Chapter 1: Overview

Back Pressure and Flow Control

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. This can occur under several circumstances. For example, if two end nodes are operating at different speeds, the switch, while transferring data between the end nodes, might need to instruct the faster end node to stop transmitting data to allow the slower end node to catch up. An example of this is when a server operating at 1000 Mbps is sending data to a workstation operating at only 100 Mbps.

How a switch signals an end node to stop transmitting data differs depending on the speed and duplex mode of the end node and switch port. A twisted pair port operating at 100 Mbps and half-duplex mode will stop an 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 end nodes to stop sending data. When the switch needs to stop a 100 Mbps, half-duplex end node from transmitting data, it forces a collision on the data link, which stops the end node. When the switch is ready to receive data again, the switch stops forcing collisions. This is referred to as back pressure.

A port operating at 100 Mbps and full-duplex mode uses PAUSE frames, as specified in the IEEE 802.3x standard, to stop the transmission of data from an end node. Whenever the switch wants an end node to stop transmitting data, it issues this frame. The frame instructs the end node to cease transmission. The switch continues to issue PAUSE frames until it is ready again to receive data from the end node. This is referred to as flow control.

The AT-GS900/8POE Gigabit Ethernet switch supports back pressure flow control in half duplex mode and pause frame flow control in full duplex mode.

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Allied Telesis GS900/8POE manual Back Pressure and Flow Control
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GS900/8POE specifications

The Allied Telesis GS900/8POE is a versatile and robust switch designed for small to medium-sized enterprises that require reliable network performance combined with power over Ethernet (PoE) capabilities. As part of the GS900 series, this switch offers a range of features, technologies, and characteristics that cater to modern networking demands.

One of the main features of the GS900/8POE is its eight 10/100/1000BASE-T Ethernet ports, which enable seamless connectivity for a variety of devices. Each port is equipped with Power over Ethernet capabilities, allowing the switch to deliver up to 15.4 watts of power per port. This is particularly beneficial for powering devices such as IP cameras, wireless access points, and VoIP phones without the need for additional power sources, minimizing installation costs and clutter.

The GS900/8POE also supports advanced Layer 2 switching capabilities, including VLAN (Virtual Local Area Network) support, which enhances network segmentation and security. By allowing administrators to create isolated networks within a larger network, VLANs reduce broadcast traffic and improve overall performance. Additionally, the switch supports Quality of Service (QoS) features, ensuring that time-sensitive applications, such as video streaming and voice communications, receive priority over less critical data.

Another noteworthy characteristic of the GS900/8POE is its fanless design, which makes it ideal for environments where noise levels need to be kept to a minimum. This design approach not only enhances the aesthetics of the deployment area but also increases reliability by reducing moving parts that could potentially fail.

Security is also a significant focus in the GS900/8POE's design, with features like MAC address filtering and DHCP snooping to protect against unauthorized access and potential network attacks. These safeguards ensure that network administrators can maintain control over connected devices and mitigate risks associated with security breaches.

In summary, the Allied Telesis GS900/8POE switch is a feature-rich solution that meets the demands of modern networks. Its PoE capabilities, advanced Layer 2 features, fanless operation, and robust security measures make it an excellent choice for businesses looking to expand their network infrastructure while keeping operational efficiency and reliability at the forefront. Whether used in a small office setting or for branch locations, the GS900/8POE provides a compelling balance of performance and functionality.
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