SCHC over Sigfox LPWAN

3.1. Network Architecture

Figure 1 represents the architecture for compression/decompression (C/D) and fragmentation/reassembly (F/R) based on the terminology defined in [RFC8376], where the Radio Gateway (RG) is a Sigfox Base Station and the Network Gateway (NGW) is the Sigfox cloud-based Network.¶

  Sigfox Device                                           Application
+----------------+                                     +--------------+
| APP1 APP2 APP3 |                                     |APP1 APP2 APP3|
+----------------+                                     +--------------+
|   UDP  |       |                                     |     |  UDP   |
|  IPv6  |       |                                     |     | IPv6   |
+--------+       |                                     |     +--------+
| SCHC C/D & F/R |                                     |              |
|                |                                     |              |
+-------+--------+                                     +--------+-----+
        $                                                       .
        $   +---------+     +--------------+     +---------+    .
        $   |         |     |              |     | Network |    .
        +~~ |Sigfox BS|     |Sigfox Network|     |  SCHC   |    .
            |  (RG)   | === |    (NGW)     | === |C/D & F/R|.....
            +---------+     +--------------+     +---------+

In the case of the global Sigfox Network, RGs (or Base Stations) are distributed over multiple countries wherever the Sigfox LPWAN service is provided. The NGW (or cloud-based Sigfox Core Network) is a single entity that connects to all Sigfox base stations in the world, providing hence a global single star network topology.¶

The Device sends application flows that are compressed and/or fragmented by a SCHC Compressor/Decompressor (SCHC C/D + F/R) to reduce headers size and/or fragment the packet. The resulting SCHC Message is sent over a layer two (L2) Sigfox frame to the Sigfox Base Stations, which then forward the SCHC Message to the Network Gateway (NGW). The NGW then delivers the SCHC Message and associated gathered metadata to the Network SCHC C/D + F/R.¶

The Sigfox Network (NGW) communicates with the Network SCHC C/D + F/R for compression/decompression and/or for fragmentation/reassembly. The Network SCHC C/D + F/R shares the same set of rules as the Dev SCHC C/D + F/R. The Network SCHC C/D + F/R can be collocated with the NGW or it could be located in a different place, as long as a tunnel or secured communication is established between the NGW and the SCHC C/D + F/R functions. After decompression and/or reassembly, the packet can be forwarded over the Internet to one (or several) LPWAN Application Server(s) (App).¶

The SCHC C/D + F/R processes are bidirectional, so the same principles are applicable on both uplink (UL) and downlink (DL).¶

3.2. Uplink

Uplink Sigfox transmissions occur in repetitions over different times and frequencies. Besides time and frequency diversities, the Sigfox network also provides space diversity, as potentially an uplink message will be received by several base stations.¶

Since all messages are self-contained and base stations forward all these messages back to the same Sigfox Network, multiple input copies can be combined at the NGW providing for extra reliability based on the triple diversity (i.e., time, space and frequency).¶

A detailed description of the Sigfox Radio Protocol can be found in [sigfox-spec].¶

Messages sent from the Device to the Network are delivered by the Sigfox network (NGW) to the Network SCHC C/D + F/R through a callback/API with the following information:¶

• Device ID¶
• Message Sequence Number¶
• Message Payload¶
• Message Timestamp¶
• Device Geolocation (optional)¶
• RSSI (optional)¶
• Device Temperature (optional)¶
• Device Battery Voltage (optional)¶

The Device ID is a globally unique identifier assigned to the Device, which is included in the Sigfox header of every message. The Message Sequence Number is a monotonically increasing number identifying the specific transmission of this uplink message, and it is also part of the Sigfox header. The Message Payload corresponds to the payload that the Device has sent in the uplink transmission.¶

The Message Timestamp, Device Geolocation, RSSI, Device Temperature and Device Battery Voltage are metadata parameters provided by the Network.¶

A detailed description of the Sigfox callbacks/APIs can be found in [sigfox-callbacks].¶

Only messages that have passed the L2 Cyclic Redundancy Check (CRC) at network reception are delivered by the Sigfox Network to the Network SCHC C/D + F/R.¶

               +---------------+-----------------+
               | Sigfox Header | Sigfox payload  |
               +---------------+---------------- +
                               |   SCHC message  |
                               +-----------------+

