6
Wi-Fi Integration – Network Discovery and Selection
6.1. Mechanisms defined by 3GPP organization
6.1.1. ANDSF function
The selection of the access network and the management of the traffic between LTE (Long-Term Evolution) access and Wi-Fi (Wireless Fidelity) access are supported by the ANDSF (Access Network Discovery and Selection Function) server.
The configuration data are provided by the ANDSF server and are organized in a hierarchical structure called the device management tree. The exchange of the management object (MO) between the ANDSF server and the mobile is based on extensible markup language (XML).
The mobile can reach the ANDSF server via Wi-Fi access to the Internet or via Wi-Fi access or LTE access to the 4G mobile network.
The ANDSF server can push the information to the mobile (push mode) or the mobile can interrogate the ANDSF server and receive the corresponding information (pull mode). If the mobile submits a request, then it may also include other information in its request, such as its location and the list of discovered radio access networks.
The mobile can discover the ANDSF server in one of the following configurations:
- – static configuration;
- – DNS (Domain Name System) resolution for which a specific full qualified domain name (FQDN) is used
andsf.mnc <MNC> .mcc <MCC> .pub.3gppnetwork.org
- – dynamic configuration by a DHCP (Dynamic Host Configuration Protocol) server.
The ANDSF server determines the access on which the mobile must transfer the IP (Internet Protocol) flow in the following cases:
- – the mobile is able to route IP packets via a single type of access, LTE or Wi-Fi;
- – the mobile is able to route different IP packets for the same PDN (Packet Data Network) connection via different access networks;
- – the mobile is able to route IP packets for different PDN connections via different access networks.
The information provided by the ANDSF server may also be preconfigured by the home operator on the terminal or provisioned on the universal integrated circuit card (UICC).
6.1.1.1. ANDI
Following a mobile request, the ANDSF server can provide access network discovery information (ANDI) in the vicinity of the mobile (Figure 6.1):
- – the types of access technologies, such as the Wi-Fi interface;
- – the identifier of the access network, such as the service set identifier (SSID);
- – specific information on the characteristics of the radio interface, such as the frequency of the radio channel;
- – the conditions indicating when the ANDI is valid.
6.1.1.2. ISMP
The inter-system mobility policy (ISMP) provides the mobile with the rules for routing IP packets over LTE or Wi-Fi interfaces. The mobile uses these rules when it cannot access both interfaces simultaneously.
Figure 6.2 describes the structure of the management objects (MO) for the ISMP.
Each ISMP rule includes the following information:
- – the validity of the rule. These conditions may include, for example, a duration or a location area;
- – a priority list of access technologies to inform the mobile of the order in which they are preferred or restricted access for the connectivity to the evolved packet core (EPC);
- – a rule priority to inform the mobile of the level of priority of this rule compared to other ISMP rules provided by the same 4G mobile network.
Figure 6.1. ANDI
Figure 6.2. ISMP
6.1.1.3. ISRP
The inter-system routing policy (ISRP) is a set of operator-defined rules that determine how the mobile should route traffic across multiple radio access interfaces.
The IFOM (IP Flow Mobility) rules identify a prioritized list of radio access technologies that should be chosen by the mobile to route the different IP packets of a PDN connection that corresponds to an access point name (APN).
As the PDN connection is anchored in the EPC network, seamless continuity for each IP flow is provided between LTE and Wi-Fi accesses.
Figure 6.3 describes the structure of managed objects (MO) for IFOM rules.
An IFOM rule can also identify which radio accesses are restricted for the traffic, for example, IP packets containing voice must not use Wi-Fi interface.
Each IFOM rule can identify the traffic based on the IP address of the source or destination, the transport protocol, the port numbers of the source or destination, or the DSCP (DiffServ Code Point) or TC (Traffic Class) field.
Figure 6.3. IFOM rules
The MAPCON (Multi-Access PDN Connectivity) rules identify a prioritized list of radio access technologies that should be used by the mobile to route each PDN connection that corresponds to an access point name (APN).
