1000BASE-T: Gigabit Ethernet
Over Category 5 Copper Cabling

Technical Fundamentals

By Philippe Ginier-Gillet, 3Com Corporation, and Christopher T. Di Minico, Cable Design Technologies Corporation

How can network managers deploy bandwidth-intensive applications over their local area networks (LANs) when they have tight budgets and must leverage their existing infrastructure? The latest Ethernet technology, 1000BASE-T (Gigabit Ethernet over Category 5 copper cabling), helps network managers boost their network performance in a simple, cost-effective way.

1000BASE-T is Ethernet that provides speeds of 1000 Mbps over Category 5 copper cabling, the most widely installed LAN cabling infrastructure. The IEEE Standards Committee formally ratified 1000BASE-T as an Ethernet standard in June 1999.

This paper is for network managers who want a technical understanding of the fundamentals of 1000BASE-T. It explains how 1000BASE-T has been designed by the IEEE 802.3ab Task Force to run over Category 5 cabling and how to implement 1000BASE-T over existing Category 5 cable infrastructure and over Enhanced Category 5 (Cat 5e) in new sites.

Contents:

Why 1000BASE-T: Gigabit Ethernet over Category 5 Copper Cabling?
Exploding Bandwidth Requirements
Significant Investment in Ethernet/Fast Ethernet Infrastructure
A Simple, Cost-Effective Performance Boost on Existing Category 5 Cabling
1000BASE-T Technical Fundamentals
Gigabit Ethernet Media Specifications
1000BASE-T Key Specifications
Detailed 1000BASE-T Cable Specifications
1000BASE-T Design
Preparing for Deployment over Existing Cabling
Testing the Installation of Existing Category 5 Cabling
Adjusting Existing Category 5 Cabling to Run 1000BASE-T
Preparing for Deployment over New Copper Cabling: Category 5e
Migrating Ethernet/Fast Ethernet Networks Toward High-Speed Networking
Conclusion
Appendix A: Gigabit Ethernet Structure and Support for Different Media
Appendix B: Key Performance Parameters for 1000BASE-T Cabling
Appendix C: 1000BASE-T Physical Layer Implementation
Appendix D: Category 5 Cabling Field Mitigation Procedures
Acronyms and Abbreviations

Why 1000BASE-T:Gigabit Ethernet over Category 5 Copper Cabling?
1000BASE-T, Gigabit Ethernet over Category 5 copper cabling, is an attractive option for network managers for several reasons. It addresses the exploding bandwidth requirements on current networks that are the result of implementing new applications and the increasing deployment of switching at the edges of the network. Gigabit Ethernet leverages the organization’s existing investment in Ethernet and Fast Ethernet infrastructures, and it provides a simple, cost-effective performance boost while continuing to use the dominant horizontal/floor cabling medium.

Exploding Bandwidth Requirements
New, bandwidth-intensive applications are being deployed over Ethernet and Fast Ethernet networks. These applications include the following:

• Internet and intranet applications that create any-to-any traffic patterns, with servers distributed across the enterprise and users accessing Web sites inside and outside the corporate network. These applications tend to make traffic patterns and bandwidth requirements increasingly unpredictable.
• Data warehousing and backup applications that handle gigabytes or terabytes of data distributed among hundreds of servers and storage systems.
• Bandwidth-intensive, latency-sensitive groupware applications such as desktop video conferencing or interactive white-boarding.
• Publication, medical imaging, and scientific modeling applications which produce multimedia and graphics files that are exploding in size from megabytes to gigabytes to terabytes.

Figure 2. Current Building Backbone Cable Types

1000BASE-T Technical Fundamentals
Gigabit Ethernet Media Specifications
Gigabit Ethernet cost-effectively leverages existing cabling infrastructures. It can be implemented in floor, building, and campus networks because it offers a wide range of connectivity media and connection distances. Specifically, Gigabit Ethernet is designed to run over four media:

• Single-mode fiber, with connections up to at least 5 kilometers
• Multimode fiber, with connections up to at least 550 meters
• Balanced, shielded copper, with connections up to at least 25 meters
• Category 5 cabling, with connections up to at least 100 meters

Figure 3. Gigabit Ethernet Media Options and Standards

1000BASE-T Key Specifications
The 1000BASE-T standard leverages the existing cable infrastructure as it is specified to operate up to 100 meters on Category 5 cabling.

