WiMAX Interview Questions and Answers

WiMAX -Worldwide Interoperability for Microwave Access

WiMAX (Worldwide Interoperability for Microwave Access) is the wireless technology based on 802.16 standard. It’s also called 4G sometimes. It works pretty similar to wireless network, but with much bigger coverage (quite similar to cell phone network coverage), higher speed and greater number of users supported.

That’s why it’s suitable to be used as broadband wireless access service in rural areas that lack of DSL/Cable broadband service. In order to connect to WiMAX network, the most convenient way is to install a PCI card or USB dongle on the computer to connect to WiMAX transmitting station. Also external WiMAX antenna can be installed to connect to WiMAX network. The specification (802.16e) latest standard of WiMAX offers more than 70 GHz range. It is providing scalable orthogonal frequency-division multiple accesses and is showing much better performance than fixed standard.

The 802.16a standard for 2-11 GHz is a wireless metropolitan area network (MAN) technology

that will provide broadband wireless connectivity to Fixed, Portable and Nomadic devices.

It can be used to connect 802.11 hot spots to the Internet, provide campus connectivity, and provide a wireless alternative to cable and DSL for last mile broadband access.

WiMAX Technology is an IP based, wireless broadband access technology that provides performance similar to 802.11/Wi-Fi networks with the coverage and QOS (quality of service) of cellular networks.

WiMAX technology is a telecommunications technology that offers transmission of wireless data via a number of transmission methods; such as portable or fully mobile internet access via point to multipoint links. The WiMAX technology offers around 72 Mega Bits per second without any need for the cable infrastructure. WiMAX technology is based on Standard that is IEEE 802.16, it usually also called as Broadband Wireless Access. WiMAX Forum created the name for WiMAX technology that was formed in Mid-June 2001 to encourage compliance and interoperability of the WiMAX IEEE 802.16 standard. WiMAX technology is actually based on the standards that making the possibility to delivery last mile broadband access as a substitute to conventional cable and DSL lines.

WiMAX 2 technology is improving with IEEE 802.16m mobile WiMAX standard which is called WiMAX 2. Well Known business leaders make known proposal to speed up WiMAX 2 Technology Solutions built upon IEEE 802.16m. WiMAX 2 Technology is in the final stage of approval and WiMAX 2 technology may be beginning in 2011 and possibly commercialized or deployed in 2012.

The objective behind WiMAX 2 Technology are technology cooperation and mutual presentation benchmarking, dual testing of 4G Technology requests over WiMAX 2 Technology solution, and untimely network level interoperability testing. There are several companies have announced WiMAX 2 technology with primary benefit of speed such as ZTE, Intel, Samsung, Motorola. There are two type of existed WiMAX, Fixed WiMAX which is faster than Wi-Fi. Mobile WiMAX, which is used as a substitute of 3G mobile phone operator. Mobile WiMAX can transport 144 Mbps download and 35Mbps uploading speed. While WiMAX 2 technology intended to meet the advanced 4G standards within 1Gbps access speed to fixed and 100Mbps to fast moving mobile nodes.

• WiMAX is High Speed Network
• Best coverage
• Potential and development
• Best Infrastructure
• Low cost
• Homeland Security
• High Quality Features
• Wi-Fi
• Lack of history
• Ultra-wide Band
• Smart antenna and Mesh Topology
• Multi-functionality within WiMAX Technology

WiMAX technology can operate on both licensed and non-licensed frequencies. WiMAX Technology is powerful mobile technology but are facing some disadvantages discussed below.

• WiMAX Bandwidth
• Expensive network
• Lack of Quality
• Weather condition
• Wireless equipment’s
• WiMAX range: WiMAX offer 70Mbps in range with moving station but in practice it is quite different because it is possible only in specify or ideal circumstances. If a user staying away from the specified environment then speed can drop considerably.
• Data Rate: The data rate of WiMAX as compared to other network such as fiber optics, satellite, cables etc are very slow.
• Power consuming

The WiMAX family of standards (802.16) concentrate on two types of usage models a fixed WiMAX usage model and a mobile WiMAX usage model. The basic element that differentiates these systems is the ground speed at which the systems are designed to manage. Based on mobility, wireless access systems are designed to operate on the move without any disruption of service; wireless access can be divided into three classes; stationary, pedestrian and vehicular.

WiMAX system has been introduced as a replacement of DSL technologies. WiMAX is a broadband wireless technology used mainly for providing internet connectivity over the air. There are different physical layers and frequency bands of operation based on country and applications. It utilizes concept of OFDM modulation technique to achieve the higher data rate by transmitting data over multiple subcarriers closely packed without leaving much subcarrier spacing in between.

WiMAX operates similarly to Wi-Fi systems but on a much greater scale.  The WiMAX system has two components, the WiMAX receiver and the WiMAX tower.  The receiver is a small box or card that would often be built into a device such as a laptop or installed on a home for personal use and a WiMAX tower would be similar to telephone towers.  The towers would give signals to customers within a certain radius at a lower frequency.  This allows for transfer of data through objects.  When a customer accesses the internet using a WiMAX system, they are receiving a signal from a local tower.  Each tower is also capable of sending signals to other towers over a much higher frequency to provide a stronger signal.  This means that with the use of towers it is possible to grant internet access to rural areas of the country relatively cheap and effectively.  These towers connect with the internet service provider’s network which is connected to an internet backbone.

