This chapter discusses topology rules that determine the maximum cable lengths allowed between SuperStack II Hub 100 units and other network devices. This chapter also covers cabling requirements for the two transceiver interface modules that are compatible with the Hub 100.
You can use the SuperStack II Hub 100 in many network environments. The IEEE 802.3u specifications define the Fast Ethernet cabling and topology requirements for the various network configurations.
The Fast Ethernet specification preserves the 100-meter maximum unshielded twisted-pair (UTP) cable length from the hub to an end node (data terminal equipment [DTE], switch, bridge, or router) that is also a requirement of the 10 Mbps Ethernet standard. (See "10BASE-T Standard" in Appendix A for information on 10 Mbps Ethernet requirements.) In addition, the following important 100 Mbps topology rules specify cabling requirements for the SuperStack II Hub 100.
The SuperStack II Hub 100 TX can operate as a Class II repeater. According to the 802.3u specification, there can be no more than two Class II repeaters per collision domain. A collision domain is defined as a length of cable with a switch, bridge, or router attached to one end of the cable and end nodes attached to the other end through a hub (or repeater). Therefore, two SuperStack II Hub 100 TX units or hub stacks can be in a single local area network (LAN) segment.
Both SuperStack II Hub 100 TX models described in this guide (3C250C-TX-12 and 3C250C-TX-24) can operate as Class I or Class II repeaters. These models replace the Hub 100 TX 3C250B-TX, which can also operate as a Class I or Class II repeater. An earlier Hub 100 TX model, 3C250A-TX/I, operates only as a Class I repeater.
A LAN segment is defined as the length of cable (copper or fiber-optic) that connects any two end nodes. A LAN segment with one or two Class II repeaters can be interconnected with another LAN through a switch, a router, or a bridge. See "Considerations for Multisegmented Networks" in Appendix B for additional information on repeaters and collision domains.
In a single collision domain containing one Class II repeater using TX signaling, the maximum network diameter is 200 meters using Category 5 UTP cable, or 308.8 meters combining fiber-optic and UTP cable. See Table 3-1 for a summary of cable types and connectors required for connecting the hub to any source or destination of data on the LAN.
In a single collision domain containing two Class II repeaters using TX signaling, the maximum network diameter is 205 meters using UTP cable, or 216.2 meters combining fiber-optic and UTP cable.
The SuperStack II Hub 100 T4 is a Class I repeater. According to the 802.3u specification, there can be only one Class I repeater per collision domain. Therefore, only one SuperStack II Hub 100 T4 or hub stack can be in any single LAN segment. A LAN segment with a Class I repeater can be interconnected with another LAN through a switch, a router, or a bridge. See "Considerations for Multisegmented Networks" in Appendix B for additional information on repeaters and collision domains.
In a single collision domain containing a Class I repeater using T4 signaling, the maximum network diameter is 200 meters using only Category 3, 4, or 5 UTP cable, or 231 meters with a combination of fiber-optic and UTP cable. See Table 3-1 for a summary of cable types and connectors required for connecting the hub to any source or destination of data on the LAN.
A hub stack composed of both Hub 100 TX (Class II) and Hub 100 T4 (Class I) units functions as a Class I repeater.
Table 3-1 summarizes the possible schemes for connecting the SuperStack II Hub 100 in a 100BASE-T network. See Appendix B for rules governing maximum cable lengths for each type of connection.
Once installed, the hub can support up to 12 (or 24) end-station connections through the front panel. Figure 3-1 shows 11 PCs and one server connected to the RJ-45 ports on the hub's front panel. You can connect any combination of PCs and servers to the RJ-45 ports.
The thirteenth port (or twenty-fifth port for the 24-port hub) permits an additional connection through the transceiver interface module located on the hub's rear panel. See "Installing the Transceiver Interface Modules" in Chapter 2.
