TXC-06880BIOG Transwitch Corporation, TXC-06880BIOG Datasheet - Page 71

• Manufacturer Part Number: TXC-06880BIOG
• Manufacturer: Transwitch Corporation

Specifications of TXC-06880BIOG

• Operating Supply Voltage (typ): 1.8/3.3V
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Lead Free Status / RoHS Status: Not Compliant

PRELIMINARY TXC-06880-MB, Ed. 4
February 2005
6.5 ETHERNET FULL DUPLEX
6.4.6 Half-Duplex Flow Control
6.5.1 Overview
6.5.2 PAUSE Control Frame Reception
There is no IEEE 802.3 compliant backpressure mechanism for Half Duplex. The common
industry implementation is the “Raise Carrier” method. The Envoy-CE2 MAC uses the
configurable “Raise Carrier” method for flow control in Half-Duplex mode. In the event the
Envoy-CE2 port needs to backpressure the transmission medium, it raises carrier by
transmitting the preamble. Other devices on the transmission medium defer to the carrier. If a
collision occurs due to the raised carrier, the congestion is resolved using the standardized
collision-detect, backoff method. The Host can not initiate flow control in half-duplex mode.
The Envoy-CE2 implements the IEEE 802.3 compliant full-duplex flow control mechanism.
This mechanism is used to prevent frame loss, resulting from frame discard due to a buffer
becoming full. It is intended for a Full-Duplex link between two devices; for example, a link
between a switch and an end device (e.g., host). The mechanism involves the generation and
operation of specific Ethernet Control frames called “PAUSE Control frames”. A PAUSE
control frame has certain values for the Destination Address field, Type field, Opcode field,
MAC control parameter field and must have a valid FCS. Following are the required PAUSE
control frame field values.
The Destination address must either be the multicast address (01-80-C2-00-00-01) or the
unicast address of the device the frame is being sent to. The Type field should have a
reserved value of 0x8808 (MAC Control frame type) and the Control Opcode should be equal
to 0x0001. A MAC control parameter called pause_time, specifies the time period, in 512-bit
times, for which the device receiving the PAUSE control frame is paused.
The Flow control operation in the Envoy-CE2 is divided into two operations:
1) The action taken when a PAUSE control frame is received from a remote device
2) The action taken when Envoy-CE2 generates a PAUSE control frame and transmits it to a
remote device.
The remote device is connected to the Envoy-CE2 via a full duplex link. Envoy-CE2 PAUSE
frame processing needs to be enabled for the PAUSE frame flow control mechanism to take
effect. An option is provided in the device to filter out the received PAUSE control frame.
Remote devices receiving Ethernet frames from Envoy-CE2 can backpressure Envoy-CE2,
by sending a PAUSE control frame to Envoy-CE2. The backpressure may be needed if the
buffer of the remote device on the link is becoming full. Upon reception of the PAUSE control
frame, Envoy-CE2 will stop transmitting frames to the remote device. If Envoy-CE2 is in the
middle of transmission of a frame, when it receives a PAUSE control frame, it will complete
transmission of the frame and then pause transmission of any further Data frames. When
Envoy-CE2 is in the pause state, it allows transmission of PAUSE control frames from the
Envoy-CE2. The period for which Envoy-CE2 stays in a PAUSE state is defined by the
pause_time parameter in the PAUSE control frame. The pause_time parameter is used as
the initial value of an internal count down timer. This timer is started as soon as the
pause_time parameter is loaded into the count down timer. Envoy-CE2 will stay in this
- Operation -
Envoy-CE2 Device
DATA SHEET
TXC-06880
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