Fixed-mobile convergence

Fixed-mobile convergence

Fixed-mobile convergence (FMC) is a change in telecommunications that removes differences between fixed and mobile networks.

In the 2004 press release announcing its formation, the Fixed Mobile Convergence Alliance said:[1]

Fixed Mobile Convergence is a transition point in the telecommunications industry that will finally remove the distinctions between fixed and mobile networks, providing a superior experience to customers by creating seamless services using a combination of fixed broadband and local access wireless technologies to meet their needs in homes, offices, other buildings and on the go.[2]

In this definition “fixed broadband” means a connection to the Internet, like DSL, cable or T1. “Local access wireless” means Wi-Fi or something like it. BT’s initial FMC service[3] used Bluetooth rather than Wi-Fi for the local access wireless. The advent of picocells and femtocells means that local access wireless can be cellular radio technology.

The term “seamless services” in the quotation above is ambiguous. When talking about FMC, the word “seamless” usually refers to “seamless handover,” which means that a call in progress can move from the mobile (cellular) network to the fixed network on the same phone without interruption, as described in one of the FMCA specification documents:

Seamless is defined as there being no perceptible break in voice or data transmission due to handover (from the calling party or the called party”s perspective).[4]

The term “seamless services” sometimes means service equivalence across any termination point, fixed or mobile, so for example, dialing plans are identical and no change in dialed digits is needed on a desk phone versus a mobile. A less ambiguous term for this might be “network agnostic services.”

The FMCA is a carrier organization, mainly oriented to consumer services. Enterprise phone systems are different. When Avaya announced its “Fixed Mobile Convergence” initiative in 2005, it was using a different definition. What Avaya and other PBX manufacturers were calling FMC was the ability for a PBX to treat a cell phone as an extension, and the ability for a cell phone to behave like a PBX extension phone:

Extension to Cellular technology: software seamlessly bridges office phone services to mobile devices, permitting the use of just one phone number and one voice mailbox. Client software extends the capabilities of the PBX to a mobile smartphone, creating a virtual desk extension. This software runs on Nokia Series 60 phones and works in conjunction with Extension to Cellular.[5]

In other words, this new definition of FMC didn’t include local access wireless and it didn’t include fixed broadband technology. The only defining characteristic it shared with the previous definition was seamless services, albeit without seamless handover.



Each vendor appears to have its own definition of enterprise FMC, but all their products consist of one or more of the following capabilities:

Session redirection

Session redirection means moving a call in progress from a cell phone to a desk phone or vice-versa, in much the same way as a call can transfer from one extension to another. For example, one is in a car on the way to work, listening to a conference call on a cell phone. One walks into the office, sits down, and redirects the call (session) to a desk phone. Depending on the implementation, control of the process might be from a cell phone, a desk phone or a PC, using touch-tones or something more user-friendly.

PBX mobility

This is what the Avaya press release terms “extension to cellular,” and some other vendors term “PBX extension.” The cell phone number is entered into the PBX (or third party PBX mobility device, see the paragraph below headed “PBX agnostic”), and then when someone calls the related office number, the PBX dials a cell phone over the PSTN and bridges the call. The PBX treats the cell phone as if it is an analog extension, so PBX can invoke features like hold and transfer by touch-tone commands. Thus, one can use any cell phone and any carrier (see the paragraphs below headed “handset agnostic” and “carrier agnostic”).

Treating the mobile phone as an analog extension to the PBX opens up several more possibilities. Various flavors of this service might include features like single number, simultaneous ringing and single voicemail.

Single number

Single number means that the mobile phone and the desk phone share an extension number. So only one phone number need be given out to receive calls on either a mobile or desk phone. But the cell phone likely still has its own number, it’s just that one need not give it out to anyone. To make business calls from a cell phone, one dials an access number at the office, gets a new dial tone, and then dials the destination number. This allows taking advantage of corporate least-cost-routing, and shows the office number on the caller ID display of the called person.

Single voicemail

Single voicemail is the option to use the corporate voice mail rather than the cell phone’s voice mail. This only works on calls made to an office number.

Simultaneous ringing

Simultaneous ringing means that when someone calls an office number, a desk phone and a mobile phone ring simultaneously.

When a cell phone receives a call made to an office number, the caller ID display would normally show the office as the caller, since the call is routed though the PBX. When the client software on the cell phone can pre-empt the built-in phone software (depends on the handset and client software vendor) this caller ID is suppressed and the mobility controller passes the correct calling number and name to the client software on the handset using the cellular data channel. Alternatively, depending on the PBX and carrier, the system may be able to insert the caller ID of the person calling into the regular caller ID notification (caller ID spoofing). This will show the 'correct' caller ID even on the built-in handset interface.

Client software

PBX mobility on a regular cell phone is not particularly user friendly, what with the touch-tone interface and the access number prefixing. With a smartphone things get a lot better. The definition of a smartphone is that it can run third-party software. If one has a smartphone, and it is a model supported by an enterprise FMC system, it will be able to run a “client application” that puts a friendly user interface on the PBX mobility features, allowing easy use of PBX features like 4 digit dialing to other extensions.

If the phone supports it, well written client applications can fully hide the native phone user interface. Otherwise users will have two different screens from which to dial calls: the built-in one and the client application.

RIM has built PBX signaling features into its handsets running firmware version 4.2.1 or above. Thus, Blackberries can access PBX features through menus rather than touch-tones, even without add-on client software.

