Board Gateway Profile,1
A Study on WiFi and WiMAX Layred Structures
A Study on WiFi and WiMAX Layred Structures
Dr.Hari Ramakrishna
Professor, Department of CSE,
Chaitanya Bharathi Institute of technology
Gandipet -500 075, Hyderabad,
dr.hariramakrishna@rediffmail.com
K.Ravi
Asst. Professor
Dept. of Informatics
Alluri Institute of Management Sciences
ABSTRACT
The connectionless approach of WiFi, it was designed to be capable of providing high-rate, high-quality data services to mobile users in medium to wide areas at very reasonable service charges. Mobile WiMAX is very new, with the first IEEE 802. This paper details with the network technologies adopted by Mobile WiMAX for the implementation of IP-based broadband mobile wireless access and the WiFi technologies for IP-based wireless local network access. These access and local technologies have many things in common, most prominently that both are oriented toward IP traffic and standardized by IEEE 802 working groups. In this paper we first introduce WiFi and WiMAX and later we see the similarities and differences between these two wireless communication technologies.
Keywords: WiFi, WiMAX, IEEE 802.16e, Physical Layer, MAC Layer, Network Configuration, Connectionless Service, Protocol
1. INTRODUCTION
Mobile WiMAX and WiFi were generated independently each other by different standards groups and or different target frequency bands. In addition, the goals were different: Mobile WiMAX was intended to offer a wireless access means to wide area networks, whereas WiFi was intended to function as a wireless extension to the existing local area networks. Consequently, the employed PHY and MAC technologies were different from each other as each was optimized to its own targeted goals. Nevertheless, they possess an important commonality: both standard groups are under the same umbrella of IEEE 802 LAN/MAN committee, which has developed various access and connectivity protocols on the foundation of the same packet-mode (or IP-mode) concept, as opposed to other existing cellular wireless systems that employ the circuit-mode concept.
2. WiFi and WiMAX
Standardization of Mobile WiMAX and WiFi was dictated by IEEE 802.16 and 802.11 Working Groups (WGs), respectively, both under the IEEE 802 LAN/MAN committee. The IEEE 802.16 standardization dates back to 1999, with the first IEEE 802.16 standard published in 2002. A series of IEEE 802.16 standards soon followed, with the 802.16a, 802.16d, and 802.16e standards published in 2003, 2004, and 2006, respectively. In contrast, the IEEE 802.11 standardization was started much earlier, with the first baseline standard of IEEE 802.11 published in 1997. Subsequently, the IEEE 802.11a and 802.11b standards were published in 1999 and 802.11g in 2003.
2.1 IEEE 802.16/WIMAX STANDARDIZATION
The IEEE 802.16 standard has been developed by the IEEE 802.16 WG on broadband wireless access (BWA) since 1999. This standard was initially designed for fixed wireless services and was expanded to support mobility feature in IEEE 802.16e. In parallel with that, the Telecommunications Technology Association (TTA) of Korea worked on the standardization of WiBro system in harmonization with IEEE 802.16e, thereby positioning it as a special profile of the IEEE 802.16e standard. WiMAX system, the commercial profile of IEEE 802.16 standard, has been developed by WiMAX Forum. Among IEEE 802 standard committees responsible for standardization of PHY and MAC layers of LAN/MAN, IEEE 802.16WGhas developed BWA standards since it was formed in 1999.
This standard was initially designed to support fixed BWA service in LOS environment of 10–66-GHz band and IEEE 802.16-2001 [8] was approved by the Standard Board of the IEEE Standard Association (IEEE-SA) in 2001. Later, in the NLOS environment of the 2–11-GHz band, IEEE 802.16a standard [9], which includes three types of physical layers, SCa, OFDM, and OFDMA was developed in 2003 These standards were later revised and consolidated by IEEE 802.16 Task Group d (TGd) and its final version, IEEE 802.16, was approved.
On the other hand, IEEE 802.TGe was organized to enhance the standards by including mobility in 2002. In the beginning, it started with incorporating a limited mobility to the existing OFDMA specification with 1,024-point fast Fourier transform (FFT) and 5-MHz bandwidth. Later, it was expanded to encompass full mobility.
2.2 FREQUENCY SPECTRUM FOR WIMAX
The IEEE 802.16 standard system operates in two different frequency bands: one is in the 10–66-GHz band, and the other is the “below 11 GHz” band, or the 2–11-GHz band, specifically.
