Proceedings of 5th Intl. Workshop on Mobile Multimedia Communication MoMuc’98, October 12-14 1998, Berlin
A Framework for Mobile Wireless Networks with an Adaptive QoS Capability J. Chan, R. De Silva, S.Zhou and A. Seneviratne
Abstract – The rapid deployment of mobile computing and interactive multimedia applications leads to the need for a mobile wireless network with a wide range of QoS support. This paper introduces the concept of Adaptive QoS, which provides a flexible and robust environment for mobile users. An Adaptive QoS based system can be realized through the use of proxy modules and a pre-allocation scheme. The proxy modules called PRIMATEs, act as “software glue” to enable the seamless use of different network protocols when a mobile client roams through numerous networks. The pre-allocation scheme uses a Prediction Confidence Ratio, which allows the user to have some control of the changes in the received QoS. The paper introduces the proposed Adaptive QoS architecture, discusses the different components and presents some experimental results. Index Terms – Adaptive QoS, user mobility, application adaptivity, resource pre-allocation, heterogeneous network.
I. INTRODUCTION With the rapid growth of mobile computing and interactive multimedia applications, users will require the new generation of wireless networks to provide a wide range of Quality-of-Service (QoS) levels to their mobile terminals. Before this can be realized, it is necessary to find solutions to at least the following two questions: • •
How to provide end-to-end data flows with QoS support? How to maintain QoS while “on the move”?
Mobile clients do not access the network at a fixed point. They move from one location to another and join the wired network either directly or via a wireless connection. This implies that the data traffic will traverse different networks, e.g. a wireless subnet, a departmental LAN, the corporate
J. Chan and A. Seneviratne are with Faculty of Engineering, University of Technology, Sydney, PO Box 123, Broadway NSW 2007, Australia Email: [jchan aps]@eng.uts.edu.au R. De Silva is with School of Computing Science, University of Technology, Sydney, PO Box 123, Broadway NSW 2007, Australia Email:
[email protected] S. Zhou is with CSIRO Telecommunications & Industrial Physics, PO Box 76, Epping NSW 2121, Australia Email:
[email protected]
network and a public network. To simplify this scenario without losing generality, consider the case in which a mobile user conducts an active session to a remote site while walking from one network management domain to another. Assume that each management domain consists of a local network and a wireless subnet, and is connected to a backbone network as shown in Figure 1. A.
End-to-end QoS
To provide the necessary support for real-time multimedia services to a mobile user, all the network components in Figure 1 have to be QoS enabled. There are two basic techniques for providing the necessary QoS support. One is to use ATM (Asynchronous Transfer Mode) network technology, which has long been recognized as one that provides the required QoS support. In the past few years, numerous schemes to extend ATM capability across the wireless interface have been proposed [1, 2]. This has led to the development of the concept of Wireless ATM (WATM) [3]. In WATM, it is assumed that both the local and backbone networks are ATM based, with end-to-end QoS provisioning. To enable roaming of users the core ATM network however needs switches that are specially designed to provide the functionality necessary to re-route the connections. The other technique is to use IP technology. Mobile-IP [4] supports user mobility in IP networks. To provide the necessary QoS support, is possible to use Resource Reservation Protocol (RSVP) [5] and/or the emerging differential services [6] concepts.
Backbone Network (DIFFSERV / ATM) Local Network 1 (ATM)
Local Network 2 (RSVP)
Wireless Subnet 2 (IP) Wireless Subnet 1 (IP)
Figure 1. Study scheme of a mobile user roaming across a heterogeneous network infrastructure
Proceedings of 5th Intl. Workshop on Mobile Multimedia Communication MoMuc’98, October 12-14 1998, Berlin
Although the delivery of QoS is very different between the ATM and IP schemes, we believe that the future mobile computing systems will not exclusively be based on either of the above transport technologies, but will rather use a combination of the two. Thus like the current global network infrastructure, in the foreseeable future, it will consist of a combination of ATM and IP equipment. Recently, researchers reported some integrated schemes such as “IP over WATM” [7] and “RSVP over ATM” [8]. Therefore, we have considered the following network architecture in our work to explore the feasibility of end-toend QoS support: • • •
B.
