One of the biggest trends in the electronics market is adding wireless
functionality and connectivity to products. Products as varied as
electric, gas or water meters, home security systems, TV remote
controls or exercise equipment have added wireless functionality.
This is partly due to convenience for the user such as a wireless
remote control that can operate anywhere in a house as opposed to a
line of sight (LOS) infrared remote.
Another driving factor is avoiding the expensive rewiring that would
be required to retrofit a home with a security system. Other trends
such as automated meter reading (AMR)
or advanced metering infrastructure
(AMI) rely on low power protocols where battery lifetime can be
measured in years.
In this three part series we will provide an overview of wireless
network protocols, but will focus on ZigBee,
802.15.4 and compare it with
one example proprietary protocol, SimpliciTI,
all designed for low power applications.
In Part 1, here, we will review network basics including common
networking terminology, the Open Systems Interconnect (OSI) network
model and considerations to keep in mind when deciding on a network for
a particular application.
In Part 2,
a set of network selection criteria will be presented
upon which the comparison of the three protocols will be framed.
Finally, in Part 3, the 802.15.4, ZigBee
and the proprietary SimpliciTI
protocols will be described in detail and some specific examples
will be described, using these selection criteria to determine
the optimum network for
particular applications.
The list of released wireless network protocols is a long one. The
following is just a partial list of network protocols that are
available:
* Standards-based protocols such as WiFi (802.11b), Bluetooth,
ZigBee, ZigBee Pro, 802.15.4, RFID and Wireless USB
* Proprietary protocols from silicon vendors, third parties, or what
this paper calls "do-it-yourself" networks, built and controlled
completely by the company that uses the network in their own products.
Figure 1 below shows a
table with various protocols, the applications these are focused on,
typical resource requirements and so forth. When comparing the low
power networks such as ZigBee/802.15.4 against other protocols, it is
important to focus on the key features of a low power wireless network
that distinguishes it from its peers: low data rates, reduced operating
range, a low frame overhead, power management considerations built
directly into the protocol, and low complexity.
Each design consideration serves as a basis for the end goal of
reducing the power consumption per individual node on the network. As
the figure shows, this is a perfect fit to the applications within
environmental monitoring and control markets in which the devices would
be optimally battery powered and be offered at price points
considerably lower than hardware capable of supporting larger, more
complex protocols.
Figure
1--Wireless protocols parameters and focus applications (courtesy of
ZigBee Alliance)
Low power networks were designed chiefly to provide wireless
connectivity between products with battery lifetime in months or years.
In most low power wireless systems, the element that uses the most
power is the radio transmitting and receiving data. It is, therefore,
extremely important to minimize the power used by the radio to maximize
battery life.
Since the distance a signal can be transmitted and received is a
function of the power input to the antenna, the wireless network range
between individual nodes is typically limited. Wireless protocols also
tend to be less complex to reduce computational overhead and the need
for larger memories, resulting in lower cost.
Network basics Figure 2 below shows a block
diagram of a typical low power network node. In general there is a
sensor (or sensors) collecting data or status and/or providing a user
interface to the system. A microcontroller interfaces the sensor and
controls the radio (the CC1100 or CC2500 in this particular case) while
the radio transmits and receives status.
Figure
2--a typical low power network block diagram, shown for an automatic
meter reading application
All networks on which these nodes communicate, wired or wireless,
can be conceptually described by what is referred to as the Network
Open Systems Interconnect (OSI) Basic Reference Model, depicted in Figure 3 below.
Developed in the late 1970's by the International Organization for
Standardization (the ISO), this model separates the components of a
network protocol implementation into software layers. For two
applications on separate devices to communicate, a message must travel
down from the application layer, across the physical layer, and up the
other side. Each layer communicates only with the layers above and
below it.
Figure
3--Elements of a network
One way to conceptualize a layered software architecture cold be by
describing the mailing of a letter by mail. The letter itself can be
considered application data. The individual drops it off to be picked
up by the mailman in the post-office box so that it may be transported
to the post office.
The post office separates all mail according to its destination
before shipping it, by sea, air and land to its final destination. For
the letter to arrive to the recipient, it must work its way back up the
other side, being dropped off at the post office, sorted by
destination, transported to the recipient's mailbox, to be interpreted
by the reader.
