diff --git a/CGMBLEKit Example/AppDelegate.swift b/CGMBLEKit Example/AppDelegate.swift
index 3f8028cc..e872997d 100644
--- a/CGMBLEKit Example/AppDelegate.swift	
+++ b/CGMBLEKit Example/AppDelegate.swift	
@@ -11,8 +11,8 @@ import xDripG5
 import CoreBluetooth
 
 @UIApplicationMain
-class AppDelegate: UIResponder, UIApplicationDelegate, TransmitterDelegate {
-    
+class AppDelegate: UIResponder, UIApplicationDelegate, TransmitterDelegate, TransmitterCommandSource {
+
     var window: UIWindow?
 
     static var sharedDelegate: AppDelegate {
@@ -28,14 +28,20 @@ class AppDelegate: UIResponder, UIApplicationDelegate, TransmitterDelegate {
                 )
                 transmitter?.stayConnected = UserDefaults.standard.stayConnected
                 transmitter?.delegate = self
+                transmitter?.commandSource = self
 
                 UserDefaults.standard.transmitterID = id
             }
+            glucose = nil
         }
     }
 
     var transmitter: Transmitter?
 
+    let commandQueue = CommandQueue()
+
+    var glucose: Glucose?
+
     func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
 
         transmitterID = UserDefaults.standard.transmitterID
@@ -82,6 +88,18 @@ class AppDelegate: UIResponder, UIApplicationDelegate, TransmitterDelegate {
         return dateFormatter
     }()
 
+    func dequeuePendingCommand(for transmitter: Transmitter) -> Command? {
+        return commandQueue.dequeue()
+    }
+
+    func transmitter(_ transmitter: Transmitter, didFail command: Command, with error: Error) {
+        // TODO: implement
+    }
+
+    func transmitter(_ transmitter: Transmitter, didComplete command: Command) {
+        // TODO: implement
+    }
+
     func transmitter(_ transmitter: Transmitter, didError error: Error) {
         DispatchQueue.main.async {
             if let vc = self.window?.rootViewController as? TransmitterDelegate {
@@ -91,6 +109,7 @@ class AppDelegate: UIResponder, UIApplicationDelegate, TransmitterDelegate {
     }
 
