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4.5G: Integration of LTE and Wi-Fi networks


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With LTE-A getting ready to meet the IMT-Advanced requirements and fulfilling the role of promised "4G", we believe the next phase of evolution before 5G will be successful interworking of LTE and Wi-Fi networks. This whitepaper explores this feature, we call 4.5G, in detail.

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4.5G: Integration of LTE and Wi-Fi networks

  1. 1. 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
  2. 2. 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
  3. 3. 3web: email: tel: +44 (0) 1582 635026 Explaining Technology 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.
  4. 4. 4web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  5. 5. 5web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  6. 6. 6web: email: tel: +44 (0) 1582 635026 Explaining Technology  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
  7. 7. 7web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  8. 8. 8web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  9. 9. 9web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  10. 10. 10web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  11. 11. 11web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  12. 12. 12web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  13. 13. 13web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  14. 14. 14web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  15. 15. 15web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  16. 16. 16web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  17. 17. 17web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  18. 18. 18web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  19. 19. 19web: email: tel: +44 (0) 1582 635026 Explaining Technology 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-
  20. 20. 20web: email: tel: +44 (0) 1582 635026 Explaining Technology 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-
  21. 21. 21web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  22. 22. 22web: email: tel: +44 (0) 1582 635026 Explaining Technology 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
  23. 23. 23web: email: tel: +44 (0) 1582 635026 Explaining Technology UTRAN UMTS Terrestrial Radio Access Network Wi-Fi Wireless Fidelity WLAN Wireless Local Area Network WISP Wireless Internet Service Provider
  24. 24. Complete WLAN Offload test As data consumption continues to explode, innovative solutions are required to avoid a looming capacity crunch. Network operators are lookingtosmallcelltechnologiesforasolution,offloadingdatatoWi-Fi enabled devices is one cost effective means. The MD8475A is designed to provide multi-standard testing in just one unit. LTE-A (FDD/TDD), WCDMA/HSPA, GSM/EGPRS and 2 cell interRAT/intraRAT,highlevelapplicationsandservices(IMS/VoLTE,RCS, web), and turn-key solutions (Wi-Fi, battery test, video & video quality) are integrated into the MD8475A. The SmartStudio GUI provides an easy to use interface hiding perfectly the complexity and power of the testing available. The MD8475A is the ideal solution for WLAN Offload testing, where a combination of technologies is required. Hosting the mobile network architecture on the MD8475A and a WLAN Offload server application on an external PC the whole system configuration is achieved. The MD8475A governs the functionality, allowing you to focus on the results. WiFiTM Offload Extending the cellular network capacity Understanding WLAN Offload - Download your FREE Guide, learn more. Sales Offices: Europe 44 (0) 1582-433433, USA and Canada 1-800-ANRITSU, Japan 81 (46) 296-1208, Asia-Pacific (852) 2301-4980, ©2014 Anritsu Company Since 1895