11
Chapter
177
I-WLAN
I-WLAN overview
An
Interworking
Wireless
Access
Network
(I
‐
WLAN)
is
a
wireless
data
access
solution
that
allows
data
traffic
to
flow
between
WLANs
and
3GPP
systems.
Clients
accessing
such
a
WLAN
might,
for
example,
be
mobile
computing
devices
requiring
public
Internet
access.
A
SEG
can
facilitate
this
by
providing
interworking
between
the
clients
and
a
GPRS
backbone.
All
of
the
backbone’s
existing
facilities
can
then
be
utilized
including
charging
for
data
traffic
flows
using
standard
mobile
billing
methods.
The
overall
solution,
illustrated
below,
is
known
as
I
‐
WLAN.
Figure 12. A typical I-WLAN configuration
The
security
gateway
acts
as
a
Tunnel
Terminating
Gateway
(TTG)
in
this
I
‐
WLAN
solution,
with
the
SEG
providing
interworking
through
its
support
for
VPN
tunneling
on
the
client
side
and
the
GPRS
Tunneling
Protocol
(GTP)
on
the
GPRS
backbone
side.
GTP
is
used
by
the
SEG
to
communicate
with
a
GPRS
Service
Support
Node
(GGSN)
within
the
GPRS
backbone
network.
GTP
handles
both
signalling
and
data
transfer
in
the
network
and
is
implemented
as
a
layer
on
top
of
the
UDP
protocol.
SEG
GTP
support
means
that
the
security
gateway
behaves
like
a
Serving
GPRS
Support
Node
(SGSN),
allowing
traffic
flows
between
the
GPRS
Backbone
and
the
clients
connected
to
the
Access
Network
(usually
the
public
Internet).
As
explained
in
more
depth
later,
the
SEG
uses
a
feature
called
Stitched
Interfaces
to
facilitate
these
flows.
I
‐
WLAN
clients
connect
to
the
security
gateway
via
a
VPN
tunnel
using
IKEv2
security
negotiation
and
IPsec
encryption.
Authentication
of
the
client
is
by
EAP
‐
SIM
or
EAP
‐
AKA
via
the
security
gateway
to
an
AAA
server.
Authentication
of
the
security
gateway
is
by
certificates
sent
to
the
client.