The other key specifications of 1000BASE-T make it a cost-effective, nondisruptive, and high-performing technology. First, it supports the Ethernet MAC, and is thus backward compatible with 10/100 Mbps Ethernet. Second, many 1000BASE-T products will support 100/1000 auto-negotiation, and 1000BASE-T can thus be incrementally deployed in a Fast Ethernet network. Third, 1000BASE-T is a high-performing technology with less than one erroneous bit in 10 billion transmitted bits (this bit error rate of less than 10^–10 is the same error rate as that of 100BASE-T).

Detailed 1000BASE-T Cable Specifications
1000BASE-T is specified to run over four pairs of Category 5 balanced cabling. The four pairs of Category 5 balanced cabling are specified in ANSI/EIA/TIA-568-A (1995). Additional link performance parameters (return loss and ELFEXT) are specified in TIA/EIA-TSB-95. Figure 4 details the standards of reference for the specification of 1000BASE-T cable performance parameters. For additional information, see Appendix B. Category 5 cabling is also specified in ISO/IEC 11801:1995 (“Information Technology: Generic Cabling for Customer Premises”). The second edition of ISO/IEC 11801:1995 will include the additional cabling performance parameters specified to support Gigabit Ethernet.

Figure 4. Standards of Reference for 1000BASE-T Performance Parameters

1000BASE-T Design
1000BASE-T is designed to run over Category 5 copper cabling. The transmission of 1 Gbps is possible thanks to the use of four twisted-pair links with 250 Mbps of throughput on each pair (250 Mbps x 4 = 1 Gbps).

1000BASE-T transmits at the same clock rate as 100BASE-T (125 MHz) but uses a powerful signaling and coding/decoding scheme that enables the transmission of double the amount of data as 100BASE-T. Following is a comparison of the two specifications:

• 1000BASE-T: 125 MHz x 2 bits = 250 Mbps
• 100BASE-TX: 125 MHz x 1 bit-symbol = 125 Mbit-symbol/s

Table 1. 100BASE-T Technologies Used in 1000BASE-T Technology/Method 1000BASE-T 100BASE-TX 100BASE-T2 Multi-level signaling Five-level PAM   Five-level PAM Symbol clock rate 125 MHz 125 MHz   Transmit spectrum MLT-3–like MLT-3   Digital signal processing Yes Available Yes Transmission Bidirectional   Bidirectional

Preparing for Deployment over Existing Cabling
Preparing existing Category 5 copper cabling for running 1000BASE-T is a straightforward process. The first step is a simple test of the adequacy of the cable installation. In the unlikely event that an existing installation does not meet one of the performance parameters specified by 1000BASE-T, standard corrective actions can be implemented.

Testing the Installation of Existing Category 5 Cabling

• Cable testing information is specified in the ANSI/TIA/EIA-TSB-67 standard, “Transmission Performance Specifications for Field Testing of Twisted-Pair Cabling System,” which has been used by cabling installers since 1995.
• The additional test parameters of return loss and ELFEXT for 1000BASE-T are specified in the ANSI/TIA/EIA-TSB-95 Bulletin, “The Additional Transmission Performance Guidelines for 100 Ohm 4-Pair Category 5 Cabling.”
• These additional tests are incorporated into the current versions of cable test tools (for a list of them, refer to the Gigabit Ethernet Alliance Web site). Field testing is performed by connecting the two handheld devices—one at each end of the cabling under test (see Figure 18 in Appendix D)—with a field test cord, and then activating the 1000BASE-T test function. A pass or fail will be indicated for the 1000BASE-T test and for the specific test parameters under test. Many field testers include diagnostic functions to help identify the cause of failures. Corrective actions are provided in Appendix D.

Preparing for Deployment over New Copper Cabling:Category 5e
The Gigabit Ethernet Alliance recommends that all new cable installations designed for 1000BASE-T deployment should be specified as Category 5e (enhanced Category 5). Category 5e cabling is manufactured to meet all 1000BASE-T transmission performance parameters. When field testing for Category 5e cabling performance is performed, 1000BASE-T–specific field testing is not required.

