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AN1066

INTRODUCTION

Implementing applications with wireless networking is

now common. From consumer devices to industrial

applications, there is a growing expectation that

devices will have built-in the ability to communicate

with each other without a hard-wired connection. The

challenge is to select the right wireless networking

protocol and implement it in a cost-effective manner.

The Microchip MiWi™ Wireless Networking Protocol

Stack is a simple protocol designed for low data rate,

short distance, low-cost networks. Fundamentally

based on IEEE 802.15.4™ for Wireless Personal Area

Networks (WPANs) later expanded to support

Microchip proprietary RF transceivers, the MiWi

protocol provides an easy-to-use alternative for

wireless communication. In particular, it targets smaller

applications that have relatively small network sizes,

with few hops between. MiWi protocol now is one of the

wireless communication protocols that are supported in

MiWi™ Development Environment (DE). It uses

MiMAC interface to communicate with Microchip RF

transceivers and uses MiApp interface to interact with

application layer.

For more information, please refer to the Microchip

application note AN1283 “Microchip Wireless Media

Access Controller - MiMAC” (DS01283) and AN1284

“Microchip Wireless Application Programming

Interface - MiApp” (DS01284).

This application note covers the definition of the MiWi

Wireless Networking Protocol Stack and how it works.

For completeness, this document also introduces

several aspects of wireless networking, as well as key

features of IEEE 802.15.4. However, it is assumed that

the user is already familiar with the C programming

language and IEEE 802.15.4. You are strongly advised

to read the IEEE 802.15.4 specification and MiMAC/

MiApp application notes in detail prior to using the

Microchip MiWi Wireless Networking Protocol Stack.

FEATURES

The current implementation of the MiWi protocol has

these features:

• Support all Microchip RF transceivers on different

frequency bands.

• Portable between various Microchip MCU

families.

• RTOS and application independent

• Out-of-box support for the MPLAB® C18, C30 and

C32 compilers

• Easy-to-use API

CONSIDERATIONS

A network using the MiWi protocol is capable of having

a maximum of 1024 nodes on a network. Each

coordinator is only capable of having 127 children, with

a maximum of 8 coordinators in a network. Packets can

travel a maximum of 4 hops in the network and 2 hops

maximum from the PAN coordinator.

If, after reading this application note, you determine

that you require a standardized wireless platform,

larger network sizes or common marketing logos,

please refer to the application notes AN1232

“Microchip ZigBee-2006 Residential Stack Protocol”

(DS01232) and AN1255 “Microchip ZigBee PRO

Feature Set Protocol Stack” (DS01255). Alternatively,

users may consider using the basic MiWi protocol and

modifying it to suit their own applications.

For more information on the most up-to-date list of

limitations of the Stack, refer to the Readme file located

with the Stack download at http://www.microchip.com/

miwi.

TERMINOLOGY

In describing the MiWi protocol, two specific terms are

used throughout that are borrowed from the IEEE

standard.

The first term is cluster, which refers to a grouping of

nodes that form a network. A MiWi protocol cluster can

be up to 3 nodes deep and is controlled by a cluster-

head. In the current implementation of the MiWi protocol,

the cluster-head is always the PAN coordinator. (For

more information, see Table 2.)

The second term is socket, also referred as “Indirect

Message” in MiApp interface. It refers to a virtual

connection between two devices. Rather than have an

exclusive hard-wired connection between devices,

many devices with many types of sockets share a

common communications medium and use some

common method to associate applications and

devices. When a new device or application is added to

the network, it requires configuration to communicate

Author: David Flowers and Yifeng Yang

Microchip Technology Inc.

Microchip MiWi™ Wireless Networking Protocol Stack

AN1066

to other devices or applications. By using sockets,

nodes in the network can find communication partners

dynamically without having to know any information

about them.

MiWi PROTOCOL OVERVIEW

The MiWi protocol is based on the MAC and PHY

layers of the IEEE 802.15.4 specification, and is

tailored for simple network development in the 2.4 GHz

and subGHz ISM frequency bands. The protocol

provides the features to find, form and join a network,

as well as discovering nodes on the network and route

to them. It does not cover any application-specific

issues, such as how to select which network to join to,

how to decided when a link is broken or how often

devices should communicate.