     func transmitter(_ transmitter: Transmitter, didRead glucose: Glucose) {
+        self.glucose = glucose
         DispatchQueue.main.async {
             if let vc = self.window?.rootViewController as? TransmitterDelegate {
                 vc.transmitter(transmitter, didRead: glucose)
diff --git a/CGMBLEKit Example/Base.lproj/Main.storyboard b/CGMBLEKit Example/Base.lproj/Main.storyboard
index 62b8b23b..be42d9a1 100644
--- a/CGMBLEKit Example/Base.lproj/Main.storyboard	
+++ b/CGMBLEKit Example/Base.lproj/Main.storyboard	
@@ -1,8 +1,11 @@
-<?xml version="1.0" encoding="UTF-8" standalone="no"?>
-<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="11201" systemVersion="15G1004" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" useTraitCollections="YES" colorMatched="YES" initialViewController="BYZ-38-t0r">
+<?xml version="1.0" encoding="UTF-8"?>
+<document type="com.apple.InterfaceBuilder3.CocoaTouch.Storyboard.XIB" version="3.0" toolsVersion="13771" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" useTraitCollections="YES" colorMatched="YES" initialViewController="BYZ-38-t0r">
+    <device id="retina5_5" orientation="portrait">
+        <adaptation id="fullscreen"/>
+    </device>
     <dependencies>
         <deployment identifier="iOS"/>
-        <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="11161"/>
+        <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="13772"/>
         <capability name="Constraints to layout margins" minToolsVersion="6.0"/>
         <capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
     </dependencies>
@@ -10,31 +13,36 @@
         <!--View Controller-->
         <scene sceneID="tne-QT-ifu">
             <objects>
-                <viewController id="BYZ-38-t0r" customClass="ViewController" customModule="xDripG5_Example" customModuleProvider="target" sceneMemberID="viewController">
+                <viewController id="BYZ-38-t0r" customClass="ViewController" customModule="CGMBLEKit_Example" customModuleProvider="target" sceneMemberID="viewController">
                     <layoutGuides>
                         <viewControllerLayoutGuide type="top" id="y3c-jy-aDJ"/>
                         <viewControllerLayoutGuide type="bottom" id="wfy-db-euE"/>
                     </layoutGuides>
                     <view key="view" contentMode="scaleToFill" id="8bC-Xf-vdC">
-                        <rect key="frame" x="0.0" y="0.0" width="375" height="667"/>
+                        <rect key="frame" x="0.0" y="0.0" width="414" height="736"/>
                         <autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
                         <subviews>
                             <stackView opaque="NO" contentMode="scaleToFill" axis="vertical" distribution="equalSpacing" alignment="center" translatesAutoresizingMaskIntoConstraints="NO" id="mqJ-p6-sSU">
+                                <rect key="frame" x="20" y="40" width="374" height="676"/>
                                 <subviews>
                                     <view contentMode="scaleToFill" translatesAutoresizingMaskIntoConstraints="NO" id="K5I-e5-MvQ">
+                                        <rect key="frame" x="15.666666666666657" y="0.0" width="343" height="52"/>
                                         <subviews>
                                             <label opaque="NO" userInteractionEnabled="NO" contentMode="left" horizontalHuggingPriority="251" verticalHuggingPriority="251" text="Transmitter ID" textAlignment="natural" lineBreakMode="tailTruncation" baselineAdjustment="alignBaselines" adjustsFontSizeToFit="NO" translatesAutoresizingMaskIntoConstraints="NO" id="5y6-vU-qC3">
+                                                <rect key="frame" x="8" y="14.333333333333334" width="249" height="20.333333333333329"/>
                                                 <fontDescription key="fontDescription" style="UICTFontTextStyleBody"/>
                                                 <color key="textColor" red="0.0" green="0.0" blue="0.0" alpha="1" colorSpace="custom" customColorSpace="sRGB"/>
                                                 <nil key="highlightedColor"/>
                                             </label>
                                             <view contentMode="scaleToFill" translatesAutoresizingMaskIntoConstraints="NO" id="oUg-jb-mCK">
+                                                <rect key="frame" x="265" y="42" width="70" height="1"/>
                                                 <color key="backgroundColor" red="0.33333333333333331" green="0.33333333333333331" blue="0.33333333333333331" alpha="1" colorSpace="custom" customColorSpace="sRGB"/>
                                                 <constraints>
                                                     <constraint firstAttribute="height" constant="1" id="Rjq-qn-DaB"/>
                                                 </constraints>
                                             </view>
                                             <textField opaque="NO" clipsSubviews="YES" contentMode="scaleToFill" contentHorizontalAlignment="left" contentVerticalAlignment="center" text="500000" textAlignment="center" minimumFontSize="17" translatesAutoresizingMaskIntoConstraints="NO" id="e0T-ru-tWD">
+                                                <rect key="frame" x="265" y="8" width="70" height="34"/>
                                                 <constraints>
                                                     <constraint firstAttribute="width" constant="70" id="1v1-n5-aT4"/>
                                                     <constraint firstAttribute="height" constant="34" id="CVt-L0-Ts1"/>
@@ -59,13 +67,16 @@
                                         </constraints>
                                     </view>
                                     <stackView opaque="NO" contentMode="scaleToFill" spacing="8" baselineRelativeArrangement="YES" translatesAutoresizingMaskIntoConstraints="NO" id="4lp-eb-b4C">
+                                        <rect key="frame" x="74.333333333333329" y="129" width="225.33333333333337" height="31"/>
                                         <subviews>
                                             <label opaque="NO" userInteractionEnabled="NO" contentMode="left" horizontalHuggingPriority="251" verticalHuggingPriority="251" text="Stay connected" textAlignment="natural" lineBreakMode="tailTruncation" baselineAdjustment="alignBaselines" adjustsFontSizeToFit="NO" translatesAutoresizingMaskIntoConstraints="NO" id="NSs-9e-3Sl">
+                                                <rect key="frame" x="0.0" y="0.0" width="168.33333333333334" height="31"/>
                                                 <fontDescription key="fontDescription" style="UICTFontTextStyleBody"/>
                                                 <color key="textColor" red="0.0" green="0.0" blue="0.0" alpha="1" colorSpace="custom" customColorSpace="sRGB"/>
                                                 <nil key="highlightedColor"/>
                                             </label>
                                             <switch opaque="NO" contentMode="scaleToFill" horizontalHuggingPriority="750" verticalHuggingPriority="750" contentHorizontalAlignment="center" contentVerticalAlignment="center" translatesAutoresizingMaskIntoConstraints="NO" id="DR7-3h-biq">
+                                                <rect key="frame" x="176.33333333333337" y="0.0" width="51" height="31"/>
                                                 <connections>
                                                     <action selector="toggleStayConnected:" destination="BYZ-38-t0r" eventType="valueChanged" id="Wci-5s-BJm"/>
                                                 </connections>
@@ -73,26 +84,59 @@
                                         </subviews>
                                     </stackView>
                                     <stackView opaque="NO" contentMode="scaleToFill" spacing="8" baselineRelativeArrangement="YES" translatesAutoresizingMaskIntoConstraints="NO" id="lPZ-HJ-Wji">
+                                        <rect key="frame" x="74.333333333333329" y="237.33333333333331" width="225.33333333333337" height="31"/>
                                         <subviews>
                                             <label opaque="NO" userInteractionEnabled="NO" contentMode="left" horizontalHuggingPriority="251" verticalHuggingPriority="251" text="Run alongside G5 app" textAlignment="natural" lineBreakMode="tailTruncation" baselineAdjustment="alignBaselines" adjustsFontSizeToFit="NO" translatesAutoresizingMaskIntoConstraints="NO" id="3i8-2m-QFG">
+                                                <rect key="frame" x="0.0" y="0.0" width="168.33333333333334" height="31"/>
                                                 <fontDescription key="fontDescription" style="UICTFontTextStyleBody"/>
                                                 <color key="textColor" red="0.0" green="0.0" blue="0.0" alpha="1" colorSpace="custom" customColorSpace="sRGB"/>
                                                 <nil key="highlightedColor"/>
                                             </label>
                                             <switch opaque="NO" contentMode="scaleToFill" horizontalHuggingPriority="750" verticalHuggingPriority="750" contentHorizontalAlignment="center" contentVerticalAlignment="center" translatesAutoresizingMaskIntoConstraints="NO" id="mjL-My-zyc">
+                                                <rect key="frame" x="176.33333333333337" y="0.0" width="51" height="31"/>
                                                 <connections>
                                                     <action selector="togglePassiveMode:" destination="BYZ-38-t0r" eventType="valueChanged" id="vGB-RZ-H3T"/>
                                                 </connections>
                                             </switch>
                                         </subviews>
                                     </stackView>
-                                    <activityIndicatorView opaque="NO" contentMode="scaleToFill" horizontalHuggingPriority="750" verticalHuggingPriority="750" hidesWhenStopped="YES" animating="YES" style="gray" translatesAutoresizingMaskIntoConstraints="NO" id="v7k-HB-Hm0"/>
+                                    <activityIndicatorView opaque="NO" contentMode="scaleToFill" horizontalHuggingPriority="750" verticalHuggingPriority="750" hidesWhenStopped="YES" animating="YES" style="gray" translatesAutoresizingMaskIntoConstraints="NO" id="v7k-HB-Hm0">
+                                        <rect key="frame" x="177" y="345.33333333333331" width="20" height="20"/>
+                                    </activityIndicatorView>
                                     <label opaque="NO" userInteractionEnabled="NO" contentMode="left" horizontalHuggingPriority="251" verticalHuggingPriority="251" text="––" textAlignment="center" lineBreakMode="tailTruncation" baselineAdjustment="alignBaselines" adjustsFontSizeToFit="NO" translatesAutoresizingMaskIntoConstraints="NO" id="zAF-A7-1Oc">
+                                        <rect key="frame" x="173.66666666666666" y="442.33333333333331" width="27" height="31.333333333333314"/>
                                         <fontDescription key="fontDescription" style="UICTFontTextStyleTitle1"/>
                                         <color key="textColor" red="0.0" green="0.0" blue="0.0" alpha="1" colorSpace="custom" customColorSpace="sRGB"/>
                                         <nil key="highlightedColor"/>
                                     </label>
+                                    <stackView opaque="NO" contentMode="scaleToFill" distribution="fillEqually" alignment="center" spacing="20" baselineRelativeArrangement="YES" translatesAutoresizingMaskIntoConstraints="NO" id="VKn-a6-Gbj">
+                                        <rect key="frame" x="74" y="551" width="226" height="30"/>
+                                        <subviews>
+                                            <button opaque="NO" contentMode="scaleToFill" contentHorizontalAlignment="center" contentVerticalAlignment="center" buttonType="roundedRect" lineBreakMode="middleTruncation" translatesAutoresizingMaskIntoConstraints="NO" id="HEQ-Bp-kx7" userLabel="Start">
+                                                <rect key="frame" x="0.0" y="0.0" width="62" height="30"/>
+                                                <state key="normal" title="Start"/>
+                                                <connections>
+                                                    <action selector="start:" destination="BYZ-38-t0r" eventType="touchUpInside" id="CoA-cH-SFe"/>
+                                                </connections>
+                                            </button>
+                                            <button opaque="NO" contentMode="scaleToFill" contentHorizontalAlignment="center" contentVerticalAlignment="center" buttonType="roundedRect" lineBreakMode="middleTruncation" translatesAutoresizingMaskIntoConstraints="NO" id="HYW-Ex-7LC">
+                                                <rect key="frame" x="82" y="0.0" width="62" height="30"/>
+                                                <state key="normal" title="Calibrate"/>
+                                                <connections>
+                                                    <action selector="calibrate:" destination="BYZ-38-t0r" eventType="touchUpInside" id="5Ft-BY-tyB"/>
+                                                </connections>
+                                            </button>
+                                            <button opaque="NO" contentMode="scaleToFill" contentHorizontalAlignment="center" contentVerticalAlignment="center" buttonType="roundedRect" lineBreakMode="middleTruncation" translatesAutoresizingMaskIntoConstraints="NO" id="JAM-9K-KeE" userLabel="Stop">
+                                                <rect key="frame" x="164" y="0.0" width="62" height="30"/>
+                                                <state key="normal" title="Stop"/>
+                                                <connections>
+                                                    <action selector="stop:" destination="BYZ-38-t0r" eventType="touchUpInside" id="hWH-qi-m3s"/>
+                                                </connections>
+                                            </button>
+                                        </subviews>
+                                    </stackView>
                                     <label opaque="NO" userInteractionEnabled="NO" contentMode="left" horizontalHuggingPriority="251" verticalHuggingPriority="251" text="Waiting for first reading" textAlignment="center" lineBreakMode="tailTruncation" numberOfLines="0" baselineAdjustment="alignBaselines" adjustsFontSizeToFit="NO" translatesAutoresizingMaskIntoConstraints="NO" id="9p7-LX-EMK">
+                                        <rect key="frame" x="-20" y="658" width="0.0" height="18"/>
                                         <fontDescription key="fontDescription" style="UICTFontTextStyleSubhead"/>
                                         <color key="textColor" red="0.0" green="0.0" blue="0.0" alpha="1" colorSpace="custom" customColorSpace="sRGB"/>
                                         <nil key="highlightedColor"/>
diff --git a/CGMBLEKit Example/CommandQueue.swift b/CGMBLEKit Example/CommandQueue.swift
new file mode 100644
index 00000000..0f9c7b5f
--- /dev/null
+++ b/CGMBLEKit Example/CommandQueue.swift	
@@ -0,0 +1,34 @@
+//
+//  CommandQueue.swift
+//  CGMBLEKit Example
+//
+//  Created by Paul Dickens on 25/03/2018.
+//  Copyright © 2018 LoopKit Authors. All rights reserved.
+//
+
+import Foundation
+import xDripG5
+
+
+class CommandQueue {
+    private var list = Array<Command>()
+    private var lock = os_unfair_lock()
+
+    func enqueue(_ element: Command) {
+        os_unfair_lock_lock(&lock)
+        list.append(element)
+        os_unfair_lock_unlock(&lock)
+    }
+
+    func dequeue() -> Command? {
+        os_unfair_lock_lock(&lock)
+        defer {
+            os_unfair_lock_unlock(&lock)
+        }
+        if !list.isEmpty {
+            return list.removeFirst()
+        } else {
+            return nil
+        }
+    }
+}
diff --git a/CGMBLEKit Example/ViewController.swift b/CGMBLEKit Example/ViewController.swift
index 642d31b5..37a25b1d 100644
--- a/CGMBLEKit Example/ViewController.swift	
+++ b/CGMBLEKit Example/ViewController.swift	
@@ -62,6 +62,60 @@ class ViewController: UIViewController, TransmitterDelegate, UITextFieldDelegate
         UserDefaults.standard.passiveModeEnabled = sender.isOn
     }
 
