In a relatively short space of time, network connections have evolved from shared or switched Ethernet to shared or switched Fast Ethernet and shared or switched Gigabit Ethernet to accommodate rising bandwidth demand. Now, within enterprise networks, business applications are advancing to embrace high-resolution graphics, video, and other rich media types that exceed the capacity of even Gigabit Ethernet performance.
The driving forces behind such a rapid pace of development include medical imaging applications that produce multimedia and graphics files that are ballooning from gigabytes to terabytes in size.
In addition, Internet and intranet applications and bandwidth-intensive groupware solutions - such as desktop video conferencing, interactive whiteboarding, and real-time video that support mission-critical business applications - require more raw bandwidth than ever before to be effective.
The rapid rise in demand for more speed and capacity is at often at odds with network managers views on future proofing their network infrastructures.
Most of the cabling installed today inside buildings is Category 5 UTP copper. Migration to high speed fibre or wireless media, while perfectly acceptable from a technology perspective, would be a costly exercise.
New standard
This is why a new benchmark for Gigabit Ethernet has been developed. The 1000BASE-T Gigabit Ethernet over copper standard (also known as IEEE Std 802.3ab) defines Gigabit Ethernet operation over distances of up to 100 meters using four pairs of Category 5 balanced copper cabling.
As a result, network managers and planners will be able to run 1000BASE-T over existing Category 5 cabling.
The IEEE
1000BASE-T has been developed by the IEEE, the world`s largest technical professional society. The members and volunteers of the IEEE are the technical and scientific professionals making the revolutionary engineering advances that are reshaping our world today.
Since its inception the technical goal of the IEEE 1000BASE-T development task force has been to support legacy Category 5 cabling.
According to the industry experts that make up the IEEE 1000BASE-T Task Force, any link that is currently using 100BASE-TX should easily support 1000BASE-T.
Fast Ethernet (100BASE-TX) achieves 100 Mb/s operation by sending three-level binary encoded symbols across the link at 125 Mbaud.
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.
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.
Setting up the infrastructure
In addition to existing performance criteria for Cat 5 field testing, users planning to use existing Cat5 cabling for 1000BASE-T are advised to test each link for return loss and Equal-Level Far End Crosstalk (ELFEXT) as specified in 1000BASE-T.
Return loss is a measure of the reflected energy caused by impedance mismatches in the cabling system. Far-End Crosstalk is noise on a wire pair at the far end from the transmitter (ie 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 1000BASE operation. (Return loss and ELFEXT can also affect the operation of 100BASE-TX.)
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.
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