Wireless Convergence Architecture: A Case Study Using GSM and ...
Mobile Networks and Applications 7, 259–267, 2002
2002 Kluwer Academic Publishers. Manufactured in The Netherlands.
Wireless Convergence Architecture: A Case Study Using GSM
and Wireless LAN
NIKOS A. NIKOLAOU
Bell-Labs, EMEA, Botterstraat 45, 1270 AA, Huizen, The Netherlands
KONSTANTINOS G. VAXEVANAKIS, SOTIRIOS I. MANIATIS and IAKOVOS S. VENIERIS
National Technical University of Athens, Heroon Polytechniou 9, 157 73, Athens, Greece
NICHOLAS A. ZERVOS
Ellemedia Technologies, Syggrou 223, 171 21, Athens, Greece
Abstract. The evolution of wireless networks has motivated the expansion of the static business environment to a mobile and wireless
one. However, current and forthcoming wireless technologies are characterized by different attributes, regarding coverage area, offered
bandwidth and delay. The transparent conjunction of various wireless technologies into a single mobile terminal can further boost the
wireless explosion. This paper presents the Wireless Convergence Architecture (WCA) that incorporates different wireless interfaces under
the same mobile terminal. Depending on the location and the availability of the underlying wireless network, automatic and seamless
switching is performed to the most appropriate wireless network interface. Special care is taken to maintain continuous connectivity at the
transport layer (TCP). WCA introduces software components both at the terminal and network side. A speciﬁc implementation is presented,
based on two complementary wireless technologies – in terms of coverage area – an IEEE 802.11-compliant in the short local and a GSM
in the wide area. To demonstrate the operational correctness of the architecture, experiments were performed using standard and proprietary
Keywords: wireless networks convergence, TCP/IP, mobility, wireless LAN, GSM
1. Introduction ious wireless systems could be utilized in a complementary
manner. More particularly, for in-building, as well as in-
The trend in network evolution favours the presence of var- premises communication (indoors), a Wireless LAN (WLAN)
ious network technologies that reveal different characteris- technology is appropriate. Existing solutions in the local area
tics, in terms of coverage area, underlying physical medium, include the IEEE 802.11 , ETSI HIPERLAN , and the
medium access control (MAC), available bandwidth and de- Digital Enhanced Cordless Telecommunications (DECT) sys-
lay. Local and wide area, wireline and wireless, narrow-band tem . The two former wireless networks are succeeded
and broadband, are pairs of opposing attributes, which are ex- by IEEE 802.11a and ETSI BRAN , respectively. More-
pected to co-exist in the plethora of network systems. Despite over, it is an initiative of both working groups to allow mu-
those variations, the Internet Protocol (IP) appears to be the tual accesses to each groups’ technical documentation, so that
common denominator. a global standard will arise. In the future, the Wireless ATM
At the same time, various wireless technologies, with dif- standard  could also provide efﬁcient wireless data com-
ferent pros and cons, are becoming more popular, as they munications in the local area.
eliminate the disadvantage of having the user restricted in Regarding the wide area (outdoors), the most predominant
a particular location. Wireless systems constitute a mature technology today in Europe is the Global System for Mobile
solution that can be deployed easily, decreasing the overall communications (GSM ). However, recent advances in
installation cost. Based on these observations, it is foreseen the cellular communications include the General Packet Ra-
that, in the near future, the expansion of the working envi- dio Service (GPRS)  and the High Speed Circuit Switched
ronment to a global one will be based on different wireless Data (HSCSD) , which provide much higher bandwidth
access technologies, interworking with each other transpar- and quality than GSM. Next generation realizations are mov-
ently. The vision for the next generation business environ- ing towards the Universal Mobile Telecommunications Sys-
ment comprises of various wireless technologies that create tem (UMTS) , while global access can also be provided
a uniﬁed virtual world for the end-user (ﬁgure 1). through satellite communication.
Current wireless networks provide connectivity, targeting A common difﬁculty faced by mobile users, which actu-
speciﬁc environments that pose restrictions in terms of avail- ally contradicts their “mobile” nature, is that they are con-
able bandwidth, coverage area and cost. As a result, the mo- strained by the coverage area of the wireless technology used.
bile user is limited by the wireless technology, although var- A straightforward approach to address this issue is to utilize
260 N.A. NIKOLAOU ET AL.
Figure 1. Vision for the next generation mobile environment.
multiple wireless adapters that have complementary coverage vironment, along with the various connectivity scenarios that
areas. For instance, one adapter could be active, while in- can be supported. Section 4 discusses the experiments and
doors, and a different one while outdoors. the obtained results, along with some performance evaluation
Introducing two or more wireless interfaces in the same issues. Finally, conclusions and future extensions are summa-
mobile terminal does not automatically solve the problem. rized in section 5.
