Wi-Fi Offload Mechanisms for Networks

|

Telecom-Cloud LTE & WiMAX Blog

Much ink has been spilt on how AT&T is using the Wi-Fi network as a means of offloading the macro network, as a slew of smartphones are clogging the network pipes. The rise of the Wi-Fi as a mechanism for offload is not new nor game-changing but part of orthogonal thinking that is here to stay until there are other solutions to relieve the macro network. Traffic as we know on 3G networks have grown exponentially in the last 2-3 years since smartphones, tablets with 3G-HSPA+ capabilities but how have the wireless operators changed ? There is multitude of optimization techniques happening all the same time over 3G networks – Carrier addition to the NodeBs, Fiber to Site (AAV), Direct tunnel for GGSN, PCRF, IMS, etc. Every single one of the changes takes up resources either as part of the spectrum, Channel elements on RNCs, power or bandwidth on the core.

Many mechanisms have been explored with cost to benefit ratios calculated but the most cheapest though not the easiest of mechanisms to use has been Wi-Fi – no spectrum licensing costs, low capex/opex costs with improved customer performance. Pico cells are another option but they offer only slightly more advantages with a main drawback of licensed spectrum usage, and not Wi-Fi download speeds in ideal radio conditions.

Why Offload?

3G traffic trends have been growing with smartphones and dongles, and some of the mechanisms to offload the traffic are – HSPA+ upgrade along with Hybrid Iub, Femtocells, Wi-Fi and upgrade path to all-IP 4G systems. Since 4G systems offer all IP super-core optimized for packet data transmission, an easy path would to upgrade to these systems, easier said than that as LTE ecosystem is still developing while WiMAX deployment seems to be dying down.

Meanwhile as the standard bodies as well as the Vendors work it out to bring new devices and Infrastructure certified and ready for deployment for 4G, the current networks have to struggle and use different mechanisms to offload data traffic. Below is a typical device type distribution of a WCDMA service provider.

UE TypePenetratio

n [%]CS Call

[%]CS Erlang

[%]PS Call

[%]PS Erlang

[%]DL Data

[%]Smartphones 63% 66% 71% 96% 94% 87%

Data cards 1% 1% 2% 3%Other phones 36% 33% 29% 3% 5% 10%

Based on the tables above it can be inferred that -

2 www.harishvadada.wordpress.com – Please share and give me feedback – via my blog!

Data sessions come mainly from smartphones Data cards usage is very small, both in device penetration and usage Total on-air time (Erlang) for PS is approx double that of CS averaged over all devices.

Traffic modeling based from the table above, network on-air time for PS services are now on par with or higher than on-air time for CS services. For smartphones, the PS Erlang is even higher than the CS Erlang, hence significant capacity savings could be achieved if a portion of the PS traffic could be carried by Wi-Fi instead of the 3G network as soon as it enters the Wi-Fi zone.

Let us spread the deck of cards that we have available and do a techno- analysis of which is the most suitable for this place an time. Networks can be offloaded by various other wireless ‘ancillary’ technologies and standards starting with – Wi-Fi, UMA (unlicensed mobile access), Femtocells & their variants with Rel.10 and possible other solutions like Whitespaces and smartgrid-based neighborhood area networks (NAN). LTE-Advanced also gives some congestion ‘relief’ techniques like relays which come in three forms – amplify & forward, Selective Decode & Forward as well as Demodulation & Forward.


Wi-Fi Architecture and Seamless Handover

Wi-Fi bubbles have grown enormously worldwide and some of the places we now see the growth are at home, office, airports, Starbucks, libraries etc. Some of them are open to connect and many are closed by the administrators. Let us explore the case of iPhone and how AT&T is using Wi-Fi to offload the traffic. The new iPhone 3.0 OS software contains the ability for iPhones to autoswitch from 3G to Wi-Fi when an AT&T Wi-Fi network is in range, without user intervention.

AT&T made a statement saying they would allow "seamless transition from their 3G network to any one of their 20,000 Wi-Fi wireless hotspots". While no protocol was stated, it is likely the way that the iPhones will switch back and forth between 3G and Wi-Fi. There is always the possibility that they are doing "dumb handoffs" as well which would just be like when a normal iPhone enters an area which a recognized network is present. The iPhone finds the network, knows the password and switches

over. AT&T* today announced it will support auto-authentication for iPhone OS 3.0 users connecting to AT&T Wi-Fi Hot Spots. Auto-authentication allows iPhone users to seamlessly switch from AT&T’s 3G network to an AT&T Wi-Fi Hot Spot without being prompted.

