Wireless Networking with Android

Uichin LeeKAIST KSE

Review

• Wireless link characteristics• 802.11 WLAN (a, b, g, n)

– channels, associations, scanning – multiple access: CSMA/CA (RTS-CTS-DATA-ACK)

• PAN: Bluetooth– master vs. slave– peer discovery (inquiry) + connection setup

(paging)– piconet (up to 7 slaves)

SNR

• RSS: received signal strength– Measure of the power present in a received radio signal (at the

receiver antenna)– Represented in decibels above a reference level of 1 mW (dBm) – WiFi:

• 100mW (20dBm): EIRP for IEEE 802.11b/g Wireless LAN 20Mhz-wide channels in the 2.4GHz ISM band (5mW per MHz).

• Typical range (−60 to −80 dBm) of wireless received signal power– -60dBm => 0.0000001 mW

• SNR: signal to noise ratio– SNR = Eb/No where Eb the energy per bit, and No noise power

spectral density• SNR = (Signal Power/Bit Rate) / (Noise Power/Filter Bandwidth)

– CNR = Signal Power / Noise Power – BER is a function of SNR; e.g., BER(BPSK) = ½erfc(sqrt(SNR))

.

Switches

• Filtering: whether a frame should be forwarded to some interface or should be dropped

• Forwarding: determining the interfaces to which a frame should be directed; and then moves the frame to those interfaces

• Switch filtering and forwarding are done with a switch table– ADDR X: Interface Y: address X comes from interface Y

• Elimination of collisions: – No wastage of bandwidth thanks to buffers– Switch buffers frames and never transmit more than one frame

on a segment at any one time– Switch provides a significant performance improvement over

LANs with broadcast links

Internet

SwitchSwitch

Managing network connectivity

• Android broadcasts Intents that describe the changes in network connectivity – 3G, WiFi, etc.

• There are APIs for controlling network settings and connections

• Android networking is handled by ConnectivityManager (a network connectivity service)– Monitor the state of network connections– Configure failover settings– Control network radios

Managing network connectivity

• Accessing connectivity manager

• App needs read/write access permission on network states

String service = Context.CONNECTIVITY_SERVICE;ConnectivityManager connectivity =(ConnectivityManager) getSystemService(service);

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/><uses-permission android:name="android.permission.CHANGE_NETWORK_STATE"/>

Accessing network status

// Get the active network information.NetworkInfo activeNetwork = connectivity.getActiveNetworkInfo();int networkType = activeNetwork.getType();

switch (networkType) {case (ConnectivityManager.TYPE_MOBILE) : break;case (ConnectivityManager.TYPE_WIFI) : break;default: break;

}

// Get the mobile network information.int network = ConnectivityManager.TYPE_MOBILE;NetworkInfo mobileNetwork = connectivity.getNetworkInfo(network);

NetworkInfo.State state = mobileNetwork.getState();NetworkInfo.DetailedState detailedState = mobileNetwork.getDetailedState();

Preferred networks

• Preferred network configuration:– getNetworkPreference()– setNetworkPreference()

• If the preferred connection is unavailable, or connectivity on this network is lost, Android will automatically attempt to connect to the secondary network.

• Control availability of the network types using the setRadio() method. connectivity.setRadio(NetworkType.WIFI, false);connectivity.setRadio(NetworkType.MOBILE, true);

int networkPreference = connectivity.getNetworkPreference();connectivity.setNetworkPreference(NetworkPreference.PREFER_WIFI);

WiFi

Based on “Programming Android 2”

Managing your WiFi

• WifiManager: represents the Android WiFi connectivity service– Configure WiFi network connections– Manage current WiFi connection– Scan for access points– Monitor changes in WiFi connectivities

Monitoring WiFi connectivity

• Accessing the WiFi Manager

• Monitoring and changing Wifi state– State: enabling, enabled, disabling, disabled, and

unknown

String service = Context.WIFI_SERVICE;WifiManager wifi = (WifiManager) getSystemService(service);

<uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/><uses-permission android:name="android.permission.CHANGE_WIFI_STATE"/>

if (!wifi.isWifiEnabled())if (wifi.getWifiState() != WifiManager.WIFI_STATE_ENABLING)

wifi.setWifiEnabled(true);

Monitoring WiFi connectivity

• WifiManager broadcasts Intents whenever connectivity status changes– WIFI_STATE_CHANGED_ACTION

• Wi-Fi h/w status has changed: enabling, enabled, disabling, disabled, and unknown• EXTRA_WIFI_STATE, EXTRA_PREVIOUS_STATE

– SUPPLICANT_CONNECTION_CHANGED_ACTION:• Whenever connection state with the active supplicant (access point) changes• Fired when a new conn is established, or existing conn is lost (EXTRA_NEW_STATE =

true/false)

– NEWTWORK_STATE_CHANGED_ACTION:• Fired whenever wifi connectivity state changes• EXTRA_NETWORK_INFO: NetworkInfo obj for current network status• EXTRA_BSSID: BSSID of the access point that you’re connected to

– RSSI_CHANGED_ACTION: • Monitor the signal strength of the connected WiFi network• EXTRA_NEW_RSSI: current signal strength

Monitoring active connection details

• Once you connected to an access point, use getConnectionInfo of WifiManager to find info of that connection– Returns “WifiInfo” object

WifiInfo info = wifi.getConnectionInfo();

if (info.getBSSID() != null) {int strength = WifiManager.calculateSignalLevel(info.getRssi(), 5);int speed = info.getLinkSpeed();String units = WifiInfo.LINK_SPEED_UNITS;String ssid = info.getSSID();String cSummary = String.format("Connected to %s at %s%s. Strength %s/5",ssid, speed, units, strength);

}

Scanning hotspots

• Use WifiManager to scan access points using startScan() • Android will broadcast scan results with an Intent of

“SCAN_RESULTS_AVAILABLE_ACTION” // Register a broadcast receiver that listens for scan results.registerReceiver(new BroadcastReceiver() {

@Overridepublic void onReceive(Context context, Intent intent) {

List<ScanResult> results = wifi.getScanResults();ScanResult bestSignal = null;for (ScanResult result : results) {

if (bestSignal == null ||WifiManager.compareSignalLevel(bestSignal.level,result.level)<0)

bestSignal = result;}String toastText = String.format("%s networks found. %s isthe strongest.",results.size(), bestSignal.SSID);Toast.makeText(getApplicationContext(), toastText, Toast.LENGTH_LONG);

}}, new IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));

// Initiate a scan.wifi.startScan();

Creating a WiFi configuration

• To connect to a WiFi network, a WiFi configuration must be created and registered– Normally a user does this, but app can do this

• Network configuration is stored as WifiConfiguration object– SSID (service set ID, e.g., IPv4_KAIST)– BSSID (MAC addr of an AP)– networkId (unique ID that the supplicant uses to

identify this network configuration entry)– priority (priority of this access point)– status (current status: ENABLED, DISABLED, CURRENT)

Power Management

• Android supports its own Power Management (on top of the standard Linux Power Management) – To make sure that CPU shouldn't consume power if

no applications or services require power

• Android requires that applications and services request CPU resources with "wake locks" through the Android application framework and native Linux libraries.

• If there are no active wake locks, Android will shut down the CPU.

Power Management

http://www.kandroid.org/android_pdk/power_management.html

WakeLock

Flag value CPU Screen Keyboard

PARTIAL_WAKE_LOCK On Off Off

SCREEN_DIM_WAKE_LOCK On Dim Off

SCREEN_BRIGHT_WAKE_LOCK On BRIGHT Off

FULL_WAKE_LOCK On Bright Bright

// Acquire handle to the PowerManager service PowerManager pm = (PowerManager)mContext.getSystemService(

Context.POWER_SERVICE); // Create a wake lock and specify the power management flags for screen, timeout, etc.PowerManager.WakeLock wl = pm.newWakeLock( PowerManager.SCREEN_DIM_WAKE_LOCK | PowerManager.ON_AFTER_RELEASE, TAG); // Acquire wake lockwl.acquire(); // ... // Release wake lockwl.release();

