Πέμπτη, 11 Ιουλίου 2013



Dilbert on Standards
One of my favorite online comics, along with xkcd and PhDcomics, is Dilbert (http://www.dilbert.com) , who is a capable engineer  in a dead-end and no-future company. His boss is the pointy-haired guy, who is a moron, has no technical education and keeps assigning impossible tasks to Dilbert. The bald-headed guy with the glasses is the department’s lawyer and the little dog is the evil Dogbert, who in different strips has different role, from Human Resources Manager to CEO. Here are my top 4 Dilbert comic strips about patents and standards:
Source: http://www.dilbert.com/dyn/str_strip/000000000/00000000/0000000/100000/20000/9000/800/129847/129847.strip.gif
Source: http://www.dilbert.com/dyn/str_strip/000000000/00000000/0000000/000000/60000/6000/400/66480/66480.strip.zoom.gif
Source: http://www.dilbert.com/dyn/str_strip/000000000/00000000/0000000/100000/60000/2000/800/162864/162864.strip.gif
Source: http://dilbert.com/dyn/str_strip/000000000/00000000/0000000/100000/30000/3000/100/133183/133183.strip.gif


[1] M. Nakamura, T. Chujo, T. Saito, Standardization Activities for WiMax, Fujitsu Science Technology Journal, July 2008
[2] K. Etemad, M.Y. Lai, WiMax Technology and Network Evolution, Chapter 1
[3] M. Riegel, Interpretation of the WiMax Standards, WiMax Congress presentation, Amsterdam, June 2009
[4] M. Nakamura, WiMax Standardization, Fujitsu Laboratories presentation, December 2008
[5] RF Spectrum Utilization in WiMax, Fujitsu Microelectronics America White Paper, November 2004
[6] S. Omerovic, WiMax Overview, University of Ljubljana Report
[7] The History of WiMax: A Complete Survey of the Evolution in Certification and Standardization for IEEE 802.16 and WiMax
[8] J. Hasan, Security issues of IEEE 802.16 (WiMax), Australian Information Security Management Conference ,  Edith Cowan University, 2006
[9] J.M. Smits, Regulations and Standards for Wireless Communications (0EL70), Eindhoven University of Technology, course’s slides 2013
[10] H.d.Vries, Fundamentals of Standards and Standardization
[11] Wikipedia WiMax webpage: http://en.wikipedia.org/wiki/WiMAX
[12] Wikipedia IEEE 802.16 webpage: http://en.wikipedia.org/wiki/802.16
[13] Tutorialspoint WiMax webpage: http://www.tutorialspoint.com/wimax/index.htm
[14] WiMax Forum webpage: http://www.wimaxforum.org/about
[15] J. Grivolas, J. Hellstrom, S. Nicoletti, P. Warner, WiMax Regulation: an overview, Ovum, 2006
[16] WiMax Forum Air Interface Specifications, WiMax Forum proprietary, March 2013
[17] M. Moghazi, WiMax Spectrum, NTRA Egypt presentation
[18] M. Nohara, IEEE 802.16/WiMax Broadband Wireless Access, ITU-T Workshop “NGN and its transport networks”, April 2006
1. Source [3], slide 3
2. Source [5], page 5
3. Source [17], slide 4
4. www.tutorialspoint.com/images/wimax_reference_network.gif
6. Source [18], slide 9
7. Source [3], page 7
8. Source [1], page 2
9. Source [4], slide 7
10. Source [4], slide 8
11. Source [7], page 1190
12. Source [7], page 1195
13. Source [1], page 287
14. Source [2], page 9
1. www.tutorialspoint.com/images/wimax_ieee_standards.gif

