Cellular Wi-Fi Integration—A comprehensive analysis—Part I

From Competitive Positions to Shared Goals

Cellular and Wi-Fi radio technologies originated and evolved from two fundamentally different objectives. The former was motivated by the desire to make telephony technology mobile, and the latter by the desire to make data communications wireless. Over time, each has trended towards the other, with wireless data a central use of cellular technology today while over-the-top services provide voice over data networks. This confluence seems headed towards a fundamentally integrated cellular and Wi-Fi landscape, but the evolutionary nature of the trend has resulted in a broad variety of approaches and solutions.

Significant advancements in data over wireless began to emerge in the early 1980s. InterDigital helped pioneer this capability, with TDMA access in a DSP-based implementation in the 1980s laying the technological blocks for the digital wireless revolution of the 1990’s, followed by InterDigital’s WCDMA system using CDMA in channels as wide as 20 MHz to provide high-capacity fixed wireless access. With the introduction of the ETSI’s GSM system (a digital TDMA system with DSP-based devices), the vision of ubiquitous connectivity was at last beginning to appear real.  By the late 1990’s, ETSI and then 3GPP were already looking towards providing packet connectivity, first as the GPRS “add-on” system to GSM and then, with UMTS (a CDMA system), as a fundamental capability. 

At the same time a completely different vision of wireless access emerged.  This vision was based on the success of Local Area Networks – particularly Ethernet – in connecting the enterprise “intranet” into a highly capable local network.  Using the “free-for-all” ISM bands, first allocated by the US FCC in 1985, the IEEE 802.11 Working Group began looking at taking the Ethernet wireless. With no reliance on expensive licensed spectrum and fewer concerns regarding QoS, Wi-Fi technology emerged as a strong consumer – and, increasingly, enterprise – wireless solution by the mid-2000s.

The initial response from the cellular community was purely defensive.  Rather than embracing and integrating the new technology, 3GPP embarked on a fundamental re-architecture of its system which resulted in a completely IP-based packet-focused Evolved Packet Core (EPC).  Combined with OFDM-based LTE access technology, this was supposed to be the answer to the threat posed by Wi-Fi. 

The response from the Wi-Fi community to the customer demand for integration with mobile communication solutions was similarly lukewarm.  With the MIP family of IP enhancement, IETF took the lead in trying to provide mobility support to IP-based systems such as Wi-Fi.  However, the limited traction these protocols did find was in 3GPP where MIP and PMIP have been adopted as solutions for the EPC.   Until very recently the focus of the Wi-Fi community remained on delivering access to ever-greater bandwidth (with 802.11n), while the need for QoS management and mobility was completely ignored. 

That mutual defensiveness has now reversed, fuelled both by consumer demand regarding data to new device types and operator efforts to relieve network pressure. From the cellular side, small, localized cells, such as Wi-Fi AP coverage regions, are growing in importance.   As an IP-based system, the EPC is well positioned for integration of multiple heterogeneous access technologies and 3GPP is now moving towards taking advantage of EPC to delivery solution for real integration of technologies such as Wi-Fi.  With ANDSF, an integrated policy-based management system for how devices access spectrum 3GPP has already taken a first step in that direction. 

On the other hand, the Wi-Fi community has finally acknowledged that when using mobile devices (for example, the iPhone), consumers expect to receive the same quality of service whether they use Wi-Fi, 3G or LTE.   By extension, Wi-Fi must provide operators with the tools to manage Wi-Fi networks in the same way that they can manage their own 3G or LTE networks, and the recent Hotspot 2.0 profile and the associated PassPoint certification program sees the Wi-Fi Alliance beginning to move towards delivering such “carrier-grade” Wi-Fi solutions to the market. 

Finally, the trend towards true integration is beginning to come to the fore. The need is particularly acute in small cells – designed to address the high spectrum needs of local consumer and enterprise networks and Hotspots. Thus, through its work on Integrated Femto-Wi-Fi (IFW), the Small Cells Forum is already taking steps towards defining the near-future of such spectrum integration. What will this future hold? A combination of integrated small-cell solutions, smart connection management at the terminal, within a policy framework that provides management controls to both users and operators.

Wi-Fi Evolution

Initially conceived in the late 1980’s as a wireless extension of Ethernet, initial WLAN installations used the then recently FCC established unlicensed frequency bands and were primarily confined to fixed enterprise deployments.  With the establishment of the 802.11 Working Group by the Institute of Electrical and Electronics Engineers (IEEE) in 1991, the increasing speeds of 802.11a and 802.11b (operating on the unlicensed 5 and 2.4 GHz bands respectively), and the Wireless Ethernet Compatibility Alliance (WECA) coining the name “Wi-Fi”, and initiated an industry marketing, interoperability and certification program in 1999, Wi-Fi was successfully launched as a broadly-adopted wireless standard.

