Page 97 - DCAP311_DCAP607_WIRELESS_NETWORKS

Unit 6: Wireless PAN Systems

Notes
Figure 6.1: Breakdown of Packet Delay Components

Source: http://www.cse.wustl.edu/~jain/cse574-06/ftp/time_sync/
As shown in Figure 6.1, all the wireless synchronization schemes have four basic packet delay
components: send time, access time, propagation time, and receive time. The send time is that
of the sender constructing the time message to transmit on the network. The access time is that
of the MAC layer delay in accessing the network. This could be waiting to transmit in a TDMA
protocol. The time for the bits to by physically transmitted on the medium is considered the
propagation time. Finally, the receive time is the receiving node processing the message and
transferring it to the host. The major problem of time synchronization is not only that this packet
delay exists, but also being able to predict the time spent on each can be difficult. Eliminating any
of these will greatly increase the performance of the synchronization technique.
As illustrated there are many different variations of time synchronization or wireless networks.
They range from very complex and difficult to implement to simpler and easy to implement. No
matter the scheme used, all synchronization methods have the four basic components: send time,
access time, propagation time, and receive time.
There are many synchronization protocols, many of which do not differ much from each other. As
with any protocol, the basic idea is always there, but improving on the disadvantages is a constant
evolution. Three protocols will be discussed at length: Reference Broadcast Synchronization
(RBS), Timing-sync Protocol for Sensor Networks (TPSN), and Flooding Time Synchronization
Protocol (FTSP). These three protocols are the major timing protocols currently in use for wireless
networks. There are other synchronization protocols, but these three represent a good illustration
of the different types of protocols. These three cover sender to receiver synchronization as well as
receiver to receiver. Also, they cover single hop and multi hop synchronization schemes.

6.1.2 Streaming Multimedia

A wireless sensor network with multimedia capabilities typically consists of data sensor nodes,
which sense, for instance, sound or motion, and video sensor nodes, which capture video of
events of interest. In this survey, we focus on the video encoding at the video sensors and the
real-time transport of the encoded video to a base station. Real-time video streams have stringent
requirements for end-to-end delay and loss during network transport. In this survey, we
categorize the requirements of multimedia traffic at each layer of the network protocol stack and
further classify the mechanisms that have been proposed for multimedia streaming in wireless
sensor networks at each layer of the stack. Specifically, we consider the mechanisms operating
at the application, transport, network, and MAC layers. We also review existing cross-layer
approaches and propose a few possible cross-layer solutions to optimize the performance of a
given wireless sensor network for multimedia streaming applications.

6.1.3 Control

A Wireless PAN network does not use wires. They are more reliable as they have less number
of cables.

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