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Worldwide Interoperability for Microwave Access (WIMAX) is an emerging wireless telecommunication technology devised to serve the purpose of wireless broadband access to the home and business users. It is a technology similar to Wi-Fi but outstands it with the quality like higher bandwidth, strong encryption technology and access to far longer distance with the aid of base stations. WIMAX can be used for key applications like high speed internet, video and VOIP. Its main advantage is that it can be implemented in a location where cable broadband is unreachable and can provide high level of bandwidth as the cable broadband does. 
WIMAX falls under the 802.16 standard of IEEE and the organization called WIMAX Forum plays a vital role in development of the standards and technology for WIMAX. WIMAX works both in Line of Sight (LoS) and Non Line of Sight (NLoS) technology where LOS offers more bandwidth and stable connection than the NLOS technology. The LOS technology operates at very high frequency between 11-66 GHz and NLOS operates at the frequency between 2-6 GHz. WIMAX generally operates in OFDM technology which consumes less power but also has adaptive nature in its physical layer to operate at end to end in SC and OFDMA mode to support both the single user and multi users. In a broad sense there are two types of WIMAX. They are the fixed WIMAX and the mobile WIMAX. Fixed WIMAX is an IEEE 802.16d-2004 version of standard which has been designed to be a supplement or replacement of the cable broadband and works under LOS technology. It can provide bandwidth access up to 50 km. Mobile WIMAX on the other hand is an IEEE 802.16e-2005 version of standard which supports fixed broadband as well as supports nomadic devices to provide portable high speed bandwidth access to the devices. Mobile WIMAX works under the NLOS technology and can provide bandwidth access from 5-15 km. The standard data transmission rate for WIMAX is 72 Mbps which is enough to support more than 60 businesses with T1 connectivity but in practicality the data transmission rate is around 15 Mbps. 
WIMAX falls under the 802.16 standard category of IEEE. The 802.16 family of standard is officially called Wireless MAN but commercially known as WIMAX by the effort of industry alliance called WIMAX Forum. WIMAX Forum is the alliance of group of companies involved in WIMAX dedicated to promote and certify the interoperability and compatibility of the broadband wireless products based on WIMAX standards.
The IEEE under the IEEE 802.16 standard committee initiated the 802.16 working group for the global deployment of broadband Wireless Metropolitan Area Network and later on for Wi-MAX. The publication of various standards at different periods has brought many changes and various innovations. One of the most important and popular released standard is the 802.16e-2005 that added mobility to the previous fixed WIMAX structure. The following table shows the fundamental released standards that indicates various changes in WIMAX from its inception period. 
Provides air interface for Fixed Broadband Wireless Access on the frequency 10-63 GHz band
Defines Physical layer and enhanced MAC layer functionality operating in 2-11 GHz band
Defined originally to enhance the standard 802.16 by extending the licence exempt frequency. The project was later on merged to 802.16a standard project.
Defined for maintenance and system profile of 2-11 GHz band but later on merged into 802.16-2004 standard.
Defined to provide analysis criteria for system profiling and performance on frequency of 10-66 GHz band.
Consolidates the 802.16 and its amendments by covering LOS and NLOS application in the frequency of 2-66 GHz band. This standard is the roll up of 802.16.2001, 802.16a, 802.16c and 802.16d standards. Also, the first standard to provide OFDM with 256 carriers.
The most popular standard that provides nomadic and mobile support for users on the frequency band of 2-6 GHz. Supports hard handover and two types of soft handover i.e. MDHO and FBSS.
Defines the Management Information Base (MIB) for 802.16-2004 standard.
Defines management plane procedures and services
Defines the improved coexistence mechanism for the license exempt operation.
Defines the mobile Management Information Base. The project was later on merged into 802.16-2009 standards.
Amendment standard of IEEE 802.1D that defines the bridging of 802.16
Defines the air interface for Fixed and Mobile Broadband Wireless Access System. Also, this standard is the roll up of 802.16-2004, 802.16e, 802.16g, 802.16f, and 802.16i standards.
Amendment of the 802.16-2009 standard that defines the multihop relay specifications.
Defines the improved coexistence mechanisms for the Licensed Exempt Operation.
