Integrating Wi-Fi RANs into the Mobile Packet Core

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Integrating Wi-Fi
RANs into the
Mobile Packet Core
There has been a great deal of interest of late in using Wi-Fi to offload traffic from
heavily congested mobile networks. Early deployments consisted of building a parallel
Wi-Fi offload network that takes traffic directly to the Internet. The mobile network
operator would implement a proprietary client of some kind that would manage the
offload function. Many subscribers have implemented their own offload strategy by
selecting Wi-Fi when it’s available.
Now the industry is shifting its focus toward integrating Wi-Fi RANs into the mobile
packet core. In this approach, Wi-Fi would take its place alongside 3G/LTE as a
cornerstone technology in the mobile world. The mobile device selects the best radio
access technology based on the conditions (typically signal strength, application type,
default to Wi-Fi, etc.) and the subscriber is automatically authenticated and connected.
This is a manual process today, but the industry is moving rapidly toward automating
all this under a combination of operator and subscriber control. All RAN traffic is
brought back into the mobile packet core, and from there it goes to the mobile
operator’s service complex, the Internet, or a corporate intranet. To make this HetNet
vision a reality, the experience of connecting to Wi-Fi must be made as simple and
secure as when connecting to cellular. The services must also be the same, and
it should even be possible to seamlessly handoff between Wi-Fi and cellular RAN
technologies. This vision requires that operators maintain the same level of control
regardless of the RAN type.
Integrating Wi-Fi RANs into
the Mobile Packet Core
The services that are available in the mobile packet core include:
• Pre-paid and post paid billing (with zero rating)
FIGURE 1: Untrusted Wireless LAN Access Using TTG, PDG, or
ePDG Functionality
Mobile Core
• Lawful intercept
• Deep packet inspection and analytics
• Content filtering (including parental controls)
• IP address assignment (to support seamless inter-RAT
• Roaming1
• DNS, NAT, Firewall, etc.
• HLR/HSS Subscriber Database
• IP Multimedia Subsystem (IMS)
• Mobility management
• Voice over IMS
• Etc.
3GPP (the 3rd generation partnership project) has developed two
different approaches to integrating Wi-Fi into the mobile packet
core. The first assumes untrusted WLAN access and the second
assumes trusted WLAN access. The work on untrusted WLAN
access first appeared as part of the I-WLAN effort in 3GPP
Release 6, which defined a TTG (tunnel termination gateway)
and a PDG (packet data gateway) to provide the interworking
function. This work was extended in 3GPP Release 8 with the
introduction of the ePDG (evolved packet data gateway) for LTE.
Beginning in Release 11 (TS 23.402 V11.3.0 (2012-06), 3GPP has
introduced a new architecture for trusted WLAN access based
on the work of the S2a Mobility based on GTP (SaMOG)2 working
group. This paper will examine both approaches in more detail.
Untrusted WLAN access
The assumption in untrusted WLAN access is that the mobile
operator need not know anything about the Wi-Fi network that
originated the connection. That operator could be a hotel, airport,
cable operator, aggregator, etc. The mobile operator would have
to have a roaming arrangement with that entity, but didn’t need
to know much else. In most early Wi-Fi deployments the security
was modest to non-existent, so this was a valid approach.
3GPP’s solution was to have the mobile device set up an IPsec
1 In this document we will use the mobile definition of roaming which refers to the ability to use your
mobile device on the network of an operator for which you do not have a business arrangement,
versus the Wi-Fi definition which means to be handed off from one AP to another.
2 The SaMOG work is described in 3GPP TR (technical report) 23.852 V1.1.0 (2012-05). That work
was then incorporated into TS (technical specification) 23.402 V11.3.0 (12-06). SaMOG can also
support mobility over PMIP.
page 2
session using IKEv2 for authentication and tunnel over the Wi-Fi
access network. These tunnels would terminate on a massively
scalable IPsec concentrator back in the mobile operator’s data
center. This concentrator function is integrated into the TTG part
of a PDG (a PDG consists of a TTG and selected elements of a
GGSN). When connecting back into an existing GGSN, only the
TTG function is utilized. The TTG/PDG constructs are specific to
3G, and are replaced with an ePDG when moving to LTE. This is
sometimes called an overlay model and it requires nothing more
from the access layer than a simple bit pipe. It does, however,
put a significant burden on the mobile device and the mobile
packet core.