IEEE 802.15.4 MAC

The MiWi protocol uses IEEE Standard 802.15.4 as

reference to develop its MAC layer.

Similar to IEEE 802.15.4, MiWi protocol uses an

Acknowledged data transfer mechanism in the MAC.

This method uses a special ACK flag in the packet

header. When this flag is set, Acknowledgement to the

transmitter by its receiver is required; this ensures that a

frame is, in fact, delivered. If the frame is transmitted with

an ACK flag set and the Acknowledgement is not

received within a certain time-out period, the transmitter

will retry the transmission for a fixed number of times

before declaring an error.

It is important to note that the reception of an

Acknowledgement simply indicates that a frame was

properly received by the MAC layer. However, it does not

indicate that the frame was processed correctly. It is

possible that the MAC layer of the receiving node

received and Acknowledged a frame correctly, but due

to the lack of processing resources, a frame might be

discarded by upper layers. As a result, the upper layers

of the application may require additional

Acknowledgement response.

Device Types

IEEE 802.15.4 defines devices based on their overall

functionality. There are basically two device types as

shown in Table 1.

The MiWi protocol defines three types of MiWi protocol

devices, based on their functions in the network: PAN

Coordinator, Coordinator and End Device. The MiWi

Wireless Networking Protocol Stack functionality helps

to determine the type of IEEE functionality that the

device requires. The MiWi protocol device types and

their relationship to IEEE device types are shown in

Table 2.

TABLE 1: IEEE 802.15.4™ FUNCTIONAL DEVICE TYPES

TABLE 2: MiWi™ PROTOCOL DEVICE TYPES

Device Type Services Offered Typical Power Source Typical Receiver Idle

Configuration

Full Function Device (FFD) All or Most Mains On

Reduced Function Device

(RFD)

Limited Battery Off

Device Type IEEE Device Type Typical Function

PAN Coordinator FFD One per network. Forms the network, allocates network

addresses, holds binding table.

Coordinator FFD Optional. Extends the physical range of the network. Allows

more nodes to join the network. May also perform monitoring

and/or control functions.

End Device FFD or RFD Performs monitoring and/or control functions.

AN1066

MiWi PROTOCOL NETWORK

CONFIGURATIONS

Of the three device types defined in the MiWi protocol,

the most essential type to networking is the PAN

coordinator. The PAN coordinator is the device that

starts the network, and selects the channel and the

PAN ID of the network. All other devices joining onto

the PAN have to obey the instructions of the PAN

coordinator.

Star Network Configuration

A star network configuration (Figure 1) consists of one

PAN coordinator node and one or more end devices. In

a star network, all end devices communicate only with

the PAN coordinator. If an end device needs to transfer

data to another end device, it sends its data to the PAN

coordinator, which in turn, forwards the data to the

intended recipient.

Cluster Tree Network Configuration

In a cluster tree network (Figure 2) there is still only one

PAN coordinator; however, other coordinators are

allowed to join on to the network. This forms a tree-like

structure, where the PAN coordinator is the root of the

tree, the coordinators are the branches of the tree and

the end devices are the leaves of the tree. In a cluster

tree network, all of the messages sent through the

network follow the path of the tree structure. Since

messages may be routed through more than one node

to reach their eventual destination, cluster tree networks

are sometimes also referred to as multi-hop networks.

FIGURE 1: STAR NETWORK CONFIGURATION

FIGURE 2: CLUSTER TREE TOPOLOGY

PAN Coordinator

FFD End Device

RFD End Device

Legend

PAN Coordinator

FFD End Device

RFD End Device

Coordinator

Legend

AN1066

Mesh Network Configuration

A mesh network (Figure 3) is similar to a cluster tree

configuration, except that Full Function Device (FFDs)

can route messages directly to other FFDs instead of

following the tree structure. Messages to Reduced

Function Device (RFDs) must still go through the

RFD’s parent node. The advantages of this topology

are that message latency can be reduced and reliability

is increased. Like cluster tree networks, mesh networks

are multi-hop.

Multi-Access Networks

An IEEE 802.15.4 network is a multi-access network,

meaning that all nodes in a network have equal access

to the medium of communication. There are two types

of multi-access mechanisms: beacon and non-beacon.