+    @IBAction func start(_ sender: UIButton) {
+        let dialog = UIAlertController(title: "Confirm", message: "Start sensor session.", preferredStyle: .alert)
+
+        dialog.addAction(UIAlertAction(title: "OK", style: .default, handler: { (action: UIAlertAction!) in
+            AppDelegate.sharedDelegate.commandQueue.enqueue(.startSensor(at: Date()))
+        }))
+
+        dialog.addAction(UIAlertAction(title: "Cancel", style: .cancel))
+
+        present(dialog, animated: true, completion: nil)
+    }
+
+    @IBAction func calibrate(_ sender: UIButton) {
+        let dialog = UIAlertController(title: "Enter BG", message: "Calibrate sensor.", preferredStyle: .alert)
+
+        let unit = HKUnit.milligramsPerDeciliter
+
+        dialog.addTextField { (textField : UITextField!) in
+            textField.placeholder = unit.unitString
+            textField.keyboardType = .numberPad
+        }
+
+        dialog.addAction(UIAlertAction(title: "Calibrate", style: .default, handler: { (action: UIAlertAction!) in
+            let textField = dialog.textFields![0] as UITextField
+            let minGlucose = HKQuantity(unit: HKUnit.milligramsPerDeciliter, doubleValue: 40)
+            let maxGlucose = HKQuantity(unit: HKUnit.milligramsPerDeciliter, doubleValue: 400)
+
+            if let text = textField.text, let entry = Double(text) {
+                guard entry >= minGlucose.doubleValue(for: unit) && entry <= maxGlucose.doubleValue(for: unit) else {
+                    // TODO: notify the user if the glucose is not in range
+                    return
+                }
+                let glucose = HKQuantity(unit: unit, doubleValue: Double(entry))
+                AppDelegate.sharedDelegate.commandQueue.enqueue(.calibrateSensor(to: glucose, at: Date()))
+            }
+        }))
+
+        dialog.addAction(UIAlertAction(title: "Cancel", style: .cancel))
+
+        present(dialog, animated: true, completion: nil)
+    }
+
+    @IBAction func stop(_ sender: UIButton) {
+        let dialog = UIAlertController(title: "Confirm", message: "Stop sensor session.", preferredStyle: .alert)
+
+        dialog.addAction(UIAlertAction(title: "OK", style: .default, handler: { (action: UIAlertAction!) in
+            AppDelegate.sharedDelegate.commandQueue.enqueue(.stopSensor(at: Date()))
+        }))
+
+        dialog.addAction(UIAlertAction(title: "Cancel", style: .cancel))
+
+        present(dialog, animated: true, completion: nil)
+    }
+
     // MARK: - UITextFieldDelegate
 