Defines Advanced Air Interface that can provide data rate of 100 Mbps for mobile interface and 1 Gbps for the fixed interface.
Standard to provide the higher reliable networks.
Table 1: WIMAX Standards Family
The development of the mobile WIMAX was started in 2001 and was published in 2002. This publication provided the last mile air interface specification for fixed subscribers only. Also, the interface was only the LOS interface between the subscriber station and the base station. Later on after two years, the amendments in the 802.16a and 802.16c extended the MAC and PHY features.
The 802.16-2004 standard defined the air interface including the MAC and PHY layer functionality for BWA (Broadband Wireless Access) system. The highlight of this system was the introduction of new features like ARQ (Automatic Repeat Request) and mesh topology in the MAC layer. Multi frequency access technology like SC (Single Carrier), OFDM (Orthogonal Frequency Division Multiplexing), OFDMA (Orthogonal Frequency Division Multiple Access) were supported and this standard also had the support for NLOS connectivity. 
An important standard that makes the base for the current research is the 802.16e standards. It supports the mobility of the subscriber stations. 802.16e-2005 standard was published in 2006 which was the amendment of 802.16-2004 standard. The standard is not backward compatible so the equipment used for the previous standard need to be updated in order to be compatible with this standard. 802.16e-2005 standard consists all that needed factors to make the the subscriber station mobile with the base station. So, it is also referred to as the 'Mobile WIMAX'. Mobile WIMAX supports for sub-channelization and SOFDMA with the capacity upto 2048 bit FFT (Fast Fourier Transform) that enables the dynamic channel bandwidth from 1.25 KHz to 20KHz. Other highlighted features of 802.16e-2005 are support for smart antenna systems, such as AAS (Advanced Antenna System), MIMO (Multiple Input Multiple Output) and beam forming and also the support for power management. The combination of all these technology was a real leap into the stepping of new mobile broadband wireless system. The possibility of handover such as hard handover and soft handovers like MBDO (Macro Based Diversity Handover) and FBSS (Fast Base Station Switching) were only possible after the release of 802.16e-2005 standard. 
The next entrant in the WIMAX standard family that beats the features of 802.16e standard is the 802.16-2009 standard. It's the amendment of the 802.16j and includes the powerful feature of multihop relay specification. With the implementation of this specification, network coverage and performance can be enhanced. This specification introduces the RS (Relay Station) i.e. the unit station like a base station which enables SS to connect to the MR-BS (Multi Relay Base Station) and then to the backbone network station.
The on-going and under development standards are the 802.16m and 802.16n standards. The 802.16m is the amendment to IEEE Std 802.16-2009, developed by Task Group m and aimed to provide Advanced Air Interface which will be able to provide data rate of up to 100 Mbps in mobile state and up to 1 Gbps in the fixed state of the subscribing station. The draft 802.16n standard is aimed to provide the highly reliable network.
WIMAX Protocol Architecture
The WIMAX protocol is almost similar to the OSI (Open Source Interconnect) model but has only the first two layering defined in relation to the OSI model. The function defined in each layer of the WIMAX standard guides the stations to attend the various air traffic interfaces. Each layering of the stack does the function like data convergence, data filtering, data security and various other functions to meet the all the connectivity requirements. The fundamental layering in the WIMAX architecture are the Physical Layer, Data Link Layer and Upper Layer. The following figure shows the protocol architecture of WIMAX.
Fig1: WiMAX Protocol Architecture
The data in the wireless network first reaches the physical layer. The physical layer performs the conversion of the data to the physical medium understandable format and also makes the coordination of the reception and transmission of these physical signals. The data are received from the upper layer in the physical layer which are converted into physical format such as frames and burst and then transmitted to the other layers. The fundamental features that makes physical layer important in WIMAX protocol architecture are as follows.
The physical layer is scalable that supports the of bandwidth 1.25 MHz, 5MHz, 10MHz and 20 MHz on the 128, 512, 1024 or 2048 bit FFT (Fast Fourier Transform) respectively using the OFDMA technology. [6..]. Physical layer also supports AMC (Adaptive Modulation and Coding ) depending on the different burst of data which helps to make the network robust or makes efficient access of the network to provide QOS in varying condition. There are four fundamental QOS scheme. They are 64 QAM, 16 QAM, QPSK and BPSK. Each scheme has its own speciality where 64 QAM offers highest bandwidth and QPSK offers more robust connection to aid in highest distance of the serving station.
