The authors compares briefly the differences among physical layers and MAC of 802.15.4 vs. 802.15.4a. The goal of the 802.15.4a standard for low data rate networks is to provide joint communications and high accuracy positioning in future sensor networks. In the same time, IEEE 802.15.4b Task Group focused on refining 802.154 and completed its activities in 2006 by releasing an updated version of the original standard.
PHY Layer :
The channelization scheme :
– 1 channel (Channel 0) was defined in 868 Mhz band.
– 10 Channels (Channels 1 – 10) were defined in the 915 MHz band, with a channel spacing og 2 MHz.
– 16 channels (Channels 11-26) were defined in the 2.4 GHz band, with a channel spacing of 5 MHz.
The channel defined in the first two bands were intended for very low bit rate operations, with user rates of 20 kb/s and 40 kb/s perchannel for the 868 MHz and the 915 MHz bands respectively. The 2.4 GHz ISM band allowed bit rates up to 250 kb/s per channel. The improved version of the standard released in 2006 introduced new modulation schemes for the channels in the 868 MHz and 915 MHz bands allowing to achieve 250 kb/s per channel for these two band as well.
Access Strategies :
In the case of a nonbeacon-enabled network, when a device needs to send data it picks a random backoff delay, defined as a multiple of a backoff time unit. When the backoff delay expires, the device performs a Clear Channel Assessment (CCA) operation, consisting in listening to the channel in order to determine if its idle. If the channel is idle the defice immediately transmits the data packet.
In other hand, in the case of a backoff-enabled mode, if the channel is busy the defice repeats the procedure by picking a new backoff delay, larger than the previous one.
Overview if the IEEE 802.15.4/4a standards for low data rate Wireless Personal Data Networks.
ByLuaca De Bardis and Maria-Gabriella
Note : This resume is created for self-learning only. Author and Publisher hold copyrights
October 27, 2008
High Speed Network Lab