Wireless Spectrum Needs Vs. Wi-Fi Offload Solutions

Cellular data networks operators are faced with the significant challenge of increasing data usage and flat ARPU. The emergence and proliferation of smartphones and mobile devices are taxing the capacity of cellular networks, given the limited spectrum holding of the carriers and the associated channel bandwidth. A 100% growth of smartphones is projected by 2017, bringing the number of devices to 2.4B. At the same time, ARPU is projected to have a gap of more than 25% by 2017, compared to $150 today for quad services.

Wireless bandwidth demand will grow from 500M bytes per month per user to 2G+ by 2017, and the peak data rate demand on the wireless network is expected to face a 100X gap from today’s 50Mbps.

To future proof the mobile networks of tomorrow, a transformational & heterogeneous network architecture is required by leveraging the best of macro cellular networks for mobility, and the best of wire-line network for bandwidth availability and efficiency, integrated through small cells (e.g., Wi-Fi offload on unlicensed and/or metro cells on licensed spectrum) in a seamless manner to preserve the mobile lifestyle for wireless data.

Network operators use small cells to extend their service coverage and/or increase network capacity. Carrier-class Wi-Fi is an attractive complement to 2G/3G/LTE small cells architecture, since it is preserving the spectrum, and the Wi-Fi chipset is already built into the smartphones. The target architecture (comprised of LTE, Wireline and Small Cells) allows the mobile devices to switch the IP traffic to use (e.g., Wi-Fi connection instead of the cellular data connection and vice versa). In this approach, the two networks are in practice totally separated, but fully integrated, and network selection is done in the heterogeneous network. Studies show that a significant amount of data can be offloaded in this manner to Wi-Fi networks even when users are mobile.

Heterogeneous network architecture allows mobile operators to move traffic from the cellular network, where the capacity constraints are most acute, to cheaper shorter range small cells network, connected over a variety of backhaul connections. The environment for mobile connectivity is therefore becoming a more complex mix of technologies in both the air interface (e.g., HSPA, LTE, 2.4/5GHz 802.11x), backhaul (e.g., Ethernet, DSL, FTTC, FTTP) and core, which is further complicated by the more complex inter-operator roaming agreements enabled by emerging technologies such as Hotspot2.0 and non-3GPP access for Evolved Packet Core (EPC), which provides security mechanisms such as IPsec tunneling of connections with the user device over an untrusted non-3GPP access.

The process for the offload must be transparent to the users. In that, the end device, such as a smartphone, must automatically be authenticated to access a known network, as defined by it’s Wi-Fi SSID. Next, the traffic must be redirected from the Access Point (AP) thru the wired network to internet, and not passed thru the cellular core network. The technology evolution starts with a “hard handover” for data connection at this time vs. dynamic offloading, and over time will evolve to a seamless “handover” back and forth between the wireless core and wire-line core.

The target architecture for a Wi-Fi offload should not require a client on the device side (i.e., the device OS should include support for functionality to allow the offload). It is important to adopt a solution based on handover with IP address consistency which will not require a client on the device side.

In addition, the offload should preserve the service continuity across radio access types, in that a network connection should not be dropped as a result of the offload. Full dynamic offloading would require the device to support Wi-Fi and xG transmission concurrently.

Business Drivers for Wi-Fi Offload

From the carrier perspective, carrier-Wi-Fi is a strategic carrier decision and not a tactical move to meet short term spikes in traffic. The key drivers for this strategy are:

a) Differentiated QoS, and Customer retention. Customer retention through higher speed user experience and better QoS. Most cellular operators own a fraction of spectrum (e.g., 20 MHz) vs the unlicensed spectrum availability for Wi-Fi in excess of 600 MHz.

b) Incremental ARPU thru tapping into non-SIM devices such as tablets and laptops.

c) Lower cost of delivering wireless internet connectivity.

d) Up to 10X improvement in capital unit cost for Wi-Fi offload vs. cellular alternative is expected. One key smartphone application requiring excellent Wi-Fi support is the streaming of HD video in high density areas which would otherwise overload the cellular network.

Business Model for Wi-Fi Offload

The business model for Wi-Fi-offload must include both SIM-enabled and non-SIM devices. The non-SIM devices include lap tops, tablet and other hand-held devices that present a significant monetization opportunity for the carriers.


  • “Best effort” for free, without consumption limit
  • Premium service for a fee (with consumption limit)
  • Wi-Fi for 2/3G subscribers without a 2/3G data plan


  • Managed carrier-class Wi-Fi service subscription
  • Temporary usage based: hourly, daily, etc.
  • Premium application specific streaming (e.g., video) service

3GPP And IEEE Standards for Wi-Fi Offload

Interconnection of the cellular technologies such as 3GPP LTE and Wi-Fi of IEEE 802.11n is not trivial. 3GPP LTE is full scale 7-layer protocol, while 802.11n is only defined for lower protocol layers, and the higher layers are left to the vendors. Standardization efforts have focused on specifying tightly or loose coupling between the cellular and the Wi-Fi networks.

  • 3GPP-based Enhanced Generic Access Network (EGAN) architecture applies tight coupling as it specifies rerouting of cellular network signaling through Wi-Fi access networks. This makes Wi-Fi a 3GPP RAN.
  • Loose coupling is based on direct connection to internet without having to interwork the two networks.

The 3GPP has defined two types of non-3GPP (i.e., Wi-Fi) access network types: “trusted—SIM or USIM-based authentication to take place over the Wi-Fi network” and “non-trusted” access.

