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This chapter describes the Catalyst 4912G Gigabit port configuration and contains these sections:
The Gigabit Ethernet ports can be configured with any combination of shortwave (SX) and longwave/long-haul (LX/LH) Gigabit Interface Converters (GBICs).
The Gigabit ports on these modules are used primarily for backbone interconnection of other high-performance switches and routers.
The following sections describe the features of the Gigabit ports and GBIC interfaces. For GBIC cabling information, see the "Port Cabling Specifications" section. For GBIC installation information, see the "GBIC Installation, Care, and Handling" section.
A GBIC is a hot-swappable input/output device that plugs into the Gigabit port module, linking the module with the fiber-optic network (see Figure 5-1). The following GBIC media types are supported:
This section describes how to install the GBICs into a Gigabit port module.
Read the installation information in this section before you install a GBIC.
Table 5-1 provides cabling specifications for the GBICs that you install in the Gigabit port modules. Note that all GBIC ports have SC-type connectors, and the minimum cable distance for all GBICs listed (MMF and SMF) is 6.5 feet (2 meters).
| GBIC | Wavelength (nm) | Fiber Type | Core Size (micron) | Modal Bandwidth (MHz.km) | Cable Distance |
|---|---|---|---|---|---|
SX1 | 850 | MMF
| 62.5 62.5 50.0 50.0 | 160 200 400 500 | 722 ft (220 m) 902 ft (275 m) 1640 ft (500 m) 1804 ft (550 m) |
LX/LH | 1300 | MMF2
SMF (LX/LH) | 62.5 50.0 50.0 9/10 | 500 400 500 - | 1804 ft (550 m) 1804 ft (550 m) 1804 ft (550 m) 32,810 ft (10 km) |
| 1MMF only. 2Patch cord required (refer to the "Patch Cord" section for details). |
When using the LX/LH GBIC with 62.5-micron diameter MMF, you must install a mode-conditioning patch cord (Cisco product no. CAB-GELX-625 or equivalent) between the GBIC and the MMF cable on both the transmit and receive ends of the link. The patch cord is required for link distances greater than 984 feet (300 meters).
Figure 5-2 shows a typical configuration using the patch cord.
Plug the end of the patch cord labeled "To Equipment" into the GBIC (see Figure 5-3). Plug the end labeled "To Cable Plant" into the patch panel. The patch cord is 9.84 feet (3 meters) long and has duplex SC-type male connectors at each end.
This section describes how to install, remove, and maintain GBICs.
 | Caution Unnecessary removal or insertion of a GBIC could lead to premature failure of the GBIC. A GBIC has a lifetime of 100 to 500 removals or insertions. |
A switch can be shipped with or without GBICs installed.
| Caution When removing or inserting a GBIC, always wear an electrostatic discharge (ESD) wrist strap connected to the ESD wrist strap connector. |
To install a GBIC, perform these steps:
Step 1 Remove the GBIC from its protective packaging.
Step 2 Verify that the GBIC is the correct type for your network by checking the part number. The number indicates whether it is 1000BaseSX, 1000BaseLX/LH, or other.
Step 3 Grip the sides of the GBIC with your thumb and forefinger; insert the GBIC into the slot on the front of the switch (see Figure 5-4).
Step 4 Slide the GBIC into the slot until you hear a click. The click indicates the GBIC is locked into the slot.
 | Warning Invisible laser radiation may be emitted from the aperture ports of fiber-optic modules when no cable is connected. Avoid exposure and do not stare into open apertures. |
Step 5 When you are ready to attach the fiber-optic cable, remove the plugs from the GBIC and save them for future use.
| Caution Do not remove the plugs from the GBIC optical bores or the fiber-optic cable until you are ready to connect the cable. The plugs protect the GBIC optical bores and cable from contamination. |
To remove a GBIC, perform these steps:
Step 1 Disconnect the fiber-optic cable from the GBIC SC-type connector.
Step 2 Release the GBIC from the slot by simultaneously squeezing the plastic tabs (one on each side of the GBIC).
Step 3 Slide the GBIC out of the slot.
