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Note: The following technical article was current at the time it was published. However, due to changing technologies and standards updates, some of the information contained in this article may no longer be accurate or up to date.



Q: How can I be sure that the category 5 cabling system I already have installed will support Gigabit Ethernet?

A: TSB95 "Additional Transmission Performance Guidelines for 100 ohm 4-pair Category 5 Cabling" was drafted to provide test requirements that allow for verification that installed or "legacy" category 5 cabling is compatible with full duplex applications, such as Gigabit Ethernet. It is expected that two-connector channel models compliant with ANSI/TIA/EIA-568-A will meet these recommendations. Three- or four-connector models will also be supported provided they meet the pending TSB95 specifications, which include:

  • Return Loss
  • ELFEXT
  • Power Sum ELFEXT
  • Propagation Delay
  • Delay Skew

Many hand held test manufacturers now feature an Autotest setting which can be used to verify existing cabling system compatibility with full duplex applications.

Q: When are the standards for category 5e and category 6 going to be released?

A: The document outlining the pending specifications for category 5e (SP-4195-A to be published as ANSI/TIA/EIA-568-A-5 [ĎA-5]) is currently in circulation for technical comments and is expected to be published in the 4th quarter of 1999.

The pending specifications for category 6/class E are also in the committee draft stage and are expected to be released for industry ballot in the 4th quarter of 1999.

Q: What is the difference between category 5 and category 5e?

A: The most significant difference between category 5 (with the proposed additional specifications of TSB95) and category 5e is the specification of NEXT loss and return loss as illustrated in Table 1. Category 5e specifies an additional 3dB of NEXT loss (which results in a similar +3dB change to PSNEXT, ACR and PSACR) and 2dB of return loss over category 5.

Industry Standards Performance Comparison Chart
(Worst case channel performance at 100 MHz)

Parameter

Category 5 and Class D
with pending additional requirements TIA PN-4292 (TSB95) and ISO/IEC SC 25 N 487

Category 5e
TIA/SP-4195 ('A-5)

Proposed Category 6
TIA TR42.7

Specified Frequency Range

1 - 100 MHz

1 - 100 MHz

1 - 250 MHz

Attenuation

24 dB

24 dB

21.7 dB

NEXT

27.1 dB

30.1 dB

39.9 dB

Power sum NEXT

N/A1

27.1 dB

37.1 dB

ACR

3.1 dB

6.1 dB

18.2 dB

Power sum ACR

N/A

3.1 dB

15.4 dB

ELFEXT

17 dB2

17.4 dB

23.2 dB

Power sum ELFEXT

14.4 dB2

14.4 dB

20.2 dB

Return Loss

8 dB2,3

10 dB

12.0 dB

Propagation Delay

548 ns

548 ns

548 ns

Delay Skew

50 ns

50 ns

50 ns



The current proposed category 5 (TSB95) return loss and ELFEXT requirements are based upon a 3-connector channel model (not the worst case 4-connector channel specified in ANSI/TIA/EIA-568-A).The 3-connector model was used based on the assumption that the majority of installed category 5 cabling utilizes an interconnect instead of a cross-connect or does not contain a consolidation point. The category 5e specification was developed based upon the full 4-connector model as defined by ANSI/TIA/EIA-568-A. For many manufacturers, the category 5e specification has now become the de facto standard for cable, connecting hardware, and system performance.

Q: What is backwards compatibility and why is it important?

A: In order to comply with the pending category 5e and 6 standards, many manufacturers optimize their components to achieve desired category performance. If component design does not take into consideration the existing range of category 5 performance, it is possible that the use of these products with lower category components may actually degrade the performance of the system below that of the lowest category component (for example, the use of a category 6 modular cord with a category 5e outlet could produce performance below category 5e).

Backward compatibility is an extremely important design consideration and will be required for all system components to ensure robust system operation.


Q: What is FEXT and why is it important?

A: Far-end crosstalk (FEXT) loss is defined as a measure of the unwanted signal coupling from a transmitter at the near-end to a neighboring pair measured at the far-end (see Figure 1 below). Applications employing simplex transmission only transmit in one direction for a given pair, so they are sensitive to crosstalk noise generated by the transmitter at the near-end as illustrated in Figure 2 (below). With the introduction of full duplex transmission, signals are being simultaneously sent and received by multiple pairs on both ends of the cabling channel. Therefore, designers of these applications must be aware of noise coupled not only from the near-end of the channel, but also the resultant noise generated from the far-end.

Equal level far-end crosstalk (ELFEXT) is defined as a measure of the unwanted signal coupling from a transmitter into another pair measured at the far-end relative to the received signal strength (ELFEXT = FEXT loss - Attenuation). To system designers, ELFEXT is a much more important parameter than FEXT loss. Similar to ACR, ELFEXT provides a direct indication of signal-to-noise ratio (SNR).


Figure 1: Far End Crosstalk (FEXT)

Figure 2: Near End Crosstalk (NEXT)

Q: What is the proper separation between communications cabling and power conductors?

A: ANSI/TIA/EIA-569-A section 10.3.1 states that the co-installation of telecommunications cable and power cable is governed by applicable electrical code for safety. For minimum separation requirements of telecommunications cable from typical branch circuits (120/240V, 20A), Section 2 of Article 800-52 of the ANSI/NFPA 70 1999 National Electrical Code specifies the following:

"Communications wires and cables shall be separated at least 2 in. (50.8 mm) from conductors of any electric light, power, Class 1, non-power-limited fire alarm, or medium power network-powered broadband communications circuits."

There are 2 exceptions that involve various means of isolating the two:

"Exception No. 1: Where either (1) all of the conductors of electric light, power, Class 1, non-power-limited fire alarm, or medium power network-powered broadband communications circuits are in a raceway or in metal-sheathed, metal-clad, nonmetallic-sheathed, Type AC or Type UF cables, or (2) all of the conductors of communications circuits are encased in a raceway.

Exception No. 2: Where the communications wires and cables are permanently separated from the conductors of electric light, power, Class 1, non-power-limited fire alarm, or medium power network-powered broadband communications circuits by a continuous and firmly fixed nonconductor, such as porcelain tubes or flexible tubing, in addition to the insulation on the wire."

This separation is required for safety purposes only. For installations where additional separation is possible, chapter 21, page 26 of the 8th Edition of the Building Industry Consulting Services International's (BICSI) Telecommunications Distribution Methods (TDM) Manual states:

"A minimum clearance of 61 cm (24 in.) between electrical power cables and telecommunications cables is recommended."