3. Towards Reliable and Available Wireless Networks

This section preserves the RFC text for RAW technologies, including Wi-Fi 6/7, IEEE 802.11, TSCH, 6TiSCH, 5G NR, TSN/TSC integration, UE, gNB, RAN, UPF, PDU sessions, LDACS, PHY/MAC terms, figures, tables, and security considerations.

Original RFC Text

3.  Towards Reliable and Available Wireless Networks

3.1.  Scheduling for Reliability

   A packet network is reliable for critical (e.g., time-sensitive)
   packets when the undesirable statistical effects that affect the
   transmission of those packets (e.g., delay or loss) are eliminated.

   The reliability of a deterministic network [RFC8655] often relies on
   precisely applying a tight schedule that controls the use of time-
   shared resources such as CPUs and buffers, and maintains at all times
   the number of the critical packets within the available resources of
   the communication hardware (e.g., buffers) and the transmission
   medium (e.g., bandwidth, transmission slots).  The schedule can also
   be used to shape the flows by controlling the time of transmission of
   the packets that compose the flow at every hop.

   To achieve this, there must be a shared sense of time throughout the
   network.  The sense of time is usually provided by the lower layer
   and is not in scope for RAW.  As an example, the Precision Time
   Protocol (PTP), standardized as IEEE 1588 and IEC 61588, has mapping
   through profiles to Ethernet, industrial and SmartGrid protocols, and
   Wi-Fi with IEEE Std 802.1AS.

3.2.  Diversity for Availability

   Equipment (e.g., node) failure can be the cause of multiple packets
   being lost in a row before the flows are rerouted or the system
   recovers.  Examples of equipment failure include a broken switch, an
   access point rebooting, a broken wire or radio adapter, or a fixed
   obstacle to the transmission.

   Equipment failure is not acceptable for critical applications such as
   those related to safety.  A typical process control loop will
   tolerate an occasional packet loss, but a loss of several packets in
   a row will cause an emergency stop.  In an amusement ride (e.g., at
   Disneyland, Universal Studios, or MGM Studios parks), a continuous
   loss of packets for a few 100 ms may trigger an automatic
   interruption of the ride and cause the evacuation of the attraction
   floor to restart it.

   Network availability is obtained by making the transmission resilient
   against hardware failures and radio transmission losses due to
   uncontrolled events such as co-channel interferers, multipath fading,
   or moving obstacles.  The best results are typically achieved by
   pseudorandomly cumulating all forms of diversity -- in the spatial
   domain with replication and elimination, in the time domain with ARQ
   and diverse scheduled transmissions, and in the frequency domain with
   frequency hopping or channel hopping between frames.

3.3.  Benefits of Scheduling

   Scheduling redundant transmissions of the critical packets on diverse
   paths improves the resiliency against breakages and statistical
   transmission loss, such as those due to cosmic particles on wires and
   interferences on wireless.  While transmission losses are orders of
   magnitude more frequent on wireless, redundancy and diversity are
   needed in all cases for life- and mission-critical applications.

   When required, the worst-case time of delivery can be guaranteed as
   part of the end-to-end schedule, and the sense of time that must be
   shared throughout the network can be exposed to and leveraged by
   other applications.

   In addition, scheduling provides specific value over the wireless
   medium:

   *  Scheduling allows a time-sharing operation, where every
      transmission is assigned its own time/frequency resource.  The
      sender and receiver are synchronized and scheduled to talk on a
      given frequency resource at a given time and for a given duration.
      This way, scheduling can avoid collisions between scheduled
      transmissions and enable a high ratio of critical traffic (think
      60% or 70% of high-priority traffic with ultra low loss) compared
      to statistical priority-based schemes.

   *  Scheduling can be used as a technique for both time and frequency
      diversity (e.g., between transmission retries), allowing the next
      transmission to happen on a different frequency as programmed in
      both the sender and the receiver.  This is useful to defeat co-
      channel interference from uncontrolled transmitters as well as
      multipath fading.

   *  Transmissions can be also scheduled on multiple channels in
      parallel, which enables the use of the full available spectrum
      while avoiding the hidden terminal problem, e.g., when the next
      packet in a same flow interferes on a same channel with the
      previous one that progressed a few hops farther.

   *  Scheduling optimizes the bandwidth usage.  Compared to classical
      collision avoidance techniques, there is no blank time related to
      Interframe Space (IFS) and exponential back-off in scheduled
      operations.  A minimal clear channel assessment may be needed to
      comply with the local regulations such as ETSI 300-328, but that
      will not detect a collision when the senders are synchronized.

   *  Scheduling plays a critical role in saving energy.  In the
      Internet of Things (IoT), energy is the foremost concern, and
      synchronizing the sender and listener enables always maintaining
      them in deep sleep when there is no scheduled transmission.  This
      avoids idle listening and long preambles, and it enables long
      sleep periods between traffic and resynchronization, allowing
      battery-operated nodes to operate in a mesh topology for multiple
      years.