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PBX
FMC Mobiity Appliance
Media Gateway
Public Switched Telephony Network (PSTN)
Telephone Lines
T
e
l
e
ph
one
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s
Telephone Lines
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atewa
y
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h
Base

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on
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ontrolle
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Assigned Extension: 1111
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ssigned Number:
(
408
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O
wns Number:
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Owns Number:
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Ca
rri
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Enterprise Network
Extensions
C
ellular Radio
FMC Dialing Software
Wi-Fi Radio
Landline
(650) 222-2222
1) Phone registers using
SIP to PBX
2) Incoming call is routed
to PBX
3) Incoming call is routed to
Wi-Fi radio phone. FMC Dialing
Software runs the SIP engine
and handles the call.
Figure 7.5: An In-Building Call with Enterprise-Centric FMC
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PBX
FMC Mobiity Appliance
Media Gateway
Public Switched Telephony Network (PSTN)
Telephone Lines
Telephone Lines
Telephone Lines
Gateway
Switch
Base Station

Access Point
Base Station
Controller
Phone
A
ssigned Extension:
1111
Assigned Number: (408) 987-6543
Owns Number:
(408) 987-6543
Owns Number:
(408) 555-1111
Carrier Network Enterprise Network
Extensions
Cellular Radio
FMC Dialing Software
Wi-Fi Radi
o
A
E
Landline
(650) 222-2222
1) Phone registers using
SIP to PBX. The message
crosses over the Internet.
3b) The mobility appliance
sends a message to the
FMC software on the phone,
informing it of the true
identitiy of the incoming call

2) Incoming call is routed
to PBX
3a) Incoming call is routed
to FMC appliance, which
bridges the call to the
cellular telephone number
by constructing a hairpin.
4) The PBX sends the call
over the public network,
where it gets routed to the
cellphone.
Figure 7.6: An Out-of-Building Call with Enterprise-Centric FMC
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The phone call reaches the mobile phone. Instead of the normal mobile phone application
and display showing an incoming call, however, the FMC dialing software takes over and
shows its own display. This display provides the correct information about the call—if the
normal phone application had shown the incoming call as is, it would have shown the
phone number the mobility appliance used to place the outgoing call, or possibly some
other incorrect enterprise number.
Outgoing calls from the cellphone are performed similarly. When the phone is in the Wi-Fi
network, the call works as usual. However, when the phone is in the carrier’s network,
the FMC dialing software must route the call back to the enterprise, where the mobility
appliance will then bridges the call back to the final destination. This extra step is necessary
to ensure that the caller ID is correct.
Notice the similarity of the concept used behind the roaming mechanism to the one the
mobile carriers used when setting up their service on top of the public landline telephone
network. The enterprise-centric FMC solution regards the carrier as just a pipe to place
telephone calls across.
7.3.1.1 Enterprise FMC Features and Benefits

There are strong reasons for using enterprise-centric FMC. With enterprise-centric FMC, the
enterprise retains ownership of the number and the service. Because the carrier is not
involved uniquely in providing the FMC features, the enterprise remains able to change
carriers as they wish, without worrying about significant loss of service—providing that the
phones continue to work on the new carrier. Furthermore, the management of the voice
mobility network is greatly simplified, as there are a limited number of management “touch
points” between the two technologies, and the entire operation can be run out of the
enterprise by the same IT or voice staff that run the existing networks.
Enterprise-centric FMC also provides direct access to the PBX features. This allows the
user to have access to direct-extension dialing, also known as four-digit dialing. Because
the outbound calls from the cellphone are routed directly to the mobility appliance, which
then bridges the call, the PBX does not know that the phone is roaming. The mobility
appliance can then use all of the in-building PBX dialing features to complete the call, even
though the call itself is coming from the outside. A user can dial just the extension of the
other party, using the same abbreviated dialing plan as if his or her phone were in the
enterprise.
Furthermore, by bridging the call, enterprise-centric FMC solutions ensure that the PBX
remains in the call path, whether the phone is local or roaming. PBX features such as call
transfer, conferencing, and autoattendant are available to the outside user. As far as the
outside user is concerned, the phone is a true part of the enterprise PBX, precisely because
the call is always going through the PBX.
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On the other hand, this model locks the voice mobility network into the specific FMC
vendor, and further constrains voice mobility planners by working on a subset of available
dual-mode phones, and not being able to provide the same feature set and quality across
each phone. Enterprise-centric FMC is in its early days. The FMC vendors have to
interoperate with both the PBX vendor and the handset vendor, and often, because of the
FMC vendors’ sizes, this interoperability is not a relationship of peers. What works with
today’s phones may not continue to work in the next generation, and every upgrade cycle

