4.5G: Integration of LTE and Wi-Fi networks

Integration of LTE
and Wi-Fi networks
Authors Sponsor
Dr Triantafyllos Kanakis
Technical Trainer
Zahid Ghadialy
MD and CTO
4.5G:
v 1.0.0

Explaining Technology
10. Concluding remarks 20
168. Challenges with Wi-Fi integration to EPC
189. Wi-Fi and EPC tighter integration
1. Introduction 3
1.11.1 Introduction to WiIntroduction to Wi--FiFi 3
1.2 Need for Wi1.2 Need for Wi--FiFi
2. Interoperability between 3GPP and Wi-Fi
2.1.2 Discovery Information
4
5
22.1.1 Access Network Discovery and Selection FunctionAccess Network Discovery and Selection Function 6
8
2.1.3 UE Location 8
3. Network Architecture 10
Contents
2.1.1 Policy 7
2.1.4 Intersystem Routing Policy 9
2.1.5 UE Profile 10
3.1 Trusted network architecture3.1 Trusted network architecture 10
3.2 Untrusted network architecture3.2 Untrusted network architecture 11
3.3 Trusted with SaMOG network architecture3.3 Trusted with SaMOG network architecture 12
4. Seamless connectivity—Mobility 12
44.1.1 Mobile IPMobile IP 12
55..22 Proxy Mobile IPProxy Mobile IP 13
5. Simultaneous access to 3GPP and non-3GPP 13
55.1.1 MAPCONMAPCON 14
55..22 IFOMIFOM 14
6. Hotspot 2.0 15
66.1.1 RoamingRoaming 15
66..22 OperationOperation 15
7. Authentication and Carrier Wi-Fi 16
11. List of References 20
12. List of abbreviations 22

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1. Introduction
1.1 Introduction to Wi1.1 Introduction to Wi--FiFi
Wi-Fi, also known as WLAN, is a wireless data communication network, standardized by IEEE and specified by
the IEEE 802.11 family of technology which defined the physical layer (PHY) and medium access control
(MAC). Wi-Fi was introduced as 802.11 standards in 1997, in an attempt to replace the wired Ethernet con-
nections of the LANs and since then it has become inseparable from portable computers, mobile devices, tab-
lets and peripherals (e.g. Printers). Due to its wireless nature it became popular very quickly, by becoming a
norm for the new laptops being released at the time. The main success factor attributed to Wi-Fi is the low
cost of equipment and the fact that it uses ISM radio band, a portion of the spectrum reserved internationally
to be used for Industrial, Scientific and Medical purposes other than telecommunications. This means that
any equipment using any flavour of 802.11 does not need to pay any sort of fees to the government or any
other authority anywhere in the world for the lease of the spectrum. This fact alone made a collaboration be-
tween the IEEE and 3GPP impossible since cellular networks operate on dedicated (and generally expensive)
spectrum where they do not expect interference from any other technology. However, in the recent years, a
notable effort has been made by both standardization bodies in the direction of integration of Wi-Fi and 3GPP
cellular networks. Moreover, free spectrum is the main driver for LTE-U, a flavour of LTE-A operating in the
unlicensed bands proposed by Qualcomm and backed by many vendors and operators worldwide. Nowadays,
Wi-Fi access points (AP) can be found in enterprise and domestic use; in stores, offices, shopping malls, hotel
foyers, streets or stadiums.
Wi-Fi is present almost everywhere around us (Figure 1) as part of a modern connected world. Public trans-
port stations (1) are equipped with Wi-Fi, allowing visitors to access the internet during their stay in the prem-
ises. Wi-Fi hotpots are even present within the newer public transport vehicles for passengers to be con-
nected on the move. As a large part of productivity, information and entertainment are now passing through
the internet, telecom providers are choosing to have a strong presence in places with high concentration of
people who are of course their potential customers. So access to wireless networks can often be found in sta-
diums, sports venues, shopping malls, exhibition centres (2), domestic areas (3), airports (4), shops, cafes, res-
taurants (5), hotels, flat blocks, offices (6) just to mention a few.
Figure 1: Presence of the Wi-Fi.

In the past, Wi-Fi was used as an alternative to using the cellular data. Their research, development and stan-
dardization happened independently of each other. Whereas Wi-Fi was believed to be inferior in quality and
security, offering no additional benefits like seamless connectivity and roaming, it was free or comparatively
very cheap. In the recent years this thinking has changed and the cellular community has realised that Wi-Fi
can very well complement cellular data. Based on this revised understanding, the cellular and Wi-Fi standardi-
sation bodies have been working closely together for the cellular devices to be able to take advantage of the
Wi-Fi offering.
Mobidia reports that a typical iPhone user uses approximately 4 GB of data per month where the 82% is con-
sumed over Wi-Fi and only 18% over cellular networks. A typical Android user on the other hand, uses 2.9 GB
of data per month, 66% of which is transferred over Wi-Fi [1]. Other research published by Maravedis—
Rethink shows that a typical smartphone user uses approximately 4 GB of data per month with only a quarter
of it being transferred over cellular networks [2]. It is more than obvious that in the future a typical network
should be a combination of macrocells for mobility sensitive applications (such as voice calls) while small cells
and Wi-Fi will be used for offloading and coverage improvement.
Over the past few years, 3GPP has been working on new functionality that will allow a Wi-Fi AP to connect on
the EPC. As a result, the operators are able to offer a carrier grade Wi-Fi that allows the cellular subscribers to
offload part of their traffic. Wi-Fi roaming has also become possible recently, thanks to the developments in
Wi-Fi standards. The next big challenge is to enable simultaneous use of cellular and Wi-Fi to allow the best
access network for different individual data streams.
1.1.22 Need for WiNeed for Wi--FiFi
The increasing number of interconnected devices has led to the experts forecasting up to 50 billion connected
devices by 2020 [4] whilst the volume of data transmission is forecasted to increase ten times in the same pe-
riod. The operators are shifting their focus on to the 4G networks due to its ability to handle higher data vol-
umes, in addition to higher speeds and lower latency while on the other hand the legacy networks are shrink-
ing. As the amount of data transfer increases, offloading is becoming increasingly important. The obvious
choices for offloading are small cells and Wi-Fi. Note that residential Wi-Fi is generally not considered as an
offload. In the year 2013 alone, 34% of the mobile traffic was offloaded to alternative means. Cisco in its Vir-
tual Networking Index (VNI) predicted that by 2018 [5], more than half of the mobile traffic would be off-
loaded to alternative technologies like Wi-Fi.
Virgin Media, UK based quad-play service provider, runs a public Wi-Fi network throughout the London Under-
ground in the UK, serving more than a million connections daily. Online since the summer 2012, ready on time
Figure 2: Data traffic orientation

