PPS Calculator

Bits per second tell only half of a network device’s story. Every packet takes a fixed amount of work from a switch, router or firewall, so a link full of tiny packets can overwhelm a device that handles the same bit rate of large ones without effort. The page converts between link speed, frame size and packet rate.

What a frame costs on the wire

An Ethernet frame is sent after a preamble of 7 bytes and a start-of-frame delimiter of 1, 8 bytes together, and the sender must then leave a gap equivalent to 12 bytes before the next frame. Neither is part of the frame size that tools report, but both use link time. The minimum frame is 64 bytes, so it occupies 64 + 8 + 12 = 84 bytes, 672 bits, of wire time. At 1 Gbit/s that is 672 nanoseconds per packet, or 1,488,095 packets per second. At 10 Gbit/s the figures become 67.2 nanoseconds and 14,880,952 packets per second.

Two calculations

The default mode, “Packets per second at line rate”, takes the link speed in Mbit/s and the frame size in bytes (64 is the minimum, 1518 the classic maximum). The other mode, “Bandwidth needed for a packet rate”, works backwards: given a rate in packets per second and a frame size, it returns the line rate required and, separately, the bandwidth of the frame payload alone without preamble and gap. Presets load gigabit Ethernet with 64-byte frames, 10 gigabit with 1518-byte frames and 2 million packets per second of 512-byte frames.

The table

Below the answer, a table gives the maximum packet rate for your link speed at eight frame sizes. At 1000 Mbit/s the rates are 1,488,095 at 64 bytes, 844,595 at 128, 452,899 at 256, 234,962 at 512, 119,732 at 1024, 96,154 at 1280, 81,274 at 1518 and 13,858 at 9000 bytes. The rate falls roughly in proportion to the frame size, which is why jumbo frames cut the load on a CPU by a large factor for bulk transfers.

A worked example

With the first preset, a 1 GbE link with 64-byte frames, the page shows 1,488,095 packets per second, a time per packet of 672.0 ns, and 84 bytes on the wire per frame (64 + 8 + 12). That is the figure to compare with a device’s forwarding rate, which vendors usually quote for 64-byte frames. For ten times the speed, multiply by ten: 14,880,952 packets per second.

Limits

These are theoretical maxima for Ethernet. Real traffic contains a mixture of sizes, with many acknowledgements near the minimum and bulk data near the maximum. A device that can sustain line rate for 64-byte frames in a data sheet may still drop packets under features such as deep inspection, encryption or logging. Over VLAN tags, tunnels or other media the overhead per frame differs. For bandwidth rather than packet counts, see the Network Goodput Calculator.

Related tools

• Network Goodput Calculator shows how much of a link carries data.
• MTU and MSS Calculator sizes packets for tunnels.
• VoIP Bandwidth Calculator is a case of many small packets.
• Packet Header Decoder reads the headers inside those frames.