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I recall (?) Bc should be large enough to cover maximum packet size. If correct, one reason for that, you cannot send parts of a frame/packets.
Also, with Ethernet, minimum size is 64 bytes. But let's assume your packet size is only 8 bits, and we have six of them, back-to-back, and line speed is 64 Kbps.
First packet arrives, current allowance for first Tc is your 32 bits, so it is transmitted. Second packet arrives, current allowance for first Tc is now 24 bits, so it too is transmitted. 3rd and 4th are transmitted too.
When the 4th packet is transmitted, your allowance is now zero. So the 5th and 6th packets are queued until the next Tc period when your allowance is reset to 32 bits. But how long are they held?
Well the first 4 packets, combined 32 bits, were transmitted at 64 Kbps, which takes 32 / 64000 = .5 ms. So the introduced delay, before the 5th packet begins to be transmitted is 4 ms - .5ms = 3.5 ms. The 5th and 6th packets only take another .25ms to transmit.
So, to recap, your first 4 packets were transmitted in .5 ms, although your CIR is 8 Kbps. All 6 packets, though, took 4.25 ms to transmit.
As only 32 bits, per 4 ms are allowed to be transmitted, your long term average is maintained, but individual 8 bit packets are still transmitted at wire-speed.
Now for even more fun, lets assume your first 8 bit packet arrives 3.5 ms into the first Tc period. The first 4 packets are transmitted, the Bc is reset at the 4 ms interval, and so the 5th and 6th packets are immediately transmitted. So, all 6 packets are transmitted, back-to-back, in .75 ms, i.e. none were delayed!
PS:
BTW, the above doesn't address shapers that carry unused Bc credits.
Suppose the first packet arrives just at the end of the first Tc and the rest of that period wasn't used. So although the next 5 packet have 40 bits to transmit, and a Tc only provides 32 bits per Tc, if we add in the "unused" credit of 24 bits from the first period, we could transmit the 6th packet w/o delay too. Again, all six 8 bit packets would be transmitted at wire speed, w/o any additional delay!