for the 2012 London Olympic Games, 92 “tube” stations had free Wi-Fi connectivity, with 137 hotspots cur-
rently in operation. The Cloud (a BSkyB company) brings Wi-Fi to more than 56 London Overground stations
[6]. At the same time BT in the UK reports on their website a massive 5 million Wi-Fi hotspots nationwide. In
the Wi-Fi sharing community, FON alone reports 13 million Wi-Fi APs worldwide; widely successful in the UK,
France, Portugal, Poland, Italy and Japan along with most of the other large cities globally [7]. Contrary to
what some analysts have predicted in previous years, Wi-Fi will not be replaced by small cells, instead cellular
and Wi-Fi are expected to be deployed hand-in-hand. It is now believed that in the following years, Wi-Fi will
become increasingly important, playing the role of the third RAN and will be the most reliable data offload
technology. While some operators like China Mobile (with over 4 million Wi-Fi AP’s available for their custom-
ers) prefer having their own APs, others like Telstra (tie-up with FON) and Verizon (tie-up with Boingo) are
happy to work with third party Wi-Fi service providers. Often, operators find it easy to start providing Wi-Fi in
partnerships with the third party Wi-Fi providers and then, install their own APs when they gain confidence in
the interworking of the technologies. UK mobile operator O2 is one such example where they had a partner-
ship with BT Openzone Wi-Fi network but later on installed their own APs for their customers to use.
There was a time when there were many disagreements between the cellular and Wi-Fi community. Opera-
tors discounted Wi-Fi as an inferior technology because of the limited channel interference control in the unli-
censed band being unable to guarantee the Quality of Service (QoS). Users on the other hand saw Wi-Fi as a
free resource and were unwilling to pay for it, unless for business use. Recently there has been a change in
attitude of both the parties. Wi-Fi is being seen as an alternative access technology, complementing the cellu-
lar technology, if it is seamless, providing a service similar to those of cellular networks. Wi-Fi generally pro-
vides higher speed internet access than cellular networks, that are often insufficient to provide a broadband
internet to all users simultaneously especially during peak hours. The shorter range of the Wi-Fi AP means that
there is a much lower number of users in an equivalent area, as compared to a macrocell. This generally trans-
lates to most users getting a better throughput. The advantage of mobile broadband on the other hand is
seamless connectivity while on the move, even when travelling at high speeds.
A general question often asked is, why not deploy Small Cells rather than Wi-Fi AP’s. Since the main focus is
dealing with the capacity, rather than coverage, Small Cells deployment in co-channel will give rise to Interfer-
ence. There are interference management techniques available in the standards but may not work well with
the legacy devices already in use. Wi-Fi on the other hand uses the ISM band in the 2.4GHz and 5.8GHz which
does not interfere with any cellular bands. Wi-Fi can do a better job in this scenario. This big slice of 5.x GHz
spectrum available for use free of charge is the main motivation of the unlicensed LTE (LTE-U). It should be
pointed out that the proposed LTE-U implementation refers exclusively to small cells deployment which due
to the higher carrier frequency, will not interfere with the macro-cell.
2. Interoperability between 3GPP and Wi-Fi
From cellular point of view, the interoperability between mobile and Wi-Fi networks need to support:
 Simultaneous access on both 3GPP and non-3GPP networks
 Seamless Connectivity between 3GPP and non-3GPP networks

 Unified authentication and security mechanism and
 Traffic Offloading
For all of the above to happen, the UE should be aware of the non-3GPP network presence in its vicinity along
with information about the operator’s policies towards each of the above. While a contemporary UE can easily
locate Wi-Fi networks in its vicinity, it is not in a position to know the individual roaming or interoperability
agreement a mobile operator has with the Wi-Fi networks in its vicinity. Furthermore, in the presence of mul-
tiple Wi-Fi networks the UE will not be in a position to perform the appropriate selection and will have to de-
cide purely on received power criteria.
3GPP realised the need to develop a mechanism for the interoperability between the 3GPP and non-3GPP
networks that would speed up the convergence of all cellular and wireless communication systems towards
LTE. Access Network Discovery and Selection Function (ANDSF) was introduced in 3GPP Rel. 8 [8]. It is an op-
tional entity within the EPC and its main function is to assist the UEs to discover and select non-3GPP networks
for offloading traffic.
2.1 Access Network Discovery and Selection Function2.1 Access Network Discovery and Selection Function
ANDSF is used to optimize the discovery of non-3GPP networks, such as Wi-Fi, by allowing the UE to interface
with the ANDSF server and retrieve the necessary roaming, billing and priority list information. It is a stand-
alone entity and interfaces with the UE over the 3GPP standardized interface S14 as shown in Figure 3.
Figure 3: The ANDSF entity
Figure 4: The ANDSF high level description

