[primer] rename Router/REED to Active/Standby Mesh Extender

Update the Thread Primer documentation to transition terminology from
Router and Router Eligible End Device (REED) to Active Mesh Extender
and Standby Mesh Extender, respectively.

- Rename "Router" to "Active Mesh Extender" (or "Mesh Extender" where
  referring generally to the device capability).
- Rename REED (Router Eligible End Device) to Standby Mesh Extender.
- Rename "Router-Router links" to "Active Mesh Extender links".
- Adjust roles, device types, addressing, and MLE attach descriptions
  to use the new terminology.
- Fix grammatical errors (e.g., "a Active" to "an Active") and typos
  introduced during the transition.

Author: jwhui

site/en/guides/images/ot-primer-roles.png

@@ -34,20 +34,22 @@ location in the network topology.
 
 ### How a Routing Locator is generated
 
-All devices are assigned a Router ID and a Child ID. Each Router maintains a
-table of all their Children, the combination of which uniquely identifies a
-device within the topology.  For example, consider the highlighted nodes in the
-following topology, where the number in a Router (pentagon) is the Router ID,
-and the number in an End Device (circle) is the Child ID:
+All devices are assigned a Router ID and a Child ID. Each Parent
+maintains a table of all their Children, the combination of which
+uniquely identifies a device within the topology.  For example,
+consider the highlighted nodes in the following topology, where the
+number in a Parent (pentagon) is the Router ID, and the number in an
+End Device (circle) is the Child ID:
 
 <figure>
 <a href="../images/ot-primer-rloc-topology_2x.png"><img src="../images/ot-primer-rloc-topology.png" srcset="../images/ot-primer-rloc-topology.png 1x, ../images/ot-primer-rloc-topology_2x.png 2x" border="0" width="600" alt="OT RLOC Topology" /></a>
 </figure>
 
-Each Child's Router ID corresponds to their Parent (Router). Because a Router is
-not a Child, the Child ID for a Router is always 0. Together, these values are
-unique for each device in the Thread network, and are used to create the RLOC16,
-which represents the last 16 bits of the RLOC.
+Each Child's Router ID corresponds to its Parent (Active Mesh
+Extender). Because an Active Mesh Extender is not a Child, the Child ID for an
+Active Mesh Extender is always 0. Together, these values are unique for each
+device in the Thread network, and are used to create the RLOC16, which
+represents the last 16 bits of the RLOC.
 
 For example, here's how the RLOC16 is calculated for the upper-left node (Router
 ID = 1 and Child ID = 1):
@@ -81,9 +83,10 @@ This same logic can be used to determine the RLOC for all highlighted nodes in t
 However, because the RLOC is based on the location of the node in the topology,
 the RLOC of a node can change as the topology changes.
 
-For example, perhaps node `0x400` is removed from the Thread network. Nodes
-`0x401` and `0x402` establish new links to different Routers, and as a result
-they are each assigned a new RLOC16 and RLOC:
+For example, perhaps node `0x400` is removed from the Thread
+network. Nodes `0x401` and `0x402` establish new links to different
+Mesh Extenders, and as a result they are each assigned a new RLOC16
+and RLOC:
 
 <figure>
 <a href="../images/ot-primer-rloc-topology-change_2x.png"><img src="../images/ot-primer-rloc-topology-change.png" srcset="../images/ot-primer-rloc-topology-change.png 1x, ../images/ot-primer-rloc-topology-change_2x.png 2x" border="0" width="600" alt="OT Topology after Change" /></a>
@@ -272,7 +275,7 @@ What you've learned:
 *   A Thread device has multiple unicast IPv6 addresses
     *   An RLOC represents a device's location in the Thread network
     *   An ML-EID is unique to a Thread device within a partition and should be used by applications
-*   Thread uses multicast to forward data to groups of nodes and routers
+*   Thread uses multicast to forward data to groups of nodes
 *   Thread uses anycast when the RLOC of a destination is unknown
 
