# Does PTP 650/700 have a packets per second limit?

PTP 600/650/700/800 have a hardware-based data path between the Ethernet ports and the wireless port, and the end to end capacity is not limited by the packet processing performance of a CPU or DSP. It's tempting to say that these products have infinite PPS performance. However, very little in this life is really infinite, and the maximum capacity in packets per second can be calculated as a simple function of the link capacity and the Ethernet frame size.

For example, a PTP 650 link with 45 MHz bandwidth, 256QAM 0.81 DualĀ modulation and zero range has a one-way capacity of 226.1 Mbit/s. With 1518 byte Ethernet frames, this is equivalent to 226,100,000 / (1518 x 8) = 18,618 packets per second.

To work out the packets per second for a different frame size, we need to know the link capacity at that frame size. PTP 600/650/700/800 capacity changes slightly with frame size because each Ethernet frame incurs a two byte overhead as part of the MAC layer at the wireless ports. This is already taken into account for the standard 1518 byte frame size.

The capacity of the same link at the MAC layer (not counting the two byte overhead) is 226,100,000 x (1518 + 2) / 1518 = 226,379,892 bit/s.

The packets per second limit for the same link with 64 byte frames is 226,379,892 / ((64 + 2) x 8) = 428,784 packets per second. If we aggregate the two directions, this is 857,568 packets per second.

The one-way capacity for 64 byte Ethernet frames isĀ 428,784 x 64 x 8 = 219,537,408 bits/s.

The published capacity for PTP products is dependable and accurate enough for most network capacity calculations. If you're really particular, and the network traffic contains a large proportion of small Ethernet frames, you might want to take into account the slightly reduced capacity for smaller frames sizes using the method described above.

Find out about capacity with an arbitrary mixture of frame sizes here: How do I calculate PTP 650/700 capacity for an IMIX?

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