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Out-of-band Signalling Channel for Efficient Multicast Service Delivery in Heterogeneous Wireless Networks Alexander Gluhak, Klaus Moessner, Rahim Tafazolli Centre for Communication Systems Research Mobile Communications Research Group The University of Surrey Guildford, GU2 7XH, UK Email: {a.gluhak,k.moessner, r.tafazolli}@surrey.ac.uk
The reminder of the paper is structured as follows. Section 2 outlines the application scenario, motivating the employment of the proposed MSC. In section 3 the MSC is presented, providing an analysis of the employment gains of the MSC. Furthermore the proposed scheme for receiver subset addressing on the MSC is described in more detail. Then guidelines for signalling network selection are provided in section 4. Concluding remarks are given in section 5 outlining future research directions.
Abstract— Multicast delivery in heterogeneous wireless networks requires careful coordination, in order to take full advantage of the resources such an interworking network environment can offer. Such coordination may demand interworking signalling from coordinating network entities to receivers of a multicast service. Scalable delivery of interworking signalling to receivers of multicast user services is of great importance, since the numbers of receivers may be very large. This paper therefore investigates the use of a multicast signalling channel (MSC) to carry such interworking signalling in a scalable manner. Analytical evaluations are performed, comparing required signalling load on the MSC to unicast signalling. In order to further increase the efficiency of the MSC a novel approach is proposed that allows efficient addressing of a subset of receivers within a multicast group. Furthermore guidelines for the selection of a suitable signalling network carrying the MSC are provided.
2. APPLICATIONS FOR MULTICAST CONTROL SIGNALLING Previous work within the MIRAI [2] project, has proposed an out-of-band signalling mechanisms and network entities to facilitate seamless network interworking [3]. The control signalling, also called basic access signalling (BAS) is used for user registration, radio access network discovery, location update, session setup, and media handover. Such signalling can be provided over a dedicated basic access network (BAN), used only for the signalling purposes, or over one of the available radio access networks. The proposed mechanisms, however, mainly targeted unicast communication. Our work draws on the concepts and experiences of MIRAI and aims to provide an efficient signalling solution for coordinated multicast service delivery in HWN. Previous work in the Core 3 research program of Mobile VCE [5] has therefore proposed and investigated a more network-centric approach [4], which allows the coordination of multicast user services by (possibly distributed) network management entities. Using scenario based analysis the required signalling for advanced interworking functionality such as dynamic network selection or load balancing have been studied. It has been found that in the case of multicast service delivery in HWN, the same interworking signalling information may have to be delivered to multiple receivers. The following signalling applications involving groups of receivers have thus been identified:
Key words: Multicast Service Delivery, Control Signalling, Heterogeneous Wireless Networks 1. INTRODUCTION While looking for different ways to improve the efficiency of service delivery, mobile network operators have recently discovered multicast delivery as promising solution in mobile networks and are currently standardising Multimedia Broadcast and Multicast Services [1] in UMTS Release 6. Furthermore in order to offer superior services to their mobile customers, operators with different access network technologies, e.g. DVB and WLAN, are likely to cooperate in a future network scenario. Such a heterogeneous wireless network (HWN) environment, however, requires new mechanisms to facilitate efficient network interworking for multicast service delivery. This paper investigates the use of a multicast signalling (MSC) channel as a scalable approach for delivering downlink signalling for network interworking support to groups of heterogeneous receivers. Our analysis reveals two main issues that are crucial for the design of such an MSC, namely the selection of an appropriate signalling network and effective addressing of subset of receivers subscribed to the signalling channel. We give guidelines for the selection of an appropriate signalling network and propose a novel efficient mechanism for addressing a subgroup of receivers within a multicast group. The proposed mechanism minimises the required signalling load by allowing an aggregation of receivers, which is based on context information receivers may have in common.
