CORDIS Archive
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Vincenzo Marziale
v.marziale@roma.alespazio.it
Alenia Aerospazio Div. Spazio, Italy
Giacinto Losquadro
g.losquadro@roma.alespazio.it
Alenia Aerospazio Div. Spazio, Italy
Introduction
ACCORD: achieving a Global Mobile Broadband System
ACCORD Multi-Mode Terminal
Mobility Management: InterSegment Roaming and InterSegment Handover
Multimedia Services in the ACCORD System
ACCORD Trials: demonstrating and validating the R&D activity
Outstanding issues to be addressed in the IST framework
Conclusions
References
In the last ten years the new, very promising, market of multimedia services has been attracting more and more computing, entertainment and communication industries. The incredible rate of growth which characterises the demand for multimedia service is driving the R&D sector towards the definition and implementation of telecommunication systems able to guarantee efficient and simultaneous exchange of voice, video and data information. Working practices, education, and healthcare represent examples of fields which could widely exploit the new opportunities offered by the emerging technologies.Beside this growing demand for multimedia services, the impressive evolution and penetration of cellular technology (e.g. GSM) has shown that the provision of user mobility is a requirement which each new telecommunication system cannot afford to ignore. Such mobility should be guaranteed not only in a local context but also on a global world-wide scale. This means that the same telecommunication system should be able to provide its services in a wide range of environments, from open countryside fields to densely populated cities and built up areas, as well as in the indoor environment. This objective can only be reached by complementing the local coverage provided by mobile terrestrial networks with satellite global coverage. Satellite access is considered a complement to terrestrial mobile access networks for both tariff and physical reasons. In fact, terrestrial systems are still very cost effective and the availability of satellite radio links in urban environments is still problematic, especially due to the building shadowing effect. Nevertheless, the presence of a satellite component is unavoidable where terrestrial networks cannot be deployed or where their first installation is not cost effective.
The final aim is therefore the creation of a Global Mobile Broadband System (GMBS) able to provide multimedia services to a generic user at any time and anywhere, i.e. able to guarantee to subscribing users, regardless of their position, the simultaneous exchange of voice, video and data information.
It appears evident that the flexible working field could heavily exploit the opportunities offered by the GMBS. Services like teleworking, desktop videotelephony and business communications (banking, insurance) could be provided in any environment thanks to "mobile office" terminals enabling, for instance, businessmen to operate on the move as if they were at their desks.
The ACCORD Project activity addresses the scenario above. ACCORD aims at the definition of a Global Mobile Broadband System which will assure genuinely global (indoor and outdoor, near and far building) coverage and service provision. This is realised by integrating an ATM based Core Network with four different satellite/terrestrial Access Networks and by developing a Multi-Mode Terminal able to provide connectivity through the most suitable access segment on each occasion.
ACCORD is an ACTS Project aimed at creating a global mobile broadband system. The network architecture proposed in ACCORD is obtained from the integration of four different (terrestrial/satellite) Access Systems and a Core Network, all developed in the framework of ACTS projects. The core network is the Mobile Broadband Core Network, which is ATM-based implementing IN (Intelligent Network) functionality, developed in the framework of the EXODUS project. The Access Networks (also referred to as "segments") are the following ones:The four Access Networks, which are mutually complementary in the type of services supported and coverage, provide full radio coverage enabling a mobile user located anywhere to have, while roaming within the multi-environment coverage, seamless access to the EXODUS Core Network. Figure 22 shows the target architecture of the ACCORD System.
Figure 22: ACCORD System Architecture
The rationale of the ACCORD project is to create a network which is able to guarantee, in each possible scenario and condition, communication support between a mobile broadband user terminal and the EXODUS Core Network supplying the required multimedia services. This communication support is provided, depending on the environment where the user is located, by one of the four envisaged Access Networks (SECOMS, MEDIAN, SAMBA, EXODUS/DECT). Depending on the location of the user, on the bandwidth of the required service and on economic factors (a cheaper link should have a higher priority), the connection with the EXODUS Core Network will be supported by a different segment. For instance, a user moving within an indoor environment will exploit a MEDIAN link while, at the opposite extreme, a user in an open rural area, not covered by any mobile terrestrial network, will use the satellite link provided by the SECOMS system.
