Cell phones are everywhere, and femtocells
may follow. Designed for homes and small
businesses, these desktop cell-phone basestations
connect to the cell-phone network
via an existing high-speed Internet connection
using a DSL or cable TV modem.
The femtocell is a home version of the
micro and pico cells used in buildings and
other densely populated environments. They enable the network
to handle more subscribers, and they improve indoor
handset performance. And not only will they greatly improve
home cell-phone connectivity, they’ll also help unburden the
carriers’ already heavily loaded backhaul channels.
Femtocells promise unparalleled in-building coverage (i.e.,
five bars guaranteed in your home or office), dedicated highspeed
3G data services, and aggressive bundled tariff packages
that allow flat-fee rates for unlimited voice and data calls
while using your femtocell, for example. Beyond these three
major benefits, femtocells are seen as a platform for service
innovation, primarily by exploiting the fact that the device will
be deployed in the customer’s residence or office.
Multiple architecture options are already on the market and
under consideration for standardization (see the table). These
include the Universal Mobile Telecommunications System
(UMTS), Iub-over-IP, Iu-over-IP, Unlicensed Mobile Access/
Generic Access Network (UMA/GAN), and the IP Multimedia
Subsystem (IMS).
UMTS-BASED FEMTOS
Proponents of the UMTS-based architecture
cite its ability to leverage existing
mobile core network equipment, standards,
and capabilities to deliver femtocell
service faster with minimal added
equipment investment. UMTS-based
approaches provide an accelerated path
to macro-network equivalent services,
so operators can trial the femtocell value
proposition with customers earlier.
The first UMTS-based approach is
the Iub-over-IP architecture (Fig. 1). The
femtocell access point (FAP), also known
as the Home NodeB (HNB), takes on
the role of the Node B (i.e., 3G basestation).
The femtocell gateway (FGW),
also known as the Home NodeB Gateway (HNB-GW), lies
between the FAP and the radio network controller (RNC).
This approach specifically suits small-office/home-office
(SOHO) or single-home environments where only a few subscribers
would access services through the FAP. Depending
on the number of FAPs connected to the FGW, the RNC and
FGW could even be collapsed into one device.
The FAP communicates with the FGW using a 16-bit cell
ID that uniquely identifies a femtocell. The FGW multiplexes
the traffic coming from different FAPs and forwards it to the
RNC using the Framing Protocol (FP). The FGW doesn’t
modify any of the FP packets, especially the FAP identifier.
At startup, the FAP establishes a security association with
the FGW to avoid compromising subscriber information over
the public IP network. The FAP could use TR-069 or some
other similar mechanism to discover and obtain IP addresses
from an auto-configuration server (ACS).
The RNC would handle the resource management (bearer
and control) functionality. The RNC and FGW would take care
of delay jitter for the bearer traffic and control signaling (specifically
forced/hard handover mobility management).
To communicate with the pre-R4 Core Network (CN),
which doesn’t support IP transport, the RNC will have to
accomplish IP-to-ATM and vice-versa transport conversion.
In this architecture, the FAP looks like a normal basestation
to the CN. Also, all of the mobility management (MM) and call control (CC), including handover,
remains in the CN equipment domain.
The second UMTS-based approach
is Iu-over-IP (Fig. 2). The FAP takes
on the role of both the NodeB and the
RNC. A collapsed architecture makes it
possible to offload existing RNCs, freeing
up macro-network capacity while
still leveraging existing protocols and
cellular MM and CC procedures.
The configuration suits small and
medium business (SMB) and multiple
dwelling unit (MDU) environments,
where multiple customers can access
services through the FAP. The Iu-over-
IP configuration creates a greater
demand on the FAP to handle frequent
MM and resource management (RM)
procedures and places an even greater
premium on advanced quality-of-service
(QoS) capability in the FAP itself.
The FAP communicates with the
FGW using a 12-bit RNC-ID. The FGW
tunnels Radio Access Network Application
Part (RANAP) signaling from
the FAP to the CN. It may also convert
the IP transport from/to ATM transport
using SIGTRAN functionality if the CN
doesn’t support IP transport.
At startup, the FAP establishes a
security association with the FGW to
avoid compromising subscriber information
over the public IP network. The
FAP could use TR-069, or some other
similar mechanism could be employed
to discover and obtain IP addresses
from an ACS for the FGW. In addition,
the FAP uses the ACS to obtain RM
parameters and algorithms to be exercised
in the femtocell environment.
In this approach, the FAP handles
most of the RM (i.e., bearer and control)
functionality within the femtocell
environment. It defers to the CN for
these procedures during femtocell-tomacrocell
handover.
UMA/GAN-BASED FEMTOS
The UMA/GAN-based architecture
takes advantage of a proven and
standards-based approach for providing
cellular-grade services over the
best-effort Internet (Fig. 3). UMA is an
approach to fixed-mobile convergence
that bridges cellular radio and unlicensed
802.11 Wi-Fi technologies.
The standard UMA architecture
requires a dual-mode phone with specialized
client software for managing
handover and call control when moving
from cellular to Wi-Fi networks and vice
versa. The UMA/GAN-based femtocell
architecture moves the specialized client, or UMA client, from the handset to
the FAP, eliminating the requirement for
a dual-mode phone while leveraging
UMA’s signaling and QoS innovations.
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