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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™.

AN1066

FIGURE 7: DECISION TREE FOR

PACKET FORWARDING

Broadcast Messages

When a MiWi protocol network coordinator receives a

broadcast packet, it will rebroadcast the packet as long

as the Hops counter (the first byte of the header) is not

equal to zero.

Broadcast packets should always have their ACK

request bits (in both the MiWi protocol header and the

MAC header) set to ‘ ’. Coordinators that receive0

broadcast packets should then process the packet after

retransmitting it.

Know the

Know the node’s

Send it

Have a next hop

Send it

YES

node directly?

parent directly?

for them (if coord)

to Them Directly

to Their Parent

to the Next Hop

to My Parent

or parent?

AN1066

MiWi Protocol Reports

The MiWi protocol transports packets between devices

by using special packets called reports. The protocol

allows for the implementation of up to 256 Report

Types, and up to 256 separate Report IDs for each

Report Type. The Report ID is the specification function

of the packet.

Report Type 00h is reserved for MiWi Wireless

Networking Protocol Stack packets, which have packet

payload that is directed to the Stack. For example, a

MiWi protocol ACK has a Report Type of 00h (because

it is a Stack packet) and a Report ID of 30h. All other

Report Types are available for the user.

The Report Type and Report ID are defined in the packet

header, as previously described in the Section “Packet

Format”. The size and contents of the payload of a

report depends on the particular Report ID. In this

implementation of the MiWi protocol, the payload size

varies from 0 bytes (i.e., sending a packet with a specific

Report Type/ID is essentially the entire message) to 10

bytes, with multi-byte payloads being delivered Least

Significant Byte (LSB) first. A list of the implemented

reports in the current version of the protocol is provided

in Table 4, with detailed descriptions immediately

following.

TABLE 4: REPORTS IMPLEMENTED IN THE MiWi™ PROTOCOL

OPEN_CLUSTER_SOCKET_REQUEST

The destination address of the MiWi protocol header

should be the PAN coordinator (0000h). The source

address of the MiWi protocol header should be the

address of the device that is initiating the request. The

Requesting EUI Address field specifies the EUI of the

device initiating the request, LSB first.

OPEN_CLUSTER_SOCKET_RESPONSE

The destination address of the MiWi protocol header

should be the original requesting device. The source

address should be the PAN coordinator (as this packet

will only originate from the PAN coordinator). The

Resulting EUI Address field specifies the EUI of the

device that responded to the request (LSB first). Note

that this is a different address than the MiWi protocol

destination address.

The Resulting Short Address field is the short address

of the device that responded to the request. With the

combination of EUI and short address sent to both

requesting nodes, they should be able to communicate

on the network and find each other if either of them

happens to move in the network. Once the

OPEN_CLUSTER_SOCKET_RESPONSE is sent out,

the PAN coordinator will no longer maintain any of the

socket information.

Report Type Report ID Name

00h

10h OPEN_CLUSTER_SOCKET_REQUEST

11h OPEN_CLUSTER_SOCKET_RESPONSE

12h OPEN_P2P_SOCKET_REQUEST

13h OPEN_P2P_SOCKET_RESPONSE

20h EUI_ADDRESS_SEARCH_REQUEST

21h EUI_ADDRESS_SEARCH_RESPONSE

30h ACK_REPORT_TYPE

40h CHANNEL_HOPPING_REQUEST

41h RESYNCHRONIZATION_REQUEST

42h RESYNCHRONIZATION_RESPONSE

01h-FFh 00h-FFh Available for use

Report Type (1 byte) Report ID (1 byte) Requesting EUI Address (8 bytes)

00h 10h The EUI of the initiating device.

Report Type

(1 byte)

Report ID

(1 byte)

Resulting EUI Address

(8 bytes)

Resulting Short Address

(2 bytes)

00h 11h The EUI of the resulting device. The short address of the resulting device.

AN1066

OPEN_P2P_SOCKET_REQUEST

Both the source and destination information in the MiWi

protocol header should be FFFFh. The Hops field of the

MiWi protocol header should be 00h in order to prevent

rebroadcast of the packet. The MAC level source and

destination PANID should be FFFFh. The MAC

destination short address should be FFFFh. The MAC

level source address should be Long Address mode.

OPEN_P2P_SOCKET_RESPONSE

Both the source and destination information in the MiWi

protocol header should be FFFFh. The Hops field of the

MiWi protocol header should be 00h. The MAC level

source and destination PANID should be FFFFh. The

MAC destination and source addresses should be both

long addresses (EUIs).

EUI_ADDRESS_SEARCH_REQUEST

The destination short address and PANID of the MiWi

protocol header should be the broadcast address,

FFFFh. The source address and PANID of the MiWi

protocol header should be the address information that

is requesting the search. On reception of this packet, a

coordinator in the network will rebroadcast this packet if

the number of hops is more than 00h. The coordinator

will decrement the Hops counter before rebroadcasting

the packet. The coordinator will not change the value of

the MiWi protocol sequence number when broadcasting

the packet.

