UMA Today Magazine: UMA Jumpstarts Femtocells Page 11

"iU-b over ip"-bAsed solUTions

The

earliest femtocell solution proposals used a device-to-core
network connectivity approach generally referred to as "Iu-
b over IP." These solutions looked to leverage the existing
3GPP defined Iu-b interface that exists between 3G Radio
Network Controller (RNCs) and 3G radio antennas (Node
Bs). Primarily proposed by RNC vendors, these solutions
allowed operators to leverage the same RNC to support
femtocells in addition to macro network NodeBs.

As Iu-b over IP-based solutions enable operators to

leverage their existing core networks through standard
interfaces (Iu-CS and Iu-PS), they meet the operator
requirement for full-service transparency, as well as the
requirement for low initial deployment cost and network
disruption. As a result, these solutions were initially
viewed positively.

However, they eventually fell out of favor as they were

not able to cost-effectively scale. The challenge with
scaling this approach is in the basic design of RNCs, which
are typically optimized to support a relatively low number
of very high-capacity macro base stations.

Unfortunately, femtocells place the exact opposite design

requirement on a network controller, which is support for
a very large number of low-capacity base stations.

In addition, while 3GPP does define the Iu-b interface,

it is largely a proprietary interface specific to each RAN
vendor. As a result, there has been hesitancy to fully
standardize the Iu-b interface in order to open up the
femtocell market.

sip/iMs-bAsed solUTions

One alternative

proposal for femtocell device-to-core network connectivity
is to use a new SIP-based protocol between the mobile core

network and the femtocells. Operators would deploy a new
SIP-based core network that operates in parallel with their
existing circuit and packet-based core network. When a
handset is connected to a femtocell, it receives all of its
services from the new SIP core network.

As many operators believe they will eventually transition

their core networks toward an IMS and SIP-based
infrastructure, these solutions were also initially viewed
positively. SIP-based approaches also hold the promise of
cost-effective support for large-scale deployments.

However, as handsets are served by a different core

network when connected to femtocells versus when they
are on the macro radio network, service transparency
becomes a significant challenge for SIP-based solutions.
For every service an operator makes available on its macro
network, it now needs to replicate that service on its SIP
network.

Further, as the SIP-based approach requires operators

to acquire and integrate a new core service network,
the initial deployment costs and overall 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-

FALL 2007

UMA TodAY

Figure 2: Comparison of Device-to-Core Network Connectivity Approaches

Service Transparency

Excellent

Poor

Initial Cost/Disruption

Good

Excellent

Poor

Ability to Scale

Poor

Excellent

Path to Standards

Poor

Excellent

Good

lub over IP

RAN Gateway

SIP