Allied Telesis AT-GS900/8PS manual Auto-Negotiation Mode Flow Control and Back Pressure

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Chapter 1: Overview

Auto-Negotiation Mode

Flow Control and Back Pressure

Each AT-GS900/8PS switch port is configured for Auto-Negotiation mode which determines the speed and duplex mode. Each end-node connected to a twisted-pair port should also be configured for Auto-Negotiation mode. This insures that the speed and duplex mode is matched on each end.

Note

If an end-node connected to a AT-GS900/8PS twisted-pair port is set to a fixed speed and duplex, a speed and duplex mismatch may occur which can cause a reduction in the data flow.

When two end-nodes are set to different speeds, an Ethernet switch with the slower data rate capability may randomly be flooded with more data than it can process and may need to signal the opposite end-node to stop sending data until it is ready again. How a switch signals an end-node to stop transmitting data differs depending on the duplex mode of the Ethernet ports.

A port operating in full-duplex mode uses PAUSE frames, as specified in the IEEE 802.3x standard, to momentarily stop the transmission of data from an end-node. Whenever a switch port wants an end-node to stop transmitting data because it is being flooded by data, it issues a PAUSE frame. This frame instructs the end-node to temporarily cease transmission. The switch continues to issue PAUSE frames until it is ready to receive data again from the end-node. This is referred to as flow control.

Atwisted pair port operating at half-duplex mode stops its end-node from transmitting data by forcing a collision. A collision on an Ethernet network occurs when two end-nodes attempt to transmit data using the same data link at the same time. A collision causes the end-nodes to momentarily stop sending data. When a switch port has received more data than it can process, it needs to temporarily stop its end-node from transmitting data. The port does this by forcing a collision on the data link, which stops the opposite end-node from transmitting any more data. After the switch is ready to receive data again, the switch stops forcing collisions. This is referred to as back pressure.

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Contents AT-GS900/8PS Installation Guide Copyright 2012 Allied Telesis, Inc Electrical Safety and Emissions Standards Translated Safety Statements Contents Page Figures Figures Tables Tables Preface Symbol Conventions Contacting Allied Telesis Preface Overview ChapterFeatures LEDs PoE Ports SFP SlotsPower over Ethernet Front and Back Panels Back PanelMDI/MDI-X Mode Twisted Pair PortsAuto-Negotiation Mode Flow Control and Back Pressure SFP Port LEDs DescriptionTotal PoE delivered power is between 70 W Describes the LEDs for the SFP slots SFP Slot LEDs Power Over Ethernet PoE Power Budget PoE Guard Band Overview Wiring Implementation Power Supply Installation Reviewing Safety Precautions AT-GS900/8PS Switch Installation Guide Selecting a Site for the Switch Unpacking the Switch AT-GS900/8PS Switch Items Installing the Switch on a Desktop Installing the Switch in an Equipment Rack AT-GS900/8PS Switch Installation Guide Installing the Switch on a Wall AT-GS900/8PS Switch Installation Guide Installation Installing Optional SFP Transceiver Installation Cabling the Switch Powering On the Switch Twisted Pair Port LINK/ACT LED Off TroubleshootingPower LED OFF SFP Port LINK/ACT LED Off Slow Network Performance LINK/ACT LED Intermittently BlinkingPowered Device Not Receiving Power Resetting Blinking PoE Port LED Troubleshooting Technical Specifications Physical SpecificationsEnvironmental Specifications Power SpecificationsPoE Specifications Safety and Electromagnetic Emissions CertificationsConnectors and Port Pinouts Pin SignalPin Pair Signal