Figure 2 shows a SCHC Message sent over Sigfox, where the SCHC Message could be a full SCHC Packet (e.g. compressed) or a SCHC Fragment (e.g. a piece of a bigger SCHC Packet).¶

3.3. Downlink

Downlink transmissions are Device-driven and can only take place following an uplink communication that so indicates. Hence, a Device explicitly indicates its intention to receive a downlink message using a downlink request flag when sending the preceding uplink message to the network. After completing the uplink transmission, the Device opens a fixed window for downlink reception. The delay and duration of the reception opportunity window have fixed values. If there is a downlink message to be sent for this given Device (e.g. either a response to the uplink message or queued information waiting to be transmitted), the network transmits this message to the Device during the reception window. If no message is received by the Device after the reception opportunity window has elapsed, the Device closes the reception window opportunity and gets back to the normal mode (e.g., continue UL transmissions, sleep, stand-by, etc.)¶

When a downlink message is sent to a Device, a reception acknowledgement is generated by the Device and sent back to the Network through the Sigfox radio protocol and reported in the Sigfox Network backend.¶

A detailed description of the Sigfox Radio Protocol can be found in [sigfox-spec] and a detailed description of the Sigfox callbacks/APIs can be found in [sigfox-callbacks].¶

3.4. SCHC-ACK on Downlink

As explained previously, downlink transmissions are Device-driven and can only take place following a specific uplink transmission that indicates and allows a following downlink opportunity. For this reason, when SCHC bi-directional services are used (e.g. Ack-on-Error fragmentation mode) the SCHC protocol implementation needs to consider the times when a downlink message (e.g. SCHC-ACK) can be sent and/or received.¶

For the UL ACK-on-Error fragmentation mode, a DL opportunity MUST be indicated by the last fragment of every window (i.e. FCN = All-0, or FCN = All-1). The Device sends the fragments in sequence and, after transmitting the FCN = All-0 or FCN = All-1, it opens up a reception opportunity. The Network SCHC can then decide to respond at that opportunity (or wait for a further one) with a SCHC-ACK indicating in case there are missing fragments from the current or previous windows. If there is no SCHC-ACK to be sent, or if the network decides to wait for a further DL transmission opportunity, then no DL transmission takes place at that opportunity and after a timeout the UL transmissions continue. Intermediate SCHC fragments with FCN different from All-0 or All-1 MUST NOT use the DL request flag to request a SCHC-ACK.¶

3.5. SCHC Rules

The RuleID MUST be included in the SCHC header. The total number of rules to be used affects directly the Rule ID field size, and therefore the total size of the fragmentation header. For this reason, it is recommended to keep the number of rules that are defined for a specific device to the minimum possible.¶

RuleIDs can be used to differentiate data traffic classes (e.g. QoS, control vs. data, etc.), and data sessions. They can also be used to interleave simultaneous fragmentation sessions between a Device and the Network.¶

3.6. Fragmentation

The SCHC specification [RFC8724] defines a generic fragmentation functionality that allows sending data packets or files larger than the maximum size of a Sigfox payload. The functionality also defines a mechanism to send reliably multiple messages, by allowing to resend selectively any lost fragments.¶

The SCHC fragmentation supports several modes of operation. These modes have different advantages and disadvantages depending on the specifics of the underlying LPWAN technology and application Use Case. This section describes how the SCHC fragmentation functionality should optimally be implemented when used over a Sigfox LPWAN for the most typical Use Case applications.¶

As described in section 8.2.3 of [RFC8724], the integrity of the fragmentation-reassembly process of a SCHC Packet MUST be checked at the receive end. Since only UL messages/fragments that have passed the Sigfox CRC-check are delivered to the Network SCHC C/D + F/R, integrity can be guaranteed when no consecutive messages are missing from the sequence and all FCN bitmaps are complete. With this functionality in mind, and in order to save protocol and processing overhead, the use of a Reassembly Check Sequence (RCS) as described in Section 3.6.1.3 is RECOMMENDED.¶

