51-20-98 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 in- formation. 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 tech- nology 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 sat- isfy organizational requirements. 08/99 Auerbach Publications © 1999 CRC Press LLC 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 tech- nique literally opens the collision window and permits a much more practical 200-meter network diameter, the carrier symbols occupy band- width and adversely affect the ability of Gigabit Ethernet to transport in- formation 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-du- plex, nonshared media Gigabit Ethernet uses separate paths for the trans- mission 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 administra- tors will focus their attention on the less expensive technology. After all, 1Gbps of shared media bandwidth represents a considerable improve- ment 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 net- work 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 max- imum 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 net- work. 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 num- ber of bytes capable of being transported in maximum length and mini- mum 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 maxi- mum of 812 frames per second can flow on that network and each max- imum 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 trans- fer 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 cor- responds 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.