Wifi background data transfer

• Background data transfer:– Wifilock + Wakelock (partial)

// http://developer.android.com/reference/android/net/wifi/WifiManager.WifiLock.html WifiManager.WifiLock wifiLock = null;PowerManager.WakeLock wakeLock = null;// acquireif (wifiLock == null) {

WifiManager wifiManager = (WifiManager) context.getSystemService(context.WIFI_SERVICE);

wifiLock = wifiManager.createWifiLock("wifilock");wifiLock.setReferenceCounted(true);wifiLock.acquire();

PowerManager powerManager = (PowerManager) context.getSystemService(context.POWER_SERVICE);wakeLock = powerManager.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, "wakelock");wakeLock.acquire();

} // releaseif (wifiLock != null) {

wifiLock.release();wifiLock = null;wakeLock.release();wakeLock = null;

}

Background data transfer

• Setting > Accounts & sync settings > background data setting

• If this setting is off, an application cannot transfer data only when it is active and in the foreground – Services cannot transfer data (by definition)

• Use connectivity manager to check this:– boolean backgroundEnabled = connectivity.getBackgroundDataSetting();

• App can listen to changes in the background data transfer preference: registerReceiver(

new BroadcastReceiver() {@Overridepublic void onReceive(Context context, Intent intent)// do something..

}, new IntentFilter(ConnectivityManager.

ACTION_BACKGROUND_DATA_SERVICE_CHANGED));

Bluetooth

Based on “Programming Android 2”Android Developer Guide: http://developer.android.com/guide/topics/wireless/bluetooth.html

Main classes

• BluetoothAdapter: – local Bluetooth device

• BluetoothDevice: – representing each remote device with which we

want to communicate with

• BluetoothSocket: – call “createRfcommSocketToServiceRecord()” on a

remote Bluetooth Device object to create a Bluetooth socket

• BluetoothServerSocket: – creating a Bluetooth server socket by calling

“listenUsingRfcommWithServiceRecord()” method

Overview

Master(initiator)

Slave

Peer discovery: get a list of neighboring peers

Connect to a slave:socket.connect(“one of slaves”);(act as a client)

Setting BT as “discoverable” => respond to peer inquiry

Accepting incoming connections: Socket.accept()(act as a server)

Bluetooth permissions

• To use Bluetooth features, at least one of two Bluetooth permissions must be declared:– BLUETOOTH

• Requesting/accepting a connection, and data transfer

– BLUETOOTH_ADMIN• More powerful than BLUETOOTH; includes device

discovery, manipulating Bluetooth settings (on/off), etc. <manifest ... >

  <uses-permission android:name="android.permission.BLUETOOTH" />  ...</manifest>

Setting up Bluetooth

• Bluetooth setup:1. Get the BluetoothAdapter

2. Enable Bluetooth

BluetoothAdapter mBluetoothAdapter = BluetoothAdapter.getDefaultAdapter();if (mBluetoothAdapter == null) {    // Device does not support Bluetooth}

if (!mBluetoothAdapter.isEnabled()) {    Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);    startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT);}

@Overrideprotected void onActivityResult(int requestCode, int resultCode, Intent data) {

if (requestCode == REQUEST_ENABLE_BT)if (resultCode == RESULT_OK ) // if user pressed “YES” if (resultCode == RESULT_CANCELED) { // if user pressed “NO”

}

Setting up Bluetooth

• App can also listen for “ACTION_STATE_CHANGED” broadcast intent– Android will broadcast whenever the Bluetooth state has

changed

• Broadcast contains the extra fields:– EXTRA_STATE: current state– EXTRA_PREVIOUS_STATE: previous state– State values:

• STATE_TURNING__ON• STATE_ON• STATE_TURNING_OFF• STATE_OFF

• Enabling “discoverability” will automatically enable Bluetooth

Discoverability (slave)

• Enabling “discoverability” – New intent with “ACTION_REQUEST_DISCOVERABLE”

