Until recently, the fastest 802.11ac routers were labeled 'AC1750' or something along those lines. That refers to the combined theoretical bandwidth of the 2.4 GHz and the 5 GHz bands. On 2.4 GHz that is 450 Mbps and on 5 GHz that is 1.3 Gbps. The new routers have the same speed on the 5 GHz frequency, but on 2.4 GHz they're now capable of 600 Mbps. The ASUS RT-AC68U and the Linksys EA6900 we will look at today are the first two routers of that new generation.
If you're familiar with the 802.11n standard you'll know that a router has a maximum data rate of 150 Mbps per data stream. That's why there are 150N, 300N and 450N routers. If you apply that logic to the 600 Mbps the new routers can achieve on 2.4 GHz, it would seem that a fourth data stream has been added. That's not the case, however.