     func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
diff --git a/xDripG5.xcodeproj/project.pbxproj b/xDripG5.xcodeproj/project.pbxproj
index 62b33750..8f159a59 100644
--- a/xDripG5.xcodeproj/project.pbxproj
+++ b/xDripG5.xcodeproj/project.pbxproj
@@ -73,6 +73,11 @@
 		43F82BD41D037227006F5DD7 /* SessionStartRxMessageTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 43F82BD31D037227006F5DD7 /* SessionStartRxMessageTests.swift */; };
 		C19084BB203932BD00AA47F3 /* Main.storyboard in Resources */ = {isa = PBXBuildFile; fileRef = C19084B9203932BD00AA47F3 /* Main.storyboard */; };
 		C1CD8B0C203931AD00A8F498 /* NSUserDefaults.swift in Sources */ = {isa = PBXBuildFile; fileRef = C1CD8B0A203931AD00A8F498 /* NSUserDefaults.swift */; };
+		E71BFF7E20673BD700C876A2 /* CommandQueue.swift in Sources */ = {isa = PBXBuildFile; fileRef = E71BFF7D20673BD700C876A2 /* CommandQueue.swift */; };
+		E752B4102063C31B0063027D /* Command.swift in Sources */ = {isa = PBXBuildFile; fileRef = E752B40F2063C31B0063027D /* Command.swift */; };
+		E75DB6AE20419B5D00FBE04E /* CalibrateGlucoseRxMessage.swift in Sources */ = {isa = PBXBuildFile; fileRef = E75DB6AD20419B5D00FBE04E /* CalibrateGlucoseRxMessage.swift */; };
+		E76FD69C205C75780056DA5B /* Calibration.swift in Sources */ = {isa = PBXBuildFile; fileRef = E76FD69B205C75780056DA5B /* Calibration.swift */; };
+		E76FD69E205C7A200056DA5B /* CalibrationDataTxMessage.swift in Sources */ = {isa = PBXBuildFile; fileRef = E76FD69D205C7A200056DA5B /* CalibrationDataTxMessage.swift */; };
 /* End PBXBuildFile section */
 
