82801DB Datasheet, PDF (84/587 Pages) Intel Corporation – Intel 82801DB I/O Controller Hub 4 (ICH4)

Functional Description
5.2.4
5.2.4.1
5.2.4.2
5.2.4.3
CSMA/CD Unit
The ICH4 integrated LAN controller CSMA/CD unit implements both the IEEE 802.3 Ethernet
10 Mbps and IEEE 802.3u Fast Ethernet 100 Mbps standards. It performs all the CSMA/CD
protocol functions (e.g., transmission, reception, collision handling, etc.). The LAN controller
CSMA/CD unit interfaces to the 82562ET/EM 10/100 Mbps Ethernet through the ICH4’s LAN
Connect interface signals.
Full Duplex
When operating in full-duplex mode, the LAN controller can transmit and receive frames
simultaneously. Transmission starts regardless of the state of the internal receive path. Reception
starts when the LAN Connect component detects a valid frame on its receive differential pair. The
ICH4 integrated LAN controller also supports the IEEE 802.3x flow control standard, when in full-
duplex mode.
The LAN controller operates in either half-duplex mode or full-duplex mode. For proper operation,
both the LAN controller CSMA/CD module and the discrete LAN Connect component must be set
to the same duplex mode. The CSMA duplex mode is set by the LAN controller Configure
command or forced by automatically tracking the mode in the LAN Connect component.
Following reset, the CSMA defaults to automatically track the LAN Connect component duplex
mode.
The selection of duplex operation (full or half) and flow control is done in two levels: MAC and
LAN Connect.
Flow Control
The LAN controller supports IEEE 802.3x frame-based flow control frames only in both full
duplex and half duplex switched environments. The LAN controller flow control feature is not
intended to be used in shared media environments.
Flow control is optional in full-duplex mode and is selected through software configuration. There
are three modes of flow control that can be selected: frame-based transmit flow control, frame-
based receive flow control, and none.
Address Filtering Modifications
The LAN controller can be configured to ignore one bit when checking for its Individual Address
(IA) on incoming receive frames. The address bit, known as the Upper/Lower (U/L) bit, is the
second least significant bit of the first byte of the IA. This bit may be used, in some cases, as a
priority indication bit. When configured to do so, the LAN controller passes any frame that
matches all other 47 address bits of its IA, regardless of the U/L bit value.
This configuration only affects the LAN controller specific IA and not multicast, multi-IA, or
broadcast address filtering. The LAN controller does not attribute any priority to frames with this
bit set, it simply passes them to memory regardless of this bit.
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Intel^® 82801DB ICH4 Datasheet