Converging Cellular and Wi-Fi using Small Cells

Wi-FiFor several years now, Wi-Fi has been an alternative and usually completely independent method of accessing the Internet than through the cellular network. There has been little or no synergy or commonality between the two. Some visionaries talk of Wi-Fi as just another RAN (Radio Access Network), which fits alongside 3G and LTE, albeit operating in unlicenced spectrum.

With the growing uptake of Carrier Wi-Fi, we look at various technical options of how Wi-Fi and Licenced Cellular are converging.

Four areas of co-habitation

There are four main touchpoints where both Wi-Fi and Cellular can reside together:

  1. The Smartphone: I'd be surprised if any are sold today without Wi-Fi onboard and the vast majority of current models would be dual band (2.4GHz and 5GHz). In most cases, the user can manually switch on or off the Wi-Fi and connect to private or public networks with passwords, splash screens or VPN clients. Apps from Wireless ISPs or network operators can help automate the login process.
  2. The Access Point: Wi-Fi is mostly delivered by a separate and independent set of access points from the cellular service. Recent small cell models, especially those designed for Enterprise/In-building use, incorporate a Wi-Fi chipset. Examples include Cisco's Aironet 3600 with 5310 small cell module. These two radio access bands work completely independently within the access point – there is no direct interaction between them today. Enterprise and Urban Small Cells now incorporate higher quality/carrier grade Wi-Fi modems.
  3. The local (onsite) network: Residential and some small businesses may share their local network (LAN) openly with Wi-Fi and Small Cells. Larger Enterprise IT departments are likely to segregate these two access methods either physically or logically, but in future may move towards adopting common Ethernet wiring for both. Modern DAS systems provide an IP backhaul feed for adjacent Wi-Fi access points, but don't physically integrate them.
  4. The mobile operator's core network: Most Wi-Fi bearer traffic is dumped directly onto the Internet, whereas cellular data would be routed through the core network gateway. Hotspot 2.0 allows Wi-Fi logon using the SIM Card to authenticate, promoting the use of the HSS (formerly the HLR) as a central point of authentication for both Celluar and Carrier Wi-Fi service. We can expect more Wi-Fi traffic to be routed through the mobile operator's core network in the future.

Lets look at how each of these may evolve:

Smartphones and Tablets

Most smartphones are capable of remembering Wi-Fi networks they've previously logged onto and can automatically rediscover and logon. This is typically useful at home or in the office, where the majority of data traffic is handled, but is also helpful when frequenting the same haunts. A downside can be hgher battery drain when out and about, as the smartphone constantly searches for potential Wi-Fi opportunities but isn't able to attach to them.

A surprising number of smartphones never have Wi-Fi enabled – one vendor reports levels of 30% - leading to solutions for operators that remotely activate the Wi-Fi and setup a profile which encourages higher usage.

Both the 3G/LTE and Wi-Fi access are each allocated their own separate IP address, with the traffic routed differently. The switch to/from Wi-Fi is a roaming transaction, not a seamless cellular handover. This can add complexity to Apps, especially when both connections are active simultaneously. Some, such as Apple Siri, are capable of selecting the one with the lowest latency for short time critical interactions. Skype is smart enough to simulate handover between them. Other audio and video streaming sessions are most likely to be disrupted. Most Apps are unaware and leave it to the operating system.

Hotspot 2.0, which can use the SIM card to authenticate with Carrier Wi-Fi access points, will help streamline use and make Wi-Fi a more seamless service. Most of the recent models of smartphones (eg Android, Apple, Samsung, Huawei etc.) are capable of supporting this service either natively or with a software update. It's been included in iOS7 (so works with any iPhone 4 or later) and Samsung 2.0, 3.0 and 4.0.

This won't solve all cases of Wi-Fi access because not all Wi-Fi access points will be HotSpot 2.0 enabled. Many will be open with or without password protection. Another concern is the quality of service that you might get, making it better to stay connected to the cellular network. This leads to opportunities for Wi-Fi management services that can remotely manage and monitor the quality of service achieved, actively directing the smartphone towards the most appropriate access point and providing login credentials where needed.

Access Points

The costs of physically installing a box on the wall, providing power and data connectivity together with ongoing oversight and maintenance are probably much the same regardless of function. It could be a burglar alarm, video camera, fire detector, Wi-Fi access point or Small Cell. In my office, I can see all of those present, all completely independent and no doubt serviced by completely different organisations.

While I'm not suggesting there's a market for a combined box that performs all five of those functions (and makes the coffee at the same time), there is a very good case for combining Wi-Fi access points with cellular Small Cells. Products already exist which have a small form factor and can be remotely powered via a single Ethernet cable that also delivers data connectivity. The Small Cell Forum published findings that confirm there is no technical barrier to combining both types of radio transmitters within the same box.

Typically the number of Wi-Fi access points will far exceed the number of Small Cells. A case study from Wisconsin reported 800 Wi-Fi access points installed throughout an educational campus which could adequately be covered by less than 10 small cells. (Clearly the total capacity would be less). It seems likely to me that even where Small Cells have Wi-Fi hardware embedded, it might only be enabled on 10-30% of units.