In cases of national or international roaming the UE has IP access on both Home and Visited ANDSF commonly
referred to as the H-ANDSF and V-ANDSF respectively. ANDSF provides the UE with Discovery Information by
sharing a list of networks that may possibly be available in its current location. If the UE discovers an additional
network that is not listed, it can report it back to the ANDSF server for investigation. With the discovery infor-
mation message a network priority list shall also be sent to the UE where in the presence of multiple Wi-Fi
networks the UE should be in a position to make a non-random selection. As shown in Figure 4, ANDSF mes-
sages are composed of six sets of information and they can be initiated either by the UE or the network.
2.1.1 Policy2.1.1 Policy
The policy set of information represents the Intersystem Mobility Policies (ISMP) with at least one active rule
at any time. Policy practically indicates the supported Access Networks while it also provides the appropriate
priority rules and Access Network IDs. It also defines the geographical area where a UE may be eligible for
making use of alternative RANs by means of Tracking Area Code and Cell ID, WLAN SSID or Geographical coor-
dinates fed from the UE’s GPS receiver. This is presented in more details in section 2.1.3. The policy rules have
a number of validity conditions and possible results e.g. the validity conditions may include the locations and
the exact time throughout the day a particular set of discovery information is valid.
Figure 5: The ANDSF Policy
Figure 6: The ANDSF Discovery Information

2.1.2 Discovery Information2.1.2 Discovery Information
Discovery information node specifies the RATs in the vicinity of the UE while the access network area is ex-
pressed as TAC and Cell ID, WLAN SSID or GPS geographical coordinates. The UE shall initiate the provision of
discovery information from the ANDSF server over S14 interface. The ANDSF server will report at least one
network in the operational area of the UE, while the UE is responsible for discarding the information from the
unsupported systems.
2.1.3 UE Location2.1.3 UE Location
The UE may send location information to the ANDSF server which will be based on either of the following op-
tions:
The UE gets location information from the System Information Blocks (SIB) of the macro cell. The Public Land
Mobile Network (PLMN) identity, Tracking Area Code (TAC) and cell identity can provide location information
to the ANDSF server.
Location information is also given by the Wi-Fi network by HESSID, SSID, BSSID messages sent over the bea-
con. Some Access Networks share Latitude and Longitude information with the UE over Radius. Latitude and
Longitude information can also be taken by GPS receivers, since most UEs have them nowadays.
Once the UE switches on a 3GPP network, it follows the PLMN selection procedure as specified by 3GPP, be-
fore any other Access Network discovery procedure is initiated. Once PLMN is chosen, the UE shall first select
an Access Network and then determine the presence of such network in the local area. The selection of an
Access Network is made based upon a priority list. According to the standards, if a higher priority Access Net-
work is detected and is connected to the selected PLMN (or a PLMN with a higher priority), then the UE shall
attempt to attach via that network. The Access Network type of interest in this document is WLAN which is
assumed to be the one with the highest priority.
For the detection of the supported WLAN Specific Identifiers (WSIDs) of the WLAN, the UE will initiate either
the passive or active scanning functions defined by IEEE std 802.11 [2007]. In passive scan operation, the UE
Figure 7: The ANDSF UE Location

monitors the wireless medium for beacon frames that provide the UE with timing and advertising information.
In this type of scanning, the UE listens to every channel of the wireless medium, one at a time. In active scan
operation the UE takes the initiative to associate with an AP by sending a Probe Request message on each
probed channel, one by one, and waits for a Probe Response message from the reachable APs. If no Response
messages are received within the timer expiry period, the UE assumes that the channel is inactive and moves
on to the next one. The WLAN name is provided in the SSID information element.
Upon successful discovery procedure the UE shall attempt to camp on cellular and WLAN cells. Therefore, lo-
cation information shall be provided.
2.1.4 Intersystem routing policy2.1.4 Intersystem routing policy
Inter-system routing policy (ISRP) shown in Figure 8 has been developed by 3GPP and it is part of the ANDSF. It
is used to provide the UE with necessary information about routing certain types of traffic. In fact, operators
must offer the best service to every user with a high level of QoE, depending of course on their subscription.
Therefore, the management of data traffic to and from their network must be carried out in the best possible
way. The ISRP will indicate to the UE which type of traffic should be routed through the cellular access net-
work and which should be routed through WLAN. ISRP rules have a home PLMN leaf and a roaming leaf given
that PLMN allows roaming. At any given time at least one IRSP rule applies which is referred to as the “active
rule” while in roaming situations, a Visited PLMN rule applies on top of the Home PLMN rule. In this case, the
active rule shall be the one from the Visited access network.
The ISRP information is divided into 3 categories depending on whether the operator allows and supports
seamless mobility between access networks or not.
The first category (ForFlowBased) specifies the routing individual flows of data packets carrying traffic to and
from the same distant IP address. It is likely some data packets will be forwarded through the Wi-Fi route
while some others are routed through the cellular core network. The choice of the access network for each
flow of the data packet is a choice of the access network selection policy for each operator. ForFlowBased is
designed for IP Flow Mobility (IFOM) offloading, as discussed later in the document.
Figure 8: ANDSF ISRP