runs the risk of causing the FMC solution to suffer.
The area of greatest risk lies with the phone software. This dialing software is required to
replace, or interfere with, the standard telephone dialing application. The reason is to ensure
that outgoing calls are routed first to the enterprise, and that incoming calls have the
requisite caller information displayed. Furthermore, the software needs to take over or use
the correct set of speakers and microphones. Cellphones often have two speakers, one
dedicated for phone calls and another used to play music or ringtones. For some of the
older models of phones, it was difficult or impossible for the FMC software to access the
voice speaker, and the audio would come out the speakerphone speaker instead. Issues
such as these restrict the phones that the FMC vendors have available to produce the
highest-quality experience. Problems with audio routing, layered applications, and user
interfaces can lead to widely different experiences on phones from even the same vendor.
Finally, there is an education process required to convince users of dual-mode phones to use
the FMC dialer and not the standard telephone dialer. The FMC vendors do a good job
making their application look and feel like a native dialer, but the presence of two dialers
can cause severe issues with user experience. If the caller uses the wrong dialing software,
the digits dialed may make no sense, the call may go through the wrong path, and the
hand-in and hand-out processes may be disrupted. This is a critical problem for calls placed
in-building, but on the wrong dialer, when FMC is explored as a way to improve the
in-building voice coverage. Dialing on the wrong network can lead to rejected or poor-
quality calls.
Enterprise-centric FMC also has an issue of draining enterprise resources. Whereas the
previous problems are all shorter-term issues, and, over time, the enterprise-centric FMC
players will be able to work through these engineering challenges, enterprise-centric FMC
has a major long-term challenge.
Enterprise-centric FMC does occupy additional PBX and telephone line resources that
would not otherwise be occupied. Every phone that is out-of-building requires twice the
number of resources when in a call than when it is in-building. This is not a significant
resource drain when the number of callers who are roaming is small, but if the FMC option
grows in popularity among the users of a voice mobility network, planners and

administrators may find that a large percentage of their calling resources become occupied
performing just the hairpinning (Figure 7.7). The enterprise PBX is usually not provisioned
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to perform the sort of bridging that a cellular gateway switch is. Classic PBX provisioning
techniques rely on assuming that most calls stay in-building, and do not occupy outside
lines. With every indoor extension possibly consuming one outside line, the demand on
provisioning of both the PBX and the links to the PSTN can become greatly inflated. This
is a distinct problem with enterprise-centric FMC that will not likely be resolved, because
the FMC appliance will bridge the call, and the hairpin cannot be torn down.
Public Switched
Telephony Network (PSTN)
Base Station
Phone
Landline
PBX
FMC Mobiity Appliance
Media Gateway
Hairpinned
Resources
Hairpinned
Resources
Figure 7.7: Hairpinning Resource Exhaustion
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7.3.2 Cellular-Centric FMC
The alternative to enterprise-centric FMC is cellular-centric FMC. In cellular-centric FMC
(Figure 7.8), the mobile operator owns the phone number. The phone is given one and only
one phone number. In this case, the enterprise PBX is not at all involved in the FMC
operation.

In cellular-centric FMC, all of the changes are in the carrier. The carrier adds another
platform, an FMC mobility gateway, to their network. This gateway fits into their existing
architecture, but is given an IP address and access directly to the Internet.
When the phone is in the cellular network, it behaves as a typical mobile phone. However,
when the phone roams into the enterprise, its carrier-centric software—built-in to the phone,
so there is no distinction between the cellular and Wi-Fi software—registers over the
Internet, back to the preprovisioned address of the cellular FMC gateway. The FMC
gateway updates the cellular network’s systems, informing them that any incoming calls
should be directed to the FMC gateway. When an incoming call is placed, the mobile
operator’s switch routes the call to the FMC gateway, which converts the voice into IP
packets and sends those packets across the wide-area Internet, to the enterprise. The packets
are placed on the Wi-Fi network, and arrive at the cellular telephone, which decodes the
packets and plays them out.
Outgoing calls are performed in the same way. The phone uses the Wi-Fi radio to signal the
start of the call, and then directs the bearer traffic over the Wi-Fi network, through to the
Internet, where the FMC gateway converts them back to the digital circuit and through the
mobile operator’s network.
Cellular-centric FMC fits directly into the existing mobility architecture for the carrier. As
far as the rest of the carrier’s network is concerned, the phone is using a typical base station
on a typical base station controller. The mobility gateway just happens to be that controller,
and tunnels the signaling and bearer traffic over IP, rather than continuing down on circuits
to the remaining parts of the network. The elements of the base station itself is left out, as
the air interface has been replaced with an IP interface.
7.3.3 Cellular-Centric FMC Features and Benefits
Cellular-centric FMC’s advantage is that it does not use or tax the enterprise PBX. There
are no voice network provisioning requirements for cellular-centric FMC. This is a great
advantage to environments where either a large percentage of the workforce will be roaming
outside the enterprise. It is also an advantage to organizations where their existing PBX
infrastructure does not have the capacity to provide the mobility forwarding, or even the
extensions for the mobility devices. This latter part is true for organizations that are just

