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Download miễn phí Khóa luận Evaluation of existing algorithms scheduling for Real-Time service flows for WiMAX uplink
Tables of Contents
Tables of Contents 1
Acknowledgements 4
List of figures 5
List of tables 6
Glossary 7
Acronyms 9
Chapter 1. 12
1.1 Quality of Service Fundamentals 13
1.2 QoS in IEEE 802.16 14
1.2.1 Admission Control 16
1.2.2 Scheduling 16
1.3 Research Motivation and objectives 16
1.4 Thesis organization 17
Chapter 2. IEEE 802.16 Standards 18
2.1 Protocol layer in 802.16 18
2.1.1 Physical layer 18
2.1.2 MAC Layer 20
2.2 Admission Control 23
2.2.1. Objective 24
2.2.2 Overview of Admission Control 24
2.2.3 Admission Control Policy 24
2.3 Services and service flows 26
2.3.1 Connections and service flow 26
2.3.2 Connection Identifiers (CIDs) 27
2.3.3 Service Flows 29
2.4 QoS architecture model 35
Chapter 3. Scheduling and Admission for Real-Ttime Traffic 39
3.1 Scheduling and admission for real-time traffic. 39
3.1.1 System models [18]. 40
3.1.2 Loss rate for preemptive EDF 44
3.1.3 Non-preemptive EDF 47
3.2 Some current scheduling algorithms for real-time polling services 49
3.2.1 EDF Broadband Wireless Access Scheduling Algorithm 51
3.2.2 Admission Control 54
Chapter 4. Simulation Results 57
4.1 Theoretical Performance of single queue EDF scheduling algorithms 57
4.2 Simulation of NP-EDF scheduling algorithm for rtPS services in WiMAX 59
4.3 Conclusion 61
References 62
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s.2.2.2 Overview of Admission Control
I express the need for the admission control in order to control the usage and allocation of bandwidth resources for various traffic classes requiring certain QoS guarantee. Admission control is a key component in determining whether a new request for a connection can be granted or not according to the current traffic load of the system. This assumes great significance when the BS needs to maintain a certain promised level of service for all the connections being admitted (served). If the admission control admits too few connections, it results in wastage of system resources. On the other hand. If too many connections are allowed to contend for resources, then the performance of the already admitted connections degrades rapidly in the presence of new connections. Therefore, judicious decision making mechanism for allocating bandwidth to different classes of service is needed.
In IEEE 802.16, before an SS can initiate a new connection or changing or deleting an already admitted connection, it must first make a request to the BS. As mentioned earlier in chapter 1, four types of MAC layer services exist in IEEE 802.16. These service flows can be created, changed, or deleted through the issue of dynamic service addition (DSA), dynamic service creation (DSC) and dynamic service deletion (DSD) messages. Each of these actions can be initiated by the SS or the BS and are carried out through a two or three-way-handshake.
2.2.3 Admission Control Policy
The task of admission controller is to accept or reject the arriving requests for a connection in order to maximize the channel utilization, by accepting as many connections as possible, while keeping the QoS level of all connections at the level specified in their traffic profile. In other words it ensures that already admitted connections QoS will not be affected by the decision made. Although it may seems to be very intuitive and simple procedure, it has great influence on QoS of the admitted connections.
This issue have been studied and researched extensively in the context of wired and wireless networking. Although the focus of this research was not admission control, whenever it comes to IEEE 802.16, putting a well rounded introduction seems to be indispensable, thus we have concisely introduced some of the fundamental works that need to be done in this area as there is no defined procedure in IEEE 802.16.
Admission control algorithms can be categorized into three classes of complete sharing, complete partitioning and hybrid policies which is a combination methods of the other two.
Complete sharing (CS): allows all users equal access to the bandwidth available at all times. This strategy results in maximum utilization of the available bandwidth, specially in high traffic networks, which is what network providers aiming at. However, at the same time, it does not differentiate between connections of different priority that is a perverse outcome when connection of one class needs significantly less bandwidth than others. At this situations it might be desirable to reject calls of this type to increase the probability of future acceptance of a larger call. In other word it is not fair strategy to the wider bandwidth users as all request would be dealt with the same priority.
Complete partitioning (CP): divides up the available bandwidth into non-overlapping pools of bandwidth in accordance with the type of user’s connection. Therefore, number of existing users in each class would be prohibited to a maximum number M which admission decision will be made upon. This policy allows for more control of the relative blocking probability at the expense of overall usage of the network.
