HP 100BaseT Guide de l'utilisateur télécharger pdf (Page 31)

HP 100Base-T Hubs Installation and Reference Guide

Product Overview 1-11

As a device functioning on the media access control (MAC)

layer, the Switch Port Modules are protocol independent, and

therefore compatible with TCP/IP, NetWare, DECnet and XNS

protocols. They can provide a connection between

conventional 10Base-T and high-speed 100Base-TX networks;

or can serve as a bridge in a 100Base-TX or 100Base-FX

environment. Moreover, where traditional 100Base-TX

networks restrict the maximum distance between end-nodes to

205 meters, these modules break down this barrier. By

breaking your network into smaller and more manageable

segments, each linked to the larger network with a Switch Port

Module, the maximum distance for communications between

end-nodes is unlimited (depending on the specific timing

requirements of your network applications).

Switching Methods

Traditional bridges and routers use a switching method called

store-and-forward in which the entire frame must be received

before the device can perform a table look-up for the

destination node and forward the packet to the corresponding

port. As a result, each packet experiences a significant delay.

It may be necessary to use store-and-forward when a lot of data

errors are occurring over the network, or when connecting to

very slow devices. Compared to this “safe” mode of operation,

cut-through switching significantly reduces packet

transmission delay by picking the destination address out of the

header as soon as it is received, and directing the frame to the

appropriate segment (or the backbone) long before the full

packet has been received. The remaining technique, employed

to improve data reliability, is called fragment-free cut-

through. It uses a more conservative approach to cut-through

that waits until the collision window has elapsed (i.e., the first

full 64 bytes have been received) before processing the packet.

This prevents runts from being passed along, effectively

cleaning up the data stream.

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HP 100BaseT Guide de l'utilisateur Page 31