First of all, the terminal needs extra intelligence, so that it can
accurately determine the boundaries between wireless net-
works and switch from the one that ceases to be operational to 2. Wireless Convergence Architecture overview
the most appropriate. Thus, location transparency is the ﬁrst
There are many different wireless technologies that provide a
requirement that must be met.
stable networking solution for the mobile user. However, they
Additionally, the fact that wireless networks are more sus-
usually pose location restrictions (e.g., WLAN or DECT) that
ceptible to link breaks (e.g. due to physical obstacles) should
make them unavailable when moving outside their coverage-
be carefully considered. Temporal link breaks may disrupt
area. Even when they provide global access (e.g., GSM or
established TCP sessions – due to time-outs – forcing them to
Satellite), they have the disadvantage of an extremely high
termination. Thus, a second requirement to be fulﬁlled is the cost that discourages their usage in a regular basis.
resilience against wireless link breaks. In order to free the mobile user from location restrictions
This paper introduces the Wireless Convergence Architec- the mobile terminal must be equipped with multiple wire-
ture (WCA) that caters for the aforementioned requirements, less network adapters. Depending on the current location
introducing new software components, both at the terminal of the user, the most appropriate will be activated. How-
and the local Intranet. WCA is ﬂexible enough to incorporate ever, due to hardware limitations, mobile terminals cannot
different wireless interfaces under the same mobile terminal. incorporate an unlimited number of wireless Network Inter-
Depending on the location and the availability of the under- face Cards (NICs). Following the distinction between indoors
lying wireless network, automatic and seamless switching is and outdoors communications, it seems reasonable that two
performed to the most appropriate wireless interface, while adapters – one for each case – would be sufﬁcient. Neverthe-
transport sessions (TCP) are maintained alive. less, this argument should not be considered as an architec-
Seeking for complementary technologies that could pro- tural restriction.
vide connectivity for both indoors and outdoors, two spe- Using multiple wireless interfaces provides a working
ciﬁc wireless technologies have been selected, namely the framework to support user mobility, but it does not solve all
IEEE 802.11 Wireless LAN and the Global System for Mo- the problems. There are two major issues that must be suf-
bile communications. Combining these technologies, connec- ﬁciently resolved, namely the location transparency and the
tivity can be achieved not only while being indoors (e.g., in- resilience to temporal wireless link disconnection. To ade-
side the company’s building), but also outdoors (e.g., when quately address them, new software components have to be
moving away from the corporate premises). Taking into incorporated both at the terminal and the network side (local
account this speciﬁc implementation, a reference network en- Intranet), as shown in ﬁgure 2. As far as the network side is
vironment has been determined and experiments were per- concerned, all the modiﬁcations are introduced in a dedicated
formed using standard and proprietary networking applica- machine called Gateway/Proxy.
tions. Location transparency involves the automatic switching
The rest of the paper is organized as follows. In section 2 from one wireless interface to the other, when moving from
we present the overall architecture, covering all the functional indoors to outdoors and vice-versa. This kind of function-
components, both at the terminal and network side. In sec- ality is provided, mainly, by a combination of two compo-
tion 3, we give an overview of the targeting operational en- nents, one residing at the terminal side (IP Mobility Enhance-
WIRELESS CONVERGENCE ARCHITECTURE 261
Figure 2. Wireless Convergence Architecture protocol stack.
ment Client – IPMEC) and a second one residing at the Gate- 2.2. Resource Repository
way/Proxy (IP Mobility Enhancement Server – IPMES).
At the same time it is imperative for WCA to pro- The Resource Repository (RR) is used for storing critical in-
tect end-user sessions from being disrupted by short signal formation for the proper operation of the mobile terminal. For
losses. This could harm the operation of Transport layer ses- instance, connectivity status, signal-strength thresholds, ac-
sions, especially, when a protocol with time-out mechanisms tive wireless interface, IP and MAC addresses are some of
(e.g., TCP) is used. To face this problem a separate pair the vital parameters found in RR. All the information is or-
of modules has been introduced, consisting of the Abstract ganized in tree-like structure, thus, making future extensions
Socket Layer (ASL) and the Communication Manager (CM), easy.
placed at the mobile terminal and the Gateway/Proxy, respec- Additionally, RR provides an interface through which reg-
tively. ASL and CM can hide the side effects of temporal istered modules or user-applications are asynchronously no-
wireless link breaks from end-user applications. tiﬁed when the value of a speciﬁc parameter changes. Net-
Finally, two more components have been identiﬁed and in- working applications that are aware of this interface may use
troduced in the terminal side, namely the Supervisor and the it to adapt their behaviour according to the capabilities of the
Resource Repository (RR). The former maintains the over- active wireless NIC.