– This will show a user permission dialog:– Activity will receive the response through “onActivityResult()”

• Activity.RESULT_CANCELLED (if a user canceled)

– The device will be “discoverable” for the specified duration (default: 2 minutes)

• App can be notified of mode changes by registering a BroadcastReceiver for the “ACTION_SCAN_MODE_CHANGED” Intent

– EXTRA_SCAN_MODE– EXTRA_SCAN_MODE_PREVIOUS– Values: SCAN_MODE_CONNECTABLE_DISCOVERABLE (inquiry scan + page scan on),

SCAN_MODE_CONNECTABLE (page scan on), SCAN_MODE_NONE (none)

• App does not need to enable device “discoverablity” if you will be initiating a connection to a remote node

– Only necessary when other parties discover an app that hosts a server socket for accepting incoming connections

Intent discoverableIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_DISCOVERABLE);discoverableIntent.putExtra(BluetoothAdapter.EXTRA_DISCOVERABLE_DURATION, 300);startActivityForResult(discoverableIntent, DISCOVERY_REQUEST);

Discovering devices (master)

• Discovering devices– Call “startDiscovery()” – async method (about 12 second inquiry)

BluetoothAdapter bt = BluetoothAdapter.getDefaultAdapter();bt.startDiscovery();

– App registers a BroadcastReceiver for the ACTION_FOUND Intent to receive information about each device discovered

• It carries the extra fields: EXTRA_DEVICE (BluetoothDevice) and EXTRA_CLASS (BluetoothClass)

– Call “cancelDiscovery()” to cancel discovery (check first with isDiscovering())// Create a BroadcastReceiver for ACTION_FOUNDprivate final BroadcastReceiver mReceiver = new BroadcastReceiver() {    public void onReceive(Context context, Intent intent) {        String action = intent.getAction();        // When discovery finds a device        if (BluetoothDevice.ACTION_FOUND.equals(action)) {            // Get the BluetoothDevice object from the Intent            BluetoothDevice device = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);            // Add the name and address to an array adapter to show in a ListView            mArrayAdapter.add(device.getName() + "\n" + device.getAddress());        }    }};

// Register the BroadcastReceiverIntentFilter filter = new IntentFilter(BluetoothDevice.ACTION_FOUND);registerReceiver(mReceiver, filter); // Don't forget to unregister during onDestroy

Device pairing (master/slave)

• Device pairing (or bonding)– When a connection is made with a remote device

for the first time, a pairing request is automatically presented to the user

– When paired, basic information about a device is kept locally such as device name, class, MAC addr, some secret keys (for secure comm)

– Android Bluetooth APIs require devices to be paired before an RFCOMM connection can be established

Socket communications (slave)

• Server must hold an open BluetoothServerSocket• Procedure:

1. Get BluetoothServerSocket by calling listenUsingRfcommWithServiceRecord(String, UUID)• String is a name of your service written into Service

Discovery Protocol (SDP) DB• UUID is an entry in SDP DB (unique service identifier)

2. Listening for connection requests by calling accept()• Blocking call; return a connection if accepted• Matching if a remote device sent a req with a UUID

matching the one registered with this listening server socket

Socket communications (slave)private class AcceptThread extends Thread {    private final BluetoothServerSocket mmServerSocket;    public AcceptThread() {        BluetoothServerSocket tmp = null;        try {            // MY_UUID is the app's UUID string, also used by the client code            tmp = mAdapter.listenUsingRfcommWithServiceRecord(NAME, MY_UUID);        } catch (IOException e) { }        mmServerSocket = tmp;    }    public void run() {        BluetoothSocket socket = null;        // Keep listening until exception occurs or a socket is returned        while (true) {            try {                socket = mmServerSocket.accept();            } catch (IOException e) { break; }            // If a connection was accepted            if (socket != null) {                // Do work to manage the connection (in a separate thread)                manageConnectedSocket(socket);                mmServerSocket.close();                break;            }        }    }    /** Will cancel the listening socket, and cause the thread to finish */    public void cancel() {        try {            mmServerSocket.close();        } catch (IOException e) { }    }}