The Category 5e specification includes transmission parameters that are only informative recommendations for Category 5. (These parameters are the measures for return loss and ELFEXT, described in Appendix B.) Cable manufacturers such as Mohawk/CDT ( http://www.mohawk-cdt.com) with its MegaLAN™ cable can be held accountable for the transmission specifications of Category 5e. Category 5e also provides a further enhanced margin over the worst-case 1000BASE-T link requirements.

The 1000BASE-T standard specifies operation over the installed base of Category 5 cabling. 1000BASE-T will also run over Category 6 and Category 7 copper cabling systems (see box, “Other Cabling Categories”).

Other Cabling Categories A Category 6 cabling standard,specified to 250 MHz,is under development in the ANSI/TIA/EIA TR-42.7.1 Copper Cabling Systems Working Group and in the International Standard Committee ISO/IEC/SC25/WG3. Network managers and cable system planners may want a cabling infrastructure that provides greater bandwidth or “headroom ” to accommodate future high-speed technologies. A Category 7 cabling standard,specified to 600 MHz,is under development in the International Standard Committee ISO/IEC/SC25. Category 7 cable is constructed with individually shielded pairs with an additional shield over the pairs.Category 7 cabling requires termination to a shielded connector. The Category 7 standard is still in the early stages of development. 1000BASE-T will operate on the cabling specified in the current draft 5 of ANSI/TIA/EIA-Category 6 (estimated release, late 2000)and the current draft of the Category 6 specifications proposed for the second edition of ISO/IEC 11801:1995 (estimated release, late 1999)and Category 7 (estimated release, late 2000).

Migrating Ethernet/Fast Ethernet Networks Toward High-Speed Networking
1000BASE-T allows a simple performance boost to support exploding bandwidth requirements on today’s networks. 1000BASE-T is best suited for unclogging network bottlenecks that occur in three main areas:

• Workgroup aggregation
• Connections to high-speed servers
• Desktop connections

Conclusion
1000BASE-T, Gigabit Ethernet over Category 5 copper cabling, helps network managers boost their network performance in a simple, cost-effective way while enabling migration of today’s Ethernet/Fast Ethernet networks toward high-speed networking. Following is a summary of Gigabit Ethernet characteristics:

• 1000BASE-T is Ethernet, providing speeds of 1000 Mbps.
• 1000BASE-T is designed to run over Category 5 copper cabling, the most widely installed LAN cabling infrastructure.
• 1000BASE-T leverages the design of proven, cost-effective existing Fast Ethernet and modem technologies.
• 1000BASE-T can be progressively deployed in a Fast Ethernet network since 100/1000 auto-negotiation will be supported in many 1000BASE-T products.

Appendix A:
Gigabit Ethernet Structure and Support for Different Media

Gigabit Ethernet specifies technology covering the bottom two layers of the OSI model:

• The data link layer, which controls access to the physical medium of transmission
• The physical layer, which controls the actual transmission over the physical medium

Appendix B:
Key Performance Parameters for 1000BASE-T Cabling

Performance Parameters Specific to 1000BASE-T Cabling
The additional cabling performance parameters of return loss and far-end crosstalk (FEXT) specified for 1000BASE-T and not specified for 10BASE-T and 100BASE-TX are related to differences in the signaling implementation. 10BASE-T and 100BASE-TX signaling is unidirectional—signals are transmitted in one direction on a single wire pair. In contrast, Gigabit Ethernet is bidirectional—signals are transmitted simultaneously in both directions on the same wire pair; that is, both the transmit and receive pair occupy the same wire pair (Figure 8). 1000BASE-T uses bidirectional signaling on four wire pairs. Bidirectional data transmission on a single pair is enabled by devices called hybrids. The hybrid stops the local transmitted signals from being mixed with the local received signals.

Figure 8. Unidirectional and Bidirectional Transmission

Bidirectional transmission on the same wire results in echo (Figure 9). Echo is the combined effect of the cabling return loss and the hybrid function.

Figure 9. Echo in Bidirectional Transmission Systems

Return loss is a measure of the reflected energy caused by impedance mismatches in the cabling system. Echo is countered by echo cancellation. Echo cancellation is proven in established phone technologies.

