100 Mbps (Fast) Ethernet

Classic Ethernet, at 10 Mbps, is quite slow by modern standards, and so by 1995 the IEEE had created standards for Ethernet that operated at 100 Mbps. Ethernet at this speed is commonly known as Fast Ethernet; this name is used even today as “Fast” Ethernet is being supplanted by Gigabit Ethernet (below). By far the most popular form of 100 Mbps Ethernet is officially known as 100BASE-TX; it operates over twisted-pair cable.

In the previous analysis of 10 Mbps Ethernet, the bandwidth, minimum packet size and maximum network diameter were all interrelated, in order to ensure that collisions could always be detected by the sender. Increasing the speed means that at least one of the other constraints must be scaled as well. For example, if the network physical diameter were to remain the same when moving to 100 Mbps, then the Fast-Ethernet round-trip time would be the same in microseconds but would be 10-fold larger measured in bits; this might mean a minimum packet size of 640 bytes instead of 64 bytes. (Actually, the minimum packet size might be somewhat smaller, partly because the “jam signal” doesn’t have to become longer, and partly because some of the numbers in the 10 Mbps delay budget above were larger than necessary, but it would still be large enough that a substantial amount of bandwidth would be consumed by padding.) The designers of Fast Ethernet felt that such a large minimum-packet size was impractical.

However, Fast Ethernet was developed at a time (~1995) when reliable switches (below) were widely available; the quote above at 2 Ethernet from [MB76] had become obsolete. Large “virtual” Ethernet networks could be formed by connecting small physical Ethernets with switches, effectively eliminating the need to support large-diameter physical Ethernets. So instead of increasing the minimum packet size, the decision was made to ensure collision detectability by reducing the network diameter instead. The network diameter chosen was a little over 400 meters, with reductions to account for the presence of hubs. At 2.3 meters/bit, 400 meters is 174 bits, for a round-trip of 350 bits. The slot time (and minimum packet size) remains 512 bits – now 5.12 µsec – which is safely large enough to ensure collision detection.

This 400-meter diameter, however, may be misleading: the specific 100BASE-TX standard, which uses so-called Category 5 twisted-pair cabling (or better), limits the length of any individual cable segment to 100 meters. The maximum 100BASE-TX network diameter – allowing for hubs – is just over 200 meters. The 400-meter distance does apply to optical-fiber-based 100BASE-FX in half-duplex mode, but this is not common.

The 100BASE-TX network-diameter limit of 200 meters might seem small; it amounts in many cases to a single hub with multiple 100-meter cable segments radiating from it. In practice, however, such “star” configurations could easily be joined with switches. Ethernet Switches, switches partition an Ethernet into separate “collision domains”; the network-diameter rules apply to each domain separately but not to the aggregated whole. In a fully switched (that is, no hubs) 100BASE-TX LAN, each collision domain is simply a single twisted-pair link, subject to the 100-meter maximum length.

Fast Ethernet also introduced the concept of full-duplex Ethernet: two twisted pairs could be used, one for each direction. Full-duplex Ethernet is limited to paths not involving hubs, that is, to single station-tostation links, where a station is either a host or a switch. Because such a link has only two potential senders, and each sender has its own transmit line, full-duplex Ethernet is entirely collision-free. Fast Ethernet (at least the 100BASE-TX form) uses 4B/5B encoding, covered in 4.1.4 4B/5B. This means that the electronics have to handle 125 Mbps, versus the 200 Mbps if Manchester encoding were still used.

Fast Ethernet 100BASE-TX does not particularly support links between buildings, due to the maximumcable-length limitation. However, fiber-optic point-to-point links are an effective alternative here, provided full-duplex is used to avoid collisions. We mentioned above that the coax-based 100BASE-FX standard allowed a maximum half-duplex run of 400 meters, but 100BASE-FX is much more likely to use full duplex, where the maximum cable length rises to 2,000 meters.