AT&T customers with qualifying iPhone data plans have unlimited access to the nation’s largest Wi-Fi network — more than 20,000 U.S. AT&T Wi-Fi Hot Spots. The new process eliminates the previous two-step authentication, making it easier and faster for iPhone customers to connect to AT&T Wi-Fi. Auto-connect is established once a customer connects their iPhone to an AT&T Wi-Fi Hot Spot the first time. The addition of auto-authentication for iPhone users comes at a time when Wi-Fi usage continues to experience rapid growth, driven by the proliferation of Wi-Fi enabled devices. More than 4 million connections were made at AT&T’s U.S. Hot Spots with smartphones, including the iPhone 3G, in the first quarter of 2009 alone.


MIP and 802.21 (MIH/Vertical Handover) standards

Two other handover mechanisms used in mobile IP communications are Mobile IP and Sea Moby an evolving standard in IP based mobility. Mobile IP (MIP) is where a Home agent (HA) and foreign agent (FA) are used to seamlessly handover between two networks with an anchor IP address at the FA, with a Care-of-address generated at this location.

Two types of MIPs are supported as standards in many of the networks –

Client Mobile IP: Each device supports a local Mobile IP client that interacts with HA via FA so that incoming data can be successfully routed by HA to the device.

Proxy Mobile IP: Instead of having a Mobile IP client on the device, an instance of this is created in the ASN(in WiMAX). Proxy MIP eliminates the constraint of requiring Mobile IP client on the WiMAX device. Proxy MIP allows MIP registration which takes place by the DAP/CAPC on behalf of the Client/CPE.

A Media Independent Handover Function (MIHF) that provides three services for efficient handovers Information Service:

Provides query/response model to obtain useful network data e.g. “list of neighboring networks”


Event Service: delivers link layer conditions & triggers e.g. “link going down” Command Service: controls link behaviors e.g. “scan link”

What 802.21 provides:

Standardized interface between upper and lower layer mobility protocols Triggers fast detection (discovery) of neighboring L2 networks of different technologies Detects current L2 link status Quickly informs upper layers of new L2 network point of attachment Allows setup of multiple L2 links for Make Before Break handling Allows quick teardown of unused L2 links

This will result in a relevant frame-work to allow efficient multi-RAT mesh network

Case Study – Stoke Solutions (www.stoke.com)

Mobile broadband operators are revisiting Wi-Fi in order to help manage the high costs of data services delivery. And the cost implications of the growing mobile broadband data glut are nowhere more prevelant than on the operator radio access network — the RAN. And with highly loaded cell sites often located in dense urban locations, Wi-Fi can offer a low cost alternative to continued cell splitting. Moreover, IP enabled Wi-Fi enables lower cost backhaul options as well.

Stoke's SSX-3000 offers high density user-to-network and network-to-network secure session termination and provides the ideal security gateway for Wi-Fi deployments for either 3GPP or 3GPP2 networks.

Stateful management of tens of thousands of secure, reliable, high-quality subscriber sessions over multiple access types requires modern product design. New challenges - such as cost-effective scaling of multi-access subscriber connections, granular session management, and improved service creation and control - must be addressed at the system level.


http://www.stoke.com/

With this perspective, and in compliance with leading standards including 3GPP/2, ITU and PCMM, the Stoke Session Exchange (SSX) was designed and developed to deliver a wide range of mobile operator gateway functions to meet the challenges of rapidly growing mobile broadband data services for 3G and 4G network deployments. The SSX mobile broadband gateway offers fundamental feature, price and performance advantages over re-purposed routers or legacy session-oriented platforms.

Femto Architecture changes with 3GPP Rel.10

Femtocells have long been used for providing two main benefits indoor coverage and offloading of macrocells. The first Pico cells made it into the market with GERAN with Abis over IP, and they have morphed into 3G Femtocells for UTRAN as well as EVDO. But with Release 10 of 3GPP three very important concepts have been introduced - Local IP Access (LIPA), Selected IP Traffic offload (SIPTO) and IP Flow mobility (IFOM) Let us see how the architecture has evolved to provide relief to congestion on the network.

LIPA

The features of the architecture for LIPA for HeNB subsystem are -

For LIPA traffic, a Local Gateway (L-GW) function for EPS that can be either collocated on the HeNB or as a standalone node includes partial P-GW function and S-GW downlink data buffering function; it could also be possible, if required in the future, to be used as anchor point for inter-HeNB mobility;

The S-GW in the core network serves for the CN traffic; The L-S11 interface between the L-GW and the MME is used to manage the session for LIPA

traffic; For the LIPA PDN connection, the L-GW needs to be selected close to the HeNB, and establish

the connection through L-S11 interface. The L-GW selection mechanism has been specified in section 6.1;

When the UE is in idle mode, the LIPA downlink packets are buffered in the L-GW; When UE enters connected mode, the packets buffered in L-GW are forwarded on the path

between the L-GW and the HeNB.


For UMTS the direct tunnel functionality can be used to manage the PDP connection between the L-GW and the HNB. It is assumed the User plane does not go through the HNB GW for the direct tunnel.

