Contenido esencial del derecho a probar

The basic access mechanism in 802.11x protocol is the well-known Currier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism which was firstly introduced in Ethernet technologies. In CSMA/CA a station that wants to transmit, senses the medium and starts the transmission only if no other station is transmitting [52]. Wireless Lan system are All-Ip based networks and follow the IP-systems philosophy. IEEE 802.11 protocols transmit in the 2.4-2.5 GHz frequencies and 5.5 GHz frequency with achievable speeds of up to 300 Mbps. For a station to join an existing Cell it can either wait for a beacon from an AP with synchronization information or make an active search by transmitting Probe Requests and awaiting Probe Replies from the AP. Once the base station is located then the authentication process starts in which a series of message exchange occurs where each side provides knowledge of a pass- word. There are three encryption protocols which lead to association: WEP, WPA and WPA2 with pre-shared Key. After a successful authentication the next step is the association process where a set of information regarding the station and BSS capabilities are exchanged. Once association is completed the station can transmit and receive frames [52]. The authentication process can either take place locally between the AP (based on a local database) and the client, usually in home environments, or by using a Network Authentication Server (NAS) and a RADIUS server in larger scale networks. A RADIUS server which stands for Remote Authentication Dial In User Service is a networking protocol that provides centralized Authentication, Authorization and Account- ing management. NAS server is a single point of access to a remote resource and is used as a trusted gateway in the network for credential exchange with new clients. RADIUS is a client/server protocol operating in the application layer of OSI stack, using UDP as transport. The three main functions of RA- DIUS are to authenticate clients before granting access to a network, authorize the client for resource requests and to account the clients usage. Each of the three functionalities is based on a series of request/response messages between the client and the server. When a user wants to connect to a WLAN, the NAS server will prompt for the users username and password and will then sent the credentials to the RADIUS server where a database check will notify the NAS server to allow or deny access to the user.

3.6. Wi-Fi Sollution - An overview 27

3.6.1

Wi-Fi 802.11af (Wifi using TVWS)

Since September 2010 when the Federal Communications Commission (FCC) revised the rules regarding access to unused UHF spectrum industry and stan- dardization bodies have shown interest in using TVWS to provide extended Wi-Fi like service. Cambridge TV White space consortium which aims to dis- cover how TVWS can be used to provide broadband services in the UK [54] and the Microsofts attempt of the WhiteFi network are good examples of the ongoing effort [55]. Also a working group named IEEE 802.11af was thus cre- ated to define a standard for the protocol implementation [56]. In general, it is believed that operating in the free white spaces of TV, Wi-Fi will benefit from greater speeds, extended range and better QoS [53]. A study made in [57] on the feasibility of large-scale Wi-Fi networks operating at TVWS shows that is viable and less expensive than alternative/completive cellular next generation networks. However, a recent work [53],that takes into account inter AP interfer- ence and congestion shows that operating Wi-Fi hotspots in outdoor rural areas is more viable and possible than outdoor urban areas where the user density is higher, with interference limiting the network capacity. For indoor deployment scenarios, the authors argue that due to the better propagation and wall pen- etration features of low frequency TVWS, WhiteFi can be used but a careful techno-economic analysis is necessary before any call.

3.6.2

Femtocell Wi-Fi integration

As both femtocells and Wi-Fi technologies were developed for indoor small area coverage to provide general Internet access to end users at first sight it seemed that these two technologies are competing each other. However it was soon re- alized that they could actually co-exist and complement each other with various benefits for the users and the providers. Some of the benefits are drive scale of economics of integrated devices and services, increased user satisfaction and alternative connection technology and service continuity on 3G UMTS voice and data applications to Wi-Fi [51]. In Figure 3.2, an abstract IFW system architecture is shown.

A 3G WiFi/Femto cell is operating in a hotspot providing both cellular and WiFi cervices to end users. It is important to note that the Wi-Fi con- troller which is responsible for the AP co-ordination is placed within the hotspot premises and uses SNMP protocol for management whereas FAPs are managed by the Femto Provisioning Server using the TR-069 3GPP standard. IPsec is used to encapsulate packets traversing the Internet towards the Core Net- work for Femtocell originated traffic and the GRE protocol for the WiFi ones. The ubiquitous management and provision of both technologies is challenging and opportunistic at the same time as a single entity for the management of CM/AM/PM is desirable. On solution is to proxy the WiFI protocol over TR- 069 by mapping SNMP MIB to TR-069 DM or the other way around. In IFW networks it is possible to use WiFi for data offload towards the Femtocell which differs from femtocell offload towards MNOs with potential advantages when the femtocell is operating in licensed mode. Additionally, seamless handover is possible from Femto to WiFI and vice versa based on handover protocols at the IP level. Furthermore, a unique feature is the flow segregation technique where http traffic is directed to the WiFi interface and Voice traffic to the Femto in-

28 Chapter 3. Background

Figure 3.2: From smallcell forum white paper on Integrated Femtocell Wifi systems.

terface [51]. The TS 23.234 protocol is describing the internetworking between cellular and wifi networks with handover procedures being standardized in TS 23.237. The TS 23.403 protocol describes how untrusted WLANs can access an Evolved Packet Core (EPC) network.