The L2 Word Size used by Sigfox is 1 byte (8 bits).¶

3.7. SCHC-over-Sigfox F/R Message Formats

This section depicts the different formats of SCHC Fragment, SCHC ACK (including the SCHC Compound ACK defined in [I-D.ietf-lpwan-schc-compound-ack]), and SCHC Abort used in SCHC over Sigfox.¶

           |- SCHC Fragment Header -|
           +------------------------+---------+
           |   RuleID   |    FCN    | Payload |
           +------------+-----------+---------+
           |   3 bits   |  5 bits   | 88 bits |

       |--------  SCHC Fragment Header -------|
       +--------------------------------------+--------------+
       | RuleID | FCN=ALL-1 |  RCS   |  b'000 |   Payload    |
       +--------+-----------+--------+--------+--------------+
       | 3 bits |  5 bits   | 5 bits | 3 bits | 0 to 80 bits |

                Sender-Abort
           |------ Header ------|
           +--------------------+
           | RuleID | FCN=ALL-1 |
           +--------+-----------+
           | 3 bits |  5 bits   |

             |-- SCHC Fragment Header --|
             +--------------------------+---------+
             | RuleID |   W    |  FCN   | Payload |
             +--------+--------+--------+---------+
             | 3 bits | 2 bits | 3 bits | 88 bits |

  |-------------  SCHC Fragment Header -----------|
  +-----------------------------------------------+--------------+
  | RuleID |   W    | FCN=ALL-1 |  RCS   |b'00000 |   Payload    |
  +--------+--------+-----------+--------+--------+--------------+
  | 3 bits | 2 bits |  3 bits   | 3 bits | 5 bits | 0 to 80 bits |

               |---- SCHC ACK Header ----|
               +-------------------------+---------+
               | RuleID |    W   | C=b'1 | b'0-pad |
               +--------+--------+-------+---------+
               | 3 bits | 2 bits | 1 bit | 58 bits |

|--- SCHC ACK Header ---|- W=w1 -|...|----- W=wi ------|
+------+--------+-------+--------+...+--------+--------+------+-------+
|RuleID| W=b'w1 | C=b'0 | Bitmap |...| W=b'wi | Bitmap |  00  |b'0-pad|
+------+--------+-------+--------+...+--------+--------+------+-------+
|3 bits| 2 bits | 1 bit | 7 bits |...| 2 bits | 7 bits |2 bits|

     Losses are found in windows W = w1,...,wi; where w1<w2<...<wi

                |---- Sender-Abort Header ----|
                +-----------------------------+
                | RuleID | W=b'11 | FCN=ALL-1 |
                +--------+--------+-----------+
                | 3 bits | 2 bits |  3 bits   |

   |- Receiver-Abort Header -|
   +---------------------------------+-----------------+---------+
   | RuleID | W=b'11 | C=b'1 |  b'11 |  0xFF (all 1's) | b'0-pad |
   +--------+--------+-------+-------+-----------------+---------+
   | 3 bits | 2 bits | 1 bit | 2 bit |  8 bit          | 48 bits |
             next L2 Word boundary ->| <-- L2 Word --> |

        |------- SCHC Fragment Header ------|
        +-----------------------------------+---------+
        | RuleID |    W   |  FCN   |  0000  | Payload |
        +--------+--------+--------+--------+---------+
        | 6 bits | 2 bits | 4 bits | 4 bits | 80 bits |

        |--------- SCHC Fragment Header -------|
        +--------------------------------------+--------------+
        | RuleID |    W   | FCN=ALL-1 |  RCS   |    Payload   |
        +--------+--------+-----------+--------+--------------+
        | 6 bits | 2 bits |  4 bits   | 4 bits | 8 to 80 bits |

          |---- SCHC ACK Header ----|
          +-------------------------+---------+
          | RuleID |    W   | C=b'1 | b'0-pad |
          +--------+--------+-------+---------+
          | 6 bits | 2 bits | 1 bit | 55 bits |

|--- SCHC ACK Header ---|- W=w1 -|...|---- W=wi -----|
+--------+------+-------+--------+...+------+--------+------+-------+
| RuleID |W=b'w1| C=b'0 | Bitmap |...|W=b'wi| Bitmap |  00  |b'0-pad|
+--------+------+-------+--------+...+------+--------+------+-------+
| 6 bits |2 bits| 1 bit | 12 bits|...|2 bits| 12 bits|2 bits|

   Losses are found in windows W = w1,...,wi; where w1<w2<...<wi

    |- SCHC ACK Header -|-W=00 -|      |-W=01 -|...
    +------+------+-----+-------+------+-------+...
    |RuleID| W=00 |C=b'0|Bitmap | W=01 |Bitmap |...
    +------+------+-----+-------+------+-------+...
    |6 bits|2 bits|1 bit|12 bits|2 bits|12 bits|...

