Understanding Gigabit Ethernet Performance

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DATA COMMUNICATIONS MANAGEMENT UNDERSTANDING GIGABIT ETHERNET PERFORMANCE

Gilbert Held

INSIDE Ethernet Frame Flow; The Gigabit Ethernet Frame

OVERVIEW Gigabit Ethernet represents the latest Institute of Electrical and Electronic Engineers (IEEE) Carrier Sense Multiple Access with Collision Detection (CSMA/CD) 802.3 standard. In developing the Gigabit Ethernet standard, the IEEE decided that it was important to retain the CSMA/CD access pro- tocol and frame format to provide a truly scalable technology, ranging from Ethernet’s 10Mbps operating rate to the 100Mbps operating rate of Fast Ethernet and to the 1Gbps operating rate of Gigabit Ethernet. Retain- ing the same media access and frame format simpliﬁes equipment design as the differences between each version of Ethernet become primarily one of speed adaptation on clocking. Unfortunately, retaining the frame structure of Ethernet and Fast Ether-

net introduced a new problem in the PAYOFF IDEA form of a collision domain. Gigabit Ethernet represents an increase in the op- Both Ethernet and Fast Ethernet erating rate of Fast Ethernet by an order of magni- provide sufﬁcient time between the tude. Although Gigabit Ethernet operates faster than Fast Ethernet, there are certain situations next-to-last bit in a frame being where the actual performance of the more mod- placed on a network and the receipt ern technology may not provide any signiﬁcant of a propagated collision signal to improvement when one considers the actual abil- enable a cabling length of 100 meters ity of each networking technology to transfer information. Thus, it is important for network man- from a workstation to a hub port. In agers and LAN administrators to understand how a Gigabit Ethernet environment, the Gigabit Ethernet transfers data and those situa- higher operating rate restricts the di- tions that inhibit its ability to transfer information. ameter of a network to approximate- Knowing the operation of Gigabit Ethernet will al- ly ten meters, as cabling beyond that low one to select an appropriate networking technology to satisfy organizational requirements distance closes the collision window without basing the decision on the operating rate and makes it possible for a station to of the network, which, under certain situations, transmit every bit in a frame while a can result in the inability of the technology to satisfy organizational requirements.

collision occurred on some prior bits and the collision signal is still prop- agating toward the transmitting station. Because a ten-meter diameter would be overly restrictive, the IEEE standardized a technique referred to as carrier extension technology that adds special symbols to the end of relatively short frames on a Gigabit network. While the resulting technique literally opens the collision window and permits a much more practical 200-meter network diameter, the carrier symbols occupy bandwidth and adversely affect the ability of Gigabit Ethernet to transport information per unit of time under certain conditions. Thus, the trade-off was between obtaining a reasonable network cabling diameter and low- ering the ability of the network to transport information under certain network situations. It should be noted that carrier extension technology is only applicable to half-duplex, shared-media Gigabit Ethernet. This is because full-duplex, nonshared media Gigabit Ethernet uses separate paths for the transmission and reception of data, precluding the possibility of collisions. However, because shared-media Gigabit Ethernet is copper based and costs significantly less per port than full-duplex, nonshared media, it is reasonable to expect that many network managers and LAN administrators will focus their attention on the less expensive technology. After all, 1Gbps of shared media bandwidth represents a considerable improvement over 100Mbps Fast Ethernet. Or does it? To answer this question, one must first examine the potential frame flow on different types of Ethernet networks.

ETHERNET FRAME FLOW Exhibit 1 illustrates the composition of an Ethernet frame. Note that the “Data” field must be a minimum of 46 bytes in length and is filled with pad characters if the number of bytes to be transported is less than 46. Thus, the minimum length of an Ethernet frame is 72 bytes. In some literature one may have read that the minimum length of an Ethernet frame is 64 bytes. That metric references the IEEE standard that defines the frame prior to its placement on the network at the physical layer. Because network adapter cards automatically add the “Preamble” and “Start of Frame Delimiter” fields that add eight bytes to the frame for synchronization, the minimum length of the frame as it flows on the network is 72 bytes. Returning to the Ethernet frame’s “Data” field, its maximum value is 1500 bytes. When one considers the 26 overhead bytes in each frame, this means that the maximum frame length on the network is 1526 bytes. Many publications reference a maximum frame length of 1518 bytes. Once again, such publications are referencing the length of the frame pri- or to its placement on the network and do not consider the additional eight bytes added by the network adapter card in the form of a “Pream- ble” field and a “Start of Frame Delimiter” field for synchronization.

