Reliable and Available Wireless Architecture

Abstract

Due to uncontrolled interferences, including the self-induced multipath fading, deterministic networking can only be approached on wireless links. The radio conditions may change -way- faster than a centralized routing can adapt and reprogram, in particular when the controller is distant and connectivity is slow and limited. RAW separates the routing time scale at which a complex path is recomputed from the forwarding time scale at which the forwarding decision is taken for an individual packet. RAW operates at the forwarding time scale. The RAW problem is to decide, within the redundant solutions that are proposed by the routing, which will be used for each individual packet to provide a DetNet service while minimizing the waste of resources.¶

1. Introduction

Bringing determinism in a packet network means eliminating the statistical effects of multiplexing that result in probabilistic jitter and loss. This can be approached with a tight control of the physical resources to maintain the amount of traffic within a budgetted volume of data per unit of time that fits the physical capabilities of the underlying technology, and the use of time-shared resources (bandwidth and buffers) per circuit, and/or by shaping and/or scheduling the packets at every hop.¶

Wireless networks operate on a shared medium where uncontrolled interference, including the self-induced multipath fading, adds another dimension to the statistical effects that affect the delivery. Scheduling transmissions can alleviate those effects by leveraging diversity in the spatial, time, code, and frequency domains, and provide a Reliable and Available service while preserving energy and optimizing the use of the shared spectrum.¶

Deterministic Networking is an attempt to mostly eliminate packet loss for a committed bandwidth with a guaranteed worst-case end-to-end latency, even when co-existing with best-effort traffic in a shared network. This innovation is enabled by recent developments in technologies including IEEE 802.1 TSN (for Ethernet LANs) and IETF DetNet (for wired IP networks). It is getting traction in various industries including manufacturing, online gaming, professional A/V, cellular radio and others, making possible many cost and performance optimizations.¶

The "Deterministic Networking Architecture" [RFC8655] is composed of three planes: the Application (User) Plane, the Controller Plane, and the Network Plane. Reliable and Available Wireless (RAW) extends DetNet to focus on issues that are mostly a co"ern on wireless links, and inherits the architecture and the planes. A RAW Network Plane is thus a Network Plane inherited by RAW from DetNet, composed of one or multiple hops of homogeneous or heterogeneous technologies, e.g. a Wi-Fi6 Mesh or one-hop CBRS access links federated by a 5G backhaul.¶

RAW networking aims at providing highly available and reliable end-to-end performances in a network with scheduled wireless segments. Uncontrolled interference and transmission obstacles may impede the transmission, and techniques such as beamforming with Multi-User MIMO can only alleviate some of those issues, so the term "deterministic" is usually not associated with short range radios, in particular in the ISM band. This uncertainty places limits to the amount of traffic that can be transmitted on a link while conforming to a RAW Service Level Agreement (SLA) that may vary rapidly.¶

The wireless and wired media are fundamentally different at the physical level, and while the generic "Deterministic Networking Problem Statement" [RFC8557] applies to both the wired and the wireless media, the methods to achieve RAW must extend those used to support time-sensitive networking over wires, as a RAW solution has to address less consistent transmissions, energy conservation and shared spectrum efficiency.¶

The development of RAW technologies has been lagging behind deterministic efforts for wired systems both at the IEEE and the IETF. But recent efforts at the IEEE and 3GPP indicate that wireless is finally catching up at the lower layer and that it is now possible for the IETF to extend DetNet for wireless segments that are capable of scheduled wireless transmissions.¶

The intent for RAW is to provide DetNet elements that are specialized for short range radios. From this inheritance, RAW stays agnostic to the radio layer underneath though the capability to schedule transmissions is assumed. How the PHY is programmed to do so, and whether the radio is single-hop or meshed, are unknown at the IP layer and not part of the RAW abstraction.¶

Still, in order to focus on real-worlds issues and assert the feasibility of the proposed capabilities, RAW will focus on selected technologies that can be scheduled at the lower layers: IEEE Std. 802.15.4 timeslotted channel hopping (TSCH), 3GPP 5G ultra-reliable low latency communications (URLLC), IEEE 802.11ax/be where 802.11be is extreme high throughput (EHT), and L-band Digital Aeronautical Communications System (LDACS). See [RAW-TECHNOS] for more.¶

The establishment of a path is not in-scope for RAW. It may be the product of a centralized Controller Plane as described for DetNet. As opposed to wired networks, the action of installing a path over a set of wireless links may be very slow relative to the speed at which the radio conditions vary, and it makes sense in the wireless case to provide redundant forwarding solutions along a complex path and to leave it to the Network Plane to select which of those forwarding solutions are to be used for a given packet based on the current conditions.¶