                ...       |-W=10 -|      |-W=11 -|
                ...+------+-------+------+-------+-+
                ...| W=10 |Bitmap | W=11 |Bitmap |0|
                ...+------+-------+------+-------+-+
                ...|2 bits|12 bits|2 bits|12 bits|

    Losses are found in windows W = w1,...,wi; where w1<w2<...<wi

        |---- Sender-Abort Header ----|
        +-----------------------------+
        | RuleID |   W    | FCN=ALL-1 |
        +--------+--------+-----------+
        | 6 bits | 2 bits |  4 bits   |

   |- Receiver-Abort Header -|
   +---------------------------------+-----------------+---------+
   | RuleID | W=b'11 | C=b'1 |  0x7F |  0xFF (all 1's) | b'0-pad |
   +--------+--------+-------+-------+-----------------+---------+
   | 6 bits | 2 bits | 1 bit | 7 bit |  8 bit          | 40 bits |
             next L2 Word boundary ->| <-- L2 Word --> |

           |-- SCHC Fragment Header --|
           +--------------------------+---------+
           | RuleID |   W    | FCN    | Payload |
           +--------+--------+--------+---------+
           | 8 bits | 3 bits | 5 bits | 80 bits |

 |-------------- SCHC Fragment Header -----------|
 +-----------------------------------------------+--------------+
 | RuleID |    W   | FCN=ALL-1 |  RCS   |  000   |    Payload   |
 +--------+--------+-----------+--------+--------+--------------+
 | 8 bits | 3 bits |  5 bits   | 5 bits | 3 bits | 8 to 72 bits |

            |---- SCHC ACK Header ----|
            +-------------------------+---------+
            | RuleID |    W   | C=b'1 | b'0-pad |
            +--------+--------+-------+---------+
            | 8 bits | 3 bits | 1 bit | 52 bits |

|-- SCHC ACK Header --|- W=w1 -|...|---- W=wi -----|
+------+------+-------+--------+...+------+--------+------+-------+
|RuleID|W=b'w1| C=b'0 | Bitmap |...|W=b'wi| Bitmap | 000  |b'0-pad|
+------+------+-------+--------+...+------+--------+------+-------+
|8 bits|3 bits| 1 bit | 31 bits|...|3 bits| 31 bits|3 bits|

     Losses are found in windows W = w1,...,wi; where w1<w2<...<wi

           |---- Sender-Abort Header ----|
           +-----------------------------+
           | RuleID |   W    | FCN=ALL-1 |
           +--------+--------+-----------+
           | 8 bits | 3 bits |  5 bits   |

  |-- Receiver-Abort Header -|
  +-----------------------------------+-----------------+---------+
  | RuleID | W=b'111 | C=b'1 | b'1111 |  0xFF (all 1's) | b'0-pad |
  +--------+---------+-------+--------+-----------------+---------+
  | 8 bits |  3 bits | 1 bit | 4 bit  |  8 bit          | 40 bits |
              next L2 Word boundary ->| <-- L2 Word --> |

                   SCHC Fragment
                |--    Header   --|
                +-----------------+---------+
                | RuleID |  FCN   | Payload |
                +--------+--------+---------+
                | 3 bits | 5 bits | 56 bits |

 |--------- SCHC Fragment Header -------|
 +--------------------------------------+--------------+
 | RuleID | FCN=ALL-1 |  RCS   |  000   |    Payload   |
 +--------+-----------+--------+--------+--------------+
 | 3 bits |  5 bits   | 5 bits | 3 bits | 0 to 48 bits |

                 SCHC ACK
            |--   Header   --|
            +----------------+---------+
            | RuleID | C=b'1 | b'0-pad |
            +--------+-------+---------+
            | 3 bits | 1 bit |  4 bits |

  |---- SCHC ACK Header ----|
  +--------+-------+--------+---------+
  | RuleID | C=b'0 | Bitmap | b'0-pad |
  +--------+-------+--------+---------+
  | 3 bits | 1 bit | 31 bits|  5 bits |

                Sender-Abort
           |----   Header   ----|
           +--------------------+
           | RuleID | FCN=ALL-1 |
           +--------+-----------+
           | 3 bits |  5 bits   |