embracing voice mobility, and may have had a smaller number of desk phones but are
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FMC Mobiity Gateway
Public Switched Telephony Network (PSTN)
Telephone Lines
Internet Traffic
Gateway
Switch
Base Station Access Point
Base Station
Controller
Phone
Dedicated Lines
Assigned Number: (408) 987-6543
Owns Number:
(408) 987-6543
Carrier Network Enterprise Network
Extensions
Cellular Radio Wi-Fi Radio
Figure 7.8: The Cellular-Centric FMC Architecture
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looking for expanding the reach of the network at the same time as providing mobility, as a
part of a packaged upgrade.
The economic advantages are still preserved in cellular-based FMC. When the phone is
using the Wi-Fi network, the mobile operator will usually charge either less for the call or
not at all. They do this in part because it frees up resources on their network, allowing them
to add extensions for nearly free, and thus making a higher profit on the monthly service
fees. This can provide a method for them to expand their business without expanding their

expensive cellular resource allocation step-by-step.
Furthermore, the integration of the technology is far more complete than with enterprise-
centric FMC. There are no issues with having software to install or manage on the phone
itself, as the software is a part of the phone’s operating system. There is only one dialing
application, one address book, and one feature set for a user to be concerned with. This
provides both less usability headaches and reduced management complexity.
There are two obvious challenges for cellular-based FMC. The first one, a short-term
challenge, concerns the lack of enterprise PBX integration. Cellular-based FMC today has
no integration with the enterprise network. PBX features are not available, therefore, and
every extension becomes a public phone. Cellular-based FMC has primarily, up until now,
been targeted mostly at consumers. As carriers enter the enterprise market, they are likely
to add other methods of enterprise integration. Most certainly, given that many of the
cellular-centric FMC phones on the market today are enterprise-oriented phones such as
the Research in Motion BlackBerry, the enterprise data integration is already strong.
Specifically, Research in Motion offers software solutions to extend the PBX dialing plan
and features out to the phone, with or without FMC. The need for enterprise PBX
integration is somewhat diminished, and the avenue for further enterprise services
integration is available. Even in environments with well-provisioned private telephone
networks, there are some where users will insist on trying a person’s cellphone first, and the
desk phones remain mostly idle. These environments will do well with cellular-centric
FMC.
The second challenge is one of provisioning a high-quality Internet link to the carrier.
Cellular-centric FMC relies on sending real-time streaming video over the Internet, a
network not directly designed for quality-of-service traffic. Especially as the number of
in-building users grows, the traffic on this one link to the outside can become congested.
Just as enterprise-centric FMC makes additional demands on the PBX and PSTN link,
cellular-centric FMC makes additional demands on the router and Internet link. Additional
access bandwidth to the Internet may be required, as cellular-based FMC becomes more
adopted. While the FMC bearer packets are within the enterprise, they are stamped with the
correct DSCP tags to allow routers, switches, and the access points to provide the proper

