End to end IP over IEEE 802.16 QoS Framework

Oumarou Halidou and Radouane Mrabet Mohammed V-Souissi University

National Engineer School of Computer Science and System Analysis Agdal, Rabat, Maroc

oumarhalid@gmail.com mrabet@ensias.ma

Abstract—This electronic document is a “live” template. In this paper we are interested to IEEE 802.16, the standard for broadband (high-speed) wireless access (BWA) in a metropolitan area. Many carriers all over the world have been deploying Mobile WiMAX infrastructure and equipment. This technology makes it possible to meet escalating business demand for rapid Internet connection and integrated data, voice and video services. With increasing deployment of wireless broadband infrastructure, it is not enough to count on the MAC layer QoS mechanism provided in the last mile wireless connection. Therefore we propose architecture to provide end to end QoS control for IEEE 802.16 standard. To achieve our goal, we defined mapping rules to interface IP and 802.16 layers, and scheduling mechanisms for different service classes. We also propose an SLA management model based on SOA paradigm and policy rules to administrate a network domain. (Abstract)

Keywords-component; IEEE 802.16; WiMAX; QoS; DiffSer; IntServ; SLA; PDP; PEP (key words)

I. INTRODUCTION Recently, broadband wireless access network is emerging

for wireless communication for user requirements such as high quality data/voice service, fast mobility, wide coverage, etc. The IEEE802.16 working group develops standards and recommended practices to support the development and deployment of broadband wireless metropolitan area networks.

Thus IEEE 802.16e Mobile WiMAX becomes the standard for broadband (high-speed) wireless access (BWA) in a metropolitan area. Many carriers all over the world have been deploying Mobile WiMAX infrastructure and equipment. For interoperability testing, several WiMAX profiles have been developed by WiMAX Forum [1].

Besides Quality of Service (QoS) support, the IEEE 802.16 standard is currently offering a nominal data rate up to 100 Mega Bit Per Second (Mbps), and a covering area around 50 kilometres. Therefore a deployment of multimedia services such as Voice over IP (VoIP), Video on Demand (VoD) and video conferencing is now possible, which will open new markets and business opportunities for vendors and service providers.

As the deployment of IEEE 802.16 access networks progresses, users will be connected to IP networks. While the

IEEE 802.16 standard defines the encapsulation of an IPv4/IPv6 datagram in an IEEE 802.16 Media Access Control (MAC) payload, a complete description of IPv4/ IPv6 operation and deployment is not present.

However, the fact that these new technologies use IP for maintaining connectivity increases the problem of QoS of IP network. Actually, the Internet offers a point-to-point delivery service, which is based on the "best effort" delivery model. In this model, data will be delivered to its destination as soon as possible, but with no commitment as to bandwidth or latency. This is inadequate for applications requiring timeliness. For example, distributed multimedia applications need to communicate in real-time and are sensitive to the quality of service they receive from the network. For these applications to perform adequately and be widely used, QoS must be quantified and managed, and the Internet must be modified to support real-time QoS and controlled end-to-end delays. the intent of this paper is to propose a functional architecture for QoS provisioning in IP over 802.16 deployment scenarios.

This paper is organized as follows: Section II briefly describes the IEEE 802.16 standard, while Section III introduces our network architecture model. Finally, the conclusions are drawn in Section IV.

II. IEEE 802.16 STANDARD The IEEE 802.16 standard primarily supports a Point-to-

multipoint (PMP) architecture, with an optional mesh topology. In the PMP mode, communication is possible only between a Base Station (BS) and Subscriber Station (SS). In mesh mode, multi-hop communication is possible between SSs. In this paper, however, we focus on the PMP mode. As shown in Figure 1, the Mac layer is composed of three sub-layer:

• Service Specific Convergence Sublayer (SSCS): The SSCS sublayer defines convergence services for upper layers. It adapts the SDU (Service Data Unit) of the upper layers for use in the MAC layer. It has two convergence services, one for ATM services, and a second to ensure the correspondence packet services (services IP version 4 or 6, service VLANs, Ethernet, PPP etc.).. SSCS is also responsible for classifying the

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packets by source and destination in order to spread the good connection MAC.

• MAC Common Part Sublayer (MAC CPS): This sub-layer is the heart of the MAC layer in that it deals with the establishment and maintenance of connections and the allocation of bandwidth. It receives packets from the SSCS that it will be mapped to connections with different levels of service quality.