      Receiver-Abort
    |---  Header  ---|
    +----------------+--------+-----------------+
    | RuleID | C=b'1 | b'1111 |  0xFF (all 1's) |
    +--------+-------+--------+-----------------+
    | 3 bits | 1 bit | 4 bit  |  8 bit          |

3.8. SCHC-Sender Abort

• As defined in [RFC8724], a SCHC-Sender Abort can be triggered when the number of SCHC ACK REQ attempts is greater than or equal to MAX_ACK_REQUESTS. In the case of SCHC/Sigfox, a SCHC-Sender Abort MUST be sent if the number of repeated All-1s (i.e., with the same bitmap) sent in sequence is greater than or equal to MAX_ACK_REQUESTS.¶

3.9. SCHC-Receiver Abort

• As defined in [RFC8724], a SCHC-Receiver Abort is triggered when the receiver has no RuleID and DTag pairs available for a new session. In the case of SCHC/Sigfox a SCHC-Receiver Abort MUST be sent if, for a single device, all the RuleIDs are being processed by the receiver (i.e., have an active session) at a certain time and a new one is requested, or if the RuleID of the fragment is not valid.¶
• A SCHC-Receiver Abort MUST be triggered when the Inactivity Timer expires.¶
• MAX_ACK_REQUESTS can be increase when facing high error rates.¶
• Although a SCHC-Receiver Abort can be triggered at any point in time, a SCHC-Receiver Abort downlink message MUST only be sent when there is a downlink transmission opportunity.¶

3.10. Padding

The Sigfox payload fields have different characteristics in uplink and downlink.¶

Uplink frames can contain a payload size from 0 to 12 bytes. The Sigfox radio protocol allows sending zero bits, one single bit of information for binary applications (e.g. status), or an integer number of bytes. Therefore, for 2 or more bits of payload it is required to add padding to the next integer number of bytes. The reason for this flexibility is to optimize transmission time and hence save battery consumption at the device.¶

Downlink frames on the other hand have a fixed length. The payload length MUST be 64 bits (i.e. 8 bytes). Hence, if less information bits are to be transmitted, padding MUST be used with bits equal to 0.¶

4.1. Uplink No-ACK Examples

The FCN field indicates the size of the data packet. The first fragment is marked with FCN = X-1, where X is the number of fragments the message is split into. All fragments are marked with decreasing FCN values. Last packet fragment is marked with the FCN = All-1 (1111).¶

Case No losses - All fragments are sent and received successfully.¶

        Sender                              Receiver
          |-------FCN=6 (00110), Seq=1-------->|
          |-------FCN=5 (00101), Seq=2-------->|
          |-------FCN=4 (00100), Seq=3-------->|
          |-------FCN=3 (00011), Seq=4-------->|
          |-------FCN=2 (00010), Seq=5-------->|
          |-------FCN=1 (00001), Seq=6-------->|
          |-------FCN=31 (11111), Seq=7------->| All fragments received
        (End)

When the first SCHC fragment is received, the Receiver can calculate the total number of SCHC fragments that the SCHC Packet is composed of. For example, if the first fragment is numbered with FCN=6, the receiver can expect six more messages/fragments (i.e., with FCN going from 5 downwards, and the last fragment with a FCN equal to 15).¶

Case losses on any fragment except the first.¶

        Sender                      Receiver
          |-------FCN=6, Seq=1-------->|
          |-------FCN=5, Seq=2----X--->|
          |-------FCN=4, Seq=3-------->|
          |-------FCN=3, Seq=4-------->|
          |-------FCN=2, Seq=5-------->|
          |-------FCN=1, Seq=6-------->|
          |-------FCN=31, Seq=7------->| Missing Fragment - Unable to reassemble
        (End)

4.2. Uplink ACK-on-Error Examples: Single-byte SCHC Header

The single-byte SCHC header ACK-on-Error mode allows sending up to 28 fragments and packet sizes up to 300 bytes. The SCHC fragments may be delivered asynchronously and DL ACK can be sent opportunistically.¶

Case No losses¶

The downlink flag must be enabled in the sender UL message to allow a DL message from the receiver. The DL Enable in the figures shows where the sender should enable the downlink, and wait for an ACK.¶

