What’s required for SIP based femtocells?
If a mobile operator already has an IMS core network, then SIP based femtocells can be almost directly connected to it. There is no need for a femto gateway – instead all that’s required is a security gateway (which terminates the IPsec tunnels) and a Session Border Controller (SBC), which acts as sort of super firewall/quality control/policing function (for want of a better term).
SIP based femtocells also support existing 3G mobile phones – the femtocell translates the standard signalling messages into SIP format so that the phone doesn’t know it’s even using SIP.
Where SIP capable handsets are used, the more sophisticated capabilities of IMS can be used (such as the Rich Communication Suite mentioned ealier and native SIP/IMS sessions).
Also a SIP-based femtocell architecture requires an element to provide seamless hand-off and hand-in to the macro cellular RAN. This element, known as a convergence server, also provides support for the rich voice, messaging and supplementary services that the mobile core supports.
Do will the standards support it?
The current focus for 2009 in the 3GPP mobile standards body is defining a SIP architecture in Release 9 using an IMS femtocell approach. UMTS operators are not really considering SIP until it is defined in the 3GPP architecture.
3GPP2 (which looks after CDMA standards) is also working on a CDMA/IMS approach. CDMA operators are more likely to adopt a SIP approach given their longer term move to IMS and the extensible nature of SIP core for other services and applications.
Voice calls are likely to terminate in the femtocell unless there is a non-voice interactive session. This allows easy handover to and from the existing 2G/3G core network.
Interworking between SIP/IMS and existing/legacy systems
With an installed base of over 4 Billion subscribers, the existing GSM and CDMA core networks and supporting systems aren’t about to be switched off anytime soon. Those using SIP based phones and services will want compatibility when in areas or countries that don’t support this mode. There are two main scenarios:
- When making a call using a phone in SIP mode, and then moving outside of the coverage area which supports it.
- When making a call outside a SIP based area, and then moving into a coverage area which supports it.
Companies such as Maviner and Tatara Systems provide “convergence servers” which interwork between the SIP/IMS and current core networks, mapping and translating the call handling protocols for sessions which bridge the two. This supports call handover between the two networks using a standard called Voice Call Continuity (VCC), as well as accessing your voicemail and/or SMS text messages held by systems in one or other scheme.
ENUM – translating between phone numbers and SIP addresses
There is also an issue of what your primary contact number/address is – should it be your existing telephone number or change to a text based SIP format. The ENUM standard combines the existing international telephone number standard known as E.164 with the universally adopted internet addressing standard known as DNS (Domain Naming Scheme or Domain Name Server). ENUM stands for E.164 NUmber Mapping.
In a nutshell, this standard maps a telephone number to a DNS address. This is best illustrated as an example:
| International Phone Number
||+1 555 123 2038
| ENUM DNS
| SIP address
The DNS entry can then also return other associated addresses including email etc.
This information is stored in a DNS server, which could be a function provided by an HSS (Home Subscriber Store). These supercede HLR (Home Location Registers) in IMS networks and contain all the subscriber specific data required to register and authorise subscriber activities in real-time.
However new vendors are appearing which specialise in these systems. Nominum and NetNumber both offer ENUM specific routing databases.
The idea is that our business cards will show a SIP address (which might well be the same as our email address) as well as a direct contact number. Over time, more people will use SIP and the numeric telephone number will become a thing of the past.