During the last year, WiMAX has been marketed as the future broadband wireless standard. Many Wireless Internet Service Providers (WISPs) running solutions based on IEEE 802.11 are considering investing in WiMAX based solutions but they are not sure of what exactly WiMAX can offer them and to what price. Is WiMAX the latest “techno-hype” or does it open new opportunities for broadband wireless connectivity?
This section aims to serve as a reference for some of the technical differences between IEEE 802.11 and IEEE 802.16. It is assumed that the reader is already familiar with IEEE 802.11 based solutions and want to know what IEEE 802.16 can offer.
IEEE 802.16 has been designed specifically for point to multi-point outdoor environments which a single media access control (MAC) that can accommodate different physical layers (PHY) in the frequency range of 11-66 Ghz. IEEE approved the initial IEEE 802.16 standard for wireless MAN in the 11-66 GHz frequency range in December 2001. The 802.16a extension for sub-11 GHz was approved in January 2003. The 802.16-2004 standard was ratified by the IEEE in June 2004. The 802.16e standard is being reviewed by IEEE and is expected to be approved late 2005. Industry speculation suggests the standard will be officially named 802.16 2005. The purpose of 802.16e is to add data mobility to the current standard, which is designed mainly for fixed operation.
In simple words, although the radio modulation technique changes depending on the frequency of operation, the packet format, medium sharing or the error control techniques are independent of the frequency of operation. The “electronics” used in the IEEE 802.16 MAC (ISO layer 2 data link) are not dependent of the frequency of operation.
IEEE 802.16 does not only aim to satisfy the wireless ISP and industry requirements in almost all possible scenarios but also to become the de facto broadband outdoor wireless standard. Having that said, it does not necessary mean that other technologies should automatically be considered obsolete. In many cases the discussion is about spectrum efficiency, regulatory aspects, access to technology or investment costs. In this document we have deliberately decided to focus on the main technical differences, as we believe that is a good starting point for the majority of the readers.
So, what is the answer to the question “WiFi or WiMAX?”: It depends on. It depends on what you need today and what you expect to need tomorrow.
IEEE 802.11 is a wireless LAN (indoor) protocol that was designed to operate in small cells (up to 100 meters) and that in the design phase never was considered as a point-to-multipoint outdoor solution. IEEE 802.11 MAC suffers from the hidden-node problem and is known for bad performance in long distance links with many stations.
The access method in IEEE 802.11 (CSMA/CA) assumes that all nodes that are communicating with the access point can hear each other to avoid collisions. Collisions in IEEE 802.11 can be avoided if all nodes can effectively sense if the channel is occupied or not. Unfortunately, this requirement can not always be satisfied when implementing IEEE 802.11 based network in an outdoor environment. When more than ten <someone might say twenty> stations are associated to the same access point and the rate of collisions increases, the consequent backoffs and retransmissions introduce a significant waste of airtime resources. IEEE 802.11 performs bad when many users are associated to an access point in an outdoor environment. In order to solve some of this problems, proprietary solutions based on the principle of “polling the clients” or bandwidth reservations in the IP layer has been implemented. By introducing “polling” in IEEE 802.11, the access point decides in which moment a station is granted to talk to the access point. The hidden node problem is nothing new and as soon as IEEE 802.11 was standardized there were already modifications of the IEEE 802.11 MAC to solve the problem (e.g. Karlnet TurboCell, WORP etc.). Many other proprietary solutions became available but interoperability between vendors was not guaranteed. In the recent standard IEEE 802.11e the MAC was enhanced to include “polling” and make implementations interoperable. On the contrary, IEEE 802.16 was born to be a wireless metropolitan outdoor solution and was designed as an outdoor solution from the beginning. IEEE 802.16 is designed to operate in a typical cell size of 7 to 10 km and can handle distances up to 50 km. The hidden node problem was solved from the very early design phase by including DAMA-TDMA for the uplink where the base station allocates slots to each station. IEEE 802.16 DAMA-TDMA uses the same principle as a satellite network where the stations (clients) can not hear each other.
To be able to operate better in non light-of-sight environments (NLOS), IEEE 802.16 included a more complex modulation based on 256-points of Fast Fourier Transform )(FFT) of OFDM instead of the 64-points in IEEE 802.11a/g. By including 256 points instead of 64, IEEE 802.16 is equipped with a better non-line of sight capability. IEEE 802.16 can tolerate 10 times more multi-path delay spread than 802.11. IEEE 802.16 can make better use of the available channel resources in an outdoor environment as the base station schedules the subscribers using dynamic scheduling algorithms. The number of subscribers does not effect the number of collisions and retransmissions of packets.
As mentioned before IEEE 802.11 coverage is limited by the hidden node problem. IEEE 802.11 performs well in a indoor environment or in point-to-point solutions but is not optimal for an outdoor point-to-multipoint solution.
The possibility in IEEE 802.16 to dedicate a certain bandwidth to a subscriber by means of TDMA, without worrying about hidden nodes, allows the introduction of smart antennas. A smart antenna combines multiple antenna elements with a signal-processing capability and can optimize its beam pattern automatically. IEEE 802.16 will allow advanced antenna techniques and hence better cell planing.
IEEE 802.16 has also included support for mesh networking. In mesh networking each subscriber access point is also part of the routing infrastructure. IEEE 802.16 makes a smarter “adaptive” modulation than IEEE 802.11 and enables optimization of each subscriber’s data rate by allowing the base station to set modulation schemes on a link-by-link basis. A subscriber station close to the base station can use high data rate modulation as 64QAM, while the weaker signal from a more remote subscriber might only permit the use of 16QAM or QPSK. The adaptive modulation included in the IEEE 802.16 MAC also allows to have different modulation method for downlink and uplink bursts.
While IEEE 802.11 has a fixed channel bandwidth of 20 Mhz, IEEE 802.16 has the flexibility of allocating different bandwidth in each radio channel, from very narrow channels of 1.5 Mhz to a maximum of 20 Mhz. The possibility of setting different channel bandwidth enables frequency reuse and better cell planning. While the number of non-overlapping channels in IEEE 802.11b is 3 and 5 in IEEE 802.11a, the number of non-overlapping channels in IEEE 802.16 is limited by the total available spectrum.
When it comes to data rates, IEEE 802.11 can provide a peak data rate of 2.4 bps/Hz. In the 20 Mhz channel that implies a maximum of 54 Mbps. IEEE 802.16 allows a theoretical maximum of 70 Mbps in a 20 Mhz channel. The level of actual throughput will depend on Light-of-sight, distance, air quality, interference and other factors (real values of 50 Mbps are expected).
IEEE 802.11 includes quality of service in the new standard IEEE 802.11e (products of a profile of the 11e standard known as Wireless Multimedia of WMM are already in the market). Unfortunately IEEE 802.11e will only support a limited prioritization on a single connection between the IEEE 802.11 access point and the station. In WMM, QoS is achieved by including shorter Interframe Space (IFS) for multimedia traffic.
IEEE 802.16 has implemented QoS in a “per-flow” basis, where multiple connections between a subscriber station and a base station can have different QoS attributes. QoS in IEEE 802.16 is achieved by means of “polling”. The base station polls the subscribers stations for bandwidth requests and schedules the traffic according to their responses.
Four types of scheduling services are supported in IEEE 802.16 depending on the type of traffic.