Interconnection-Between-UMTS-and-WLAN-Through-Virtual-Access-Point-VAP

The VAP reverses the roles played by the UMTS and WLAN in the first two
interconnection architectures. Here, the WLAN is the master network, and
UMTS is the slave network. Figure 2.18 depicts the architecture of this interconnection
type.
The difference of this interconnection compared to the other architectures
is the existence of a VAP instead of RNC/3G-SGSN emulators. This is
the duality of the RNC/3G-SGSN emulators. Mobility is managed according
to the IEEE 802.11 WLAN and Interaccess Point Protocol (IAPP) specifications
by the WLAN standard. The intertechnology roaming that the
WLAN observes is between different access points in the extended service set
and the VAP that appears as yet another access point to the IEEE 802.11 WLAN. From the WLAN point of view, the entire UMTS network appears
as a basic service set or a picocell associated with another access point (in this
case, VAP). The function of the VAP is to communicate with MSs connected
through UMTS, de-encapsulate their packets, and transmit them
on the LAN. After this is done, the packets will reach the final destination
through the router attached to the LAN. The protocol stack architecture
is a modified version of that in Figure 2.13, only the VAP entity protocol
stack is placed after the GGSN protocol stack. The protocol stack of the VAP
entity is presented in Figure 2.19(a). The VAP-based interconnection also
requires some modification on the MS protocol stacks, which is presented in
Figure 2.19(b).
In this interconnection, the VAP becomes an adopted unit in the user
plane protocol of the UMTS architecture. For this reason, the IEEE 802.11
MAC protocol is implemented on top of the protocols of the UMTS GGSN
part. This is done for both the MS and the VAP entity. From the GGSN part of the UMTS network, all protocols up to GTP-U level will be mapped onto
IEEE 802.3 MAC so that the WLAN network sees the VAP as an access
point. On the VAP side, the UDP/TCP protocol is on the top of the stack.
The 802.11 MAC protocol that is implemented in the MS is a level below
the UDP/TCP. Hence, the 802.11 MAC protocols and the protocols below
it are mapped to the 802 logical link control (LLC). The 802.3 MAC protocol
in the GGSN part is mapped onto VAP below the IP/Point-to-Point Protocol
(PPP) protocol in the VAP protocol stack.