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3 TECHNOLOGY
The wireless VoIP phone is an interesting product in that it combines various
technologies. This includes IEEE802.11(Wavelan), OPENH323, and Voice over IP.
3.1 IEEE802.11:
The Wavelan IEEE Silver implements the IEEE802.11 standard of wireless networking.
This standard specifies a Physical Layer and a Media Access Control (MAC) layer.
3.1.1 Physical Layer
At the physical (PHY) layer, IEEE 802.11 defines three physical characteristics for
wireless local area networks: diffused infrared, direct sequence spread spectrum (DSSS),
and frequency hopping spread spectrum (FHSS). The latter two operate in 2.4GHz R-F
frequency band also known as ISM band. Wavelan uses the Digital Spread Spectrum
modulation technique. Spread Spectrum technique spreads the power of a signal over its
frequency bandwidth increasing its signal to noise ratio while sacrificing bandwidth.
Direct Spread Spectrum makes use of a chipping code in order to send a stream of bits for
every bit of data that is sent. This increases the tolerance of the signal to noise. This can
enable data rates as high as 11Mbps which is supported by the Lucent Wavelan card.
3.1.2 Media Access Control
The 802.11 MAC layer, supported by an underlying PHY layer, is concerned primarily
with rules for accessing the wireless medium. Two network architectures are defined: the
Infrastructure Network and the Ad Hoc Network. An Infrastructure Network is a network
architecture for providing communication between wireless clients and wired network
resources. The transition of data from the wireless to the wired medium is via an Access
Point. The coverage area is defined by an Access Point (AP) and its associated wireless
clients, and together all the devices form a Basic Service Set.
An Ad Hoc network is an architecture that is used to support mutual communication
among wireless clients. Typically created spontaneously, an ad hoc network does not
support access to wired networks, and does not need an AP to be part of the network.
The primary services provided by the MAC layer are as follows:
• Data transfer
Wireless clients use a Collision Sense Multiple Access with Collision Avoidance
(CSMA/CA) algorithm as the media access scheme.
• Association
This service enables the establishment of wireless links between wireless clients
and APs in Infrastructure Networks.
• Reassociation
This takes place in addition to association when a wireless client moves from one
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