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Masugi Inoue National Institute of Information and Communications Technology 3-4 Hikarino-oka, Yokosuka Kanagawa, 239-0847 Japan Email:
[email protected]
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Network-initiated establishment of multicast bearers. In order to allow advanced interworking functionality such as dynamic access network and bearer selection, network operators need to initiate the establishment of suitable multicast bearers. Intelligent algorithms [6] could select suitable bearer paths, e.g. according to availability of current network resources or terminal capabilities and then trigger the establishment of those at interested receiver groups. Vertical network handoff for groups of receivers. During a multicast session conditions may arise where a
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change of current access network may be required. An example of a network initiated vertical handoff would be a load balancing situation: In order to free up their own network resources, an operator may decide to handover a group of receivers in several cells of an access network to an alternative access network of an interworking operator. Thus signalling has to be delivered to the respective receivers in order to initiate a change of the delivery network. Flow handoff for groups of receivers. Often a change of access network may lead to a change of the delivery characteristics e.g. bandwidth availability. In such cases the flow within a multicast session may change, e.g., different QoS of a media flow or addition/removal of media flows. Furthermore to increase the delivery flexibility, e.g. in emergency situations, receivers may be temporarily downgraded to a lower rate flow in order to free up network resources or may be upgraded if sufficient network resources are available. Radio access network discovery: Network assisted radio access network discovery allows terminals to have knowledge of available access networks, without the need for frequent scans on all network interfaces. Thus significant power saving can be achieved [7]. Groups of receivers at a similar location can be provided with the same network discovery information.
IP Header (Unicast)
(a)
IP Header (Multicast)
Addressing Expr.
Signalling Payload
(b)
Figure 1. message.
Format of a unicast(a) and a multicast(b) signalling
as addition to the signalling message. The addressing expression allows to identify the subset of receivers, for which the signalling message is actually intended. Receivers evaluate the addressing expression above the network layer, and based on the outcome of the assessment, accept or discard the signalling message. Figure 1 shows the format of a control message sent via unicast (a) and a multicast message (b) on the MSC. 3.2. Analytical Evaluation In order to employ the proposed MSC successfully, its advantages and disadvantages need to be fully understood. This section therefore presents an analytical evaluation of the signalling load of the MSC approach, by comparing the signalling load requirements on the MSC to unicast delivery. The analysis considers only the bandwidth requirements on the last hop wireless link and neglects the core network resources. This assumption can be justified, since the wireless resources usually represent the bottle neck within a wireless network. Parameters for the analytical comparison of required signalling load for both cases are described in the following: let Cuni be the signalling cost for message delivery in unicast case and Cmulti be the respective signalling cost for delivery over the multicast signalling channel. Furthermore let Nuni denote the number of bits for an IP header in the unicast case, Nmulti the number of bits for an IP header in the multicast case, Nexp the size of the addressing expression required for receiver subset addressing on the MSC and Nmsg the number of bits of the actual higher layer message payload. Sending a message to r receivers, the following signalling costs can be identified for the unicast delivery case:
In order to provide the required control signalling in HWN to a potentially large group of receivers, a scalable way of delivery is essential. In the next section we explore the deployment of a MSC to carry control signalling from coordinating network entities to groups of receivers. 3. MULTICAST SIGNALLING CHANNEL 3.1. Principles The key idea for the deployment of a MSC is to reduce the number of signalling messages and hence the signalling load in the network, when the same signalling message needs to be delivered to a group of receivers. Instead of delivering a signalling message via unicast individually to all affected receivers, a single message would be send to a specific multicast group address, identifying the MSC. Receivers, which need to receive the signalling message would subscribe to the multicast group address of the MSC over which the message is sent. Ideally only receivers, for which a control signalling message is intended, should be subscribed to the MSC at the time the signalling message is sent. While this concept may work perfect in theory, it exhibits some practical problems. In most cases a control message will not be intended for all receivers of a multicast user service. Rather a signalling message, such as a request for vertical handoff would target only a subset of receivers, e.g. only receivers which are capable of establishing a multicast bearer in the new access network. This would require a way to inform the receivers to subscribe or unsubscribe from the MSC, whenever a new message needs to be sent. While such notifications will add to the required signalling load, frequent ’joins’ and ’leaves’ of receivers to multicast groups will further increase the signalling load, thus reducing the foreseen benefits of the MSC. It is therefore more realistic to assume that all receivers of a particular multicast service are subscribed to the same MSC for the lifetime of the session. In case of access network discovery, receivers at certain geographic areas receive their information from the same MSC. Within the MSC an addressing expression is used
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Signalling Payload
Cuni = r ∗ (Nuni + Nmsg )
(1)
While the cost for delivering a control signalling message via unicast is directly proportional to the number of receivers, the signalling cost for a multicast message is independent of the number of receivers as seen in equation 2. In contrast the signalling cost for delivering a message via multicast is proportional to the number of cells m, in which wireless access network resources are utilised. Cmulti = m ∗ (Nmulti + Nexp + Nmsg )
(2)
Assuming that the signalling network used for the MSC consists of multiple cells, a multicast bearer needs to be established in all cells, which host receivers that are subscribed to the MSC. A signalling message is sent in each of the cells, regardless of the presence of receivers from the subset, for which the messages was actually intended. A main difficulty is thus to find the break-even point, which justifies the utilisation of a MSC. Comparing the signalling load of a single message delivery, the use of an MSC can be justified if the following applies:
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Control Signalling Load − Payload : 1000 bits − Addressing Expression: 1000 bits
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Signalling Load (kBytes)
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Figure 2. Signalling load for unicast and multicast delivery for different addressing expression sizes in a single cell.
