IEEE 802.3 Quick Primer
14
Thin (10BASE2) Ethernet
When configuring thin coax segments, IEEE 802.3 specifications allow 30 or
fewer MAU attachments per cable segment spaced at no less than 0.5 meters
(1.64 ft.) between each attachment. The thin (10BASE2) cable length can not
exceed 185 meters (606 ft.) per thin (10BASE2) cable segment. The worst
case propagation delay for a 185 meter (606 ft.) thin Ethernet segment is
950.9 ns. The propagation delay for thin (10BASE2) Ethernet cable is 5.14 ns/
meter. Both ends of the segment must be terminated with a 50
Ω
termination
with a power rating of 0.5 watts or greater. The segment shield must be
grounded only at one point on the cable.
Thick (10BASE5) Ethernet
When configuring thick coax segments, IEEE 802.3 specifications allow 100
MAU attachments or less, spaced at multiples of 2.5 meters (8.2 ft.) measured
accurately from the cable end (50
Ω
terminator included). The thick cable
segment cannot exceed 500 meters (1640 ft.) in length. Worst case end to end
propagation delay of a thick coax segment is 2165 ns. Propagation delay of
thick Ethernet coax is calculated at 4.33 ns/meter. Both ends of the segment
must be terminated with a 50
Ω
termination with a power rating of 0.5 watts
or greater. Earth grounding of the segment shield must take place only at one
point on the cable.
Propagation
Propagation delay is the time it takes a signal to travel from the input of a
system component to the output. Propagation delay is usually measured in
nanoseconds. IEEE 802.3 has specific propagation delay maximums for the
Ethernet components. Cable length plays a major role in propagation delay.
i.e, a 50 meters (164 ft.) AUI cable has a maximum propagation delay of 257
ns. The propagation delay of cable is dependent on length and velocity factor
of the cable type.
Round-trip propagation delay through-out the entire cable system, from
farthest ends, may not exceed 51.7
µ
s,
±
4
µ
s.
Four Repeater Rule
Up to four repeater units may be connected for a total of five segments
provided that two of the segments are link segments. Link segments have no
MAU attachments. If all segments are coax segments, having MAU
attachments, then a maximum of two repeater units and three segments are
allowed.