In a beacon enabled network, nodes are allowed to

transmit in predefined time slots only. The PAN

coordinator periodically begins with a superframe,

identified as a beacon frame, and all nodes in the

network are expected to synchronize to this frame. Each

node is assigned a specific slot in the superframe, during

which, it is allowed to transmit and receive its data. A

superframe may also contain a common slot during

which all nodes compete to access the channel.

In a non-beacon enabled network, all nodes in a

network are allowed to transmit at any time as long as

the channel is Idle. The current version of the Microchip

MiWi Wireless Networking Protocol Stack supports

only non-beacon networks.

FIGURE 3: MESH NETWORK

PAN Coordinator

FFD End Device

RFD End Device

Coordinator

Legend

AN1066

ADDRESS ASSIGNMENT

The MiWi protocol uses the addresses provided by

IEEE 802.15.4. There are three different addresses

defined by the specification:

1. Extended Organizationally Unique Identifier

(EUI): This is an 8-byte number that is globally

unique. Every device shipped using the IEEE

802.15.4 specification, should have a unique EUI

address. The upper 3 bytes of the EUI are

purchased from IEEE (see link for the site in the

Section “References” to buy them). The lower

5 bytes of the EUI are available for the user, as

they see fit, as long as they are globally unique.

For SubGHz proprietary RF transceivers, the EUI

address length is in the range of two to eight

bytes, defined by the application.

2. PAN Identifier (PANID): The PANID is a 16-bit

address that defines a group of nodes. All

nodes in the PAN share a common PANID. A

device assumes the PANID for a network when

it selects to join that PAN.

3. Short Address: Also known as the device

address, this is a 16-bit (2-byte) address that is

assigned to a device by its parent. This short

address is unique within a PAN and is used for

addressing and messaging within the network.

IEEE specifies that the PAN coordinator always

has an address of 0000h. The address

allocation is up to the PAN coordinator from that

point forward.

The MiWi protocol uses the 16 available bits in the

short address to help with routing and exchanging node

information. The bit fields within the address are shown

in Figure 4.

The Parent’s Number field (bits 10-8) is unique for each

coordinator on the network, including the PAN

coordinator. As the Parent’s Number field is only 3-bits

long, this limits the number of coordinators in a network

to 8.

The Child’s Number field (bits 6-0) of any coordinator

on the network will be 00h. This indicates that they are

operating as a coordinator. Other values for this field

are determined by the type of device (FFD or RFD), as

well its function within the PAN. Figure 5 gives a

general idea of how short addresses are determined.

The RxOffWhenIdle field (bit 7) is the inverse of the

IEEE 802.15.4 defined property of RxOnWhenIdle.

When this bit is set, it indicates that this device will turn

off its transceiver when it is Idle and will be unable to

receive packets. Any device, other than this device’s

parent, should route any packets that have this bit set

to the device’s parent. The target device’s parent will

buffer the message for the child until it wakes up and

requests the data. If this bit is not set in the device’s

address, then this device is always capable of receiving

packets.

Bits 15 through 11 are always ‘0’ in this implementation.

FIGURE 4: BIT FIELD ARRANGEMENT FOR THE MiWi™ PROTOCOL SHORT ADDRESS

Reserved Parent’s Number Child’s Number

00000xxxxxxxxxxx

RxOffWhenIdle

bit 15 bit 0

AN1066

FIGURE 5: ASSIGNING SHORT ADDRESSES WITHIN A TYPICAL MiWi™ PROTOCOL NETWORK

MiWi PROTOCOL MESSAGING

Once a network has been formed, the next major

concern is how to send messages through the network.

Any device that is a member of a MiWi protocol network

will use its short address to communicate through the

network. This short address helps other devices in the

network to determine the location of the node and how

to route to that device.

Packet Format

For a IEEE 802.15.4 compliant RF transceiver, MiWi

protocol uses 16-bit short address to transmit and

receive messages whenever possible. The packets

should be constructed according to Section 7.2 of the

IEEE 802.15.4 specification. Proprietary SubGHz RF

transceiver, however, use EUI address in MAC layer.