2. Source [3], slide 11

Conclusions and Overview of WiMax Standardization

Worldwide Interoperability for Microwave access is a wireless communication standard operating at a frequency of a few Gigahertzes (mainly at 2.5 , 3.5 and 5.8), which could initially provide 30 to 40 megabits per  second and increased to 1 Gigabit with the 2011 update. It can operate at a radius of several kilometers (in principle up to ~50 kilometers with a bitrate of a couple of megabits), covering very large areas. The first final version of WiMax was IEEE 802.16-2004 in 2004 and the most recent is IEEE 802.16m in 2011. WiMax has achieved some considerable penetration in developing countries mainly in Southeastern Asia, but has until today failed to compete GSM/CDMA in the Western World. Even though no one can be sure about the future, it seems to be very difficult for WiMax to gain a big market share in the West. Nevertheless, the developing countries around the world sum up to a very big percentage of the global population and can create a critical subscribers’ mass for WiMax. Until now, it can probably be considered to be a failure in general (maybe because of the great initial expectations created), but things can change in the future [7].
The standardization of WiMax involves three different entities: IEEE with the 802-16 Working Group, the WiMax Forum and the International Telecommunications Union. IEEE 802.16 WG develops the basic technical specifications, closer to the physical layer. WiMax Forum adds some extra features, closer to the application layer and issues compatibility and interoperability certificates for vendors. ITU gives a more formal and official recognition, mainly to the spectrum allocated and used for WiMax worldwide. WiMax standardization allows vendors to produce equipment compatible with each other’s in big quantities, that drop the manufacturing cost. Furthermore, as good practices are adopted in the standard, the hardware and software used achieve the best possible result with the minimum price (value for money), thus decreasing even more the costs [1].

What has been learnt from the WiMax standardization process is that developing a new standard should not be blind to new features and characteristics that can dramatically change the way forward, with the example of the spectrum, as IEEE was initially researching the 10-66 GHz spectral region, which would not allow a very large coverage area from a base station and would cause many problems in both amplifying the signal and processing it digitally, compared to the region of 2-11 GHz that was later researched. The importance of the certification from the WiMax Forum is also significant, as interoperability is a key feature for the widespread penetration of any technology. Finally, the collaboration with other international organizations (like ITU) can rise the esteem of a standard and facilitate new deployments and development.

Alternative WiMax applications and security issues

The first obvious and straightforward application of WiMax is the retail wireless Internet access at a mobile basis, replacing today 3G Internet access of GSM and CDMA worldwide.  Apart from that though, WiMax can be a real alternative for 4G wireless networks [11]. The main problem in that is that there is not a market of an adequate size from vendors for personal mobile devices. This can be overcome if the demand rises, but it is a “chicken and egg” problem (the demand is waiting for the industry and the industry is waiting for the demand). WiMax was proposed as an alternative to DVB-T for the digital television, but DVB-T was preferred.
A very promising alternative use for WiMax is as a substitute for the last mile [11]. The copper from the central offices of a provider to the subscriber’s premises is, usually in the Western World, public-owned and each provider pays a monthly fee for every local loop they use. As the management of the last mile across the country is a relatively difficult operation for a non-telecommunications company, in most cases the management (extension, upgrade, maintenance and of course the lease) is conducted by the incumbent telecommunications provider of each country ( eg British Telecom for the UK, Deutsche Telekom for Germany, KPN for the Netherlands, NTT for Japan etc), which is a now a privatized company and a former state monopoly. The contradiction of the incumbent of being simultaneously retail provider and manager of the access network used by its competitors (so the incumbent is responsible for activating the subscribers’ connections of its own competitors) leads to competition problems (margin squeeze, unethical competition with deliberately delayed activations and unfixed failures etc). The monopoly of the incumbent in the last mile cannot be easily overcome in many cases, even though many regulatory efforts have been made. The only 100% effective solution is to change the physical medium of the last mile. One solution is fibers, with Fiber To The Home (FTTH) networks, but when we talk about wireless solutions, this could be WiMax. One Base Station with a WiMax antenna can cover an area of tens of square kilometers ( , with r the cell radius) and (in theory even) hundreds of thousands of potential subscribers. In this case, the activation of the services would not depend on the last mile manager company, but on the provider itself, as only a terminal device (Customer’s Premises Equipment) and the credentials would be needed. Unfortunately for the competition, this has not worked in almost any country. It is characteristic that for example in Greece, the incumbent (OTE, former state monopoly and Deutsche Telekom owned today) has bided for a WiMax license, won it, but never used it commercially, for some analysts just in order to block the WiMax penetration and to keep its competitors dependent on the last mile that OTE manages.
Regulation in WiMax is mainly focused on the spectrum used. The channels at 2.5 and 3.5 GHz need a license, while the ones at 5.8 GHz do not. Licenses in countries are either given in auctions or at a first-come-first-serve basis [15]. Just like GSM and all other wireless communications, spectrum shortage is a critical issue. Especially for the lower frequencies (which are more attractive, as they experience less attenuation over distance), there is a significant spectral congestion with loads of technologies claiming part of the spectrum. 
There are a few security issues in WiMax mainly coming mainly from the fact that the physical medium (air interface) is shared and not dedicated for each user. Confidentiality and resistance to interception and eavesdropping are the main concerns. Message authentication is needed in order to ensure the integrity of both the message and the sender, while Denial of Service (DoS) attacks affect the availability of the service. The main types of attacks are: man-in-the-middle attack, message replay attack on authentication and authenticated key formation protocols, parallel session attack, interleaving attack, attack due to type flaw, reflection attack, attack due to name omission and attack due to misuse of cryptographic services. In the IEEE 802.16 standard, the privacy sublayer (PS) is on top of the physical layer, so the PS guards only the data link layer and not the physical layer, leaving it in general vulnerable to attacks [8].
Jamming is conducted with a source of strong noise in order to decrease the channel capacity, causing DoS problems, but it is easily detected with radio analyzers. Scrambling is jamming for a short period for time, but it is not trivial to implement it, because synchronization at certain time intervals is needed. Identity theft is also an issue in WiMax, which is done by reprogramming a device with the hardware address of another device. This is also difficult to be done, as the attacker must keep transmitting at the exact timeslots that the Base Station is, of course with a stronger signal. Water torture attack is also possible, where a series of frames that drain the receiver’s battery are transmitted. This can be neutralized with a data authenticity technology. There are two types of certificate in WiMax: one of the manufacturer’s and one at the Subscriber Station. There is not a certificate for the Base Station, so the Subscriber Station certificate is verified with a public key, making the scheme vulnerable. If there also is a Base Station certificate, making a mutual authentication, this vulnerability will vanish. Furthermore, the state of the Security Associations (SA) does not differentiate from one timeslot to the other, so a replay attack is possible. Also, the Cipher Block Chaining uses a 56-bit key, which can be decrypted with brute force with present computing power. Finally, an Authorization Key (AK) lasts for up to 70 days, while a Traffic Encryption Key (TEK) for 30 minutes, so a data Security Associations can use 3.360 TEK’s over the AK’s lifetime. The Security Associations Identifier is 2 bits long, but for 3.360 at least 12 bits are needed ( different TEK’s) [8].