This is a four-tiered communication protocol where the individual
writing the letter represents a single layer, the transport to and from
the local post office represents the second layer, the post office
sorting the mail represents the third layer, and the shipping
methodologies represent the fourth.
Each layer concerns itself only with its own task, and communicates
only with its adjacent layers. Only by working its way down one side
and up the other will the contents of the letter (or application data )
be successfully communicated between individuals.
The OSI model services seven separate layers. An application layer
services the direct interface to the user. The presentation layer
formats the message to or from a network format, oftentimes implemented
as message encryption and/or encoding.
The session layer creates and manages the logical link between any
two devices on the network. The transport layer is responsible for
providing reliable end-to-end communication. If the transport layer
fails too often, perhaps the channel is too noisy or the link is bad,
and it should inform the session layer to create a new link between the
failing nodes.
The network layer is responsible for network routing mechanisms,
leaving the responsibility of message transmission between individual
devices to the data link layer. The data link layer assures
peer-to-peer delivery of a message, but leaves the actual transmission
of messages across the physical medium to be managed by the physical
layer. And so, a message is transmit down one side of the OSI model and
up the other.
A designer has the option to select protocols that have different
numbers of layers implemented, and can choose to customize the
remaining layers to his or her application as needed.
Most networking implementations today do not actually implement
these layers wholesale, and may mix the functionality of certain layers
according to the requirements of the protocols.
In fact, the OSI model is best to consider as a framework for
conceptualizing the complexity and functionality of a protocol's
architecture The designer should always be aware of what functionality
his or her solution implements and which functionality it is
consciously leaving out.
Giving structure to a wireless
network
To give structure to selecting a wireless protocol when deciding
between ZigBee, 802.15.4, or a proprietary network, the following
criteria are helpful to keep in mind.
1. Application
considerations 2. Robustness and reliability 3. Ease of use 4. Hardware and RF
considerations
Application considerations
The initial steps in network design, much like any other system design,
are to define the high-level requirements for the application. The
following list captures some of the most important network parameters
that should be defined before considering any wireless protocol as the
solution of choice.
The possible implementations addressing these application
considerations can then be traced throughout the paper, as the detailed
explanation of the selection criteria and the protocols themselves
require further elaboration.
1. Network topology a. How many
nodes and in what general organization within the network does the
application require? 2. Reliability of communications a. How
critical is the reception of each data packet within the network? 3. Network security a. Does the
data need to be secure? If so, how critical is the absolute security of
the network data transmissions? 4. Customization and design
freedom a. How
customized does the networking protocol solution need to be to fit the
application, and does the protocol offer the design freedom to do so? 5. Development time and
protocol complexity a. Along
the same lines of #4, what complexity exists within the protocol in
question? b. How much
time will be spent learning and integrating the protocol in question to
the existing application versus the time that would be spent developing
a more customized solution? 6. Interoperability a. Would the
end product benefit from interoperability with other vendors, or will
it be a completely proprietary solution?
Figure 4 below presents four
of the most common wireless network topologies available for
implementation.
Figure
4--low power network topologies
A peer-to-peer networktopology is one that supports either uni- or
bi-directional links between individual nodes on the network. Nodes
will only communicate if they are within range of each other, as they
must maintain direct physical links for communication; the only
exception being a broadcast message, which could be re-broadcast and
propagated through the network.
A tree network topology is one each node in the network associates
itself to a parent node, and the network addressing is reflected as
such, much like an IP internet address. This allows for more efficient
routing algorithms to be implemented, as the more significant digits of
a node's network address can identify the node's location in relation
to its peers.
A star network identifies a single node as the network coordinator,
responsible for a variety of possible network management controls, such
as node associations, network join and linking permissions, message
forwarding, and security exchanges. A star network depends upon the
coordinator to keep the network communicating and is susceptible to
disruption if the coordinator node goes down.
A mesh network, in its most general sense, is defined as a network
in which there are at least two pathways to each node and a fully
meshed network means that every node has a direct connection to every
other node in the network.
The latter case somewhat unreasonable in many cases, as this would
quickly limit the size of a network to the minimum range of the weakest
device, and the former is perhaps too strict a requirement.
Instead, one tends to see interpretations that exist somewhere in
between the two cases, where a central node is responsible for starting
the network, and a tree addressing technique is used to locate nodes
and manage the associations between them.
