Intel PCI-X, Intel Gigabit Ethernet Controllers Carrier Extension 1000 Mb/s Only, Packet Bursting

Ethernet Interface

For receives, the Ethernet controller supports carrier extended packets and packets generated during packet bursting operations (see Section 8.4.2.1 and Section 8.4.2.2). The Ethernet controller can be configured to transmit in packet burst mode via the TCTL.PBE bit in the Transmit Control register (see Section 13.4.46).

Carrier extension is only defined in the IEEE 802.3z standard for half-duplex operation for operation frequencies above 100 Mb/s (Gigabit Ethernet).

8.4.2.1Carrier Extension (1000 Mb/s Only)

One of the objectives of the IEEE 802.3z standard development was to support a maximum collision domain of 200 m and retain the IEEE 802.3 Ethernet frame format. The scaling of the line transfer rate by 10x to 1 Gb/s reduced the bit time by 10 and effectively reduced the theoretical collision domain to an unusable size with the minimum packet size of 64 bytes. To overcome this, the 802.3z specification development added the notion of carrier extension to the standard.

Carrier extension provides a method to increase the duration of the carrier event to a minimum usable duration in order to meet the collision domain objective. Packets that are signaled from the CSMA/CD layer that do not meet the minimum slot time of 512 bytes have extension bytes appended to them in order to meet this minimum slot time requirement. The extension bytes are defined within the context of the frame encapsulation discussion of the 802.3z standard and are recognized by 802.3z compliant devices (see Figure 8-1).

Preamble SFD	DA	SA	T/L	Data/Pad	FCS
			Minimum Frame Size
				Slot Time
			Duration of Carrier Event

Extension

Figure 8-1. Carrier Extended Frame Format

The Ethernet controller supports the reception and transmission of carrier extended packets. Carrier extension is implemented via specifying the collision distance parameter, COLD, in the Transmit Control register (TCTL). Note that this field is evaluated whether in full- or half- duplex operation.

8.4.2.2Packet Bursting

In an attempt to recover some of the lost overhead encountered with short duration packets using carrier extension, the IEEE 802.3z standard incorporates the implementation of packet bursting. Packet bursting is a mechanism that allows a transmitting device to “own-the-wire” for a longer duration and “pack” extra packets in a burst without relinquishing ownership of the medium. A burst length timer is implemented which allows the Ethernet controller to continue to send packets until the timer expires (if packets are available for transmission).

In the case where a transmitting station has more than one packet to send, it can transmit the first packet (extending to 512 bytes if necessary) and then begin the transmission of subsequent packets. Packet transmission can continue until either there are no more packets ready for transmission, or the burst timer has expired. The burst timer limit is specified as 8 KB.

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