FEXT is the noise induced by a transmitter at the near-end into a far-end receiver due to unwanted signal coupling (Figure 10). FEXT can be a factor in multi-pair, bidirectional signaling such as 1000BASE-T. It is countered by the use of cancellation.

Equal level far-end crosstalk (ELFEXT) is defined as the measure of the unwanted signal coupling from a transmitter at the near end into a neighboring pair measured at the far end relative to the received signal level measured on that same pair (Figure 10).

Figure 10. Far-End Crosstalk (FEXT)and Equal- Level Far-End Crosstalk (ELFEXT)

Performance Parameters for 10BASE-T, 100BASE-TX,and 1000BASE-T Cabling
The transmit signal is subject to impairments introduced by the cabling and external noise sources (Figure 11). In order for the receiver to operate reliably, the impairments to the transmit signal need to be controlled. The signal-to-noise ratio (SNR), the ratio between the impairments (typically referred to as noise) and the transmit signal, is maintained in order to achieve an acceptable bit error rate (BER).

Figure 11. Impairments on the Transmit Signal

The following key cabling performance parameters characterize the signal impairments:

• Attenuation is a reduction in signal power due to cabling losses, expressed in decibels (Figure 12). Attenuation is a function of frequency and is proportional to the cable length; that is, at a given frequency, the attenuation of 50 meters is one-half of the attenuation of 100 meters. The effect of attenuation is countered at the receiver by the equalization of the signal, which compensates for the cabling losses. Ethernet, Fast Ethernet, and Gigabit Ethernet signals are subject to cabling attenuation.

  
  Figure 12. Attenuation

• Near-end crosstalk (NEXT) is the noise induced by a transmitter to a neighboring receiver due to unwanted signal coupling (Figure 13). NEXT can impair both Ethernet/Fast Ethernet and Gigabit Ethernet. It is countered by the use of NEXT cancellation. Gigabit Ethernet technology employs the use of NEXT cancellation, while Fast Ethernet does not. The addition of NEXT cancellation to 1000BASE-T is an enhancement to Ethernet/Fast Ethernet because it provides additional immunity to noise.

  
  Figure 13. Near-End Crosstalk (NEXT)

• Delay is the time it takes for a signal to travel through a medium compared to the speed of light. Delay skew is the difference in delay between pairs (Figure 14). The delay skew specification ensures that the transmit signals divided across the four pairs can be reassembled in Ethernet/Fast Ethernet and Gigabit Ethernet.

  
  Figure 14. Delay and Delay Skew

Summary
All Ethernet twisted-pair technologies are subject to signal impairment. But in the case of 1000BASE-T, these disturbances are cancelled. For example, 1000BASE-T echo cancellation uses established, proven technologies leveraged from telecommunications. In addition, 1000BASE-T crosstalk cancellation uses digital signal processing (DSP) technology that has been used by many advanced modems and digital subscriber line (DSL) devices. Table 3 summarizes the signal impairments and corrective actions implemented in 1000BASE-T.

Table 3. 1000BASE-T Signal Impairments and Corrective Actions Signal Disturbance Signal Integrity Restoration Attenuation Adaptive equalizers NEXT NEXT cancellers FEXT FEXT cancellers Return Loss Echo cancellers

Appendix C:
1000BASE-T Physical Layer Implementation

As described earlier, the 1000BASE-T physical layer is composed of various sublayers.

From the MAC layer, frames of 8 bits are transmitted to the physical coding sublayer (PCS) through the Gigabit Media Independent Interface (GMII). To encode eight GMII bits, 2⁶ = 256 codes are needed. A two-level signal used on each of the four pairs of transmission would enable the coding of 2⁴ = 16 data codes. Similarly a three-level signal would give 3⁴ = 81 codes. A five-level signal gives 5⁴ = 625 potential codes and has been chosen by the IEEE for implementation. The specific five-level signal used by 1000BASE-T is Pulse Amplitude Modulation 5 (PAM-5), which was already implemented in 100BASE-T2 (Fast Ethernet over two pairs of Category 3).

