Integrating Wi-Fi and Cellular Radio Access

Wi-FiPublic access Wi-Fi continues to evolve. Until recently, it has been more or less a parallel universe. Pretty much every smartphone has some sort of Wi-Fi capability, and users frequently take advantage of free, bundled or low cost Wi-Fi service in order to save money and/or access better data rates.

There has been a lot of talk about integrating Wi-Fi with 3G (and LTE) onboard the same small cells, particularly for public access areas. How will this work and what are the issues?

Wi-Fi in the home or office is popular and works well

Using Wi-Fi in the home or office is by far the most common scenario today. Security is excellent using WPA2 encryption, and switching to Wi-Fi access can be seamless because the smartphone remembers the configuration details and passkey or IEEE 802.1X/EAP credentials. Most homes and offices have only one owner, so there are fewer issues with conflicting Wi-Fi hotspots from different networks. However, this can become a problem in dense urban areas or multi-tenanted buildings, especially with the longer range achieved from more recent Wi-Fi products.This should be alleviated through wider deployment of the 5GHz band.

Having switched from 3G to Wi-Fi, your smartphone will be allocated a different IP address which will route traffic over Wi-Fi rather than cellular. Each interface in a computer or smartphone has its own IP address, allowing the network to differentiate between the alternate routes. Many data applications aren't affected by that because they don't maintain a continunous data session. A web browser typically creates one or more temporary TCP/IP sessions when downloading each inidividual web page, which are closed once the page is loaded. However, some applications rely on long lived TCP socket connections and this socket will typically be bound to a physical interface. Such an application will have a problem trying to sustain a change between cellular and WiFi access.

However, some session based applications are becoming smarter about this. Skype is reputed to be able to handle a call, maintaining parallel session as it switches between different interfaces, whether hard-wired Ethernet, Wi-Fi or cellular.

But public access Wi-Fi raises a number of issues

It's when trying to access public Wi-Fi services when away from base that can be the most problematic. Sometimes it works extremely well and is faster/better/cheaper than the cellular equivalent while at other times or places you may receive a less effective result. Visitors may need to take time to register and pay up front for a service of uncertain speed or quality. By contrast, you can switch on your mobile phone in almost any country and receive immediate service with the charges billed to your existing home account, but the value for money can be highly variable – you may have to pay significant roaming fees.

The major issues boil down to:

  • Hotspot Selection: There are so many hotspots to choose from in some areas, making it difficult to know which one(s) are compatible and can provide the service required.
  • Security: This is a two way issue. Do you (or your smartphone) trust a Wi-Fi hotspot that is advertising its service enough to part with your credit card details and/or email address? Similarly, while some establishments don't restrict access to their Wi-Fi with even a password, others have some level of security to protect against abuse.
  • Encryption: Today's Public access hotspots necessarily have WPA encryption turned off, so that anyone can access them. The default is that all traffic is sent in the clear and can be seen by others in the area. While your web browser may hide secure pages accessed through SSL, and possibly your email client may do also, browser cookies may be sent in the clear and so you really need a VPN setup to ensure privacy – something that few consumers would know about.
  • Ease of use: Often, you'll need to open a web page and fill in your details and credit card number, registering and paying for service in advance. Frequent users might have an account which simplifies this, but the widespread variety of Wi-Fi providers makes this impractical for many.
  • Billing: You may need to have a separate account (or multiple accounts) with different Wi-Fi aggregators. In some cases, access is granted for those with cable or landline accounts – extending their privileges from another telecom service. In other cases, such as foreign business travel, aggregators such as iPass offer comprehensive use of large numbers of Wi-Fi networks.
  • Quality of Service: Having bought and paid for a Wi-Fi service, the throughput and performance can be highly variable. It's very frustrating to have invested the time and money to find the service is effectively unusable. A much simpler and streamlined connection process might allow the quality to be quickly established and a choice made to revert to standard cellular service where appropriate.

Since today's Wi-Fi don't use the same protocols, commercial processes or methods as cellular networks, the future Wi-Fi and cellular systems will need to interwork much more closely.

