that indicates the type of high-level protocol data we are carrying within the Data Field of the Ethernet Frame.
For example, if the Type field is set to 0x0800, then the Data Field (within this particular Ethernet frame) is transporting data that supports the IP (Internet Protocol).
In another blog post, I present a list of standard values for the Type field.
The IEEE 802.3 (Basic) and the other IEEE 802.3-compliant Ethernet frames all use the Type/Length for this 2-byte position within the Ethernet frame.
I have defined the Destination Address, Source Address, Data Field, and Frame Check Sequence fields in other blog posts.