The second category (ForServiceBased) specifies the routing of data packets carrying any type of traffic to and
from different IP addresses simultaneously. Hence, in this case, the UE might simultaneously use cellular and
Wi-Fi resources, where each access network is used to carry different types of traffic. ForServiceBased is de-
signed for Multiple Access PDN Connectivity (MAPCON) offloading as discussed later in the document.
The third category (Non-seamless Offload) specifies the traffic behaviour for non-seamless offloading. In this
case the UE is able to choose between access networks on a per IP flow basis however, the WLAN traffic is not
routed through the P-GW. Hence traffic is routed on the PDN via alternative route not involving 3GPP entities.
Session continuity and QoE cannot be guaranteed since WLAN is not controlled by the operator nor is it a
roaming partner.
2.1.5 UE Profile2.1.5 UE Profile
By UE profile, ANDSF stores information regarding the UE including the device capabilities and the supported
RATs along with the Operating System information by means of OS ID. Therefore, operators should be aware
of the UE’s OS family and version. Anyhow, it is the UE’s responsibility to periodically re-evaluate ANDSF poli-
cies and update the server with the most current information.
3. Network Architecture
3.1 Trusted network architecture3.1 Trusted network architecture
The UE might connect on either the LTE or the Wi-Fi network. The Wi-Fi APs are connected to Mobility Con-
troller Gateways (MC-GW), a form of Wi-Fi concentrators interfacing with the AAA server for authentication,
Figure 9: UE Profile
Figure 10: Integration of a trusted Wi-Fi network to EPC

authorization and accounting purposes and with the P-GW to gain access to the PDN. A MC-GW entity is
equivalent to a S-GW in the EPC; it is fully managed by the WLAN service provider and interfaces to the Wi-Fi
AP over SWu which is also managed by the WLAN provider. The interface between the MC-GW and P-GW is
the S2c which is managed by the cellular operator if different from the WLAN service provider. As it is shown
in Figure 10, the UE will always route its traffic through P-GW which acts as the mobility anchor between
3GPP and non-3GPP networks. Users will be authenticated for access on both WLAN and EPC, through the
AAA server of the cellular network.
3.2 Untrusted network architecture3.2 Untrusted network architecture
In a similar manner to the trusted network architecture, the UE is still capable of choosing between an un-
trusted non-3GPP WLAN and the LTE networks, given that the cellular operator and the Wi-Fi service provider
have some sort of a roaming agreement between each other. Since the EPC does not have a fully managed
secure interface with the WLAN network, a new network entity is introduced, the evolved Packet Data Gate-
way (ePDG). The ePDG acts as a S-GW for the entire Wi-Fi network as shown in Figure 11 and is connected on
the P-GW over S2b interface, commonly known as SMOG (S2b Mobility over GTP) [10]. The main function of
the ePDG is to secure the transmission between the UE and P-GW when traffic is routed through the WLAN
and is transported through a secure IPSec tunnel to TWAG and a GTP tunnel over S2b interface to the P-GW.
Figure 11: Integration of an untrusted Wi-Fi network to EPC with SMOG
Figure 12: Integration of a trusted Wi-Fi network to EPC with SaMOG

3.3 Trusted with SaMOG network architecture3.3 Trusted with SaMOG network architecture
For a safer mobility between 3GPP and non-3GPP networks that have a trusted relationship, 3GPP introduced
S2a-based Mobility Over GTP (SaMOG) [11], shown in Figure 12, allowing UE to seamlessly handover between
cellular and Wi-Fi networks. With SaMOG, the MC-GW will not directly connect onto the P-GW as WLAN net-
works are lacking security in comparison to cellular networks. For the extra protection, a Trusted Wireless Ac-
cess Gateway (TWAG) entity is used that acts as the perimeter security entity of the EPC network and con-
nects to the P-GW over a secure GTP tunnel.
4. Seamless connectivity—Mobility
A lot of work has been done in the last few years towards achieving seamless connectivity, often referred to as
the IP session continuity in Wi-Fi. A number of Wi-Fi related techniques have been developed to allow mobil-
ity between different WLANs.
4.1 Mobile IP4.1 Mobile IP
The simplest Wi-Fi mobility technique is Mobile IP (MIP) as shown in Figure 13a where the UE moves from a
Wi-Fi AP to another. For this purpose a temporary IP address is assigned to the UE in its new location which is
often referred to as the Care-of Address (CoA). However, for IP session continuity, traffic should still be routed
to the UE’s original IP address known as the Home Address (HoA). A Home Agent (HA) entity is used to provide
information about the UE’s location at any time and is responsible for associating the HoA with the CoA, a pro-
cedure known as “binding”, which once completed, sends an acknowledgment message back to the UE. The
Correspondent Node (CN) which is located within the PDN sends traffic to the UE HoA which is tunnelled by
the HA to the CoA. On the other hand, the UE can transmit either using the CoA and update routing or keep
the same routing and communicate through the established tunnel via the HA. Therefore, MIP routes data
packets to and from a UE by providing session continuity by means of the HA. MIP was initially designed for
use with IPv4 networks but it was later extended to support IPv6 addresses too (MIPv6) and further extended
to support dual stack (IPv4 and IPv6) commonly known as DSMIPv6. The anchor point for the mobility be-
Figure 13: (a) Wi-Fi seamless connectivity with MIP (b) Seamless connectivity between Wi-Fi and EPC with MIP