Security mechanisms are always expensive processes. They require extensive research, evaluation and implementation outcomes. The mobility of IEEE 802.16e makes WiMax more vulnerable to attacks, so more precautions must be taken. Nevertheless, we must always keep in mind that “what locks, can be unlocked”. 

Τετάρτη, 10 Ιουλίου 2013

Overview of the standardization and importance

A standard, according to De Vries (1997) is “an approved specification of a limited set of solutions to actual or potential matching problems, prepared for the benefits of the party or the parties involved, balancing their needs and intended and expected to be used repeatedly or continuously during a certain period, by a substantial number of the parties for whom they are meant” [10]. The six basic principles of standardization are: voluntary, open, consensus, public, general purpose for the society and compatibility between generation [9]. According to Simons and De Vries (2002), a standard is considered to be “good” when:
·         it provides a solution for a matching problem
·         it fulfills the need of parties (workable and acceptable)
·         there is more than one party involved
·         its lifetime is longer than the process for creating the standard
·         it is not in contradiction with other valid, operational standards
·         it has backwards compatibility
·         it does not block a priori future improvements and developments
·         it is easily readable and unambiguous
·         it fits for repetitive, frequent application
WiMax standardization aims towards these targets and has achieved most of them. To start with, the initial suggestions within the IEEE differ quite a lot from the final standards. At the first years of 802.16 WG, the focus was mainly on the spectrum of 10 to 66 GHz, before shifting to the lower frequencies of 2 to 11 GHz [1]. IEEE includes three different physical layers (SC, OFDM and OFDMA), but only one MAC protocol, with new features still being added at every single major release and amendment. IEEE has maintained close contacts with ITU, which has included WiMax in IMT-2000 and later in the 4G wireless technologies. This provides WiMax with global recognition and prestige. WiMax Forum is responsible for the fixed WiMax and the mobile WiMax profiles, with extra features apart from those originally implemented by the IEEE 802.16 WG. The WiMax certification ensures interoperability and global roaming among the vendors’ equipment [7].

