Tham khảo tài liệu krone - white paper - truenet kuk , công nghệ thông tin, quản trị mạng phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Where did all the Bandwidth go? Day after day, we hear about the need for ever greater bandwidth. We fear applications as yet unthought of, but which we are sure will appear in three, four or five years time with even more bandwidth-hunger than we have so far seen. And so we invest heavily, now, in the latest and greatest infrastructure such as Category 6 cabling – even in the knowledge that the standard is far from finalised – in the hope that this will buy us extra safety margin for the future. But, as KRONE have recently unveiled, there’s a lot more to “How many megabits per second will my network pass” than meets the eye. But is what you get actually what it says on the box? It should be of course. Any properly installed Cat 5 network should happily handle 100Megabit/s and likewise Cat 5e should give you 1000 megabit/s however, in practice quite often they don’t. According to Krone, the main reason for this is Bit Errors in the channel. Bit errors – more bandwidth hungry than any application! When bit errors occur. “1s” arrive as “0s”, “0s” arrive as “1s” and either way the chunk of data that we were trying to send ends up as garbage. Ethernet and other protocols are thankfully not stupid and can detect when they receive a packet of garbage. This is OK for one or two garbled packets, but as the incidence of garbling increases so the requests for retransmissions, and the retransmissions themselves, become a significant but invisible part of the network traffic. In fact they quickly become the major part of network traffic (Table 1). According to recent research at Krone’s labs, the major cause of these Bit Errors is signal reflections caused by impedance changes – or mismatches – at various points through the channel. And with the ‘Cat’ specs allowing individual components to vary from 85 ohms impedance to 115 ohms, there’s a lot of scope for reflections and Bit Errors. TABLE 1 % of Retransmissions Data rate 0% 100 Mbit/s 1% 20Mbit/s 2% 4Mbit/s 3% 800Kbit/s 4% 160Kbit/s 5% 32Kbit/s (Modem speed!) Is there a solution? Readers who have had any dealings with radio – and after all our Cat 5 and Cat 6 systems are actually operating at VHF radio frequencies – will know that closely matching the impedance of all elements of the system will reduce reflected power, as well as distortion and standing waves. This rids us of the majority of the problems, and this is what Krone say they’ve now done. Zero Bit Error Rate TrueNet is a physical layer solution, impedance matched to within +/- 3 ohms throughout, both in the time domain and the frequency domain, which exhibits such low levels of reflection, attenuation deviation and alien crosstalk that Krone is prepared to guarantee the network to be completely Bit Error-free for five years (in addition to the normal twenty year warranty). Krone has even coined the term ZeBER, standing for zero bit error rate, to describe the result! Diagram: The reflection effect of impedance mismatch Diagram: Impedance variation – Normal system Diagram: Impedance variation – KRONE TrueNet system Bandwidth gone? Page 5 of 8