50 0
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Figure 3. Unicast vs multicast delivery of a control signalling message if receivers are distributed in multiple cells. 3.3. Context Information Based Receiver Subset Addressing
Cmulti ≤ Cuni
The signalling expression on the MSC is used to identify a receiver subset, in case the control message does not apply to all subscribed receivers. In order to minimise the signalling load on the MSC, the required addressing expression should be kept as small as possible. The simplest way to identify a receiver subset is to explicitly encode unique identifiers of each receiver in the addressing expression. Examples of unique identifiers are unicast IP address, as used in the XCast approach [8], or host identity [9]. The approach, however, does not scale well, if the signalling message needs to address a large number of receivers. Assuming 10.000 receivers and the use of a IPv4 unicast address the explicit addressing expression would be as large as 320kBits! Therefore more powerful expressions to aggregate addressing for large subset of receivers are necessary. Analysing the signalling application scenarios, following important observation has been made: In many cases the control signalling targets a group of users with common context e.g. all receivers in a certain area/cells of a network or all receivers currently receiving a flow with the same quality of service. This has motivated to propose a receiver aggregation mechanism for the MSC that is based on receiver context information. A subset of receivers on the MSC is described by context information that those receivers have in common. The requirement for such a mechanism to work is that the network entity, sending the control messages via the MSC has access to the context information of these receivers. Moreover receivers need to evaluate the specification of the context information provided in the addressing expression and be able to infer, wether or not the specification applies to them. Useful context information, which can be used for such address aggregation has been identified and is described in the following: • Receiver location: Often control signalling will target a set of receivers at a geographic location. Geographic locations can be described logically by specifying network and cells or by GPS information. For example load balancing will try to free up resources in a cell or cell cluster of a network by switching the multicast bearer to another access network with more available resources. Users in a cell cluster could be easily
(3)
Substituting (3) by (2) and (1), and with Nmulti = Nuni = Nip the threshold can be rewritten showing the multicast dependent parameters on the left and the unicast dependent on the right hand side: m∗(
Nexp + 1) ≤ r Nip + Nmsg
(4)
Figure 2 shows the signalling load required for delivering a single control message in a single cell as a function of the number of targeted receivers and the size of required addressing expression. A signalling payload of 1000 bits as well as the use of IPv4 headers has been assumed. The figure depicts the signalling load required for the unicast case as well as for the multicast delivery using the MSC. When an addressing expression of 1000 bit size is used to identify a receiver subset, the use of an MSC becomes already efficient, if two or more receivers are targeted by a signalling message. Even for large addressing expression sizes such as 10000 bits (ten times the size of the signalling payload), the use of an MSC can be justified if 10 or more receivers are targeted by a control message. In order to allow efficient delivery also to a small number of targeted receivers, the addressing expression should be kept as small as possible. Therefore a novel technique for efficient receiver subset addressing on the MSC is presented in section 3-3. Most likely not all of the receivers, which are subscribed to the MSC will be located in a single cell of the utilised signalling network. Receivers will be scattered in several cells, for which network resources need to be utilised. Figure 3 shows the overall signalling load for a control message as a function of the number of targeted receivers and the number of cells in which this receivers are distributed. A payload size of 1,000 bits and an addressing expression size of 1,000 bits have been used in the example. At least 94 receivers need to be targeted in the case of 50 cells, in order to gain an advantage by employing a MSC. Therefore the use of an access network with large cell coverage is advantageous if receivers are geographically wide distributed. Section 4 provides a more detailed discussion on signalling network selection.
