Information Technology Reference
In-Depth Information
for transmissions from other devices. If a transmission is received that matches the
device's address, it is processed in accordance with higher-level protocols. If not, the
transmission is simply ignored. When a device wishes to transmit, it first “listens”
for another existing transmission. If none is present, it transmits its message, and
then notes the voltage level on the line to see if another device happened to trans-
mit at the same time. This would be called a
collision
. If a collision occurs, each
device waits a short, random time period and retransmits, after checking for an
existing transmission. Thus, many devices have access to the network (multiple
access), they each listen before transmitting (carrier sense), and they retransmit if a
collision occurs (collision detect).
Newer 100 and 1000 Mbps Ethernet technologies use variations of this basic
scheme to encode data and detect carrier and/or collisions.
Thicknet (10Base5) Cabling
The original implementation of Ethernet used a large, 50-ohm coax trunk cable that
is now referred to as thicknet. This cable is specified in the 10Base5
2
standard of
IEEE 802.3, and is used at the 10 Mbps signaling rate.
A 10Base5 network consists of a thicknet backbone cable that is tapped with
a series of
transceivers
or
media attachment units
(MAUs). Each workstation or
server is connected to a single transceiver with a transceiver cable. Thicknet is a
tapped bus topology. A typical thicknet installation is shown in Fig B.1. The trans-
ceiver literally taps directly into the Ethernet cable by means of a drilled hole and a
probe contact. The transceiver has a 15-pin connector, which carries the Ethernet 10
Mbps LAN signal, error and status signals, and power. This interface is called the
attachment unit interface
(AUI). The transceiver is connected by a transceiver cable,
often called an AUI cable, to the workstation or server.
This AUI cable is a stranded, 20-gauge, 4-twisted-pair, shielded cable termi-
nated at each end by a 15 pin d-shell connector with a special locking mount. The
AUI connections are shown in Fig. B.2.
The thicknet cable is run in a continuous fashion, from a starting point to an
area near each workstation or server that is to be connected. If a cable is cut, or if
another length of cable is to be attached, both cable ends are connectorized and a
barrel connector is used to join the two sections. Type-N connectors are used with
thicknet. The backbone cable must be terminated at each end by a 50-ohm termi-
nating resistor, which is usually incorporated into a coax connector. The terminat-
2
The “5” in 10Base5 indicates the segment maximum length of 500 m. The 10 indicates the symbol
rate of 10 Mbps. In Manchester coding, the symbol rate is the same as the signaling rate (in MHz).
