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I know this is a simple and possibly stupid question but i'm not quite sure what to search up on google to find the right answer.

Basically i have a gigabit router (Dlink DIR 825) and have multiple laptops/clients connected to it (wired cat6/cat5e ethernet) and was wondering about the total speed the router can transfer at once:

eg, if i had 4 computers connected (wired, all on gigabit speeds) and transferred a 1 GB file from Computer A to computer B while transfering a file from computer C to D all at the same time, would both a + b, c + d bandwidth be halved (eg, 500 Mbps)? or would both A-B, D-C get a chance at 1Gbps? (Yes, i'm aware that 1Gbps is the theoretical "up to" speed of gigabit ethernet and that HD, NIC, Computer specs can all effect the actual speed)

Please comment if i'm not explaining this clearly.

All answers are appreciated.

Thank you.

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migrated from serverfault.com Dec 9 '12 at 10:09

This question came from our site for professional system and network administrators.

4 Answers 4

You're talking about the difference between a hub and a switch. With an Ethernet hub, all of the traffic is sent to every port, and so the total available bandwidth is shared between every port. Switches detect which port needs to receive traffic and only send it to that port, so the bandwidth is per port. Your router is probably a switch rather than a hub, meaning that you can have 1GBps on A-B and C-D at the same time.

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My Router is dlink.com.au/products/?pid=dir-825 "DIR-825 XTREME N™ DUALBAND BROADBAND ROUTER WITH 4-PORT GIGABIT SWITCH" –  Daniel Dec 9 '12 at 7:30
    
I don't think any Gigabit hubs have ever existed. –  David Schwartz Dec 10 '12 at 10:46
    
@Daniel - So you answered your own question its a switch not a hub –  Ramhound Dec 11 '12 at 14:32

Your router has full non-blocking bandwidth between its four switch ports. When traffic is local to a subnet and two machines are talking directly to each other, each wired to one of those four switches and operating at Gigabit speed, the full port speed is available in both directions on all ports.

If you have machines A, B, C, and D connected to those four ports and all are at gigabit speed, you can have traffic from A to B, B to C, C to D, and D to A all at the same at near gigabit speeds.

This is all done in hardware by the switch and requires no work by the router's CPU. However, when you start moving traffic between the wireless and wired ports or between machines on the LAN and machines on the WAN, you start to run into limitations due to CPU power.

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If you were on an old hub then yes, you would share the bandwidth like in your 500/500 scenario, but on a switch traffic is routed from one port to another, not broadcast across all ports...basically means each port gets dedicated bandwidth not shared bandwidth (sort of).

To answer the other part of the question, each switch has a theoretical and stated maximum "Switch Capacity". I don't know what the capacity of your particular switch is but I know it uses the RealTek rtl8366sr switch inside, so you can look up details if needed. For example, I purchased a ZyXel 24 port gigabit switch which has a 48GB Switch Capacity. Meaning, it can send 1GB and receive 1GB per port (Full Duplex)- these are non-blocking switches. So that would mean each port gets a chance at the full 1GB. Some switches may have more ports but less switch capacity which means not all ports would get the chance at a full GB.

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TL:DR: Theoretically/Marketing-wise yes. In reality, no.

What you are talking about is LAN/LAN throughput. This all depends on the processing capability of the hardware. A website I have found helpful is http://www.smallnetbuilder.com/

While the LAN/LAN throughput is not measured here, your Router looks to have a WAN/LAN routing throughput of 228.5Mb/s. Hardly gigabit, but faster than 100Mb/s. You will find that almost all "Gigabit" devices, barring rack-mounted enterprise level routers/switches never have actual gigabit speeds. You will also find that all "Gigabit" means for consumer network equipment is "Faster than 10/100"

Theoretically, if all routing is performed in hardware and not in the CPU, you should reach 1Gbps for A->B and C->D, however unless you are using high quality CAT6 cable, you will never achieve those speeds. There may be that much data being passed but there could be a very high bit error rate. That transfer alone would be in excess of 256MB/s due to all the extra ethernet packet informatioon. Add processing on that to verify that the packet is in tact and ready to be passed to another port. Look up the address of the destination, etc. and you are looking at quite a few CPU clock cycles for the router/switch to process.

The processor on that router is 600MHz. That means that you have approximately 2 cycles for every byte of data that needs to be buffered/processed/redirected. Whether the packet redirection is done in hardware or software, I highly doubt that it could process that much simultaneous data. Not to mention performing all the other tasks the router has to do in the background.

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