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Table 1. Context information types, associated attribute types and example values.
aggregated by an expression such as ’all receivers in cells 4,5,6,7 of a UMTS network’. Terminal capabilities: Terminal capabilities are attributes that can be useful to express commonness among users. Terminal capabilities include available network access interfaces (NAI), maximum supported QoS for a connection on a NAI, available memory, software and codecs etc.. A control message initiating a vertical handoff should move all receivers with an appropriate NAI to a different access network. An expression such as ’all receivers with NAI type UMTS’ represents a short but powerful aggregation for such a use case. Receiver preference: As with terminal capabilities, receiver preferences are attributes, which can be used to identify a subset of receivers. Preferences can be expressed in ’delivery network’ or the ’QoS’ for the service flows. A control message using preferred access network as a receiver aggregation could initiate the establishment of a multicast bearer service on the respective access network for all receivers, which have their preferences in common. Communication context: Communication context describes the multicast bearers and flows of a receiver that are currently associated with the reception of a multicast service. Communication context can be described by delivery network, multicast group address, source address, QoS of received flows and source and destination ports. Receivers are aware of their communication context and the information can be used to efficiently aggregate receivers, e.g. receivers subscribed to a common multicast bearer or receiving the same service flow. Note that unlike the previously mentioned context information, communication context does not exist before a communication is established. Therefore communication context can be only used for addressing expressions of control messages, which are sent during a session.
Attribute Type
Example Value
Location
Network Cell GPS
Terminal Capabilities
Network Interface Supported QoS
UMTS,DVB 256k
Service Preferences
Delivery Network Desired QoS
DVB 384k
Communication Context
Delivery Network Current QoS Multicast Addr Source Addr Port
WLAN 384k 239.27.0.8 123.44.32.2 1500
DVB 1,2,5 ◦ N49 00’ 32´’. . .
to maximise the flexibility in defining a receiver subset with an addressing expression, a combined addressing expression using explicit addressing and context information based aggregation can be used. We have implemented a prototype of the MSC utilising the proposed addressing scheme for receiver subset addressing and performed successful tests in a network environment. In the initial version, XML has been used to encode addressing expressions. XML has been chosen due to its good human readability and extensibility by new context information types. Figure 5 shows an example of an XML encoded addressing expression. During the experiments with the prototype it was found that the use of XML adds considerable overhead to the context information based addressing expression. The overhead comes mainly from the tags, which defined the structure and nature of the addressing information. Since the addressing expression size is critical for the performance of the MSC, improvements in the representation of contextual information will have to be made, before deployment in a real world system. One way would be the replacement of the currently used tags by shorter ones at costs of the readability, which would not matter once the experimental phase is concluded. A more effective alternative would be the definition of proprietary binary encoded information elements.
The above list of context information can be easily extended as new useful context information for aggregation becomes available. Within the addressing expression, context information is expressed as a key-value pair, with the type of context being the key, and a context attribute representing the value. Context attributes can be further composed of attribute-types and associated attributevalues. For example the context type ’location’ can be expressed by attribute types ’network’ and ’cell’, with ’UMTS’ and ’7,8,9’ being examples of the respective attribute-values. Two expressions of context information can be combined by logical operators to characterise a receiver subset more specifically. Furthermore the notion of a compound attribute is introduced, which allows arbitrary combination of key-value pairs with logical operators. Table 1 shows an overview of commonly used context types, associated attribute-types and typical values.
4. SIGNALLING NETWORK SELECTION The selection of an appropriate signalling network is an important means to reduce the overall signalling load required for the delivery of a message on the MSC. A signalling message, which is sent on the MSC is transmitted in every cell in which the MSC is established. Thus available bandwidth and hence radio resources are utilised in each of those cells. Although all receivers of a multicast user service would subscribe to a respective MSC,
Figure 4 shows an example of a context information based addressing expression. The expression describes all receivers with a DVB network interface present in cells 6,7 and 8 of a UMTS network. In a load balancing scenario a control message with this addressing expression could thus initiate a vertical handoff to a DVB network to free up resources in cells 6,7,8 of the UMTS network. As demonstrated in the example, a receiver subset can be accurately identified with a relatively short expression, instead of using hundreds or more explicit addresses. In order
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Context Information Type
{ [’Location’:([’Network’:’UMTS’] & [’Cells’:’6,7,8’])] & [’Interface’:’DVB’] } Figure 4. Example of a context information based addressing expression.
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5. CONCLUSIONS
UMTS 1,2,4,5 AND DVB AND DVB Figure 5.
In this paper a multicast signalling channel has been proposed to efficiently carry out-of-band interworking signalling to groups of receivers for coordinated multicast service provisioning in HWN. The selection of an appropriate signalling network as well as efficient mechanisms for addressing a subset of receivers have been identified as crucial design issues for such a signalling channel. While giving guidelines for the selection of a suitable signalling network, we also propose a novel efficient mechanism for addressing a subset of receivers within a multicast group, by aggregation of receivers based on context information they have in common. Despite the potential signalling performance gains of the MSC, situation may occur, where unicast signalling would be preferable, e.g. very small subset of receivers to be targeted by a control message. In such cases, a hybrid solution may be employed, which could switch between multicast and unicast signalling delivery based on a threshold. Such mechanisms however require further investigation.
