Gigabit Ethernet

Gigabit Ethernet

As if 100 Mbps is not enough, yet another higher bandwidth technology was unleashed on the industry in June of 1998. Gigabit Ethernet (IEEE 802.3z) specifies operations at 1000 Mbps, another tenfold bandwidth improvement. We discussed earlier how stations are hard-pressed to fully utilize 100 Mbps Ethernet. Why then do we need a Gigabit bandwidth technology? Gigabit Ethernet proponents expect to find it as either a backbone technology or as a pipe into very high speed file servers. This contrasts with Fast Ethernet in that Fast Ethernet network administrators can deploy Fast Ethernet to clients, servers, or use it as a backbone technology. Gigabit Ethernet will not be used to connect directly to clients any time soon.

Some initial studies of Gigabit Ethernet indicate that installing 1000 Mbps interfaces in a Pentium class workstation will actually slow down its performance due to software interrupts. On the other hand, high performance UNIX stations functioning as file servers can indeed benefit from a larger pipe to the network.

In a Catalyst network, Gigabit Ethernet interconnects Catalysts to form a high-speed backbone. The Catalysts in Figure 1-9 have low speed stations connecting to them (10 and 100 Mbps), but have 1000 Mbps to pass traffic between workstations. A file server in the network also benefits from a 1000 Mbps connection supporting more concurrent client accesses.

Figure 1-9. Gigabit Ethernet Backbone Between Catalysts

Gigabit Architecture

Gigabit Ethernet merges aspects of 802.3 Ethernet and Fiber Channel, a Gigabit technology intended for high-speed interconnections between file servers as a LAN replacement. The Fiber Channel standard details a layered network model capable of scaling to bandwidths of 4 Gbps and to extend to distances of 10 kms. Gigabit Ethernet borrows the bottom two layers of the standard: FC-1 for encoding/decoding and FC-0, the interface and media layer. FC-0 and FC-1 replace the physical layer of the legacy 802.3 model. The 802.3 MAC and LLC layers contribute to the higher levels of Gigabit Ethernet. Figure 1-10 illustrates the merger of the standards to form Gigabit Ethernet.

Figure 1-10. The Formation of the Gigabit Ethernet Standard

The Fiber Channel standard incorporated by Gigabit Ethernet transmits at 1.062 MHz over fiber optics and supports 800 Mbps data throughput. Gigabit Ethernet increases the signaling rate to 1.25 GHz. Further, Gigabit Ethernet uses 8B/10B encoding which means that 1 Gbps is available for data. 8B/10B is similar to 4B/5B discussed for 100BaseTX, except that for every 8 bits of data, 2 bits are added creating a 10-bit symbol. This encoding technique simplifies fiber optic designs at this high data rate. The optical connector used by Fiber Channel, and therefore by Gigabit Ethernet, is the SC style connector. This is the push-in/pull-out, or snap and click, connector used by manufacturers to overcome deficiencies with the ST style connector. The ST, or snap and twist, style connectors previously preferred were a bayonet type connector and required finger space on the front panel to twist the connector into place. The finger space requirement reduced the number of ports that could be built in to a module.

  • Note
    A new connector type, the MT-RJ, is now finding popularity in the fiber industry. The MT-RJ uses a form factor and latch like the RJ-45 connectors, supports full duplex, has lower cost than ST or SC connectors, and is easier to terminate and install than ST or SC. Further, its smaller size allows twice the port density on a face plate than ST or SC connectors.

Full-Duplex and Half-Duplex Support

Like Fast Ethernet, Gigabit Ethernet supports both full- and half-duplex modes with flow control. In half-duplex mode, though, the system operates using CSMA/CD and must consider the reduced slotTime even more than Fast Ethernet. The slotTimes for 10BaseX and 100BaseX networks are 51.2 microseconds and 5.12 microseconds, respectively.

These are derived from the smallest frame size of 64 octets. In the 100BaseX network, the slot-time translates into a network diameter of about 200 meters. If the same frame size is used in Gigabit Ethernet, the slotTime reduces to .512 microseconds and about 20 meters in diameter. This is close to unreasonable. Therefore, 802.3z developed a carrier extension that enables the network distance to extend further in half-duplex mode and still support the smallest 802.3 packets.

