VT6508 datasheet(22/48 Pages) ETC1

22 / 48 page

VIA Technologies, Inc.

Preliminary VT6508 Datasheet

-22-

Enabled only when the flow control feature is enabled and the full-duplex port’s party has

flow control capability.

Note that as both the flow control feature and the drop control feature are enabled, the

drop control feature is not turned on if the full-duplex port’s party has flow control

capability.

(2) backpressure for half-duplex ports

Enabled only when the flow control feature is enabled and the port is in half-duplex mode.

Note that as both the flow control feature and the drop control feature are enabled, the

drop control feature is not turned on if the port is in half-duplex mode.

(3) drop control for private buffer reservation

Enabled when the flow control feature and the drop control feature are enabled, but the

full-duplex port’s party has not flow control capability.

Also enabled when the flow control feature is disabled and the drop control feature is

enabled.

XON/XOFF Window and Drop Window

The flow control and backpressure are based on XON/XOFF window. Within XON

window, the source port is in non-congestion status so that any incoming packets are

forwarded normally. If an incoming packet violates the constraint of congestion control, the

source port will leave the XON window and enter the XOFF window after this enqueue

operation is served. Within XOFF window, the source port is in congestion status so that any

incoming packet will trigger a congestion-control operation that depends on the used policy.

The drop control mechanism, residing in Forwarding Control and enabled by DROP_EN = 1,

is based on every ports’ congestion windows that are defined as (1) any one is in XOFF

window, (2) a destination port is in congestion status if its reservation buffer count == 0.

The XON/XOFF windows and the DROP ON/OFF windows are maintained by TMAC,

according to port status and input signals from Queue Control. Figure 6 shows an example

XOFF window status (named as XOFF_WINDOW[0]) of port 0, where XOFF[0] and

XON[0] is an event generated by queue control to notify this source port should enter XOFF

state, and XON[0] are events generated by queue control to notify this source port should

enter or leave XOFF state, respectively. If the source port is within XOFF window as an

XOFF event arrives, an flow control frame with pause time = 0xffff will be sent out. If the

source port is within XON window, the arriving XON event will be ignored.

VT6508 Datasheet(PDF) 22 Page - List of Unclassifed Manufacturers