• Privacy Sublayer (PS): PS Underlayment protects data with the using of IP PKM (Privacy Key Management) [2] which takes into account several encryption methods, even the most complex such as AES ( Advanced Encryption Standard) [3].

Figure 1. 802.16 protocol stack

The IEEE 802.16 Mac protocol is connection-oriented. All traffic, including connectionless traffic, is mapped onto connections which are uniquely identified by a 16-bit Connection Identifier (CID). The MAC layer defines a signaling mechanism for information exchange between the BS and the SSs. This signaling mechanism allows the SSs to request bandwidth from the BS. A request can be sent as stand-alone message in response to a poll from the BS, or it can be piggybacked in data packets.

Concerning QoS support, the 802.16 standard proposes to classify, at the MAC layer, the applications according to their QoS service requirement (real time applications with stringent delay requirement, best effort applications with minimum guaranteed bandwidth) as well as their packet arrival pattern (fixed /variable data packets at periodic /aperiodic intervals). For this aim, the IEEE 802.16 standard defines four types of service flows. Each connection between the SS and the BS is associated with a single service flow. the four types of service flows are:

• Unsolicited grant service (UGS): supports Constant Bit Rate (CBR) services, such as T1/E1 emulation and VoIP without silence suppression.

• Real-time polling service (rtPS): supports real-time services with variable size data on a periodic basis, such as MPEG and VoIP with silence suppression.

• Non Real-Time Polling service (nrtPS): supports non real-time services that require variable size data bursts on regular basis, such as File Transport protocol (FTP) service.

• Best effort (BE): for applications that do not require QoS such as Hyper Text Transfer Protocol (HTTP).

The IEE 802.16e [1], adds a new service flow called extended real-time Polling Service (ertPS). This service is designed for real-time traffic with variable data rates, such as VoIP service with silence suppression. This service uses a grant mechanism similar to that of UGS connections. Moreover, grants allocated periodically can be used to send bandwidth request to inform the BS of the grant size. The BS does not change the size of uplink allocations until it has received another bandwidth request from the SS.

Each SS requiring a connection has to include its needs on QoS by specifying which service flow will be used.

The convergence sublayer classifies the incoming SDUs by their type of traffic (voice, web surfing, ATM CBR,) and assigns them to a service flow using a 32-bit Service Flow ID (SFID). Here, if the IP CS is used, then the SDUs are classified according to: IP addresses (destination and source), Transport (TCP or UDP) Ports (destination and source) or Type of service field. When the service flow is admitted or active, it is mapped to a MAC connection that can handle its QoS requirements using a unique 16-bit CID. A service flow is characterized by a QoS Parameter Set that describes its latency, jitter and throughput assurances. With Adaptive Burst Profiling, each service flow is assigned a PHY layer configuration (i.e. modulation scheme, Forward Error Correction scheme, etc) to handle the service.

Once the service flow is assigned a CID, it is forwarded to the appropriate queue. Uplink packet scheduling is done by the BS through signaling to the SS. At the SS, the packet scheduler will retrieve the packets from the queues and transmit them to the network in the appropriate time slots as defined by the UL-MAP sent by the BS. This is illustrated in Figure 2.

Figure 2. MAC procedures

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III. QOS PROVISIONING ARCHITECTURE IN IP OVER 802.16 STANDARD

The IEEE 802.16 standard provides the air interface for WiMAX but does not define the full end-to-end WiMAX network. The WiMAX Forum's Network Working Group (NWG), is responsible for developing the end-to-end network requirements, architecture, and protocols for WiMAX, using IEEE 802.16 as the air interface.

The WiMAX NWG has developed a network reference model [4] to serve as an architecture framework for WiMAX deployments and to ensure interoperability among various WiMAX equipment and operators.

The network reference model envisions unified network architecture for supporting fixed, nomadic, and mobile deployments and is based on an IP service model. Below is simplified illustration of IP based WiMAX network architecture. The overall network may be logically divided into three parts as depicted in Figure 3:

• Mobile Stations (MS) used by the end user to access the network.

• The access service network (ASN), which comprises one or more base stations and one or more ASN gateways that form the radio access network at the edge.

• Connectivity service network (CSN), which provides IP connectivity and all the IP core network functions.

Figure 3. Network reference model

This model does not take into account the QoS in IP. So, to obtain end-to-end QoS, supplementary mechanisms should be developed to interconnect the WIMAX cells thigh IP net work.