 /* Begin PBXContainerItemProxy section */
@@ -180,6 +185,11 @@
 		43F82BD31D037227006F5DD7 /* SessionStartRxMessageTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = SessionStartRxMessageTests.swift; sourceTree = "<group>"; };
 		C19084BA203932BD00AA47F3 /* Base */ = {isa = PBXFileReference; lastKnownFileType = file.storyboard; name = Base; path = Base.lproj/Main.storyboard; sourceTree = "<group>"; };
 		C1CD8B0A203931AD00A8F498 /* NSUserDefaults.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = NSUserDefaults.swift; sourceTree = "<group>"; };
+		E71BFF7D20673BD700C876A2 /* CommandQueue.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = CommandQueue.swift; sourceTree = "<group>"; };
+		E752B40F2063C31B0063027D /* Command.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Command.swift; sourceTree = "<group>"; };
+		E75DB6AD20419B5D00FBE04E /* CalibrateGlucoseRxMessage.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = CalibrateGlucoseRxMessage.swift; sourceTree = "<group>"; };
+		E76FD69B205C75780056DA5B /* Calibration.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Calibration.swift; sourceTree = "<group>"; };
+		E76FD69D205C7A200056DA5B /* CalibrationDataTxMessage.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = CalibrationDataTxMessage.swift; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -216,6 +226,7 @@
 			children = (
 				C1CD8B0A203931AD00A8F498 /* NSUserDefaults.swift */,
 				437AFEF92038EC43008C4892 /* AppDelegate.swift */,
+				E71BFF7D20673BD700C876A2 /* CommandQueue.swift */,
 				437AFEFB2038EC43008C4892 /* ViewController.swift */,
 				437AFF002038EC43008C4892 /* Assets.xcassets */,
 				C19084B9203932BD00AA47F3 /* Main.storyboard */,
@@ -263,7 +274,9 @@
 				43CABE0E1C350B2800005705 /* BluetoothManager.swift */,
 				43CABE0F1C350B2800005705 /* BluetoothServices.swift */,
 				431CE7621F8EEF6D00255374 /* CBPeripheral.swift */,
+				E76FD69B205C75780056DA5B /* Calibration.swift */,
 				43E3978A1D5668BD0028E321 /* CalibrationState.swift */,
+				E752B40F2063C31B0063027D /* Command.swift */,
 				43E3978E1D566B170028E321 /* Glucose.swift */,
 				43CABDF81C3506F100005705 /* Info.plist */,
 				430D64C41CB7846A00FCA750 /* NSData+CRC.swift */,
@@ -307,6 +320,8 @@
 				43CE7CCB1CA73BCC003CC1B0 /* BatteryStatusTxMessage.swift */,
 				43CABE1B1C350B3D00005705 /* BondRequestTxMessage.swift */,
 				43CE7CD11CA73CBC003CC1B0 /* CalibrateGlucoseTxMessage.swift */,
+				E75DB6AD20419B5D00FBE04E /* CalibrateGlucoseRxMessage.swift */,
+				E76FD69D205C7A200056DA5B /* CalibrationDataTxMessage.swift */,
 				43846AC51D8F896C00799272 /* CalibrationDataRxMessage.swift */,
 				43CABE1C1C350B3D00005705 /* DisconnectTxMessage.swift */,
 				43CE7CC71CA73AEB003CC1B0 /* FirmwareVersionTxMessage.swift */,
@@ -491,6 +506,7 @@
 			files = (
 				43E5292C2060C4FA00ACEB3B /* HKUnit.swift in Sources */,
 				C1CD8B0C203931AD00A8F498 /* NSUserDefaults.swift in Sources */,
+				E71BFF7E20673BD700C876A2 /* CommandQueue.swift in Sources */,
 				43D140CE2047AA930032346D /* Data.swift in Sources */,
 				437AFEFC2038EC43008C4892 /* ViewController.swift in Sources */,
 				437AFEFA2038EC43008C4892 /* AppDelegate.swift in Sources */,
@@ -502,8 +518,10 @@
 			buildActionMask = 2147483647;
 			files = (
 				43CE7CCA1CA73B94003CC1B0 /* TransmitterVersionTxMessage.swift in Sources */,
+				E76FD69C205C75780056DA5B /* Calibration.swift in Sources */,
 				43E3978B1D5668BD0028E321 /* CalibrationState.swift in Sources */,
 				431CE7631F8EEF6D00255374 /* CBPeripheral.swift in Sources */,
+				E76FD69E205C7A200056DA5B /* CalibrationDataTxMessage.swift in Sources */,
 				43CABE2D1C350B3D00005705 /* TransmitterTimeRxMessage.swift in Sources */,
 				43CABE291C350B3D00005705 /* GlucoseRxMessage.swift in Sources */,
 				43CE7CDC1CA77468003CC1B0 /* TransmitterStatus.swift in Sources */,
@@ -527,12 +545,14 @@
 				43CABE2E1C350B3D00005705 /* TransmitterTimeTxMessage.swift in Sources */,
 				43880F981D9E19FC009061A8 /* TransmitterVersionRxMessage.swift in Sources */,
 				43CABE2C1C350B3D00005705 /* TransmitterMessage.swift in Sources */,
+				E75DB6AE20419B5D00FBE04E /* CalibrateGlucoseRxMessage.swift in Sources */,
 				43CABE131C350B2800005705 /* BluetoothServices.swift in Sources */,
 				43E3978F1D566B170028E321 /* Glucose.swift in Sources */,
 				43CABE151C350B2800005705 /* Transmitter.swift in Sources */,
 				43CABE281C350B3D00005705 /* DisconnectTxMessage.swift in Sources */,
 				43CABE121C350B2800005705 /* BluetoothManager.swift in Sources */,
 				43CABE2B1C350B3D00005705 /* KeepAliveTxMessage.swift in Sources */,
+				E752B4102063C31B0063027D /* Command.swift in Sources */,
 				43CABE241C350B3D00005705 /* AuthChallengeTxMessage.swift in Sources */,
 				43CE7CD21CA73CBC003CC1B0 /* CalibrateGlucoseTxMessage.swift in Sources */,
 				43F82BD01D035D68006F5DD7 /* SessionStopRxMessage.swift in Sources */,
diff --git a/xDripG5/BluetoothManager.swift b/xDripG5/BluetoothManager.swift
index e5f876e0..7e7ec5e5 100644
--- a/xDripG5/BluetoothManager.swift
+++ b/xDripG5/BluetoothManager.swift
@@ -175,7 +175,7 @@ class BluetoothManager: NSObject {
 extension BluetoothManager: CBCentralManagerDelegate {
     func centralManagerDidUpdateState(_ central: CBCentralManager) {
         peripheralManager?.centralManagerDidUpdateState(central)
-        log.info("%{public}@: %{public}@", #function, String(describing: central.state))
+        log.info("%{public}@: %{public}@", #function, String(describing: central.state.rawValue))
 
