Propagation delay &
its relationship to max. cable length
Introduction
You may
know that the minimum frame size in an Ethernet network
is 64 bytes or 512 bits, including the 32 bit CRC. You
may also know that the maximum length of an Ethernet
cable segment is 500 meters for 10BASE5 thick cabling
and 185 meters for 10BASE2 thin cabling. It is, however,
a much less well known fact that these two
specifications are directly related. In this essay, we
will discuss the relationship between minimum frame size
and maximum cable length.
Propagation Delay
Before we
discuss frame size and cable length, an understanding of
signal propagation in copper media is necessary.
Electrical signals in a copper wire travel at
approximately 2/3 the speed of light. This is referred
to as the propagation speed of the signal. Since we know
that Ethernet operates at 10Mbps or 10,000,000 bits per
second, we can determine that the length of wire that
one bit occupies is approximately equal to 20 metres or
60 feet via the following maths:
-
speed of light in a vacuum = 300,000,000 metres/second
-
speed of electricity in a copper cable = 200,000,000
metres/second
-
(200,000,000 m/s) / (10,000,000 bits / s) = 20
metres per bit
We can
further determine that a minimum size Ethernet frame
consisting of 64 bytes or 512 bits will occupy 10,240
metres of cable.
The Relationship
The only
time that an Ethernet controller can detect collisions
on the wire is when it is in the transmit mode. When an
Ethernet NIC has finished transmitting and switches to
receive mode, the only thing it listens for is the 64
bit preamble that signals the start of a data frame. The
minimum frame size in Ethernet is specified such that,
based on the speed of propagation of electrical signals
in copper media, an Ethernet card is guaranteed to
remain in transmit mode and therefore detecting
collisions long enough for a collision to propagate back
to it from the farthest point on the wire from it.
Take, for
example, a length of 10BASE5 thick Ethernet cabling
exactly 500 meters long (the maximum that the spec
allows) with two stations, Station A and Station B,
attached to the farthest ends of it.
If Station
A begins to transmit, it will have transmitted 25 bits
by the time the signal reaches Station B, 500 meters
away. If Station B begins to transmit at the last
possible instant before Station A's signal reaches it,
the collision will reach Station A 25 bit-times later
(the time it takes for the signal on the wire to travel
one bit-length -- 20 metres in copper cable). Station A
will have transmitted only 50 bits when the collision
reaches it -- nowhere near the 512 bit boundary for an
early collision.
Upon closer
examination, however, a peculiarity arises. If a normal
collision happens before the 512 bit boundary, Station A
would have to be over 5000 metres away from Station B
before a late collision occurred. Examine the maths for
yourself: 512 bits times 20 metres/bit = 10,240 metres.
That's 256 bits or approximately 5000 metres for the
signal to propagate from Station A to Station B and 5000
metres for the collision event to propagate back to and
be detected by Station A. It seems like a late collision
would never occur with a maximum cable length of only
500 meters. What is the reason for the overhead?
The reason
for the overhead is twofold. First of all, while the
maximum possible cable segment length in Ethernet is 500
metres, it is possible to extend that length with up to
4 repeaters before the IEEE 802.3 spec is violated. This
means that the signal may have to travel through as much
as 2500 metres of cable to reach Station B, or 5000
metres of cable round trip. The second and final reason
for the overhead lies solely in the carefulness of
Ethernet's inventors. Generally the spec is twice as
strict as it needs to be, allowing ample room for
errors.
Herein lies
one of the greatest strengths and weaknesses of
Ethernet. It is a strength in that if you need to, you
can probably get away with violating the specs -- an
extra length of cable here, an extra repeater there and
your network continues to run normally. It is a weakness
in that while you can get away with violating the specs,
there is a very fine line between a network that is
violating the specs and is running and a network that is
violating the specs and is crippled by late collisions
and you never know which extra bit of wire or extra
repeater is going to cross the line.
Despite
this dire warning, there are some general rules for
violating specs:
-
If your vendor tells you you can violate the spec
and you're not mixing vendors, it's probably ok. If
you mix vendors, obey the strictest vendor.
-
If something is wrong with your network and you know
that it violates the spec in places, those places
should be the first ones you check. Try segmenting
the network with a bridge and see which side of the
bridge the problems are on.
|
