Xem mẫu

  1. = = = = = = WHITE PAPER 1 of 4 pages = Do the Standards go far enough? Current position and focus of Standards versus what we now know regarding component matching. For some time now we have had ratified standards throughout the world relating to generic, or structured cabling. We have seen progress from the initial Category 3 cable and connectors, to Category 4 albeit very briefly, to Category 5 where we are today. We have also seen a shift of emphasis from component specifications to link and channel specifications which is somewhat more meaningful to the end user. In more recent times we have seen many drafts of Cat. 5, or Cat 5e, Category 6 and 7. All of these specifications focus heavily on bandwidth, with progression from 16 MHz for Cat 3, to 20 MHz for Cat 4 and for Cat.5 or Cat. 5e still focuses on 100MHz of bandwidth, with some parameters tightened and some new parameters added. Category 6 doubles this bandwidth to 250 MHz, so it is a quantum leap over Cat 5/ Cat 5E. Category 7, if it ever comes to fruition, takes a further giant leap to 600MHz, however it will be a heavily screened solution with possibly non-RJ45 connecting hardware. As previously mentioned, we have seen many drafts of both Cat 5e and Cat 6, but the actual ratified published versions seem to be forever coming but never arriving. In fact, as this paper is being written, we are up to draft 11 of Cat.6! So why the delay? Well, a couple reasons come to mind. Firstly, and most importantly, there is an ongoing problem with a relatively new parameter called return loss. Return loss is a signal echo resulting from inconsistencies in the channel structure, mainly due to poor component impedance matching. The other reason is that it is no easy feat to produce a Category 6 jack that performs adequately at 250MHz and that remains backwards compatible with Cat 5e patch cords.
  2. = = = WHITE PAPER 2 of 4 pages Return Loss (RL) Let me come back to the issue of Return Loss (RL). For some time now the standards bodies have been aware of the problem of poor performance of patch cords when introduced to the basic link (which then becomes the channel). Now we have the hard evidence to support this concern. KRONE have identified this problem and the effect on the channel, particularly the data through-put, through the use of a new test instrument called the Le Croy Newsline. This device has the ability to check both passive and active channels, and when testing active channels has the ability to look at all seven layers of the OSI model. In looking specifically at the passive channel, the Newsline can show any impedance anomalies throughout the cabling infrastructure, including the cable, connectors and patch cords. Published standards specify components at 100Ω +/- 15 Ω, which includes cable both solid and stranded, connectors and plug/socket assemblies. The problem comes about from the fact that it appears
  3. = = = WHITE PAPER 3 of 4 pages this impedance spread is too broad, resulting in excessive mismatches of components which affects data throughput on today’s high speed networks. The diagrams below illustrate this point. All components in the above diagrams are standards compliant, however the return loss introduced by the signal reflections seriously distorts data signals leading to excessive bit error rates (BER’s), resulting in re-transmission of data packets and an insidious slowing down of your network. This problem often manifests itself when a company upgrades its network from 10baseT to 100baseT, which uses more complex and less robust data protocols. The MLT3 encoding used for 100baseT can very quickly degrade due to these mismatches, resulting in receivers not being able to recognise the signal. CRC errors become evident and the resultant retransmissions begin to mount. The two diagrams below are actual traces of the data signal at the transmit end (left diagram) and the receive end (right diagram). It’s hard to believe they are supposed to be the same signal, although the right image would be attenuated somewhat in normal circumstances. Tuned Individual Pairs Within The Cable KRONE have solved this problem. We have truly matched the cable and connectors, and also tuned the individual pairs within the cable to ensure a much tighter impedance spread. The obvious resultant benefit is less return loss therefore less signal degradation, significantly improving network performance. Even the match of the Network Interface Card (NIC) to the cable plant is improved by this tighter tolerance, often proving to be the saviour of a marginal NIC that may otherwise be discarded and replaced.
  4. = = = WHITE PAPER 3 of 4 pages Another excellent feature of the Le Croy Newsline is its ability to look at actual data signals and transmission speed. Many dual speed cards have indicator LED’s showing the transmission speed of the link, either 10Mb/s or 100Mb/s. The Newsline has shown that these LED indicators are not accurate, often indicating 100Mb/s when in reality they are running at only 10Mb/s (sometimes even less!). Many Network Administrators, when investigating complaints of slow networks, look at these cards and see the “fast” light on and go off on a wild goose chase to trouble-shoot their network. Cabling, on many occasions, accounts for only about 5% of total network costs, yet is often taken for granted. Money is regularly found for upgrading servers, software and interface cards in a vain attempt to improve poor data throughput. Network Administrators assume cable is cable and rarely look the cabling infrastructure when diagnosing network problems. Similarly, cable experts have a “blind faith” in the active electronics and rarely give any consideration to the compatibility of the active equipment to the passive cable plant. The Newsline evidence strongly suggests that we should be looking at the whole network concurrently, as this is the only way we can address the compatibility issue adequately. Further detailed information on this issue can be obtained on the at :www.kroneasia.com
nguon tai.lieu . vn