CS8900A
Crystal LAN™ Ethernet Controller
attempting transmission. The CS8900A sup-
ports two schemes for determining when to ini-
not in Backoff, transmission begins the 9.6 µs
IPG). If no transmit packet is pending, the
tiate transmission: Two-Part Deferral, and MAC continues to monitor the network. If activ-
Simple Deferral. Selection of the deferral ity is detected before a transmit frame is ready,
scheme is determined by the 2-partDefDis bit
(Register 13, LineCTL, Bit D). If the 2-partDef-
Dis bit is clear, the MAC uses a two-part defer-
ral process defined in section 4.2.3.2.1 of the
Ethernet standard (ISO/IEC 8802-3, 1993). If
the 2-partDefDis bit is set, the MAC uses a
simplified deferral scheme. Both schemes are
described below:
the MAC defers to the transmitting station and
resumes monitoring the network. Figure 11 di-
agrams the simple deferral process.
Start Monitoring
Network Activity
3.9.5.2 Two-Part Deferral
In the two-part deferral process, the 9.6 µs In-
ter Packet Gap (IPG) timer is started whenev-
er the internal Carrier Sense signal is
deasserted. If activity is detected during the
first 6.4 µs of the IPG timer, the timer is reset
and then restarted once the activity has
stopped. If there is no activity during the first
6.4 µs of the IPG timer, the IPG timer is al-
lowed to time out (even if network activity is
detected during the final 3.2 µs). The MAC
then begins transmission if a transmit packet is
ready and if it is not in Backoff (Backoff is de-
scribed later in this section). If no transmit
packet is pending, the MAC continues to mon-
itor the network. If activity is detected before a
transmit frame is ready, the MAC defers to the
transmitting station and resumes monitoring
the network.
Ye s
Network
Active?
No
Start IPG
Timer
IPG
Timer =
Yes
6.4 s?
µ
No
W ait
3.2
µ
s
No
Network
Active?
Ye s
The two-part deferral scheme was developed
to prevent the possibility of the IPG being
shortened due to a temporary loss of carrier.
Figure 10 diagrams the two-part deferral pro-
cess.
No
Tx
Frame
Ready and Not
in Backoff?
No
Ye s
Network
Active?
3.9.5.3 Simple Deferral
Ye s
In the simple deferral scheme, the IPG timer is
started whenever Carrier Sense is deasserted.
Once the IPG timer is finished (after 9.6 µs), if
a transmit frame is pending and if the MAC is
Transm it
Fram e
Figure 10. Two-Part Deferral
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