Information Technology Reference
In-Depth Information
An extension of NLPs is fast link pulses. These do not perform link tests, but instead are employed in
the autonegotiation process to advertise a device's capabilities. Autonegotiation on 1000BaseX networks
works at only 1000 Mbps, so the only feature “negotiated” is for full- or half-duplex operation. There
may be new vendor implementations on the market that can autonegotiate speeds 10 to 1000BaseX, but
at this time they are not widely deployed.
A backup alternative, called parallel detection, works for 10/100 speeds if autonegotiation is disabled or
is unsupported. This is basically a fallback mechanism that springs into action when autonegotiation
fails. The interface capable of autonegotiation will configure itself for bare bones 10-Mbps half-duplex
operation.
Physical Connections
IEEE 802.3 specifies several different physical layers, whereas Ethernet defines only one. Each IEEE
802.3 physical layer protocol has a name that summarizes its characteristics. The coded components of
an IEEE 802.3 physical layer name are shown in Figure 4-1.
Figure4-1
IEEE 802.3 Physical Layer Name Components
A summary of Ethernet Version 2 and IEEE 802.3 characteristics appears in Tables 4-3 and 4-4.
Table4-3
Ethernet Version 2 and IEEE 802.3 Physical Characteristics
IEEE 802.3 Values
10Base5
Ethernet
Value
Characteristic
10Base2
1Base5
10BaseT
10Broad36
Data rate (Mbps)
10
10
10
1
10
10
Signaling
method
Baseband
Baseband
Baseband
Baseband
Baseband
Broadband
Maximum
segment
length (m)
500
500
185
250
100
1800
Media
50-ohm
coax
(thick)
50-ohm
coax
(thick)
50-ohm
coax (thin)
Unshielded
twisted-
pair wire
Unshielded
twisted-
pair wire
75-ohm coax
Topology
Bus
Bus
Bus
Star
Star
Star
