TC9208M-DS-R06 datasheet(23/53 Pages) ETC1

23 / 53 page

TC9208M

Preliminary Data Sheet

After recognizing and executing appropriate flow control actions these frames will be also filtered. The

port address is obtained by adding the port’s number to the base address contained within EEPROM.

When no EEPROM is present and DisFdFC shared configuration pin is configured to high state, the

switch will inhibit its ability to send flow control packets on all ports while preserving its ability to receive

and act upon the incoming flow control packets. If this pin is configured to low state the switch will

execute symmetrical PAUSE operation as defined in 802.3x.

The function of enabling/disabling the flow control in the EEPROM enabling/disabling the flow control is

now available on a per port basis rather than setting flow control globally for all ports globally and separate

enabling/disabling flow control ability can be performed on either receive or transmit side of a port.

TC9208M can be instructed to ignore the auto-negotiation result for full duplex flow control ability. When the

FrcFdFC shared configuration pin or the equvalent register in the EEPROM is equal to 1, the link partner

will be considered to have full duplex flow control capable no matter of auto-negotiation result. The

FrcFdFC setting is effective only for ports configured in 10/100 Mbps speed modes. When the FrcGbFC

and the equivalent register in the EEPROM is equal to 1, the link partner will be considered symmetric and

asymmetric towards link partner full duplex flow control capable no matter of auto-negotiation result. The

FrcGbFC setting is effective only for ports configured in 1000 Mbps speed mode.

The TC9208M executes backpressure algorithm for half duplex flow control, supporting both

collision-based and carrier-based backpressure. For collision-based backpressure the switch will be

forced to send collision signals to the terminal that sends packets to TC9208M. While TC9208M detects

an incoming frame that it wishes to backpressure with carrier sense signals, the switch will start

transmission to that port. If no packet is available at that moment for transmission then the MAC layer will

generate short jamming frames. Additionally, an aggressive backoff will be executed by the transmit

MAC after each of the forced collisions. The transmit MAC will chose between 0 and 1 slot times to

backoff. This will grant a fast recovery for the switch's congested port and will secure the channel for the

congested port in case it wishes to transmit (empty its buffers). If desired, the backoff can be completely

disabled using shared configuration pin DisBPBk or EEPROM. In this case the switch will start

transmitting with minimum IFG after carrier sense is deasserted and followed after collision.

For carrier-based backpressure the switch will use the deferral mechanism rather than the collision

backoff mechanism. The transmit MAC will jam the line by sending continuous preamble. The link

partner will see the channel busy and thus it will defer transmission without imposing any additional

backoff delay. The jamming procedure will have short break to avoid jabber condition and the break will

also be short enough to prevent the other stations from starting transmission. Preamble can be sent this

way as long as necessary. If valid packets became available for transmission during this period then

jamming will be interrupted and the packets will be transmitted with standard IFG (Inter-Frame Gap). In

this case backpressure is executed the same way as collision based mechanism. Carrier based

backpressure can be selected using shared configuration pin CrBP or EEPROM.

Backpressure operation can be disabled globally using the shared configuration pin DisBkPr or per port

basis using the EEPROM. By default forward pressure is also enabled whenever back pressure is

enabled. Forward pressure is executed only in extreme congestion conditions that normally do not occur

often. This flow control procedure is highly efficient in minimizing the packet loss. If desired, the forward

pressure can be disabled by the EEPROM settings.

If a HUB is connected to many workstations, one of the ports may be partitioned in heavy traffic when the

switch executes backpressure. TC9208M can prevent this by discontinuing the backpressure process after

a predefined number of consecutive collisions have reached. This function can be enabled using the

shared configuration pin FullBP or adjusting EEPROM setting. Unlike other settings, to enable this feature

the pin/bit should be set to '0'. The respective number of collisions defaults to 28 and can be specified using

the EEPROM. In addition, when this feature is enabled the MAC will either grant receiving the next packet

without colliding it, after which, it will resume the backpressure, or will completely quit backpressure waiting

for a new XOFF command from internal flow control management device.

Confidential.

23/53

August 1, 2003

TC9208M-DS-R06

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