Enterprise IT departments want visibility of where and which units have the cellular small cells fitted and enabled. Cisco's Wi-Fi access point will be able to report that status to its existing management system. Standalone access points may need some other method to achieve that.

Enterprise Network

A segregated VPN is often assigned within a larger Enterprise for dedicated use of the Small Cells. This allows the same physical Ethernet switches and cables to be shared throughout the building, with each VPN having its own IP addressing, DNS, DHCP and other IP supporting services.

Flow control and traffic shaping isn't commonly used within an Enterprise but could be added if required, for example to prevent individual guests grabbing too many resources.

It seems likely that the same VPN could be shared for both Wi-Fi and Small Cell use rather than keeping this traffic segregated. For larger Enterprises, a local controller may be present in the building to handle the Small Cells. This may be combined or co-located with the Wi-Fi access controller. At present, these functions are logically quite separate.


Network Architecture Diagram courtesy of Cisco

Operator's Core Network Wi-Fi traffic handling

There are several Standards projects working on how Wi-FI traffic could be routed through the operator's core network and gateways. This would provide greater control (e.g. filtering out unsuitable sites) and visibility of user behaviour.

Three new network interfaces called S2a, S2b and S2c treat the traffic from Wi-Fi Access Point to Core Network in different ways.

  • SaMOG (S2a Mobility based on GTP) which simulates a cellular data session by tunnelling the traffic from a trusted Wi-Fi access point to the mobile operator's core network. This is a 3GPP release 12 feature (so is due to be published during 2014).
  • ePDG (evolved Packet Data Gateway) using S2b. Similar to SaMOG but uses an additional gateway for traffic from untrusted Wi-Fi access points. The Smartphone directly encrypts the data using IPsec.
  • S2c is a new protocol that can handle both untrusted and trusted access points, using a modified form of Mobile IP. This option has been heavily promoted by Qualcomm and allows channel bonding (i.e. can use both cellular and Wi-Fi at the same time for higher throughput). Extending the concept further, Qualcomm have also proposed LTE-U, which would operate LTE radio technology on unlicenced (e.g. Wi-Fi) spectrum. As presently proposed, LTE-U must have at least one active connection to a macrocell (not a small cell). It would need new software on smartphones and is not backward compatible with today's installed base. Industry commentators are not confident of this option becoming a reality. More coverage of the topic reported here

Mobile Operator Wi-Fi Authentication

Mobile networks have always had an HLR (Home Location Register) which authenticates (verifies the identify of) and authorises (grants access permission for) network access. The HLR has evolved to become an HSS (Home Subscriber Server) which can also includes the AAA function to authenticate for Wi-Fi access and would also support SIM card authentication for HotSpot 2.0.

I'd expect many mobile operators to support HotSpot 2.0 authentication during 2014, even when they have no Wi-Fi hotspots themselves. This service can be delivered at minimal additional cost and is worthwhile even where there is no direct revenue from the roaming partner – the benefits of offloaded traffic from the macrocell network and increased customers loyalty make this more than beneficial.

Once in place, this works with any HotSpot 2.0 capable Wi-Fi access point, whether it is combined with a licenced Small Cell or not. There does need to be a commercial roaming agreement between the HotSpot owner and your home mobile network operator. The live demo at Mobile World Congress worked well for those "in the club", such as AT&T, China Mobile etc, but sadly wasn't open to anyone from a UK based mobile network. Usage of such roaming networks is charged as part of your mobile plan/bundle – one operator expected certain tier price plans to include (say) 1Gbyte of roaming Wi-Fi traffic that was separate from your cellular use.


Mobile operator's perception of Wi-Fi has developed over recent years, and it is now an acknowledged part of the total solution for wireless data services. Quality of experience is key for commercial service providers, who want to ensure a high quality, easy to use, consistent and ubiquitous wireless service.

Combining Wi-Fi and Small Cells into the same box initially drives down costs, saving installation and physical management of diverse equipment. The incremental hardware cost of the Wi-Fi is relatively insignificant.

For larger Enterprise installations, combining the Wi-Fi and Small Cell controller functions should also reduce network management overheads.

In the short term, Wi-Fi and cellular traffic will continue to be routed through one or other method. In the longer term, we may see channel bonding which will allow higher overall data rates. I'd question whether this is really urgent, given the fast speeds achievable with 802.11ac and LTE-A. Various standards bodies are looking at alternative options on how to achieve this.

Mobile operators can regain more visibility and provide more continuity of service to their customers if they embrace methods to encourage seamless logon to Wi-Fi. This includes Hotspot 2.0 and other proprietary methods, but will also need some checks and balances to monitor quality of service if customers are not to be disappointed by rogue Wi-Fi that fails to deliver. Third party vendors such as Devicescape and Birdstep have made great strides to achieve that vision.

References and further reading:


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