A backbone network – DIFFSERV or ATM based Two local networks – one ATM and one RSVP based Two wireless subnets – both IP based with QoS support at the medium access control (MAC) QoS on the Move
The end-to-end QoS is primarily influenced by the user mobility, as movement may result in: •
•
•
Volatile wireless links: The wireless network link is likely to be the bottleneck in end-to-end connections, and the user’s movement can cause a further reduction of throughput, e.g. due to temporarily fading or handover latency. Lack of resources: There will be cases when a mobile client moves away from one base station, and the only base station(s) available do not have sufficient free bandwidth to support every on-going application. Long processing overheads: As users move from one management domain to another in a heterogeneous environment, some time-consuming processes may take place, such as transport protocol conversion and access authentication.
We argue that the QoS delivered to mobile users cannot be strictly guaranteed for the lifetime of an application. Thus we propose an “Adaptive QoS”, i.e. a less stringent QoS support, for mobile users in a heterogeneous network environment. C.
Adaptive QoS
The term “Adaptive QoS” does not mean a total abandoning of an established QoS agreement, but implies a two-way effort to better serve a mobile user. The network should safeguard (at least statistically) the QoS agreements despite the user’s movement and the heterogeneous nature of the underlying networks. In the case where the desired QoS cannot be maintained, an adaptive mechanism has to be invoked to obtain the maximum utility from the available resources, i.e. maximize user satisfaction. In this paper, we will discuss the realisation of Adaptive QoS in the heterogeneous mobile computing environment depicted in Figure 1. The rest of the paper is organized as follows. Section two introduces the proxy-based
architecture that provides Adaptive QoS functionality. Then section three presents a mobility prediction scheme that provides the necessary support to pre-configure the system to minimize the call-dropping probability and handover latency. Section four describes our proof-of-concept implementation, and provides some preliminary results. Finally, section five provides some concluding remarks and describes our future work plan. II. A REALISATION OF ADAPTIVE QOS FUNCTIONALITY A proxy or agent is a software entity that can perform a wide range of activity on a user’s behalf. Thus it can be used to provide a fault tolerant environment for user applications. In our framework, we use a proxy module called “PRoxy Intelligent Module for Adapting Traffic Efficiently” (PRIMATE) as a “software glue” to hold together different network protocols of the different networks. PRIMATE modules are placed at various network boundaries1 and offers services in three areas, namely user mobility, protocol conversion and application adaptation, which will be discussed in the following sections. A.
User Mobility Support
As been mentioned in Section I-A, a network can support user mobility via mobility enhanced ATM switches or mobile-IP agents. In a general ATM network, one cannot assume all switches to be mobility enhanced to handle extra signaling and provide the databases necessary to support mobility. To deal with this, the WATM Requirements Specification [9] suggests the handover procedure to be implemented by base stations with switching functionality. Similarly, location databases can be separated from the switch while the protocol for updating and querying databases can run over established virtual connections. In our framework, PRIMATEs located at the base station are ideal for implementing the above functions to support mobility. In a large backbone network, connection re-establishment can be a lengthy procedure. Moreover, the desired resources may no longer be available once the existing backbone connection is torn down. Therefore, PRIMATE at the gateway of the current local network can simply extend the existing backbone connection to an adjacent management domain once an inter-domain handover takes place. The extension of the path will be more efficient than restarting the session if the length of the extended segment2 is shorter than the length of existing connection. Figure 2 demonstrates the operation of the above using a fixed anchor point handover scheme [9]. Assume that connections (x) and (y) were setup from the mobile 1
we do not assume that PRIMATE is installed at the remote site as it may not be autonomously owned and operated 2 length is considered to be the number of network segments
Proceedings of 5th Intl. Workshop on Mobile Multimedia Communication MoMuc’98, October 12-14 1998, Berlin
QoS class, i.e., best effort traffic even if the underlying networks can support different QoS classes.