EUI_ADDRESS_SEARCH_RESPONSE

The EUI_ADDRESS_SEARCH_RESPONSE should

be unicast back to the address of the device that

originally sent the request (the device mentioned in the

MiWi protocol source fields of the request packet).

ACK_REPORT_TYPE

The MiWi protocol source address of the MiWi protocol

ACK packet should equal the MiWi protocol destination

address of the packet that requires Acknowledgement.

The MiWi protocol destination address of the MiWi

protocol ACK packet should equal the MiWi protocol

source address of the packet that requires

Acknowledgement.

Report Type (1 byte) Report ID (1 byte)

00h 12h

Report Type (1 byte) Report ID (1 byte)

00h 13h

Report Type

(1 byte)

Report ID

(1 byte)

Search EUI Address

(8 bytes)

00h 20h The EUI of the device that is being searched for.

Report Type

(1 byte)

Report ID

(1 byte)

Search EUI Address

(8 bytes)

Search Results PANID

(2 bytes)

Search Results Short

Address (2 bytes)

00h 21h The EUI of the device that is

being searched for.

The resulting device’s

PANID.

The resulting device’s

short address.

Report Type (1 byte) Report ID (1 byte)

00h 30h

AN1066

CHANNEL_HOPPING_REQUEST

CHANNEL_HOPPING_REQUEST is the command

that PAN Coordinator broadcast to every node on the

network to move to a different channel. This is the start

of frequency agility process.

RESYNCHRONIZATION_REQUEST

When a sleeping device wakes up but cannot

communicate with its parent continuously, it may be

due to the fact that its parent has hopped to a different

channel due to frequency agility capability.

RESYNCHRONIZATION_REQUEST is the request

sent from the sleeping device and request

recommunicate with its parent within all possible

channels.

RESYNCHRONIZATION_RESPONSE

When a sleeping device wakes up but cannot

communicate with its parent continuously, it may be

due to the fact that its parent has hopped to a different

channel due to frequency agility capability.

RESYNCHRONIZATION_RESPONSE is the response

to the RESYNCHRONIZATION_REQUEST command

from the parent to its child.

Report Type (1 byte) Report ID (1 byte) Current Channel (1 byte) Channel to hop (1 byte)

00h 40h Current operating channel Channel to hop to

Report Type (1 byte) Report ID (1 byte) Current Channel (1 byte)

00h 41h Current operating channel

Report Type (1 byte) Report ID (1 byte)

00h 42h

AN1066

STACK MESSAGES AND SERVICES

Providing address allocation and routing services are

just the beginning of a wireless network solution. In

addition to these basic features, the MiWi protocol

provides other optional services that assist developers

to more rapidly reach a solution. These services

include dynamically creating connections between two

devices without having to know any information about

the device ahead of time (i.e., sockets), and the ability

to search the network for a specific device’s long

address.

Discovering Nodes in the Network by EUI

When two nodes communicate over a MiWi protocol

network, they use their short addresses. If the network

topology ever changes, it is useful to be able to find that

device again. Because the short address of a device is

assigned to it by its parent, the short address of the

destination device may have changed. If this is the case,

then the new short address must be discovered before

communication can be re-established. Unlike the short

address, the EUI of a device never changes and is

globally unique. If one device knows another device’s

EUI, it will be able to distinguish that device from any

other device. Searching the network for a specific EUI

then becomes important in reestablishing

communication with nodes that have moved. The MiWi

protocol provides such a feature to search the network

for a specific EUI.

There are two Stack packets that are defined in order

to assist with searching for a specific EUI on the

network: EUI_ADDRESS_SEARCH_REQUEST and

EUI_ADDRESS_SEARCH_RESPONSE. The Search

Request is unicast to the first coordinator (Figure 8,

sequence 1) and then broadcast among the

coordinators into the network with the EUI of the device

that needs to be located (sequence 2). It is repeated by

all of the coordinators until the Hops counter dies

(sequence 3).

If one of the coordinators currently has this device as a

child, it returns a

EUI_ADDRESS_SEARCH_RESPONSE

packet with the device’s EUI and its short address. The

EUI_ADDRESS_SEARCH_RESPONSE

is sent unicast,

node by node, back to the MiWi protocol source

address of the packet that originally sent the packet

(sequence 4).

Opening a Socket to a Device

Another feature that may be important to some

networks is the ability to dynamically form

communication links on the network. This is useful in

networks where preconfiguration of nodes needs to be

minimal.

As an example, a user may wish to add a new light

switch to a lighting control system. How does that light

switch know which light to send its packets to? One

way would be to have some type of interface where the

device installer manually programs controller and

target addresses into the devices.

CLUSTER SOCKET

A more dynamic method would be to have a push button

on both the light and the light switch. You first press the

button on the light, and then the light switch, within a

specified time interval. This lets these two devices know

that they need to communicate with each other. This

allows for dynamic run-time changes in the behavior of

the network in a simple, user-friendly method.

The MiWi protocol defines this dynamically formed

communication link as a cluster socket. A cluster

socket exists between two nodes that are members of

the network and is formed based on the short address.