There are five physical interfaces defined in wimax standard the summary of which is given in the table below.
WirelessMAN - SC
10-66 GHz using LOS
FDD and TDD
WirelessMAN - SCa
Below 11 GHz using NLOS, licensed bandwidth
FDD and TDD
AAS, ARQ, STC
WirelessMAN - OFDM
Below 11 GHz using licenced bandwidth
FDD and TDD
AAS, ARQ, STC, mesh mobility
WirelessMAN - OFDMA
Below 11 GHz using licenced bandwidth
FDD and TDD
AAS, ARQ, STC, HARQ, mobility
Below 22 GHz with licence exempt
Table2: Physical interfaces of 802.16 standard 
Medium Access Control (MAC) Layer
The functionality of the MAC layer makes an important impact on the traffic management, quality of service and security of the overall network system. There are three sub layers in MAC layer each dealing with specific functions. They are described in details below.
MAC Convergence Sub layer (CS)
MAC Convergence Sub layer (CS) resides on the top of the MAC layer and has the following five fundamental functions.
Receiving PDUs (Packet Data Units) from the network layer which is the higher layer.
Classification of the received PDUs. The classification is done according to one of the three available CS specifications which have been explained later in this section.
If required processing the higher level PDUs. The processing may include, for example, PHS (Payload Header Suppression).
Making the waypoint to the lower layer by delivering the CS PDUs to the right MAC SAP (Service Access Point).
Receiving the CS PDU from peer entity.
The Payload Header Suppression (PHS) helps to reduce the overhead by removing the unwanted repetitive payload header at the transmitter. The header then restored back to the header information by reinserting the repetitive data.
The three fundamental functions of CS specification are ATM CS, Packet CS and Generic Packet CS. The availability of these specifications makes possible for CSs to map 802.16 MAC on standard network interface. This makes MAC a versatile and compatible with multiple network technologies . The fundamental CS specifications are explained in brief below:
The ATM CS does the function of receiving ATM cells from higher layer. It then performs classification, and PHS if needed and does forward the CS PDUs to the MAC SAP. Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) identifies the ATM connection. This identification is used for mapping the CS data to the appropriate connection identifier (CID ) of the lower MAC sub layer. The classification and CID mapping aid in the QOS enhancement function of the WIMAX.
Another CS specification is the Packet CS which does the function of providing support for packet based protocol such as IPv4, IPv6, Ethernet and Point to Point Protocol (PPP). Its main function is the MAC SDU classification, delivering CS PDU to the MAC SDU and receiving CS PDU from peer MAC SAP. Packet CS may also perform the PHS function but its optional again. [8..]
Generic Packet Convergence Sub layer (GPCS)
The GPCS supports multiple packet based protocol over Wimax standard interface because it is independent from various higher layer networking protocols.
MAC Common Part Sublayer
The function that the MAC Common Part Sublayer performs makes it one of the vital part of the 802.16 network. In general, it does the multiple functions of traffic management and QOS maintenance between network layer and physical layer. Some fundamental functions of this sub layer are described below.
Point to Multipoint (PMP) Traffic
WIMAX operates in a connection oriented basis. So, there is a need of connection between BS and SS for all the data and other services communication to take place. Hence, connection is established between BS and SS in a Point to Multipoint basis. According to this protocol, the central BS broadcasts the DL data in an independent; frequency separated technique so that multiple SS receive their own respective data. The MAC PDU consists of the DL-MAP in the beginning of the frame that specifies which portion of the DL frame that is assigned to the specific connection. The BS controls the UL from the SS in a request/ allocation basis. So, this procedure makes BS able to control traffic of all the SS at a particular time to avoid congestion in data transmission and also maintain the QOS.
There are two types of connection. They are management connection and transport connection used in network entities for message and user data transmissions respectively. The 16 bit CID helps to identify the connection regardless of the connection type.
