e value corre- sponding to the difference between the arrival of the token and the TTRT value. To transmit asynchronous frames, a station must have a token and the THT timer must not have expired. VALID TRANSMISSION TIMER A third FDDI station timer is the valid transmission timer (TVX). The function of this timer is to time the peri- od between valid transmissions on the FDDI ring. If a token is lost, excessive noise occurs on the ring, or another impairment results in the TVX expiring, the station assumes a problem occurred and initiates a ring initialization operation to restore the ring to an operational status. Summary FDDI represents a well-thought-out but complex fiber-optic LAN tech- nology. While it can support a dual ring up to 100 km (62 miles), which makes it attractive for MAN operations, its primary use is in a LAN environment. Unfortunately, its relatively high cost because of its com- plexity has limited its appeal in comparison to the Fast Ethernet and Gigabit Ethernet, which benefit from lesser complexity and greater economies of scale; the latter advantage is due to the considerably larger market for the Ethernet products. Nevertheless, FDDI continues to be used in mission-critical areas where reliability as well as the ability to transmit data near the maximum LAN operating rate is critical. Ethernet and Fast Ethernet Anyone who has not been in a prolonged period of isolation would know that the battle for LAN dominance is over, with Ethernet in its several flavors representing the victor. However, what we may not realize is the fact that fiber-optic components have a long association with Eth- ernet, dating from their use as a transmission extender for 10 Mbit/s Chapter Five Ethernet to the preferred media used to construct a Gigabit Ethernet network. In this section we will focus on the use of optical components to extend the capabilities of different types of Ethernet networks. How- ever, because fiber-optic media are used primarily to extend transmis- sion distance in an Ethernet and Fast E