Example of an XML encoded addressing expression.
ACKNOWLEDGMENT The work reported in this paper has been part of a collaboration of the Virtual Centre of Excellence in Mobile & Personal Communications, Mobile VCE, www.mobilevce.co.uk and National Institute for Information and Communication Technolgy (NICT). This collaboration has been made possible by Japanese Society for Promotion of Science (JSPS), whose funding support is gratefully acknowledged.
a signalling message would often target only a subset of those receivers. In the worst case this subset of receivers is located in a single cell, thus the message is unnecessarily transmitted in the remaining other cells. In contrast no resources would be wasted if receivers of the destined subset are present in each of the cells, where the MSC is established.
REFERENCES
In order to utilise radio resource efficiently, the area covered by the cells of the signalling network carrying the MSC should approximately match the area occupied by the receivers subscribed to the MSC. Ideally a signalling network providing the MSC would cover all receivers subscribed to that channel by a single cell. That way the receivers could be reached by a single transmission, utilising network resources only in a single cell. Therefore networks employing larger cell structures such as broadcasting networks, e.g. DVB, would be suitable candidates for carrying an MSC. The fewer cells are needed to provide the MSC to the receiver group, the less overall network resources are claimed for the transmission of a message. However networks with a large cell sizes do not provide a good granularity. In some cases networks with hierarchical network structure, e.g. employing both micro and macro cell overlays would provide the highest flexibility. Thus micro and macro cells could be combined for providing the MSC in the required area, without unnecessarily trading in radio resource and coverage efficiency.
[1] T. S. G. Services and S. Aspects, 3GPP TS 22.246 V6.2.0: Multimedia Broadcast/Multicast Services (MBMS) user services; Stage 1 (Release 6). 3rd Generation Partnership Project 3GPP, Sept. 2004. [2] G. Wu, M. Mizuno, and P. Havinga, “Mirai architecture for heterogenoeus networks.” IEEE Communications Magazine, pp. 126–134, Feb. 2002. [3] M. Inoue, K. Mahmud, H. Murakami, M. Hasegawa, and H. Morikawa, “Novel out-of-band signalling for seamless interworking between heterogeneous networks.” IEEE Wireless Communication Magazine, pp. 56–63, Apr. 2004. [4] A.Gluhak, K.Moessner, and R.Tafazolli, “Controlled multicast service delivery in heterogeneous wireless networks.” in Proceedings of the IEICE General Conference, Osaka, Japan. IEICE, Mar. 21–24 2005. [5] “Mobile Virtual Centre of Excellence, www.mobilevce.com.” [6] A.Gluhak, K.Chew, K.Moessner, and R.Tafazolli, “Multicast bearer selection in hetergenoues wireless networks.” in Accepted for publication by the International Conference on Communication, ICC 2005 , Seoul, Korea. IEEE, May 16–20 2005. [7] K. Mahmud, M. Inoue, H. Murakami, M. Hasegawa, and H. Morikawa, “Energy consumption meassurement of wireless interfaces in multi-service user terminals in heterogeneous wireless networks.” IEICE Transactions on Communications, pp. 1097–1109, Mar. 2005. [8] R. Bovie and et al., draft-ooms-basic-spec-00.txt: Explicit Multicast (XCast) Basic Specification. Internet Engineering Task Force IETF, Dec. 2000. [9] R.Moskowitz, P. Nikander, P.Jokela, and T. Hendersson, draftmoskowitz-hip-08.txt: Host Identity Protocol(HIP). Internet Engineering Task Force IETF, Oct. 2003.
Finally, problems selecting a suitable signalling network may arise from the heterogeneity of receivers. The above discussion was based on the assumption that the MSC would be provided to all receivers via a a common signalling network. This however may not be feasible in a HWN environment, since receivers may not have the same network interfaces. In some cases there may not be a common access network on which all receivers could be reached. Also a common access network may not always be the best choice to carry a MSC. In such cases a combination of two or more access networks may be required to carry the MSC. Furthermore in some cases it may be feasible to provide a hybrid signalling solution consisting of multicast and unicast delivery e.g. by serving the majority of receivers via an MSC on a common signalling network and providing the message to the remaining receivers via unicast.
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