Table 1: Frequency Spectrum Allocation for Wireless Communication Services in Korea
3. BROADBAND WIRELESS ACCESS NETWORKS
IEEE 802.16 generated a family of standards for BWA, among which the IEEE 802.16e standard includes the mobility feature that yields the Mobile WiMAX. Mobile WiMAX supports roaming service in metropolitan and regional networks, so allows mobile connectivity to mobile users. The target mobility is 120 km/h and the peak throughput is 18.7-Mbps downlink and 5.0-Mbps uplink in the case of DL/UL ratio 29:18 and 10-MHz bandwidth. It utilizes the cell concept and the coverage of a cell is in the range a few kilometers. Equipped with such features, Mobile WiMAX is advantageous in supporting low-latency data, video and real-time voice services for mobile users at high speed.
The protocol layering of the IEEE 802.16 system consists of a MAC layer and a physical layer, with the MAC layer divided into three sublayers, namely, service specific convergence sublayer (CS), MAC common part sublayer (CPS), and security sublayer. The service-specific CS performs functions of converging user services to MACCPS. There are two CS specifications, namely ATM CS and packet CS, but the packet CS is more commonly used for transporting all packet-based protocols such as IP, point-to-point protocol (PPP), and Ethernet.
Among these CSs, only IP CS is included in the WiMAX profile. MAC CPS is the main body of the MAC layer, which supports all different types of service-specific CSs in common. It provides a mechanism that enables all the users to share the wireless medium effectively. Specifically, it provides the core MAC functionality such as system access, bandwidth allocation, connection establishment, and connection maintenance. The security sublayer (or privacy sublayer) provides authentication, privacy key exchange, and encryption functions. The security function is supported by an authenticated client/ server key management protocol (KMP) in which the BS controls the distribution
of the keying material to mobile stations.
3.1 PHYSICAL LAYER
In the IEEE 802.16 standards, four different types of physical layers for operation in different frequency bands, based on different multiple access technologies—namely, Wireless MAN-SC, Wireless- SCa, Wireless-OFDM, and Wireless-OFDMA. In addition, Wireless HUMAN PHY is specified for use in the license-exempt bands. The most commonly used among the four are the Wireless MAN-OFDM and Wireless MAN-OFDMA, which are used in the fixed and mobile BWA networks, respectively.
The OFDM/OFDMA-based IEEE 802.16 WiMAX system has several distinctive features in employing advanced technologies: First, it adopts the time division duplex (TDD) scheme for sharing communication channels between uplink and downlink, in addition to the frequency division duplex (FDD) that has been widely adopted in the existing circuit-mode mobile wireless systems. Second, it adopts the OFDMA scheme for sharing the communication link among multiple users, whereas the existing mobile wireless systems adopted TDMA or CDMA schemes.
Third, it uses AMC technology for an efficient modulation, demodulation, coding, and decoding of communication signals. AMC dynamically changes the modulation and coding techniques depending on the channel status, thereby enhancing the system efficiency in varying wireless channel conditions. Fourth, it employs multiple antenna technologies so that it can significantly enhance the system performance and increase transmission capacity by taking advantage of the space diversity, spatial multiplexing, and beam forming with interference nulling effects. In addition, it takes a larger channel bandwidth (e.g., 10 MHz) for operator allocation than the existing mobile system did, within which the operator can actively apply these technologies.
The support of the mobility, which is a very important feature of the Mobile WiMAX system, it adopts efficient technologies for battery power saving and IP-based mobility. For battery power saving, the Mobile WiMAX system adopts the sleep/idle mode terminal operation. When each MS is not in awake mode, it goes into the sleep mode, and for further power saving, it can go into the idle mode, in which case it does not register to any BS but only receives the downlink paging messages periodically. For mobility, the Mobile WiMAX can use mobile IP, which manages the location information by home and foreign agents. It realizes terminal mobility through the handover function among the neighboring BSs and it basically supports the hard handover scheme.
Supported by these advanced technologies, the Mobile WiMAX system sets aggressive requirements on system performance and data services. It supports the data transmission rate of 18.7 Mbps downstream and 5.0 Mbps upstream in case of DL/UL ratio 29:18 and 10 MHz bandwidth for non-MIMO case. The peak data rates are doubled if MIMO technology is applied. The Mobile WiMAX supports the frequency reuse factor (FRF) of 1 for all cells, the user mobility of 120 km/h, and the handover latency of 150 ms. Table 1.14 lists the features of Mobile WiMAX system
in 10 MHz bandwidth with 29:18 DL/UL ratio.