By comparison, 100BASE-T uses three-level signaling (MLT-3). Figure 15 shows the eye patterns of 100BASE-T and 1000BASE-T signaling. The eye pattern illustrated was produced by a modulated random-data wave-form, with each symbol period tracing from left to right and starting in the same place on the left. The figure shows that 1000BASE-T provides closer consecutive levels of signals and hence a greater sensitivity to transmission distortions—that is, it has a reduced signal-to-noise margin compared to 100BASE-T.

Figure 15. Eye Pattern of 100BASE-T Versus 1000BASE-T Signaling

But the reduced noise margin lost at the level of PAM-5 is recovered thanks to the use of convolution coding. Convolution coding implemented by 1000BASE-T (called Trellis coding) allows error detection and correction by the receiver (through Viterbi decoding). These are established, proven technologies defined and used in modems for more than 10 years. In comparison, 100BASE-TX uses block coding (4B5B coding, four bits coded by five symbols). Block coding uses simple codes that do not offer error detection or correction.

In fact, the use of Trellis coding and Viterbi decoding makes 1000BASE-T even more resilient to external noise than 100BASE-T, since 1000BASE-T transmits uncorrelated symbols in the transmitted symbol stream; no correlation is allowed between symbol streams traveling in both directions on any pair combination, and no correlation is allowed between symbol streams on each pair. External noise pickup is generally correlated (common) to each pair. External noise can be cancelled statistically, providing improvements in noise immunity that are not available in 100BASE-TX.

Summary
1000BASE-T is designed to run reliably on the same Category 5 cabling as Fast Ethernet because it implements powerful digital signaling and coding/decoding methods that maintain the integrity of the signal when transmitted over Category 5 cabling. Figure 16 shows the architecture of a 1000BASE-T transceiver, which implements the various technologies detailed earlier.

Figure 16. Architecture of a 1000BASE-T Transceiver

Appendix D:
Category 5 Cabling Field Mitigation Procedures

ANSI/TIA/EIA-TSB-95 (1998) defines corrective actions that can be taken to improve return loss and ELFEXT performance. These corrections apply to the different elements of the Category 5 cabling systems as specified by ANSI/TIA/EIA-568-A (Figure 17). The cross-connect is a facility (generally a type of patch panel) that allows a connection between the horizontal cable and the equipment cable using a patch cord or jumper. Cross-connections are not generally used for high-speed data communication cabling solutions. 1000BASE-T recommends an interconnect cabling solution as illustrated in Figure 18. The interconnect predominates in data communication cabling. The interconnection (patch panel) provides for the direct connection of the horizontal cable to the equipment cable. The following five steps are to reduce the maximal configuration to the minimal configurations. They reference the elements in Figures 17 and 18.

1. Replace the patch cord with a cord constructed from a Category 5e patch cable, a patch cable designed to comply with the return loss and ELFEXT parameters.
2. Reconfigure the cross-connect as an inter-connect.
3. Replace the transition point or consolidation point connector with a Category 5e transition point or consolidation point connector.
4. Replace the work area outlet connector with a Category 5e work area outlet connector.
5. Replace the interconnect with a Category 5e interconnect.

A retest for compliance is recommended after each option is implemented. In practice, some flexibility in the order of implementation of these options is possible. For example, it may be more convenient to substitute a new patch cord as a first step while the tester is at that location.

Acronyms and Abbreviations
ANSI
American National Standards Institute
BER
bit error rate
DSL
digital subscriber line
DSP
digital signal processing
ELFEXT
equal level far-end crosstalk
FEXT
far-end crosstalk
GMII
Gigabit Media Independent Interface
IEEE
Institute of Electrical and Electronics Engineers
IP
Internet Protocol
ISO
International Organization for Standardization
LAN
local area network
MAC
Media Access Control
MLT
Multilevel Transmit Signal
MM
multimode
NEXT
near-end crosstalk
NIC
network interface card
OSI
Open System Interconnection
PAM
Pulse Amplitude Modulation
PCS
physical coding sublayer
PMA
physical medium attachment
PMD
physical medium dependent
SM
single-mode
SNR
signal-to-noise ratio
STP
shielded twisted pair
UTP
unshielded twisted pair

Reproduced with kind permission 3COM