EXHIBIT 1 — The Ethernet Frame Format

By understanding that the minimum length of an Ethernet frame is 72 bytes and its maximum length is 1526 bytes, one can compute the maximum number of minimum length and maximum number of maximum length frames that can ﬂow on an Ethernet network. Because many read- ers may be from the “Show Me” state of Missouri, the computations are given here. At a 10Mbps operating rate, the Ethernet standard requires a “dead time” of 9.6 µs between frames, while the bit duration is 100 ns. Thus, the time required to transmit a 72 byte minimum length Ethernet frame becomes:

9.6 µs + 72 bytes × 8 bits/byte × 100 ns/bit or 67.2 × 10–6 s. Thus, in one second, there can be a maximum of 1/67.2 × 10–6, or 14,880 minimum size 72-byte frames flowing on an Ethernet 10Mbps network. Similar to the manner by which the maximum number of minimum length frames that can flow on a 10Mbps Ethernet network was calculat- ed, one can also compute the maximum number of maximum length frames that can flow on that network. To do, first compute the time per frame as follows:

9.6 µs + 1526 bytes × 8 bits/byte × 100 ns/bit or 1.23 ns. Then, in one second, there can be a maximum of 1/1.23 ns or 812 maximum length frames that can ﬂow on a 10Mbps Ethernet network. In examining Fast Ethernet technology, one notes that the dead times between frames and bit duration are one tenth of 10Mbps and that both technologies use the same frame format with the same minimum frame length of 72 bytes and maximum frame length of 1526 bytes. Due to this, the maximum frame rate obtainable on Fast Ethernet is ten times the frame rate of 10Mbps Ethernet. In addition to determining the maximum frame rate on Ethernet and Fast Ethernet, one can also compute their information transfer capability. To do so, multiply their maximum and minimum frame rates by the number of bytes capable of being transported in maximum length and minimum length frames. For example, consider the maximum frame rate when minimum length frames are transported on an Ethernet network. Since a minimum length 72 byte frame is capable of transporting 46 bytes and 14,880 minimum length frames per second can ﬂow on an Ethernet network, the data transfer rate becomes:

14,880 frames/s × 46 bytes/frame × 8 bits/byte = 5.476Mbps

EXHIBIT 2 — Ethernet Frame and Information Transfer Capability

Average Frame Size Maximum Frame Maximum Information (Bytes) Rate Transfer

Ethernet 72 14880 5.476 Mbps 1526 812 9.744 Mbps

Fast Ethernet 72 14880 54.76 Mbps 1526 8120 97.44 Mbps

Similarly, one can also compute the information transfer capability of 10Mbps Ethernet for maximum length 1526-byte frames. Since a maximum of 812 frames per second can ﬂow on that network and each maximum length ﬁeld can transport 1500 bytes, then the maximum data transfer rate for maximum length frames becomes:

812 frames/s × 1500 bytes/frame × 8 bits/byte = 9.744Mbps

Exhibit 2 summarizes the maximum frame rates and information transfer capability for minimum length and maximum length frames flowing on Ethernet and Fast Ethernet networks. Because Fast Ethernet uses the same frame format and only differs from Ethernet by placing bits on the network faster by an order of ten, its transfer rate is ten times that of Ethernet. Thus, although one could compute the information transfer of Fast Ethernet in the same manner as for the information transfer for Ethernet, this will not be done. Instead, note that because the maximum frame rate on Fast Ethernet is ten times that of Ethernet, this results in the information transfer capability also being ten times that of Ethernet. Returning to the examination of the entries in Exhibit 2, one can now focus on the maximum information transfer capability of Ethernet and Fast Ethernet. Note that when the minimum frame length occurs that corresponds to the transmission of frames carrying interactive traffic, the maximum information transfer on Ethernet and Fast Ethernet is slightly more than half of the operating rate of each network. At the opposite end of the range of frame lengths, the transmission of maximum length frames that corresponds to file transfers results in the information transfer rate approaching the operating rate of each network. Thus, the average frame length flowing on a network has a significant effect on the information transfer rate obtainable on an Ethernet network. With this in mind, one can focus on Gigabit Ethernet by first examining the extension of the Ethernet frame via the use of carrier extension technology.