As the PDN connection is anchored in the EPC network, seamless continuity for each PDN connection is provided between LTE and Wi-Fi accesses.
Figure 6.4 describes the structure of managed objects (MO) for MAPCON rules.
A MAPCON rule can also identify which radio access is restricted for PDN connections; for example, Wi-Fi interface must not be used for certain types of access points (APN).
Figure 6.4. MAPCON rules
The NSWO (Non-Seamless WLAN Offload) rules identify which IP packets should be offloaded by Wi-Fi access to the Internet network without crossing the EPC network.
Because streams are not anchored in the EPC network, seamless continuity for each IP stream is not assured between LTE and Wi-Fi accesses.
It is possible to restrict or allow the offloading of traffic to specific Wi-Fi access networks.
Figure 6.5 describes the structure of managed objects (MO) for NSWO rules.
Figure 6.5. NSWO rules
6.1.1.4. IARP
The IARP (Inter-APN Routing Policy) rules determine which traffic should be routed across different PDN connections and which traffic should be offloaded by Wi-Fi access to the Internet network (Figure 6.6).
The rules for the access point (APN) identify a prioritized list of access point names that should be used by the mobile to route traffic that matches IP traffic filters.
The rules for NSWO identify which traffic should be offloaded for Wi-Fi access to the Internet network.
When the mobile has both the IARP rule and the ISRP rule simultaneously, it first evaluates the IARP rules to determine how to route an IP stream. If the IP stream does not match any IARP rules, the mobile evaluates the active ISRP rules to determine how to route the IP stream.
Figure 6.6. IARP rules
6.1.1.5. WLANSP
The WLANSP is a set of rules that determine how the mobile selects a Wi-Fi access network.
Figure 6.7 describes the structure of the managed objects (MO) for the WLANSP.
Figure 6.7. WLANSP
Each WLANSP rule includes the following information:
- – the conditions indicating when the rule is valid. The conditions of validity can include the time, geolocation and location of the network, such as the location area;
- – the selection criteria that must be fulfilled by the Wi-Fi access network to be eligible, such as cell load or transmission network throughput.
6.1.1.6. Wi-Fi access network preferences
Network preferences include information that helps the mobile to select a Wi-Fi access network.
The network preferences indicate whether the network prefers the mobile to establish a PDN connection using the S2a architecture or not.
In the case of a PDN connection using the S2b architecture, the network preferences indicate the identity of the evolved packet data gateway (ePDG).
The EHSP (Equivalent Home Service Providers) information contains a list of service providers that are equivalent to the mobile home network. Each service provider is identified with a domain name.
The PSPL (Preferred Service Provider List) information contains a preferred list of service providers.
6.1.2. RAN assistance
The evolved node base station (eNB) can provide mobile support information. This information includes the following parameters:
- – the thresholds for access to the LTE interface;
- – the thresholds for the Wi-Fi interface;
- – the offload preference indication (OPI).
The thresholds for the LTE interface define the high and low values of the radio parameters, for example the average value of the reference signal received power (RSRP).
The thresholds for the Wi-Fi interface define the high and low values of the access parameters, such as the received signal strength indication (RSSI) of the beacon, the transmission network throughput and the load of the radio channel.
The OPI is a one-dimensional bitmap that can be used by mobiles to determine when they should move certain traffic to Wi-Fi access or LTE access. The meaning of each bit is operator specific.
The thresholds and parameters can affect the validity of ANDSF rules and thus make these rules subject to the conditions defined by the eNB entity.
The thresholds and parameters can be used by the following ANDSF rules:
- – the ISRP rules, including IFOM rules, MAPCON rules and NSWO rules;
- – the IARP rules, including rules related to the access point name (APN) and NSWO.
The selection of the Wi-Fi interface and the routing behavior for the mobile must be controlled either by the ANDSF rules or by the rules provided by the eNB entity, and not by a combination thereof.
The only exception is the simultaneous enforcement of the rules provided by the eNB entity and the IARP rules for the APN.
6.2. Mechanisms defined by IEEE and WFA organizations
Before associating with an access point, the mobile requires information on the services provided by the Wi-Fi access networks, from GAS (Generic Advertisement Service) frames that are Action-type management frames.