P = PRIMATE Backbone Network
(x)
(c)
P0 Local Network 1
(y)
P4 Local Network 2
(b) P1
P3
(a) P2
P5
Wireless Subnet
Figure 2. Anchor point handover scheme within and across network management domains terminal to the remote server via PRIMATE P1 at the base station and P0 at the gateway between Local Network 1 and the Backbone Network. When the user roams within the management domain of Local Network 1, the original data path is extended to two new locations which contain PRIMATEs P2 and P3. P1 performs a switching function which creates/removes the extended paths (a) and (b). When the user moves across to another management domain, i.e. Local Network 2, P0 performs switching function which removes path (y) and creates a new path (c) which extends the backbone connection (x) to the latest location via P4 and P5. It is noticeable that the services provided by P1 and P0 are very similar, except that one operates at the local network level and the other operates at the backbone network level. B.
Protocol Conversion Support
As mobile users are free to roam in a heterogeneous network environments, their application data may be transported by both IP and ATM networks. To achieve interoperability between these networks, the conventional approach is to use either “Classical IP over ATM” or “LAN Emulation”, which enables the transportation of IP traffic over ATM networks. However, they only cover a single
Backbone Network P0
In our investigations, we assumed that the wireless access system will be IP based, with some form of QoS support at the MAC layer such as PARROT [10]. Hence, user applications at the mobile terminal can issue QoS requests and setup connections to the local network via the wireless link as shown in Figure 3 (connection (a)). A PRIMATE at the serving base station, P1, will in turn maps the desired QoS of user applications into the appropriate UNI or RSVP signaling messages based on the type of local network. Then connection (b) is setup to P0 that is located at the boundary of the local and backbone networks. Similarly to P1, P0 performs appropriate QoS mappings between UNI, RSVP and DIFFSERV, depending on the type local and backbone networks. Similarly, connection (c) is setup to the remote server via the backbone network. Beside the initial end-to-end connection setup, PRIMATEs also performs protocol conversion when a mobile user moves from one management domain to another if they are using different network technologies. C.
Application Adaptation Support
As discussed in Section I-C, user applications need to be adaptive to cope with the variable network conditions. An adaptation approach is particular useful across the wireless subnet because of its relatively lower bandwidth and throughput variation, as mentioned in section I-B. Nevertheless, it is unlikely that all user applications are designed with “network awareness”, or they may not be capable of detecting changing network conditions. Similar
(c) local netw UNI RSVP
backbone netw DIFFSERV UNI
wireless link MAC QoS
(a) Wireless Subnet
Mobile Terminal
P = PRIMATE Wireless Subnet
QoS Manager
Local (b) Network P1
PRIMATE in contrast can utilize the facilities offered by the underlying networks as it uses a simple gateway based approach to bridge the wireless subnet, the local network and the backbone network together. On behalf of a mobile terminal, PRIMATE establishes connections at various network domains using their respective QoS signalling, and joins these connections by proper translation of QoS semantics and data forwarding. Once a proper method for end-to-end QoS translation has been agreed upon, PRIMATE can easily be adapted to support the agreed mappings.
local netw UNI RSVP
P
= PRIMATE
Figure 3. Protocol conversions at various network boundaries
P User App
Local Network
P an intermediate system bridging different network conditions
Figure 4. Application adaptation across the wireless subnet
Proceedings of 5th Intl. Workshop on Mobile Multimedia Communication MoMuc’98, October 12-14 1998, Berlin
However, it has been found that an accurate mobility prediction can be difficult to achieve because of the statistical randomness of user movement [13, 14]. To avoid predicting a random movement pattern, we proposed to improve the overall prediction accuracy with a QoS driven concept called Prediction Confidence Ratio (PCR) [14]. Based on different value of PCR, the fixed network will statistically reserve resources and pre-configure connections to one or more adjacent locations in which a mobile user is likely to visit.
to the architectures proposed by Diot [11], our framework decouples these adaptive functions from a user application and places them in a pair of PRIMATEs located at the mobile terminal and an intermediate system [12]. Since it may not be possible to have control of the remote site, the PRIMATEs can be placed in special servers located within the network. For the ease of demonstration, we have assumed that PRIMATES will be located at the base station as shown in Figure 4. Either the applications or the mobile users need to specify their desired level of service to the underlying network. This task is usually accomplished via a QoS Manager, which in turn control the adaptation functionality of PRIMATE (see Figure 4). A further discussion of this topic is in Section IV.