Close coordination between the IEEE 802.11 Working Group and the Wi-Fi Alliance has continued to improve capabilities, and newer Wi-Fi versions like IEEE 802.11g (a 54 Mbps version of 802.11b) and IEEE 802.11n (~600 Mbps at 5GHz using a wider bandwidth and multiple antennas for transmissions and reception), coupled with backwards compatibility with the older versions of 802.11, has resulted in Wi-Fi products certified as 802.11a/b/g/n.  It is important to recognize that an IEEE 802.11 device is not “Wi-Fi” unless and until it has been certified by the WFA.

Because Wi-Fi was originally conceived as a simple extension of an Ethernet cable, it’s design provided for short-range, local area coverage, and did NOT address general network considerations such as radio measurements and statistics, device management, or Quality of Service (QoS) (although IEEE 802.11 did produce standards such as 802.11e for QoS, 802.11i for issues like improved security). With the relatively recent incorporation of Wi-Fi into most cell phones and adoption by many mobile operators, some of these considerations are starting to be addressed – as will be examined later in this paper in the Present Activities section.

Mobility Solution Toolbox from IETF

IETF (Internet Engineering Task Force) is the technical body that defines “standards” according to which the internet operates. Its “standards” are the so-called RFCs (Request for Comments), which cover various aspects of Inter-Network Transport and higher layer functionalities, in terms of a variety of protocols. For example, IETF developed the IP (Internet Protocol), which defines the structure of information packets and how they are transported between two end-to-end IP-devices across an interconnection of networks. IP protocols are designed to be agnostic to the underlying characteristics of any of the intervening networks and have provided the highway architecture for the modern internet. It is precisely this independence from the underlying network characteristics that made IP a natural choice for interworking Cellular and Wi-Fi Networks, since It is a common language that be supported by both networks.

The basic IP-protocols did not address mobility of the end-devices and these are handled in a series of Mobile-IP standards. These can be grouped in two main categories: client-based and network-based solutions. Client-based solutions require some special functionalities in the client device, and make use of a mobility agent in the network, whereas network-based solutions rely on the network for both agent and client functionalities, thus making the mobile device agnostic to these mobility functions and therefore simpler to implement. One of the main goals of any of these mobility protocols is to provide seamless mobility as the device moves from network to network. This is essentially achieved by preserving the IP address of the mobile device via the concept of Home IP Address (which stays invariant) and associated Care-Of IP Address (which changes due to mobility). The main client-based approach used to provide seamless mobility is based on the Mobile IP (MIP) protocol [RFC 6275], which lately has been extended into the Dual-Stack Mobile IP (DSMIPv6) architecture [RFC 5555]. The main network-based approaches are based on the Proxy Mobile IP (PMIP) protocol [RFC 5213], also an extension of the MIP protocol.

MIP: “Mobile IP” Protocol

MIP supports uninterrupted routing of IP-packets to and from a mobile device and provides session continuity by means of a Home Agent (HA), which is an entity located at the Home Network of the mobile device (also referred to as a Mobile Node – MN) that anchors the permanent IP address assigned to the mobile device, known as Home Address (HoA). The HA keeps the device’s HoA when the MN has moved from the home network, and redirects traffic to the device’s current location. The HA is informed of the current location by the MN, using a temporary IP address or Care-of Address (CoA) that the MN acquires from the visited network. A bi-directional IP-tunnel between the MN and the HA is then used to redirect traffic between these nodes.

MIP, defined originally for IPv4 devices and networks, was subsequently extended to MIPv6 to be applicable to IPv6 devices and networks. DSMIPv6 is a further extension of MIPv6, where the basic mobility functionality is extended to also support dual stack IPv4/IPv6 devices and networks. Accordingly, DSMIPv6 extensions are defined to also register IPv4 addresses and transport of both IPv4 and IPv6 packets over the tunnel between the HoA and the visited network. These extensions enable the mobile device to roam between IPv4 and IPv6 access networks seamlessly, and are considered crucial as IPv4 networks and devices gradually evolve to IPv6.

PMIP: Proxy MIP

PMIP and its IPv6 extension, PMIPv6, are examples of Network-based IP mobility solutions, which manage the mobility of the mobile device entirely to the network. In this way, the device is not required to perform any signaling or updates, as it changes of its point-of-attachment (i.e. visited network) due to its mobility. Hence, these changes become transparent to the mobile terminal’s IP protocol stack, resulting in simpler device solutions than those based on baseline MIP.

