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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x   Pub Date: 06/17/98
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Using the Fiber Distributed Data Interface

FDDI has been available for several years, but it is still a much newer protocol than Ethernet or Token Ring. Designed by the X3T9.5 Task Group of ANSI (the American National Standards Institute), FDDI passes tokens and data frames around a ring of optical fiber at a rate of 100Mbps. FDDI was designed to be as much like the IEEE 802.5 Token-Ring standard as possible, above the Physical layer. Differences occur only where necessary to support the faster speeds and longer transmission distances of FDDI.

If FDDI were to use the same bit-encoding scheme used by Token Ring, every bit would require two optical signals: a pulse of light and then a pause of darkness. This means that FDDI would need to send 200 million signals per second to have a 100 Mbps transmission rate. Instead, the scheme used by FDDI—called NRZI 4B/5B—encodes 4 bits of data into 5 bits for transmission so that fewer signals are needed to send a byte of information. The 5-bit codes (symbols) were chosen carefully to ensure that network timing requirements are met. The 4B/5B scheme, at a 100Mbps transmission rate, actually causes 125 million signals per second to occur (this is 125 megabaud). Also, because each carefully selected light pattern symbol represents 4 bits (a half byte, or nibble), FDDI hardware can operate at the nibble and byte level rather than at the bit level, making it easier to achieve the high data rate.

Two major differences in the way the token is managed by FDDI and IEEE 802.5 Token Ring exist. In traditional Token Ring, a new token is circulated only after a sending workstation gets back the frame that it sent. In FDDI, a new token is circulated immediately by the sending workstation after it finishes transmitting a frame, a technique that has since been adapted for use in Token-Ring networks and called Early Token Release. FDDI classifies attached workstations as asynchronous (workstations that are not rigid about the time periods that occur between network accesses) and synchronous (workstations having very stringent requirements regarding the timing between transmissions). FDDI uses a complex algorithm to allocate network access to the two classes of devices.

Although it provides superior performance, FDDI’s acceptance as a desktop network has been hampered by its extremely high installation and maintenance costs (see “Using Fiber Optic Cable” earlier in this chapter).

Using 100Mbps Ethernet

One of the largest barriers to the implementation of high-speed networking has been the need for a complete replacement of the networking infrastructure. Most companies cannot afford the down time needed to rewire the entire network, replace all the hubs and NICs, and then configure everything to operate properly. As a result of this, some of the new 100Mbps technologies are designed to make the upgrade process easier in several ways. First, they can often use the network cable that is already in place, and second, they are compatible enough with the existing installation to allow a gradual changeover to the new technology, workstation by workstation. Obviously, these factors also serve to minimize the expense associated with such an upgrade.

The two systems that take this approach are 100BaseT, first developed by the Grand Junction Corp., and 100VG AnyLAN, advocated by Hewlett-Packard and AT&T. Both of these systems run at 100Mbps over standard UTP cable, but that is where the similarities end. In fact, of the two, only 100BaseT can truly be called an Ethernet network. 100BaseT uses the same CSMA/CD media access protocol and the same frame layout defined in the IEEE 802.3 standard. In fact, 100BaseT as been ratified as an extension to that standard, called 802.3u.

To accommodate existing cable installations, the 802.3u document defines four different cabling standards, as shown in Table 1.7.

Table 1.7 100BaseT Cabling Standards.

Standard Cable Type Segment Length

100BaseTX Category 5 (2 pairs) 100 meters
100BaseT4 Category 3, 4, or 5 (4 pairs) 100 meters
100BaseFX 62.6 micrometer Multimode fiber (2 strands) 400 meters

Sites with Category 3 cable already installed can therefore use the system without the need for rewiring, as long as the full four pairs in a typical run are available.


Note:  
Despite the apparent wastefulness, in most cases it is not recommended that that data and voice traffic be mixed within the same cable, even if sufficient wire pairs are available. Digital phone traffic could possibly coexist, but normal analog voice lines will definitely inhibit the performance of the data network.

100BaseT also requires the installation of new hubs and new NICs, but because the frame type used by the new system is identical to that of the old, this replacement can be done gradually, to spread the labor and expense over a protracted period of time. You could replace one hub with a 100BaseT model, and then switch workstations over to it, one at a time, as the users needs and the networking staff’s time permits. You can even purchase NICs that can operate at both 10 and 100Mbps speeds to make the changeover even easier.

100VG (voice grade) AnyLAN also runs at 100Mbps, and is specifically designed to use existing Category 3 UTP cabling. Like 100BaseT4, it requires four pairs of cable strands to affect its communications. There are no separate Category 5 or fiber-optic options in the standard. Beyond the cabling, 100VG AnyLAN is quite different from 100BaseT and indeed from Ethernet.

While 10 and 100BaseT networks both reserve one pair of wires for collision detection, 100VG AnyLAN is able to transmit over all four pairs simultaneously. This technique is called quartet signaling. A different signal encoding scheme called 5B/6B NRZ is also used, sending 2.5 times more bits per cycle than an Ethernet network’s Manchester encoding scheme. Multiplied by the four pairs of wires (as compared to 10BaseT’s one), you have a tenfold increase in transmission speed.

The fourth pair is made available for transmission because there is no need for collision detection on a 100VG AnyLAN network. Instead of the CSMA/CD media access system that defines an Ethernet network, 100VG AnyLAN uses a brand new technique called demand priority. Individual network computers have to request and be granted permission to transmit by the hub before they can send their data.

100VG AnyLAN also used the 802.3 frame type, so its traffic can coexist on a LAN with regular Ethernet. Like 100BaseT, combination 10/100 NICs are available, and the installation can be gradually migrated to the new technology.

Support for 100VG AnyLAN has almost completely disappeared from the market due to the cost of the adapters and the popularity of 10/100Mbps Ethernet adapters.


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