Implementing admission control
5 Interconnecting domains
Even within the administration of one operator a number of domains may be defined. One reason for this can be that different flow granularities are re-quested. For example, on the access link of a house-hold the capacity is commonly restricted and there-fore performance and even admission guidelines on individual flows can be implemented. This would not be feasible in a core network where traffic related to millions of users is flowing.
An access link may thereby be supported by resource reservation, policing, packet priorities and other fea-tures as described in the previous chapter. In a core network DiffServ, in combination with MPLS, would likely be implemented. Combining a tighter regime and DiffServ can bring some benefits compared to
“simple” DiffServ. One example is that admission control can be applied at the border of the DiffServ domain. Explicit signalling per flow allows for admission control such that the flows in each traffic class receive the service level expected. Voice con-versations are examples where admission control could be fruitful to ensure that the ongoing tions get the service level and additional conversa-tions are rejected in case there are not sufficient net-work resources.
Schematically, a multi-domain path can be illustrated as in Figure 18. Note that this shows a point-to-point configuration, also referred to as a “pipe model”
where the two end-points are given. Such an illustra-tion could be used for defining a hypothetical refer-ence connection. This can be assumed for allocating service level degradation to the different network seg-ments. On the other hand, the allowed degradation may also be negotiated pair-wise between the providers in the chain on commercial terms.
The different QoS parameters can be accumulated along a path in different manners. For example, a mean delay parameter is additive; contribution from each segment can be added. For calculating loss ratio, error ratio and availability, a multiplication is com-monly applied. Delay variation is often estimated by convoluting, see e.g. ITU-T Recommendation Y.1541 amd. 2.
When explicit signalling per flow is used, policy-based control, e.g. per user and per application can be introduced in a more dynamic way. Moreover, if the router in the network is doing the packet marking sig-nalling can be used to convey the information to the router on which DiffServ class to apply for each flow.
This would particularly be useful in case IPSec is applied if the IP addresses and port numbers are not statically assigned to DiffServ classes.
5.1 User’s rights – AAA functionality When providing a service commercially and different service levels are used, they have to be supported by corresponding AAA (Authentication, Authorisation, Accounting) functionality. Accounting can be seen as included in the service provisioning process (called integrated accounting) or it can be offered as a sepa-rate service (called discrete accounting). In the for-mer the accounting is coupled to a specific service, collecting relevant information by using service spe-cific entities.
For getting access to a service, the user sends a ser-vice request to the AAA server. This checks the authorisation of the user and, assuming access is granted, forwards the necessary information to the relevant server. This server finds the information relevant for configuration of the network resources (service equipment) and distributes this information
Figure 18 Reference path illustration from ITU-T Recommendation Y.1541 IP network cloud
SRC
LAN UNI
LAN UNI
SRC DST
Network section Network section Network section End-to-end network (Bearer QoS)
User-to-user connection (Teleservice QoS)
Customer installation Customer installation
TE GW GW GW GW GW GW TE
to the network nodes. In case of DiffServ, the accounting system, QoS control and Bandwidth Broker are noted.
In ETSI-TISPAN, these actions are initiated by the Network Attachment Subsystem, NASS, ref. [ETSI-282001]. NASS provides the following functionalities:
• Dynamic provisioning of IP addresses and other terminal configuration parameters;
• Authentication taking place at the IP layer, prior to or during the address allocation procedure;
• Authorisation of network access based on user pro-files;
• Access network configuration based on user pro-files;
• Location management taking place at the IP layer.
In order to carry out its tasks, the NASS interacts with the HSS, which is a core element for storing user data, ref. [Ross06].
5.2 Managing resources
Returning to the ETSI-TISPAN reference architec-ture, the resource and admission control functionality (RACS) contains mechanisms for
• Admission control, including checking authoriza-tion based on user profiles held in the access net-work attachment subsystem, on operator specific policy rules and on resource availability;
• Gate control, including network address and port translation, priority packet marking.
In the same architecture, a Border Gateway Function (BGF) provides the interface between two IP-trans-port domains. A BGF may reside at the boundary between an access network and the customer premises equipment, between an access network and a core network or between two core networks. It sup-ports one or more of the following functionalities:
• Opening and closing gates (i.e. packets filtering depending on “IP address / port”);
• Allocation and translation of IP addresses and port numbers (NAPT);
• Interworking between IPv4 and IPv6 networks (NAPT-PT);
• Topology hiding;
• Hosted NAT traversal;
• Packet marking for outgoing traffic;
• Resource allocation and bandwidth reservation for upstream and downstream traffic;
• Policing of incoming traffic;
• Antispoofing of IP addresses;
• Usage metering.