That burden consists of a requirement that mobile device
vendors implement an IPsec/IKEv2 client on their smartphones,
and the mobile packet core vendors had to develop TTG/PDG/
ePDG technology. While the latter has happened, the former
hasn’t. Mobile device vendors have been unwilling to develop the
necessary client software, which has prevented this approach
from gaining traction. The reason for this reluctance is that
IPsec/IKEv2 is a complicated and processor intensive protocol
that loads down the mobile device. It is also makes for an overly
cumbersome connection process as the user must first authenticate to the Wi-Fi AP (using whatever protocol is required by the
AP) and then get an IP address which is used to setup an IPsec
tunnel back to the mobile packet core where it must authenticate
all over again to get yet another IP address for the actual session.
Tunneling also makes the access layer invisible, which removes
the opportunity to apply policy, quality of service, perform local
breakout, etc. There have been variations on this theme using
Mobile IP, but they all suffer from similar problems with regards to
mobile device support and complexity. The net result is that the
Integrating Wi-Fi RANs into
the Mobile Packet Core
FIGURE 2: Trusted Wireless LAN Access Using TWAG Functionality
FIGURE 3: Trusted Wireless LAN Access While Roaming
Mobile Core
Visited Network
Subscriber’s Home Network
industry has continued to look for a better solution.
Trusted WLAN access
A better solution came by way of the S2a Mobility based on
GTP (SaMOG) working group in 3GPP. Their approach has been
introduced in 3GPP Release 11 for LTE RANs, and it can easily
be extended to support 3G RANs. The focus of the SaMOG
effort was to eliminate the requirement for an IPsec/IKEv2
client on the mobile device. To do this, the WLAN needed to be
trusted3 and 802.1x was well suited to the job. This protocol has
gotten plenty of support from the mobile device and Wi-Fi AP
communities. It addresses all mobile industry concerns around
ease of use and security by utilizing EAP (extensible authentication protocol) for secure authentication and 802.11i for Wi-Fi
airlink encryption. This combination provides everything that
IPsec/IKEv2 did, but with far less complexity. EAP supports a
variety of different authentication methods including EAP-SIM,
an enhancement to EAP-AKA, and is the preferred approach by
most mobile operators. This combination of EAP authentication
and 802.11i airlink encryption are also key building blocks in the
Hotspot 2.0 initiative.
• EAP-SIM is used to authenticate SIM-based devices over
Wi-Fi, and EAP-AKA does the same for USIM-based devices.
Being able to use a SIM (2G) or a USIM4 (3G/LTE) card to
authenticate regardless of the RAN type makes for a seamless
authentication experience.
• EAP-TLS is used with X.509 certificates and EAP-TTLS with
username and password. The former is a very good option for
laptops, digital cameras, and tablets as it enables a seamless
3 The decision on whether a WLAN network is trusted or untrusted is made by the mobile operator.
The typical requirement for “trust” is that the subscriber be securely authenticated and all data
communications over the airlink be encrypted.
4 For the remainder of this document we will simply refer to SIM cards for simplicity.
page 3
connection experience without the need for SIM cards, and the
latter will always be a good option for serving drop-in customers.
The key piece of network equipment that is required for trusted
access is the Trusted WLAN Access Gateway (TWAG). On the
RAN side it connects to tens of thousands or even hundreds of
thousands of Wi-Fi APs that can optionally be encrypted. On the
mobile packet core side it connects to a PDN Gateway via the
S2a interface using GTPv2 (GPRS tunneling protocol) or a GGSN
using the Gn interface and GTPv1. Proxy Mobile IP (PMIP) can
also be used in place of GTP for CDMA-based operators moving
to LTE.