all control of the modules residing at the terminal, while the
latter provides an efﬁcient mechanism for storing useful in- 2.3. IP Mobility Enhancement Client, Server
The rest of this section describes in more detail the afore- The IP Mobility Enhancement Client and Server team up with
mentioned software components. the IP protocol to achieve location transparency and hide mo-
bility from higher layer protocols (e.g., TCP) and, eventually,
2.1. Supervisor More speciﬁcally, the IPMEC module resides within the
mobile terminal and, when it is activated, it registers with
The Supervisor is responsible for monitoring the connectiv- the IPMES. IPMEC is mainly responsible for redirecting IP
ity status of the underlying wireless adapters. It incorporates packets over the correct underlying wireless interface. It is
the necessary knowledge to decide when switching from one delegated to modify the routing table of the mobile terminal.
wireless interface to the other must occur, informing other In this respect, during the switching, all the routing entries
modules for such transitions. The selection of the appropriate directed over the current wireless interface are deleted and
wireless NIC is performed, according to predeﬁned signal- redirected over the newly selected one. IPMEC does not de-
strength thresholds. termine by itself which wireless interface is active or inac-
More particularly, the Supervisor continuously examines tive. The Supervisor, through RR’s asynchronous notiﬁcation
the signal-to-noise ratio (SNR) of the active interface. In case mechanism, conveys this kind of information to IPMEC.
the SNR drops below operational levels, then it initiates the When switching occurs, the terminal’s IP address is not
transition to the other interface. Additionally, the Supervisor modiﬁed. This is one of the prerequisites that enables the
takes into account predeﬁned priorities among the available terminal to switch between interfaces, without having the ac-
wireless interfaces, to resolve conﬂicts where more than one tive transport connections terminated. To fulﬁl this require-
wireless adapter is operational. ment, the IP address of the terminal is either associated with
Since the mobile terminal initiates the switching, the Su- the MAC address of the active wireless interface, or with the
pervisor does not need a counterpart at the Gateway/Proxy. MAC address of the Gateway/Proxy. The latter scenario is
262 N.A. NIKOLAOU ET AL.
true when the Gateway/Proxy, and particularly IPMES, acts The basic idea behind the operation of those two mod-
as a mediator between the mobile terminal and the network, ules is to split an end-to-end TCP session into three consec-
both Intranet and Internet. In this case, packets that are des- utive parts – Application-ASL, ASL-CM and CM-Server –
tined for the mobile terminal, are received by IPMES, and, as shown in ﬁgure 3. In this manner, the wireless part of
as a second step, forwarded to the terminal. IPMES asso- the communication is isolated and the possible side effects
ciates Gateway/Proxy’s MAC address with the IP address of of a temporal wireless link disconnection can be absorbed.
the mobile terminal by sending ARP packets (Request or Re- ASL resides at the mobile terminal side and provides
ply ) to the local LAN. This mechanism, known as Gra- a socket-like Application Programming Interface (API). In
tuitous ARP (GARP ), spontaneously causes other nodes this manner, every TCP socket call, initiated by a client ap-
to update their ARP cache. plication, passes through the ASL library. ASL co-operates
On the network side, the IPMES component resides in with the CM module, running at the Gateway/Proxy side,
the Gateway/Proxy and implements the complementary func- through the use of a lightweight, UDP-based, proprietary pro-
tionality of IPMEC. More speciﬁcally, following the idea tocol. The purpose of this protocol is to have ASL and CM
of the Mobile IP mobility agent , it is responsible for exchange control information that will enable them to ma-
the processing of the registration/deregistration messages re- nipulate (OPEN, ReOPEN and CLOSE) TCP sessions, initi-
ceived from IPMEC. In addition, IPMES caters for the ap- ated by the mobile terminal. The CM module, upon receiving
propriate interception of IP packets from the network and the control messages, interprets them and tries to establish the
forwarding of these packets to the proper mobile terminal. Fi- required TCP sessions that will realize the entire end-to-end
nally, IPMES implements the aforementioned GARP mecha- session.
nism. Whenever an interruption occurs, ASL re-establishes trans-
port connections to CM, and, subsequently, re-associates ses-
2.4. Abstract Socket Layer and Communication Manager sions with physical connections. The terminal’s networking
applications throughout this period are “frozen”, but not ter-
The Abstract Socket Layer (ASL) and Communication Man- minated.
ager (CM) pair of modules provide application resiliency to Figure 3 depicts the exchange of control messages, be-
wireless link disruptions. ASL and CM are capable of hiding tween ASL and CM, concerning the two most interesting sit-
temporal link breaks from the transport layer, thus protecting uations. The ﬁrst one refers to the case where a networking
TCP sessions from being terminated, due to timeouts. application at the mobile terminal initiates a new TCP session
Figure 3. ASL and CM interoperation.