As noted above the RACS applies a policy concept, see Section 4.7. The components are similar to the ones depicted in Figure 17. The Bandwidth Broker (BB) concept is similar to a Policy Decision Point (PDP) in the sense that it makes decisions regarding bandwidth provisioning. However, bandwidth bro-kers tend to operate at a higher level than PDPs.
PDPs are typically connected to a (small) number of PEPs within an administrative domain. They tend to be topology-aware as a result of their role, e.g. in the RSVP admission control process. Bandwidth Brokers are aimed more at the interfaces between domains.
A BB refers to an abstraction that automates the admis-sion control deciadmis-sions for service requests in a network domain. This means that it is responsible for keeping track of the current allocation of reserved traffic, it is configured with policies that define which traffic flows belong to which traffic classes, and it interprets new requests in the light of these policies and the current bandwidth usage. In this sense, a BB can be considered as a special type of policy server that is responsible for those related policies for a network domain. A BB is not necessarily a policy manager but policy management and bandwidth brokering will need to work together in providing integrated policy services and admission con-trol. Another important function of a BB is to configure network devices according to admitted QoS requests.
In the intra-domain case, the BB manages the re-sources based on the SLA/SLS that has been agreed upon between domains. One or more protocols are used to exchange information between a host and a BB, and a BB and a router.
In the inter-domain case, the BB is also responsible for managing inter-domain communication with BBs in neighbouring networks. This is to co-ordinate SLAs across boundaries. In order to co-ordinate bandwidth assignments across domains, a single inter-domain BB protocol must exist.
Figure 19 shows a sample network configuration. It consists of three domains AS1, AS2, and AS3 with a BB for each one (BB1, BB2 and BB3). The SLAs/
SLSs are placed between AS1 and AS2, and between AS2 and AS3. A user can be either an end-system or an application that requests bandwidth.
The Bandwidth Broker makes decisions based on the network topology and the network traffic
characteris-Figure 20 Some of the relevant data for managing SLAs and QoS
Customer data
Service descriptions
SLA
SLA templates
Collected performance data
SLA reports Customer and SLA
performance data tics. The network topology consists of a description
of all available network resources: nodes, links, link metrics, physical link capacities, e.g. link capacity that can be allocated, resource class (gold links, links only to be used for premium customers), etc. The net-work traffic characteristics are expressed as a set of traffic trunks which mainly express a bandwidth requirement between core edge nodes. This informa-tion is supplied by the policy manager, which is a storage of committed SLAs/SLSs.
5.3 SLA/SLS negotiation
Having the capability to establish SLAs rapidly, accurately and automatically is a significant contri-bution to the efficiency of a provider. This becomes even more important when the number of services and customers grows. Having adequate SLA-related mechanisms is therefore considered as a competitive edge by the providers/operators. As there are also dependencies between the providers, the SLAs need to be present throughout the set of providers involved, not only towards the end-customer.
Handling QoS and SLA in an efficient manner intro-duces a number of challenges; a few aspects are illus-trated in Figure 20. Several non-technical aspects will also be included in an agreement between the actors.
In addition to the data transfer related aspects, issues like customer support and service provisioning will often be covered.
The SLA template is used to capture a set of Service Level Objectives for a service. A Service Level Objective is a representation of the guaranteed level of service offered. It defines an individual objective for example in terms of service metric, threshold val-ues and tolerances. A service metric could be related to the entire service bundle, to a service element or to a single service interface, but is always related to something visible to the customer. Hence, the QoS requirements described for voice, video and data ear-lier come into play.
The technical part of an SLA is called a Service Level Specification, SLS, although the actual rela-tions between SLAs and SLSs can be more involved.
A service can be said to be provided to a customer by a provider. Prior to the service delivery, negotiation would commonly take place.
The components of an SLS can be grouped as (note that this assumes DiffServ-based services):
• Common unit: Describes the terms of offering the service, e.g. identifying the provider, customer, ser-vice type, etc. The period of validity is a central component.
• Topology unit: Describes the nature and number of end-points, further divided into one Service Access Point, SAP, unit and a number of graph sub-units. The SAP sub-unit gives a list of end-points that specify the topology (like hose, pipe or fun-nel). The end-points can for instance be given by IP addresses. The graph sub-unit gives a list of sources and destinations and how these are related.
Unidirectional and bidirectional relations may be described.
• QoS related unit: Describes the traffic flows and the service differentiation provided. Quantitative and qualitative service levels may be given for some or all parts of the topology unit. This unit may further be divided into: i) scope – giving the topology unit (graph sub-unit or end-point) relevant, ii) traffic descriptor – describing the traffic flows (including Figure 19 Communication among Bandwidth Brokers
BB1 BB2 BB3
RARs
SLSs SLSs
AS1 AS2 border AS3
router service
users
Inter-domain communication Intra-domain communication border router
DiffServ class, port numbers, protocol information and specification of lower layer), iii) load descriptor – giving the quantity of offered traffic, e.g. given by leaky bucket parameters, as well as treatment of excess of out-of-profile traffic, iv) QoS parameters – delay, jitter and loss of traffic flow.