Cluster Socket is also known as “Indirect Connection”

in MiApp interface. For more information, refer to the

Microchip application Note AN1284 “Microchip

Wireless Application Programming Interface – MiApp”

(DS01284).

In the previous example, pushing the button on the light

switch sends an OPEN_CLUSTER_SOCKET_REQUEST

to the PAN coordinator with that device’s information

(Figure 9, sequence 1). This notifies the PAN

coordinator that the device is looking for someone to

communicate to. The PAN coordinator keeps that

request open for an amount of time that is specified by

the application. If a similar request comes from the light

(sequence 2), the PAN coordinator combines the short

address information from both devices into one

OPEN_CLUSTER_SOCKET_RESPONSE and sends

it back to the switch and the light (sequence 3). Finally,

the PAN coordinator removes the open socket request. If

the PAN coordinator doesn’t hear a second open socket

request within the specified amount of time, then it will

terminate the open socket request without sending a

response to the requesting node.

When the devices at F and G receive the

OPEN_CLUSTER_SOCKET_RESPONSE report, they

can examine the payload of that packet and determine if

that device is, in fact, the device that they wish to talk to.

This is an application layer decision; it is not provided by

the Stack.

AN1066

FIGURE 8: SEQUENCE FOR EUI ADDRESS SEARCH REQUEST AND RESPONSE

EUI_ADDRESS_SEARCH_REQUEST From G

BCF

BACF

PAN Coordinator

FFD End Device

RFD End Device

Coordinator

Legend

Broadcast Message

ACF

EUI_ADDRESS_SEARCH_REQUEST is Broadcast

EUI_ADDRESS_SEARCH_REQUEST is Rebroadcast From All Coordinators

EUI_ADDRESS_SEARCH_RESPONSE

Unicast

Returns by Hops From C to G

AN1066

USER CONSIDERATIONS

There are several different network situations and

circumstances that are not inherently covered by the

Stack. Each of these situations should be considered.

Some of the situations must be implemented. Others

can be implemented if the system requires it.

Which Network to Join?

The network discovery feature built into the MiWi

protocol searches the available channels for networks.

It does not, however, choose which network to join.

This is left as an application decision. Some

applications will want to pick one network over another,

or a certain coordinator over another coordinator, within

the same network. After the network discovery is

complete, each device will then need to search through

the list and determine to which coordinator it will join.

Failure Recovery

Failure recovery is an interesting issue in wireless

communications. Some developers require their

networks to dynamically heal when parts of the network

fail. Other developers need the network to stay as

unchanged as possible, even when failures do occur.

Because of this variation in requirements, the MiWi

protocol Stack doesn’t default to either implementation.

The MiWi protocol provides ACKs on both the MAC

and MiWi protocol layers. This allows users to know

that their packet reached the destination correctly or to

determine that the packet did not make it to the

destination successfully. These ACKs can be used to

determine when there is a network failure.

Determining when to communicate to a node is also not

a feature of this implementation. Determining a node is

no longer available could be a failed packet ratio, a

number of consecutive packets failed, a low RSSI, a low

data throughput, and so on. The current Stack does not

implement any of these requirements. If an application

has such requirements, then it should be added in at the

application level (or change the Stack functionality, if

required).

One possible mode of failure is if the parent of a device

fails. In some networks, it is preferred that the

orphaned device find a new parent, while in other

networks, the device is left off of the network until the

parent returns online. Some implementations may

prefer to rejoin the same network in a different location,

while others may prefer to search for a new network.

Another problem that is not covered in the Stack is how

to promote a node from a coordinator (or end device) to

a PAN coordinator if the PAN coordinator fails. Because

the Stack does not determine when a device is offline,

it also cannot initiate this process. Promoting a

coordinator to a PAN coordinator can create issues in

that the coordinator’s old children must all be dropped

off of the network. Remember that the PAN coordinator

always has the address, 0000h. This means that the

coordinator that is taking over the role must change its

address, and thus, all of it previous children’s

addresses much change as well.

Which coordinator should take control of the network is

also a decision that is better suited for the application

to decide. Many networks can exist just fine without the

PAN coordinator present. Both routing and end device

joining works without the PAN coordinator. However, no

new coordinators will be allowed to join the network

while the PAN coordinator is offline.

Exchanging EUIs to Protect Against

Node Migration

Nodes in a wireless network may change their parent

for various reasons, including failure and mobility. In

this situation, the device’s short address will change.

Nodes that were communicating with the node that

moved will no longer be able to communicate with the

node. It is because of this reason that devices that care

about maintaining connectivity, despite node mobility,

may find it useful to request a node’s EUI after

establishing communications with that device.

Accepting an Open Socket Response

The Stack provides a means of forming dynamic bonds

between two devices through the request to establish

indirect connection (also called socket in MiWi protocol

term). When the request returns a result, this merely

indicates that another device was looking for a

communication partner in the same time frame. This

does not imply that the devices are meant to

communicate to one another. Deciding if the returned

node is acceptable or not is an application level

decision and may require implementation if required.

Product specificaties

Merk:	Microchip
Categorie:	Niet gecategoriseerd
Model:	PIC18F67K90

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