tween 3GPP and non-3GPP networks is the P-GW which acts as a HA in DSMIPv6 deployments while the inter-
face between the AP concentrator (MC-GW) and the P-GW (HA) is the S2c [19] as shown in section 3.1.
4.2 Proxy Mobile IP4.2 Proxy Mobile IP
Proxy Mobile IP (PMIP) has significant differences from its ancestor, MIP. The HA is being replaced by a Local
Mobility Anchor (LMA) node that is in control of all incoming and outgoing traffic on the dependent networks.
All traffic between the dependent networks and the PDN is routed through the LMA. In addition to the LMA, a
Mobile Access Gateway (MAG) entity is being introduced, responsible for providing a link between LMA and
Wi-Fi AP which are connected to a network specific MAG, which is the gateway to the LMA and the internet.
MAGs usually reside in the access routers which most of the times are the AP themselves. The LMA and the
attached MAGs together form a mobility domain, which allows the UE to move between networks in a trans-
parent mode.
Mobility between networks is detected through standard terminal operations, however the signalling associ-
ated with this movement is being taken care of by MAGs. Bi-directional tunnels are setup between the LMA
and MAGs in a such a way so that the UEs do not need to change their IP address within the mobility domain.
Due to the LMA dominant position, it is responsible for knowing the location of every UE under its mobility
domain. Any packets addressed to a specific UE are transferred to the responsible MAG over the dedicated
tunnel reducing this way the mobile device’s signalling functions while it relieves it from the need to manage
IP packet routing.
In a cellular network, the role of the LMA is been played by the P-GW [19], while the MAG is equivalent to the
TWAG as shown in section 3.3. In a similar manner to the DSMIPv6 technique shown in section 4.1, in PMIPv6
a secure dedicated traffic tunnel is formed between the UE and P-GW. The difference is that DSMIPv6 needs
to be supported by the UE while the PMIPv6 does not require any changes to the UE since TWAG runs mobile
IP functions transparently to the UE [19] over S2a interface.
5. Simultaneous access to 3GPP and non-3GPP
With the new order of things in mobile communications and with the coexistence of the two dominant radio
Figure 14: (a) Wi-Fi seamless connectivity with PMIP (b) Seamless connectivity between Wi-Fi and EPC with PMIP

access technologies in the telecommunications arena, the biggest benefit will come from collaboration be-
tween the two. So the UEs should support both 3GPP cellular and Wi-Fi communications with the aim being
the convergence of the two technologies. When the evolution towards consolidation is completed, a device
should be capable of seamless mobility between the two access networks while a combination of 3GPP and
Wi-Fi will enable smart traffic offloading and improved routing capabilities.
5.1 MAPCON5.1 MAPCON
MAPCON is developed in order to allow the UE gain simultaneous connection to more than one IP address via
both 3GPP and non-3GPP access networks subject to UE capability. MAPCON is mainly used to offload traffic
from the core network. Mobility sensitive applications (e.g. VoIP, Video streaming) shall not be offloaded as IP
connection may fail during handover. MAPCON is an EPC function and it does not depend on MIP.
5.2 IFOM5.2 IFOM
IFOM is a function that allows traffic to be routed through either 3GPP or non-3GPP access network, with indi-
vidual flows to the same PDN connection. IFOM is based on network policies, where different types of traffic is
being forwarded to and from the UE through different Access Networks via individual flows. IFOM requires UE
to be compatible with MIP family stack.
Figure 15: Simultaneous Access to LTE and Wi-Fi with MAPCON
Figure 16: Simultaneous Access to LTE and Wi-Fi with IFOM

6. Hotspot 2.0
In 2010, Hotspot 2.0 Task groups in Wi-Fi Alliance was formed; they created a set of standards to improve the
end user experience, interoperability and roaming issues. HS2.0 can limit MAC or user name and password
based authentication. It is often referred to as HS2.0 and Wi-Fi Certified Passpoint. HS2.0 is based on IEEE
802.11u “Interworking with External Networks” and it defines functions and procedures aiding network dis-
covery and selection. Mobile devices will automatically join a Wi-Fi network whenever it is available as HS2.0
allows Wi-Fi roaming; it provides the end user with a better bandwidth and ultimately offloads macrocell. With
millions of Wi-Fi APs available worldwide, especially in the high density areas, seamless WLAN mobility is
made possible with HS2.0. Therefore the time a UE spends in WLANs is extended reducing the “ping pong”
effect whereby the UE moves between WLAN and cellular very often, as shown in section 8.
6.1 Roaming6.1 Roaming
Although in cellular communications “roaming” mostly refers to making use of an international visited net-
work , with HS2.0 a UE is allowed to use the WLAN network of either a national or an international roaming
partner. HS2.0 handles roaming between the Wi-Fi APs. The UE can move outside the coverage of the home
network, entering into the coverage of a HS2.0 roaming partner seamlessly, without the need of authentica-
tion in the new AP. HS2.0 will handle roaming mobility by allowing a UE to maintain its IP address regardless of
the number of HS2.0 associated APs used, assuring this way an uninterrupted switch between roaming part-
ners.
6.2 Operation6.2 Operation
The HS2.0 Wi-Fi AP broadcasts a Beacon message practically advertising the HS2.0 support. If the UE is em-
bedded with IEEE 802.11u, it will pick up this message and will try to camp on the AP by initiating the Access
Network Query Protocol (ANQP) signalling. ANQP is a query and response protocol that allows a Wi-Fi enabled
UE to discover the available Wi-Fi APs within coverage. The UE will send an ANQP Query message utilizing the
Generic Access Service (GAS) protocol, part of the 802.11u. The AP will forward the Query to the ANQP Server
which will respond with “ANQP Response” message back to the UE, via the Wi-Fi AP, containing a capability
list, expressed in a set of values that practically define the type of authentication supported, types of services
supported, the domain name and the supported roaming partner list. The Home PLMN (HPLMN) operator will
decide which non-3GPP Access Network shall be selected.
Figure 17: Hotspot 2.0

