[6 pages — 4338 words]


INTRODUCTION

There has been much discussion recently regarding the applicability of using Ethernet at various levels of the control hierarchy. Since Ethernet is so prevalent in the office and is frequently used as the enterprise network for high-end controllers, it would seem to be natural to use Ethernet at the control level or even at the device level as proposed by some in our industry. The arguments for its application include low cost, good connectivity and simple migration to higher speed networks. The cry to use "standard" Ethernet for control applications requires an understanding of the fundamentals of Ethernet.


What is standard Ethernet?

I am not sure what standard Ethernet is, but it certainly is not the 2.94 Mbps version that came out of Xerox's Palo Alto Research Center (PARC) in the early 70s. In 1980, Digital Equipment Corporation (DEC), Intel and Xerox published the DIX V1.0 standard which boosted the speed of Ethernet to 10 Mbps while maintaining Ethernet's thick trunk cabling scheme. In 1982 the DIX V2.0 standard was released and is now commonly referred to as Ethernet II. Xerox then relinquished its trademark.

At the time of the first DIX standard, the Institute of Electrical and Electronic Engineers (IEEE) was attempting to develop open network standards through the 802 committee. In 1985 the IEEE 802.3 committee published "IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications." This technology is called 802.3 CSMA/CD and not Ethernet; however, it is frequently referred to as Ethernet even though the frame definition differs from DIX V2.0. Although 802.3 and DIX frames can coexist on the same cable, interoperability is not assured. Therefore, when discussing "Ethernet," it is necessary to distinguish between 802.3 frames and DIX V2.0 frames.

To further confuse issues, standard Ethernet sometimes means an attached protocol—mainly TCP/IP. Ethernet only defines the data link and physical layers of the Open Systems Interconnect (OSI) Reference Model, whereas TCP/IP defines the transport and network layers, respectively, of the same model. Therefore, when the suggestion is made to use standard Ethernet for control does this mean TCP/IP connectivity as well?

APPLICATION
PRESENTATION
SESSION
TRANSPORT
NETWORK
DATA LINK
PHYSICAL

Figure 1 — Ethernet defines the lower two layers of the OSI Reference Model.


ETHERNET FRAMES

The two types of Ethernet frames used in industry are similar. The DIX V2.0 frame, frequently referred to as the Ethernet II frame, consists of an eight-byte preamble, six-byte source and destination addresses, a two-byte type field used to identify higher layer protocols, a variable data byte field followed by a four-byte frame check sequence (FCS) field. The IEEE 802.3 frame divides the preamble into a seven-byte preamble followed by a single byte start of frame delimiter (SFD). The two-byte type field now becomes a two-byte length field. The data field now includes an 802.2 Logical Link Control (LLC) field that precedes the actual data. The FCS remains the same.


  Ethernet II DIX Frame
64 bits
48 bits
48 bits
16 bits
368 to 12000 bits
(46 to 1500 bytes)
32 bits
Preamble
Individual / Group Address Bit
Destination Address
Source Address
Type
Data
Frame Check Sequence

  IEEE 802.3 Frame
56 bits
8 bits
48 bits
48 bits
16 bits
368 to 12000 bits
(46 to 1500 bytes)
32 bits
Preamble
SFD
Individual/ Group Address Bit
Globally/ Locally Administered Address Bit
Destination Address
Source Address
Length
LLC/Data
Frame Check Sequence

Figure 2 — Two types of Ethernet frames are used in industry.

Preamble

The DIX preamble consists of 64 bits of alternating "1s" and "0s", but ending with two "1s" to indicate that a valid frame is to begin. This creates a 10 MHz signal that synchronizes the receivers on the network before actual data arrives. Ethernet uses Manchester encoding.

The IEEE redefined the preamble to be seven bytes of preamble, the same as the DIX preamble, followed by a one-byte start of frame delimiter (SFD) which looks like the last byte of the DIX preamble. There is no change in operation between the DIX preamble and the IEEE preamble and SFD byte. Both preambles are considered not part of the frame when calculating the size of the overall frame.

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