The management connection does the function of transportation of MAC management message between BS and SS which includes various types of messages such as handover, ARQ and network entry messages. Transport connection does the function of transporting application data between the entities. The link to the service flow of the transport connection makes possible for applying QOS on each connection basis. 
Bandwidth requests and allocations
The SS requests the BS about the need of the UL bandwidth allocation. The bandwidth requests may be transmitted in a standalone BRH method or piggybacked with the normal PDU. The BS's scheduler may grant bandwidth resource on per SS basis. It is then the duty of SS to decide on how to allocate bandwidth to each of its connecting entities. The bandwidth can be granted by the BS following the unicast polling mechanism or the multicast and broadcast polling mechanism. In unicast mechanism, the BS allocates bandwidth to each SS individually where as in multicast and broadcast polling bandwidth requests may be granted to multicast or broadcasts CID in the UL- MAP.
Automatic Repeat Request (ARQ) helps for the maintenance of the QOS of the connectivity due to its error control mechanism. The mechanism works on per connection basis in which the flawed or dropped packets are transmitted to the receiver back. ARQ is active in MAC layer of the WIMAX which helps control error in the following way. The transmitter after sending the packets waits for the acknowledgement from the receiver. If it doesn't receive acknowledgement or receive the negative acknowledgement due to flawed or dropped packets, then it retransmit the packets to the receiver. The ARQ feedback message may be sent as a separate management message or may also be piggybacked with the PDU. 
During multicasting and broadcast polling, there are chances that there might occur collision between the SSs because any SS may request bandwidth during the request interval assigned for multicast and broadcast allocated CID in the UL - MAP. Hence, to avoid the collision and to enable the SSs to participate and multicasting and broadcast polling, contention resolution algorithm is used. 
The idea of the QOS in WIMAX network is realized through the combinational functionality from the various features like connection oriented architecture, scheduling and service flow management. On each of the UL or DL connection between BS and SS, a service flow is attached. The service flow acts as a transport service that defines various QOS parameters to be used. The parameters may be maximum tolerated jitter, maximum latency, maximum sustained traffic and traffic priority that helps to ensure specific level of service. The Service Flow Identifier (SFID) which is of 32-bits makes the identification of each service flow possible. 
The connection management is done by the BS which makes well scheduled traffic of UL and DL with the SS. The BS controls the UL traffic from SS with the use of bandwidth request/grand mechanism that allows the SS to transmit only in the pre-allocated time period. Also, the band allocation in per SS basis enables specific bandwidth allocation to various services like VOIP, real time video or simply web browsing. The 802.16 standard specifies five scheduling types implied on different applications. They are explained below.
Unsolicited Grant Service (UGS): The UGS helps to minimize the need for bandwidth request for a SS by providing fixed size UL allocation thus reducing the latency and overhead. This mechanism is useful for real time application like live video streaming.
Real Time Polling Service (rtPS): The rtPS makes useful for video transmission by supporting for real time UL transport. A cost of slight overhead makes possible for data transport efficiency in this mechanism.
Extended Real Time Polling Service (ertPS): ertPS also allows unsolicited unicast grant for SS as like UGS but also allows dynamic size UL allocation. This makes smooth running for service like real time video and voice application (eg. VOIP with activity detection).
Non Real Time Polling Service (nrtPS): nrtPS enables the user to request unicast bandwidth that makes possible to run the application even in congestion when the resources are limited.
Best Effort (BE): The BE allows the SS to use connection request opportunity. This scheduling is effective when the application has no minimum connection requirement like the web browsing application.
There are two power modes defined in 802.16 standard for the mobile equipment i.e. sleep mode and the idle mode. Both the system of power management is associated with turning off of the equipment when the equipment is not in use. The functions of the two power modes are given below. 
Sleep Mode: When in sleep mode, the mobile station turns off its devices for pre-set period which is also informed to the BS. When there is no data available then the MS changes its status into listening period in which its still able to receive and transmit the data. The listening period is then followed by the sleeping period. Three power saving classes are defined by the WIMAX standard. In Power Saving Class 1, if no data is perceived then the sleeping period is exponentially increased. In Power Saving Class 2, there are fixed listening and sleeping lengths whereas Power Saving Class 3 is useful with multicast or management connection which has only single sleep period.