The PMIP-enabled mobile IP network architecture consists of a central entity, called Local Mobility anchor (LMA), and a number of Mobile Access Gateways (MAGs), which together define a mobility domain. The LMA plays the role of a local HA (as in DSMIP networks) and anchors the IP prefixes used by the MNs. MAGs reside close to the mobile node, usually in the Access Routers (which in turn are either collocated with the Access Points or directly connected to them).

Detection of movement of mobile devices as well as implementing associated signaling is done by the MAGs. Typically, the MAG detects mobility through standard terminal operations, such as router and neighbor discovery or by means of link-layer support, without any mobility specific support from the device. Bi-directional tunnels between the LMA and the MAGs are set up, so that the mobile device is able to keep the originally assigned IP address within the mobility domain despite any location changes. Since the LMA is aware of the actual location of the mobile device, any packets addressed to the device are tunneled to the appropriate MAG, relieving the mobile device of the need to manage the IP packet routing due to its own mobility.

Extensions to Mobility of Multi-connection Devices

The underlying assumption in basic MIP and all their derivatives (such as DSMIP [RFC5555], an extension of DSMIP to support simultaneous IPv4/IPv6 operation) is that a mobile device has a single Home Address and a single Care of Address (which may change due to MN mobility). However, modern devices, such as smart phones, can support multiple IP connections, for example via Cellular and Wi-Fi network interfaces. Clearly, the DSMIP cannot support mobility of such devices, and IETF standardized the basic components required to remove such limitations. These components are: multiple care-of address registration support [RFC 5648], flow bindings support [RFC 6088], and traffic selectors definition [RFC 6089].

Multiple care-of-address registration enables a device with multiple IP connections to be registered with a single Home-Address and multiple Care-of-Addresses.  This allows the management of mobility of one or more network connections, by updating the corresponding care-of-addresses with the Home Agent. Flow bindings concepts enable the association (or binding) of individual IP-Flows to specific care-of-addresses (or network interfaces). IP-flows in turn are defined by the notions of traffic selectors, defined in RFC6089.  These concepts extend the concept of mobility to individual IP-Flows and allow one to move IP-Flows dynamically from one network interface to another (e.g. mobility of IP-Flows from Cellular to Wi-Fi and vice versa).

In the case of network-based mobility solutions, the mobility management control is not located in the mobile device but in the network. PMIPv6 and GTP are network-based mobility protocols and they provide some basic support for handling multiple interfaces. However, they do not support mobility at an IP flow granularity. For this reason, extensions are being defined in IETF [ IETF draft-ietf-netext-pmipv6-flowmob,  IETF draft-ietf-netext-logical-interface-support ] and the recently approved 3GPP study item on network-based flow mobility (NB-IFOM). The first extension allows a mobile device being attached to two different media access gateways (MAG) with two different interfaces (e.g. cellular and Wi-Fi) in the same PMIPv6 domain. The second extension allows a MAG to forward traffic to a mobile device, even if the IP address (e.g. IPv6 prefix) was originally delegated to the mobile via a different MAG.

Extensible Authentication Protocols (EAP)

Extensible Authentication Protocol (EAP) is a simple authentication protocol defined in [RFC 3748] which is designed to provide a generic framework for user authentication in a network within IP-Networks.   A key aspect of EAP is that it itself does not define how authentication is done – rather it defines a framework which can be used to define a specific authentication protocol.  Such protocols are referred to as EAP methods and in most cases the protocol involves authentication with a remote server. 

Examples of such EAP methods include EAP-SIM [RFC 4186] and EAP-AKA [RFC 4187, RFC 5448], which are used for authenticating cellular subscribers over Wi-Fi networks. Specifically, these use the SIM or USIM credentials of cellular subscribers for authentication. These protocols also provide for mutual authentication, meaning that UE is authenticated to the mobile network, but, at the same time, the UE is able to verify the identity of the network as well.

As an example, a Wi-Fi network performing EAP-SIM authentication would consist of a Wi-Fi Access Point (AP) to which the UE is attached. The AP is also connected to an AAA-server, which in turn is connected to a mobile operator’s HLR/HSS for the purpose of authenticating the user. The AP asks the UE to identify itself via EAP message exchange. On hearing back from the UE, the AP passes on the UE responses to the AAA Server via RADIUS messages, which are further passed on to HLR/HSS. Upon successful authentication, the HLR/HSS informs the AAA Server, which in turn communicates the results to the AP and eventually to the UE.

This concludes a review of efforts by the internet community to integrate cellular technology. In the second part of this paper, we’ll look at the reverse: how the GSM community and 3GPP have worked to integrate Wi-Fi.

Source: https://www.edn.com/design/communications-networking/4390437/Cellular-Wi-Fi-Integration-A-comprehensive-analysis-Part-I?page=2 – Prabhakar Chitrapu, Alex Reznik, Juan Carlos Zuniga- 07.16.2012 July 16, 2012