• Monitoring unit: Defines a set of parameters that are to be collected and reported between the cus-tomer and provider. The structure might be similar to the QoS-related unit.
6 Concluding remarks
Is introducing IP QoS worth while? QoS-related mechanisms imply a more efficient network and wider spectre of service portfolio. Hence, savings on the cost of network resources as well as potential additional revenues are within the reach by imple-menting such mechanisms.
There is a trend towards multiple applications on IP networks covering all components of a multimedia application. In the long run, this requires that QoS-related mechanisms are smoothly operated and thus being a motivation for this paper. Considering a multi-provider environment this becomes more chal-lenging. Industry initiatives, e.g. IPSphere, have been started to address these issues. Hence, reflecting on the motivation and effort, it seems to be more a ques-tion of how fast the concerned IP QoS-related mech-anisms need to be introduced.
References
[22.105] ETSI. Universal Mobile Telecommunica-tions Systems (UMTS); Service aspects; Services and Service Capabilities. 2000. (ETSI TS 122 105)
[E.721] ITU. Network grade of service parameters and target values for circuit-switched services in the evolving ISDN. Geneva, 1999. (ITU-T Recommenda-tion E.721)
[E.860] ITU. Framework for a service level agree-ment. Geneva, 2002. (ITU-T Recommendation E.860)
[ES.282] ETSI. Telecommunications and Internet converged Servies and Protocosl for Advanced
Networks (TISPAN); NGN Functional Architecture Release 1. Draft, April 2005. (ETSI ES 282 001)
[EU.P806] EQoS – A Common Framework for QoS/Network Performance in a multi-Provider Environment. EURESCOM project 806. URL:
http://www.eurescom.de/public/projects/P800-series/P806/default.asp
[G.1010] ITU. End-user multimedia QoS categories.
Geneva, 2001. (ITU-T Recommendation G.1010)
[Jens03] Jensen, T. Planning Dependable Network for IP/MPLS over Optics. Telektronikk, 99 (3/4), 128–162, 2003.
[Kell00] Kelly, F, Key, P, Zachary, S. Distributed Admission Control. IEEE Journal on Selected Areas in Communications, 18 (12), 2617–2628, 2000.
[RFC2475] IETF. An Architecture for Differentiated Services. 1998. (RFC 2475)
[RFC2722] IETF. Traffic Flow Measurement: Archi-tecture. 1999. (RFC 2722)
[RFC2753] IETF. A Framework for Policy-based Admission Control. 2000. (RFC 2753)
[Ross06] Rossebø, J, Sijben, P. Security issues in VoIP. Telektronikk, 102 (1), 130–145, 2006. (This issue)
[Ulse06] Ulseth, T, Stafsnes, F. VoIP speech quality – better than PSTN? Telektronikk, 102 (1), 119–129, 2006. (This issue)
[Y.1540] ITU. Internet protocol data communication service – IP packet transfer and availability perfor-mance parameters. 1999. (ITU-T Recommendation Y.1540)
[Y.1541] ITU. Network performance objectives for IP-based services. 2002. (ITU-T Recommendation Y.1541)
[Øste2005] Østerbø, O. Performance Modeling of an Upstream Link in Open Access Networks. Broad-band Wireless Access Network Workshop, Florida, USA, June 2005. (www.ist-oban.org)
Dr. Terje Jensen is Senior Research Scientist at Telenor Research and Development. In recent years he has mostly been engaged in strategy studies addressing the overall network and system portfolio of an operator.
Besides these activities he has been involved in internal and international projects in various areas, including network planning, performance modelling/analyses and dimensioning.
email: [email protected]
Introduction
The customers of a VoIP service have expectations to the speech quality. These expectations are based on the experiences made when making telephone calls over the present circuit-switched network (PSTN/
ISDN). However, the success of mobile systems offering a lower quality than fixed network systems shows that the users may accept quality degradations if there are other benefits (e.g. mobility). On the other hand, the introduction of the GSM Enhanced Full Rate Codec indicates that there is a demand for a quality that at least is similar to that of the fixed network.
In the beginning VoIP had a reputation for being cheap and of low speech quality. The low speech quality may be acceptable to enthusiasts in the initial phase, but is hardly acceptable on a longer term.
This article presents an overview of how speech qual-ity is measured or assessed and factors that are influ-encing the user perceived speech quality. Finally, an attempt is made to answer the basic question ‘Is the VoIP speech quality better (or poorer) than the PSTN speech quality?’.