The ACCORD System is based on ATM technology in the sense that ATM is the transport mechanism used in the EXODUS Core Network and the SECOMS, MEDIAN and SAMBA Access Networks have been developed to transparently support data generated by a ‘whatever’ user application through an ATM Mobile Broadband UNI (MB-UNI). The main ACCORD commitment is that integration of the various Access Networks and Core Network has to be performed, whilst limiting, as much as possible, modifications to the segments which are already working. In order to reach this objective, an "interworking" approach has been followed, aimed at de-coupling the Access Network specific procedures from the EXODUS Core Network ones. In fact, the Access Networks (SECOMS, MEDIAN, SAMBA and EXODUS/DECT) have to provide physical support for the ATM cells. User information and EXODUS procedures in charge of the call and mobility management travel transparently on that physical link.
One of the main advantages in the use of ATM technology, from a user perspective, concerns the possibility of assuring a pre-established Quality of Service (QoS) for the required applications. The approach towards the QoS implies several possible alternative technical solutions to select, at any time, the most suitable access segment (among the ones which can provide radio connectivity) supporting the call. The most challenging selection criteria take into account the following parameters:
The integration/inter-working of the access and core networks proposed in the ACCORD project is achieved through the exploitation of advanced networking concepts and technologies such as the Intelligent Network (IN). Mobility management, representing one of the most challenging aspects in ACCORD, is supported by the IN capabilities which guarantee not only correct routeing within the Core Network but also inter-segment roaming and inter-segment handover among the four different Access Networks. In fact, IN functionality supports both Personal and Terminal mobility:
Intelligent Network capabilities also ensure flexibility in service offerings by enabling the rapid creation and deployment of new multimedia services. User profiles can be interrogated and modified by users at any time in order to customise services to their personal preferences. These features seem to be mandatory in the current multimedia scenario, which presents a continuously and rapidly changing market. Such an aspect is even more important in the context of flexible working where business needs require to be satisfied as quickly as possible in order to guarantee high competitiveness.
ACCORD is a project based on the integration of pre-existing Core and Access Networks. Such integration is realised by following an "interworking" approach aimed at limiting modifications to already working segments as much as possible.Each Access Network (ie Segment specific terminal + Segment specific Base Station; see Figure 22) has to execute its own procedures in order to provide a physical medium through which the ATM cells can be transparently exchanged. Such an ATM cell flow takes place between the ATM User Terminal and the Core Network (in particular the Service Switching Point (SSP)) which exploit the connection "as if it was a wired link" in order to execute EXODUS specific procedures and to access/provide the multimedia services subscribed to.
This rationale has heavily influenced the design of the ACCORD Multi-Mode Terminal whose architecture is shown in Figure 22.
Figure 23: ACCORD Multi-Mode Terminal Architecture
The proposed ACCORD Multi-Mode Terminal consists of:
The working methods of the ATM UT, on the one side, and of the SECOMS, MEDIAN, SAMBA and DECT terminals, on the other side, do not need modification, with respect to the behaviour envisaged in the relevant projects, once they are integrated in the ACCORD Multi-Mode Terminal. As a matter of fact, all the ACCORD specific functionality is grouped in the ACCORD Segment Selector which is in charge of managing the correct "inter-working" of the four Access Networks. In order to execute this task, the ACCORD Segment Selector retrieves specific terminal information from the Segment about the availability of the radio links (information processed by the Segment Selection Handler) and information about the current location of the Multi-Mode Terminal (information processed by the Location Area Handler). On the other interface, the ACCORD Segment Selector interacts with the ATM UT to correctly trigger mobility management procedures.
On the basis of the radio link availability information relevant to the EXODUS, MEDIAN, SAMBA and DECT Access Networks, and taking into account a priority scheme, the ACCORD Segment Selector automatically selects the segment which will support the required multimedia call. An option could also be envisaged to enable the user to manually select his/her own preferred access segment among those available at a certain time.