WIRELESS CONVERGENCE ARCHITECTURE 263
to a server. As a respond, ASL establishes a local TCP ses- wireless terminal is equipped with a WLAN NIC, for in-
sion with the application, stores information regarding com- doors communication, and a combined GSM/PSTN adapter,
munication end-points and sends an OPEN message to CM. for outdoors communication. To complete the picture, two
Having received the OPEN message, CM initiates a proce- other devices are present, the Gateway/Proxy and the WLAN
dure to establish two separate TCP sessions, one with ASL base station. The former also maintains the control of a pool-
and a second with the server application. Subsequently, CM of-modems, providing access through the public GSM/PSTN
associates those two sessions and returns an answer (ANS) network.
message to ASL, indicating the successful completion of the The reference network environment has been established
process. At this point the end-to-end connection is actually within a real corporate network (Nationale Nederlanden,
established and data starts ﬂowing from each session to its Athens, Greece ), giving access to resources both at the
adjacent one. local Intranet and Internet. Based on this installation, four
When a temporal wireless link break occurs, the TCP ses- different connectivity scenarios are supported for accessing
sion between ASL and CM is interrupted. This is depicted in the corporate network, including indoor (WLAN) and outdoor
the second scenario of ﬁgure 3. In this case, ASL and CM ex- (GSM/PSTN) access. More particularly:
change control information to reestablish the TCP session, as
soon as the wireless link becomes active. More particularly, (A) The mobile terminal is located in the network segment,
ASL sends to CM a ReOPEN message, identifying the con- in which its IP address belongs to, and access is achieved
nection that must be reopened. CM compiles the ReOPEN by means of the WLAN interface.
message, retrieves the connection properties from its data-
base, and reestablishes the required session with ASL. As a ﬁ- (B) The terminal is connected to the corporate LAN, utilizing
nal step, an ANS message is sent to ASL, acknowledging the a GSM physical link to the GSM service provider. At the
successful reconnection. GSM service provider’s local exchange, trafﬁc is routed,
via a direct PSTN connection, to a modem pool, which
is managed by the corporate Gateway/Proxy.
3. Reference network environment
(C) The physical connection is achieved through the PSTN
Figure 4 presents the reference network environment derived network only. In this case, mobility is restricted, as the
from the Wireless Convergence Architecture. It accommo- terminal uses a telephone line to connect to the corporate
dates two wireless networks, based on different technologies, network. However, it may have lower cost, compared
namely the IEEE 802.11 Wireless LAN and the GSM. Each with scenario (B).
Figure 4. Reference network environment.
264 N.A. NIKOLAOU ET AL.
(D) A logical connection to the Internet is established uti- 4. Experimental results
lizing the services of an Internet Service Provider (ISP).
The physical link of the connection is achieved through The implementation of the mobile terminal components, pre-
the GSM or the PSTN interface. This scenario is quite sented in section 2, has been realized in two different Oper-
close to cases (B) and (C). The main difference is that in ating Systems (OS), including Windows NT 4.0 and Linux
scenario (D), an ISP router comes in between the termi- (SuSE 6.1). The former was selected because it has a large
nal and the corporate Gateway/Proxy. base of end-users and, at the time of the decision, it was the
most stable among the various clones of MS Window OSs.
While the mobile terminal is within the corporate premises, The Linux OS was considered so as to prove the implemen-
WLAN is always favoured. When it moves towards the tation feasibility and the generality of WCA. Regarding the
boundaries of WLAN and the Wireless LAN SNR drops be- components residing within the Gateway/Proxy, they were
low a predeﬁned threshold, the system switches to GSM. implemented on the Linux platform, because of its increased
More particularly, it sets up a GSM connection and reg- ﬂexibility.
isters the terminal to the Gateway/Proxy (IPMES), in or- The testing scenarios were based on standard network-
der to tunnel all terminal packets via the GSM. During the ing applications, including Telnet, Web browsing (Internet
switching period, all running sessions are kept alive, so Explorer for Windows NT and Netscape for Linux) and
that they can continue their operation after the new physi- videoconference (NetMeeting for Windows NT). Addition-
cal connection is established. A reverse switching is per- ally, a custom-built, light-FTP application utilizing directly
formed, when the WLAN signal rises to operational lev- the ASL library was used in the Windows NT environment.