Figure 3-1 SuperStack II Hub 100 Supporting 11 Users and a Server
The maximum distance allowed between a single SuperStack II Hub 100 and any end node using UTP cable is 100 meters, making the maximum collision domain diameter 200 meters. With a combination of 100BASE-TX UTP cabling and 100BASE-FX multimode fiber-optic cabling, the total network span from a Hub 100 TX can be up to 216.2 meters with two Hub 100 TX (Class II) repeaters in a collision domain. (See Figure B-7.) If you use a single Hub 100 TX repeater in a collision domain, the total network span can be up to 308.8 meters. (See Figure B-5.)
With a combination of 100BASE-T4 UTP cable and 100BASE-FX multimode fiber-optic cable, the total network span from a Hub 100 T4 can be up to 231 meters. See Appendix A for more information about the 100BASE-T cabling standard. See Appendix B for more information about cabling rules for Class I and Class II repeaters.
The UTP cable used for hub-to-end node connections is a straight-through connection; that is, no crossovers should be present. The pin assignments for a straight-through cable for both 100BASE-TX and 100BASE-T4 cabling are shown in Figure 3-2.
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Figure 3-2 Pin Assignments for Straight-Through Cabling
The pin assignments for the 100BASE-TX transceiver interface module are the same as the pin assignments for the RJ-45 ports on the front panel of the SuperStack II Hub 100 TX. Figure 3-3 shows the RJ-45 connector pin assignments on the hub side for 100BASE-TX.
Figure 3-3 RJ-45 Connector Pin Assignments for 100BASE-TX
Figure 3-4 shows the RJ-45 connector pin assignments for 100BASE-T4.
Figure 3-4 RJ-45 Connector Pin Assignments for 100BASE-T4
You can interconnect up to eight hubs in a stack, using expansion cables to form one logical unit. (The hub expansion cable is also called the inter-hub bus [IHB] cable.) Interconnecting eight 24-port hubs will provide a maximum of 200 ports (192 RJ-45 ports from the front panels and 8 ports from the transceiver interface modules inserted in the hubs' rear panels). Interconnecting eight 12-port hubs will provide a maximum of 104 ports (96 RJ-45 ports from the front panels and 8 ports from the transceiver interface modules inserted in the hubs' rear panels).
Use the hub expansion cable (part number 3C219) to connect the UP expansion port on the rear panel of one hub to the DOWN expansion port on the next hub, as shown in Figure 3-5. (The hub expansion cable must be purchased separately.) Repeat this step for each hub in the stack.
CAUTION: You do not have to turn off the power if you are connecting hubs using expansion cables. However, If network activity occurs while you are adding a hub to the stack, some packet loss may occur.
Figure 3-5 Interconnecting Hubs in a Stack
A Class II repeater is a hub with internal delay such that no more than two hubs can exist between any two end nodes within a single collision domain when two maximum-length cable segments are used. The Class II network topology allows two hubs (or hub stacks) to exist between any two end nodes.
You can interconnect up to two Class II SuperStack II Hub 100 TX logical units (including 3C250C-TX-12, 3C250C-TX-24, or 3C250B-TX models) by using a two-pair Category 5 crossover UTP cable with an RJ-45 connector. It is also possible to use the fiber-optic ports of two 100BASE-FX transceiver interface modules to make a Class II hub-to-hub connection using a two-strand 62.5/125 µm multimode fiber-optic cable with an SC connector. In either case, the length of your connecting cable must not exceed 5 meters. For more information about Class II repeater cabling rules, see Appendix B.
The UTP cable used for Class II hub-to-hub (Hub 100 TX) connections is a crossover cable. The pin assignments for 100BASE-TX crossover cabling are shown in Figure 3-6. 3Com offers a 6-ft 4-pair Category 5 UTP crossover cable (part number 3C221) that you can order separately. It supports the 100BASE-T standard, and is appropriate for 100BASE-T4, 100BASE-TX, and 10BASE-T applications.
