L3 consequences of WLAN offload in cellular networks (was - endless DHCPv6 thread)

Alexander Harrowell a.harrowell at gmail.com
Fri Dec 30 14:15:15 UTC 2011

In the DHCP v6 thread, there was some discussion of 
mobility and its IP layer consequences. As various people 
pointed out, cellular networks basically handle this in the 
RAN (Radio Access Network) and therefore at layer 2, 
transparently (well, as much as things ever are) for IP 
purposes. It therefore shouldn't be a problem. 

However, as one contributor pointed out, more and more 
cellular operators are migrating traffic onto WLAN for 
various reasons, notably:

1) Spectrum - it's unlicensed, i.e. free
2) Capex - the equipment is cheaper
3) Capacity - it's a cheap way of providing high speed
4) Signalling load - it gets rid of the signalling traffic 
associated with detaching and attaching devices from the 
core network. This is especially important in view of some 
smartphones' behaviour.

Of course much of the signalling is associated with the 
Mobility Management features, and getting rid of it by 
punting everything to WLAN implies that you lose the 
benefits of this.

That suggests that if you're going to do this on a big 
scale you need to implement Mobile IP or else keep 
backhauling traffic from the WLAN access points to the 
cellular core network (GAN/Iu interface), which has obvious 
effects on the economics of the whole idea.

Alternatively, you can work on the assumption that the WLAN 
is solely for nomadic use rather than true mobility, but a 
lot of devices will prefer the WLAN whenever possible.


The only thing worse than e-mail disclaimers...is people 
who send e-mail to lists complaining about them
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