 To learn more about Thread's IPv6 addressing, see sections 5.2 and 5.3 of the
@@ -319,12 +322,12 @@ To learn more about Thread's IPv6 addressing, see sections 5.2 and 5.3 of the
       <div>Incorrect.</div>
     </div>
     <div>
-      <div>The device is a REED.</div>
+      <div>The device is a Standby Mesh Extender.</div>
       <div>Close, but incorrect.</div>
     </div>
     <div correct>
-      <div>The device is a Router.</div>
-      <div>Correct. A Router always has a Child ID of 0.</div>
+      <div>The device is an Active Mesh Extender.</div>
+      <div>Correct. An Active Mesh Extender always has a Child ID of 0.</div>
     </div>
   </devsite-multiple-choice>
 </div>
@@ -339,10 +342,10 @@ To learn more about Thread's IPv6 addressing, see sections 5.2 and 5.3 of the
         network.</div>
     </div>
     <div correct>
-      <div>A router dropped off the network.</div>
-      <div>Correct. When a router drops off a network, the network
-        topology changes, which may result in the device promoting itself to a
-        router and obtaining a new RLOC.</div>
+      <div>An Active Mesh Extender dropped off the network.</div>
+      <div>Correct. When an Active Mesh Extender drops off a network, the network
+        topology changes, which may result in the device promoting itself to an
+        Active Mesh Extender and obtaining a new RLOC.</div>
     </div>
     <div>
       <div>The camera entered sleep mode, which changed the network topology.
@@ -381,7 +384,7 @@ To learn more about Thread's IPv6 addressing, see sections 5.2 and 5.3 of the
 <div>
   <devsite-multiple-choice>
     <div>What type of addressing and routing does Thread use to forward data to
-    groups of nodes and routers?</div>
+    groups of nodes?</div>
     <div>
       <div>unicast</div>
       <div>Incorrect.</div>

site/en/guides/images/ot-primer-roles_2x.png

@@ -24,9 +24,8 @@ When creating a new Thread network, or searching for an existing one to join, a
 Thread device performs an active scan for 802.15.4 networks within radio range:
 
 1.  The device broadcasts an 802.15.4 Beacon Request on a specific Channel.
-1.  In return, any Routers or Router Eligible End Devices (REEDs) in range
-    broadcast a Beacon that contains their Thread network PAN ID, XPAN ID, and
-    Network Name.
+1.  In return, any Mesh Extenders in range broadcast a Beacon that contains
+    their Thread network PAN ID, XPAN ID, and Network Name.
 1.  The device repeats the previous two steps for each Channel.
 
 Once a Thread device has discovered all networks in range, it can either attach
@@ -62,11 +61,12 @@ device has already been commissioned.
 
 ## Create a new network
 
-If the device elects to create a new network, it selects the least busy Channel
-and a PAN ID not in use by other networks, then becomes a Router and elects
-itself the Leader. This device sends MLE Advertisement messages to other
-802.15.4 devices to inform them of its link state, and responds to Beacon
-Requests by other Thread devices performing an active scan.
+If the device elects to create a new network, it selects the least
+busy Channel and a PAN ID not in use by other networks, then becomes
+an Active Mesh Extender and elects itself the Leader. This device
+sends MLE Advertisement messages to other 802.15.4 devices to inform
+them of its link state, and responds to Beacon Requests by other
+Thread devices performing an active scan.
 
 ## Join an existing network
 
@@ -75,13 +75,13 @@ ID, XPAN ID, and Network Name to match that of the target network via Thread
 Commissioning, then goes through the MLE Attach process to attach as a Child
 (End Device). This process is used for Child-Parent links.
 
-Key Point: Every device, router-capable or not, initially attaches to a Thread
+Key Point: Every device, Mesh Extender or not, initially attaches to a Thread
 network as a Child (End Device).
 
 1.  The Child sends a multicast [Parent Request](#1_parent_request) to all
-    neighboring Routers and REEDs in the target network.
-1.  All neighboring Routers and REEDs (if the Parent Request Scan Mask includes
-    REEDs) send [Parent Responses](#2_parent_response) with information about
+    neighboring Mesh Extenders in the target network.
+1.  All neighboring Mesh Extenders (if the Parent Request Scan Mask includes
+    Standby Mesh Extenders) send [Parent Responses](#2_parent_response) with information about
     themselves.
 1.  The Child chooses a Parent device and sends a [Child ID
     Request](#3_child_id_request) to it.
@@ -91,8 +91,7 @@ network as a Child (End Device).
 ### 1. Parent Request
 
 A Parent Request is a multicast request from the attaching device that is used
-to discover neighboring Routers and Router Eligible End Devices (REEDs) in the
-target network.
+to discover neighboring Mesh Extenders in the target network.
 
 <figure>
 <a href="../images/ot-primer-network-mle-attach-01.png"><img src="../images/ot-primer-network-mle-attach-01.png" width="350" border="0" alt="OT MLE Attach Parent Request" /></a>
@@ -113,15 +112,15 @@ target network.
     </tr>
     <tr>
       <td width="25%"><b>Scan Mask</b></td>
-      <td>Limits the request to only Routers or to both Routers and REEDs</td>
+      <td>Limits the request to only Active Mesh Extenders or to all Mesh Extenders</td>
     </tr>
   </tbody>
 </table>
 
 ### 2. Parent Response
 
 A Parent Response is a unicast response to a Parent Request that provides
-information about a Router or REED to the attaching device.
+information about a Mesh Extender to the attaching device.
 
