VoIP over WLAN; Technical and Business Aspects
Cygate Oy, Vattuniemenkatu 21, 00210 Helsinki, Finland
The usage of Voice over IP protocol technology has There are different ways how VoIP communications can
been praised by technology analysts as the next hot thing be done. Possible categories include:
in networking industry for almost a decade. However,
actual deployments in large scale have been done only • use VoIP as a replacement for trunk
during the last couple of years. This does not mean that connections between class 4/5 switches
VoIP is not suitable for production environments. It only
• use VoIP as a replacement for trunk
means that development of feature-rich and reliable
connections between PBXes
VoIP products that users can trust on their
• use VoIP for end-to-end telephony in
communications needs has taken longer than what was
enterprises (ie. use IP-PBX)
• use VoIP for end-to-end telephony for
On the other hand, the world seems to be heading consumers
towards wireless. Big hopes have been piled up to the
next generation cellular network technology. It is In addition, VoIP can be used for hosted PBX solutions
supposed to be the key to enable feature- and content (ie. IP-Centrex) and to provide add-on services like
rich mobile services. Clearly operators will be offering voice and mail integration.
services based on 3G but it is estimated that 3G will be
available only in large cities and especially contect rich Of the usage categories, especially third and fourth have
services will be quite expensive. gained more interest recently. Enterprises are replacing
their legacy PBXs with ones that are future-proof ie.
My opinion is that there is a need and opportunity for a with IP-PBXs. At the same time, old telephones are
disruptive technology to compete against 3G cellular replaced with IP-based models. With this, companies
networks. This could be wireless LAN (WLAN). WLAN usually deploy some advanced services like contact
has originally been developed as a data transfer platform software and integrated voice messaging.
but the developments in that and in VoIP mechanisms
make it a possible technology to offer both data and On the other hand, consumer telephony, which has
voice services. already moved towards mobile communications is
shifting a little bit back to fixed-line telephony.
However, this is also done with new components.
1 Introduction Alternative ways to have new kind of telephone service
at home is to have a broadband Internet connection. The
Voice over Internet Protocol technology and its usage for end device for telephony can then be:
all voice communications seems to be the ultimate goal
for data networking industry. This development seems to • traditional telephone with adapter
be well in line with Professor Christensen’s ideas of • VoIP telephone
disruptive technology [Christensen, 1997]. When IP • softphone at PC
networking was introduced, it was seen inferior
compared to other data networking technologies. This is Wireless networking is another area of rapid
one of the signs of disruptive technology and actually IP development and deployment. There are various
is used as an example in this book. When IP gained methods to do wireless data networking but among the
success, industry begin to consider it as a transport most interesting ones is WLAN. According to e.g.
vehicle for voice communications as well. There were [Synergy Research, 2004], WLAN markets are really
incentives to use Frame Relay or ATM encapsulations boosting currently.
for voice [ATM Forum, 2004; Frame Relay Forum,
1998]. Whilst these are used a lot, major trend seems to WLAN is often used in environments called hot-spots.
be shifting towards IP-based voice communications. This can be e.g. access point at home, trough which
This is confirmed by several market studies e.g. [IDC, consumer connects to a xDSL-based Internet connection.
2003]. It can also be a network of hot-spots, operated by a
service provider. In this case, the network could cover • Access Points, that are used to create wireless
public places like stations, airports, malls and café’s. cells.
• Serving IP network that is used to connect all
As said, WLAN is primarily intended to be used as a the components together.
data network. However, recent additions to WLAN • Backoffice servers to handle e.g. user
protocol specifications have led to a situation, where authentication, authorization and accounting
WLAN is also considered to be used for real-time services.
information transmission. Primary usage would be VoIP • Wireless Telephone Switch (WTS) to handle
but other possible applications include also e.g. Video calls inside wireless area and between wireless
over IP. network and outside world (PSTN).
These two strong networking trends – VoIP and WLAN The basic steps to handle wireless calls go as follows:
– seem to be converging to offer new kinds of 1) User telephone is associated with a certain AP.
communication possibilities. This paper examines VoIP 2) User wants to make a phone call. Request to do
over WLAN techniques, whether or not this is possible this is signalled to WTS that either permits or
from a technical and business perspective. The paper is denies new calls. New calls are permitted, if
organized so that chapter two is devoted to technical there is capacity available in the wireless cell.
issues and chapter three to business issues. Chapter four 3) If the call is intra-area (ie. within the wireless
is for a brief summary and conclusions. This paper system), signalling is done via WTS but the
assumes that the reader is familiar with the basic actual call flows through the serving IP
concepts of both VoIP and WLAN networking. network.
