IEEE 802.3 Quick Primer
AUI "Drop" Cables
AUI or "Drop" cables can be no longer than 50 meters (164 ft.) each.
Attachments may be made only to the cable ends at the 15 pin D-sub 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-
sub male connector at one end and a 15 pin D-sub 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.
10Base-T
10Base-T UTP media was implemented to reduce Ethernet wiring costs. UTP
wiring is installed in most buildings for telephone systems. Much of this
existing cabling is suitable for Ethernet 10 Mb data rates. Care must be taken
to ensure the buildings UTP wiring is able to carry these Ethernet data rates
before a 10Base-T network solution is implemented. Unlike coaxial Ethernet,
10Base2, 10Base5 and 10Base-T UTP is point-to-point wiring. Typically, an
Ethernet 10Base-T transceiver (MAU) is connected to one end and a 10Base-T
Hub (Repeater) is connected to the other. Straight-through cabling pinout is
used for Hub to MAU connections. Hub to Hub and MAU to MAU connections
are allowed using a UTP cable with a crossover function crossing TD+ and TD-
with RD+ and RD- signals. No other attachments are allowed along the length
of UTP wire, however, wiring patch panels and punch-down blocks are allowed
for interconnecting segments. The cable should be Type 3 UTP, 100 Ω
impedance, 22 to 24 AWG, and should not exceed 100 meters (328 ft.) in length
from one end to the other.
Thin (10Base2) Ethernet
When configuring thin coax segments, IEEE 802.3 specifications allow 29 or
fewer MAUs per cable segment spaced at no less than 0.5 meter (1.6 ft.). The
10Base2 (thin) cable length can not exceed 185 meters (606 ft.) per 10Base2
(thin) cable segment. The worst case propagation delay for a 185 meters (606
ft.) thin Ethernet segment is 960.9 ns. The propagation delay for 10Base2 (thin)
Ethernet cable is 514 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 at only one point on the cable.
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MR11x/MR12x.4Web Page 22 Monday, February 24, 1997 11:33 AM
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