        Sender                      Receiver
          |-----W=0, FCN=6, Seq=1----->|
          |-----W=0, FCN=5, Seq=2----->|
          |-----W=0, FCN=4, Seq=3----->|
          |-----W=0, FCN=3, Seq=4----->|
          |-----W=0, FCN=2, Seq=5----->|
          |-----W=0, FCN=1, Seq=6----->|
DL Enable |-----W=0, FCN=0, Seq=7----->|
      (no ACK)
          |-----W=1, FCN=6, Seq=8----->|
          |-----W=1, FCN=5, Seq=9----->|
          |-----W=1, FCN=4, Seq=10---->|
DL Enable |-----W=1, FCN=7, Seq=11---->| All fragments received
          |<------ ACK, W=1, C=1 ------| C=1
        (End)

Case Fragment losses in first window¶

In this case, fragments are lost in the first window (W=0). After the first All-0 message arrives, the Receiver leverages the opportunity and sends a SCHC ACK with the corresponding bitmap and C=0.¶

After the loss fragments from the first window (W=0) are resent, the sender continues transmitting the fragments of the following window (W=1) without opening a reception opportunity. Finally, the All-1 fragment is sent, the downlink is enabled, and the SCHC ACK is received with C=1.¶

        Sender                      Receiver
          |-----W=0, FCN=6, Seq=1----->|
          |-----W=0, FCN=5, Seq=2--X-->|
          |-----W=0, FCN=4, Seq=3----->|
          |-----W=0, FCN=3, Seq=4----->|
          |-----W=0, FCN=2, Seq=5--X-->|
          |-----W=0, FCN=1, Seq=6----->|
DL Enable |-----W=0, FCN=0, Seq=7----->| Missing Fragments W=0 => FCN=5, Seq=2 and FCN=2, Seq=5
          |<------ ACK, W=0, C=0 ------| Bitmap:1011011
          |-----W=0, FCN=5, Seq=8----->|
          |-----W=0, FCN=2, Seq=9----->|
          |-----W=1, FCN=6, Seq=10---->|
          |-----W=1, FCN=5, Seq=11---->|
          |-----W=1, FCN=4, Seq=12---->|
DL Enable |-----W=1, FCN=7, Seq=13---->| All fragments received
          |<------ ACK, W=1, C=1 ------| C=1
        (End)

Case Fragment All-0 lost in first window (W=0)¶

In this example, the All-0 of the first window (W=0) is lost. Therefore, the Receiver waits for the next All-0 message of intermediate windows, or All-1 message of last window to generate the corresponding SCHC ACK, notifying the absence of the All-0 of window 0.¶

The sender resends the missing All-0 messages (with any other missing fragment from window 0) without opening a reception opportunity.¶

        Sender                      Receiver
          |-----W=0, FCN=6, Seq=1----->|
          |-----W=0, FCN=5, Seq=2----->|
          |-----W=0, FCN=4, Seq=3----->|
          |-----W=0, FCN=3, Seq=4----->|
          |-----W=0, FCN=2, Seq=5----->|
          |-----W=0, FCN=1, Seq=6----->| DL Enable
                  |-----W=0, FCN=0, Seq=7--X-->|
      (no ACK)
          |-----W=1, FCN=6, Seq=8----->|
          |-----W=1, FCN=5, Seq=9----->|
          |-----W=1, FCN=4, Seq=10---->|
DL Enable |-----W=1, FCN=7, Seq=11---->| Missing Fragment W=0, FCN=0, Seq=7
          |<------ ACK, W=0, C=0 ------| Bitmap:1111110
          |-----W=0, FCN=0, Seq=13---->| All fragments received
DL Enable |-----W=1, FCN=7, Seq=14---->|
          |<------ ACK, W=1, C=1 ------| C=1
        (End)

In the following diagram, besides the All-0 there are other fragment losses in the first window (W=0).¶