 <figure>
 <a href="../images/ot-primer-network-mle-attach-02.png"><img src="../images/ot-primer-network-mle-attach-02.png" width="350" border="0" alt="OT MLE Attach Parent Response" /></a>
@@ -143,31 +142,31 @@ information about a Router or REED to the attaching device.
     <tr>
       <td width="25%"><b>Link Frame
         Counter</b></td>
-      <td>802.15.4 Frame Counter on the Router/REED</td>
+      <td>802.15.4 Frame Counter on the Mesh Extender</td>
     </tr>
     <tr>
       <td width="25%"><b>MLE Frame
         Counter</b></td>
-      <td>MLE Frame Counter on the Router/REED</td>
+      <td>MLE Frame Counter on the Mesh Extender</td>
     </tr>
     <tr>
       <td width="25%"><b>Source
         Address</b></td>
-      <td>RLOC16 of the Router/REED</td>
+      <td>RLOC16 of the Mesh Extender</td>
     </tr>
     <tr>
       <td width="25%"><b>Link
         Margin</b></td>
-      <td>Receive signal quality of the Router/REED</td>
+      <td>Receive signal quality of the Mesh Extender</td>
     </tr>
     <tr>
       <td width="25%"><b>Connectivity</b></td>
-      <td>Describes the Router/REED’s level of connectivity</td>
+      <td>Describes the Mesh Extender’s level of connectivity</td>
     </tr>
     <tr>
       <td width="25%"><b>Leader
         Data</b></td>
-      <td>Information about the Router/REED’s Leader</td>
+      <td>Information about the Mesh Extender’s Leader</td>
     </tr>
     <tr>
       <td width="25%"><b>Challenge</b></td>
@@ -179,11 +178,11 @@ information about a Router or REED to the attaching device.
 
 ### 3. Child ID Request
 
-A Child ID Request is a unicast request from the attaching device (Child) that
-is sent to the Router or REED (Parent) for the purpose of establishing a
-Child-Parent link. If the request is sent to a REED, it [upgrades itself to a
-Router](router-selection.md) before
-accepting the request.
+A Child ID Request is a unicast request from the attaching device
+(Child) that is sent to the Mesh Extender for the purpose of
+establishing a Child-Parent link. If the request is sent to a Standby
+Mesh Extender, it [upgrades itself to an Active Mesh
+Extender](router-selection.md) before accepting the request.
 
 <figure>
 <a href="../images/ot-primer-network-mle-attach-03.png"><img src="../images/ot-primer-network-mle-attach-03.png" width="350" border="0" alt="OT MLE Attach Child ID Request" /></a>
@@ -264,7 +263,7 @@ Child to confirm that a Child-Parent link has been established.
     </tr>
     <tr>
       <td width="25%"><b>Route
-        (REED only)</b></td>
+        (to FTDs only)</b></td>
       <td>Route propagation</td>
     </tr>
     <tr>
@@ -323,19 +322,18 @@ What you've learned:
   <devsite-multiple-choice>
     <div>What is a Parent Request used for?</div>
     <div correct>
-      <div>To discover neighboring Routers and Router Eligible End Devices
-      (REEDs) in the target network.</div>
+      <div>To discover neighboring Mesh Extenders in the target network.</div>
       <div>Correct. A Parent Request is issued by a device seeking to attach to
       a network.</div>
       </div>
     <div>
-      <div>To announce that a Router is becoming a parent.</div>
-      <div>Incorrect. A Router does not initiate a Parent-Child relationship
-      with another network device. Instead, a network device selects a Router
-      to become its Child.</div>
+      <div>To announce that a Mesh Extender is becoming a parent.</div>
+      <div>Incorrect. A Parent does not initiate a Parent-Child relationship
+      with another network device. Instead, a network device selects a Mesh Extender
+      to become its Parent.</div>    
     </div>
     <div>
-      <div>To request that a Router Eligible End Device be promoted to a Router.
+      <div>To request that a Standby Mesh Extender be promoted to an Active Mesh Extender.
       </div>
       <div>Incorrect.</div>
     </div>
@@ -378,7 +376,7 @@ What you've learned:
       <div>Incorrect.</div>
     </div>
     <div>
-      <div>REED (Router-Eligible End Device) </div>
+      <div>Active Mesh Extender</div>
       <div>Incorrect.</div>
     </div>
   </devsite-multiple-choice>