Our friend Burrakucat mentions the difference between sync rate and IP PDU speed, or IP SDU speed. These numbers are in decreasing order. The difference is because of various overheads, headers, possibly trailers and also inefficiency of coding schemes that represent sequences of bytes by more longwinded sequences of bits. A PDU is a packet where the rate number quoted includes the cost of transmitting headers, trailers and all other overheads; it’s a larger number. An SDU is the payload/content of a packet, excluding headers and ignoring any encoding; it is the data passed down to a service to be sent or the received data passed up by a service. This is a smaller number. SDU rates are what users expect from services; PDU rates measure the link itself as opposed to the accidental payload throughput that some particular protocol achieves, and different protocols achieve different amounts of SDU (ie payload) throughput.
For example IPv4 is more efficient than IPv6 because IPv4 has a 20 byte header whereas IPv6 has (at least) a 40 byte header. When including the popular TCP protocol that means PDUs maybe 1500 bytes long and an IPv4+TCP SDU will be either 1500-(20+20)=1460 bytes long or 1500-(20+20+12) bytes long if TCP uses timestamps at a cost of an additional 12 bytes of headers. So the payload of IPv4+TCP (the SDU size) will be either 1460 or 1448 bytes of user data. For IPv6+TCP the SDU size would be 1500-(40+20) or 1500-(40+20+12) bytes.
VDSL2/FTTC for example has additional overheads that are not included in the above; these include the bloat due to encoding, ethernet headers and other headers. The encoding bloat reduces efficiency to either 96.69% / 97.69% or 91% depending on protocol settings in force which may vary over time. All the header overheads can iirc vary with the ISP.
ADSL/ADSL2/ADSL2+ has very inefficient overheads. My ADSL2 service has 32 bytes of headers and an encoding system that adds on a variable amount of bloat but in the case of a long packet the bloat expansion factor is 53:48; for a short packet the expansion factor can be 100% [!], and a complex formula is required to calculate the exact length. Including all inefficiencies, a 1500 byte payload ADSL+PPP+PPPoEoA PDU can have an efficiency of ~88.44% because 1500 bytes (SDU size) expands to 1696 bytes (PDU size).