Socket communications (master)

• Must have “BluetoothDevice” of a remote device (discovered via startDiscovery())

• Procedure:1. Using BluetoothDevice, get a BluetoothSocket by

calling “createRfcommSocketToServiceRecord(UUID)”• Initialize a BluetoothSocket with a given UUID• UUID is a hard-coded string unique to the “server”

2. Initiate a connection by calling “connect()”• System will perform an SDP lookup on the remote device to

match the UUID• If the lookup is successful and the remote device accepts

the connection, it will share the RFCOMM channel for comm

Socket communications (master)

private class ConnectThread extends Thread {    private final BluetoothSocket mmSocket;    private final BluetoothDevice mmDevice;    public ConnectThread(BluetoothDevice device) {        BluetoothSocket tmp = null;        mmDevice = device;

        // Get a BluetoothSocket to connect with the given BluetoothDevice        try {            // MY_UUID is the app's UUID string, also used by the server code            tmp = device.createRfcommSocketToServiceRecord(MY_UUID);        } catch (IOException e) { }        mmSocket = tmp;    }    public void run() {        try {            // Connect the device through the socket. This will block            // until it succeeds or throws an exception            mmSocket.connect();        } catch (IOException connectException) {            // Unable to connect; close the socket and get out            try {                mmSocket.close();            } catch (IOException closeException) { }            return;        }        // Do work to manage the connection (in a separate thread)        manageConnectedSocket(mmSocket);    }   public void cancel() {        try {             mmSocket.close();        } catch (IOException e) { }    }

Managing a connection

• Each device now has a connected BluetoothSocket

• Sending/receiving data:– First get InputStream/OutputStream from

socket– Read/write data to the streams with blocking

calls, read/write(byte[])

Managing a connectionprivate class ConnectedThread extends Thread {    private final BluetoothSocket mmSocket;    private final InputStream mmInStream;    private final OutputStream mmOutStream;

    public ConnectedThread(BluetoothSocket socket) {        mmSocket = socket;        InputStream tmpIn = null;        OutputStream tmpOut = null;

        // Get the input and output streams, using temp objects because        // member streams are final        try {            tmpIn = socket.getInputStream();            tmpOut = socket.getOutputStream();        } catch (IOException e) { }

        mmInStream = tmpIn;        mmOutStream = tmpOut; }    public void run() {        byte[] buffer = new byte[1024];  // buffer store for the stream        int bytes; // bytes returned from read()

      while (true) {            try {                // Read from the InputStream                bytes = mmInStream.read(buffer);                // Send the obtained bytes to the UI Activity                mHandler.obtainMessage(MESSAGE_READ, bytes, -1, buffer)                        .sendToTarget();            } catch (IOException e) { break; }        }    }

NFCReading NDEF data from an NFC tag

Beaming NDEF messages from one device to another with Android Beam

http://developer.android.com/guide/topics/nfc/nfc.html http://developer.android.com/resources/samples/AndroidBeamDemo/src/com/example/android/beam/Beam.html

NFC: Near Field Communications

NFC Forum Technology Architecture

http://www.nfc-forum.org/events/oulu_spotlight/Technical_Architecture.pdf

NFC card emulation mode:

Smart key say for mobile local

payment

Reader/writer mode: low cost

solution to distribute

info/services

P2P mode(Android Beam):

either directly or by establishing

wireless linksNFC device NFC device

NFC device

NFC device NFC tag

NFC tag

NFC Capabilities on Android

• Currently available Android NFC phones: Google Nexus S, Samsung Galaxy S 2

• Use Android 2.3.3 (API Level 10) as the NFC APIs drastically changed from 2.3.2.

• NFC in Android:– Tag reader/writer– P2P communication (Android-specific)– Tag emulation (of certain NFC NDEF tags) – not

fully functional*

* http://stackoverflow.com/questions/6138077/can-an-android-nfc-phone-act-as-an-nfc-tag

Tag Dispatch System

• Android-powered devices are usually looking for NFC tags when the screen is unlocked, unless NFC is disabled in the device's Settings menu.