De facto standards are the standards that are used by the industry and get adopted by managing to acquire a big market share. They are market-driven standards not developed by law and official standardization organizations. On the other hand, de jure standards are developed by such organizations. All members are encouraged to take part in the development process and consensus is one of the main targets. Usually, de jure standards take more time to finalize, as they have more formal and bureaucratic processes and the members typically pay a fee in order to be members. In this context, WiMax standardization is de jure standardization, international, open and compulsory. It is conducted by IEEE, WiMax Forum and ITU, any member can take part in the standardization process, it holds in international level and the standards have to be applied to a product, so that it will be able to operate according to the WiMax technology.

ITU contribution to WiMax standardization

ITU launched the program of International Mobile Telecommunications in 2000 (IMT-2000) for third generation (3G) wireless networks defined by a set of independent ITU Recommendations. In 2006 IEEE proposed to ITU to include WiMax in IMT-2000 as IP-OFDMA, based on IEEE 802.16-2004, IEEE 802.16e-2005 and Release 1.0 of the WiMax Forum. ITU approved of that addition in 2007. Later on , the IEEE 802.16-2009 and WiMax Forum Release 1.5 were used for more up to date features (for example, the frequency division duplexing). ITU published a new version, the IMT Advanced in 2007, where the WiMax standard was again included and in December 2010 WiMax was regarded as a fourth generation (4G) technology by the ITU (along with Long Term Evolution-LTE and High Speed Packet Access Plus-HSPA+) [19].

Even though at the first glance the ITU standardization may not occur to have obvious advantages, the truth is far from this. The standardization itself can be conducted by the IEEE and the WiMax Forum, but the recognition from the ITU added international credibility and global reputation, at the same level with HSPA and Code Division Multiple Access (CDMA) . Furthermore, part of the spectrum (especially around the 2.5 GHz) allocated for the IMT-2000 technologies can now be used by WiMax, as it belongs to them [7].