The carrier extension process increases the slotTime value to 4096 bits or 4.096 microseconds. The transmitting station expands the size of the transmitted frame to ensure that it meets the minimal slotTime requirements by adding non-data symbols after the FCS field of the frame. Not all frame sizes require carrier extension. This is left as an exercise in the review questions. The 8B10B encoding scheme used in Gigabit Ethernet defines various combinations of bits called symbols. Some symbols signal real data, whereas the rest indicate non-data. The station appends these non-data symbols to the frame. The receiving station identifies the non-data symbols, strips off the carrier extension bytes, and recovers the original message. Figure 1-11 shows the anatomy of an extended frame.

Figure 1-11. An Extended Gigabit Ethernet Frame

The addition of the carrier extension bits does not change the actual Gigabit Ethernet frame size. The receiving station still expects to see no fewer than 64 octets and no more than 1518 octets.

Gigabit Media Options

IEEE 802.3z specified several media options to support different grades of fiber optic cable and a version to support a new copper cable type. The fiber optic options vary for the size of the fiber and the modal bandwidth. Table 1-4 summarizes the options and the distances supported by each.

Table 1-4. Gigabit Ethernet Media Option

Standard Cable Size (Micrometers) Cable Bandwidth (MHz-Kms) Distance* (Meters)
1000BaseSX 62.5 160 220
1000BaseSX 62.5 200 275
1000BaseSX 50 400 500
1000BaseSX 50 500 550
1000BaseLX 62.5 500 550
1000BaseLX 50 400 550
1000BaseLX 50 500 550
1000BaseLX 9/10 N/A 5,000
1000BaseLH** 9/10 N/A 10,000
1000BaseZX** 9/10 N/A 90,000
*Note that the minimum distance in each case is 2 meters
**Cisco capabilities that support distances greater than the 5,000 meters specified by the IEEE 802.3z standard.


1000BaseSX uses the short wavelength of 850 nms. Although this is a LASER-based system, the distances supported are generally shorter than for 1000BaseLX. This results from the interaction of the light with the fiber cable at this wavelength. Why use 1000BaseSX then? Because the components are less expensive than for 1000BaseLX. Use this less expensive method for short link distances (for example, within an equipment rack).


In fiber optic systems, light sources differ in the type of device (LED or LASER) generating the optical signal and in the wavelength they generate. Wavelength correlates to the frequency of RF systems. In the case of optics, we specify the wavelength rather than frequency. In practical terms, this corresponds to the color of the light. Typical wavelengths are 850 nanometers (nms) and 1300 nms. 850 nm light is visible to the human eye as red, whereas 1300 is invisible. 1000BaseLX uses 1300 nm optical sources. In fact, the L of LX stands for long wavelength. 1000BaseLX uses LASER sources. Be careful when using fiber optic systems. Do not look into the port or the end of a fiber! It can be hazardous to the health of your eye.

Use the LX option for longer distance requirements. If you need to use single mode, you must use the LX.


Not included in Table 1-4 is a copper media option. 1000BaseCX uses a 150-Ohm balanced shielded copper cable. This new cable type is not well-known in the industry, but is necessary to support the high-bandwidth data over copper. 1000BaseCX supports transmissions up to 25 meters. It is intended to be used to interconnect devices collocated within an equipment rack very short distances apart. This is appropriate when Catalysts are stacked in a rack and you want a high speed link between them, but you do not want to spend the money for fiber optic interfaces.


One final copper version is the 1000BaseT standard which uses Category 5 twisted-pair cable. It supports up to 100 meters, but uses all four pairs in the cable. This offers another low cost alternative to 1000BaseSX and 1000BaseLX and does not depend upon the special cable used by 1000BaseCX. This standard is under the purview of the IEEE 802.3ab committee.

Gigabit Ethernet Interface Converter

A Gigabit Ethernet Interface Converter (GBIC) is similar to an MII connector described in the Fast Ethernet section and allows a network administrator to configure an interface with external components rather than purchasing modules with a built-in interface type. With a GBIC interface, the administrator has flexibility to change the interface depending upon his needs. GBIC transceivers have a common connector type that attaches to the Gigabit device, and the appropriate media connector for the media selected: 1000BaseSX, 1000BaseLX, or 1000BaseCX.

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