You are designing an update to your client's wireless network. The existing wireless network uses $802\mathrm{.}11$b equipment, which your client complains runs too slowly. She wants to upgrade the network to run up to $600$ Mbps$.$ Due to budget constraints, your client wants to upgrade only the wireless access points in the network this year. Next year, she will upgrade the wireless network boards in her users' workstations. She has also indicated that the system must continue to function during the transition period. Which $802\mathrm{.}11$ standard will work BEST in this situation?