Architecture above illustrates the LIPA architecture including the L-GW function in UMTS for the HNB, where the L-GW is physically co-located with the HNB and no Gn is supported, i.e., LIPA for the residential or single HNB scenario. In the case of the enterprise network where mobility is supported, the L-GW is located above the HNB but still physically within the enterprise network, and the L-GW now includes the functionality to support a Gn interface to the HNB.

SIPTO

SIPTO for Macro-Cellular breakout point is close to the UE's point of attachment to the access network, and that it shall be possible to support mobility for offloaded traffic, which means that the breakout point is "at or above RAN". Moreover, SIPTO for H(e)NB Subsystem allows the breakout to be located either in the residential/enterprise network as LIPA, or "above" H(e)NB in the hierarchical view of the mobile operator network i.e. in the backhaul or at the H(e)NB-GW.As a consequence, two types of breakout architectures are distinguished:

Architectures with breakout "at or above RAN" (covering macro and some H(e)NB SIPTO scenarios);

Architectures with breakout "in the residential/enterprise IP network" (covering LIPA and some H(e)NB SIPTO scenarios).

In addition, selected IP traffic offload for the Home (e)NodeB Subsystem may support the following three scenarios:Scenario 1: Home (e)NodeB Subsystem and backhaul are provided by the same operator;Scenario 2: Home (e)NodeB Subsystem and backhaul are provided by different operators;Scenario 3: Local Breakout point (L-PGW) for LIPA/SIPTO is located in a private address domain, e.g. behind a NAT gateway.

When Local Breakout is active, at all mobility events involving the core network the [source] SGSN shall re-evaluate the eligibility of Local Breakout and disconnect any PDP contexts for which the specific


breakout point is no longer allowed. If one or more PDP contexts are no longer allowed for the current breakout point:

At intra-SGSN mobility, the SGSN shall trigger the SGSN initiated PDP context deactivation procedure; otherwise

Mobility to a new SGSN/MME, the target SGSN deactivates the PDP contexts determined as not applicable for local breakout.

The behavior should be the same as for PS domain emergency handling for HO into a restricted area when ordinary contexts/bearers are active. During mobility between different CSGs, the SGSN shall determine whether the CSG-ID has changed, if it has changed the SGSN shall re-evaluate whether the existing local breakout can still be applied for the target CSG.

IFOM

The increased data demand, caused by the increased use of 3rd party applications and Internet browsing is creating interest for new operator tools to lower the cost on providing data access. The increased availability of WLAN radio in many terminals and the increasing availability of WLAN access networks in many geographical locations provide means to achieve this goal. When the subscriber happens to be under WLAN coverage, it is beneficial for the operator to offload some traffic (e.g. best effort) to the WLAN access. At the same time it may be beneficial to still keep some traffic (e.g. VoIP flow) in the cellular access. With this IP flow mobility solution the operator can lower it data access costs while the subscriber just experiences maximized bandwidth without any service disruption or interruption.

It is therefore of interest to 3GPP community to specify a solution for operators for a seamless WLAN offloading via IP flow mobility. Based on this solution, operators can use WLAN as a seamless extension of their cellular access and thus increase the overall system capacity while minimizing the access cost. The MAPIM Study Item documented in TR 23.861 provides a technical solution for seamless WLAN offload which is mature enough to specify this capability as part of 3GPP normative specification.Additionally it is possible to provide a limited non-seamless WLAN offload as done in current deployments via a transient IP connection via WLAN (referred also as Direct IP Access in I-WLAN). This implies that the UE uses the WLAN IP address and no IP address preservation is provided between WLAN and 3GPP accesses. While most details of this scenario are outside the scope of 3GPP as they are confined into the non-3GPP access, it is useful to define operator's policies in 3GPP to guide the behavior of the UE.

UMA (Unlicensed Mobile Access)

UMA has been around for a while now, though commercially it has not been as successful as initially thought out to be due to lack of vendor support and development in that space as well as due to excessive power usage by the handsets on the onset. In the US, T-Mobile has been the only Tier 1 carrier that has supported UMA from its launch. I have extensively discussed UMA technology in my last blogpost here.


Conclusion

Wi-Fi, whitespaces, Femto and other indoor solutions alike shall be used as mechanisms to offload the macro networks whether they are 3GPP/3GPP2 or WiMAX based networks due to influx of data demand. It remains to be seen how operators evolve their solutions and cater to the’ always-on’ devices that need broadband everywhere. Spectrum as we know is always a scare resource but ‘intelligent’ use of unlicensed spectrum along with licensed spectrum will the main play that operators would need to adopt and push the vendors to bring these solutions into the market fast enough to reap benefits. Wireless broadband is in an exciting phase where devices and customers are demanding more at a faster rate pushing the operators into a defensive mode unlike in the past.

10

www.harishvadada.wordpress.com – Please share and give me feedback – via my blog!

Documents

Wi-Fi Offload Mechanisms for Networks