6
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
and 10BASE5, 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 cross-over 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 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 0.5 watts or greater. Earth grounding of the
segment shield must take place at only 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 at only one point on the cable.
MR125DBook Page 6 Wednesday, August 21, 1996 4:24 PM
