Page 167 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 10: Wireless MAN Technologies

Differentiating the IEEE 802.16a and 802.11 Standards - WiFi versus WiMAX Scalability Notes
At the PHY layer the standard supports flexible RF channel bandwidths and reuse of these
channels (frequency reuse) as a way to increase cell capacity as the network grows. The standard
also specifies support for automatic transmit power control and channel quality measurements
as additional PHY layer tools to support cell planning/deployment and efficient spectrum use.
Operators can re-allocate spectrum through sectorization and cell splitting as the number of
subscribers grows. Also, support for multiple channel bandwidths enables equipment makers
to provide a means to address the unique government spectrum use and allocation regulations
faced by operators in diverse international markets. The IEEE 802.16a standard specifies channel
sizes ranging form 1.75MHz up to 20MHz with many options in between.
WiFi based products on the other hand require at least 20MHz for each channel (22MHz in the
2.4GHz band for 802.11b), and have specified only the license exempt bands 2.4GHz ISM, 5GHz
ISM and 5GHz UNII for operation. In the MAC layer, the CSMA/CA foundation of 802.11,
basically a wireless Ethernet protocol, scales about as well as does Ethernet. That is to say -
poorly. Just as in an Ethernet LAN, more users results in a geometric reduction of throughput, so
does the CSMA/CA MAC for WLANs. In contrast the MAC layer in the 802.16 standard has been
designed to scale from one up to 100’s of users within one RF channel, a feat the 802.11 MAC was
never designed for and is incapable of supporting.
z z Coverage: The BWA standard is designed for optimal performance in all types of
propagation environments, including LOS, near LOS and NLOS environments, and
delivers reliable robust performance even in cases where extreme link pathologies have
been introduced. The robust OFDM waveform supports high spectral efficiency (bits per
second per Hertz) over ranges from 2 to 40 kilometers with up to 70 Mbps in a single RF
channel. Advanced topologies (mesh networks) and antenna techniques (beam-forming,
STC, antenna diversity) can be employed to improve coverage even further. These
advanced techniques can also be used to increase spectral efficiency, capacity, reuse, and
average and peak throughput per RF channel. In addition, not all OFDM is the same. The
OFDM designed for BWA has in it the ability to support longer range transmissions and
the multi-path or reflections encountered.
In contrast, WLANs and 802.11 systems have at their core either a basic CDMA approach
or use OFDM with a much different design, and have as a requirement low power
consumption limiting the range. OFDM in the WLAN was created with the vision of
the systems covering tens and maybe a few hundreds of meters versus 802.16 which is
designed for higher power and an OFDM approach that supports deployments in the tens
of kilometers.
z z QoS: The 802.16a MAC relies on a Grant/Request protocol for access to the medium and
it supports differentiated service levels (e.g., dedicated T1/E1 for business and best effort
for residential). The protocol employs TDM data streams on the DL (downlink) and TDMA
on the UL (uplink), with the hooks for a centralized scheduler to support delay-sensitive
services like voice and video. By assuring collision-free data access to the channel, the 16a
MAC improves total system throughput and bandwidth efficiency, in comparison with
contention-based access techniques like the CSMA-CA protocol used in WLANs. The 16a
MAC also assures bounded delay on the data (CSMA-CA by contrast, offers no guarantees
on delay).

Notes The TDM/TDMA access technique also ensures easier support for multicast and
broadcast services. With a CSMA/CA approach at its core, WLANs in their current
implementation will never be able to deliver the QoS of a BWA, 802.16 system.

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