Ethernet - Computer Busses - page 433

ages as positive voltages. Thus, there are some codes that have a different number of nega-
tives and positives, these include:

0100 0001 +0-00+
0111 1001 +++-0-

Most transceiver circuits use a transformer to isolate the external equipment from the com-
puter equipment. These transformers do not allow the passage of DC current. Thus if the line
code has a sequence which consecutively has more positives than negatives, the DC current
will move away from its zero value. As this does not pass across the transformer, the receive
bitstream on the output of the transformer can reduce the amplitude of the received signal
(and may thus cause errors). This phenomenon is known as DC wander. A code that has one
more positive level than the negative levels is defined as having a weighing of +1.
The technique used to overcome this is to invert consecutive codes that have a weighing
of +1. For example, suppose the line code were

+0++-- ++0+-- +++--0 +++--0

it would actually be coded as

+0++-- --0-++ +++--0 ---++0

The receiver detects the -1 weighted codes as an inverted pattern.

26.9.3 100VG-AnyLAN
The 100VG-AnyLAN standard (IEEE 802.12) was developed mainly by Hewlett Packard
and overcomes the contention problem by using a priority-based round-robin arbitration
method, known as demand priority access method (DPAM). Unlike Fast Ethernet, nodes
always connect to a hub which regularly scans its input ports to determine whether any nodes
have requests pending.
100VG-AnyLAN has the great advantage over 100BASE in that it supports both IEEE
802.3 (Ethernet) and IEEE 802.5 (Token Ring) frames and can thus integrate well with exist-
ing 10BASET and Token Ring networks.
100VG-AnyLAN has an in-built priority mechanism with two priority levels: a high pri-
ority request and a normal priority request. A normal priority request is used for non real-
time data, such as data files, and so on. High priority requests are used for real-time data,
such as speech or video data. At present, there is limited usage of this feature and there is no
support mechanism for this facility after the data has left the hub.
100VG-AnyLAN allows up to seven levels of hubs (i.e. one root and six cascaded hubs)
with a maximum distance of 150 m between nodes. Unlike other forms of Ethernet, it allows
any number of nodes to be connected to a segment.

Connections
100BASE-TX, 100BASE-T4 and 100VG-AnyLAN use the RJ-45 connector, which has eight
connections. 100BASE-TX uses pairs 2 and 3, whereas 100BASE-T4 and 100VG-AnyLAN
use pairs 1, 2, 3 and 4. The connections for the cables are defined in Table 26. 3. The
white/orange colour identifies the cable which is white with an orange stripe, whereas or-
ange/white identifies an orange cable with a white stripe.