# SANbox2 Extended Credits

The extended credits feature provides for a port’s buffer credits to be "donated" to another port to create a larger receive-buffer capacity for that port. Each port is supported by a data-buffer with a 12-credit capacity; that is, 12 maximum sized frames. For fibre optic cables, this enables full bandwidth Class 2 service over a distance of 20 kilometers at 1 Gbps, or 10 kilometers at 2 Gbps. Longer distances can be spanned at full bandwidth by borrowing credits from designated donor ports thus pooling the buffer capacities. This is called credit extension. Each donor port contributes 11 credits to the pool from which the recipient ports can draw. For example, one donor port contributes 11 credits to the pool from which a recipient draws for a total of 23 credits (11+12).