The above considerations highlight the fact that the Multi-Mode Terminal has been devised according to a modular approach aimed at permitting easy plug-in and disconnection of the various segment specific terminal parts, as well as of the ATM UT. This approach presents the undeniable advantage of Multi-Mode Terminal reconfigurability. In fact, since all the ACCORD specific functionality in charge of managing the interworking among the access segments is contained in the ACCORD Segment Selector, it is possible to modify some aspects of the inter-segment way of working without impacting on the segment specific terminals.
Since the ACCORD objective is the creation of a mobile telecommunication system on a world-wide scale, mobility management is one of the key aspects of the project. A great deal of effort has been devoted to investigation of the mechanisms needed to handle mobility among different Access Networks. Inter-Segment Roaming (Personal and Terminal Mobility) and Inter-Segment Handover are proposed in the ACCORD System to provide full interworking between the SECOMS, SAMBA, MEDIAN and EXODUS/DECT access segments and therefore to guarantee broadband multimedia services in a seamless fashion.
Mobile specific functionality is executed with an exchange of messages between the ATM User Terminal and the EXODUS Service Control Point (SCP) hosting the Intelligent Network entities. These messages are Call Unrelated and are supported by the Q.2932 Connection-Less - Bearer Independent protocol. This means that mobility related information is encapsulated in Q.2932 Facility and transmitted in a connection-less way trough the EXODUS Core Network to the Intelligent Network node.
Mobility management is based on a cellular approach. Several Location Areas (LAs) have been defined, each one characterised by an optimal dimension. The radio coverage guaranteed in each Location Area is provided by only one kind of access network. Figure 24 shows an example of a coverage zone served by the SAMBA and the MEDIAN segments.
At the Multi-Mode Terminal, switching on of a User Registration procedure is executed. One of the results of this procedure is that the identifier of the Location Area where the terminal is currently roaming is stored in the core network databases (Service Data Point). From the moment that a mobile terminated call set-up is generated, a paging message is only transmitted in the Location Area where the MMT is currently registered. After successful completion of the paging and call set-up procedures, the ATM flow in the core network is correctly addressed towards the location area where the user is currently roaming. The physical radio link supporting ATM cell transmission will be provided by the access segment serving that area.
In the case of overlapping location areas the approach is the same. The Multi-Mode Terminal, prior to executing User Registration, selects the most suitable access segment on the basis of a predefined algorithm and consequently selects the location area in which to register.
When moving, a user in stand-by mode can change Location Area. In this case a Location Update procedure is triggered. The main aim of this procedure is to keep the information contained in the core network database consistent with the actual position of the user. In this way such a user will be continuously reachable and successfully paged.
Mobile controlled Inter-Segment HandOver is one of the main challenging features of the ACCORD System. When the quality of the radio link currently supporting the call in progress decreases below a fixed threshold, the Multi-Mode Terminal (in particular the Segment Selector) triggers a procedure aimed at handing over the call to a different access segment in a seamless way. This could be executed not only when the user, due to his/her motion, changes location area and therefore serving access segment, but also when, in a overlapping radio coverage zone, the segment currently in use becomes unavailable due to, for instance in the satellite case, shadowing effects.
The main ACCORD Project commitment is to develop a platform able to provide multimedia services to a user in any location. No constraint concerning mobility is imposed on the user who can access subscribed services both in a steady state and "on the move".Several examples of services supported by the ACCORD System can be given:
The following table summarises the main characteristics of the set of services taken into consideration (ITU-T Recommendation I.362). This set covers several relevant cases of multimedia characteristics, required data rates and adopted protocols.
