9.2 100-Mbps Media Systems

Compared to the 10-Mbps specifications, the 100-Mbps system results in a factor of ten reduction in the bit-time, which is the amount of time it takes to transmit a bit on the Ethernet channel. This produces a tenfold increase in the speed of the packets over the media system. However, the other important aspects of the Ethernet system including the frame format, the amount of data a frame may carry, and the media access control mechanism, are all unchanged.

The Fast Ethernet specifications include mechanisms for Auto-Negotiation of the media speed. This makes it possible for vendors to provide dual-speed Ethernet interfaces that can be installed and run at either 10-Mbps or 100-Mbps automatically.

There are three media varieties that have been specified for transmitting 100-Mbps Ethernet signals.

FIGURE 9.1 The three 100-Mbps Ethernet media varieties

The three media types are shown with their IEEE shorthand identifiers. The IEEE identifiers include three pieces of information. The first item, "100", stands for the media speed of 100-Mbps. The "BASE" stands for "baseband," which is a type of signaling. Baseband signaling simply means that Ethernet signals are the only signals carried over the media system.

The third part of the identifier provides an indication of the segment type. The "T4" segment type is a twisted-pair segment that uses four pairs of telephone-grade twisted-pair wire. The "TX" segment type is a twisted-pair segment that uses two pairs of wires and is based on the data grade twisted-pair physical medium standard developed by ANSI. The "FX" segment type is a fiber optic link segment based on the fiber optic physical medium standard developed by ANSI and that uses two strands of fiber cable. The TX and FX medium standards are collectively known as 100BASE-X.

The 100BASE-TX and 100BASE-FX media standards used in Fast Ethernet are both adopted from physical media standards first developed by ANSI, the American National Standards Institute. The ANSI physical media standards were originally developed for the Fiber Distributed Data Interface (FDDI) LAN standard (ANSI standard X3T9.5), and are widely used in FDDI LANs.

Rather than "re-inventing the wheel" when it came to signalling at 100-Mbps, the Fast Ethernet standard adapted these two ANSI media standards for use in the new Fast Ethernet medium specifications. The T4 standard was also provided to make it possible to use lower-quality twisted-pair wire for 100-Mbps Ethernet signals.

Quick Reference Guide to 100-Mbps Media Systems - 09 SEP 95

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