3.2 MAC LAYER
Mobile WiMAX is connection-oriented system, which enables it to tightly control the resource allocation and QoS, as well as the security function, needed for broadband wireless access. The MAC function of Mobile WiMAX is divided into three sublayers, namely service-specific CS, CPS, and security sublayer. The service specific CS performs the functions needed for converging user services to MAC CPS, including the reception, classification, and processing of the higher layer PDUs, the delivery of CS PDUs to the appropriate MAC SAP, and the receiving of CS PDUs from the peer entity. The MAC CPS performs the core MAC functionality including system access, bandwidth allocation, connection establishment, and connection maintenance, as well as effective user sharing of the wireless medium.
For the enhancement of reliability of data transmission, Mobile WiMAX adopts both ARQ and HARQ mechanisms. ARQ is a primitive form of error recovery technique that totally relies on the retransmission of the erred packets, and HARQ is an enhanced form of ARQ that utilizes FEC for the improvement of detection capability. Specifically, HARQ exploits the information in the original message to aid the decoding of the retransmitted messages.
ARQ in Mobile WiMAX is enabled on a per-connection basis and is specified and negotiated during connection setup. Mobile WiMAX defines four ARQ feedback types to signal ACK/NAK, namely, selective ACK, cumulative ACK, cumulative with selective ACK, and cumulative ACK with block sequence ACK. In the case of HARQ, both Chase combining and incremental redundancy (IR) HARQ methods are defined in the IEEE 802.16e standards but only Chase combining HARQ is included in the WiMAX profile.
Table 2: System Feature of the Mobile WiMAX System (10-MHz BW)
The principal mechanism of Mobile WiMAX for providing QoS is to associate packets traversing the MAC interface into a service flow. The MS and BS provide the QoS according to the QoS parameter set defined for the service flow. As the mechanisms of providing QoS services, Mobile WiMAX defines several bandwidth allocation types, which reflect the delay requirements and traffic characteristics, and their corresponding data delivery services, such as unsolicited grant service (UGS), extended real-time variable-rate (ERT-VR) service, real-time variable-rate (RT-VR) service, nonreal-time variable-rate (NRT-VR) service, and best effort (BE) service.
In order to enhance the efficiency of the bandwidth usage, the Mobile WiMAX adopts well-organized bandwidth request, grant, and polling mechanisms, which are supported by these five different types of delivery services. The downlink bandwidth is solely managed by the downlink scheduler at the BS, but the uplink bandwidth is allocated by BS to MSs through the resource request and grant process. For the implementation of QoS services, Mobile WiMAX employs the enforcement functions such as scheduling, CAC, and policing. These functions are designed to maximize the QoS satisfaction, minimize the QoS violation, and protect the QoS of the contract-conforming connections.
For the mobility, Mobile WiMAX basically supports the hard handover scheme, as it is optimized for IP data traffic, but it also supports soft handover (in the standard, but not in the WiMAX profile). Handover is performed in two main processes, namely, network topology acquisition process and handover execution process: The network topology acquisition periodically updates the parameter values needed for making handover decision, and the handover execution practically executes the handover through a series of processes such as neighbor scanning, handover capability negotiation, MS release, and network re-entry. Power saving is crucial to terminal mobility, and Mobile WiMAX supports both sleep mode and idle mode operations: the sleep mode allows MS to be absent from the serving BS air interface while not in use, and the idle mode allows MS to be mostly idle and only listen to the paging messages periodically.
3.3 NETWORK CONFIGURATION
The WiMAX system is originally designed to be a data-centric network based on the IP technology, different from the existing voice-centric mobile communication networks that used circuit-mode technology. It adopts an all-IP network structure tailored for Internet service provision, so the network structure is simple and is adequate for provision of diverse set of services.
The following figure illustrates the configuration of Mobile WiMAX network. As the network is designed based on the all-IP network concept, the network configuration is very simple and the network construction cost is low. The network has a star architecture, with the mobile stations located at the end of the branches. The IP packets sent by MSs get accessed to the Internet via the BS to access service network gateway (ASN-GW) path. This demonstrates how simple it is to provide Internet services over the WiMAX network. Consequently, a diverse set of services can be provided over the WiMAX network at low cost, with the voice service provided in VoIP form.
Figure 1 Illustration of Mobile WiMAX network configuration.
