"A" will send at the full rate. If the switch doesn't use flow control, the buffers will eventually fill and frames will be lost. However, the TCP protocol also has it's own flow control mechanisms, that cause it to back off, when switch congestion causes delays or lost frames. This will reduce the amount of data that "A" tries to send through the switch.
Forgot to mention: look up IEEE 802.3x flow control. This flow control mechanism uses a "pause" MAC control frame to tell another device to stop sending for a specified period of time. However, flow control is optional, so not all switches use it.
Hello,
I have a hypothetical question with practical implications: Two computers, 'A' and 'B', are plugged into a 100baseTX switch. Computer 'A' has a 100baseTX interface and computer 'B' has a 10baseT card. When 'A' is talking to 'B' through the switch, does it send its frames at the slower 10 Mbit/sec rate, or does it send to the switch at the full 100 Mbit/sec and the switch shift it out slower to 'B'?
- Andy Ball
it is a switch, so each port operates at the local line rate. a device cant "see" the line rate of other devices whether they are on the same switch or across the world.
the switch will have some internal buffering, so there is some Q capacity between the 2 ports.
either a higher layer protocol will control the amount of data sent so that the connection operates with a tolerable average loss rate (e.g. TCP), or the traffic stream is uncontrolled, so the Q will overflow and the switch will throw away some packets.
It's optional at 100 Mbps but mandatory at 1 Gbps and above.
However, most folks caution against enabling 802.3x for any ports other than those connected directly to an end-station because it causes head of line (HOL) blocking.
Anoop
Hello Stephen,
S> it is a switch, so each port operates at the local line > rate. a device cant "see" the line rate of other > devices whether they are on the same switch or across > the world. the switch will have some internal > buffering, so there is some Q capacity between the 2 > ports.
That's good to hear. Hopefully it means that a server with a 100baseTX port can service a few workstations that each have 10baseT ports without one workstation being able to saturate the server's connection to the switch. Is it safe to assume that the same would apply if the server had 1000- baseCX and the workstations had 100baseTX, or the server had
10 gigabit and the workstations had 1000baseCX ports (given wire-speed switches) ?- Andy.
Hello James,
JK> Forgot to mention: look up IEEE 802.3x flow control. > This flow control mechanism uses a "pause" MAC control > frame to tell another device to stop sending for a > specified period of time. However, flow control is > optional, so not all switches use it.
Thanks, that's useful information. Something to look for on switch data sheets too.
- Andy.
yes - with the very big assumption that the network is the bottleneck and not the server :)
in practice 10/100 ports are the minimum available on modern Ethernet switches - and 10/100/1000 ports are now standard on some low to mid range laptops....
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