4) If the call is destined to some other network
2 Voice over WLAN; Technical (other wireless system, PSTN, mobile network
etc.), a gateway is used to handle this.
Voice over Internet Protocol is a complex issue of it’s As said, there are numerous technical problems in this
own. When an unreliable wireless link that has limited otherwise quite simple architecture. Next I will introduce
capacity as a transmission medium is used for the most significant of them.
transmission increases this complexity. I will investigate
each of the possible technical limiting factors one at the 2.1 Cell coverage
time. But before that it is good to take a look at the
general topology of a VoIP-WLAN network. Figure 1 Current WLAN physical layer techniques include
illustrates the main components of such a network. 802.11b, 802.11g and 802.11a/h. Each one of these have
certain characteristics in terms of the area that one cell
can cover. This range also influences the available
speeds. Table one, taken from [Cisco 2004], shows one
measurement of the bandwidth-distance relation for
different technology brands.
Table 1: Range of certain 802.11 flavors
Data rate 802.11a 802.11g 802.11b
54 13 m 27 m -
48 15 m 29 m -
36 19 m 30 m -
24 26 m 42 m -
18 33 m 54 m -
12 39 m 64 m -
11 - 48 m 48 m
9 45 m 76 m -
Figure 1: VoIP-WLAN architecture
6 50 m 91 m -
5,5 - 67 m 67 m
The main components in a VoIP-WLAN network are: 2 - 82 m 82 m
• Mobile nodes ie. devices that are capable of 1 - 124 m 124 m
handling VoIP calls over WLAN media.
As can be seen from the table, one cell can cover at especially new physical layers for 802.11 networking
maximum on area that is about 48 000 square meters. such as 802.11n  will propably change this in the
But since the signal is quite weak and sensitive to future.
physical obstructions, this range is usually reduced a lot.
This might be a problem, since this means that WLAN- 2.3 Interference
based solutions can be used only in city areas. Even
One of the potential problems related to WLAN
there, it might be problematic. For example, Helsinki´s
networking is that especially 802.11b and 802.11g are
land area is 186 km2. This means that in ideal case we
using unlicensed 2,4 GHz ISM band, which is totally
would need 3850 APs to cover Helsinki area. Not only is
free to be used by anyone. This might lead to
the equipment needed expensive but all the connections
interference in transmissions.
from the serving IP network to APs would also cost a lot.
But we will touch these issues more in business section
The risk of interference becomes especially big, if
of this paper.
802.11 networks are implemented by many operators in
the same geographical areas. Since the bandwidths and
2.2 Capacity channels are not usually planned between operators, this
What follows immediately from the discussion of the will lead to problems. The only way to prevent these
coverage area, is the capacity of the cells. Table 1 problems is to have some sort of system through which
showed also the maximum bandwidths that can be used interference is minimized.
in different distances from the AP. However, this figure
does not show directly how many simultaneous calls can Interference can be reduced by using channels that are
be active within the range of one AP. Of course, this not too occupied. IEEE 802.11a/h that use 5 GHz
depends heavily on the amount of bandwidth one call frequency band are in a much better position than 2,4
takes. To have an idea of that, table 2 [www.erlang.com, GHz alternatives.
2004] shows the most important codec´s IP bandwidth
usage. 2.4 Channel MAC
802.11 networks have two different MAC mechanisms
Table 2: Bandwidth used by different voice codecs defined. These are DCF (Distrubuted Coordination
Function) and PCF (Point Coordination Function). DCF
Coding Name Bandwidth Sample IP is the one that is used by the APs that are currently on
(kbps) (ms) bandwidth the market. DCF is offering MAC services for normal
(kbps) data traffic. PCF was planned to offer features that allow
G.711 PCM 64 0,125 80 real-time traffic transmissions. PCF has not been
G.723.1 ACELP 5,6 30 16,3 currently implemented by any 802.11 device in common
6,4 17,1 markets. One reason for this is that despite its premises,
G.726 ADPCM 32 0,125 48 PCF has been shown to be inadequate for the task it was
G.728 LD- 16 0,625 32 designed to handle.
