Path: utzoo!attcan!uunet!zaphod.mps.ohio-state.edu!sdd.hp.com!hplabs!hpcc05!hpcuhb!hpda!hpcupt1!hprnd!pat
From: pat@hprnd.rose.hp.com (Pat Thaler)
Newsgroups: comp.dcom.lans
Subject: Re: how far can you run ethernet over fiber
Message-ID: <2230123@hprnd.rose.hp.com>
Date: 30 Oct 90 23:41:08 GMT
References: <7178@cica.cica.indiana.edu>
Organization: HP Roseville Networks Division
Lines: 55

> 
> How far can you run ethernet over fiber before the collision
> detecting mechanism breaks down?  I came up the following which
> seems too far:
> 
Min frame size is actually preamble + start of frame delimiter + 512,
but the transmitting DTE has to receive the collision signal early
enough to send a 32 bit jam and stop transmitting before it has
transmitted the 512th bit and to stop transmitting soon enough that
collision indication on the media ends before the 512th bit.  This means
that the round trip from first bit transmitted to collision detected
should occur by the 512 bit transmitted including preamble.

However, not all of those 512 can go into media delay.  Delays in
MAUs/transceivers, hubs, AUI/transceiver cables (if any), and DTEs
have to come out of that.  The amount available for the fiber will
vary depending on the type of fiber MAU, presence of AUI cables, etc.
It will be less than 480 bit times.

FOIRL is spec'ed for up to 1 Km.  Some manufacturers support an extended
distance, but about the most I've seen is about 2 Km.  You have to allow
for delay in any repeaters/hubs you use to connect fiber segments together.

For instance, in a rough calculation assuming a single repeater, no AUI
cables, and 4 MAUs end-to-end, I get about 420 bit times available for
the fiber delay.  (This was based on delays for a FOIRL (Fiber Optic
Inter Repeater Link) MAU which is defined in IEEE 802.3 for connections
between repeaters.  Using such a MAU to connect to a DTE is not covered
by the standard, but a number of manufacturers sell such a product.)

Speed of light in fiber is closer to .67c or 2 * 10^8  m/s

So, the calculation for fiber distance in that situation would be:

    420 BT * 100 ns/BT * 2*10^8 m/s / 2 = 4.2 Km.

Now this was a pretty back-of-the-envelope calculation and I may
have forgotten something.  Such a network would be outside of what
is allowed by the current 802.3 standard since the FOIRLs were allowed
to connect directly to the DTEs and the fiber distance is more than
1 Km / segment.

The standard allows up to five segments and four repeaters end-to-end.
Up to three of those can be coax segments of up to 500 m.  The rest
can be link segments (FOIRL or 10BASE-T) with the FOIRL segments allowed
up to 500 m.  There will be 10 MAUs and 10 AUI cables of up to 50 m each.
So the total distance spanned will be:

    5 segments * 500 m/segment + 10 AUIs * 50 m/AUI = 3 Km.

Speed of light is actually a bit faster on thick coax (0.77c), but
collision propagation is slower.  This calculation also had 3 more
repeaters and 6 more MAUs.

Pat Thaler