WIMAX, being connection oriented, a prompt solution is to couple it with IntServ [5], and to reserve resource for each service flow. This solution matches better to a small network because of IntServ scalability problem.

We propose a functional architecture for multi-layer QoS (802.16 and IP layer) shown in Figure 4. This architecture is based on DiffServ [6] and IntServ models. We consider that the BS acts as edge router. We have defined two functional blocks:

the SPM (Service Provider Module) and RCM (Resources Control Module).

Figure 4. Functional architecture

A. Service Provider Module (SPM) The SPM module includes functional components such as

the SLA (Service Level Agreements) management module, the bandwidth broker server, the Policy Management Tool, the Policy Storage Service and the Policy Consumers. It is an agent that has some knowledge of the network domain priorities and policies and allocates QoS resources with respect to those policies. In order to achieve an end-to-end allocation of resources across separate domains, the SPM managing a domain will have to communicate with its adjacent peers, which allows end-to-end services to be constructed out of purely bilateral agreements.

The SLA management module is responsible for all SLA-related activities and it is comprised of two main sub-blocks: SLA Subscription block and SLS Invocation block. SLA subscription is the process of customer registration, a customer being possibly a service provider. This concerns the SLA, containing prices, terms and conditions and the technical parameters of the SLA named SLS (Service level Specification). The subscription could provide the required authentication information for Authentication, Authorization and Accounting (AAA) purposes, when an SLS is eventually invoked. SLS Invocation is the process of dealing with the flow dynamically. It performs admission at run-time as requested by the user and delegated the necessary rules to the traffic conditioning elements in the same architecture.

To process the SLA subscription and negotiation, we use web services technologies. Such a technology defines a plenty of standards (SOAP, UDDI, WSDL, BPEL4WS, etc) permitting respectively to invoke, to find/to publish, to describe and to compose services. To elucidate the utilization of web service, let us consider a video-on-demand service. A customer requests to view a movie at home via his WiMAX connection. The service is operated by the SPM, who has a settlement

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arrangement with a movie content provider , as well as the RCM. The message sequence is shown in the diagram.

Figure 5. Video on Demand message sequence

The web service technologies in this scenario can be considered to be the means by which the service is automated. Without this technologies, the interactions between the four actors in the system would have to be co-ordinated manually.

The SPM also maintains a number of management information bases (MIB) for the purpose of QoS control and management of the network domain. For example, the topology information base contains topology information that the SPM uses for route selection and other management and operation purposes; and the policy information base contains policies and other administrative regulations of the network domain. It acts as a Policy Decision Point (PDP) [7], whilst the RCM acts as Policy Enforcement Points (PEPs) to police traffic

B. RCM, Resources Control Module The RCM is the central component of our architecture. It

must be present in all edge routers. It can be seen as an middle layer between the Service Specific Convergence Sublayer of 802.16 and DiffServ/IntServ layer. It is responsible of mapping QoS parameters between the two levels. It also includes the Authorization Module (AM) that deals with local political control and admission control based on the resources it was allocated by the SPM.

The RCM incorporates at the same time the functionality of a PDP and a policy enforcement points (PEP). Seen by the SPM, the RCM is PEP equipped with a local PDP (LPDP). The advantage is that the RCM can itself take a first decision in certain circumstances (loss of connection between the RCM and the SPM, too long awaiting a decision from SPM…) to apply a local decision. However, the SPM remains the authoritative decision point at all times. This means that the

relevant local decision information must be relayed to the SPM. That is, the SPM must be granted access to all relevant information to select a final policy decision. To facilitate this functionality, the RCM must send its local decision information (using its LPDP) to the remote SPM.

After a SLA subscription with the SS, the SPM sends instructions to RCM to create a new service flows and to allocate him the necessary bandwidth. These services will then be forwarded to the SS with the QoS parameters and a SFID in a Dynamic Service Addition request (DSAREQ) (see Figure 6)

Figure 6. Service flow creation

When the SS decides to create a service flow, the Authorization Module ensures first that the contract of SS allows this creation. Whether the application will be accepted. If necessary, the RCM sends a request to the SPM module to ask for permission. SPM may establish an SLA negotiation phase with SS before sending a final decision to the RCM.

C. Mapping rules In the IntServ domain, as illustrated in figure7, the sender

will send a PATH message including traffic specification (TSpec) information. The parameters such as up/bottom bound of bandwidth, delay and jitter can be easily mapped into parameters in DSA message such as Maximum Sustained Traffic Rate, Minimum Reserved Traffic Rate, Tolerated Jitter and Maximum Latency.