WiMax Forum contribution to WiMax standardization

Wimax Forum is an industry-led, not-for-profit organization that certifies and promotes the compatibility and interoperability of broadband wireless products of IEEE 802.16. Its goal is to accelerate the adoption, deployment and expansion of WiMax technologies all over the world, while facilitating roaming agreements, sharing best practices within its members and certifying products [14].
WiMax forum has three main areas of standardization development:
·         air interface specifications ,which focus on the physical and the data link layer (layers 1 and 2 of the OSI reference model) and are based on IEEE 802.16
·         network specifications, which apply to higher layers and are not based on 802.16, but developed by WiMax Forum
·         roaming specifications, which deal with the roaming business framework with functions for wholesale rating etc
The relationship between IEEE 802.16 and the WiMax Forum with respect to the OSI reference model is shown in Figure 12 [7].
Figure 12: IEEE 802.16 and WiMax Forum standardization areas of the OSI reference model
The discussions in the WiMax Forum is carried out by industrial delegates. It releases the Wimax Forum’s Mobile WiMax System Profile, which does not describe precise specifications (unlike the IEEE WirelessMAN standard), but only lists functionalities and parameters that are to be used. This list enhances common functionalities and interoperability among WiMax products. The relationship between the WiMax standardization of these two entities is illustrated in Figure 13. A very representative example of the standardization differences of them lies in the physical layer, where IEEE 802.16 defines three options (Single Carrier, Orthogonal Frequency Division Multiplexing and Orthogonal Frequency Division Multiple Access), while the WiMax Forum refers only to OFDMA, as it provides suitable operation only for mobile users [1].
Due to the potential for very high growth and utilization, the WiMax Forum focuses its efforts on Multichannel Multipoint Distribution Service, the 3.5 GHz licensed bands and the unlicensed 5 GHz band.
Figure 13: WiMax Forum and IEEE 802.16 WiMax standardization
The 9 Working Groups of the WiMax Forum and their main functions are as follows:
·         the Network WG, that develops network architecture specification, network procedures and protocols. It includes the Network Interoperability Testing TG, which defines network-level interoperability testing specifications.
·         the Technical WG, which develops technical conformance specifications, system profiles and certification test suites for the air interface.
·         the Application WG, that benchmarks, characterizes and demonstrates best-practice solutions among different classes of applications.
·         the Service Provider WG, which develops the requirements on air-interface and network systems.
·         the Marketing WG, that promotes global adoption of the WiMax broadband wireless technology and informs about the progress and development of WiMax
·          the Certification WG, which manages the WiMax Forum certification program with the supervision of the test labs, the evaluation of the general tests themselves and especially the network-related interoperability tests.
·         the Regulatory WG, that is in charge of the spectrum policy and regulatory issues worldwide, promoting access to a spectrum “fit for purpose” .
·         the Global Roaming WG, which is responsible for the frameworks needed to achieve global roaming across the various WiMax networks with the respective roaming guidelines, interfaces and tests.
·         The Technical Steering Committee (or Technical Plenary), that is promoting better cross-working-group coordination and decision making on crucial issues.
The WiMax Forum issues profiles, releases and certificates, each of the three with a different role. The profiles are based on the IEEE 802.16 and ETSI HiperMAN standards [3]. Features that are optional in standards can be compulsory in a profile. At the beginning, the WiMax Forum was focusing on the spectrum of 10-66 GHz, but later on smaller frequencies were included. There are currently two system profiles: the fixed WiMax system profile for systems based on IEEE 802.16-2004, with 256 carriers in OFDM and the mobile WiMax profile for systems of IEEE 802.16e-2005 amendment. The certification profiles are derived by the system profile and depend on the spectrum used (eg 2.5 GHz or 3.5 GHz), the channel size (eg 1.25 MHz or 10 MHz etc) and the duplexing mode (frequency or time division duplex). These three parameters are dependent on the local regulatory rules. Complying to the same certification profiles ensures interoperability among products of different vendors [7].
Table 2: major WiMax releases and their features
WiMax Forum develops releases and waves within these releases, in order to implement new features .The equipment that meets the standards of a release has to be compatible with older releases. The release development process consists of six stages. The requirements of the first stage are created by the Service Provider WG, of the second to the fifth stage by the Technical WG, the Network WG and the Global Roaming WG concerning the air interface, network and roaming specifications. The major releases are 1, 1.5, 1.6 and 2, which are shown on Table 2 [3] .
WiMax Forum issues certificates for vendors, after they have submitted their products for testing at the six WiMax Forum designated certification labs around the world, which are in Spain, Taiwan, China, the USA, Malaysia and South Korea. The testing itself takes place in these labs without the WiMax Forum involving directly [7]. The certification process is described with the flow chart of Figure 14. After the equipment is submitted by the vendor, conformance and interoperability tests are conducted, before the certification can be issued. If any of the two tests fails, the process halts and the vendor is asked to make the necessary changes in order to comply with the requirements.
Figure 14: flow chart of the WiMax Forum certification

When the certification is finally issued, the product is added to the WiMax Forum Certified Product Registry [7].