els. The purpose of this application was to collect and evaluate
The selection of the aforementioned wireless interfaces has performance measurements.
been triggered mainly by two factors. First of all, the GSM Two testing scenarios were used, depicted in ﬁgure 5:
technology is mature, well established and used by millions
Scenario A. The mobile terminal is connected to the network
of people in a daily basis, thus, giving marketing potentials
through path (a). The selected networking applications
to the overall system. The second reason is correlated with
are executed and operate normally. At this point, an au-
the requirement to have two different wireless interfaces that
tomatic transition from WLAN to GSM link is performed,
can be used in a complementary manner, in terms of coverage
due to a deliberate degradation of the WLAN SNR. After
area. To this respect, WLAN and GSM are a perfect match,
the switching, trafﬁc is routed through path (b). Alterna-
because they provide wireless communication for indoors and
tively, the PSTN dial-up connection, path (c), can be uti-
lized. The desired result for the running applications is
However, WLAN and GSM do not exhibit the same perfor-
a seamless handover.
mance, in terms of operational cost and bandwidth availabil-
Scenario B. It is similar to scenario A, only this time the
ity. WLAN appears to be superior in both metrics. It can sup-
reverse transition from GSM to WLAN is performed
port bit rates that range from 2 to 11 Mbps (or up to 54 Mbps
(from path (b) or (c) to (a)), due to an increase of the
when 802.11a will be available as a product), while GSM
WLAN SNR. The uninterrupted operation of the selected
provides limited bandwidth (9.6 Kbps). Moreover, the use
networking applications is the desired result.
of the WLAN has almost zero operational cost, apart from
the initial installation, whereas GSM can be quite expen- Table 1 summarizes the outcome of the aforementioned
sive. In addition, the coverage area of WLAN is a sub-area scenarios for the Windows NT OS. Regarding scenario A, all
within the GSM coverage, but, whenever it is feasible, the the applications continue their normal operation unhindered.
WLAN must be favoured over the GSM. The selection of However, this is not the case for scenario B. It has been ob-
the wireless interface highly depends on these issues and it served that during switching from GSM to WLAN, TCP con-
is performed in an automatic way that is transparent to the nections are automatically terminated. This happens because
end-user. as soon as Windows NT detects that the GSM (PSTN) link is
The transition from GSM to WLAN implies that more down, it releases all resources referring to the Point-to-Point
bandwidth becomes available for the running applications. protocol (PPP ), with the imminent result of terminating
However, this is not the case when the reverse switching any previously active TCP connection. At this point the ASL,
is performed, where the bandwidth is reduced dramatically which resides above the standard socket API, undertakes the
from Mbps to Kbps, imposing restrictions to active applica- responsibility to hide and absorb this side effect. However,
tions. To eliminate this inconvenience, the API provided by due to Windows NT restrictions, the ASL could not be fully
RR can be exploited. Through that API user-level applica- integrated in the protocol stack of the mobile terminal. As
tions are informed about the bandwidth capabilities of the ac- a result, not all of the existing applications could operate
tive wireless interface and warned when a switching is going above the ASL. For that reason, some applications (e.g., telnet
to be performed. In this manner, an application can adapt its and videoconference) failed to continue in scenario B, while
behaviour according to the capabilities of the WLAN or GSM others (e.g., browser) continued uninterrupted. Regarding the
adapter. custom ftp application, which was using the ASL library di-
WIRELESS CONVERGENCE ARCHITECTURE 265
Figure 5. Testbed infrastructure.
Experimental results with the mobile terminal running Windows NT OS.
Telnet Videoconference Browsing Custom application
Scenario A Continued Continued Continued Continued
Scenario B Interrupted Interrupted Continued Continued
Experimental results with the mobile terminal running Linux OS.
Scenario A Continued Continued
Scenario B Continued Continued Figure 6. Total time needed to transfer ﬁles of various sizes with and without
(dotted line) the use of the ASL–CM pair.
rectly, no problems have been revealed during the execution
of scenario B.
Table 2 presents the results for the same testing scenarios,
but this time the mobile terminal operates in Linux OS. Under
these conditions, two representative networking applications
were used: a common telnet application and the Netscape
browser. The results, after applying scenarios A and B, are
that both applications continue their operation unhindered,
even if the switching of media occurred, while downloading
an image (browsing).
