Propagation Delay

If your transferring data from one PC to another over a network the sending machine needs time to read the data from its hard drive, break the data into packets, and add the packaging needed to get the data to the receiving computer. The receiving computer will collect packets until its receive window (RWin) is full and then it will send an acknowledgment to the sender asking for the next “block” of packets. All of those functions takes time – and during that time your network is sitting idle, unused. The amount of time it takes for data to propagate through a piece of equipment is called propagation delay – and it’s yet one more limiting factor in sending data across a net. Just because your network is rated at 10 Mb/s that doesn’t mean that you will be able to supply data fast enough to keep it busy. It really depends on the speed of the computers on your network – the speed of every component in them. If your data has to go through a hub, switch, and/or router to reach its destination then the propagation delay induced by every piece of equipment on the network will lower your effective bandwidth. When connecting to a site on a network like the Internet you are competing with everyone else for effective bandwidth and resources on the server that you are trying to reach – more delay…

Wrappin’ it up

One last term to cover that is used heavily by marketing: Full Duplex. Full duplex means that a node can transmit at the same time that it is receiving a packet – and it’s a good thing. But from time to time someone will attempt to list the bandwidth of a 100 Mb/s full duplex node as 200 Mb/s – not a chance! Just because I can receive while transmitting that doesn’t mean I can accept data twice as fast – a 100 Mb full duplex node can only transmit or receive at a rate of 100 Mb/s – theoretically, of course…

– DaLANTech –

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