7. Authentication and Carrier Wi-Fi
The Authentication Key Agreement (AKA) is defined by the 3GPP and it is the security procedure of the 3GPP
networks. Extensible Authentication Protocol (EAP) is defined by the IEEE as the authentication mechanism of
Wi-Fi and HS2.0. EAP-AKA creates the same authentication challenges as the EPS-AKA used by the 3GPP net-
works and is the most popular option used by the mobile operators providing carrier Wi-Fi services to their
subscribers as shown in Figure 18. Hence, users can freely move between cellular and carrier Wi-Fi networks
(of the same operator) using a common authentication mechanism. At the same time, if the carrier Wi-Fi is
HS2.0 enabled, the UE will be able to roam to Wi-Fi partners without the need for authentication.
HS2.0 is based on the IEEE 802.11u/I specifications and is standardized in two phases. The Passpoint Release 1
certification, completed in 2012, contains the Hotspot Security and Automatic Login, Network Selection proce-
dure giving a priority selection of Wi-Fi Networks and Network Selection by ANQP. The Passpoint Release 2
certification contains the online sign-up and the operator specific policy functionality. Due to the availability of
Passpoint, 3GPP has been able to provide the hooks in the cellular networks to link corresponding functional-
ity, such as RAN access, authentication, policy, charging, and roaming. This in turn will allow the cellular net-
work operators to offer carrier Wi-Fi solutions, either by themselves or in conjunction with other Wi-Fi service
providers.
8. Challenges with Wi-Fi integration to EPC
Having shown the network traffic offloading techniques, the simultaneous access to 3GPP and non-3GPP net-
works and how seamless connectivity can be achieved, we are ready for 4.5G as a result of the integration of
Wi-Fi into EPC. Although it seems that the conditions have matured enough so that cooperation between LTE
and Wi-Fi could be as smooth as possible, there are still challenges to be addressed while designing the net-
work, as can be seen in Figure 19. It is clearly a decision of the operator to decide which type of traffic should
be offloaded on Wi-Fi and which should be routed through the cellular network, guaranteeing a certain QoS.
The main challenges to be considered are as follows:
Figure 18: WLAN to LTE joint authentication

1. The UEs are often mandated by the network to automatically camp on Wi-Fi when the power received
by the Wi-Fi AP is higher than those received by the LTE eNB. The problem in this scenario is that the UE
only monitors the air interface link neglecting the backhaul capacity (C) of the base stations (eNB and
AP) to the core network. Hence, if the backhaul connection speed of the Wi-Fi AP is lower than that of
the cellular network, the UE has probably made a poor decision offloading onto the Wi-Fi network. Con-
versely, if the Wi-Fi AP backhaul connection speed is sufficient to match that of the cellular network, it
should result in an improved QoE for the end user. An end-to-end connection speed evaluation prior to
traffic offloading would significantly improve the decision making process, thereby reducing any down-
grading of the QoE.
2. In a similar case to that of the first scenario, the UE might be instructed to select the heavily loaded (L)
Wi-Fi AP. In this case, the backhaul may be sufficient or better than that of the cellular network; how-
ever, the wireless channels might be heavily loaded. This scenario is most likely to happen when many
UEs are using this AP. In this case too, the solution of the problem would be to perform end-to-end con-
nection speed evaluation prior to traffic offloading.
3. In high density urban areas, where Wi-Fi APs are literally present everywhere but very often with limited
over-lapping, the UEs end up bouncing between the cellular and Wi-Fi networks, significantly reducing
the user QoE. A great amount of resources is wasted on signalling, increasing the signalling traffic at the
network side while dramatically degrading the QoE for the user. An obvious solution to this challenge
would be for the cellular network to hold on to the UE when high mobility is detected within a highly
Figure 19: Challenges with Wi-Fi to Cellular mobility

dense urban area.
4. As shown in scenarios 1 and 2, the UEs might be mandated to automatically switch to Wi-Fi when avail-
able. However, UE might handover / offload to a Wi-Fi AP a bit prematurely, when the power (P) re-
ceived by the Wi-Fi AP will still be low providing the UE with limited channel capacity capabilities. The
premature handover will result in a QoE degradation, especially if the UE will not move any closer to the
Wi-Fi AP. Network testing prior to handover is the solution to this problem too.
In addition to the above mentioned challenges the operator may have to address other minor challenges that
are beyond the scope of this paper, such as excessive battery use due to continuous scanning to detect Wi-Fi
in their vicinity etc.
9. Wi-Fi and EPC tighter integration
In order to be in a position to reach a point where all wireless and mobile communication systems will co-
verge into LTE, studies have been conducted to achieve the cooperation between 3GPP and non-3GPP sys-
tems. With LTE and LTE-A failing to meet the minimum requirements to be considered the '4G' mobile com-
munication system; integration of Wi-Fi, the most successful wireless access technology and the biggest com-
petitor to the cellular systems, should be speeded up in the 3GPP based cellular systems. Not only will this
help solve coverage, capacity and offloading issues, it would only meet and better the '4G' requirements laid
out by ITU. The successful selection and offloading between cellular and Wi-Fi will lead to networks being able
to provide higher capacity and much better data rates, thereby leading to 4.5G, an intermediate step before
the imminent arrival of 5G. Rel. 12 of the 3GPP standards contains many improvements of the existing func-
tionality and introduces new functionality helping enable a smoother coexistence of Wi-Fi and LTE under a sin-
gle network.
Starting with ANDSF, which already exists since the 3GPP Rel. 8 and has been the vehicle of convergence for
the non-3GPP into 3GPP networks, despite slow progress in the past years, major improvements and new ele-
Figure 20: Evolution of ANDSF in 3GPP standards since Rel. 8