THE GIGABIT ETHERNET FRAME As previously mentioned, the higher operating rate of Gigabit Ethernet resulted in the IEEE extending its frame through the use of carrier extension technology for shared-media, half-duplex operations. This frame extension occurs by the addition of carrier extension symbols to ensure all frames have a minimum length of 512 bytes in the adapter and 520 bytes when placed onto the network. Through the use of carrier extension technology, a 200-meter network diameter can be supported. Otherwise, the need to ensure that a station could hear a collision propagate on the network between transmitting the next-to-last bit in a short frame and the last bit in the frame would have significantly reduced the allowable cabling distance to 10 meters (or approximately 33 feet). Exhibit 3 illustrates the Gigabit Ethernet frame format. Note that the carrier extension extends the frame timing to guarantee at least a 512- byte slot time (520 bytes on the network) for half-duplex, shared-media operations. The increase in the minimum length frame does not change the frame size and only alters the time the frame is on the network. This enables compatibility to be maintained between Ethernet, Fast Ethernet, and Gigabit Ethernet. Although the carrier extension scheme permits the support of a more reasonable and practical network diameter, that extension is not without a price. That price is one of additional network bandwidth utilization by carrier extension symbols, which reduces the information transfer capability of Gigabit Ethernet when carrier extension symbols are required. One can determine the frame rate for Gigabit Ethernet in the same manner as for Ethernet; that is, first compute the transmission time required for placing a Gigabit Ethernet frame on the network. Once this is accomplished, divide that time into one second to determine the number of frames per second that can flow on the network. Because Gigabit Ethernet also has minimum length and maximum length frames, perform two computations to determine the maximum number of minimum length frames and maximum number of maximum length frames that can flow on a Gigabit network. Once this is accomplished, multiply the frame rates by the number of bytes in the “Data” field of each frame to compute the information transfer rate for minimum length and maximum length frames. Obtaining this information will illustrate the effect of transporting certain types of data on the performance of Gigabit Ethernet, which will in turn illustrate the importance of throughput above the operating rate of a network. To compute the frame rate on Gigabit Ethernet, note that the minimum length frame is now 520 bytes when placed on the network. Be- cause carrier extension symbols are added to the end of an Ethernet frame, the actual composition of the frame is not altered. This means that the “Data” field will vary between 46 and 1500 bytes. Thus, the minimum number of bytes that can be transported in a minimum length Gigabit

EXHIBIT 3 — Half-Duplex Gigabit Ethernet Using Carrier Extension to Extend Timing

Ethernet frame is 46, which is the same as Ethernet and Fast Ethernet. However, instead of being 72 bytes in length, the minimum frame is 520 bytes on the network to include 448 carrier extension symbols. To compute the maximum number of minimum length frames that can ﬂow on a Gigabit Ethernet network, use a dead time between frames of 0.096 µs, a frame length of 520 bytes, and a bit duration of 1 ns. Thus, the time per minimum length frame becomes:

0.096 × 10–6 + 520 bytes × 8 bits/byte × 1 ns/bit = 4.256 × 10–6 s

Then, in one second, there can be a maximum of 1/4.256 × 10–6 or 234,962 frames per second. Because each frame can transport up to 46 bytes of data, the maximum information transfer capability for minimum length Gigabit Ethernet frames becomes:

234,962 frames/s × 46 bytes/frame × 8 bits/byte = 86.46Mbps

In comparing the value just computed to the entries in Exhibit 2, note that the ratio of Gigabit Ethernet’s Information transfer to that of Fast Ethernet for minimum length frames is 86.46/54.76, or approximately 1.58 to 1. Thus, although Gigabit Ethernet’s operating rate is ten times the operating rate of Fast Ethernet, when used to transport relatively short interactive queries, its information transfer capability is only slightly better than Fast Ethernet. A similar computation for the maximum length Gigabit frame shows that Gigabit Ethernet indeed provides ten times the information transfer capability of Fast Ethernet. This results from the fact that as the “Data” field increases, the number of carrier extensions decreases until a “Data” field transporting 494 or more bytes of data requires no carrier extension symbols and provides exactly ten times the information transfer capability of Fast Ethernet. Thus, for all frames with “Data” fields between 494 and 1500 bytes, half-duplex, shared-media Gigabit Ethernet provides what one would normally expect from the technology — an information transfer rate ten times that of Fast Ethernet and 100 times that of Ethernet.

RECOMMENDED COURSE OF ACTION While Gigabit Ethernet provides an operating rate ten times that of Fast Ethernet, its information transfer rate can be much lower. This means it is important to examine the type of data being transmitted on a network prior to considering the use of a shared-media, half-duplex Gigabit Ether- net backbone. If the organization’s average frame length is relatively low, one may need to consider installing a more expensive, full-duplex Giga- bit Ethernet backbone or consider an alternate technology such as ATM.

Gilbert Held is an internationally known author and lecturer. Gil is the author of over 40 books and 250 technical articles. Some of Gil’s recent titles include Ethernet Networks, 3rd ed.; Data and Image Compression, 4th ed.; and Working with Network Based Images, all published by John Wiley & Sons of New York and Chichester, En- gland. Gil can be reached via email at [email protected].