RAW distinguishes the longer time scale at which routes are computed from the the shorter forwarding time scale where per-packet decisions are made. RAW operates at the forwarding time scale on one DetNet flow over one path that is preestablished and installed by means outside of the scope of RAW. The scope of the RAW WG comprises Network plane protocol elements such as OAM and in-band control to improve the RAW operation at the Service and at the forwarding sub-layers, e.g., controlling whether to use packet replication, Hybrid ARQ and coding, with a constraint to limit the use of redundancy when it is really needed, e.g., when a spike of loss is observed. This is discussed in more details in Section 4.3 and the next sections.¶

2. Terminology

RAW defines the following terms:¶

PAREO:

Packet (hybrid) ARQ, Replication, Elimination and Ordering. PAREO is a superset Of DetNet's PREOF that includes radio-specific techniques such as short range broadcast, MUMIMO, constructive interference and overhearing, which can be leveraged separately or combined to increase the reliability.¶

Flapping:

In the context of RAW, a link flaps when the wireless connectivity is interrupted for short transient times, typically of a subsecond duration.¶

RAW reuses terminology defined for DetNet in the "Deterministic Networking Architecture" [RFC8655], e.g., PREOF for Packet Replication, Elimination and Ordering Functions.¶

RAW also reuses terminology defined for 6TiSCH in [6TiSCH-ARCH] such as the term Track. 6TiSCH defined a Track as a complex path with associated PAREO operations.¶

This document reuses terms that are well-defined in the context of automation to networking and packet delivery, in particular for reliability and availability. In the context of the RAW work, they are defined as follows:¶

Reliability:

Reliability is a measure of the probability that an item will perform its intended function for a specified interval under stated conditions. For RAW, the service that is expected is delivery within a bounded latency and a failure is when the packet is either lost or delivered too late. RAW expresses reliability in terms of Mean Time Between Failure (MTBF) and Maximum Consecutive Failures (MCF).¶

Availability:

Availability is a measure of the relative amount of time where a path operates in stated condition, in other words (uptime)/(uptime+downtime). Because a serial wireless path may not be good enough to provide the required availability, and even 2 parallel paths may not be over a longer period of time, the RAW availability implies a path that is a lot more complex than what DetNet typically envisages (a Track).¶

3. Use Cases and Requirements Served

[RFC8578] presents a number of wireless use cases including Wireless for Industrial Applications, Pro-Audio and SmartGrid. [RAW-USE-CASES] adds a number of use cases that demonstrate the need for RAW capabilities in new use cases including Pro-Gaming and drones.¶

The use cases can be abstracted in two families, radio access protection and Track protection in a wireless mesh.¶

                                   ***   *
                RAN 1  -----  ***     **  **
             /              *    **        ****
   +----+  /              *           **      ***
   |    |-                *                      **
   |    |--zzz- RAN 2 -- *      Internet       ***
   |    |-                 *                  ***
   +----+  \                *              ***
             \               ***   **     **
                RAN n  --------  ***   ***

zzz = flapping now

         A-------B-------C-----D
        /  \   /       /        \
       I ----M-------N--zzzzz--- E
        \      \   /            /
         P--zzz--Q-------------R

zzz = flapping now

5. RAW Architecture Components

                 (A)   (C)   (E)

   source (S)                       (R) (root)

                 (B)   (D)   (F)

               ===> (A) => (C) => (E) ===
             //        \\//   \\//       \\
   source (S)          //\\   //\\         (R) (root)
             \\       //  \\ //  \\      //
               ===> (B) => (D) => (F) ===

                                Timeslot
+---------++------+------+------+------+------+------+------+
| Channel || 0    | 1    | 2    | 3    | 4    | 5    | 6    |
+---------++======+======+======+======+======+======+======+
| 0       || S->A | S->B | B->C | B->D | C->F | E->R | F->R |
+---------++------+------+------+------+------+------+------+
| 1       ||      | A->C | A->D | C->E | D->E | D->F |      |
+---------++------+------+------+------+------+------+------+

               ===> (A) ====> (C) ====> (E) ====
             //     ^ | \\                      \\
   source (S)       | |   \\                      (R) (root)
             \\     | v     \\                  //
               ===> (B) ====> (D) ====> (F) ====

7. IANA Considerations

This document has no IANA actions.¶

8. ConTributors

Xavi Vilajosana:

Wireless Networks Research Lab, Universitat Oberta de Catalunya¶

Remous-Aris Koutsiamanis:

IMT Atlantique¶

Nicolas Montavont:

IMT Atlantique¶

9. Acknowledgments

The authors wish to thank¶

10. References