larryme@tektronix.UUCP (Larry Meneghin ) (11/02/85)
In article 1116 eric@unmvax.UUCP writes: >> Does a standard exist for the so call "thin Ethernet" ... >> >> ... I have seen several companys talk of it (e.g. 3com) but >> also none of them have quoted the same segment length, number >> of supported stations per segment, etc. >> In article 1119 drk@well.UUCP (Doug Kaye) writes: > To the best of my knowledge, there is no published spec for RG-58 ... It turns out that there is a spec. in the works. It is IEEE Draft Standard 802.3 section 10. Section 10 is the specification for what has been known unofficially as "cheapernet". The copy I have is Draft E, dated Nov. 84'. It not the latest, (Draft E Re- vised, Mar. 85'), but I put a call in to Don Loughry, Chairman of the IEEE 802.3 committee, to confirm that it hasn't changed in any of its essentials. Here are the details. MAU "normally contained within the DTE". 10 Megabits per second. RG 58 A/U or RG 59 A/U cable. 185m (600 ft) max. cable length per segment. 30 nodes max. per segment. I personally wouldn't mix "thin" and "thick" cable on the same net. At least not in an arbitrary fashion, and not without know- ing exactly what the characteristics of the adaptor were. I might consider it in a net that is short and small and is going to stay that way but I think I would limit myself to one or two thick/thin transitions and I would TDR the cable after installa- tion. Cable design is a complex subject and I don't pretend to be an expert in it, but some of the issues I am aware of are as fol- lows. Transfer impedance, which is a function of C and L per unit length, is a measure of how much the cable will distort your sig- nal. Manchester code contains very strong components at 5 and 10 Mhz and your cable looks like a low pass filter. This is a sig- nificant contributor to signal jitter in long networks. Transfer impedance is also a strong determining factor in a cables EMC performance. Attenuation per unit length, of course has to do with signal am- plitude at the receiver. Velocity of propagation helps determine the maximum round trip propagation time and therefore the inter-frame gap time and minimum frame length. Since you use the D.C. component of the signal to detect colli- sions, D.C. loop resistance affects the size of your collision window. Larry Meneghin tektronix!scargo!larryme Tektronix Inc. P.O. Box 500 Beaverton, OR 97077 D.S. 50-761 (503) 627-7479