G.729(A) CS- 8 10 24 For this reason, 802.11e task group has been designing
ACELP new MAC mechanisms for the past couple of years. Two
new possibilities are EDFC (Enhanced DCF) and HCF
Several studies have been done to determine, how many (Hybrid Coordination Function) [Garg, 2003]. Each of
simultaneous calls one AP can handle. The figure is these provide eight different classes of service which is
disappointedly low. According to [Chen, 2002], the adequate for VoIP purposes. In addition, HCF takes also
number of simultenous calls is 15 before observed care of DiffServ/IntServ architecture support.
speech quality is lowered via increased delay. The same
kind of figure is also considered as the upper limit in The real problem here is to finalize 802.11e and get it
[Anjum, 2003; Baek 2003]. One of the reasons for this implemented by different 802.11 vendors. This might
phenomenon is that even if we have 11 Mbit/s (802.11b) take some time thus leading to a situation, where
or 54 Mbit/s (802.11a, g) physical layer capacity, 802.11e cannot be much of a help to provide better
goodput is much lower. Goodput varies depending e.g. service for interactive traffic classes such as VoIP.
on the radio conditions. For example, for TCP
(Transmission Control Protocol) connections, goodput 2.5 Roaming
has been measured to be at maximum half of the nominal
speed [Gast, 2003]. Roaming is one of the major issues that need to be taken
care of in mobile networks. In 802.11 networks, there are
Bandwidth will be a limiting actor on VoIP-WLAN two roaming possibilities. One is within an ESS
networks for a while. However, new voice codecs and (Extended Service Set) and the other is between them.
2.7 Mobile Telephone Switch and
Roaming within an ESS is already specified in 802.11 gateways
standards. However, the standard defines roaming quite
loosely and therefore there is ongoing work that tries to One technical issue is MTS and gateways that connect
address this issue. The basic idea behind fast roaming is the whole VoIP-WLAN service area to the rest of the
efficient surveillance of radio conditions. How this is network. There are three main alternatives here. One is
done in 802.11 environments is specified by 802.11k to implement a real SoftSwitch with all the features and
[Drew, 2003] task group. functionality as in real legacy class4/5 telephone switch.
This is not very cost-effective although if there are a lot
When a WLAN station moves inside one ESS, it of users, it might be needed.
preserves it´s IP address and also all the upper layer
connections. Second alternative is to buy an IP-PBX and use it as a
tool to handle customers. Current IP-PBXes can handle
Roaming – or actually mobility between ESSes is quite some 15 000 – 100 000 subscribers depending on the
another story. Once a station moves to a new ESS, it will server’s physical configuration. Via IP-PBX extended
have to change IP address. This leads us to a situation, functionality can be provided to VoIP users.
where all upper layer connections will drop unless there
is another mechanism to take care of them. For WLANs Third possibility is to forget functionality and offer just
the mechanism that will be used is same that is used by enough intelligence in the network to get calls done. This
3GPP2 ie. Mobile IP [Mobile IP Working Group, 2004]. alternative is handled by a SIP (Session Initiation
Protocol) server [see SIP Forum, 2004].
Based on these two roaming and mobility possibilities, I
argue that this issue is taken care of in WLAN networks. All these different alternatives need a way to connect
VoIP-WLAN network to traditional networks. This is
done via a gateway component. There are multiple
2.6 Security telephony gateways in the market.
During the past few years, trade press has announced
that WLANs are inherently insecure and they should not Be the choice any of the possibilities for offering central
be deployed and used without extreme precautions. This and gateway services, all these can be done. So, these
has been true but situation has changed and security of should not limit the deployment of VoIP-WLAN
WLAN networks is improving constantly. networks.
For VoIP-WLAN service, we would need the following 2.8 VoIP-WLAN end user equipment
two major issues to be taken care of:
• mutual authentication (user and network) There is not much to be done with a VoIP-WLAN
network, if there are no terminals that can function in
• encrypted radio network transmissions
one. Fortunately, lately there has been quite a lot of
development in this area. There is a bunch of VoIP-
The authentication methods that are planned and already
WLAN capable devices from different vendors and the
in use in 802.11 networks are based on WPA
amount of them is steadily increasing.
specification [Wi-Fi Alliance, 2003]. WPA specifies that
authentication is based on 802.1x general authentication
However, the first generation VoIP-WLAN equipment
scheme. Other important components are EAP
seem to suffer from multiple problems. The main issues
(Extensible Authentication Protocol) and RADIUS
seem to be related to power consumption (short
(Remote Access Dial-In User Service). EAP allows the
functioning times), battery heating (unpleasant to speak
usage of any actual authentication mechanism. The most
with a hot phone) and coverage. The last question we
interesting proposals include PEAP (Protected EAP),
have already discussed in section 2.1. The other
EAP-TTLS (EAP – Tunneled Transport Layer Security)
problems will propably be fixed over time.
and EAP-SIM (GSM SIM-card based authentication).