For more information on the packet format in MAC

layer for Microchip proprietary RF transceiver, refer to

the Microchip application note AN1283 “Microchip

Wireless Media Access Controller - MiMAC”

(DS01283).

Above this layer resides the MiWi protocol header that

contains information needed for routing and packet

processing. This header format is shown in Figure 6.

It is comprised of the following components:

•Hops: The number of hops that the packet is

allowed to be retransmitted (00h means don’t

retransmit this packet – 1 byte).

•Frame Control: The Frame Control field is a

bitmap that defines the behavior of this packet.

The individual bits are defined in Table 3 (1 byte).

•Dest PANID: The PANID of the final destination

node (2 bytes in the MiWi protocol).

•Dest Short Address: The final destination’s short

address (2 bytes).

•Source PANID: The PANID of the node that

originally sent the packet (2 bytes).

•Source Short Address: The short address of the

node that originally sent the packet (2 bytes).

•Sequence Number: A sequence number that can

be used to track the status of packets as they

travel through the network (1 byte).

FIGURE 6: MiWi™ PROTOCOL PACKET HEADER FORMAT

BCF

PAN Coordinator

FFD End Device

RFD End Device

Coordinator

Legend

0000h

0100h 0200h 0282h

0300h 0201h

0381h xxxxh Short Address

Hops

Frame Control

Dest PANID

Dest

Short Address

Source PANID

Source Short Address

Sequence Number

Report Type

Report ID

1 1 1 12 2 2 2 1

Legend: Numbers indicate packet component size in bytes.

AN1066

Routing

Routing in wireless networks can be a very difficult and

resource intensive task. The MiWi protocol solves this

problem by using the address allocation to indicate the

parent of the device you want to send the packet to,

and by using the already provided IEEE services to

help exchange and relay routing information in the

network.

LEARNING ABOUT NEIGHBORING

COORDINATORS

One of the tasks of a routing algorithm is determining

the next hop for any outgoing packet. The MiWi

protocol uses the IEEE network join mechanism, in

addition to regular network traffic, to discover these

paths. When any device is joining onto the network, it

first sends out a beacon request packet. All of the

coordinators that hear the beacon request packet

sends out a beacon packet informing neighboring

devices of their network information.

In the MiWi protocol, three bytes of additional

information are attached to the beacon payload to

assist with routing:

•Protocol ID (1 byte): This helps distinguish MiWi

protocol networks from other IEEE 802.15.4 net-

works that may be operating in the same radio

range. Protocol ID should always be 4Dh.

•Version Number (1 byte): The version number of

the specification.

•Local Coordinators (1 byte): This field is a

bitmap that indicates which coordinators are

currently visible by the coordinator that is sending

the beacon. Each bit position directly represents

one of 8 possible coordinators. Bit 0 is 0000h (the

PAN coordinator). Bit 1 indicates that this

coordinator can talk directly to 0100h, and so on.

For example: Coordinator 0x200 is capable of talking to

0x500 and the PAN coordinator. The Local

Coordinators field would be ‘b00100101.

Through the Local Coordinators field of the beacon

payload, all of the coordinators on the network will learn

about various possible routes to all of those nodes

without having to send out unique requests.

ROUTING TO OTHER DEVICES

Routing in MiWi protocol networks becomes easy once

we have knowledge of the neighboring coordinators, as

well as what those coordinators can see. Sending a

packet to another node follows the logic, as shown in

Figure 7.

TABLE 3: FRAME CONTROL BIT FIELD

0 0 0 0 0 x 1 0

r r r r r ACKREQ INTRCLST ENCRYPT

bit 7 bit 0

bit 7-3 Reserved: Maintain as ‘0’ in this implementation

bit 2 ACKREQ: Acknowledge Request bit

When set, the source device requests an upper layer Acknowledgement of receipt from the destination device.

bit 1 INTRCLST: Intra Cluster bit

Reserved in this implementation, maintain as ‘1’.

bit 0 ENCRYPT: Encrypt bit

When set, data packet is encrypted at the application level.

Note: Abbreviated bit names are for convenience of display only; they are not an official part of IEEE 802.15.4™.

Product specificaties

Merk:	Microchip
Categorie:	Niet gecategoriseerd
Model:	PIC16F88

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