quality of service. Nevertheless, there is no natural quality-of-service on the path out from
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the enterprise to the carrier. It remains to be seen if carrier-centric FMC operators will work
with Internet service providers to provide a more reliable and quality-protected network
between the enterprises and the mobile operator’s networks.
7.4 Handoff Between Different Networks: Handing In
and Handing Out
The descriptions so far have concentrated on the two types of architectures for creating an
FMC voice mobility network. Both architectures address the problem of finding a mobile
phone, when it is roaming in a different network than the one that owns the phone number.
However, because FMC provides the illusion of seamlessness or homogeneity between the
carrier and enterprise networks, users may expect that they can carry a live phone call
between the networks, without having to drop it. Just as with cellular operators alone, the
combined solution must provide a way to hand off between the two networks, or at least
allow the user to predict where one network ends and another begins. This is the FMC
handoff problem.
7.4.1 The Handoff Problem
The problem of call continuity itself—transferring the call without disruption between the
two networks—may not need to be solved, for many FMC voice mobility networks. For
networks and organizations where users are either in-building or on the road, the lack of
call continuity may not be a concern. Of course, it is inconvenient to lose a call as the user
walks inside or outside a building, but physically entering or exiting a building is sufficient
an event that the user can be trained to expect a disruption, and plan accordingly.
However, all FMC technologies work to provide call continuity, as one of the expected
benefits of a unified system and as a powerful selling tool. Each of the two architectures
handles the call connection or transfer in a different way, but the principles on actually
deciding when or how to make the transition are similar for both.
The problem the phone needs to solve is to determine when it makes sense to put in for a
transition. Unlike with straight mobile networking, where different base stations can see the

phone and can possibly coordinate to decide when the best transition time is, with FMC
each network can only see its own side of the solution. The cellular network cannot use
pure cellular control—not without substantial mobile assistance—because only the phone
knows whether it is in range of the appropriate Wi-Fi network. On the other side of the
puzzle, the Wi-Fi network is good any knowing where the client is, but does not know
about any other network.
From the phone’s point of view, it is given two tasks that must be performed
simultaneously: evaluate the Wi-Fi network for suitability for voice, and ensure that the
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cellular connection is good. If either side breaks, the phone must immediately register back
with the central mobility platform to inform it of the loss of a link. When a call is in
progress, this can be a substantial task.
Wi-Fi complicates the picture. Cellular networks are designed to keep the phone
associated with the base station that will provide continuous access to voice resources. The
phone’s cellular engine will, under the network’s control, maintain a connection to the
mobile operator unless the phone is truly going out of coverage. The Wi-Fi radio
does not work that way. Unfortunately, as you saw in Chapter 5, the principles of Wi-Fi
require that Wi-Fi make numerous handoffs as the mobile makes its typical motions
throughout the network. These Wi-Fi to Wi-Fi handoffs, also called horizontal handoffs,
because they work within the technology that is providing service, appear to the phone as
constant service disruptions. These disruptions may be brief, or they may last for long
periods of time, but they are generally false, as the phone is not leaving coverage from the
network itself, but just leaving coverage from elements of the network and entering
coverage of other elements. Because the phone cannot know when it is leaving the network
for good, versus when it is just exiting one small Wi-Fi cell and entering another, the
phone’s FMC engine has to weigh whether the waning Wi-Fi coverage is sufficient enough
to warrant a handout, or a handoff outside of the Wi-Fi network. Should the phone and
supporting services make the wrong decision, the phone can alternatively end up with no
Wi-Fi coverage when the phone has not prepared to transition to cellular, or end up on

cellular when the phone is within the building. The latter can have a direct monetary impact
on the voice mobility network, because minutes spent on the carrier’s network may not be
free, and the point of many FMC solutions is to minimize the amount of money spent on
the carrier.
There are broadly two methods that this is being handled by FMC today, both of which can
be complimentary. One method works specifically for enterprise-centric FMC, and involves
using Wi-Fi intelligence on the phone and FMC appliance to make a better call. The general
idea is to require the voice mobility planner to enable location tracking on the Wi-Fi
infrastructure, using location training (walking from point to point and marking where the
point is on a digital map of the building) to let the FMC appliance learn where the edges of
the buildings are. When the phone remains within the building, the FMC appliance and the
phone will resist handing out to the cellular network, prioritizing Wi-Fi over cellular. This
does subject the caller to whatever handoff quality the Wi-Fi network has, which in itself
can be difficult to tolerate, but it does allow the enterprise to save money on the transitions.
As the phone enters the edge of the building, the appliance and the phone switch the
preference, favoring the cellular network, if not requiring it outright. Therefore, as a user
heads to the edge of the building, the mobility appliance will detect this and start preparing
the phone for a potential transition to the outside. In reverse, as a phone enters the building,
the mobility appliance can detect this and allow the phone to predictably hand in, or hand