|
APPLICATION |
Quality/standard |
Maximum end to end Delay |
Traffic category |
Integrity & Continuity of the Service |
|
|
BER 10-4-10-6 |
5 min |
NI&UBR |
No |
|
Paging |
BER 10-4-10-6 |
5 min |
NI&UBR |
No |
|
Data Base access |
BER 10-6 |
500ms for file transfer |
NI&UBR |
No |
|
Video libraries |
MPEG1, MPEG2, HDTV |
500ms |
NI&UBR |
No |
|
Telephony |
RP-LP-LPC to G.722 |
250ms |
I&CBR(VBR) |
Yes |
|
Video Telephony |
H.221, H261, MPEG1 |
200 ms* |
I&CBR(VBR) |
Yes |
|
Video conferencing |
H221, BER 10-6 (for data) |
200 ms* |
I&CBR (VBR) |
Yes |
|
Tele medicine |
H221, BER 10-6 (for data) |
200 ms* |
I&CBR (VBR) |
Yes |
|
Video Broadcasting |
HDTV,EDTV,MPEG2 10-9 - 10-10 |
500 ms |
I&CBR |
Yes |
|
Audio Broadcasting |
MUSICAM MPEG |
500ms |
I&CBR |
Yes |
|
Data Diffusion |
BER 10-6 |
1s |
NI&ABR. |
No |
* quality of conversational service does not degrade appreciably in the user’s perception if the transmission delay does not exceed » 280ms
Multimedia Service Characteristics
It is worth noting that these services involve several fields such as working practices, entertainment, education, healthcare etc. which are currently very appealing from a commercial exploitation point of view.
In particular, ACCORD is naturally suited to providing a platform to support Flexible Working services. Business customers often need to access data and still and moving images during their work. This access should be guaranteed anywhere and anytime. For instance, a travelling businessman could need to access a database server containing professional multimedia information in the form of video, audio, pictures, text, etc; at the same time he would like to have a face-to-face dialogue and exchange multimedia documents with colleagues. The global coverage of the ACCORD System guarantees that these kinds of services are available everywhere. The integration of terrestrial access networks with satellite systems proposed in ACCORD permits coverage of any possible environment and ensures that a user can move from an indoor site up to an open rural site without losing the connection with the serving network.
For instance a businessman spending most of the day travelling could need to work "on the move" with his terminal as if he was at his desk. The ACCORD Multi-Mode Terminal in such a case behaves like a proper small "mobile office" guaranteeing access to the required multimedia services in different environments. Imagining a businessman’s trip, during the taxi journey he will exploit the SAMBA access segment, in the airport lounge the MEDIAN segment, while on board the aeroplane he will use the SECOMS satellite link. It is worth highlighting that the Inter-Segment Handover functionality foreseen in the ACCORD system guarantees that the call is not interrupted when moving through the different environments.
The currently ongoing trial campaigns envisaged in the ACCORD framework are aimed at obtaining meaningful data and information regarding a multi-access scenario. Several test-bed configurations have been devised to explore several environments.The general approach for the ACCORD trials is based on the use of three different Dual-Mode Terminals able to provide access to the SECOMS satellite link and, in turn, to one of the three terrestrial access segments (i.e. SAMBA, MEDIAN and EXODUS/DECT). In this way the SECOMS-SAMBA, SECOMS-MEDIAN and SECOMS-EXODUS/DECT coverage scenarios are being investigated.
The target is to execute measurements at different protocol layers: i.e. ATM, TCP/UDP/ICMP and Application level. The collected data is very useful from an industrial exploitation point of view since it permits evaluation of the behaviour of protocols in a wireless environment which were originally devised for fixed networks. This approach gives support to implementers for strategic decisions since it highlights which one (if any) of the protocols investigated is naturally suitable for use on a radio link and which one (if any) needs to be adapted.
Moreover, the trials which are foreseen allow the collection of results, from time to time, on a different pair of satellite/wireless-terrestrial radio links. Dual-Mode Terminal mobility is investigated by moving the device on a predefined path under the coverage provided by two different access segments. Data collected at the coverage area border, where, due to the motion of the terminal, one radio link becomes more and more unusable while the other one increases its power level, permits investigation of how the radio coverage influences the behaviour of the higher layer and how this affects the terminal mobility.