ments have been introduced in 3GPP Rel. 12 to allow the mobile operator to gain a greater control over the
RAN selection and Wi-Fi offloading policies [27].
 The first additional element in ANDSF is WLANSP (WLAN Selection Policy) which is a list of prioritized op-
erator set rules wherein at any point of time only one rule applies, the “active rule”. The most impor-
tant elements of the WLANSP are the selection criteria that are operator prioritized and contain infor-
mation about the preferred roaming partners, minimum backhaul requirement for a UE to camp on a Wi
-Fi AP, the maximum acceptable channel load and the preferred SSID list. Note that the backhaul (UL/DL)
capabilities and channel utilization are advertised by the Wi-Fi AP while the selection criteria are user
specific and are associated with the rates each set of credentials are charged with. With WLANSP, the
two most important challenges of Wi-Fi integration to EPC are addressed as shown in Section 8 and de-
picted in Figure 19.
 The second new element in ANDSF is the IARP (Inter-APN Routing Policy) that consists of two major sets
of rules, one for the inter-APN (Access Point Name) routing and one for the non-seamless offload similar
to the one found in ISRP. IARP allows the operator to set the routes for different types of traffic, the vari-
ous applications and the traffic ports used, the appropriate routing criteria per validity area and time of
day and APN based routing rules along with a priority list.
 The rule selection information refers to the WLAN selection rules when the UE is roaming to a visited
PLMN (VPLMN). The UE will receive information from both H-ANDSF and V-ANDSF for the active rule to
be decided. In this case, rule selection information will instruct the UE on how to select an active ISMP,
ISRP and WLANSP.
 Finally, the Home operator preference information is a set of rules that contain S2a Connectivity prefer-
ence information. The most important one being the S2a connectivity preference node indicating to the
UE whether the operator would prefer to establish a PDN connection over WLAN or the cellular route.
This element is mostly applicable to operators that maintain a carrier Wi-Fi network that interfaces with
EPC through a TWAG, operating the enhanced SaMOG (eSaMOG).
eSaMOG is the second major set of improvements in 3GPP Rel. 12 and can be considered as the foundation
for '4.5G' network. TWAG becomes a far more active entity of the network by taking the responsibility to At-
tach and Detach a UE on the Carrier Wi-Fi network, assign it with an IP address and negotiate with the EPC the
handover and offload capabilities. A set of different modes of operation have been introduced. Based on the
mobile operator’s policies, the network negotiates with the UE, the mode of operation as part of the authenti-
cation procedure, based on extensions to the EAP-AKA. In addition TWAG is used to allocate the UE with an IP
address depending on the mode of operation that has been a selection of:
 Single connection mode: the TWAG establishes a tunnel between the UE and either the P-GW or the
Offload route based on the UE’s request.
 Transparent single connection mode: the TWAG establishes a tunnel between the UE and either the P-

GW or the Offload route based on the network’s selection which is transparent to the UE. In other
words, the UE does not care if the traffic will be routed through the EPC or the WLAN backhaul.
 Multi-connection mode: the TWAG establishes two tunnels, one between the UE and the P-GW and an-
other one between the UE and the Offload route given that the UE can be simultaneously connected on
both access networks.
Additional signalling has been introduced as part of the EAP-AKA authentication where, apart from the con-
nection mode request, the UE requests the PDN type (IPv4, IPv6, both), handover information and indicates to
the network whether or not 'Non-Seamless WLAN offload' is supported. Finally, TWAG is responsible for the
attach and detach of UE on the WLAN, handover from 3GPP to WLAN and back and non-seamless WLAN off-
load in compliance to the mobile operator’s policies advertised to the UE through the ANDSF. The improved
TWAG and S2a interface functionality are the main drivers for the eSaMOG leading to a Tighter Integration of
Wi-Fi into EPC.
10. Concluding remarks
It is shown in this paper that cellular and Wi-Fi together can be used to increase system capacity and provide
subscribers with a seamless and fast connectivity over the internet. With the help of tight integration between
the two systems, '4.5G' is born, capable of satisfying the ITU’s criteria for 4G and even bettering it, laying foun-
dation for the imminent arrival of 5G. Mobile operators worldwide are either already investing or getting
ready to invest in the carrier Wi-Fi networks, allowing the subscribers to seamlessly and transparently move
between cellular and WLAN networks. Along with offloading, coverage is also improved by the low cost infra-
structure (as compared to small cells) whereas capacity is significantly improved.
The development of HS2.0 marks a new era in WLAN networks. Wi-Fi is gradually becoming as secure as cellu-
lar and the cellular devices can now transparently move between cellular and Wi-Fi without the need to enter
any login and password information. The predicted 50 billion connected devices [4] would need much more
than just small cells and Wi-Fi integration in the core networks and these are the first steps in the direction
towards 5G or 'beyond 2020' mobile communications.
The future hyper dense networks will most likely operate in a mesh topology where any connected device will
be capable of communicating with the peers in the same way as the operator's network. Without a doubt,
seamless connectivity, development of HetNets and integration of new RANs into the existing cellular core
network will help the transition towards 5G.
11. List of Reference
[1] Understanding today’s smartphone user, Demystifying data usage trends on cellular and Wi-Fi networks,
Informa Telecoms & Media—Mobidia White Paper
[2] Caroline Gabriel, “Integrating WiFi in the HetNet”, Maravedis Rethink, Mobile Operator Strategies Analy-

