17
Appendix A
IEEE 802.3 Quick Primer
IEEE 802.3 AUI Drop
Cables
AUI or "drop cables" can be no longer than 50 meters (164 feet) each. Attachments
may be made only to the cable ends at the 15 pin D-shell connector. AUI drop
cables may have a maximum 257ns propagation delay for computing the worst
case propagation delay of a cable system. AUI cable propagation delay is about 5.13
ns/meter. This cable internally consists of four shielded twisted pair wires with an
overall shield and drain wire; a 15 pin D-shell male connector at one end and a 15
pin D-shell female connector at the other end. Cable impedance is typically 78
Ω
.
The AUI cable typically connects a transceiver attached to a coaxial segment to a
Data Terminal Equipment (DTE) workstation.
IEEE 802.3/10BASE2
(Thin Ethernet)
When configuring thin coax segments, IEEE 802.3 specifications allow 29 or fewer
Media Attachment Units (MAUs) per cable segment spaced at no less than .5
meter. The IOBASE2 (thin) cable length can not exceed 185 meters (600 feet) per
10BASE2 (thin) cable segment. The worst case propagation delay for a 185 meter
thin Ethernet segment is 950.9 ns. The propagation delay for 10BASE2 (thin
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 .5 watts or greater. Earth grounding
of the segment shield must take place only at one point on the cable.
IEEE 802.3/10BASE5
(Thick Ethernet)
When configuring thick coax segments, IEEE 802.3 specifications allow 100 MAU
attachments or less, spaced at multiples of 2.5 meters measured accurately from
the cable end (
50
Ω
terminator included). The thick cable segment can not exceed
500 meters (1640 feet) 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 .5 watts or greater. Earth grounding of the
segment shield must take place only at one point on the cable.
Propagation
Round-trip propagation delay through-out the entire cable system, from farthest
ends, may not exceed 51.7
µ
s to -4
µ
s.
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 meter
AUI cable has a maximum propagation delay of 257ns.) The propagation’s delay of
cable is dependent on length and velocity factor of the cable type.