Carrier Wi-Fi adopts Cellular industry procedures

Now that cellular carriers have adopted Wi-Fi for use outside the home and workplace, there was a growing business incentive to develop standards and procedures to address these issues and provide a seamless and secure end-user experience. The two main Wi-Fi industry bodies (the Wi-Fi Alliance and Wireless Broadband Alliance) worked together with their counterparts in the cellular industry (3GPP and GSMA) to complete the HotSpot 2.0 (also known as Next Generation Hotspot) program.

The Wi-Fi Alliance developed the technology - standards and software protocols for Hotspot 2.0, together with a certification scheme for vendors to prove compliance and compatibility.

The Wireless Broadband Alliance adopted standard procedures for roaming, billing and settlement to facilitate roaming and enable easy access to Wi-Fi charged to your home account.

The main features and benefits of this approach for the end user are:

Passpoint 1.0: Hotspots will broadcast their properties without the need to attach to them. Smartphones can pre-determine if the service can be paid for through their Service Provider, and whether it has enough capacity (including backhaul and congestion levels) to deliver the service required.

This will lead to a move away from configuring hundreds of SSIDs (Station Identification Identities) on the same access points.

Roaming, clearing and settlement processes: Complementary to the back end administration processes developed by the GSMA, billing records log usage similar to TAP records for cellular. There are processes to identify and deal with fraud and internetwork authentication and authorisation.

The signalling and IP traffic between cellular operators is routed through international gateways, called GRX (GPRS Roaming Exchanges) and IPX (IP roaming exchanges). In parallel, the WRIX (Roaming IP Exchange) performs a similar function for Wi-Fi and the GRX/WRIX can interwork.

Security: The 3GPP Technical Report 23.852 for Wi-Fi is claimed to be as secure as LTE. Operators have to be sure of the identity of each device that accesses their networks, and authentication is the mechanism to achieve that. Equally, smartphones have to be sure they are connecting to valid, trusted networks. The WBA agreed on four different methods (EAP-SIM, EAP-AKA, EAP –TLS, EAP-TTLS). EAP-SIM and EAP-AKA uses the cellular SIM card, directly related to your existing mobile phone account. Both EAP-SIM and EAP-AKA ensure mutual authentication between the SIM and the home network.

These features enable a much more seamless and secure carrier Wi-Fi service to be offered, backed with commercial processes that can directly operate with an existing mobile phone account. It remains to be seen how commercially successful this is, where factors around marketing, pricing and competitive alternatives have equal or greater impact.

We are only at the beginning

Despite the progress already made, there are many aspects to be considered and improved. There will be lots of innovation (and IPR) to come. Aspects include:

  • Who and where is the intelligence to switch to/from Wi-Fi and make the most appropriate selection. It's probably best to make that decision nearer to the smartphone, rather than in the core.
  • It's not easy to handover and retain the same session, for the reasons given above. This is particularly the case for secure sessions including VPNs.
  • Lawful Intercept doesn't apply for private Wi-Fi sessions but regulatory requirements may change if the carrier has control over WiFi network selection.
  • Enterprise and FemtoZone services may be expected to work equally well on Wi-Fi as for cellular.
  • Wi-Fi has a perceived low price point. How will this be reconciled with similar network operator costs (apart from spectrum)?
  • Network Management: There is still work to do around managing carrier Wi-Fi networks. Hotspot 2.0 has its own management object, as does ANDSF and legacy OMA-DM WiFi provisioning still relies upon SSIDs. There is some early work looking at how to converge these different models into a coherent framework.
  • Closer integration between the Wi-Fi and 3G/LTE subsystems onboard the same small cell.
  • Use of caching within a small cell, applying to content for both Wi-Fi and Cellular access

Cisco have been involved in these initiatives from an early stage. They brought the specification to WFA and worked with the WBA to trial the technology on the path to adoption. My thanks to Mark Grayson, Distinguished Consulting Engineer responsible for leading Cisco's mobile architecture strategy, and Dave Stephenson, Senior Technical Leader, Cisco who contributed substantially to this article.

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