Allied Telesis AT-8500 Series manual Store Forward, Back Pressure and Flow Control

Store and

Forward

Back Pressure and Flow Control

AT-8500 Series Layer 2+ Fast Ethernet Switches Installation Guide

transmits data. If an end node can receive or transmit data, but not both simultaneously, the end node is operating in what is referred to as half- duplex mode. If an end node can both receive and transmit data simultaneously, the end node is said to be operating in full-duplex mode. Naturally, an end node capable of operating in full-duplex can handle data much faster than an end node that can only operate in half-duplex mode.

The twisted pair ports on the AT-8500 Series switches can operate in either half- or full-duplex mode. The twisted pair ports are IEEE 802.3u- compliant and will Auto-Negotiate the duplex mode setting for you.

By allowing the switch to configure the duplex mode for each port, you will not need to change the setting for a port on the switch should you replace an end node with an end node that has a different duplex mode capability. With Auto-Negotiation, the switch automatically resets the port to a new duplex mode setting.

If desired, you can disable Auto-Negotiation on the switch ports so that you can set the duplex mode manually through the switch’s management software.

Note

In order for a switch port to successfully Auto-Negotiate its duplex mode with an end node, the end node should also be using Auto- Negotiation. Otherwise, a duplex mode mismatch can occur. A switch port, using Auto-Negotiation, defaults to half-duplex if it detects that the end node is not using Auto-Negotiation. This results in a mismatch if the end node is operating at a fixed duplex mode of full-duplex.

Consequently, when you connect an end node with a fixed duplex mode of full-duplex to a switch port, you should use the AT-S62 management software to disable Auto-Negotiation on the port and set the port speed and duplex mode manually.

These Fast Ethernet switches use store and forward as the method for receiving and transmitting frames. When an Ethernet frame is received on a switch port, the switch does not retransmit the frame out the destination port until it has received the entire frame and stored the frame in a port buffer. It then examines the frame to determine if it is a valid frame. Invalid frames, such as fragments or runts, are discarded by the switch. This ensures that only valid frames are transmitted out the switch ports and that damaged frames are not propagated on your network.

To maintain the orderly movement of data between the end nodes, an Ethernet switch may periodically need to signal an end node to stop sending data.

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