Mobile WiMAX network consists of access service network (ASN) and connectivity service network (CSN). ASN consists of three basic building blocks, namely, MS, BS, and ASN-GW, and the CSN consists of various servers and core routers/switches. So the Mobile WiMAX network configuration is much simpler than existing circuit-based mobile communication networks such as the IS-95/EV-DO family system, which includes base station controller (BSC), mobile switching center (MSC), or the GSM/WCDMA family system, which includes radio network controller (RNC), serving GPRS support node (SGSN), and gateway GPRS support node (GGSN), in place of ASN-GW
To be more specific, the BS collects user terminal data via wireless path, passes it to the ASN-GW in the upstream, and distributes the data received from the ASN-GW to the MSs in the downstream. The functions of BS include wireless access processing, radio resources management and control, mobility support for seamless services while moving, QoS support for stable service quality, and overall equipment control and management. On the other hand, the ASN-GW connects the BS with the various servers and core routers/switches in the CSN. It performs the routing function transferring data between the BS and the CSN and the control function controlling the MSs, services, and mobility.
4. WIFI: WIRELESS LOCAL AREA NETWORKS
IEEE 802.11 WLAN or WiFi is probably the most widely accepted broadband wireless networking technology, providing the highest transmission rate among standard- based wireless networking technologies. Today’s WiFi devices based on IEEE 802.11a and 802.11g provide transmission rates up to 54 Mbps and, further, a new standard IEEE 802.11n, which supports up to 600 Mbps, is being standardized. The transmission range of a typical WiFi device is up to 100m, where its exact range can
vary depending on the transmission power, the surrounding environments, and others. The 802.11 devices operate in unlicensed bands at 2.4 and 5 GHz, where the exact available bands depend on each county.
Most of today’s laptop computers as well as many PDAs and smart phones are shipped with embedded WLAN interfaces. Moreover, many electronic devices including VoIP phones, personal gaming devices, MP3 players, digital cameras, and camcorders are being equipped with WLAN interfaces as well. The most typical applications of the 802.11 WLAN should be the Internet access of portable devices in various networking environments including campus, enterprise, home, and hot-spot environments, where one or more access points (APs) are deployed to provide the Internet service in a given area.
The 802.11 can be used for a peer-to-peer communication among devices where APs are not deployed. For examples, laptops and PDAs in proximity can use the 802.11 to share their local files. Also, people in proximity can do networked gaming using their gaming devices with the 802.11 interface. It is primarily being used for the indoor purpose. However, it can be also used in outdoor environments, and some level of mobility (e.g., the walking speed) can be also supported.
The IEEE 802.11 WG has generated a family of standards for WLAN. The 802.11 pacifications are limited to PHY and MAC layers, and the existing higher layer protocols, which were originally developed for wireline networking technologies, can work on top of the 802.11 since it was basically developed to provide the service similar to the 802.3 Ethernet. At one point, this technology was referred to as “Wireless Ethernet.” In typical 802.11 devices, the 802.2 LLC protocol sits on top of the 802.11 MAC, where IP sits on top of the LLC. Through its evolution, the 802.11 is becoming much more than Ethernet.
For example, the 802.11e MAC enables multimedia applications such as voice over IP (VoIP)
over WLAN. The protocols for seamless mobility are being developed since the support of seamless mobility became quite critical along with the emergence of WLAN-based VoIP phones In fact, people are also trying to use this technology for vehicular networking (e.g., car-to-car and car-to-roadside) as well.
4.1 PHYSICAL LAYER
The IEEE 802.11 PHYs have been evolving dramatically. The baseline standard of IEEE 802.11 defined three different PHY protocols, namely, direct-sequence spread-spectrum (DSSS), frequency-hopping spread-spectrum (FHSS), and IR, where all three PHYs supported only the transmission rates of 1 and 2 Mbps. The extensions of the 802.11 PHY include the 802.11a supporting up to 54 Mbps based on the OFDM, the 802.11b (published in 1999) supporting up to 11 Mbps based on the complementary code keying (CCK), and the 802.11g (published in 2003) again based on OFDM to support up to 54-Mbps transmission rates.
