Interconnection Between 3G-SGSN and WLAN Access Point

With this interconnection, the IEEE 802.11 WLAN network is connected to
the UMTS CN via the Iu-PS interface. Figure 2.13 shows this heterogeneous
network architecture.
IEEE 802.11 WLAN–based RAN is connected via an interworking unit
(IWU), as shown in Figure 2.13, which is an RNC emulator. This is needed
to exchange the packets between IEEE 802.11 WLAN network and UMTS.
The function of the IWU is similar to an RNC in the (UMTS terrestrial
RAN) UTRAN. It has to relay the Iu bearer service on the CN side to the
distribution network [IEEE 802.3 local area network (LAN)] bearer service
on the other side. The adapted UMTS bearer concept includes an appropriate
location and mobility management for the terminals in the IEEE 802.11
WLAN coverage area. Due to the very small cell size of IEEE 802.11 WLAN
systems, the access points are not directly connected to the UMTS CN. This
reduces the signaling load caused by mobility and location management. A
distribution network connects the WLAN access points and enlarges the coverage
area of this radio access form. The IEEE 802.11 WLAN is treated as a
routing area associated with the 3G-SGSN. Thus, the WLAN looks like an
RNC to the UMTS network. A user, whether connected to the UMTS network
or the WLAN, will always be treated as a UMTS user. The UMTS mobility management will have to maintain information about the user even
when it is connected to a WLAN network. The IWU entity is the RNC
emulator, which is presented in Figure 2.14.
The RNC emulator could be a LAN entity or a UMTS entity implemented
in the networks. The LAN entity avoids encapsulation for routing to
the UMTS network. For this type interconnection, a dual IEEE 802.11
WLAN/UMTS mode mobile station is required to use both networks, as
shown in Figure 2.15.
The intertechnology roaming arises when the user is connected to the
WLAN network. For this interconnection, the users have to interface to the
UMTS Packet Data Convergence Protocol (PDCP) network through the
RNC emulator. UMTS-specific protocol such as is on top of the IEEE
802.11 MAC and the PHY layers implemented. UMTS-related signaling
protocols are carried out between the protocols in the mobile station (MS)
and the RNC emulator. The RNC emulator is a black box that hides
WLAN-specific features from the UMTS network. The IP protocol is used
to transfer packet-switched data over the Iu interface as well as in the CN.
The GPRS Tunneling Protocol for UMTS (GTP-U) on the top of this transport
IP layer provides a tunneling service through the CN until the access
network encapsulates the user data. Hence, if IP packets are transmitted on
user level, two IP layer exist in the packet-switched architecture. The IEEE 802.11 WLAN coverage area is represented as one routing
area for the CN. If the mobile node leaves or enters a routing area, an update
message is sent to the CN of UMTS. Hence, the 3G-SGSN can simply distinguish
the different RANs via the routing areas. Running IP sessions are
not interrupted because the IP address of a terminal is not changed. The procedure
is completely transparent to the user. However, if a mobile leaves the
IEEE 802.11 WLAN coverage area, the service quality will degrade, especially
for those sessions that made use of the high throughput capabilities of
IEEE 802.11 WLAN system.
The current UMTS approach foresees that within the CN, differentiated
services (DiffServ) are used on the transport IP level to differentiate between
different traffic classes. This approach can be mapped quite easily on both the
IEEE 802.11 WLAN distribution network and the IEEE 802.11 WLAN
bearer. If switched Ethernet implements the IEEE 802.11 WLAN distribution
network, the DiffServ classes can be mapped onto IEEE 802.1p priorities and
then to IEEE 802.11 WLAN MAC connections, and vice versa. Figure 2.16
shows the UMTS bearer concept [18] with IEEE 802.11 WLAN access integrated.
The UMTS bearer is not changed with respect to the different radio
interfaces. The RAB must be adapted to the new, underlying distribution network
(DN) bearer and the IEEE 802.11 WLAN bearer.