Encryption of traffic in radio link is still handled by RC4 2.9 Summary of technical issues
cipher. However, significant changes has been made to Based on the previous discussion, I state that range
it. For example IV (Initialization Vector) is longer and a (coverage), capacity, interference and MAC seem to be
message integrity code has been added. problematic for current VoIP-WLAN network
implementations. Of these, capacity and MAC will be
Together these two aforementioned issues make WLAN improved in future releases. Interference can be
networks secure enough for even sensitive usage like decreased by using relatively free 5 GHz, which is
phone calls. specified as the band for 802.11a/h. Range will be a
problem unless transmission power can be increased. But Based on these answers, VoIP-WLAN can continue to
this is up to regulators, not a technical issue per se. the second step that has also two questions. Both of these
need to answered positively.
The following issues seem to be already in place:
Roaming, security and network components and For enterprises:
addional VoIP mechanisms. 1) It seems that this segment is more or less overserved.
But the determination of worse product needs to be
3 Voice over WLAN; Business issues considered. As discussed in section 2.8, functionality and
features of existing VoIP-WLAN equipment seems to be
Next I will turn the focus to business issues. I will base worse than those of the existing cellular phones.
this section on a discussion about the evaluation methods However, if we consider mobile communications as a
for disruptive technologies that can be found from service, it seems that most of the enterprises would be
[Christense, 2003]. Professor Christensen argues that to happy to go for a cheaper service even if the quality is a
be a disruptive innovation, one has to pass the litmus test little bit lower. With own VoIP-WLAN network inside
illustrated in figure 2. firm’s office, a company could tailor different kinds of
add-on services like follow-me.
2) I assume that it is difficult to build a business case
where an operator would provide VoIP-WLAN
infrastructure services for multiple companies but as
discussed above, a company can easily justify
investments to build own VoIP-WLAN network as a
way to reduce otherwise high mobile communications
1) Since the advent of MVNOs (Mobile Virtual Network
Operators), it seems that mobile communication needs of
ordinary consumers are satisfied and this group is not
Figure 2: Determining whether a technology is overserved. However, since mobile communication is
disruptive (according to Christensen) still rather expensive (around 10 cents per minute), a
I will study the technology on hand from three different fixed fee –based rating would be effective. Fixed fees
perspectives. These are a) enterprises, b) consumers and with certain limitations is actually what Finnish MNOs
c) network operators. and MVNOs are trying currently to attract customers.
Maybe extra low-cost fixed montly fees could work for
First step of the flowchart states two questions. The VoIP-WLAN operators as well.
answers for these would be:
2) Since VoIP-WLAN network is rather simple
For enterprises: (compared to e.g. 3G network), building and operating it
1) Yes, mobile communications have been bought as a should not be too costly. Therefore I assume that even
service from different mobile operators. with moderately low monthly fees, a profitable business
2) No, mobile communications have been available case can be found.
everywhere (it is the idea of them!)
For consumers: 1) Yes. All the mobile operators are overserved by
1) This depends. Traditionally mobile communications existing mobile network equipment vendors. Actually,
have been bought as a service (and it has been available vendors want to serve operators so well that they even
to everybody). But mobility within a limited area (say, take care of network operations on behalf of the
inside one´s home) can be done with VoIP-WLAN operators. What is left to operators is customer
without operators. relationships and billing. However, all this comes with a
2) No. price. To be able to build a low-cost network with small
operational expenses could attract some parties. Or
For operators: actually it already has. In Finland, Vantaan Energia
1) For legacy mobile operators, no. But for operators that (local electricity provider) has announced that as soon as
do not have own mobile network, this opens up new WLAN phones are ready, they will offer VoIP-WLAN
kinds of possibilities to offer services to consumers. services.