        Sender                      Receiver
          |-----W=0, FCN=6, Seq=1----->|
          |-----W=0, FCN=5, Seq=2--X-->|
          |-----W=0, FCN=4, Seq=3----->|
          |-----W=0, FCN=3, Seq=4--X-->|
          |-----W=0, FCN=2, Seq=5----->|
          |-----W=0, FCN=1, Seq=6----->|
DL Enable |-----W=0, FCN=0, Seq=7--X-->|
      (no ACK)
          |-----W=1, FCN=6, Seq=8----->|
          |-----W=1, FCN=5, Seq=9----->|
          |-----W=1, FCN=4, Seq=10---->|
DL Enable |-----W=1, FCN=7, Seq=11---->| Missing Fragment W=0 => FCN= 5, 3 and 0
          |<------ ACK, W=0, C=0 ------| Bitmap:1010110
          |-----W=0, FCN=5, Seq=13---->|
          |-----W=0, FCN=3, Seq=14---->|
          |-----W=0, FCN=0, Seq=15---->| All fragments received
DL Enable |-----W=1, FCN=7, Seq=16---->|
          |<------ ACK, W=1, C=1 ------| C=1
        (End)

In the next examples, there are fragment losses in both the first (W=0) and second (W=1) windows. The retransmission cycles after the All-1 is sent (i.e., not in intermediate windows) should always finish with an All-1, as it serves as an ACK Request message to confirm the correct reception of the retransmitted fragments.¶

        Sender                            Receiver
          |-----W=0, FCN=6 (110), Seq=1----->|
          |-----W=0, FCN=5 (101), Seq=2--X-->|
          |-----W=0, FCN=4 (100), Seq=3----->|
          |-----W=0, FCN=3 (011), Seq=4--X-->|
          |-----W=0, FCN=2 (010), Seq=5----->|
          |-----W=0, FCN=1 (001), Seq=6----->|
DL enable |-----W=0, FCN=0 (000), Seq=7--X-->|
     (no ACK)
          |-----W=1, FCN=6 (110), Seq=8--X-->|
          |-----W=1, FCN=5 (101), Seq=9----->|
          |-----W=1, FCN=4 (011), Seq=10-X-->|
DL enable |-----W=1, FCN=7 (111), Seq=11---->| Missing Fragment W=0 => FCN= 5, 3 and 0, W=1 => FCN= 6 and 4
          |<--- Compound ACK, W=0,1, C=0 ----| Bitmap W=0:1010110, Bitmap W=1:0100001
          |-----W=0, FCN=5 (101), Seq=13---->|
          |-----W=0, FCN=3 (011), Seq=14---->|
          |-----W=0, FCN=0 (000), Seq=15---->|
          |-----W=1, FCN=6 (110), Seq=16---->|
          |-----W=1, FCN=4 (011), Seq=17---->| All fragments received
DL enable |-----W=1, FCN=7 (111), Seq=18---->|
          |<--------- ACK, W=1, C=1 ---------| C=1
        (End)

Similar case as above, but with fewer fragments in the second window (W=1)¶

        Sender                            Receiver
          |-----W=0, FCN=6 (110), Seq=1----->|
          |-----W=0, FCN=5 (101), Seq=2--X-->|
          |-----W=0, FCN=4 (100), Seq=3----->|
          |-----W=0, FCN=3 (011), Seq=4--X-->|
          |-----W=0, FCN=2 (010), Seq=5----->|
          |-----W=0, FCN=1 (001), Seq=6----->|
DL enable |-----W=0, FCN=0 (000), Seq=7--X-->|
       (no ACK)
          |-----W=1, FCN=6 (110), Seq=8--X-->|
DL enable |-----W=1, FCN=7 (111), Seq=9----->| Missing Fragment W=0 => FCN= 5, 3 and 0, W=1 => FCN= 6 and 4
          |<--- Compound ACK, W=0,1, C=0 ----| Bitmap W=0:1010110, Bitmap W=1:0000001
          |-----W=0, FCN=5 (101), Seq=10---->|
          |-----W=0, FCN=3 (011), Seq=11---->|
          |-----W=0, FCN=0 (000), Seq=12---->|
          |-----W=1, FCN=6 (110), Seq=13---->| All fragments received
DL enable |-----W=1, FCN=7 (111), Seq=14---->|
          |<--------- ACK, W=1, C=1 ---------| C=1
        (End)

Case SCHC ACK is lost¶

SCHC over Sigfox does not implement the SCHC ACK REQ message. Instead, it uses the SCHC All-1 message to request a SCHC ACK, when required.¶

