Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
August 2006
Developer’s Manual
Order Number: 306262-004US
235
Ethernet MACs—Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
transmitted and attempt to send the frame after a calculated back-off time, based
upon the Slot Time (SLOTTIME) Register.
• If a packet is being transmitted and a collision is detected after 64 bytes (minimum
packet size) of a packet are transmitted, the transmit engine will forward the
transmit FIFO pointer to the beginning of the next packet that is to be transmitted
and discard the current packet that is being transmitted.
In a properly-configured network, a collision should not occur after the first 64 bytes of
a packet are sent. The back-off time algorithm adheres to the IEEE 802.3 specifications
“truncated binary exponential algorithm”.
If the xMII interface is configured in half-duplex mode of operation, the transmit
interface must listen to the activity on the line for a defined wait period, called a
transmit deferral period. If there is any transmit or receive activity on the medium, the
transmit interface will defer the transmission of the packet that the transmit engine has
been requested to send.
When the transmit engine detects the medium has gone silent, the transmit engine will
continue to defer for a period of time equal to the inter-frame spacing. The deferral
period will be assigned by the transmit deferral parameters. The transmit deferral time
can be specified as a one-part, or optional two-part, deferral by manipulating bit 5 of
Transmit Control Register 1:
• Setting this bit to logic 0 enables the one-part transmit deferral.
• Setting this bit to logic 1 enables the optional, two-part transmit deferral.
If the transmit engine has a frame ready to transmit and one-part transmit deferral is
selected, the transmit engine will wait for the medium to go silent and then wait for the
time period specified in the Transmit Deferral Register (TXDEFPARS). The deferral
period will be the number of transmit clock cycles specified by the 8-bit Transmit
Deferral Register minus three transmit clock cycles. The Single Transmit Deferral
parameter specifies the Inter Frame Gap.
In the two-part deferral process, the deferral period is defined using the Transmit Two
Part Deferral Parameters 1 Register (TX2PARTDEFPARS1) and Transmit Two Part
Deferral Parameters 2 Register (TX2PARTDEFPARS2). The values specified in these two
registers when added together should not be less than the minimum Inter Frame Gap
to ensure fair access to the medium.
Transmit Two Part Deferral Parameters 1 Register is typically set to two-thirds of the
Inter Frame Gap and Transmit Two Part Deferral Parameters 2 Register is typically set
to the remaining one-third.
If the transmit engine has a frame ready to transmit and two-part transmit deferral is
selected, the transmit engine will wait for the medium to go silent and then wait for the
time period specified in the Transmit Two Part Deferral Parameters 1 Register:
• If the medium is not silent during this first part of the deferral, the deferral counter
is reset.
• If the medium is silent during the first part of the deferral, transmit engine
continues to wait for the time period specified in the Transmit Two Part Deferral
Parameters 2 Register.
When the xMII interface of the IXP45X/IXP46X network processors is configured in full-
duplex mode of operation, the two-part transmit deferral parameters and the back-off
times will not be utilized for data transmission.
Refer to IEEE 802.3 Section 4.2.3.2.1 for more information on deference.