The GTP tunnel is used to activate a session between the TWAG
and the GGSN/P-GW as part of the connection setup process.
This involves the creation of a data structure in the TWAG and in
the anchoring GGSN/P-GW. These data structures include the
subscriber’s IP address, subscriber’s IMSI, subscriber’s tunnel
endpoint ID (TEID) at the GGSN/P-GW, tunnel endpoint ID (TEID)
at the TWAG, and much more. If the subscriber is handed off to
another TWAG or an S-GW during a mobility event, the session
must follow.
One final interface of note is the STa, which relays authentication
credentials between the TWAG and the 3GPP AAA Server.
These credentials are then passed on to the HLR/HSS for
final processing.
This approach is called trusted WLAN access, because it
requires that the Wi-Fi operator implement 802.1x5 along with
802.11i. It is expected that mobile operators will implement these
protocols on their APs and will only roam with Wi-Fi partners that
also use these protocols. For a partner to roam with a mobile
operator they will need to terminate their APs on their own TWAG
5 802.1x makes use of a supplicant that runs on the mobile device, an authenticator that runs on
the TWAG, and an authentication server which runs on the HLR/HSS.
Integrating Wi-Fi RANs into
the Mobile Packet Core
and then tunnel back to the mobile operator’s GGSN/P-GW
(see Figure 3) through a global roaming exchange using GTP.
This assumes that the subscriber’s data is always tunneled
back to the home network, which is the standard in the mobile
world. 3GPP is looking at options that will support local breakout
when roaming. In this approach mobile data traffic is anchored
to a GGSN/P-GW in the visited network, instead of the normal
practice of bringing it all the way back to the home network.
Local breakout can also be implemented in the home network
by using the mobile infrastructure to authenticate the subscriber,
but then offloading their traffic directly to the Internet. Since
traffic is not being brought back into the mobile packet core,
the set of services that could be offered to the subscriber will
be very limited.
In our roaming scenario the subscriber will need to authenticate
through a local AAA server in the visited network which will
proxy it back to a AAA server in the home network, which then
connects to the HLR/HSS subscriber database.
Roaming is one of the interesting ways in which trusted WLAN
access differs from untrusted WLAN access. In the latter the
TTG/PDG/ePDG is always in the home network regardless of
where the subscriber might be, and all traffic is tunneled back
over IPsec. With trusted WLAN access, the TWAG is always in
the visited network and traffic is tunneled back over GTP to the
GGSN/P-GW in the home network (or broken out locally). This is
also how cellular networks operate when the user is roaming (the
SGSN or Serving Gateway is always in the visited network).
Trusted WLAN Access & Non-SIM Devices
There is another kind of breakout that also needs to be
supported by the TWAG gateway. Wi-Fi networks are used by
hundreds of millions of laptops and tablets that do not have
cellular modems and therefore do not have SIM cards, but still
have a great need for Internet access. Mobile operators can
address this need with EAP-TLS and x509 certificates. This
approach gives the same seamless authentication procedure
that EAP-SIM does, but no SIM is required. However, since there
is no SIM the traffic can’t be brought back to the core. Instead,
it needs to be offloaded to the Internet at the Trusted WLAN
Access Gateway (TWAG). The supporting of non-SIM devices
will require additional capabilities at the TWAG including billing,
policy, and lawful intercept support to name a few (functions that
are normally provided in the mobile packet core). The decision on
how to route the user is made at the time of authentication. By
adding this offload capability, mobile operators can now target
the hundreds of millions of non-cellular equipped Wi-Fi devices
that need seamless authentication, security, and a really good
global roaming solution. Support for non-SIM devices is probably
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best handled by distributing the TWAG function out close to the
Wi-Fi RAN, as there is no need to backhaul this traffic to the
mobile packet core.