Idle Mode: In idle mode, the equipment which are not needed are completely turned off to conserve power and radio resources. Support for mobility is well supported by idle mode than by sleep mode.
There are procedures for various types of handover defined in the MAC layer of the WIMAX standards. The procedures defined deals with how to achieve a successful handover with low latency and maintained QOS. The detailed explanation of the handover mechanism of the WIMAX standard is explained later in the section 3.0.
MAC Security SubLayer
The security of wireless system of network is always challenging as compared to the wired network. It is hard to detect the altered data and eavesdropping during the data transmission. From the user perspective, it is necessary to secure the data integrity and from the network operator perspective it is needed to secure data from being used by unauthorised personnel. The MAC security sub layer works to provide secure data transmission in the WIMAX network. There are two protocols that work for the privacy of the network. They are encapsulation protocol and the key management protocol. The encapsulation protocol provides security to the data packets by defining the pairing of data encryption , algorithm for authentication and rules to apply them on the MAC PDU. The key management protocol on the other hand is used to secure key distribution data from the BS to SS. WIMAX uses Advanced Encryption Standard (AES) to create cipher text for the data encryption. The mobule WIMAX system uses Extensible Authentication Protocol (EAP) for the authentication and Privacy and Key Management Protocol Version 2 (PKMv2) for the key exchange. 
WIMAX architecture is composed of three fundamental components. The fundamental components are the mobile station (MS), Access Service Network (ASN) and Connectivity Service Network (CSN). There is also another component i.e. the Relay Station (RS) which works in the Multihop Relay (MR) networks. The MS works as the source of network for the end users, ASN makes the radio access to all the involved BSs and CSN is associated with providing all the IP functions to the network. The figure below shows the WIMAX network architecture. 
D:\University Stuffs\CCM 4900 Dissertation\WIMAX related\Materials\wimax_reference_network.gif
Figure: WIMAX Network Architecture
The fundamental components of the WIMAX architecture are explained below:
Mobile Station (MS)
A MS also known as Subscriber Station (SS) is a Customer Premises Equipment (CPE) which is used to connect user to the WIMAX network. There are indoor and outdoor CPEs each having its own advantages and disadvantages. 
The outdoor CPE offers better reception capacity with ability to resist the obstruction from walls, RF blocking glasses or concrete of bricks. The outdoor CPE works better in response to the reception quality when compared with the indoor CPEs because the LOS technology with the BS can be utilized well by outdoor CPE than by indoor CPE. Its expensive in comparison to the indoor CPE due to number of reasons, one being the extra measures required to make the outdoor CPE weather resistant.
Fig: Outdoor CPE
The main advantage of the indoor CPE even though having less signal reception capacity than that of outdoor CPE is that it can be installed at any time by the user without waiting in queue from the network provider. The indoor CPE is less expensive than outdoor CPE which can be bought in the market by the users.
Fig: Indoor WIMAX CPE from Motorolla
Access Service Network (ASN)
ASN consists of at least one BS and at least one ASN gateway and is involved in providing network access connectivity to the SS. The ASN is also involved in providing all the authorization, authentication and accounting information of the user to the home network service provider. Also the ASN involves in providing the preferred network discovery and network service provider to the users.
The ASN gateway performs vital function in the ASN. It does the load balancing, bridging, routing and redundancy functions among the available other ASN-gateways. Other mandatory functions are also performed by the ASN-Gateway like location register with ASN, paging control, service flow management and AAA client functionality.
Base Station (BS)
The BS implements the WIMAX PHY and MAC layer specification which acts as a logical network entity to provide connectivity to the SS. The logical network consists of one or more BS where each BS is designed to cover certain distance of network coverage area. The BS controls the UL and DL connectivity with the SS and also does the tasks to achieve scheduling and QOS in data transmission. 
MR-BS (Multi Relay-Base Station) is introduced in the 802.16j standard which is an addition to the WIMAX BS family that does the controlling of both the SS and RS. The MR-BS is designed for the relay network but is also fully compatible with the 802.16e standard.