         switch central.state {
         case .poweredOn:
diff --git a/xDripG5/Calibration.swift b/xDripG5/Calibration.swift
new file mode 100644
index 00000000..fd05c906
--- /dev/null
+++ b/xDripG5/Calibration.swift
@@ -0,0 +1,27 @@
+//
+//  Calibration.swift
+//  xDripG5
+//
+//  Created by Paul Dickens on 17/03/2018.
+//  Copyright © 2018 LoopKit Authors. All rights reserved.
+//
+
+import Foundation
+import HealthKit
+
+
+public struct Calibration {
+    init?(calibrationMessage: CalibrationDataRxMessage, activationDate: Date) {
+        guard calibrationMessage.glucose > 0 else {
+            return nil
+        }
+
+        let unit = HKUnit.milligramsPerDeciliter
+
+        glucose = HKQuantity(unit: unit, doubleValue: Double(calibrationMessage.glucose))
+        date = activationDate.addingTimeInterval(TimeInterval(calibrationMessage.timestamp))
+    }
+
+    public let glucose: HKQuantity
+    public let date: Date
+}
diff --git a/xDripG5/CalibrationState.swift b/xDripG5/CalibrationState.swift
index b7a7b775..220ccb46 100644
--- a/xDripG5/CalibrationState.swift
+++ b/xDripG5/CalibrationState.swift
@@ -93,3 +93,14 @@ extension CalibrationState: Equatable {
         }
     }
 }
+
+extension CalibrationState: CustomStringConvertible {
+    public var description: String {
+        switch self {
+        case .known(let state):
+            return String(describing: state)
+        case .unknown:
+            return String(describing: self)
+        }
+    }
+}
diff --git a/xDripG5/Command.swift b/xDripG5/Command.swift
new file mode 100644
index 00000000..a741c6bf
--- /dev/null
+++ b/xDripG5/Command.swift
@@ -0,0 +1,70 @@
+//
+//  Command.swift
+//  xDripG5
+//
+//  Created by Paul Dickens on 22/03/2018.
+//  Copyright © 2018 LoopKit Authors. All rights reserved.
+//
+
+import Foundation
+import HealthKit
+
+
+public enum Command: RawRepresentable {
+    public typealias RawValue = [String: Any]
+
+    case startSensor(at: Date)
+    case stopSensor(at: Date)
+    case calibrateSensor(to: HKQuantity, at: Date)
+
+    public init?(rawValue: RawValue) {
+        guard let action = rawValue["action"] as? Int else {
+            return nil
+        }
+
+        switch Action(rawValue: action) {
+        case .startSensor?:
+            guard let date = rawValue["date"] as? Date else {
+                return nil
+            }
+            self = .startSensor(at: date)
+        case .stopSensor?:
+            guard let date = rawValue["date"] as? Date else {
+                return nil
+            }
+            self = .stopSensor(at: date)
+        case .calibrateSensor?:
+            guard let date = rawValue["date"] as? Date, let glucose = rawValue["glucose"] as? HKQuantity else {
+                return nil
+            }
+            self = .calibrateSensor(to: glucose, at: date)
+        case .none:
+            return nil
+        }
+    }
+
+    private enum Action: Int {
+        case startSensor, stopSensor, calibrateSensor
+    }
+
+    public var rawValue: RawValue {
+        switch self {
+        case .startSensor(let date):
+            return [
+                "action": Action.startSensor.rawValue,
+                "date": date
+            ]
+        case .stopSensor(let date):
+            return [
+                "action": Action.stopSensor.rawValue,
+                "date": date
+            ]
+        case .calibrateSensor(let glucose, let date):
+            return [
+                "action": Action.calibrateSensor.rawValue,
+                "date": date,
+                "glucose": glucose
+            ]
+        }
+    }
+}
diff --git a/xDripG5/Glucose.swift b/xDripG5/Glucose.swift
index 54f1b892..dbc9f24e 100644
--- a/xDripG5/Glucose.swift
+++ b/xDripG5/Glucose.swift
@@ -18,6 +18,7 @@ public struct Glucose {
         transmitterID: String,
         glucoseMessage: GlucoseRxMessage,
         timeMessage: TransmitterTimeRxMessage,
+        calibrationMessage: CalibrationDataRxMessage? = nil,
         activationDate: Date
     ) {
         self.init(
@@ -25,6 +26,7 @@ public struct Glucose {
             status: glucoseMessage.status,
             glucoseMessage: glucoseMessage.glucose,
             timeMessage: timeMessage,
+            calibrationMessage: calibrationMessage,
             activationDate: activationDate
         )
     }
@@ -34,6 +36,7 @@ public struct Glucose {
         status: UInt8,
         glucoseMessage: GlucoseSubMessage,
         timeMessage: TransmitterTimeRxMessage,
+        calibrationMessage: CalibrationDataRxMessage? = nil,
         activationDate: Date
     ) {
         self.transmitterID = transmitterID
@@ -43,6 +46,7 @@ public struct Glucose {
 
         sessionStartDate = activationDate.addingTimeInterval(TimeInterval(timeMessage.sessionStartTime))
         readDate = activationDate.addingTimeInterval(TimeInterval(glucoseMessage.timestamp))
+        lastCalibration = calibrationMessage != nil ? Calibration(calibrationMessage: calibrationMessage!, activationDate: activationDate) : nil
     }
 
     // MARK: - Transmitter Info
@@ -51,6 +55,7 @@ public struct Glucose {
     public let sessionStartDate: Date
 
     // MARK: - Glucose Info
+    public let lastCalibration: Calibration?
     public let readDate: Date
 
     public var isDisplayOnly: Bool {
diff --git a/xDripG5/Messages/CalibrateGlucoseRxMessage.swift b/xDripG5/Messages/CalibrateGlucoseRxMessage.swift
new file mode 100644
index 00000000..aaa6d8c4
--- /dev/null
+++ b/xDripG5/Messages/CalibrateGlucoseRxMessage.swift
@@ -0,0 +1,22 @@
+//
+//  CalibrateGlucoseRxMessage.swift
+//  xDripG5
+//
+//  Created by Paul Dickens on 25/02/2018.
+//  Copyright © 2018 LoopKit Authors. All rights reserved.
+//
+
+import Foundation
+
+
+public struct CalibrateGlucoseRxMessage: TransmitterRxMessage {
+    init?(data: Data) {
+        guard data.count == 5 && data.isCRCValid else {
+            return nil
+        }
+
+        guard data.starts(with: .calibrateGlucoseRx) else {
+            return nil
+        }
+    }
+}
diff --git a/xDripG5/Messages/CalibrateGlucoseTxMessage.swift b/xDripG5/Messages/CalibrateGlucoseTxMessage.swift
index 77299703..3b318689 100644
--- a/xDripG5/Messages/CalibrateGlucoseTxMessage.swift
+++ b/xDripG5/Messages/CalibrateGlucoseTxMessage.swift
@@ -9,6 +9,16 @@
 import Foundation
 
