The 1 Millisecond 5G Myth

5G must be on the steep rise part of the Gartner Hype Cycle curve as I have heard a lot of non-technical people making a lot of technical statements out of context apart from the usual Mbit/s peak data rate claims. A prime example is the 1 millisecond round trip time that 5G should/will have to enable the 'tactile' Internet, i.e. Internet connectivity that is used to remotely interact with the physical world.

Sounds all nice but physics stands a bit in the way of this and nobody seems to say so. The speed of light and electricity is limited and in one millisecond, light can only travel around 200 km through an optical cable. So even if network equipment does not add any latency whatsoever, the maximum round trip distance is 100 km. In other words, there's no way to remotely control a robot with a latency of 1 ms in one part of the world from a place halfway around the world. But then, why let physics stop you?

So perhaps what was really meant is to further reduce the latency of network components? A big step was done in LTE with an air interface that has 1 ms slices and to base all network interfaces on the IP protocol to remove protocol conversions and the resulting overhead and latency. A scheduling interval of 1 ms means the round trip time on the eNodeB is in the order of at least twice this without even forwarding the packet to another node in the network. Add to this potential HARQ (Hybrid ARQ) retransmissions so you already end up at several milliseconds. Sure one could further reduce the length of the timeslices at the expense of additional overhead. But would it really help considering the many other routers between one device and another? Have a look at this great post of Don Brown and Stephen Wilkus which goes into the details.