Τρίτη, 9 Ιουλίου 2013

The IEEE 802.16 WG Standardization of WiMax

The standardization of WiMax began with the WirelessMAN developed by IEEE 802.16 WG, which includes detailed specifications and  both mandatory and many optional functionalities in order to achieve more flexibility and possible performance enhancement for various operating scenarios. As any other IEEE standard, IEEE 802.16 fulfills the five-criteria: broad market potential, technical and economic feasibility, distinct identity and compatibility [7]. It consists of three main parts, starting from top to bottom:
·         the convergence sublayer (CS), that interfaces higher-layer protocols such as IPv4 and IPv6to the IEEE 802.16 media access control service data unit (MAC SDU)
·         the MAC common part sublayer (CPS), which conducts the fragmentation or the packing of of the MAC SDU’s in order to make them fit into MAC protocol data units (PDUs), which have a suitable format for handling by the physical layer
·         the physical layer, which defines the physical frame format, forward error correction (FEC) and modulation schemes.
These three parts are shown in Figure 8,with the physical layer at the bottom and the MAC CPS and the CS on top of it.
Figure 8: IEEE 802.16 WG standardization coverage
The IEEE standardization of WiMax has a long history, starting from 2001, with 802.16. The ancestor of it lies on the IEEE 802 Study Group on Broadband Wireless Access (BWA) in November of 1998 [7]. The first final draft document of the complete standard was 802.16 in December 2001, a huge milestone of the WiMax standardization. It was published in April of 2002 with the title “IEEE standard for local and metropolitan area networks part 16: air interface for fixed broadband wireless access systems” and it described a fixed wireless access scenario at 10-66 GHz with line-of-sight and point-to-point for a cell radius not bigger than 5 kilometers [4].
 In May 2002 802.16c introduced a number of profiles including a set of predetermined parameter values for interoperability support. 802.16c (or 802.16c-2002) is “Amendment 1: detailed system profiles for 10-66 Ghz”. In January 2003 802.16a-2003 was approved under the name “Amendment 2: medium access control modifications and additional physical layer specifications for 2-11 GHz”. As it is obvious from its name, it included frequencies lower than 11 GHz (down to 2 GHz) and it also described non-light-of-sight links. The next major update was 802.16-2004 in September 2004 , called “IEEE Standard for local and metropolitan area networks part 16: air interface for fixed broadband wireless access systems” [4].
Table 1: main initial WiMax standardization milestones
Figure 9: illustration of the 802.16-2001 development until 2005
After that, 802.16e was published in December 2005 which was “Amendment 2: physical and medium access control layers for combined fixed and mobile operation in licensed bands” and introduced mobility (at speeds less than 120 kilometers per hour) for WiMax as well as handovers, with the cell radius being 5 kilometers or less. These WiMax milestones are summarized in table 1, along with their achievable bitrates and modulation formats. The progress of the 802.16-2001 is illustrated in Figure 9 [7].
The next important update was 802.16j, which was published in June 2009. It features relay support for 802.16m network architecture. As a result, the “16jm” Ad Hoc Group was established in order to study the issues. The final report of the group was delivered in July 2008 and was approved in May 2009.It was an amendment to IEEE 802.16-2009, which was called “Amendment 1: Multihop Relay Specification” . The IEEE 802.16-2009 was “IEEE standard for local and metropolitan area networks part 16: air interface for broadband wireless access systems”. Finally, the IEEE 802.16m-2011 doubles both the user and sector throughput and supports speeds up to 350 kilometers per hour, for ultra-fast train travelers. These improvements are illustrated in Figure 10 [4].
Figure 10: 802.16j and 802.16m new characteristics
As of July 2013, two new standards are under development: 802.16n and 802.16p [12]. The first one is an amendment aiming towards higher reliability networks and the second one towards enhancements to support machine-to-machine applications. Many features of the WiMax standard are still open and under discussion and there is always a constant willing for further improvements and upgrades.
A detailed presentation of the 802.16-2001 from 2002 to 2007 standard is shown in Figure 11. “PAR” stands for Project Authorization Request, “TG” for task group and “SG” for Study Group. PARs are the means by which standards projects are started in the IEEE Standards Association [7].
IEEE 802.16 holds a plenary meeting every March July and November and 802.16 WG holds an interim meeting every January, May and September, resulting into 6 annual meetings in total, concerning the WiMax. The participants in those meetings were a few tens at the beginning (1999-2000), but exceeded the 400 in 2007 [4].

Figure 11: IEEE 802.16-2001 and its amendments until 2007

Overview of WiMax Standardization Processes

Standardization of WiMax has been carried out and is still being carried out by three different entities: IEEE 802.16 WG, WiMax Forum and International Telecommunication Union (ITU).
 The IEEE 802.16 WG was established in 1999. It has developed and published several versions of air-interface standards for wireless metropolitan area networks (Wireless MAN), with focus on the MAC and physical layer specifications. The WiMax forum was established in June 2001 by nine companies. Today it has hundreds of members, including most of the WiMax operators, component vendors and equipment vendors. It certifies products and equipment based on the IEEE 802.16 WG and European Telecommunications Standards Institute High Performance Metropolitan Area Networks (ETSI HiperMan). It can be said that the WiMax Forum is the equivalent to the WiFi Alliance for the WiMax side. WiMax Forum aims towards interoperability of WiMax devices and equipment and provides the certificate that a specific device meets the necessary requirements for that . ITU was established in 1865 in Paris as the International Telegraph Union and today constitutes a part of the United Nations (UN) for information and communication technology issues, based in Geneva, Switzerland. ITU gives recognition to WiMax standards developed by the IEEE 802.16 WG and the WiMax Forum, which allows spectrum owners to roll out WiMax easier in different countries and improves the international esteem of the technology [7].
The WiMax Forum extended the legacy 3-stage process of IEEE 802.16 WG to 6 stages in order to establish certified interoperability. This is illustrated in Figure 7.

