ethernet does not reserve bandwidth, so 15 light 100 meg users have just as much speed as 15 10 meg users, but either way the total going up the uplink port won't exceed 100 meg
on the switches EACH port gets 100 meg if needed, full duplex is possible, so the internal circuitry need to run faster than 100 meg. on the 16 port it ideally runs at 1.6 gig.
this is great if every port is just as likely to send to every other port. if all devices ONLY talk to a central server, you can be bound by the speed of the server connection. Whether you have one switch or 3 is not an issue if all devices chat with the server, the server connection will always be the bottleneck.
In reality however you often hav many servers, a router is doing DHCP and Internet access to every box, networked printers are getting print jobs from every box (either directly or via a print server), one server may hold applications while another holds data, backups may be done over the net. (all of these servers may BE one box, but they do not have to be)
once you see how many server you have you may find you have 1 meg to internet, 10 meg to a printer, 100 meg to a file server and 100 meg on a shared application. the 18 users on the 24 port switch can get to a total of 211 meg of services. the small switches can only get to 100 meg as they are bottlenecked by the uplink, which they were not in a single server situation. by putting the least demanding users on the small switches we reduce the possibility that the bottleneck ever hurts performance.
it may be of course, that your networks bottleneck's are all in typing speed and never in the elctronics. (My wife archetecural firm, whose network services are described above, has no bottlenecks that I can find except in their speed at the keyboard making drawings) then there is no difference between a designed network and one that just grew. Since it is rare that more than one person saves or loads in 10 minutes, their network is idle over 99% of the time, and a hub appears to be as fast as a switch.
in a hub, there can only be one packet moving at a time.
in a switch, each port can be sending and receiving one packet. But if all devices want to talk to one server, that server can still only recieve one packet at a time. in a switch where all services are remote, the uplink port is a 'server'
Bottom line, philosophically, I like power users 'near' the services, and idle devices remote from the services, but unless there really is a performance bottleneck, it does not matter. I tried to remain child-like, all I acheived was childish.