WiMAX deployments will use licensed Radio Frequency (RF) spectrum, positively granting them some degree of protection from unintentional interference. It is reasonably simple, however, for an attacker to use readily available tools to jam the spectrum for all planned WiMAX deployments. In addition to physical layer Denial of Service (DOS) attacks, an attacker can use legacy management frames to forcibly disconnect legitimate stations. This is similar to the de-authenticate flood attacks used against 802.11 (Wi-Fi) networks. (Joshua Wright, Dec 12 2006)

The central radio transmitter/receiver installed by service provider to broadcasts WiMax signals. These transmitters are typically mounted on towers or tall buildings. Cellular networks are based on the concept of cells (a logical division of geographical area), each such cell is allocated a frequency and is served by a base station. Base station consists of a receiver, transmitter and a control unit. Adjacent base stations use different frequencies to avoid cross-talk.

There are two types of certificate are categorized by WiMAX standard: one is for Subscriber Station (SS) certificates and the other is for manufacturer certificates but there is no provision for Base Station (BS) certificates. A manufacturer certificate identifies the manufacturer of a WiMAX device. It can be a self-signed certificate or subjected to any third party. A Subscriber certificate identifies a particular Subscriber Station and enclosed its MAC address in the subject field. Manufacturers normally create and sign Subscriber Station certificates.

Generally, the Base Station (BS) uses the manufacturer certificate’s public key to validate the Subscriber Station (SS) certificate, and therefore identify the device as genuine. This design assumes that the Subscriber Station (SS) keeps the private key related to its public key in a sealed storage, preventing attackers from easily compromise it. The major drawback of the WiMAX security design is the lack of a Base Station (BS) certificate. The only approach to defend the client against forgery or replay attack is to offer a scheme for mutual authentication. In 802.16e, EAP can be verified with specific authentication methods such as X.509 certificate.

WiMAX supports the following for encryption:

• Advanced encryption standard
• Triple data encryption standard

The following modulation schemes are supported by WiMAX:

• Binary phase shift keying modulation
• Quadrature phase shift keying modulation
• Quadrature amplitude modulation

WiMax cannot provide highest performance over 50 kilometres. As the distance increases, bit error rate thus reducing performance. Reducing distance to less than 1km allows a device to operate at higher bit rate. A user closer to base station gets better speed at around 30 mbps. Also, as an available bandwidth is shared between no of users, performance depends on number of active users connecting to that base station. So this needs a use of Quality of Service (QOS) mechanism to provide a minimum guaranteed throughput.

It is a standalone indoor device which is installed at a good reception area. It acts as a wi-fi access point for other devices such as home pc, VOIP handset etc. WiMax operators generally provide this gateway device or subscriber unit which communicates with base station and provides wi-fi access within home or office for device like laptop, Smartphone.

There are five physical layers

• OFDM
• OFDMA
• SC
• SCa
• Wireless Human

Following are the WiMAX QoS classes:

• UGS
• rtPS
• ertPS
• nrtPS
• BE

In Wimax TLV formats are used for parameters in both mac management messages as well configuration file. TLV stands for Type, length and value. Size of type field will be always 1 byte. Size of length field is either 1 byte or more than 1 byte.

Size of value field depends on length field specified. If the actual size of ‘value’ field is less than or equal to 127 bytes (length<=127) then

• Size of ‘length’ field will be 1 byte
• MSB of ‘length’ field is set to zero
• The rest of 7 bits of length field indicates actual size of ‘value’ field in number of bytes.

Microcontroller usually operates at frequencies in MHz range while today’s Microprocessor operates in GHz frequency range. This clock is provided using oscillator. Microcontroller and microprocessor both have their own applications. Hence both cannot be used for each other’s functional areas except for few exceptions (very low size code/data application).

In addition to microprocessor (CPU) functionality microcontroller will have timers, counters, RAM, ROM and Input/output ports. Most of the microprocessors have Op-codes for moving data from the external memory to the CPU; Microcontrollers may have one or two. Microprocessors have one/two types of bit handling instructions while Micro controllers have many. Microprocessor is used for general purpose applications while microcontroller is program specific and designed for particular application in consideration.

FDM is the short form of frequency division multiplexing and TDM is the short form of time division multiplexing.

• TDM is employed in PCM transmission to obtain T1 at the rate of 1.544Mbps.
• FDM is employed in satellite, Radio, HF and other wireless technologies.

Both TDM and FDM is employed in GSM cellular technology.

OFDM and OFDMA differ in the way resources are allocated to the subscribers. Also in OFDM FFT size is fixed to 256 subcarriers. In OFDMA FFT size is variable and it can take any value from 128, 512, 1024 and 2048. Frame structure is also different. There is also difference in the physical layer modules.

Both TDD and FDD are duplexing topologies used mainly to share the common central resources. In TDD, frequency is shared among subscriber’s time slot wise. In FDD, one pair of frequency is assigned for one connection one for downlink and one for uplink and hence transmission will happen simultaneously in both the directions.

There are various WiMAX ranging procedures as mentioned below.

• Initial Ranging
• Periodic Ranging
• Contention Ranging
• Non-Contention Ranging

Contention raging procedure is required by the SS to access the system for the first time and no dedicated connection resources assigned to the SS.

Non-contention raging is regulated by the BS to allow the SS to finish system access earlier when dedicated channel is provided and polled initial ranging.

Periodic raging is to maintain uplink communication with the BS and to adjust transmission parameters.

Initial raging acquires correct transmission parameters (Power adjustment, timing offset estimation and synchronization) to communicate with the BS.