       Sender                      Receiver
          |-----W=0, FCN=6, Seq=1----->|
          |-----W=0, FCN=5, Seq=2----->|
          |-----W=0, FCN=4, Seq=3----->|
          |-----W=0, FCN=3, Seq=4----->|
          |-----W=0, FCN=2, Seq=5----->|
          |-----W=0, FCN=1, Seq=6----->|
DL Enable |-----W=0, FCN=0, Seq=7----->|
      (no ACK)
          |-----W=1, FCN=6, Seq=8----->|
          |-----W=1, FCN=5, Seq=9----->|
          |-----W=1, FCN=4, Seq=10---->|
DL Enable |-----W=1, FCN=7, Seq=11---->| All fragments received
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=13---->| RESEND ACK
          |<------ ACK, W=1, C=1 ------| C=1
        (End)

Case SCHC Compound ACK at the end¶

In this example, SCHC Fragment losses are found in both window 0 and 1. However, the sender does not send a SCHC ACK after the All-0 of window 0. Instead, it sends a SCHC Compound ACK notifying losses of both windows.¶

        Sender                            Receiver
          |-----W=0, FCN=6 (110), Seq=1----->|
          |-----W=0, FCN=5 (101), Seq=2--X-->|
          |-----W=0, FCN=4 (100), Seq=3----->|
          |-----W=0, FCN=3 (011), Seq=4--X-->|
          |-----W=0, FCN=2 (010), Seq=5----->|
          |-----W=0, FCN=1 (001), Seq=6----->|
DL enable |-----W=0, FCN=0 (000), Seq=7----->| Waits for next DL opportunity
       (no ACK)
          |-----W=1, FCN=6 (110), Seq=8--X-->|
DL enable |-----W=1, FCN=7 (111), Seq=9----->| Missing Fragment W=0 => FCN= 5, 3 and 0, W=1 => FCN= 6 and 4
          |<--- Compound ACK, W=0,1, C=0 ----| Bitmap W=0:1010110, Bitmap W=1:0000001
          |-----W=0, FCN=5 (101), Seq=10---->|
          |-----W=0, FCN=3 (011), Seq=11---->|
          |-----W=1, FCN=6 (110), Seq=12---->| All fragments received
DL enable |-----W=1, FCN=7 (111), Seq=13---->|
          |<--------- ACK, W=1, C=1 ---------| C=1
        (End)

The number of times the same SCHC ACK message will be retransmitted is determined by the MAX_ACK_REQUESTS.¶

4.3. SCHC Abort Examples

Case SCHC Sender-Abort¶

The sender may need to send a Sender-Abort to stop the current communication. This may happen, for example, if the All-1 has been sent MAX_ACK_REQUESTS times.¶

        Sender                      Receiver
          |-----W=0, FCN=6, Seq=1----->|
          |-----W=0, FCN=5, Seq=2----->|
          |-----W=0, FCN=4, Seq=3----->|
          |-----W=0, FCN=3, Seq=4----->|
          |-----W=0, FCN=2, Seq=5----->|
          |-----W=0, FCN=1, Seq=6----->|
DL Enable |-----W=0, FCN=0, Seq=7----->|
      (no ACK)
          |-----W=1, FCN=6, Seq=8----->|
          |-----W=1, FCN=5, Seq=9----->|
          |-----W=1, FCN=4, Seq=10---->|
DL Enable |-----W=1, FCN=7, Seq=11---->| All fragments received
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=14---->| RESEND ACK  (1)
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=15---->| RESEND ACK  (2)
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=16---->| RESEND ACK  (3)
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=17---->| RESEND ACK  (4)
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |-----W=1, FCN=7, Seq=18---->| RESEND ACK  (5)
          |<------ ACK, W=1, C=1 ---X--| C=1
DL Enable |----Sender-Abort, Seq=19--->| exit with error condition
        (End)

Case Receiver-Abort¶

The receiver may need to send a Receiver-Abort to stop the current communication. This message can only be sent after a DL enable.¶

        Sender                      Receiver
          |-----W=0, FCN=6, Seq=1----->|
          |-----W=0, FCN=5, Seq=2----->|
          |-----W=0, FCN=4, Seq=3----->|
          |-----W=0, FCN=3, Seq=4----->|
          |-----W=0, FCN=2, Seq=5----->|
          |-----W=0, FCN=1, Seq=6----->|
DL Enable |-----W=0, FCN=0, Seq=7----->|
          |<-------  RECV ABORT -------| under-resourced
       (Error)