One of the main objectives of the ACCORD experimental research activity is related to the Inter-Segment HandOver procedure definition. In this context an extensive parallel channel measurement field campaign, mainly devoted to the investigation of radio link behaviour, is envisaged. Since this activity is exclusively addressed to the collection of radio parameters, the trial platforms which are foreseen do not require any integration with the EXODUS Core Network. The final aim is the definition of the optimal decision metrics for the Ka band satellite - wireless terrestrial handover triggering algorithm. This ambitious objective is reached by complementing the results obtained with the field measurements with the outputs of proper simulation models. In this way, useful information about the Inter-Segment Handover performances is available. On the basis of this data, a control algorithm for satellite-terrestrial handover is proposed.
This is a very challenging objective, since interworking between satellite and terrestrial networks is nowadays a very interesting topic, not only from a theoretical view point, but also for industrial exploitation. The last step of the evolution of the UMTS foresees that the radio access part will consist both of terrestrial (T-UMTS) and satellite (S-UMTS) components. Therefore, meaningful information about satellite/terrestrial network interaction, especially concerning handover issues, would be very valuable material.
A great deal of effort in the ACCORD project has been devoted to the investigation of InterSegment Handover (ISHO) related issues. The activity has been executed at three different levels: theoretical activity, radio measurements and simulation tools. The main aim is the definition of an optimal triggering and control algorithm for the handover between satellite/terrestrial networks. In this context a very interesting area of investigation could look at the practical implementation of the protocols defined in the ACCORD framework. A trial-oriented activity could foresee the execution of the proposed ISHO procedure actually handing over a predefined call between two different access segment.The ISHO algorithm applies to two access segments of any type (even both terrestrial). Nevertheless, the most interesting case takes place when a satellite and a terrestrial access network are involved. This scenario presents critical points to be investigated, mainly related to the different behaviour of the two typologies of links concerning propagation delay, localisation management, radio link reliability, shadowing effects etc. For these reasons, practical trials aimed at obtaining information about the influence of the above factors on the signalling performance could be very useful. Measurements at ATM, (S-)AAL, Q.2932 layers executed while ISHO procedures (involving access and core networks) are actually in progress could give very valuable information to complement the data obtained in the ACCORD project, where the conditions of satellite/terrestrial handover are emulated.
The procedures of ISHO, as defined in the ACCORD project, obviously have some consequences on the way of working of the Intelligent Network functionality which the EXODUS Core Network mobility management is based on. A study activity looking at the possible modifications/addition of functions to the IN entities and the impact on the core network functional architecture is a topic which could certainly provide valuable information.
The matters relevant to the QoS perceived by the users for the services they have subscribed to play a key role in a multi-environment telecommunication system. The integration of different access networks with different coverage, mobility and data rate features means that, in some situations, a service cannot be continued with the same QoS in a new segment, even if it is possible to perform an intersegment handover. This aspect is strictly related to the Virtual Home Environment (VHE) concept aimed at ensuring "a uniform appearance, or presentation of services, features and tools to a service user, or subscriber, in an identical manner independent of serving network or location". Since support of the VHE is more and more becoming a requirement to be satisfied by global telecommunication networks, the impact of intersegment handover on the negotiated QoS needs further investigation in order to be minimised.
Market pressure and emerging trends towards a new way of organising work are forcing businesses to restructure their activity by fully exploiting the opportunities offered by advanced communication technologies. Flexible and distributed working has become a necessary part of new business operations, and new telecommunication systems are now essential to link companies and business units together. Distance working - so-called "telework" - will play a fundamental role in obtaining flexibility in working hours, particularly for part-time work, and in employer-employee relationships (multiple part-time employment patterns). Competitiveness and economic development of industries will depend more and more on their ability to take advantage of advanced communication.In order to be able to cope with this growing market demand, the new telecommunication systems have to satisfy two main requirements: the provision of multimedia services, allowing a simultaneous exchange of voice, video and data information, and a global world-wide coverage area.
