Figure 3.22 illustrates the transmission distance limits associated with the use of 10BASE-T and FOIRL-compliant fiber hubs and fiber adapters. Note that a fiber network access unit installed in the system unit of a PC can communicate
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Figure 3.22 10BASE-T and FOIRL transmission distance limits.
up to 1000 meters via fiber-optic cable, either directly into a fiber hub or to a fiber adapter connected to a wire hub. Also note that in the upper right corner of Figure 3.22, it was assumed that three 10BASE-T wire hubs were installed in a wire closet. If that wire closet is located on a low floor in a building and your organization leases another floor hundreds of feet above the wire closet floor, the use of fiber-optic cable provides a mechanism to link two network segments together. Similarly, the wire closet shown in the illustration could be in one building on a university campus, with fiber used to connect that building to another building. This use of fiber-optic cable not only extends the transmission distance of the network, but also eliminates the possibility of electromagnetic interference (EMI), because fiber cable is immune to EMI.
The distance limitation associated with the use of FOIRL has nothing to do with the transmission constraints associated with optical signals. Instead, the FOIRL distance limitation is related to the timing constraints in the IEEE 802.3 standard.
Because the FOIRL standard is limited to point-to-point connections, when used to interconnect two 10BASE-2 networks you must consider the cable length of each segment. For example, assume you have two 10BASE-2 networks located in different areas on a large factory floor, as illustrated
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OR
OR
OR
Legend:
= Workstation
= Optical repeater
Figure 3.23 Connecting 10BASE-2 networks using a fiber-optic interre-peater link.
in Figure 3.23. If segment A has a cable length of 300 meters and seg-ment B has a cable length of 500 meters, what is the maximum FOIRL cable length? Because the maximum cabling distance of a 10BASE-2 net-work is 2500 meters, subtracting the sum of the cable lengths from that cable constraint (2500−(300+500)) results in a maximum fiber-optic link of 1700 meters.
Another constraint you must consider is the Ethernet repeater limitation.
Ethernet limits any interconnected network to a maximum of four repeaters between any two nodes. Thus, when considering the use of fiber-optic repeaters, you must take into consideration the maximum network length as well as the number of repeaters that can be placed between nodes. As we will shortly note, under 10BASE-F a mechanism exists that allows the 5-4-3 rule to be exceeded.
10BASE-F
The development and promulgation of the IEEE 10BASE-F standard offi-cially provided network users with the ability to use fiber-optic hubs. As previously noted in the section covering the FOIRL specification, vendors integrated FOIRL capability into hubs; however, the actual standard that officially provided this capability was the 10BASE-F standard.
Under the 10BASE-F standard, which actually represents a set of fiber-optic media standards, three types of segments for fiber-optical transmission were defined. Those segments include 10BASE-FL, 10BASE-FB, and 10BASE-FP, each of which we will now examine.
10BASE-FL
The 10BASE-FL standard provides a fiber-optic link segment that can be up to 2000 meters in length, providing that only 10BASE-FL equipment is
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used at both ends of the segment. Otherwise, the mixing of 10BASE-FL and FOIRL equipment reduces the maximum length of the optical segment to 1000 meters.
The development of the 10BASE-FL standard was intended as a replace-ment for the older FOIRL specification. Unlike FOIRL that was restricted to providing an optical connection between repeaters, 10BASE-FL enables an optical segment to be routed between two workstations, two repeaters, or between a workstation and a repeater port.
The actual connection of a copper-based network node to a 10BASE-FL seg-ment can be accomplished in one of two ways. First, a stand-alone fiber-optic MAU (FOMAU) can be connected via the 15-pin AUI connector on a network adapter to provide an electrical-to-optical conversion capability. A second method is to use a 10BASE-T/FL converter. The latter is used when only a 10BASE-T port is available on a NIC. Both the 10BASE-FL FOMAU and the 10BASE-T/FL converter include two fiber-optic connectors, one for transmit-ting and one for receiving data. Some 10BASE-T/FL converters include two RJ-45 connectors that provide a high degree of cabling flexibility. One RJ-45 connector functions as a crossover cable and allows hub-to-hub communica-tions via optical media, while the second connector is for workstacommunica-tions that use a straight-through connection.
The top portion of Figure 3.24 illustrates the connection of a workstation to a 10BASE-FL FOMAU via a 15-pin AUI connector. The lower portion of that illustration shows the use of a 10BASE-T/FL converter. Both devices provide you with the ability to transmit up to 2000 meters via a fiber link when a 10BASE-F-compliant optical device is at the other end of the optical link.
In addition to the use of a 10BASE-FL FOMAU and 10BASE-T/FL con-verter, other types of converters have been developed to extend a fiber-optic transmission capability to other types of Ethernet networks. For example, a 10BASE-2/FL converter enables you to extend the transmission distance of a 10BASE-2 network via a fiber-optic connection.
The creation of a 10BASE-F optical hub is accomplished by the inclusion of two or more FOMAUs in the hub. This results in the hub becoming an optical repeater, which retransmits data received on one port onto all other ports.
Under the 10BASE-F standard you can use multiple 10BASE-FL connections to connect several individual workstations at distances up to 2000 meters to a common hub equipped with FOMAU ports.