Hotspot 2.0
A key piece of the vision for integrating Wi-Fi into the mobile
packet core involves automating the process of discovering and
selecting an AP when roaming. This is being addressed by the
Hotspot 2.0 program, which is being driven by the Wi-Fi Alliance
and the Wireless Broadband Alliance. Phase 1 of this standard
has already been approved and it will soon be available on
mobile devices. Phase 2 will address non-SIM devices and the
downloading of operator policy into mobile devices. This work is
also being augmented by the ANDSF effort in 3GPP. The Access
Network Discover and Selection Function (ANDSF) will allow the
network to notify the mobile device about APs in its proximity.
Getting Connected
This section look at the process of getting connected when using
trusted WLAN access.
The mobile device starts by automatically scanning all available
SSIDs and when is sees one it recognizes it will begin the
authentication process. If the mobile device does not see an
SSID it recognizes, it can use Hotspot 2.0 technology to quickly
find a roaming partner. Devices with SIM cards will be authenticated with EAP-SIM. For all non-SIM devices authentication can
be via EAP-TTLS or EAP-TLS6. The mobile operator’s AAA server
will proxy the EAP-SIM authentication request back to the HLR/
HSS. Once authentication is completed and a session has been
established, the user can connect and begin transmitting. Mobile
devices with SIM cards will have their traffic backhauled to the
Internet by way of the mobile packet core and devices that don’t
have SIM cards will be offloaded to the Internet at the TWAG (or
even at the AP). Figure 4 shows the sequence of steps required
to establish on connection when using trusted WLAN access
and a SIM device. The description that follows is greatly simplified
for purposes of this document.
1.Wi-Fi access is initiated
2.E AP Authentication takes places with the AAA/HSS back
in the mobile core.
3.After successful authentication and authorization, the Wi-Fi
specific L3 attach procedure is triggered.
4.The TWAG sends a Create Session Request message to
the P-GW. This includes all parameters in the PDP context.
6 Portal based solutions using WISPr are also an option, but they require user intervention and are
not secure.
Integrating Wi-Fi RANs into
the Mobile Packet Core
FIGURE 4: Connected Sequence for Trusted Wireless Access
Non-3GPP Access
Attach Trigger
Create Session Request
CCR (Initial)
CCA (Initial)
Create Session Response
Update PDN GW Address
Attach Completed
Non-3GPP Access
GTP Tunnel
The P-GW initiates the IP CAN Session Establishment
Procedure with the PCRF (policy and charging rules function).
6.The P-GW informs the AAA Server of its P-GW identity
and the APN corresponding to the UE’s PDN connection.
The message includes information that identifies the PLMN
(public land mobile network) in which the P-GW is located.
This information is registered in the HSS.
7.The P-GW then sends a Create Session Response
message to the TWAG, including the IP address allocated
for the UE.
Authentication and Authorization
EAP Authentication
Non-3GPP Access
8.L3 attach procedure is completed and the IP address
information is provided to the UE.
The sequence will be very different in a roaming application
where traffic is backhauled to the home network, and in roaming
applications that use local breakout. The use of PMIP instead of
GTP also changes things.
Next Steps for Mobile Operators
The industry is moving forward with trusted WLAN access based
on the SaMOG model, which has been incorporated into 3GPP
Release 11. TWAG gateways will start to emerge later this year
and they will be able to interwork with mobile devices that are
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SGI IP Access
already shipping. This is a compelling solution that addresses
all of the problems with the I-WLAN approach including the
unwillingness of mobile device vendors to develop IPsec/IKEv2
clients. With trusted WLAN access, mobile operators have an
architecture that makes Wi-Fi access as simple and secure as
cellular access. It also uses protocols that are already broadly
deployed in the industry. Mobile operators can greatly increase
their footprint in a very cost effective manner by using Wi-Fi to
complement their 3G/LTE build-outs.
• The experience of getting connected will be the same.
• The set of mobile operator services will be the same, it will even
be possible to enable seamless handoff as subscribers move
from 3G/LTE to Wi-Fi and back again.
Subscribers no longer need to know or care about RAN
technologies. Instead, they get an always best-connected
experience. Operators also have the opportunity to use their
Wi-Fi assets to generate revenue from the hundreds of millions
of Wi-Fi only laptops, tablets, and digital cameras that need connectivity while on the move. Mobile operators that successfully
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