The 802.11 PHYs operate in unlicensed bands at 2.4 GHz and 5 GHz. While most of other PHYs, including DSSS, FHSS, 802.11b, and 802.11g operate at the 2.4-GHz bands, the 802.11a operates at the 5-GHz bands. The 802.11g, in fact, includes the mandatory transmission schemes of the 802.11b, while the 802.11b includes the baseline DSSS PHY. That is, the 802.11g is backward compatible with the 802.11b, while the 802.11b is backward compatible with the baseline DSSS PHY. This implies that an 802.11g device can communicate with an 802.11b device using the transmission schemes of the 802.11b. Today, the most popular 802.11 PHY is the 802.11g, thanks to its fast transmission rate as well as low-cost chipset availability even though the 2.4-GHz bands, where the 802.11g operate, are much more crowded than the 5-GHz bands of the 802.11a.
The 802.11 basically operates with a time division duplexing (TDD) scheme for the sharing between uplink and downlink transmissions. That is, a single frequency channel is used for all the transmissions in a basic service set (BSS), which is a similar concept as a cell in typical cellular networks. The transmission bandwidth depends on the PHY as well. For example, the 802.11a and 802.11g signals occupy a 20 MHz band while the 802.11b signals occupy a 22-MHz band.
The 802.11 PHYs support multiple transmission rates by using different combinations of modulation and coding schemes (MCSs). Both the 802.11a and 802.11g support up to 54 Mbps, which make the 802.11 the fastest standards-based wireless technology as of today. In fact, as discussed in Section 1.3.2, the emerging 802.11n PHY will support up to 600 Mbps by utilizing multiple antenna technologies (i.e., MIMO schemes) and channel bonding (i.e., using 40-MHz bandwidth instead of 20 MHz). As 802.11 PHYs support multiple transmission rates, selecting a rate for a given packet transmission is a very important issue for the performance optimization of the network. In general, the higher the transmission rate, the shorter the transmission range is since high-order modulation schemes require higher signal-to-interference-and-noise ratio (SINR) for successful transmissions.
The transmission power level for the 802.11 PHY depends on the regulation of the corresponding country. Each country defines the upper limit of the transmission power at particular unlicensed bands. Typical 802.11 devices emit the power up to 20 dBm.
4.2 MAC LAYER
The 802.11 baseline standard defines connectionless MAC for the best-effort service. The baseline MAC is composed of two coordination functions, namely, the mandatory contention-based distributed coordination function (DCF) and the optional contention-free point coordination function (PCF). The DCF is based on carrier-sense multiple access with collision avoidance (CSMA/CA) and the PCF is a poll-and-response MAC.
Table 3: Various PHYs of IEEE 802.11
In fact, the PCF was rarely implemented in real products due to its complexity, the lack of needs, the lack of desirable operational functions, and others. Under the DCF, which was employed by most, if not all, WiFi devices, a station transmits only when it determines that the channel is not occupied by other transmissions, and this makes this MAC a perfect fit to the operation at unlicensed bands, at which various types of devices should coexist with some etiquette.
The baseline MAC is enhanced by the 802.11e to support quality-of-service (QoS) for multimedia applications such VoWLAN, video streaming, and so forth. The 802.11e MAC is called hybrid coordination function (HCF), which comprises the contention-based enhanced distributed channel access (EDCA) and the polland- response HCF controlled channel access (HCCA). EDCA and HCCA enhance DCF and PCF, respectively. EDCA provides prioritized channel access to frames with different priorities, where lower priority frame might be transmitted before higher priority frames due to the contentious nature of the EDCA. HCCA relies on the polling and downlink frame scheduling of the AP to meet the QoS described by a set of parameters. The 802.11e also defines various features needed for QoS provisioning, including the means for admission control of QoS streams.
The carrier-sensing feature of the MAC, the 802.11 inherently supports FRF of one. That is, even if the neighboring cells use the same frequency channel, the performance degradation due to the cochineal interference will be minimal since stations transmit frames only when they determine other neighboring stations are not transmitting. Apparently, depending on how the cells are deployed and which frequency channels are used for cells, there is room to improve the networking performance. That is, it is the best if neighboring cells can operate at non overlapping channels. However, the number of available non overlapping channels varies depending on the countries. The number of non overlapping channels at the 2.4-GHz bands is only three in most countries, and hence it is almost impossible to allocate non overlapping channels to all neighboring cells. This is particularly true in multistory building environments, since the cell structure is three-dimensional.
The 802.11 MAC supports reliable transmission of frames using ARQ. The baseline MAC defines a stop-and-wait ARQ, for which a receiver of a data frame responds with an ACK frame immediately after a successful reception. The 802.11e MAC then defines an enhanced ARQ scheme (i.e., selective repeat ARQ) using a mechanism called block ACK, in which a control frame called block ACK is transmitted by the receiver after the transmission of a number of data frames. A block
ACK includes a bit map indicating which of the previous transmitted frames were successfully received and which were not.
