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With this lesson, we will return to our discussion on Ethernet by introducing the topic of Fast Ethernet. Fast Ethernet increases the data rate by 10 fold while maintaining interoperability with legacy Ethernet.

Since its introduction in the early 70’s, Ethernet has continued to evolve evidenced by the fact the IEEE 802.3-2000 standard, which describes the technology, is now over 1500 pages! Ethernet was created with a multi-drop physical layer using a thick backbone eventually operating at 10 Mbps. A thin coaxial bus topology was introduced to reduce cabling expense, but it was the twisted-pair version that received the greatest acceptance. Throughout these changes, Ethernet maintained its 10 Mbps data rate. What was going to be Ethernet’s encore?

Fiber Distributed Data Interface (FDDI) operating a 100 Mbps did exist but was considered expensive. The question was, "Could Ethernet operate at 100 Mbps?" The answer was "yes," but there were competing approaches to Fast Ethernet. One approach was not to make any changes at all, just scale the protocol to 100 Mbps. This would provide backward compatibility and was favored by most vendors. The second approach favored a redesign of the medium access control (MAC) in order to gain a feature called demand-priority at the sake of backward compatibility. In the end the "technically elegant" demand-priority solution, which utilized a token-passing protocol, lost out both in the IEEE 802.3 committee and in the marketplace. Introduced as 100VG-AnyLAN, the technology never gained widespread support outside its two proponents, Hewlett Packard and AT&T. Although the words "Fast Ethernet" are not used, the IEEE 802.3u was adopted as the Fast Ethernet standard in 1995.

How Fast is Fast?

It is generally accepted that Fast Ethernet implies Ethernet at 100 Mbps. Compared to 10 Mbps, 100 Mbps seems quite fast. In fact, to most microcontrollers it is exceptionally fast and a difficult speed to maintain. However, there are still much faster technologies. Can Ethernet scale to 1000 Mbps? Well not exactly unless you are content with a maximum network diameter of 20 m. Still many attributes of 10 Mbps Ethernet are retained in the 1000BASE-T standard. This technology is called Gigabit Ethernet and its proposed successor will operate at 10 Gigabits/second. Although 100 Mbps is not the fastest, we will still concentrate on this technology since it has the most application on the plant floor.


In Table 1 you will see a summary of attributes for both Ethernet and Fast Ethernet. You will notice the values of the various Ethernet attributes, in terms of bits or bit-times, are maintained as Ethernet is scaled from 10 Mbps to 100 Mbps. However, one bit-time at 10 Mbps is 100 ns and at 100 Mbps it is 10 ns. Look at Table 2 where the actual times replace the bit and bit-time figures. The good news is that an equivalent amount of data sent at 10 Mbps will be received in a tenth of the time at 100 Mbps. Also notice that the minimum frame size and slot time have been reduced by a factor of ten as well. These attributes are directly linked to the collision domain of Ethernet and determine the maximum network diameter of the network based upon the maximum round-trip delay of a signal propagating between the two furthest points on the network. While the Ethernet protocol scales nicely from 10 Mbps to 100 Mbps, the actual time it takes for signals to pass down wires does not. The result is that the maximum network diameter must be reduced from about 2800 m for 10 Mbps Ethernet to a low of 205 m for Fast Ethernet. The only way to increase distance while retaining 100 Mbps speed is to use full-duplex links which eliminate collisions altogether; therefore, the link segments are not limited by timing.

Various Attributes for Ethernet and Fast Ethernet
Ethernet /802.3
Fast Ethernet 802.3u
Slot Time 512 bit-times 512 bit-times
Interframe Gap 96 bit-times 96 bit-times
Attempt Limit 16 tries 16 tries
Backoff Limit 10 (exponent) 10 (exponent)
Jam Size 32 bits 32 bits
Max Frame Size 1518 bytes 1518 bytes
Min Frame Size 64 bytes 64 bytes
Address Size 48 bytes 48 bytes

Table 1 — Scaling Ethernet by 10 fold yields Fast Ethernet
without any change in attribute parameters.

Various Attributes for Ethernet and Fast Ethernet
Ethernet /802.3
Fast Ethernet 802.3u
Slot Time 51.2 Ás 5.12 Ás
Interframe Gap 9.6 Ás 0.96 Ás
Attempt Limit 16 tries 16 tries
Backoff Limit 10 (exponent) 10 (exponent)
Jam Size 3.2 Ás 0.32 Ás
Max Frame Size 1214.4 Ás 121.44 Ás
Min Frame Size 6.4 Ás 0.64 Ás
Address Size 4.8 Ás 0.48 Ás

Table 2 — In terms of actual time, there are significant differences
between Ethernet and Fast Ethernet.


Full-duplex links are the key to extending the maximum network diameter of Fast Ethernet. Full-duplex requires separate receive and transmit paths and link segments consisting of no more than two devices. These devices can be Ethernet adapters or switching hub ports. Notice that I did not mention repeating hub ports. A repeating hub is part of the collision domain and reinforces collisions received on any of its other ports. A repeating hub is not capable of full-duplex operation. Although it is possible to have just two Ethernet adapters configured for full-duplex, expansion beyond two adapters requires a switching hub capable of supporting full-duplex operation.

Half-duplex means transmitting and receiving over the same medium but not at the same time. Full-duplex allows for simultaneous sending and receiving. Coaxial-based transceivers such as 10BASE5 and 10BASE2 never were able to invoke full-duplex since they do not have separate receive and transmit paths. However, 10BASE-T and 10BASE-FL do have separate receive or transmit paths and are capable of full-duplex operation depending upon the sophistication of the Ethernet adapter or switching hub. If these interfaces are configured for half-duplex, then the simultaneous detection of receive and transmit activity will trigger collision detection. These same interfaces, configured for full-duplex, would disable this collision detection logic since full-duplex does not follow the CSMA/CD rules of shared Ethernet.

It is very important that a full-duplex link be configured properly. A station or switching hub port will send out frames at will, ignoring the CSMA/CD protocol of shared Ethernet, if it is configured for full-duplex. If the other end is configured for half-duplex, it will incorrectly detect collisions and take actions that could cause late collisions (which are not automatically resent) and CRC errors. The result is a general slowdown of the network negating the benefits of migrating to Fast Ethernet.

200 Mbps?

One virtue of full-duplex is the claim that throughput is doubled to 200 Mbps by simply invoking full-duplex over half-duplex. Although mathematically correct that two simultaneous data streams can travel on the medium—one going 100 Mbps in one direction and another going 100 Mbps in the other direction—this occurrence is not common when using industrial automation protocols. Most protocols operate as either master/slave or command/response. With these types of protocols, a station initiates a command to another station which must decode and execute the command before providing the response. This type of communication could function quite well with a half-duplex link since only a command stream or a response stream is active at a time. In backbone applications, full-duplex operation can be helpful, but do not assume that a system will double in performance by simply invoking full-duplex. The benefits of full-duplex are mostly to gain a larger network diameter and eliminate the complexity and uncertainty of the CSMA/CD protocol.

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