A common method, and one that is used for the WS-X44180-GB line cards, is oversubscription. That is where the sum of the interface speeds is greater than the backplane speed. In the case of this line card it's 4:1 oversubscription (as noted on the product data sheet).
Oversubscription works because of the fact that all ports are not ever simultaneously transmitting at full wire speed. If that were to happen for more than a few milliseconds, the switch's interface buffers would be unable to pass all the traffic and drop packets.
Some very high end switches and some line cards are referred to as "line rate" or "non-blocking". That generally means they are not built with oversubscription and you pay a premium price for that capability.
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As Marvin notes, line cards might be oversubscribed. In fact, the 4418 is a very interesting card because two of its ports are not oversubscribed, but the other 16 ports are oversubscribed 4:1.
When dealing with a device's performance capabilities, you often need to read datasheet "fine print" to know exactly what your working with. For your 4418 card, not only might we want to know which two ports support full line rate, but we might want to know how the oversubscribed ports are grouped. The latter would allow us to use just one of a group of four ports to also obtain full line rate ("wasting" three ports), or perhaps use just two of a group's four ports to obtain 2:1 oversubscription, or perhaps group one port that's busy 75% of the time (on average) with a two ports busy 10% of the time and one only busy 5% on the time resulting in no oversubscription (on average), etc.
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