Figure 7. The percentage of the degradation of the total time needed to trans-
Based on the results, we have concluded that the switching fer a ﬁle with ASL–CM compared to the case that the ﬁle is transferred with-
between different wireless interfaces, while maintaining TCP out the ASL–CM pair.
sessions intact, is accomplished successfully in almost all the
experiments, apart from the cases restricted by the operating the proprietary ﬁle transfer application. The average mea-
system’s limitations. sured time to initially set up the end-to-end communication
Moreover, measurements collected during the execution of over the ASL and CM modules is 0.463 s, instead of the
the testing scenarios revealed that the average time to set up measured 0.152 s without the use of ASL–CM. A degrada-
the GSM link is about 20 s. This big duration may have as tion on the communication speed has also been detected, even
an impact the termination of active TCP connections (due to when no interruption occurs (ﬁgure 6), especially for ﬁles that
time-out). ASL and CM gave a good solution to this problem, are small in size. Nevertheless, the degradation is decreasing
provided that the networking applications do not utilize the very quickly to accepted values (ﬁgure 7). For example, for
standard socket API, but the one offered by ASL. ﬁles larger than 20 Kbytes, the degradation percentage is less
Although ASL and CM provide resiliency to wireless link than 10%.
breaks, they also introduce extra delay. Regarding this issue In case of a wireless link break, the application is frozen
some performance measurements have been obtained using until the physical connection is reestablished and the
266 N.A. NIKOLAOU ET AL.
ASL–CM pair reassociates sessions with TCP connections. arise, as a result of the simultaneous existence of multiple ac-
The time for the former operation can be very large (20 s on tive TCP/IP ﬂows from various mobile hosts. Each active IP
the average), while the average measured time for the latter ﬂow needs minimal state maintenance in the Gateway/Proxy,
operation is about 0.3 s. To this respect, the degradation is increasing the possibility of performance problems in case the
mainly due to the procedure for the establishment of the phys- number of active IP ﬂows exceeds certain limits. To over-
ical connection. come this problem, a hierarchy of Gateway/Proxies should be
deﬁned and properly conﬁgured.
In spite of the fact that there are many open points regard-
5. Conclusions ing the provision of seamless handoff between different wire-
Wireless communications seem to be more appealing than less mediums, WCA offers a realized and tested paradigm
wired, as they liberate the mobile users to move everywhere, of the feasibility of uninterrupted connectivity. Moreover, it
while still performing any task originally carried out in the combines off-the-shelf hardware components that can be eas-
wired network. However, the evolution of networking favours ily integrated. To this respect, it also presents a low cost solu-
the existence of various network interfaces in a mobile ter- tion for seamless wireless communications.
minal. Combining the capabilities of different wireless inter-
faces, a working environment can be built-up for both indoors
and outdoors communications, allowing the user to move
seamlessly in the resulting virtual world .  IEEE Standard 802.11, Wireless LAN Medium Access Control (MAC)
On the other hand, seamless and uninterrupted connectiv- and Physical Layer (PHY) speciﬁcations (1997).
ity cannot be achieved by simply installing some network in-  ETSI ETR 133, High Performance Radio Local Area Networks
(HIPERLAN); System deﬁnition (July 1994).
terfaces on a mobile host. The mobile terminal must pro-
 DECT Forum, Digital Enhanced Cordless Telecommunications,
vide additional intelligence that caters for the selection of the http://www.dect.ch
most appropriate interface each time, through which all com-  ETSI Project – BRAN, http://www.etsi.org/bran/Sum-
munications will be performed. Roaming from one wireless mary.htm
medium to another, while still maintaining running network  A. Acampora, Wireless ATM: A perspective on issues and prospects,
applications alive, is the basic requirement for location trans- IEEE Personal Communications (October 1996).
 ETSI TBR 019, European digital cellular telecommunications system
parency. Moreover, wireless media are susceptible to inter- (Phase 2), Attachment requirements for Global System for Mobile
ruptions of any kind, as a result of the loss of the medium communications (GSM) mobile stations (March 1998).
signal. In this case, transport connections may expire, result-  ETSI EN 301 344 v7.4.1 (2000-09), General Packet Radio Service
ing in the termination of the execution of networking appli- (GPRS), Service description, Stage 2 (GSM 03.60 version 7.4.1 Re-
cations. The appropriate mechanisms for providing resilience lease 1998) (September 2000).
 ETSI TS 101 038 V7.0.0, High Speed Circuit Switched Data (HSCSD),
to interruptions should be incorporated in the host. Stage 2 (GSM 03.34 version 7.0.0 Release 1998) (August 1999).
