RAW P. Thubert, Ed.
Internet-Draft Cisco Systems
Intended status: Informational May 21, 2019
Expires: November 22, 2019

Reliable and Available Wireless Technologies


This document presents a series of recent technologies that are capable of time synchronization and scheduling of transmission, making them suitable to carry time-sensitive flows with requirements of both reliable delivery in bounded time, and availability at all times, regardless of packet transmission or individual equipement failures.

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Table of Contents

1. Introduction

When used in math or philosophy, the term "deterministic" generally refers to a perfection where all aspect are understood and predictable. A perfectly Deterministic Network would ensure that every packet reach its destination following a predetermined path along a predefined schedule to be delivered at the exact due time. In a real and imperfect world, a Deterministic Network must highly predictable, which is a combination of reliability and availability. On the one hand the network must be reliable, meaning that it will perform as expected for all packets and in particular that it will always deliver the packet at the destination in due time. On the other hand, the network must be available, meaning that it is resilient to any single outage, whether the cause is a software, a hardware or a transmission issue.

RAW (Reliable and Available Wireless) is an effort to provide Deterministic Networking on across a path that include a a wireless physical layer. Making Wireless Reliable and Available is even more challenging than it is with wires, due to the numerous causes of loss in transmission that add up to the congestion losses and the delays caused by overbooked shared resources. In order to maintain a similar quality of service along a multihop path that is composed of wired and wireless hops, additional methods that are specific to wireless must be leveraged to combat the sources of loss that are also specific to wireless.

Such wireless-specific methods include per-hop retransmissions (HARQ) and P2MP overhearing whereby multiple receivers are scheduled to receive the same transmission, which balances the adverse effects of the transmission losses that are eperienced when a radio is used as pure P2P.

2. Terminology

This specification uses a number of terms that are uncommon on protocols that ensure bets effort transmissions for stochastics flows, such as found in the traditional Internet and other statistically multiplexed packet networks.

That consistently performs as expected, the expectation for a network being to always deliver a packet in due time.
That is exempt of unscheduled outage, the expectation for a network being that the flow is maintained in the face of any single breakage.
PAREO (functions):
the wireless extension of DetNet PREOF. PAREO functions include scheduled ARQ at selected hops, and expect the use of new operations like overhearing where available.
A DODAG oriented to a destination,and that enables Packet ARQ, Replication, Elimination, and Ordering Functions.
Automatic Repeat Request, enabling an acknowledged transmission, which is the typical model at Layer-2 on a wireless medium.
Forward error correction, sending redundant coded data to help the receiver recover transmission errors.
Hybrid ARQ, a combination of FEC and ARQ .

3. On Scheduling

The operations of a Deterministic Network often rely on precisely applying a tight schedule, in order to avoid collision loss and guarantee the worst case time of delivery . 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.

3.1. Benefits of Scheduling on Wires

A network is reliable when the statistical effects that affect the packet transmission are eliminated. This involves maintaining at all time the amount of critical packets within the physical capabilities of the hardware and that of the radio medium. This is achieved by controlling the use of time-shared resources such as CPUs and buffers, by shaping the flows and by scheduling the time of transmission of the packets that compose the flow at every hop.

Equipment failure, such as an access point rebooting, a broken radio adapter, or a permanent obstacle to the transmission, is a secondary source of packet loss. When a breakage occurs, multiple packets are lost in a row before the flows are rerouted or the system may recover. This is not acceptable for critical applications such as 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 (e.g., after 4 packets lost, within a period of 1 second).

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 pseudo randomly 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.

3.2. Benefits of Scheduling on Wireless

In addition to the benefits listed in Section 3.1, scheduling transmissions provides specific value to the wireless medium.

On the one hand, scheduling avoids collisions between scheduled transmissions and can ensure both time and frequency diversity between retries in order to defeat co-channel interference from uncontroller transmitters as well as multipath fading. Transmissions can be scheduled on multiple channels in parallel, which enables to use 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.

On the other hand, scheduling optimizes the bandwidth usage: compared to classical Collision Avoidance techniques, there is no blank time related to inter-frame 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. And because scheduling allows a time sharing operation, there is no limit to the ratio of isolated critical traffic.

Finally, scheduling plays a critical role to save energy. In IOT, energy is the foremost concern, and synchronizing sender and listener enables to maintain them in deep sleep at all times when there is no scheduled transmission. This avoids idle listening and long preambles and enables long sleep periods between traffic and resynchronization, allowing battery-operated nodes to operate in a mesh topology for multiple years.

4. tech X

4.1. Provenance and Documents

4.2. General Characteristics

4.3. Applicability to Deterministic Flows

5. IANA Considerations

This specification does not require IANA action.

6. Security Considerations

Most RAW technologies integrate some authentication or encryption mechanisms that were defined outside the IETF.

7. Acknowledgments

Many thanks to the participants of the RAW WG where a lot of the work discussed here happened.

8. References

8.1. Normative References

[I-D.ietf-6tisch-architecture] Thubert, P., "An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4", Internet-Draft draft-ietf-6tisch-architecture-20, March 2019.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017.

8.2. Informative References

[IEEE80211] "IEEE Standard 802.11 - IEEE Standard for Information Technology - Telecommunications and information exchange between systems Local and metropolitan area networks - Specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications."
[IEEE802154] IEEE standard for Information Technology, "IEEE Std. 802.15.4, Part. 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks"

Author's Address

Pascal Thubert (editor) Cisco Systems, Inc Building D 45 Allee des Ormes - BP1200 MOUGINS - Sophia Antipolis, 06254 FRANCE Phone: +33 497 23 26 34 EMail: pthubert@cisco.com