The mobility support has not been a major concern of the WiFi since people rarely use their laptop or PDA to access the Internet via WLAN while they are moving around. However, some level of mobility is supported by the 802.11. For example, the walking speed mobility is surely supported. The 802.11 allows a station to be associated with a single AP at a given time. That is, a hard handoff is supported. Along with the emergence of the VoWLAN applications, supporting seamless and smooth handoffs in the 802.11 WLAN becomes a hot topic.
Power saving is one of the major concerns for portable mobile communication devices. The 802.11 MAC defines power-saving mode (PSM) operation, in which a station switches back and forth between the active and the doze states, where the station consumes minimal energy in the doze state since it can neither transmit nor receive frames while staying in the doze state. The 802.11e further enhances the power-saving scheme, thus defining a scheme called automatic power-save delivery (APSD), which allows a station to save some power even during a QoS stream operation
The baseline MAC of the 802.11 had security mechanisms for confidentiality (via encryption) and authentication, but these schemes were found to be too weak to protect the security of the WiFi users. The problems included the cryptographic weakness of the encryption scheme (called RC4), the lack of key management, and so on. For example, under the legacy security mechanism, the same security key is basically used for every station in the network, while the key is rarely changed over
time. Such a security hole of the 802.11 was a big hurdle for the wide acceptance of WiFi at one point. Especially for enterprise networking, a strong security support was a mandatory requirement. Then, IEEE 802.11i enhanced security features by defining the robust security network (RSN), which is composed of stronger encryption schemes, per-frame authentication, per-station key management, and so on.
IEEE 802.11h defines mechanisms for spectrum managements including dynamic frequency selection (DFS) and transmit power control (TPC). While the 5-GHz bands, where the 802.11a operates, are unlicensed bands, there are in fact primary users who also use these bands. Those primary users are satellite and radar systems. The regulatory body in Europe required a WLAN device to have both DFS and TPC functions to minimize the interference of the WLAN to these primary users. That is, when a radar system is detected, the WLAN devices should leave the current channel to switch to another channel, and when a satellite system is detected, the WLAN devices have to limit their transmission power to the regulatory maximum minus 3 dB.
4.3 NETWORK CONFIGURATION
The basic form of the 802.11 network is called a basic service set (BSS), which comprises a number of stations. IEEE 802.11 supports two types of network configurations, namely, infrastructure and ad hoc modes. An infrastructure BSS is composed of an AP and a number of stations that are associated with the AP
Figure 2: Illustration of WiFi network configuration: (a) ESS composed of infrastructure BSS, and (b) IBSS.
A station in an infrastructure BSS communicates with other stations or nodes outside the WLAN through its AP. IEEE 802.11 AP contains all the functions for a station, and also provides various services, including the routing of the frames from and to its stations. APs are connected through the backbone, called distribution system (DS), to form an extended service set (ESS), which can provide a seamless WLAN service to a given area. One can understand an infrastructure BSS as a cell in a cellular network. An 802.11 station can hand off from an AP to another AP while it moves around within an ESS.
Independent BSS (IBSS) is the other type of BSS, which is used for the ad hoc mode. An IBSS is composed of a number of stations that can communicate directly each other. The 802.11 does not support wireless multi hob communications. In order to support wireless multihop communications, the stations should implement a layer-3 routing function, such as by employing a mobile ad hoc network (MANET) routing protocol.
The 802.11 standards do not define how to implement the DS. That is, how to connect multiple APs is not specified in the standard. There are different ways to construct a DS. In typical deployments of the 802.11 WLAN, APs are connected via Ethernet. However, the standard also allows them to be connected wirelessly. Another issue is whether an AP is a layer-2 or layer-3 device. By default, an AP is a layer-2 bridging (or switching) device, and all the APs are connected via layer-2 bridges.
In such a case, all the APs along with the associated stations are within the same subnet. However, an AP can be implemented as a layer-3 device (or router) such that the frame forwarding is made based on the layer-3 IP address. The 802.11 MAC can be actually divided into a time critical lower MAC, including the frame transmission/reception, and a less time-critical upper MAC related with the network management. In fact, an AP can be also implemented as a lower layer-2 device. That is, an AP might include only the lower MAC functions, and then less time-critical upper MAC functions are implemented in a so-called WLAN switch, which connects multiple APs with only lower MAC functions.