Following these directives, this paper presents the WCA  UMTS Forum, The path towards UMTS – Technologies for the infor-
architecture that provides seamless and uninterrupted con- mation society (November 1998).
nectivity, within a complicated indoors and outdoors envi-  D. Plummer, An Ethernet address resolution protocol: Or converting
ronment. WCA offers the IPMEC and IPMES modules to network protocol addresses to 48 bit Ethernet addresses for transmis-
sion on Ethernet hardware, RFC 826 (November 1982).
cater for location transparency, as well as the ASL–CM pair
 W.R. Stevens, TCP/IP Illustrated, Vol. 1, The Protocols (Addison-
of modules to provide resilience to short link breaks. WCA Wesley, Reading, MA, 1994).
has been implemented under two different OSs (Windows NT  C. Perkins, IP mobility support, RFC 2002 (October 1996).
and Linux). Extensive experiments have been performed with  Nationale Nederlanden, http://www.ing.gr/en/
standard and proprietary networking applications and proved  W. Simpson, The Point-to-Point protocol (PPP) for the transmission
of multi-protocol datagrams over point-to-point links, RFC 1331 (May
the correctness and openness of the solution.
In addition to the functionality described in the paper, the  K. Pahlavan, P. Krishnamurthy, A. Hatami, M. Ylianttila, J. Makela,
architecture could be extended to various directions. To start R. Pichna and J. Vallstrom, Handoff in hybrid mobile data networks,
with, although the implementation involves two speciﬁc wire- IEEE Personal Communications 7/2 (April 2000) 34–47.
less technologies (WLAN and GSM), it can be extended to
include others. Since the overall architecture is built in a mod-
ular manner, it can easily accommodate other wireless inter- Nikos A. Nikolaou received the Dipl.-Ing. degree
in computer engineering and informatics from the
faces that ﬁt better to the speciﬁc user needs. For example, the
University of Patras, Patras, Greece, in 1996 and
substitution of GSM by GPRS would help evaluate the impact the Ph.D. degree in electrical and computer engi-
of the WCA architecture better, as GPRS does not need to neering from the National Technical University of
set-up or tear-down physical connections, like in GSM. Hand- Athens (NTUA), Athens, Greece, in 2001. In April
off between WLAN and GPRS would then be much more ef- 2000 he joined, as a System and Software Engineer,
the Forward Looking Work Department of Lucent
ﬁcient and less time-consuming.
Technologies, Huizen, The Netherlands. In January
Another major concern regarding WCA is the support of 2001 he was transferred to Advanced Technologies,
a large number of mobile hosts connecting to the home In- Bell Labs, EMEA, where he is currently working in the areas of 3G net-
tranet through the Gateway/Proxy. Scalability problems may works and protocol decomposition and proﬁling for Network Processors.
WIRELESS CONVERGENCE ARCHITECTURE 267
From 1994 to 1999, he acted as a Technical Consultant to various organiza- Iakovos S. Venieris received the Dipl.-Ing. degree
tions, including the Computer Technology Institute, the Telecommunications from the University of Patras, Patras, Greece in 1988,
Laboratory of NTUA, the Institute of Communications Systems and Com- and the Ph.D. degree from the National Techni-
puters of NTUA, Intrasoft SA and Ellemedia Technologies. Dr. Nikolaou has cal University of Athens (NTUA), Athens, Greece,
participated in many European R&D Programmes, including ACTS (projects in 1990, all in electrical and computer engineering.
MISA and PETERPAN), THERMIE (project ENTRANCE), TELEMATICS During 1991–1992 he was with the National De-
(project EUROSCOPE), ESPRIT (projects FAPSY and NOTE) and IST fense Research Center, Athens, Greece, performing
(project PRO3 ). His research interests are in the ﬁelds of network processors, research in the area of telecommunication networks
Internet QoS, 3G networks, network management and multimedia services. for military applications. From 1992–1994 he was
Dr. Nikolaou is a member of IEEE and the Technical Chamber of Greece. a research associate in the Telecommunications Lab-
E-mail: firstname.lastname@example.org oratory of NTUA. In 1994 he became an Assistant Professor in the Electrical
and Computer Engineering Department of NTUA where he is now an As-
sociate Professor. His research interests are in the ﬁelds of broadband com-
munications, Internet, mobile networks, Intelligent Networks, internetwork-
ing, signalling, service creation and control, distributed processing, agents
technology, and performance evaluation. He has over 100 publications in
Konstantinos G. Vaxevanakis received his
the above areas. Dr. Venieris has received several national and international
Dipl.-Ing. Degree in computer engineering and
awards for academic achievement. He has been exposed to standardisation
informatics from the University of Patras, Patras,
body work and has contributed to ETSI and ITU-T. He has participated
Greece, and he is currently working towards ﬁnal-
in several European Union and national projects dealing with B-ISDN pro-