The Public Action field, in the byte immediately after the Category field, differentiates the types of Action frames.
GAS frames provide transparent transport of a list with ANQP (Access Network Query Protocol) elements to communicate information.
The Interworking element in the management frames Beacon or Probe Response indicates that the GAS protocol is supported (Figure 6.9).
The Advertisement Protocol element in the management frames Beacon or Probe Response indicates that the ANQP protocol is supported (Figure 6.9).
The mobile transmits a request in a GAS Initial Request frame and the access point provides the requested information or information on how to receive the response in the GAS Initial Response frame (Figure 6.9).
The response to the GAS Initial Request frame is provided in this case in one or more GAS Comeback Response frames.
Figure 6.9. GAS/ANQP exchanges
The response to the GAS Initial Request frame shall not be shared between an Initial Response GAS frame and one or more Comeback Response GAS frames.
The access point has the information elements or transfers the request from the mobile to an ANQP server.
The IEEE (Institute of Electrical and Electronics Engineers) has defined a subset of ANQP information elements (Table 6.1).
The WFA (Wi-Fi Alliance) has completed this list as part of the Passpoint certification based on Hotspot 2.0 features (Table 6.1).
Table 6.1. ANQP information elements
ANQP information elements |
Specification |
Function |
3GPP Cellular Network |
IEEE |
Identification and authentication methods from the service provider |
NAI Realm |
IEEE | |
Roaming Consortium |
IEEE | |
Domain Name |
IEEE |
Identification of the Wi-Fi access network |
Venue Name |
IEEE | |
Operator’s Friendly Name |
WFA | |
IP Address Type Availability |
IEEE | |
WAN Metrics |
WFA |
Characteristics of the Wi-Fi access network |
Connection Capability |
WFA | |
Operating Class Indication |
WFA | |
Network Authentication Type |
IEEE | |
OSU Providers List |
WFA |
Online registration |
Icon Request & Response |
WFA | |
HS Query List |
WFA |
Capacity request |
HS Capability List |
WFA | |
NAI Home Realm Query |
WFA |
6.2.1. Information elements provided by the beacon
6.2.1.1. HESSID element
If two access points have different SSID, they are considered as different Wi-Fi networks. If two access points have the same SSID, they are considered as part of the same wireless network.
However, since SSID are not globally administered, it is possible that two access points with the same SSID concern different Wi-Fi networks.
The homogeneous extended service set identifier (HESSID) allows mobiles to detect this condition. When two access points of two different Wi-Fi networks have the same SSID, the two networks are differentiated by two different HESSID.
The HESSID is included in the Interworking element in Beacon or Probe Response frames.
The HESSID is a MAC (Medium Access Control) address. The HESSID value has the same value as the basic service set identifier (BSSID) of one of the access points.
6.2.1.2. Access Network Type field
The Access Network Type field is included in the Interworking element. Mobiles can use this information when selecting an access point.
The Access Network Type field indicates the type of network to which the access point is connected: pay public network, free public network, private network and private network with guest access.
6.2.1.3. Internet Available field
The Internet Available field is included in the Interworking element. This field informs mobiles as to whether access to the Internet is available at the access point, which may not be the case in environments where the operator (e.g. a museum) may limit Wi-Fi access to only local content.
6.2.1.4. BSS Load element
The BSS Load information element contains information on the use of the radio channels and the number of associated mobiles on the access point. The mobile uses this information when selecting a network.
6.2.2. Information elements provided by the ANQP server
6.2.2.1. 3GPP Cellular Network element
The information element 3GPP Cellular Network contains the identity of the 4G mobile network. It allows the mobile to check from its universal subscriber identity module (USIM) if the Wi-Fi network operator has a roaming agreement with the 4G mobile network operator.
The identity of the 4G mobile network consists of the mobile country code (MCC) and the mobile network code (MNC) allocated to the operator.
If the information element 3GPP Cellular Network matches any identity stored in the mobile, then it prioritizes this access point for the association.