sis, white paper
[3] HS2.0—Making Wi-Fi as easy to use and secure as cellular, Ruckus, White Paper
[4] Dave Evans, “The internet of things, How the Next Evolution of the Internet IS Changing Everything”,
Cisco Internet Business Solutions Group (IBSG), White Paper, 2011
[5] Dr R. Pepper, “Global Mobile Traffic Forecast Update”, Cisco Visual Networking Index (VNI), Feb. 2013
[6] “A revolutionary new WiFi service across London’s unique transport system helps keep the city con-
nected”, Virgin Media Business—Transport for London, Case study
[7] “FON and KT Team Up With a Global WiFi Agreement”, Media Release, FON, May 2014
[8] 3GPP TS 24.312 v 11.6.0 — Access Network Discovery and Selection Function (ANDSF) Management Ob-
ject (MO)
[9] 3GPP TS 23.261 v 11.0.0 — IP flow mobility and seamless Wireless Local Area Network (WLAN) offload.
[10] 3GPP TR 23.834 v 10.0.0—Study on General Packet Radio Service (GPRS) Tunnelling Protocol (GTP)
based S2b; Stage 2
[11] 3GPP TR 23.852 v 2.0.0 - Study on S2a Mobility based on GPRS Tunnelling Protocol (GTP) and Wireless
Local Area Network (WLAN) access to the Enhanced Packet Core (EPC) network (SaMOG)
[12] Interworking Wi-Fi and Mobile Networks, The choice of mobility solutions, Ruckus, White Paper
[13] HS2.0: Brining Cellular-like Roaming to Wi-Fi Hotspots, John Lombardi, Ruckus
[14] 3GPP TS 24.234 v 11.3.0 - 3GPP system to Wireless Local Area Network (WLAN) interworking; WLAN
User Equipment (WLAN UE) to network protocols
[15] 3GPP TS 23.402 v 11.8.0 - Architecture enhancements for non-3GPP accesses
[16] 3GPP TS 23.122 v 11.4.0 - Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle
mode
[17] 3GPP TS 23.890 v 12.0.0 - Optimized offloading to Wireless Local Area Network (WLAN) in 3GPP Radio
Access Technology (RAT) mobility
[18] 3GPP TR 37.834 v 12.0.0 - Study on Wireless Local Area Network (WLAN) - 3GPP radio interworking
[19] Architecture for Mobile Data Offload over Wi-Fi Access Networks, Cisco, White Paper
[20] WLAN Traffic Offload in LTE, Rohde & Schwarz, White Paper, Feb. 2012
[21] Mobile Traffic Offload, NEC’s cloud centric approach to future mobile networks, NEC, White Paper, 2013
[22] Wi—Fi & Packet Core Integration, Kwangwon Kim, Ericsson-LG, 2012
[23] Cellular—Wi-Fi Integration, A comprehensive analysis of the technology and standardization roadmap,
InterDigital, White Paper, June 2012
[24] Wi-Fi Consulting Services, Independent Wi-Fi Offload Network, Maravedis-Rethink, Webinar, Oct. 2013
[25] Wi-Fi Packet Core Integration, Sergei Gotchev, Djorgje Vulovic, Cisco, Mar. 2012
[26] Wi-Fi roaming—building on ANDSF and HS2.0, Alcatel-Lucent & BT, White Paper, 2012
[27] S. Rayment & J. Begstrom, “Achieving carrier-grade Wi-Fi in the 3GPP world”, Ericsson Review, 284 23-
3183
[28] An adaptive multimedia-oriented handoff scheme for IEEE 802.11 WLANs, Ahmed Riadh Rebai & Said
Hanafi, International Journal of Wireless & Mobile Networks (IJWMN) Vol. 3, No. 1, Feb. 2011
[29] Integration of Cellular and Wi-Fi Networks, 4G Americas, White Paper, Sept. 2013
[30] Understanding the Role of Managed Public Wi-Fi, in Today’s Smartphone User Experience, Informa, Mo-
bidia, White Paper, 2013

12. List of Abbreviations
AAA Authentication, Authorization and Accounting
AKA Authentication and Key Agreement
ANDSF Access Network Discovery and Selection Function
AP Access Point
EAP Extensible Authentication Protocol
eNB evolved Node B
EPC Evolved Packet Core
ePDG evolved Packet Data Gateway
eSaMOG Enhanced S2a Mobility over GTP
EUTRAN Evolved UMTS Terrestrial Radio Access Network
IFOM IP Flow Mobility
IPv4 Internet Protocol v4
IPv6 Internet Protocol v6
ISM Industrial Scientific & Medical
LIPA Local IP Access
LMA Local Mobility Anchor
LTE Long Term Evolution
MAG Mobile Access Gateway
MAPCON Multiple Access PDN Gateway
MC-GW Mobility Controller Gateway
MIMO Multiple-Input Mobile-Output
MIP Mobile IP
MME Mobility Management Entity
OFDMA Orthogonal Frequency Division Multiple Access
PCRF Policy and Charging Rules Function
PDN Packet Data Network
P-GW PDN Gateway
PLMN Public Land Mobile Network
QoE Quality of Experience
QoS Quality of Service
RADIUS Remote Authentication Dial In User Service
RAT Radio Access Technology
SaMOG S2a-based Mobility over GTP
S-GW Serving Gateway
SIPTO Selective IP Traffic Offload
SMOG S2b-based Mobility over GTP
UE User Equipment
UMTS Universal Mobile Telecommunications System

UTRAN UMTS Terrestrial Radio Access Network
Wi-Fi Wireless Fidelity
WLAN Wireless Local Area Network
WISP Wireless Internet Service Provider

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