Relay Station (RS)
Relay Station (RS) works in the Multihop Relay (MR) network where one or more RS relays the traffic and the signalling information between SS and Multihop Relay Base Station (MR-BS). A RS generally carry out the communication with other access station in LOS but may also connect the NLOS connection with the MSs. There are two types if RS. They are Transparent RS and Non-Transparent RS.
Transparent RS: The transparent RS connects the MSs and the MR-BSs by forwarding the traffic and control messages between the stations. It doesn't involve in transferring the management data but does the function of providing the extended coverage.
Non-Transparent RS: A non-transparent relay station does the function of transmitting DL frame start preamble, MAP messages, DCD/UCD messages and Frame Control Headers (FCHs). This functionality allows the RS and MR-BS to determine the bandwidth allocation to the substations of the RS by the use of the distributed scheduling.
Connectivity Service Network (CSN)
Connectivity Service Network (CSN) acts as a gateway between WIMAX ASN and other external or public networks and is maintained by the ASP. CSN provides the AAA and mobility management functions between various CSN and IP services for the network users. The mobility among different CSNs is also possible by the deployment of CSN and this is called CSN-anchored mobility. The CSN-anchored mobility consists of the intra and inter NAP mobility over the R3 reference point and may also make the inter technology handover possible. 
Network Reference Model
The Network Reference Model (NRM) defines the reference points and operational entities that make interoperability possible between the previously defined network components in the WIMAX network. The following figure shows the components and its connection architecture according to the NRM.
C:\Users\sashonline\Desktop\Network Reference Model.jpg
Fig: Network Reference Model
The networking elements are connected by the reference points R1-R5. Each reference point includes the protocol and procedures to be used between its connected elements. The protocols and procedures between MS and ASN is indicated by R1 and between MS and CSN is indicated by R2. The R2 is also associated with authorization of services, authentication and the management of the IP hosts configuration. R2 is also considered as the logical link because ASN is actually physically connected through the CSN. The R3 supports for AAA, mobility management capabilities, policy enforcements and also includes the control plane protocols between ASN and CSN. R4 includes the protocol used within ASN and also does the function of making MS mobility management between different ASN and the ASN-gateways. The R5 reference point includes the protocol to make the communication between the visited CSN and the home CSN. 
Reference Model for ASN
The figure below show the relationship of various reference points with the ASN. As stated above in the previous section, the ASN shares link of R1 reference point with MS, R3 with CSN and R4 with other ASNs. The ASN-gateways and the BS are connected with the R6 reference point and the communication between the BSs are done through R8 reference point that aid to help in seamless handover with less latency.C:\Users\sashonline\Desktop\ASN Reference Model.jpg
Fig: The ASN Reference Model
If the ASN-Gateways are optionally decomposed into two functions i.e. the Decision Point (DP) and the Enforcement Point (EP), then these two functions are connected with R7 reference point. It is shown in the figure below.
Fig: ASN-Gateway Decomposition Reference Diagram 
The Network Working Group (NWG) has created three profiles to guide the operators and manufacturers in ASN functionality and also to map the ASN functions to BS and ASN-Gateways in various manners. The operators can choose any of the profiles from the available three and it is not mandatory to choose another profile after the selection of one profile. Also, it is not mandatory to implement all the functions of a profile. The profile definition just guides where the existing functions are located. Since there is also inter vendor operations in WIMAX network, the ASN profiles even though their implementations vary should be compatible with each other.  The features of each profile are explained below. 
Profile A: The following are the fundamental features of the profile A.
Handover control at ASN-GW
Radio Resource Controller (RRC) located at the ASN-GW. This allows Radio Resource Management (RRM) with multiple base stations.
ASN anchored mobility within the BSs are done with the R4 and R6 physical connection.
Profile B: The following are the fundamental features of the profile B.
Does not make intra ASN interfaces
Makes possible for the ASN function mapping from single physical device that contains all the functions to the distribution of functions in multiple network.
The ASN in profile B operates over R3 and R4 reference point with other ASNs. R4 reference point is used for Inter ASN anchored mobility.
Profile C: The following are the fundamental features of the profile C.
Handover control and the RRC are located at the BS.
The RRM within a particular BS is possible with the help of RRC and RRA of that BS.
ASN-GW delivers the RRM message from one BS to another BS through R6 reference point with the help of RRC relay.