In our framework, this concept is applied to pre-configure PRIMATE(s) so that those mobile users with a stricter QoS requirement can receive services with a lower dropping probability and shorter handover latency. For the ease of illustration, we associate the level of QoS and PCR value with three classes: best effort (PCR = 0%), medium quality (PCR = 70%) and high quality (PCR = 90%).
III. A PREDICTION SCHEME FOR STRICTER QOS COMPLIANCE
To request a network service, the QoS Manager at the mobile terminal indicates the desired level of QoS to the PRIMATE at its serving base station. For a “best effort” service, PRIMATE P1 just establishes a connection to the backbone network without any pre-configurations (see Figure 5 (a)). For a medium quality service, P1 establishes a “better service” connection to the backbone. The QoS Manager also provides the user’s mobility pattern to P1 for its consideration of pre-assignments at adjacent PRIMATE(s). Because there is a 75% probability that the mobile user is moving to cell number 2, P1 pre-configures a connection to P2 with proper radio resources reserved at P2 (see Figure 5(b)). If a high quality service is requested, P1 performs similar tasks as in the previous case, but needs to pre-assign an additional PRIMATE P3 at cell number 3. In this scenario, we are 95% confident to offer a fast handover without call dropping and hence achieve a stricter QoS compliance (see Figure 5(c)). It is noticeable that both the level of QoS and the user’s mobility pattern determine which PRIMATE(s) to be pre-allocated.
In the previous section, we discussed PRIMATE’s support of extending QoS data flows and converting QoS semantics so that the mobile user can communicate in a heterogeneous network environment. Now we will investigate the influence of movements on QoS and how we can obtain a stricter QoS compliance. Slow handover can cause interruption to user services and call dropping is often against the desire of users. But when mobile users roam within a populated domain or across to a different domain, complete elimination of these unpleasant events could be next to impossible. To minimise the disturbance, one possible approach is to pre-configure PRIMATE at the next location so that radio and network resources can be reserved and setup in advance. This has the advantages of • • • •
Reducing the call dropping probability Decreasing the handover latency once the handover decision is made Minimising the number of unnecessary handovers Postponing a session if substantially larger bandwidth will be available in the next location. Local Network
Local Network
P0
P1
Best Effort
0
P3
3 1 (a)
Local Network
pre-configured connection
existing connection
P2
P0
P1
Medium Quality
2 0
P3
3 1 (b)
P2 (75%)
P0
P1
High Quality
2 0
P3
P2
(20%)
3 1
(75%)
2
(c)
Figure 5. The level of QoS and the user’s mobility pattern determine which PRIMATEs to be involved in the network preconfiguration and resource reservation
Proceedings of 5th Intl. Workshop on Mobile Multimedia Communication MoMuc’98, October 12-14 1998, Berlin
IV. PROTOTYPE IMPLEMENTATION AND EXPERIMENTAL RESULTS
Ethernet card
Our current testbed consists of a local network with 3 ATM switches (Fore Systems) and a few Pentium PCs used as mobile terminals, base stations and an Internet gateway. Base stations are connected to the switches via an Efficient 155 Mbps ATM card and to the mobile terminal via a Lucent Technologies 2 Mbps WaveLAN card (see Figure 6).
Gateway ATM card
F ORE LE 155
F ORE LE 155
F ORE ASX-200BX ATM card
A.
Functional Modules of PRIMATE
As been described in Section II, PRIMATE provides user mobility, protocol conversion and application adaptation supports to realise the concept of Adaptive QoS. The initial implementation focuses on the basic issues such as end-toend data connectivity, handover management and location management. In the next stage we will investigate QoS related issues like QoS mappings at the network boundaries and the PRIMATE pre-allocation scheme using Prediction Confidence Ratio. A brief description of each implementation is given below:
component
for
our
1) User Application Since PRIMATE provides the function of application adaptation and protocol conversion, we are able to use any off-the-shelf IP applications in our testbed environment with the Adaptive QoS functionality add-on.