6.2.2.2. NAI Realm element
The information element NAI Realm provides a list of domains identified by the network access identifier (NAI) for service providers that can authenticate a mobile with either a user ID or a password or a certificate.
Each entry in the NAI Realm list can identify the EAP (Extensible Authentication Protocol) methods that are supported for authentication.
6.2.2.3. Roaming Consortium element
The information element Roaming Consortium provides a list of roaming consortium identifiers and service provider partners with roaming agreements.
The information element Roaming Consortium is broadcasted in the management frame Beacon or transmitted in the Probe Response frame. A mobile may request an information element Roaming Consortium if the information received is insufficient for the selection of the network.
6.2.2.4. Domain Name element
The information element Domain Name provides a list of one or more domain names of the entity that operates the Wi-Fi network.
The mobile uses the domain name to determine whether access to this Wi-Fi network through this access point is considered access to its home network or to a visited network.
6.2.2.5. Venue Name element
The information element Venue Name provides venue names that can be used to help the mobile to select the access point. The names of the venue can be included in the same language or in different languages.
6.2.2.6. Operator’s Friendly Name element
The information element Operator’s Friendly Name provides the friendly name of the Wi-Fi network operator.
The mobile can obtain the name of the operator via GAS/ANQP requests to help the user when manually selecting access points.
6.2.2.7. IP Address Type Availability element
The information element IP Address Type Availability provides information about IP addresses and port numbers:
- – Wi-Fi access point allocates a public IPv4 address;
- – Wi-Fi access point allocates a private IPv4 address;
- – the combination of the Wi-Fi access network and the core network allocates a dual NAT IPv4 address;
- – Wi-Fi access point allocates an IPv6 address.
6.2.2.8. WAN Metrics element
The information element WAN Metrics provides information about the link that connects the access point to the Internet network: the state of the link, and the value of the bit rates for each direction of transmission.
The access point may also provide additional information, such as the load for each direction of transmission.
The mobile uses this information to make network selection decisions. The mobile determines whether the available rate level is compatible with the application need.
6.2.2.9. Connection Capability element
The information element Connection Capability provides information about the allowed values of the Protocol field of the IPv4 header or Next Header field of the IPv6 header and port numbers.
The mobile uses this information to make network selection decisions. The mobile determines whether the allowed values are compatible with the characteristics of the application.
6.2.2.10. Operating Class Indication element
The information element Operating Class Indication provides information about the radio channels and frequency bands used by the access point.
The mobile uses this information to make network selection decisions. If the mobile supports the 2.4 and 5 GHz frequency bands, and if these two frequency bands are available at the access point, then the mobile will select the 5 GHz band.
6.2.2.11. Network Authentication Type element
The information element Network Authentication Type provides a list of authentication types:
- – the network requires the user to accept the terms and conditions;
- – the network supports online registration;
- – the network infrastructure performs HTTP/HTTPS redirection;
- – the network supports a DNS redirection.
6.2.2.12. OSU Providers List element
The information element OSU Providers List contains a list of entities that offer an online registration service.
The information element OSU Providers List provides a list of available icons that can be displayed by the mobile. This list contains the definition of the image, the image type, the language and the name of the file. This information allows the mobile to determine the icon to download and the file name of the icon to recover.
6.2.2.13. Icon Request & Response element
The information element Icon Request & Response allows the mobile to send a request containing the file name of the icon and to return to the access point the answer containing the download status code, the length of the type of icon, the data length and binary icon data.
6.2.2.14. HS Query List element
The information element HS Query List is transmitted by the mobile to obtain information simultaneously on several elements of ANQP information.
The information element HS Query List is transmitted in a GAS Query Request frame.
6.2.2.15. HS Capability List element
The information element HS Capability List informs the mobile of which ANQP elements are supported by the access point.
The information element HS Capability List is transmitted in a GAS Query Response frame.
6.2.2.16. NAI Home Realm Query element
The information element NAI Home Realm Query enables the mobile to determine whether the domains for which it has security information correspond to the service providers whose networks are accessible at the access point.