Hybrid policies: basically provide a compromise between the different policies by subdividing the available bandwidth into sections. Part of the bandwidth in completely shared and the other part is completely partitioned. Depending on the policy adopted, the partitioned division would be dedicated to some or all classes. This allows more live up to the QoS requirements of the different user type while maintaining higher network utilization.
In the above mentioned admission control policies, CP and CS, have no complexities to be elaborated and decision making would be a matter of checking a single condition, though, the hybrid strategy is a more open ended problem. As an example, in the following we suggest an algorithm that could be considered as a hybrid method for admission control.
2.3 Services and service flows
2.3.1 Connections and service flow
The CS provides any transformation or mapping of external network data received through the CS Service Access Point (SAP) into MAC SDUs received by the MAC Common Part Sublayer (CPS) through the MAC SAP (see Figure 1). This includes classifying external network Service Data Units (SDUs) and associating them with the proper MAC Service Flow Identifier (SFID) and Connection Identifier (CID). Classification and mapping are then based on two 802.16 MAC layer fundamental concepts
Connection: A connection is a MAC Level connection between a BS and an SS (or MS) or inversely. It is a unidirectional mapping between a BS and an SS MAC peers for the purpose of transporting a service flow's traffic. A connection is only for one type of service (e.g. voice and email cannot be on the same MAC connection). A connection is identified by a CID (Connection IDentifier), an information coded on 16 bits.
Service flow: A Service Flow (SF) is a MAC transport service that provides unidirectional transport of packets on the uplink or on the downlink. A service flow is identified by a 32-bit SFID (Service Flow IDentifier). The service flow defines the QoS parameters for the packets (PDUs) that are exchanged on the connection.
Figure 2.2 shows the relation between the SFID and CID. The relation between the two is the following: only admitted and active service flows (see the definitions below) are mapped to a CID, i.e. a 16-bit CID. In other terms:
Figure 2.2: Correspondence between the CID and SFID
A SFID matches to zero (provisioned service flows) or to one CID (admitted or active service flow).
A CID maps to a service flow identifier (SFID), which defines the QoS parameters of the service flow associated with that connection.
The definitions of connection and service flow in the 802.16 standard allow different classes of QoS to be found easily for a given element (SS or BS), with different levels of activation (see Figure 2.3). More details will now be given about connections (and CIDs) and service flows.
Figure 2.3: Illustration of service flows and connections
2.3.2 Connection Identifiers (CIDs)
A Connection IDentifier (CID) identifies a connection where every MAC SDU of a given communication service is mapped into. The CID is a 16-bit value that identifies a unidirectional connection between equivalent peers in the MAC layers of a BS and an SS. All 802.16 traffic is carried on a connection. Then, the CID can be considered as a connection identifier even for nominally connectionless traffic like IP, since it serves as a pointer to destinations and context information. The use of a 16-bit CID permits a total of 64K connections within each downlink and uplink channel. There are several CIDs defined in the standard (see Table 1). Some CIDs have a specific meaning. Some of the procedures introduced in this table, such as ranging, basic, primary and secondary management.
Table 1: CID ranges as defined in Reference IEEE 802.16-2004.
CID
Value
Description
Initial ranging
0 × 0000
Used by SS and BS during the initial ranging process
Basic CID
0 × 0001 – m
Each SS has a basic CID and has a short delay. The same CID value is assigned to both the downlink and uplink connections
Primary management
m + 1 − 2m
The primary management connection is used to exchange longer, more delaytolerant MAC management messages
Transport CIDs and secondary management CIDs
2m + 1 − 0 × FE9F
Used for data transfer and for secondary management connection
Multicast CIDs
0 × FE9F − 0 × FEFE
For the downlink multicast service, the same value is assigned to all SSs on the same channel that participate in this connection
AAS initial ranging CID
0 × FEFF
A BS supporting AAS (Advanced Antenna System) uses this CID when allocating an AAS ranging period (using AAS_ Ranging_Allocation_IE)
Multicast polling CIDs
0 × FFOO− 0 × FFF9
An SS may be included in one or moremulticast polling groups for the purposes of obtaining bandwidth via polling. These connections have no associated service flow
Normal mode multicast CID
0 × FFFA
Used in DL-MAP to denote bursts for transmission of downlink broadcast information to normal mode SS
Sleep mode multicast CID
0 × FFFB
Used in DL-MAP to denote bursts for transmission of dow...