Range extenders, or routing nodes, exist to route messages
throughout the network and if one node or the coordinator goes down,
the network can still continue to function, sacrificing some degree of
operability. Additional features such as self-healing route discovery
and route expiration can make such routing algorithms increasingly
reliable and efficient.
Another important factor to consider may be the financial cost of
using a certain protocol. It is not uncommon that a membership or
royalty fee must be paid to the organization representing a proprietary
solution.
ZigBee, for example, does not have royalties but does require
membership in the ZigBee Alliance for a nominal, yearly fee. There can
also be a cost, both in time and money, resulting from a certification
process. Silicon vendor proprietary protocols typically require that
their devices be used in lieu of a royalty.
Robustness and reliability
The extent of the implemented robustness and reliability of a low power
wireless network protocol can be summarized into three categories:
message delivery, physical layer considerations, and the messaging
protocol.
Message delivery concerns routing methodologies that assure a
successful packet transmission and the security of network
transactions. Physical layer considerations address the interference of
noise or other transmissions within the channel of operation.
The messaging protocol then defines the division of the channel so
that all devices can use the physical medium without packets colliding
mid-transmission. All three contribute to the improved quality of
service (QOS) for a network, defined as a set of network metrics used
to gauge the efficiency, transmission rate and error rate for package
communication.
Channel scanning, or the ability to sense the amount of activity or
noise on a channel, is a physical layer consideration that network
protocol can use to find the channel within the specified frequency
band of operation that is least likely to impede communication between
nodes.
Frequency agility is the ability of a network to change the channel
of operation for all nodes on the network, so that if a channel is
bombarded with interference operation of the network may continue
without concern.
Improved message delivery can be achieved through acknowledgement
schemes, where the receiving node will transmit an ACK to the original
sender after the successful reception of the packet. Peer to peer
acknowledgements, partnered with a defined number of message retries
will highly decrease the possibility of a packet transaction being
lost.
End to end acknowledgements will provide a second layer of security
that a packet transaction will not be lost, and are especially
important in large, multi-hop networks supported by complex routing
algorithms.
The messaging protocol defines how the network bandwidth is
contended for or partitioned. Different wireless protocols will define
different partitioning of the bandwidth, and the possibilities include
division in frequency, space, time, or code.
A division in frequency parallels to a room of people talking at a
different pitch of voice; a division in space parallels to a room of
people talking in different directions; a division in time to a room
with people contending for the right to speak but backing off if
someone beats them to speak first; and a division in code to a room of
people speaking different languages in all different pitches of voice.
The protocols presented in this paper discuss only the division in
time, or Time Division Multiple Access protocol, for which there are
two possible implementations: synchronous and asynchronous
communication.
Synchronous communication is enabled by the coordinating node
broadcasting a periodic network beacon and partitioning the resulting
time in between the beacons into equal-size time slots.
A single network beacon and the time slots that occur before the
next beacon are referred to as the superframe. The time slots of the
superframe can be further partitioned into an active and inactive
period of communications, so that the coordinator can sleep in low
power modes during the inactive period. Time slots can be guaranteed or
contended for using a Channel Sense Multiple Access (CSMA), or
listen-before-speak algorithm.
A CSMA algorithm defines the protocol that will arbitrate the use of
the RF channel when multiple nodes are attempting to communicate at the
same time. The most common implementation is a CSMA/CA algorithm, where
CA stands for Collision Avoidance because a transmitting node will
avoid the transmission of its message if it senses the channel is
currently busy.
There are other implementations of the CSMA algorithm, such as
CSMA/CD (collision detection), and CSMA/CR (collision resolution), but
they are not commonly found in RF protocol implementations and are
outside the scope of this essay.
Security is also a key concern affecting the robustness of wireless
communication and may be the main function of the network. For example,
a home security network may include a garage door opener or lock and
unlock doors.
These systems need security to prevent eavesdropping, security
breaches or to maintain privacy. Security can be addressed by multiple
levels of security keying and encryption, message integrity and
authentication, and the use of a trust center, meaning that all
security is handled by a single node on the network (usually the
network coordinator) rather than a distributed scheme where individual
links exchange symmetric keys upon link creation and may allow an
attacking node entrance to the network without direct authentication
from the managing node.
