Status update for 3G CDMA Femtocell Standards

Femtocell StandardsThe two dominant mobile phone technologies, CDMA and GSM/UMTS, have developed their own standard architectures and protocols for femtocells. With Sprint rumoured to be about to launch a 3G CDMA femtocell to compete with ATT’s newly debuted 3G Microcell, we look at some of the details of the CDMA femtocell system architecture and status of the standards. Our thanks to Doug Knisely of Airvana and Phil Brown of Tatara for providing these insights to the convoluted world of 3GPP2 standards development.

Current standards status

There are two main mobile technologies which have adopted femtocells today.

  • UMTS , the 3G version of GSM, as used in most parts of the world and by AT&T, T-Mobile in the US
  • CDMA, as used in North America by Verizon, Sprint, Bell Canada, Telus and in Japan and Korea.

Initial standards for 3G domestic femtocells are pretty much done and dusted.

The UMTS femtocell standard was released in Q2 2009 as part of the Release 8 baseline, with some refinements being added in Release 9 (Q4 2009)for security procedures (mutual authentication of the femtocell with the network) and by the Broadband Forum for remote management (extending the parameters which can be controlled through the TR-069 remote management protocol in a new standard TR-196, published in Q2 2009).

Femtocells for use in business environments (enterprise femtocells) have additional requirements. We’ll provide a separate update on standards developments in this area shortly.

SIP/IMS based standards are also defined for CDMA

In the CDMA world, existing 2G CDMA femtocells use an entirely proprietary solution from Samsung.

For 3G, the 3GPP2 standards committee is working towards completion of their Phase 1 femtocell standard. This uses IMS in a relatively simple form – using the SIP based protocol for basic call control and to perform core network registration procedures, as well as to encapsulate and transport the legacy signaling to the circuit core network where required – to take advantage of offloading traffic from the legacy CS network onto the IMS network. It doesn’t provide any additional features of IMS to the end user, who will continue to use their standard 3G phone.

A Phase 2 femtocell standard would follow and is due to be published in 2010.

Which documents actually specify the standard

The standards are embodied primarily in two documents named X.S0059 and A.S0024 which are targetted for completion by the end of 2009 and will be freely available on the 3GPP2 website at www.3gpp2.org

The main document X.S0059 comprises 4 parts:

  • 000 - Overview and Architecture
  • 100 - 1xEVDO high speed data services
  • 200 - 1xRTT femtocell network and voice services
  • 400 - 1xRTT supplementary services

The annex A.S0024 is an extension of the specification describing the interface between the MSC and BSC, known as the A interface. This extension details the additional parameters uniquely required to support femtocells across this interface. These parameters and signalling messages would only ever be found between the Femtocell Convergence Server (described below) and Femtocell itself.

Separate documents address femtocell management (which uses the same TR-069 standard defined by the Broadband Forum as UMTS femtocells) and femtocell security (the security framework is specificed in S.S0132).

SIP is used to carry the existing CDMA signaling protocol messages

Signaling messages are passed through to the core network via a “Femtocell Convergence Server”, which provides the interworking between the SIP/IMS and existing 2G/3G voice switches and services based on the ANSI-41/MAP standard. At a very high level, the Femtocell Convergence Server in the CDMA architecture functions in a manner similar to the Femtocell Gateway in a UMTS network.

What's different is that the Femtocell Convergence Server also effectively looks like another MSC (voice switch) within the existing voice core network. It contains a VLR (Visitor Location Register) and communicates with other legacy CS network elements as though it were an MSC.

Benefits of the Femtocell Convergence Server approach

The benefits of this approach are that all existing mobile services are available on the femtocell, including emergency calls using E911, location services, SMS text messaging and unique calling/distinctive ringtone. It is the femtocell access points themselves which are registered and have to be authenticated on the IMS HSS server, which controls access to the network.

End user devices (handsets) register with the existing ANSI-41/MAP core network using the standard procedures to the HLR. The authentication procedures use exactly the same messages and protocols, just encapsulating the data inside SIP messages. This is partly because the SIP protocol does not support the same CAVE authentication mechanism that legacy CDMA mobiles support.

Access control of individual users (i.e. deciding which handsets/users are allowed to use each femtocell) is handled directly by the femtocell access points. These “white lists” are downloaded and configured using the TR-069 device management protocol by the operator systems themselves.

It is currently being debated whether carriers prefer this access control to be handled by a central AAA server. This approach is seen as more useful in enterprise/business environments where large numbers of users need to be managed. For the home, a short list of allowed users held in the femtocell access point is seen as more appropriate.

Packet data sessions work independently

Packet data authentication is already separate from the ANSI-41 voice network standard. Signaling is routed through to the PDSN (Packet Data Service Node) which handles this directly.

The IMS core is used to authenticate the femtocell itself

With the IMS architecture, the HSS is used to authenticate devices. This is effectively a superset of the HLR found in GSM and CDMA core networks, but in this case is used to register and validate only the femtocell access point itself – there are no end users/subscribers provisioned into the HSS and no subscriber services linked to it.

Only one SIP session is setup between the femtocell and the Femtocell Convergence Server. Each additional voice call would create an additional dialog within these sessions. This is because the SIP/IMS has no knowledge of individual users or handsets and only keeps track of the femtocell access points themselves. 

Phase 2 CDMA femtocell standards will follow

A revised version of these standards documents, known as Phase 2, will add additional features. This is likely to include aspects specifically for enterprise (business) femtocells, and specifically interworking with office telephone systems, such as IP-PBX. The likely architecture for this will be to add more functionality into the SIP client within the femtocell so that it can communicate directly with the office IP-PBX using SIP.

The probable timeframe for the published CDMA femtocell Phase 2 standards is likely to be late in 2010.

Summary

The 3G CDMA femtocell standard uses a SIP/IMS core network is a very simple way, effectively using it to pass through most of the current signaling messages to the existing 2G/3G core network. This means that all the current services and features of 3G (E911, SMS text messaging, location etc) are available in the same way. It also allows the operator to take a small step towards wider deployment of the IMS network, which is positioned as the long term core network for all telephony services, wireless and wired.

My thanks to Doug Knisely, VP Technology for Airvana and Femto Forum board member and to Phil Brown, Standards Architect at Tatara Systems. Both are very active key players in the 3GPP2 standards body and briefed me on the content of this article. 

Glossary

IMS: IP Multimedia System. A new core network standard based on IP supporting multiple sessions and accessible from any IP connection, wireless or wired. Think of it as enabling Skype or MS Communicator, with video, file sharing, chat – but working between any phone/computer on any IP network.

SIP: Session Initiation Protocol. A standard used to setup and teardown communication sessions (such as phone calls, video calls etc). Designed to replace the existing telephony protocols such as SS7 and ISDN. This is the protocol used within IMS to setup and teardown all sessions.

CDMA: Code Division Multiple Access. A mobile phone technology popular in North America, Japan and Korea.

ANSI-41: A standard which defines the core network and signaling protocols used to make/receive calls for CDMA networks.

FCS: Femtocell Convergence Server. An interworking gateway which emulates a mobile voice switch (MSC) and an IMS application server, translating between both worlds to deliver existing 3G voice services to femtocells in an IMS environment.

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