SIP femtocell architecture Part 2

SIP femtocell architectureIn this mini-series, we are looking at SIP/IMS and how it will apply to Femtocells. In the first part, we explained what SIP/IMS are and how it might apply to 3G mobile networks. This part looks at where these technologies could appear in 4G and other networks, and how networks will migrate to them.

 

Where are SIP and IMS likely to appear first?

In fact, most IMS is used by fixed network operators today. Many of the long distance calls made today are already processed using SIP somewhere along the connection. Several VoIP providers, such as Vonage are SIP based network operators.

The major equipment vendors such as Ericsson, Alcatel-Lucent and Nokia-Siemens Networks are leading vendors of IMS solutions and jointly claim hundreds of contracts. The IMS standard has also been adopted by Cable network operators who are using it to deliver business and residential telephone services.

4G may be the first step to IMS for Mobile Network Operators

In the mobile world, North American CDMA operators such as Verizon Wireless have been very enthusiastic about IMS, and have selected Nokia-Siemens Networks as their IMS supplier as part of their 4G LTE network due to go live in 2010. There are a number of good reasons for this, including:

  • CDMA operators have to make a change to their core network technology when they deploy LTE – CDMA core networks won’t support it. Therefore, why not jump direct to the latest and long term solution.
  • LTE provides a fast and low latency radio access network that is a good access technology for VoIP
  • IMS can be used for both fixed and wireline connections
  • IMS will provide a wider range of services
  • IMS can manage data traffic prioritisation extensively including differential billing

Softbank Japan – a combined wireline broadband and mobile operator - is also using SIP/IMS in its network.

How will today’s operators migrate to IMS?

At first glance, IMS changes many parts of a mobile network. The Mobile Switching Centre (MSC) which is the switch control function that sets up calls is replaced by a SIP Server that performs a similar function. These servers may combine several functional elements from the SIP standards including CSCF (Call State Control Function – the core switching capability) and the Application Servers. Initially, interconnection with other networks will be via media gateways. In the future, where multiple IMS networks are interconnected, they will only require a Session Border Gateway – radically simplifying the interconnection and costs involved. The Home Location Register (HLR), a database that keeps track of where all subscribers are at any time is extended to become a Home Subscriber Store (HSS) that also performs the same functions but is also accessible via IP.

Those supplementary services that we all use, such as voicemail and text messaging have to be accessible from both old and new network technologies depending on how we are connected to the network. There are several vendors offering application servers to bridge this gap – these are also known as convergence servers and emulate or convert the signalling between IMS and existing 3G network protocols and services.

At the time IMS was first conceived, existing 2G mobile networks were controlled by early designs of MSCs which themselves were based on existing ISDN landline switches. Over time, these monoliths have been transformed into separate MSC Servers (which handle all the control processing) and MSC gateways (which handle all the bearer traffic). Instead of being connected by circuit switched transmission, many long distance connections already use VoIP and other packet technologies. Therefore, some of the original benefits of switching to IMS have already been realised in today’s architectures.

Some argue to retain the existing core network

A lobby group are arguing to retain the current GSM model of call control instead of switching to IMS. Their target is the way in which voice is handled for the new 4G LTE networks, which had up to now assumed would use IMS. Adopting a technology developed to extend GSM mobile phone service over WiFi, called GAN (Generic Access Network) and also known under the term UMA (Universal Mobile Access), this approach would conserve existing voice services over the 4G network – simplifying the migration for current 3G UMTS operators and limiting their investment.

This group initiative is called VoLGA Forum (Voice over LTE Generic Access) and includes major vendors including Alcatel-Lucent, Ericsson and Huawei as well as operator T-Mobile. The Stage 1 specifications are published on their website. This option is available to GSM operators, but does not apply to CDMA operators who are all committed to IMS.


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