• Tag dispatch system analyzes scanned NFC tags, parses them, and tries to locate applications that are interested in the scanned data

Reading Tag Example

<intent-filter>   <action android:name="android.nfc.action.NDEF_DISCOVERED"/>   <category android:name="android.intent.category.DEFAULT"/>

// The following example filters for a URI in the form of http://developer.android.com/index.html.    <data android:scheme="http"              android:host="developer.android.com"              android:pathPrefix="/index.html" />

// for P2P beaming.. <data android:mimeType="application/com.example.android.beam" /> </intent-filter>

<manifest ... > <uses-permission android:name="android.permission.NFC" /> <uses-sdk android:minSdkVersion="10"/> <uses-feature android:name="android.hardware.nfc" android:required="true" /> </manifest>

Reading Tag Example

public void onResume() {    super.onResume();    ...

    if (NfcAdapter.ACTION_NDEF_DISCOVERED.equals(getIntent().getAction())) {        Parcelable[] rawMsgs = intent.getParcelableArrayExtra(NfcAdapter.EXTRA_NDEF_MESSAGES);

        if (rawMsgs != null) {            msgs = new NdefMessage[rawMsgs.length];            for (int i = 0; i < rawMsgs.length; i++) {                msgs[i] = (NdefMessage) rawMsgs[i];            }        }    }    //process the msgs array}

Android Beaming (P2P)

• Simple P2P data exchange between two Android-powered devices

• Sender app must be in the foreground • Receiver must not be locked • Sender’s close contact will invoke "Touch to Beam" UI• Sender can then choose whether or not to beam the

message to receiver• Note that if your activity enables Android Beam and is

in the foreground, the standard intent dispatch system will be disabled. – However, if your activity also enables

foreground dispatching, then it can still scan tags that match the intent filters set in the foreground dispatching.

Android Beaming (P2P)

• CreateNdefMessageCallBack causes a Callback to be invoked when another NFC device capable of NDEF push (Android Beam) is within range. – Instantly create an NdefMessage at the moment that another device is within

range for NFC and push it over (pushing is done automatically). – Using this callback allows you to create a message with data that might vary

based on the content currently visible to the user.

• OnNdefPushCompleteCallbackon causes a Callback as soon as sending of the NDEF Message is completed. – You should implement this method in order to let the user know, when the

transfer is complete and thus the user can put his NFC device away.

// Register callback to set NDEF message (OnCreate) mNfcAdapter.setNdefPushMessageCallback(this, this); // Register callback to listen for message-sent success mNfcAdapter.setOnNdefPushCompleteCallback(this, this);

NfcAdapter.CreateNdefMessageCallback.createNdefMessage(NfcEvent event) NfcAdapter.OnNdefPushCompleteCallbackon.NdefPushComplete(NfcEvent event)

http://developer.android.com/resources/samples/AndroidBeamDemo/src/com/example/android/beam/Beam.html

Android Beaming (P2P)// Receiving a beam from the sender    @Override    public void onNewIntent(Intent intent) {        // onResume gets called after this to handle the intent        setIntent(intent);    }    @Override    public void onResume() {        super.onResume();        // Check to see that the Activity started due to an Android Beam        if (NfcAdapter.ACTION_NDEF_DISCOVERED.equals(getIntent().getAction())) {            processIntent(getIntent());        }    } // Parses the NDEF Message from the intent and prints to the TextView    void processIntent(Intent intent) {        Parcelable[] rawMsgs = intent.getParcelableArrayExtra(                NfcAdapter.EXTRA_NDEF_MESSAGES);        // only one message sent during the beam        NdefMessage msg = (NdefMessage) rawMsgs[0];        // record 0 contains the MIME type, record 1 is the AAR, if present        mInfoText.setText(new String(msg.getRecords()[0].getPayload()));    }

http://developer.android.com/resources/samples/AndroidBeamDemo/src/com/example/android/beam/Beam.html

Summary

• Managing network connectivity• WiFi management• Bluetooth communications through

RFCOMM• NFC: tag reading and beaming