Network Media
When examining the potential use of a converter or a FOMAU, it is important to verify the type of optical media supported. Multimode fiber (MMF) is
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Ethernet NIC
AUI cable 10BASE-FL TX
FOMAU RX Optical
cable (a) Using a fiber-optic MAU (FOMAU)
Ethernet NIC
UTP cable
10BASE-T/FL
Optical cable (b) Using a 10BASE-T/FL converter
H W
Figure 3.24 Options for connection of a workstation to a 10BASE-FL segment.
commonly used, with the most popular type of fiber having a 62.5-micron (µ) fiber-optic core and a 125-µ outer cladding. This type of multimode fiber is referenced by the numerics 62.5/125µ. The wavelength of light used on a 62.5/125-µ MMF fiber link is 850 nanometers (nm), and the optical loss budget, a term used to reference the amount of optical power lost through attenuation, should not exceed the range 9.7 to 19.0 dB, with the exact amount dependent upon the type of fiber used. Table 3.3 provides a comparison of the optical attenuation for 10BASE-FL and FOIRL for six types of multimode fiber.
In examining the entries in Table 3.3, you will note that 10BASE-FL has a higher loss budget than FOIRL for each type of multimode fiber. This explains why the transmission distance of 10BASE-FL optical repeaters exceeds the distance obtainable using FOIRL repeaters.
The connectors used on the previously described multimode fiber are referred to as ST connectors. The ST connector represents a spring-loaded bayonet connector whose outer ring locks onto the connection similar to the manner by which BNC connector’s junction on 10BASE-2 segments. To facilitate the connection process, an ST connector has a key on the inner sleeve and an outer bayonet ring. To make a connection you would line up the key on the inner sleeve of the ST plug with a slot on an ST receptacle, push the connector inward and then twist the outer bayonet ring to lock the
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TABLE 3.3 Comparing the Loss Budget (Optical Attenuation) of 10BASE-FL and FOIRL
Multimode, graded
index fiber size (µm) 50/125 50/125 50/125 62.5/125 83/125 100/140
Numerical aperture .20 .21 .22 .275 .26 .30
10BASE-FL Loss budget (dB)
9.7 9.2 9.6 13.5 15.7 19.0
FOIRL
Loss budget (dB)
7.2 6.7 7.1 11.0 13.2 16.5
connection in place. This action not only results in a tight connection but, in addition, provides a precise alignment between the two portions of fiber-optic cable being joined.
It is important to note that the optical loss depends upon several factors.
First, the length of the fiber governs optical loss, with a typical fiber illuminated at 850 nm having a loss between 4 dB and 5 dB per 1000 meters. Second, these of more connectors results in a higher optical loss. Third and perhaps most important to note since this is easy to rectify, if your connectors or fiber splices are poorly made or if dirt, finger oil or dust resides on connector ends, you will obtain a higher level of optical loss than necessary.
10BASE-FB
A second 10BASE-F specification is 10BASE-FB, with the B used to denote a synchronous signaling backbone segment. The 10BASE-FB specification enables the limit on the number of repeaters previously described in the 5-4-3 rule section to be exceeded. A 10BASE-FB signaling repeater is commonly used to connect repeater hubs together into a repeated backbone network infrastructure that can span multiple 2000-m links.
10BASE-FP
A third 10BASE-F specification was developed to support the connection of multiple stations via a common segment that can be up to 500 meters in length. Referred to as 10BASE-FP, the P references the fact that the end segment is a fiber-passive system. Under the 10BASE-FP specification a single fiber-optic passive-star coupler can be used to connect up to 33 stations. Those stations can be located up to 500 meters from a hub via the use of a shared fiber segment.
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Although 10BASE-FP provides a clustering capability that might represent a practical networking solution to the requirements of some organizations, as well as reduce the cost associated with the use of multiple individual optical cables, most organizations prefer to link hubs together. Doing so also allows the use of a single pair of optical cables; however, the transmission distance to the cluster is then extended to 2000 meters.
3.4 High-Speed Ethernet
To differentiate Gigabit and 10 Gbps Ethernet from other versions of Ethernet whose operating rates exceed 10 Mbps but have a maximum operating rate one-tenth that of Gigabit, those versions of Ethernet that operate at or below 100 Mbps were classified as high-speed Ethernet and are covered in this section. This enables Gigabit and 10 Gbps Ethernet to be covered as a separate entity in Section 3.5.
There are three broad categories into which high-speed Ethernet networks fall. The first two types of high-speed Ethernet networks are represented by de jure standards for operations at 100 Mbps, while the third standard represents an extension to 10BASE-T that operates at 16 Mbps. As discussed in Chapter 2, the IEEE standardized two general types of 100-Mbps Ethernet networks, with the 802.3µ standard defining three types of 100-Mbps CSMA/CD networks, while the 802.12 standard defines a demand-priority operation that replaces the CSMA/CD access protocol. The third high-speed Ethernet network is considered as a high-speed network only when compared with the operating rate of Ethernet networks developed before 1992. This type of network, referred to asisochronous Ethernet(isoENET), operates at 16 Mbps. This section will focus upon obtaining a overview of the operation and use of each of these three types of Ethernet networks.