5. SIMILARITIES AND DIFFERENCES
Mobile WiMAX and WiFi are access technologies developed by IEEE 802.16 and 802.11 WGs, respectively, where both 802.16 and 802.11 WGs are under the umbrella of IEEE 802 LAN/MAN committee. Various access and connectivity protocols in the IEEE 802 family are developed for the packet-switched networking, and both IEEE 802.16 and 802.11 are also along the same line. This is quite different from other cellular technologies (e.g., those developed by 3GPP and 3GPP2, which evolved from voice communication-oriented circuit-switched networking). While both 802.16 and 802.11 define peer-to-peer, mesh, or ad hoc modes of operation, their primary network configuration is a star topology, where a user station communicates though its AP or BS to connect to the rest of the world.
It should be also mentioned that many people envision that these two technologies are quite complementary in that WiFi is better for lower-mobility networking while Mobile WiMAX is better for higher-mobility networking. Portable devices supporting both technologies are emerging today, and the protocols for inter working of heterogeneous access technologies like Mobile WiMAX and WiFi are being developed today (e.g., IEEE 802.21).
There are a number of differences between Mobile WiMAX and WiFi. First of all, Mobile WiMAX is developed for wireless metropolitan area network (WMAN), providing the transmission range of a few kilometers, while WiFi is for wireless local area network (WLAN) with the transmission range up to 100m. Mobile WiMAX is also mostly for commercial networks operated by service providers. However, WiFi is mainly for noncommercial networks deployed and maintained by an individual or a company.
Home and enterprise networking are good examples of WiFi. In their typical commercial deployment scenarios, Mobile WiMAX is meant for the seamless service coverage in a city or even a whole country, while WiFi is for spotty coverage provisioning at hot-spot areas, such as airports, coffee shops, and shopping malls, where many people gather. Mobile WiMAX was developed to support high mobility so that users can use this technology even inside a moving car or a train, but WiFi is mainly for nomadic users, who use this technology while mostly staying at a given place. WiFi can also support some low mobility (e.g., walking speed) but most of WiFi devices are not optimized for mobility support since people rarely use WiFi devices while moving around.
In terms of their technical operations, there are a number of differences as well. First of all, the MAC protocols are very different. The baseline MAC for WiFi relies on CSMA/CA, which is connectionless and contention-based. As WiFi operates at unlicensed bands, where various heterogeneous devices have to smoothly coexist, the adoption of CSMA/CA, which allows a device to transmit only when the channel is deemed to be free, seems a very natural and perfect choice. On the other hand, Mobile WiMAX employs a connection-oriented bandwidth request and allocation MAC. s Mobile WiMAX operates at licensed bands, for better QoS support, this type of centralized ACseems to be a good choice.
While QoS provisioning in wireless networks is always challenging due to time-varying nature of the network, it should be more feasible to provide proper QoS by using licensed bands. While Wi-Fi only supports TDD, Mobile WiMAX supports both TDD and FDD.4 As discussed in the previous sections, WiFi supports various kinds of PHYs such as 802.11 and 802.11a/b/g, whereas the Mobile WiMAX supports OFDMA PHY based on 802.16e.5 The OFDMA PHY allows multiple users to receive/transmit simultaneously by using different sub carriers. IEEE 802.11a and 802.11g are OFDM PHYs, but not OFDMA. That is, all the sub carriers are used for the transmission to a single receiver at a given time.
6. SUMMARY
WiFi and WiMAX both are the wireless transmission techniques. Both will have thee own advantages and disadvantages. Now a today’s we are using the WiFi technology and next feature technology is WiMAX which can implemented in 4G. we can implement both technologies combinable so that we can provide Quality of Service (QoS), Reliability, long distance coverage, we also overcome the signal loss problem because WiMAX provides high frequency range which are specked in the IEEE standards. The WiFi and WiMAX layers provides different functionality.
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[9] IEEE Std 802.16a-2003, Amendment 2 to Part 16: Air Interface for Fixed Broadband Wireless
[10] IEEE Std 802.16-2004, Part 16: Air Interface for Fixed Broadband Wireless Access Systems, revision of IEEE Std 802.16-2001, October 2004.
About the Author
K.Ravi
Asst. Professor
Dept. of Informatics
Alluri Institute of Management Sciences
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