Figure 7. Mapping for IntServ

According to the response of DSA message, the provisioned bandwidth can be also mapped into reserved specification (RSpec) into RESV message [8].

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For DiffServ services, DSCP code is deployed for classification. As shown in Table 1, the first 3 bits are for class selector, the middle 3 bits are for drop priority. There are three definitions of per-hop behavior (PHB) to specify the forwarding treatment for the packet. Expedited forwarding (EF) [9] is intended to provide a building block for low delay, low jitter and low loss services by ensuring that the EF aggregate is served at a certain configured rate. Assured Forwarding (AF) [10] PHB group is to provider different levels of forwarding assurances for IP packets. Four AF classes are defined, where each AF class is allocated a certain amount of forwarding resources (buffer space and bandwidth).

TABLE I. DIFFSERV CODE POINT

The table above shows the equivalence between the

different classes of service of WiMAX, IntServ and DiffServ.

TABLE II. MAPPING RULES IntServ DiffServ WiMAX

Guaranteed EF UGS

Controlled load AF rtPS,ertPS, nrtPS

BE BE BE

D. Scheduling schemas In DiffServ architecture, the differential treatment of

packets based on three basic operations: the classification of flows into classes of service, the introduction of priorities within the classes (Scheduling) and traffic management in a given class (queue Management). The second operation is ensured by the scheduling algorithms, used to control the distribution of resources among service classes. In this work, we use two types of schedulers: PQ (Priority Queuing) and WFQ (Weighted Fair Queuing).

PQ delivers the minimum delay and jitter for EF/UGS traffic and provides better bandwidth allocation for rtPS/nrtPS/AF traffic and BE traffic by priority scheduling of EF/UGS traffic and no-EF/no-UGS traffic.

Since EF/UGS traffic is given the high priority, it is guaranteed small delay and jitter. the RCM regulates the traffic to prevent the service starvation of other low priority service classes. If EF/UGS traffic exceeds certain rate limit, it will be dropped before enters the network. AF/rtPS/nrtPS and BE traffic share the “rest” of bandwidth.

Figure 8. DiffServ scheduling

Figure 9. WiMAX scheduling

IV. CONCLUSION With increasing deployment of wireless broadband

infrastructure, it is not enough to count on the MAC layer QoS mechanism provided in the last mile wireless connection. Therefore we propose an architecture to provide end to end QoS control for IEEE 802.16 standard. Both IntServ and DiffServ are supported. Compared with the traditional way of providing cross layer QoS via Wireless MAN, the proposed framework is superior in high efficiency and fastness to guarantee the throughput requirements of source traffic.

REFERENCES [1] IEEE Std 802.16e-2005- Amendment to IEEE Standard for Local and

Metropolitan Area Networks - Part 16: Air Interface for Fixed Broadband Wireless Access Systems-Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands.

[2] S. Xu et al., “ Security Issues in Privacy and Key Management Protocols of IEEE 802.16” Proceedings of the 44th annual southeast regional conference Melbourne, Florida, Mars 2006.

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[3] J. Song: “The Advanced Encryption Standard-Cipher-based Message Authentication Code-Pseudo-Random Function-128 (AES-CMAC-PRF-128) Algorithm for the Internet Key Exchange Protocol (IKE)” RFC 4615, Août 2006.

[4] WiMAX Forum Network Architecture, Stage 2: Architecture Tenets, Reference Models and Reference Points, January 2008.

[5] R. Braden, D. Clark, and S. Shenker, “Integrated Services in the Internet Architecture: an Overview,” IETF RFC 1633, June 1994.

[6] S. Blake et al., “An Architecture for Differentiated Services,” IETF RFC 2475, Dec. 1998.

[7] R. Yavatkar, D Pendarakis “A Framework for Policy-based Admission Control” RFC 2753, January 2000

[8] Jianfeng Chen, Wenhua Jiao, Hongxi Wang, “A Fair Scheduling for IEEE 802.16 Broadband Wireless Access Systems” ICC2005, May 16-20, Souel, Kerea.

[9] B. Davie et al., “An Expedited Forwarding PHB (Per-Hop Behavior),” IETF RFC 3246, Mar. 2002.

[10] J. Heinanen et al., “Assured Forwarding PHB Group” IETF RFC 2597, June. 1999.

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