PRIMATE QoS Manager
User App
CCM Manager Application Adaptation module
Protocol Suite (wireless)
Boomerang
CCM Manager Protocol Suite (wireless)
ATM card
ATM card
BS1
We have software proxy modules called PRIMATE located at the mobile terminal, the base stations and the gateway, which coordinate the communication among the user application, the WaveLAN radio link, the ATM network and the Internet. The PRIMATE has been implemented under Red-Hat Linux and the majority of the code is written in the Java language. Currently some functions of PRIMATE require C programming for the purpose of low level device access. These include the ATM network interface, the WaveLAN signal quality monitor, and the redirections of data flows from the user applications and from the Internet. The detailed components of PRIMATE in our testbed are shown in Figure 7 and their functionality will be explained in the next section.
BS3
BS2
waveLAN card
waveLAN card
waveLAN card waveLAN card MT
Figure 6. The layout of our prototype implementation 2) Boomerang Implemented in the system kernel, Boomerang [12] intercepts the data transmitted by the user application and redirects it to the PRIMATE. This allows PRIMATE to transparently provide adaptivity to all applications. 3) QoS Manager In general, the QoS management system can use a proactive approach [15] or a reactive approach [16], and a full discussion of their merits is out of the scope of this paper. A reactive framework called User Service Assistant [16] is used in this implementation to forward QoS requests from the mobile user to PRIMATE. 4) Application Adaptation Modules These modules are application-specific and located at the mobile terminal and base stations. In our framework, they provide application adaptivity for changing conditions at the wireless link. Performance results of a MPEG player specific module and a web browser specific module are provided in [12]. 5) Protocol Suite This is a collection of various network protocols to perform native QoS signalling and data transfer across different networks. Currently only TCP/IP, UDP/IP and UBR/ATM are supported, but additional protocols such as RSVP,
PRIMATE
PRIMATE
Application Adaptation module
QoS Translator
Protocol Suite (ATM)
CCM Manager
QoS Translator
Protocol Suite (IP)
ATM Local Network Protocol Suite (ATM)
Gateway Mobile Terminal
INTERNET
Base Station
Figure 7. Functional modules of PRIMATE at various places of the prototype implementation
Internet Backbone Network
Proceedings of 5th Intl. Workshop on Mobile Multimedia Communication MoMuc’98, October 12-14 1998, Berlin
DIFFSERV, CBR/ATM, VBR/ATM or any QoS capable wireless protocols can be included in the future.
Table 1. Throughput and Round-trip-time measurement at various point in the prototype Test Location
6) Connection Control and Mobility (CCM) Manager
Mobile Terminal 25.6 112.4 1.8 0.5
Base Station 1.2 1.2 6.1 3.5
Internet Gateway 1.2 N/A 9.1 N/A
Throughput PRIMATE CCM Manager plays an important role to (Mbps) Direct support user mobility in an ATM local network RTT PRIMATE without mobility-enabled switches. Its major (ms) Direct functions include connection control, re-routing control and location management. CCM Manager uses various protocol suites to setup a dedicated B. Implementation Progress and Experimental Results channel for mobility signalling which spans the mobile terminal, the base station and the gateway. At the point of writing, we have finished the testing of end7) QoS Translator This module is responsible for matching those QoS semantics between different kinds of network. At the current approach, three classes of QoS are defined – “best effort”, “medium quality” and “high quality”. A full range of QoS mappings among different networks can be included in the future once they are available. (a) Test Client
(b) Test Client
Test Server
System Kernel
System Kernel
Test Server
P
Boomerang
Mobile Terminal
Mobile Terminal
Figure 8. Testing the performance of PRIMATE at the mobile terminal
(a) Test Client
Test Server System Kernel
System Kernel
Mobile Terminal
WaveLAN
Base Station
(b) Test Client System Kernel
P
P
Test Server System Kernel
to-end data flows that spans the wireless link, the ATM local network and the Internet backbone. We are currently constructing the handover and location management functions. Common applications such as Telnet and Netscape have been running successfully in this heterogeneous environment. To further investigate the performance of this setup, we use a standard traffic generator called TTCP to measure the throughput and a simple loopback program to find the round-trip-time (RTT). As shown in Figure 8(b), 9(b) and 10, a test server (TTCP or RTT program) was inserted at the mobile terminal, the base station or the gateway. We measured the throughput and round-tip-time as experienced by a test client in the mobile terminal. To identify any overhead, these results are compared with a similar testing condition without the presence of PRIMATE (see Figure 8(a) and 8(b)). The average values of these measurements are summarised in Table 1. From Table 1, it is found that the maximum throughput across our prototype implementation was 1.2 Mbps, which is roughly equal the effective bandwidth of the WaveLAN card. This is expected since the wireless link is the bottleneck of this testbed. The maximum throughput within the mobile terminal has dropping from 112.4 to 25.6 Mbps, mainly due to the process overheads of Boomerang and the Java virtual machine. We believe that this deduction of bandwidth within the mobile terminal will not affect the overall performance of our prototype.
