Are the rating of switches the total bandwidth for all communication or between any two ports

Question

Let's say I have 4 computers on a gigabit switch. Is the switch's gigabit rating the speed for all connections added up or the speed between any two computers? For example if computer 1 was transferring a file to computer 2 at max speed and computer 3 was sending a file to computer 4 would the 1 to 2 and 3 to 4 connection each go at 1000mbit (after accounting for overhead, of course) or would the combination of the two transfers be limited to 1000mbits?

I'm assuming the answer is the latter and as such I have a follow up. If computer 1 and 2 were each connected to switch A and switch A was connected to switch B and then computers 3 and 4 were connected to switch B would we then see simultaneous gigabit speeds between (1 and 2) and (3 and 4). In this scenario though I assume you'd lose that benefit if there was a simultaneous transfer between (1 and 3) and (2 and 4) since the connection between A and B would be the binding constraint.

If my assumptions above are correct would it make sense to use 10gbe switches even though all clients are only 1000mbps for the added simultaneous throughput?

Answer 1

If you hear the term "Gigabit switch", then there is a 99.9% chance that the term is referring to the link speed for a single port. For example, consider this netgear switch:

http://www.newegg.com/Product/Product.aspx?Item=9SIA69223R2727&cm_re=gigabit_switch--33-122-111--Product

Standard 8 port gig switch. Wonderful. If you read the specifications carefully, you will see that this switch can handle a bandwidth of 16Gbps. That means there's 2Gbps for every port. (This part I'm a little fuzzy on, but should be close enough to explain things): Since it's full duplex, that's 1Gb for each direction. Hence, a Gb switch that can handle 16Gbps of simultaneous data.

Regarding your first scenario: if the switch worked how you thought, you'd actually be dealing with 4 transfers, not 2 (2x send/receive pairs). In any case, assuming the switch works as I described, no. 1Gbps is NOT the global cap.

By the way, if you want to get into 10Gbps switches, a good one is gonna START at around $800: http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100010066%20600015761%20600015762&IsNodeId=1

Answer 2

Switches are rated at the half duplex port speed. So for an 8 port Gigabit switch, the total switch fabric speed would be 16Gb. To make things more confusing, some Ethernet NICs have been advertised using the full duplex speed, so 200mb advertised speed for a standard 100mb NIC.

You are right about the link between the switches being a bottleneck. Two computers sending on one switch to the two computers on the other switch would be limited to 500Mb each. Higher end switches usually have some kind of high speed 10Gb optical link or the ability to bind multiple ports together for the this purpose.