 
-struct CalibrateGlucoseTxMessage {
-    let opcode: Opcode = .calibrateGlucoseTx
+struct CalibrateGlucoseTxMessage: RespondableMessage {
+    typealias Response = CalibrateGlucoseRxMessage
+
+    let time: UInt32
+    let glucose: UInt16
+
+    var data: Data {
+        var data = Data(for: .calibrateGlucoseTx)
+        data.append(glucose)
+        data.append(time)
+        return data.appendingCRC()
+    }
 }
diff --git a/xDripG5/Messages/CalibrationDataRxMessage.swift b/xDripG5/Messages/CalibrationDataRxMessage.swift
index 938c067f..1f77efe8 100644
--- a/xDripG5/Messages/CalibrationDataRxMessage.swift
+++ b/xDripG5/Messages/CalibrationDataRxMessage.swift
@@ -10,6 +10,9 @@ import Foundation
 
 
 struct CalibrationDataRxMessage: TransmitterRxMessage {
+    let glucose: UInt16
+    let timestamp: UInt32
+
     init?(data: Data) {
         guard data.count == 19 && data.isCRCValid else {
             return nil
@@ -18,5 +21,8 @@ struct CalibrationDataRxMessage: TransmitterRxMessage {
         guard data.starts(with: .calibrationDataRx) else {
             return nil
         }
+
+        glucose = data[11..<13].toInt() & 0xfff
+        timestamp = data[13..<17].toInt()
     }
 }
diff --git a/xDripG5/Messages/CalibrationDataTxMessage.swift b/xDripG5/Messages/CalibrationDataTxMessage.swift
new file mode 100644
index 00000000..89e42af5
--- /dev/null
+++ b/xDripG5/Messages/CalibrationDataTxMessage.swift
@@ -0,0 +1,18 @@
+//
+//  CalibrationDataTxMessage.swift
+//  xDripG5
+//
+//  Created by Paul Dickens on 17/03/2018.
+//  Copyright © 2018 LoopKit Authors. All rights reserved.
+//
+
+import Foundation
+
+
+struct CalibrationDataTxMessage: RespondableMessage {
+    typealias Response = CalibrationDataRxMessage
+
+    var data: Data {
+        return Data(for: .calibrationDataTx).appendingCRC()
+    }
+}
diff --git a/xDripG5/Messages/SessionStartTxMessage.swift b/xDripG5/Messages/SessionStartTxMessage.swift
index 8e14e66e..faa2c151 100644
--- a/xDripG5/Messages/SessionStartTxMessage.swift
+++ b/xDripG5/Messages/SessionStartTxMessage.swift
@@ -12,11 +12,16 @@ import Foundation
 struct SessionStartTxMessage: RespondableMessage {
     typealias Response = SessionStartRxMessage
 
+    /// Time since activation in Dex seconds
     let startTime: UInt32
 
+    /// Time in seconds since Unix Epoch
+    let secondsSince1970: UInt32
+
     var data: Data {
         var data = Data(for: .sessionStartTx)
         data.append(startTime)
-        return data
+        data.append(secondsSince1970)
+        return data.appendingCRC()
     }
 }
diff --git a/xDripG5/Messages/SessionStopTxMessage.swift b/xDripG5/Messages/SessionStopTxMessage.swift
index 49051fd5..0c320a57 100644
--- a/xDripG5/Messages/SessionStopTxMessage.swift
+++ b/xDripG5/Messages/SessionStopTxMessage.swift
@@ -17,6 +17,6 @@ struct SessionStopTxMessage: RespondableMessage {
     var data: Data {
         var data = Data(for: .sessionStopTx)
         data.append(stopTime)
-        return data
+        return data.appendingCRC()
     }
 }
diff --git a/xDripG5/Opcode.swift b/xDripG5/Opcode.swift
index 9bd8eab3..31249ed3 100644
--- a/xDripG5/Opcode.swift
+++ b/xDripG5/Opcode.swift
@@ -44,12 +44,12 @@ enum Opcode: UInt8 {
     case calibrationDataRx = 0x33
     case calibrateGlucoseTx = 0x34
     case calibrateGlucoseRx = 0x35
-    
+
     case glucoseHistoryTx = 0x3e
 
     case eraseTx = 0x42
     case eraseRx = 0x43
-    
+
     case transmitterVersionTx = 0x4a
     case transmitterVersionRx = 0x4b
 
diff --git a/xDripG5/Transmitter.swift b/xDripG5/Transmitter.swift
index a073a3db..a49c41bd 100644
--- a/xDripG5/Transmitter.swift
+++ b/xDripG5/Transmitter.swift
@@ -8,6 +8,7 @@
 
 import Foundation
 import CoreBluetooth
+import HealthKit
 import os.log
 
 
@@ -21,6 +22,14 @@ public protocol TransmitterDelegate: class {
     func transmitter(_ transmitter: Transmitter, didReadUnknownData data: Data)
 }
 
+/// These methods are called on a private background queue. It is the responsibility of the client to ensure thread-safety.
+public protocol TransmitterCommandSource: class {
+    func dequeuePendingCommand(for transmitter: Transmitter) -> Command?
+
+    func transmitter(_ transmitter: Transmitter, didFail command: Command, with error: Error)
+
+    func transmitter(_ transmitter: Transmitter, didComplete command: Command)
+}
 
 public enum TransmitterError: Error {
     case authenticationError(String)
@@ -41,6 +50,8 @@ public final class Transmitter: BluetoothManagerDelegate {
 
     public weak var delegate: TransmitterDelegate?
 