In the scenario described, the ACCORD System represents a viable solution to face the emerging demand. ACCORD aims at the creation of a Global Mobile Broadband System (GMBS) able to provide anytime-anywhere multimedia services. This is realised by integrating four different satellite/terrestrial Access Networks (SECOMS, SAMBA, MEDIAN, EXODUS/DECT) and a Core Network (EXODUS), all developed in the framework of ACTS projects.
In the backbone, the transport technology used is the ATM one, while service and control logic (i.e. mobility management, service control and database functions) are provided by Intelligent Network functionality.
To interconnect the access networks with the backbone, an "interworking" approach has been followed. This implies that each access segment has to provide a physical radio link where the ATM flow, supporting service provision and signalling procedures, can travel transparently. Complex protocol conversions are therefore avoided.
Depending on the area where the user is currently roaming and on the required service, one of the four access networks is selected (on the basis of a predefined algorithm) to support the call. The global coverage obtained by complementary satellite and terrestrial networks, in addition to the Inter-Segment Roaming and Inter-Segment Handover capabilities envisaged in the ACCORD System, ensures that a user located in any place, both in a steady state and in motion, can connect to the core network thanks to his/her Multi-Mode Terminal accessing the services to which he/she has subscribed.
The features described make the ACCORD System naturally suitable for supporting flexible working. In this context the Multi-Mode Terminal becomes, from a user perspective, a proper "mobile office" allowing businessmen to exchange information with their headquarters, access multimedia services, etc. recreating conditions identical to those usually available within an office.
ITU-T Recommendation Q.2931 - Broadband Integrated Services Digital Network (B-ISDN) - Digital Subscriber Signalling System n.2 (DSS 2) - User-Network Interface (UNI) Layer 3 Specification for Basic Call/Connection Control (02/95)
ITU-T Recommendation Q.2932.1 - Digital Subscriber Signalling System n.2 - Generic functional protocol: Core functions (07/96)
ATM Networks: Concept, Protocols and Applications, Händel, Huber and Schröder, Addison- Wesley Publishing Company, 1994
ATM Solutions for Enterprise Internet working, David Ginsburg, Addison Wesley, 1996
ACCORD Reference Architecture, AC348 ACCORD, CEC Deliverable AC348/UOR/DS/P/023/b1, Dec. 1998
ACCORD Target and Trial System: Service, Mobility and Coverage Requirements, AC348 ACCORD, CEC Deliverable AC348/UOR/DS/022b1, Dec. 1998
Trial Strategy Description, AC348 ACCORD, CEC Deliverable AC348/ITL/DIS/P/D31/a2, July ’99
ACCORD Evolution Path from Trial System to Target System, P.Conforto, F. Delli Priscoli, V. Marziale, C. Tocci, 4th ACTS Mobile Communication Summit, Sorrento - June 1999.
ACCORD Reference Network Architecture, P.Conforto, F. Delli Priscoli, V. Marziale, 4th ACTS Mobile Communication Summit, Sorrento - June 1999.
ACCORD Network solutions for the Global Mobile Broadband System: complementing the Satellite with Terrestrial Components, P. Conforto, G. Losquadro, G. Ferrari, L. Moretti, M. Dinis, 4th ACTS Mobile Communication Summit, Sorrento - June 1999
Architecture and Protocols for a Mobile Broadband System, P. Conforto, V. Marziale, F. Delli Priscoli, Space Communication Journal (SCJ)
Multimode Terminals for the MBS, E. Macedo, M. Barbieri, F. Delli Priscoli, S. Wolters, 4th ACTS Mobile Communication Summit, Sorrento - June 1999
ACCORD Network Protocol supporting Multi-mode Terminal, A. Kadelka, D. McNamara, S. Ghaheri-Niri, R. in’t Velt, 4th ACTS Mobile Communication Summit, Sorrento, 8-11 June 1999
ACCORD: Handover Issue and Performance in Satellite-Terrestrial Multi-layer Cell Architecture, B.G. Evans, S. Ghaheri-Niri, R. Tafazolli, 4th ACTS Mobile Communication Summit, Sorrento, 8-11 June 1999.
The next section of this document: Giving Mobile Users Access to Net-Based Services - A Mobile Agent Approach