Downstream rate

The downstream rate is measured in kilobits and includes everything arriving at the router over the WAN interface. For example, on a connection with guaranteed 768 kbps downstream, the upstream rate negotiated by the modem is 864 kbps. This still includes an overhead typical for this type of connection, which results from the modem using ATM as the transport protocol. If we adjust the 864 kbps to allow for the overhead that results from the structure of an ATM cell (48 bytes of payload for a cell length of 53 bytes), we arrive at 864 * 48/53 = 792 kbps gross downstream rate, which is transferred from the modem to the router over Ethernet. If data rates negotiated by the modem are unknown, it is possible to multiply the guaranteed data rates by 56/55 to approximate the gross data rates.