Regarding the round-trip-time, there was also a slight penalty running Boomerang and Java virtual machine at the WaveLAN Mobile Terminal Base Station mobile terminal, which incurred a RTT of 1.8ms. An additional time delay of 4.3ms was introduced when data Figure 9. Testing the performance of PRIMATEs at was looped back from the PRIMATE at the base station. the base station Finally, an extra round-trip-time of 3ms was required for data moving between the base station and the WaveLAN gateway. Since the round-trip-time across the Test Base StationTest local ATM network is considerably shorter P P P Server Client ATM (0.6ms), we believe that the Java virtual Local System System System machines at both ends introduced the Network Kernel Kernel Kernel Boomerang remaining delay of 2.4 ms while using the ATM socket calls via the Java Native Base Station G a t e w a y Mobile Terminal Interface. Figure 10. Testing the performance of PRIMATEs at the Gateway Boomerang
Proceedings of 5th Intl. Workshop on Mobile Multimedia Communication MoMuc’98, October 12-14 1998, Berlin
Comparing the RTT between PRIMATE and direct configurations in Table 1, we observe that PRIMATE introduced some extra delay along the data flows, which is unavoidable in any proxy systems. But considering PRIMATE’s additional functionality and possible Java compiler optimizations, this prototype should provide an acceptable environment for our future works. V. FUTURE WORK AND CONCLUSION In this paper we have introduced the concept of Adaptive QoS to provide a “better service” for mobile users in a heterogeneous network environment. The realisation of Adaptive QoS is achieved from a framework of multiple proxy modules called PRIMATE, which supports user mobility, protocol conversion and application adaptivity. We use a QoS-driven algorithm called Prediction Confidence Ratio to allow pre-allocation of network resources. It enables the network to have a better QoS compliance while the users are on the move. Through the prototype implementation we have shown that this framework is workable in a mixed ATM/IP environment. We intend to include a DECT wireless access system in order to evaluate the mobility issues across different wireless interface. ACKNOWLEDGEMENTS The research work of J. Chan has been partially funded by CSIRO Telecommunications & Industrial Physics. REFERENCES [1]
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at
[10] S. Zhou et al., “An Efficient Media Access Control Protocol for Wireless ATM Networks,” Proc. IEEE GLOBECOM’98, Nov. 1998. [11] C. Diot, “ Adaptive Applications and QoS Guarantees”, invited paper to IEEE Multimedia Networking, Aizu, Japan, Sep. 1995. [12] R. De Silva et al., “Total Management of Transmissions for the End-User, a Framework for User Control of Application Behaviour,” 3rd HIPPARCH’98Workshop, London, Jun. 1998. [13] G. Liu, “The Effectiveness of a Full-Mobility Architecture for Wireless Mobile Computing and Personal Communications,” Ph.D. Thesis, Royal Institute of Technology, Sweden, Mar. 1996.
[14] J. Chan, et al., “A QoS Adaptive Mobility Prediction Scheme for Wireless Networks”, Proc. IEEE GLOBECOM’98, Nov. 1998. [15] A. Vogel, et al., “Distributed Multimedia and QoS: A Survey”, IEEE Multimedia, Vol. 2, No. 1, summer 1995. [16] B. Landfeldt, et al., “User Services Assistant: an end-to-end reactive QoS architecture”, Proc. IFIP 6th International Workshop on Quality of Service (IWQoS’98), May 1998.
