UMA Today Magazine: Home Zone Issue (Spring 2008) Page 12

FEMTOcELLS

disruption to operations of the existing network are much
higher than either alternative approach.

"rAn gATeWAY"-BASeD SoLUTionS

The most recent proposals for femtocell device-to-core
network connectivity are generally referred to as RAN
Gateway solutions. The RAN Gateway approach is based
on a new, purpose-built network controller (the RAN
Gateway) that resides between an operator's existing core
service network and the IP access network, akin to an
RNC.

On its internet side, the RAN Gateway aggregates

traffic from a large number of femtocells over the new
"Iu over IP" interface. The RAN Gateway then inte-
grates the traffic into the existing mobile core network
through standard 3G Iu-CS and Iu-PS interfaces on the
core network side.

As RAN Gateway-based solutions leverage an operator's

existing core network through standard interfaces, they
allow for full-service transparency as well as a low initial
cost of deployment and low overall disruption to the
operations of an operator's existing network.

In addition, as the RAN Gateway approach employs

a "Splint-RNC" architecture in which a number of the
functions of a standard RNC are moved to the femtocell
itself, the scaling issues associated with the Iu-b over IP
approach are avoided.

Due to the service transparency, cost and scaling advan-

tages, the majority of femtocell solutions are now based
on the RAN Gateway approach for device-to-core network
connectivity.

THe nexT cHALLenge: DeFining THe
"iU over iP" inTerFAce

With the industry

now focused on the RAN Gateway approach to femtocell
device-to-core network connectivity, the key remaining
challenge is to fully define and standardize the "Iu over
IP" interface between the femtocells and a RAN Gateway.

As of mid-2007, there are two primary approaches being

discussed to address this challenge: (i) to define and stan-
dardize an entirely new interface; or (ii) to extend the existing
3GPP UMA standard, which is, for all intents and purposes, a
successful 3GPP RAN Gateway interface already used to ser-
vice dual-mode Wi-Fi handsets over the internet.

As is often the case, it helps to first define the key

operator requirements for the "Iu over IP" interface when
comparing alternative proposals, which include:

1. Provides a clear and fast path to standardization

Operators do not want to be `locked in' to a
vendor-specific interface between femtocells and
the RAN gateway. Traditional RAN vendors have
effectively created a closed market in the macro
RAN access network with vendor specific flavors
of the `Iu-b' protocol. But for the femtocell market
to flourish, an open, standardized interface
would enable operators to select from multiple
femtocell access point manufacturers, driving up
overall volumes and driving down overall costs.

2. Meets the unique challenges of mobile service

delivery over the internet As in the case of
dual-mode Wi-Fi handsets, femtocells present a
number of unique demands as they involve the

Figure 3: The "Iu over IP" Interface

UMA ToDAY

SPRING 2008