Figure 7: the 3-stage standardization of IEEE 802.16 WG, extended by the WiMax Forum

WiMax Security Issues, Mobility and Network Entities

WiMax supports Multiple-Input Multiple-Output (MIMO) schemes. In these schemes, both the Base Station and the Mobile Station establish a connection with more than one antenna.
In this way, the data rate can be increased proportionally to the minimum number of the antennae of each side. The MIMO technology improves the reception and allows for a better reach and rate of transmission. The IEEE 802.16 specification suggests and describes the use of four antennae on each side (4x4 MIMO link). The main drawback of this case, apart from the fact that more hardware concerning the antennae is physically needed, is that more advanced digital signal processing is applied, with increased processing power demanded. If the receiver has one antenna, then no better result than 1x1 is achieved. This is illustrated at Figure 5.
Figure 5: typical 2x2 MIMO WiMax channel in comparison to a 1x2 WiMax channel
Each part of a WiMax network contains several entities that form up the whole setup. The three most important ones are the Base Station, the Access Service Network Gateway and the Connectivity Service Network. The Base Station is mainly responsible for providing the air interface to the Mobile Station, but also for handoff triggering (when a Mobile Station changes from one cell to another), radio resource management, tunnel establishment, Quality of Service (QoS) policy enforcement (applies to services with higher sensitivity than the average and/or to end users with a premium subscription), traffic classification, Dynamic Host Control Protocol (DHCP) proxy, key management, session management, micromobility management functions and multicast group management. The Access Service Network Gateway acts as a layer 2 (OSI data link layer) traffic aggregation point within an Access Service Network and its main functions include intra-access service network location management and paging, radio resource management, admission control, caching of subscriber profiles and encryption keys, establishment and management of mobility tunnel with Base Stations, Quality of Service and policy enforcement and foreign agent functionality for mobile IP. The Connectivity Service Network provides connectivity to the Internet, other public and corporate networks, authenticates the connected devices, users and services. It also provides per user policy management of Quality of Service and security, manages the IP address allocation and supports roaming capabilities [13].
Figure 6: WiMax diverse usage towards full mobility
WiMax has four main user mobility scenarios:
·         nomadic, where the user is allowed to take a fixed subscriber station and reconnect from a different point of attachment
·         portable, where access is provided to a portable device  and the handover is best-effort and not guaranteed
·         simple mobility, where the user can move at speeds up to 60 kilometers per hour with brief interruptions of less than 1 second during handoff
·         full mobility, where the user can move at speeds up to 120 kilometers per hour and the handoff experiences latency less than 50 milliseconds and the packet loss is less than 1% [18]
WiMax supports nomadic and portable users (as in Figure 6) without efficiency issues, but simple and especially full mobility can severely affect the bitrate achieved at the side of the subscriber.

WiMax, as any other wireless technology, faces several security issues. The physical medium of the signal propagating is air, which is obviously spatially shared by all users. Therefore, WiMax has applied robust security, including support for privacy, device-user authentication, flexible key-management protocol, protection of control messages and support for fast handover. User data is encrypted by using Advanced Encryption Standard (AES) and Triple Data Encryption Standard (3DES) with an 128-bit or a 256-bit key, ensuring data privacy. WiMax authenticates subscriber stations and users with an authentication framework which is based on the Internet Engineering Task Force (IETF) Extensible Authentication Protocol (EAP) and supports a variety of credentials, like username-password, smart cards and digital certificates. WiMax uses the Privacy and Key Management Protocol Version 2 (PKMv2) for securely transferring keying material from the Base Station to the mobile station, by refreshing and reauthorizing the keys from time to time. The protection of control messages is secured by message digest schemes, such as AES-based Cipher-based Media Access Control (CMAC) or MD5-based (Message-Digest) Hash Message Authentication Code (HMAC). Fast handovers are achieved with the use of pre-authentication with a particular Base Station to facilitate accelerated reentry [13]. A 3-way handshake both optimizes this procedure and prevents any possible man-in-the-middle attacks.