Figure 3-6 Pin Assignments for 100BASE-TX Crossover Cabling
You can connect the hub-or a stack of up to seven hubs-to a 3Com Management Unit by using an expansion cable (part number 3C219). Place the Management Unit on top of the stack and connect the UP expansion port on the hub's rear panel to the DOWN expansion port on the Management Unit's rear panel, as shown in Figure 3-7.
The maximum number of units in a stack is eight including the Management Unit. You can add two SuperStack II Advanced RPSs to a stack of eight units (seven hubs and one Management Unit, or eight hubs with no other device).
3Com recommends that you place the Management Unit at the top of the stack, and add repeater units at the bottom of the stack. Following this guideline preserves existing nonvolatile configuration information associated with the repeaters already in the stack. Placing an additional repeater unit in the middle or on top of a managed stack may force you to reconfigure parameters contained in the Management Unit's nonvolatile RAM.
Figure 3-7 Connecting a Hub Stack to a Management Unit
You can connect a SuperStack II Hub 100 or hub stack to a backbone switch, router, or bridge through any of the front panel ports or through the optional transceiver interface module port on the rear panel.
Figure 3-8 shows single and stacked (interconnected) hubs connected to a network backbone through a switch.
Figure 3-8 Connecting Hubs to a Network Backbone with Crossover Cabling
Connecting a SuperStack II Hub 100 to a network backbone through a switch, router, or bridge requires a crossover cable or fiber-optic cable connection. Pin assignments for 100BASE-TX crossover cabling are shown in Figure 3-6. Pin assignments for 100BASE-T4 crossover cabling are shown in Figure 3-11. 3Com offers a 6-ft 4-pair Category 5 UTP crossover cable (part number 3C221) that you can order separately. It supports the 100BASE-T standard, and is appropriate for 100BASE-T4, 100BASE-TX, and 10BASE-T applications.
Figure 3-9 shows a SuperStack II Hub 100 T4 connected to a backbone with two-strand multimode fiber-optic cable, and end nodes connected to the 100BASE-T4 ports. The IEEE 802.3u specification states that the maximum cable distance between end nodes and 100BASE-T4 ports is 100 meters; therefore, the maximum network span using only UTP cabling is 200 meters. As shown in Figure 3-9, the maximum network span can be increased to 231 meters by using a combination of balanced T4 UTP and fiber-optic cabling. See Appendix B for rules governing maximum cable lengths for each type of connection.
Figure 3-9 Connecting the Hub in a Network Span of 231 Meters
If greater distances and more ports are required, you can use an internetworking device (such as a bridge, router, or switch) to isolate traffic between workgroups. This solution would divide the network into a number of independent segments, as illustrated in Figure 3-10.
Figure 3-10 Using an Internetworking Device to Extend the Network Span
The pin assignments for a crossover cable for 100BASE-T4 cabling are shown in Figure 3-11.
Figure 3-11 Pin Assignments for 100BASE-T4 Crossover Cabling
Two types of transceiver interface modules are available for connecting the SuperStack II Hub 100 to a bridge, router, switch, hub, or workstation. By inserting the appropriate transceiver interface module in the hub's rear-panel port, you can use various grades of UTP wiring or fiber-optic cabling to connect the hub to LAN devices. For example, you can connect a SuperStack II Hub 100 to a multiport bridge or switch using the 100BASE-TX module and gain access to a 10BASE-T network. (Switch connections may require a crossover cable.) Likewise, by using the 100BASE-FX module, you can attach the hub to a switch that is connected through fiber-optic cabling to the network. (See Appendix B for information about using the hub to access multisegmented networks.)
The required cabling for the two transceiver interface modules, as specified by the IEEE 802.3u standard, is as follows:
Connections to a server or PC from one of the transceiver interface modules require straight-through cabling. See "Pin Assignments for 100BASE-T Straight-Through Cabling" earlier in this chapter for details.
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