Dual-mode support

A dual-mode phone is a cell phone that also has Wi-Fi. The Wi-Fi can be for data only (as in iPhone), for voice only (as in Nokia 6086), or for both.

Three main categories of wireless extensions to PBXs exist: those that work over Wi-Fi (VoWLAN, or VoWi-Fi), those that use public mobile networks like 3G, and those that use other radio technologies like DECT. Client software can make a dual-mode smartphone act as a Wi-Fi extension to the PBX. This gives the handset a split personality: a regular cell phone and a VoIP PBX extension, each having its own phone number. These two personalities can be well integrated, fully separate or something in between. Session redirection as described above moves the call between devices; with a dual-mode phone, session redirection can occur between the two networks, keeping the call on the same handset.

Well integrated dual-mode user interfaces are sometimes described as “network agnostic” (see below).

Session continuity

Dual mode handset clients can fully hide their split personality, taking the onus of session redirection off the user, and handling it automatically. When the system senses that a phone moves into Wi-Fi coverage, it transfers the call onto the VoWi-Fi side. When a phone moves out of Wi-Fi coverage, it transfers the call onto the cellular side. This is also sometimes termed “seamless handover” or “automatic handover.” To do it imperceptibly to the user is technically challenging. This automatic, seamless type of session redirection is often termed voice call continuity (VCC). The term VCC has the disadvantage that it specifically mentions voice, while FMC systems are evolving toward multimedia sessions where voice is only one of the elements. So a better term might be session continuity.

Session continuity needs client software support in the handset, either with built-in VCC client software, or (more commonly in enterprise FMC) as a part of the client software from the enterprise FMC system vendor.

Mobility controller

VCC is a term lifted from the IP multimedia subsystem (IMS) specifications[6] published by the international bodies involved with standardizing cellular technologies. In IMS terminology, VCC is done by software termed the call continuity control function (CCCF).

Session redirection and session continuity need a device in the network that routes and reroutes the call over either the fixed or mobile network as needed; something that embodies the CCCF. Many terms for this device exist, and each of these terms can also mean something else. Also the various devices that incorporate session redirection or session continuity usually also do other things. These devices have names like “mobility server,” “mobility controller,” “mobility router,” “mobility appliance” or “mobility gateway.”

Carrier FMC compared

The path of a call transits both the service provider network and the enterprise network, and the mobility controller can be located almost anywhere on that path. If it is in the service provider network the system is termed carrier-based FMC, if in the enterprise network, enterprise FMC. This is the defining characteristic of enterprise FMC.

Most carrier FMC is aimed at the consumer market, but some implementations support enterprise features like PBX mobility. Carrier-based FMC can support PBX mobility either by installing a PBX mobility control device near the PBX in the enterprise network (the method used by Tango Networks), or by offering the PBX functionality as a network service such as Centrex (the method used by Sotto Wireless).

Carrier FMC normally uses one of two technologies to implement session continuity, VCC or unlicensed mobile access, also known as Generic Access Network or UMA. UMA is an older technology, which transports GSM packets through the IP network; the handset uses the same GSM signaling stack for Wi-Fi calls as for cellular. With the predicted conversion of the carrier networks to all-IP, UMA has been superseded by VCC, which uses SIP signaling.

Handset agnostic

We mentioned above that basic PBX mobility can work with any cellular handset. At the other extreme, carrier FMC usually only works with particular handsets. For example the T-Mobile@Home service works with only three handsets, one each from Nokia, Motorola and RIM. Client software for enterprise FMC almost always works on phones that run the Windows Mobile or S60 operating systems, particularly HTC phones and the Nokia Eseries respectively. Other smartphone operating systems that may be supported include Linux and RIM, and in the future Android and iOS (iPhone). Handset agnosticism is a major selling point. A handset agnostic system is more attractive to enterprise FMC customers than one that limits the choice of handsets.

Carrier agnostic

A system with the mobility controller in the enterprise network can work with any carrier, provided the carrier will allow the phones to connect to their network. The benefit of this is that the customer gets a wide selection of phones, and the FMC system will work on employees’ personal phones, even when those phones are on an assortment of carriers.

A system with the mobility controller in the carrier network is not carrier agnostic from the point of view of the customer. They have to buy service from that carrier.

PBX agnostic

Each of the PBX vendors offers a mobility capability. Some developed it internally. Some, like Cisco or Avaya, bought a third party developer, and some license their offering from a company like Telepo, Comdasys or Counterpath (formerly FirstHand). There is another set of vendors that offers enterprise FMC that works with any PBX, for example OnRelay, DiVitas, Agito, Tango and RIM. This is beneficial to both large and small customers. Large customers may have PBXes from multiple vendors, yet still wish to roll out a unified FMC solution. Small customers appreciate having a choice of supplier, rather than being tied to their PBX vendor.

Network agnostic interface

Some vendors use this term to mean that all features are available through a uniform user interface in both cellular and Wi-Fi networks. This means that the user should not be able to perceive which network is carrying their session on a dual-mode phone.


  1. ^ Fixed Mobile Convergence Alliance
  2. ^ Fixed Mobile Convergence Alliance press release July 2004
  3. ^ BT Fusion
  4. ^ Convergence Services over Wi-Fi GAN (UMA)
  5. ^ Avaya press release announcing their FMC initiative in 2005
  6. ^ IMS Specifications from 3GPP

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