Ethernet - Computer Busses - page 428

toring the DC (or average) voltage on the line.
When transmitting, a transceiver unit transmits the preamble of consecutive 1s and 0s.
The coding used is a Manchester coding, which represents a 0 as a high to a low voltage tran-
sition and a 1 as a low to high voltage transition. A low voltage is represented by -0.7 V and
a high is 0.7V

and -0.7 V; as illustrated in Figure 26 .9. If after the transmission of the preamble no
collisions are detected then the rest of the frame is sent.

+ 0.7 V. Thus, when the preamble is transmitted the voltage changes between +

0.1 µ s

1 0 1 0 1

+0.7 V

-0.7 V

Idle Preamble Idle

Figure 26.9 Ethernet digital signal

26.7 Ethernet types

The six main types of standard Ethernet are:

• Standard, or thick-wire, Ethernet (10BASE5).
• Thinnet, or thin-wire Ethernet, or Cheapernet (10BASE2).
• Twisted-pair Ethernet (10BASE-T).
• Optical fibre Ethernet (10BASE-FL).
• Fast Ethernet (100BASE-TX and 100VG-Any LAN).
• Gigabit Ethernet (1000BASE-SX, 1000BASE-T, 1000BASE-LX and 1000BASE-CX).

The thin- and thick-wire types connect directly to an Ethernet segment; these are shown in
Figure 26. 10 and Figure 26 .11. Standard Ethernet, 10BASE5, uses a high specification cable
(RG-50) and N-type plugs to connect the transceiver to the Ethernet segment. A node con-
nects to the transceiver using a 9-pin D-type connector and a vampire (or bee-sting) connec-
tor can be used to clamp the transceiver to the backbone cable.
Thin-wire, or Cheapernet, uses a lower specification cable (it has a lower inner conductor
diameter). The cable connector required is also of a lower specification, that is, BNC rather
than N-type connectors. In standard Ethernet the transceiver unit is connected directly onto
the backbone tap. On a Cheapernet network the transceiver is integrated into the node.
Most modern Ethernet connections are to a 10BASE-T hub, which connects UTP cables
to the Ethernet segment. A RJ-45 connector is used for 10BASE-T. The fibre optic type,
10BASE-FL, allows long lengths of interconnected lines, typically up to 2 km. They use ei-
ther SMA connectors or ST connectors. SMA connectors are screw-on types whereas ST
connectors are push-on. Table 26 .1 shows the basic specifications for the different types.