3GPP release 7 (authentication using EAP-Protocols is the recommended feature for Wi-Fi offload). It defines EAP-SIM and EAP-AKA as the authentication protocols to be used with SIM-enabled devices such as smartphones.

3GPP Release 8 (non-trusted access architecture for LTE/Wi-Fi) has a key capability for preserving the IP address when offloading from LTE to Wi-Fi.

3GPP Release 10 has a key capability for seamless LTE-Wi-Fi handover with “IP Mobility” functionality that enables IP device move between APs while preserving its IP address.

Wi-Fi Offload Solutions

There are 3 major categories of Wi-Fi: 1) private Wi-Fi—user owned APs, 2) public Wi-Fi—such as hot spots at the airports and 3) carrier-class Wi-Fi—carrier APs — the subject of this proposal.

There are numerous solutions in the market, while there is not a final architecture of Wi-Fi offload.

The first category is already being exploited for the Wi-Fi offload. It is the second and the third categories that are open for exploitation. The second category presents opportunity for the carriers for partnership to provide the service, and the third category is open for the carrier to leverage with the goal to improve customer experience, to lower the capital unit cost, and to improve ARPU.

Wi-Fi AP ranges are typically 50-70 meters. A Wi-Fi small-cell network with inter-site distances of 50-70 meters and an available bandwidth of some 600MHz will meet the wireless data demands into future. Here is the high level framework for the solution:

a) Wi-Fi APs supports 802.1x. This protocol encapsulates messages for delivery.

b) The authentication process, for SIM devices, enables the smartphone to use EAP-SIM protocol to authenticate with the HLR of the mobile provider without user interaction and without the need for a client or app in the device. The signaling path will connect through Wi-Fi AP/Access GW via EAP-SIM, to Mobile Operator Service Management platform as well as the SIM Authentication Server via EAP-SIM over RADIUS, to Mobile Network HLR through SS7/MAP.

c) The authentication process for non-SIM devices provides the service through a Service Management platform that is capable of handling both SIM and non-SIM devices. In this case the methods for authentication will use EAP-TLS and EAP-TTLS to accomplish that.

d) In 2G and 3G mobile broadband, the radio access network connects to an SGSN network node before entering the mobile core GGSN. A Wi-Fi network will emulate this architecture by making Wi-Fi an integrated sub-network of the mobile core. As in the case above non-SIM Wi-Fi traffic breaks out locally, while the EAP-SIM-authenticated Wi-Fi traffic is tunneled to the wireless carrier’s GGSN using a Wireless Access Gateway (WAG) emulating an SGSN. This is a desirable approach, as carriers are attracted to this option because it uses 3GPP specifications for interworking with Wi-Fi including a 3GPP-compliant AAA (authentication) platform as a part of the service management platform or as a stand-alone server. This method also uses policy control functions already configured in the mobile core so that ideally less system integration is required.

e) Network discovery and selection automates access to Wi-Fi networks not defined in the SSID list stored on the device.

f) Wi-Fi-offload network will Interwork with 3G/4G core network. This way the traffic will be routed to a Traffic Control Node (TCN) where carriers can retain a first degree of control over smartphone traffic inside the 3GPP. This scheme allows for non-SIM traffic to travel the usual route via local WLAN breakout while the TCN takes care of policy enforcement for SIM-based traffic. An important part of this is the routing of Wi-Fi traffic from smartphones to the mobile core instead of only allowing local WLAN breakout of Wi-Fi traffic.

g) Seamless Wi-Fi is full service continuity and device mobility across Wi-Fi and 3G / LTE networks. This involves not only the mobile and Wi-Fi network cores but also their interaction with the mobile device. As a result the IP address of the device will be preserved when the network changes so that the application can continue to run without executing their own switching routines. This way the IP traffic flow to split between Wi-Fi and 3GPP networks based on policies and QoS criteria.

Migration Path

Small cells can be used to provide in-building and outdoor wireless services for a wide range of air interfaces including GSM. Below is a potential migration path for a service provider starting with the 2G network.

  1. 2G with Wi-Fi offload: network selection is done by a client application, and the IP traffic is routed through the wireline and direct connection to internet, coupled with a common authentication architecture.
  2. 3G with small cell offload (including WiFi, Femto, Pico, metro cells etc.): Network selection is done by the Wireless Core, and a mobile device opens a VPN/IPsec tunnel from the device to the dedicated Interworking Wireless LAN server in the operator’s core network to provide the user either an access to the operator’s wireless macro services or to a gateway to the public internet, coupled with a common authentication architecture.
  3. 3G/LTE with small cell heterogeneous network: Wi-Fi becomes a de facto 3GPP RAN.

Wi-Fi Offload Technical Challenges

  • Management of QoS: in a heterogeneous network, as well as when handed off to a public Wi-Fi network
  • Site acquisition for the Wi-Fi equipment
  • Backhaul
  • Security risk
  • Continuity of service: Same service QoS support—a prime example is Voice services
  • Ease of deployment: Mounting, power, and backhaul, self-configuring.
  • Controlling Wi-Fi / 3GPP network selection

The structure of my thoughts revolves around the need for service providers to look at the strategy and consider LTE/Wi-Fi as an integrated solution rather than adjacent-based network services. I believe with the strategy outlined above, we can not only solve spectrum constraints but also use latest protocols and standards to increase speed up to 1Gbps as well.

Dr. Eslambolchi