When network managers decide on a cabling system, they make a decision which is basic to the network infrastructure, and long term in its horizon. Today, network managers are asking hard questions about 1000BASE-T for Gigabit Ethernet on Category 5 copper because they want to future proof their network infrastructures. Most of the cabling installed today inside buildings is Category 5. Migration of this Category 5 installed base to higher speed Ethernet is the primary focus of the IEEE 1000BASE-T Task Force. While networking equipment can easily be pulled from a rack, horizontal cabling can be very difficult to replace since it is located inside a wall, ceiling, or raised floor and dispersed across many wiring closets. Various copper and fiber choices are available on the market today. This document addresses directly the questions about Gigabit Ethernet operation over the installed base of Category 5 UTP.
Gigabit Bandwidth over Category 5 Cabling
Fast Ethernet (100BASE-TX) achieves 100 Mb/s operation by sending three-level binary encoded symbols across the link at 125 Mbaud. (A 125 Mbaud symbol rate is required because 100BASE-TX uses 4B5B coding.) 100BASE-T uses two pairs: one for transmit, one for receive. The next step up in speed, 1000BASE-T also uses a symbol rate of 125 Mbaud, but it uses all four pairs for the link and a more sophisticated five-level coding scheme. In addition, 1000BASE-T sends and receives simultaneously on each pair. Combining 5-level coding and 4 pairs allows 1000BASE-T to send one byte in parallel at each signal pulse. 125 M symbols/second X 1 Byte (across four pair)/symbol = 1 Gb/s. Of course, it isn't quite this simple. In addition to moving the symbols across the link, 1000BASE-T must also deal with the effects of insertion loss and link-induced interference caused by echo and crosstalk. (See the GEA 1000BASE-T white paper for more details.)
Testing the Installed Cabling
The IEEE has recommended two additional tests to qualify Category 5 cabling. The 1000BASE-T Task Force has made this recommendation for two reasons:
First, Category 5 systems installed prior to the completion of ANSI/TIA/EIA568-A in 1995 may contain connecting hardware that does not comply with the standard.
Second, the 1995 cabling standard did not specify two critical performance parameters--return loss and Far-End Crosstalk (FEXT). Return loss defines the amount of signal energy that is reflected back towards the transmitter due to impedance mismatches in the link (such as those caused by connectors). Far-End Crosstalk is noise on a wire pair at the far end from the transmitter (i.e., at the receiver) caused by signal leakage from adjoining wire pairs. It is measured at each wire pair as Equal Level Far-End Crosstalk (ELFEXT) or as Power Sum ELFEXT (PSELFEXT) which sums the total noise from all adjacent wire pairs.
Return loss, and Far-End Crosstalk have negligible impact when a Category 5 link is used to carry 10BASE-T signals, but they can significantly affect both 100BASE-TX (Fast Ethernet) and 1000BASE-T signals. Testing for return loss, ELFEXT, and PSELFEXT before using the link for high-speed protocols (and bringing the link up to the required level of performance if it is substandard) just makes good sense. Basic cable testing information is provided by ANSI/TIA/EIA-TSB-67- "Transmission Performance Specifications for Field Testing of Twisted Pair Cabling System." The additional test parameters for ELFEXT, PSELFEXT, and return loss will be published in ANSI/TIA/EIA-TSB 95.