+    public weak var commandSource: TransmitterCommandSource?
+
     // MARK: - Passive observation state, confined to `bluetoothManager.managerQueue`
 
     /// The initial activation date of the transmitter
@@ -130,13 +141,36 @@ public final class Transmitter: BluetoothManagerDelegate {
 
                         self.log.info("Bonding request sent. Waiting user to respond.")
                     }
-                    let (timeMessage, activationDate, glucoseMessage) = try peripheral.control(shouldWaitForBond: status.bonded != 0x1)
+
+                    try peripheral.enableNotify(shouldWaitForBond: status.bonded != 0x1)
+                    defer {
+                        peripheral.disconnect()
+                    }
+
+                    let timeMessage = try peripheral.readTimeMessage()
+
+                    let activationDate = Date(timeIntervalSinceNow: -TimeInterval(timeMessage.currentTime))
+
+                    while let command = self.commandSource?.dequeuePendingCommand(for: self) {
+                        do {
+                            _ = try peripheral.sendCommand(command, activationDate: activationDate)
+                            self.commandSource?.transmitter(self, didComplete: command)
+                        } catch let error {
+                            self.commandSource?.transmitter(self, didFail: command, with: error)
+                        }
+                    }
+
+                    let glucoseMessage = try peripheral.readGlucose()
+                    let calibrationMessage = try? peripheral.readCalibrationData()
+
                     let glucose = Glucose(
                         transmitterID: self.id.id,
                         glucoseMessage: glucoseMessage,
                         timeMessage: timeMessage,
+                        calibrationMessage: calibrationMessage,
                         activationDate: activationDate
                     )
+
                     self.delegateQueue.async {
                         self.delegate?.transmitter(self, didRead: glucose)
                     }
@@ -329,7 +363,7 @@ fileprivate extension PeripheralManager {
         }
     }
 
-    fileprivate func control(shouldWaitForBond: Bool = false) throws -> (time: TransmitterTimeRxMessage, activationDate: Date, glucose: GlucoseRxMessage) {
+    fileprivate func enableNotify(shouldWaitForBond: Bool = false) throws {
         do {
             if shouldWaitForBond {
                 try setNotifyValue(true, for: .control, timeout: 15)
@@ -339,32 +373,71 @@ fileprivate extension PeripheralManager {
         } catch let error {
             throw TransmitterError.controlError("Error enabling notification: \(error)")
         }
+    }
 
-        let timeMessage: TransmitterTimeRxMessage
+    fileprivate func readTimeMessage() throws -> TransmitterTimeRxMessage {
         do {
-            timeMessage = try writeMessage(TransmitterTimeTxMessage(), for: .control)
+            return try writeMessage(TransmitterTimeTxMessage(), for: .control)
         } catch let error {
             throw TransmitterError.controlError("Error getting time: \(error)")
         }
+    }
+
+    fileprivate func sendCommand(_ command: Command, activationDate: Date) throws -> TransmitterRxMessage {
+        switch command {
+        case .startSensor(let date):
+            let startTime = UInt32(date.timeIntervalSince(activationDate))
+            let secondsSince1970 = UInt32(date.timeIntervalSince1970)
 
-        let activationDate = Date(timeIntervalSinceNow: -TimeInterval(timeMessage.currentTime))
+            do {
+                return try writeMessage(SessionStartTxMessage(startTime: startTime, secondsSince1970: secondsSince1970), for: .control)
+            } catch let error {
+                throw TransmitterError.controlError("Error starting session: \(error)")
+            }
+        case .stopSensor(let date):
+            let stopTime = UInt32(date.timeIntervalSince(activationDate))
 
-        let glucoseMessage: GlucoseRxMessage
+            do {
+                return try writeMessage(SessionStopTxMessage(stopTime: stopTime), for: .control)
+            } catch let error {
+                throw TransmitterError.controlError("Error stopping session: \(error)")
+            }
+        case .calibrateSensor(let glucose, let date):
+            let unit = HKUnit.milligramsPerDeciliter
+            let glucoseValue = UInt16(glucose.doubleValue(for: unit).rounded())
+            let time = UInt32(date.timeIntervalSince(activationDate))
+
+            do {
+                return try writeMessage(CalibrateGlucoseTxMessage(time: time, glucose: glucoseValue), for: .control)
+            } catch let error {
+                throw TransmitterError.controlError("Error calibrating sensor: \(error)")
+            }
+        }
+
+    }
+
+    fileprivate func readGlucose() throws -> GlucoseRxMessage {
         do {
-            glucoseMessage = try writeMessage(GlucoseTxMessage(), for: .control)
+            return try writeMessage(GlucoseTxMessage(), for: .control)
         } catch let error {
             throw TransmitterError.controlError("Error getting glucose: \(error)")
         }
+    }
 
-        defer {
-            do {
-                try setNotifyValue(false, for: .control)
-                try writeMessage(DisconnectTxMessage(), for: .control)
-            } catch {
-            }
+    fileprivate func readCalibrationData() throws -> CalibrationDataRxMessage {
+        do {
+            return try writeMessage(CalibrationDataTxMessage(), for: .control)
+        } catch let error {
+            throw TransmitterError.controlError("Error getting calibration data: \(error)")
         }
+    }
 
-        return (time: timeMessage, activationDate: activationDate, glucose: glucoseMessage)
+    fileprivate func disconnect() {
+        do {
+            try setNotifyValue(false, for: .control)
+            try writeMessage(DisconnectTxMessage(), for: .control)
+        } catch {
+        }
     }
 
     fileprivate func listenToControl() throws {