2) Well – it depends but assuming that consumers accept It is not an easy task for a consumer to select proper
the technical disadvantages of VoIP-WLAN, then a case handsets that match his/hers usage patterns. Even more
can be done. difficult it is for companies’ ICT managers and network
operator CTOs to understand and analyse, which mobile
Last question is that is this technology truly disruptive to network(s) should be built and supported. This study of
all market players. To my best knowledge, this is a fact. one alternative VoIP-WLAN gave ideas about one
Compared to existing and forthcoming mobile technology.
communication methods, VoIP-WLAN is clearly inferior
and cheaper to build. There are no services nor operators Currently VoIP-WLAN is almost there. All the main
that could see VoIP-WLAN as sustaining technology components are ready for VoIP-WLAN to be used
development to their existing business. succesfully in corporate environments. But it still takes
some time before VoIP-WLAN is ready for operator
3.1 Summary of business aspects networks. However, with the advent of 802.11n, 802.11e
and 802.11i (security), most of the problems will be
To summarize the business aspects based on the
solved. Even when it is ready for operator usage, VoIP-
framework by professor Christensen, I argue that VoIP-
WLAN is never going to be a mainstream technology. It
WLAN is disruptive technology especially for enterprise
will be a niche technology that will be used for specific
mobile communication needs. Therefore this is the place,
user groups in specific areas.
where wise vendors will start their efforts. When the
technology matures, propably a limited-area, limited-
user VoIP-WLAN metropolitan area network can also be 5 References
Christensen, C. 1997. Innovator´s Dilemma, Harvard
3.2 Rivals of VoIP-WLAN Business School Press
Despite the fact that all the hype is still going towards
3G networking, there are developments at other fronts as ATM Forum. 2004. Approved Specifications for Voice
well. VoIP-WLAN is just one example. VoIP plans have & Telephony over ATM,
also been formed for Bluetooth. The idea here is to use http://www.atmforum.com/standards/approved.html
Bluetooth for network connectivity and transmit VoIP
packets over this connection. Another interesting Frame Relay Forum. 1998. Voice over Frame Relay
concept is IEEE 802.20, Mobile Broadband Wireless Implementation Agreement (FRF.11.1),
Access, which is building specification for efficient http://www.mplsforum.org/frame/Approved/FRF.11/frf_
packet based radio interface that is optimized for the 11.1.pdf.
transport of IP based services. Main issues for 802.20 are
that it operates on licensed bands (3,5 GHz), it can offer IDC. 2003. Voice/Data Convergence Moves from Hype
speeds up to 1 Mbps and it can be used to build similar to Reality in European WANs, says IDC, Press Release .
coverage as 2G/3G networks. What is especially nice
about 802.20 is that it is designed purely for IP and Synergy Research. 2004. WLAN Market Eclipses $2.5
VoIP. It does not have any legacy burden that 3G Billion Mark, Press Release.
systems will have. Working group for 802.20 tries to
finalize its job before the end of 2004. Cisco Systems. 2004. Capacity, Coverage and
Deployment Considerations for IEEE 802.11g,
4 Conclusions prodlit/80211_wp.pdf.
We are certainly living interesting times in the mobile
communications sector. If only had 3G networks and Www.erlang.com. 2004. Effects of coding algorithms,
handsets been ready when the first licenses were issued http://www.erlang.com/bandwidth.html
in 1999, we would have fully operational 3G networks
without any great needs to diversify our development Chen, D., Garg, S., Kappes, M., Trivedi, K. 2002.
efforts to different kinds of technologies. But since this Supporting VBR VoIP Traffic in IEEE 802.11 WLAN in
did not happen, we are facing a world in which a future PCF Mode.
mobile telecommunications consumer might face an
avalanche of options. The network can be IMT-2000 Anjum, F., et.al. 2003. Voice Performance in WLAN
based. If this is the case, then there are five different Networks – An Experimental Study, Globecom 2003,
options to be used. Then the connection might be http://www.coe.montana.edu/ee/rwolff/EE548/papers/vo
Bluetooth or WLAN. Propably 802.16 will be included ice%20over%20IP.pdf
somehow and for sure 802.20 will be available.
Baek, S., et.al. 2003. Capacity Estimation of VoIP
Channels on Wireless Networks, 2003,
Gast, M. 2003. When 54 Is Not Equal to 54? A Look at
802.11a, b and g Throughput,
Coffey, S. 2003. 802.11n: Defining a high-throughput
Garg, P., et.al. 2003. Using IEEE 802.11e MAC for QoS
over Wireless, IPCCC´03
Drew, R. 2003. Sidebar: 802.11k – Management
Standard Ahead, Computerworld
Mobile IP Working Group at IETF. 2004.
Wi-Fi Alliance. 2003. Wi-Fi Protected Access, White
SIP Forum. 2004. www.sipforum.org
Christensen, C. 2003. Innovator´s Solution, Harvard
Business School Press