ising his Ph.D. degree in electrical and computer en-
tocols, mobile networks, Intelligent Networks, ATM switching and access
gineering from the National Technical University of
techniques, Intelligent software, and Internet technologies. He is an asso-
Athens (NTUA), Athens, Greece, in the ﬁeld of high
ciate editor of the IEEE Communication Letters, member of the editorial
speed networking management. From 1997 to 1998,
board of Computer Communications (Elsevier), and has been a guest edi-
he was a Research Associate at the Telecommuni-
tor for IEEE Communications Magazine. He is a reviewer for several jour-
cations Laboratory of NTUA. During 1997, he also
nals and has been member of the Technical Program Committee and session
acted as a technical consultant to the TMN department of Intrasoft SA. While
chairman of several international conferences. Dr. Venieris is the editor and
at Intrasoft SA, Mr. Vaxevanakis was a designer of a TMN Mediation De-
co-author of two international books on Intelligent Broadband Networks (Wi-
vice (MD) implemented within OPUS project on behalf of IBM. While at the
ley, 1998) and Object oriented Software Technologies in Telecommunications
Telecommunications Laboratory of NTUA, Mr. Vaxevanakis participated, as
(Wiley, 2000). Dr. Venieris is a member of IEEE and the Technical Chamber
research engineer, in the European Research Programme ACTS in the ﬁelds
of network management for ATM, SDH and HFC networks. Since 1999, he
has been a consultant to Lucent Technologies as a senior member of techni-
cal staff of Ellemedia Technologies, a product development partner to Lucent
Technologies. At Ellemedia, Mr. Vaxevanakis worked in the exploratory de-
Nicholas A. Zervos received the Ph.D. in electrical
velopment of end-to-end home interactive multimedia consumer services. He
engineering from the University of Toronto, Canada,
worked for the development of home service platforms, wireless access, and
the M.A.Sc. in systems and computing science, Car-
core network services at the user end. He was one of the leading engineers in
leton University, Ottawa, Canada and the Dipl. in
ESPRIT NOTE project for the materialisation of a platform that incorporates
electrical and mechanical engineering from NTUA,
various wireless mediums (GSM, WaveLAN) for indoor and outdoor com-
Greece. Dr. Zervos is a Managing Director of Elle-
munications. His current work is in the ﬁeld of integrated network manage-
media Technologies (afﬁliate of Bell Laboratories of
ment for IP and WDM networks. He is a member of the Technical Chamber
Advanced Technologies EMEA) in Athens, Greece.
of Greece, IEEE and ACM.
He is currently directing exploratory development
of core network technologies in the areas of home
networking, wireline/wireless systems, and broadband networking compo-
nents. He joined the Data Communications Research Department of Lu-
cent Technologies (former AT&T) in April 1985. From 1985 to 1993, his
research was in the general area of communication and information theory
Sotirios I. Maniatis received his Dipl.-Ing. in com- with emphasis in mathematical statistics, signal space coding, and statisti-
puter engineering and informatics from the Univer- cal signal processing. His research/development work has been directed to-
sity of Patras, Patras, Greece, in 1996. He received wards state of the art digital transmission system architectures capable of
his MSc degree in information systems engineering establishing record high transmission rates over (wired and wireless/mobile)
from the University of Manchester Institute of Sci- narrowband and broadband (voiceband, radio, twisted pair, cable TV) mul-
ence and Technology, Manchester, UK, in 1997. He tiple access media. He was instrumental researcher for exploratory devel-
is currently a candidate Ph.D. student in the De- opment of high data rate loop systems over UTP. He is one of the world’s
partment of Electrical and Computer Engineering of experts in bandwidth-efﬁcient digital transmission. His research has led to
the National Technical University of Athens (NTUA) data rates up to 155 Mb/s for UTP-based ATM LAN and broadband access
and a research associate in the Telecommunications applications such as the broadband telephone network. He is the author or
Laboratory of NTUA. His primary research interests include IP network- co-author of nine patents in the areas of data transmission and digital signal
ing, IP-based 3G wireless networks, as well as network and application-level processing. He was an Editor of the IEEE Transactions on Communications,
QoS and resource management in the Internet and in 3G mobile networks. Co-Chairman and Technical Program Chairman of the 1991 IEEE Communi-
Sotirios Maniatis has received an academic excellence award from the Na- cation Theory Workshop, Chairman of the 1995 IEEE Communication The-
tional Scholarship Foundation. He is a member of IEEE and Technical Cham- ory Mini-Conference and Technical Program Vice-Chair of ICC’96. He is
ber of Greece. Co-Chairman of ICC’2002, to be held in New York.
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