Gigabit Ethernet

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In 1998, the IEEE adopted the standard defining one Gigabit (1Gb) Ethernet over fiber optic cabling. In June of 1999, the standard defining 1 Gigabit Ethernet, or 1000BaseT, over copper pairs was ratified.

The Gigabit Ethernet standard for operation over CAT-5 cabling, also known as IEEE 802.3ab, provides a seamless upgrade path from earlier versions of copper-based Ethernet. It supports such features as full and half duplex operations, the 802.3 Ethernet frame format and the CSMA/CD access method. The specification adopted for Gigabit Ethernet over fiber cabling is called 802.3z or 1000BaseX.

How is it possible to get the Gigabit rate over CAT-5 cabling? You may recall that 100BaseT Ethernet achieved higher throughput by using three-level binary encoded symbols sent at a rate of 125Mbaud across the cable. In addition, signals used separate pairs for transmit and receive, permitting full-duplex transmission capabilities. Gigabit Ethernet was developed as an extension to 100BaseT and uses the same 125Mbaud symbol rate; however, it uses five-level binary symbols operating over all four pairs of cabling. Each pair is also capable of transmitting and receiving simultaneously.

Network equipment supporting 1 Gigabit Ethernet, copper and fiber, is becoming available off the shelf. Fortunately, the network equipment designed to handle Gigabit Ethernet will also support earlier standards, such as 100BaseT and 10BaseT, so for a few extra dollars it is a wise investment to begin purchasing network hardware that will support the new standard, even if there are no current plans to upgrade the network infrastructure. Such an upgrade may make sense for several additional reasons.

  • Higher-speed/more efficient networks can be built using existing cabling.

  • There is no need for additional training of technical staff.

  • Minimal disruption to the existing network is required.

  • Copper cabling is the lowest cost method to deliver Gigabit Ethernet.

Why upgrade?

The answer to this question depends on the needs and objectives of the organization. The underlying theory is that the primary backbone provides a large pipeline to which data flows to/from one location to another. The size of this pipeline must be large enough to allow the unrestricted flow of data from the maximum amount of simultaneous users. In the classic client-server computing model, the network provided an efficient means to transfer files, share resources and access the Internet, but the actual processing of information took place on the desktop. Trends such as real-time multimedia content, storage area networking and remote application servers are placing demands on networks. In that same client-server model, workstations are connected to hubs, which are, in turn, connected to other hubs (or switches), etc. At some point, all of the data from these points will appear on the network backbone. Maintaining the proper bandwidth of this backbone and other portions of the network infrastructure is necessary for efficient operation. Gigabit Ethernet provides a means to increase the overall data throughput capabilities of a network at minimal cost.

Whether you plan to upgrade to Gigabit Ethernet now or in the future, there are some issues of which to be aware.


One of the best features of the Gigabit Ethernet standard is that it will operate over existing CAT-5 cabling, assuming that the cabling and installation practices meet or exceed the standard defined in TIA/EIA-568A. In practice, this shouldn't be a problem if the cabling was installed within the past six years and tested in accordance with 568A, adopted in 1995. Most cabling systems, currently operating properly at the 100BaseT rate should pass Gigabit Ethernet; however, the higher demands placed on the cabling operating at the gigabit rate may show problems that were passable at 100BaseT. Each existing cable span should be tested using a cable analyzer suitable to certify operation for the new Gigabit standard. In particular, each cable span should be tested for return loss and Equal Level Far End Crosstalk (ELFEXT) (see September 2001 Networks) based on the 1000BastT specification. Most current cable analysis tools can be upgraded to conform to the latest specifications through a software download or by purchasing a hardware module.

If plans call for the installation of new cabling, consider using the new generation of cable that conforms to the new CAT-5e specification. The specification for CAT-5e cabling is similar to that of CAT-5, except that it is required to meet more stringent performance requirements for return loss and ELFEXT. Several manufacturers also offer cabling meeting the more enhanced CAT-6 and -7 pre-standard cabling products, which permit transmission of data at speeds in excess of 600MHz.

In all cases, maximum length of cabling per segment is at 100 meters, as specified in the TIA/EIA-568-A standard.

Upgrade paths

Network upgrades should be performed at the top-most levels of the infrastructure, typically the level that handles the largest amount of aggregated data traffic, such as switch-to-switch or switch-to-router. Some possible scenarios follow.


Upgrading from 100BaseT to Gigabit Ethernet switches would permit more switched and shared segments. This is beneficial for networks that carry large amounts of multimedia or streaming content.


Facilities that maintain high performance and multiple servers that provide multimedia content, high availability to users, and complex database, graphical and scientific function, would benefit from an upgrade to Gigabit Ethernet from the servers to the network switch.


In a switched network environment, upgrading hubs (or workgroup switches) and routers to Gigabit Ethernet would increase the overall bandwidth, number of available segments and the total amount of nodes per segment.

User-to-hub (switch)

An upgrade would permit increased speed between high-performance workstations and Gigabit network infrastructure.


Buildings and campuses are typically interconnected via high-speed backbone using fiber (or wireless) media. Backbones using fiber, or FDDI, can be upgraded either through replacement of existing FDDI hubs to Gigabit Ethernet switches or through the addition of an FDDI switch that can be interfaced directly into Gigabit Ethernet infrastructures.

Kevin McNamara, BE Radio's consultant on computer technology, is president of Applied Wireless Inc., New Market, MD.

All of the Networks articles have been approved by the SBE Certification Committee as suitable study material that may assist your preparation for the SBE Certified Broadcast Networking Technologist exam. Contact the SBE at (317) 846-9000 or go to for more information on SBE Certification.

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