Step 4 Install the plugs in the GBIC optical bores, and place the GBIC in protective packaging.
Follow these GBIC care and handling guidelines:
| Warning Ultimate disposal of this product should be handled according to all national laws and regulations. |
| Warning Only trained and qualified personnel should be allowed to install or replace this equipment. |
To connect to the Gigabit Ethernet ports, perform these steps:
Step 1 Remove the plugs from the GBIC optical bores; store them for future use.
| Caution Do not remove the plugs from the GBIC optical bores or the fiber-optic cable until you are ready to connect the cable. The plugs protect the GBIC optical bores and cable from contamination. |
Step 2 Remove the plugs from the SC-type connector on the fiber-optic cable. Insert the connector into the GBIC (see Figure 5-5).
The following configuration information is available for configuring the Gigabit Ethernet modules:
When an unconditioned laser source (LX/LH GBIC) designed for operation on an SMF cable is directly coupled to an MMF cable, Differential Mode Delay (DMD) might occur. DMD can degrade the modal bandwidth of the fiber-optic cable causing a decrease in the link span (the distance between the transmitter and the receiver) that can be reliably supported.
The Gigabit Ethernet specification (IEEE 802.3z) outlines parameters for Ethernet communications at a Gbps rate. It offers a higher-speed version of Ethernet for backbone and server connectivity using existing deployed MMF cable by using laser-based optical components to propagate data over MMF cable.
Lasers function at the baud rates and longer distances required for Gigabit Ethernet. The IEEE 802.3z Gigabit Ethernet Task Force has identified the DMD condition that occurs in certain circumstances with particular combinations of lasers and MMF cable. The resulting characteristics create an additional element of "jitter" which limits the reach of Gigabit Ethernet over MMF cable.
With DMD, a single laser light pulse excites a few modes equally within an MMF cable. These modes, or light pathways, then follow two or more different paths. These paths may be of different lengths and have different transmission delays as the light travels through the cable. With DMD, a distinct pulse propagating down the cable no longer remains a distinct pulse or, in extreme cases, can become two independent pulses. Strings of pulses tend to interfere with each other making it difficult to recover data in a reliable fashion.
DMD does not occur in all deployed fibers. It occurs with certain combinations of worst-case fibers and worst-case transceivers. Gigabit Ethernet is the first technology to experience this problem due to its very high baud rate and its long MMF cable lengths. SMF cable and copper cable are not affected by DMD.
MMF cable has only been tested for use with LED sources. LEDs create a condition within a fiber-optic cable referred to as an overfilled launch condition. The overfilled launch condition describes the way LED transmitters couple light into the fiber-optic cable in a broad spread of modes. Similar to a light bulb radiating light into a dark room, the generated light shines in multiple directions that "overfill" the existing cable space and "excites" a large number of modes (see Figure 5-6).
Lasers launch light in a more concentrated fashion. Typically, a laser transmitter couples light into only a fraction of the existing modes or optical pathways in the fiber-optic cable (see Figure 5-6).
The solution to DMD is to condition the laser light launched from the source (transmitter) so it spreads the light evenly across the diameter of the fiber-optic cable making the launch look more like an LED source to the cable. The objective is to scramble the modes of light to distribute the power equally in all modes. This prevents the light from being concentrated in just a few modes. This is in contrast to an unconditioned launch, which, in the worst case, might concentrate all of its light in the center of the fiber-optic cable, thereby exciting only two or more modes equally.
A significant variation in the amount of DMD is produced from one MMF cable to the next. No reasonable test can be performed to survey an installed cable plant to assess the effect of DMD. Therefore, you must use the mode-conditioning patch cords for all LX/LH GBICs using MMF when the link span exceeds 984 feet (300 meters). For link spans less than 300 meters, you can omit the patch cord. We do not recommend using the LX/LH GBIC with MMF and without a patch cord for very short link distances (tens of meters). The result could be an elevated bit error rate (BER).
Posted: Thu Apr 8 13:48:27 PDT 1999
Copyright 1989-1999©Cisco Systems Inc.
