Time-Sensitive Networking
The different AVB/TSN standards documents specified by IEEE 802.1 can be grouped into three basic key component categories that are required for a complete real-time communication solution based on switched Ethernet networks with deterministic quality of service (QoS) for point-to-point connections.
The mechanism of utilizing the eight distinct VLAN priorities is retained, to ensure complete backward compatibility to non-TSN Ethernet.
The frames are scheduled very evenly, though only on an aggregate basis, to smooth out the delivery times and reduce bursting and bunching, which can lead to buffer overflows and packet drops that trigger retransmissions.
The increased buffering delay makes re-transmitted packets obsolete by the time they arrive, resulting in frame drops which reduces the quality of AV applications.
Deriving guaranteed upper bounds on delays in TSN is non-trivial and is currently being researched, e.g., by using the mathematical framework Network Calculus.
All nodes along the flow path pass the MRP Attribute Declaration (MAD) specification which describes the stream characteristics so that bridges could allocate the necessary resources.
IEEE 802.1Qch Cyclic Queuing and Forwarding (CQF), also known as the Peristaltic Shaper (PS), introduces double buffering which allows bridges to synchronize transmission (frame enqueue/dequeue operations) in a cyclic manner, with bounded latency depending only on the number of hops and the cycle time, completely independent of the network topology.
It prevents traffic overload conditions that may affect bridges and the receiving endpoints due to malfunction or Denial of Service (DoS) attacks.
The IEEE 802.1Qbv time-aware scheduler is designed to separate the communication on the Ethernet network into fixed length, repeating time cycles.
By partially or completely blocking a time-critical time slice, real-time frames can be delayed up to the point where they cannot meet the application requirements any longer.
The time-aware scheduler achieves this by putting a guard band in front of every time slice that carries time-critical traffic.
To further mitigate the negative effects from the guard bands, the IEEE working groups 802.1 and 802.3 have specified the frame pre-emption technology.
IEEE 802.3br specifies the best accuracy for this mechanism at 64 byte - due to the fact that this is the minimum size of a still valid Ethernet frame.
This minimizes the best effort bandwidth that is lost and also allows for much shorter cycle times at slower Ethernet speeds, such as 100 Mbit/s and below.
A critical shortcoming is some delay incurred when an end-point streams unsynchronized data, due to the waiting time for the next time-triggered window.
However synchronizing TSN bridge frame selection and transmission time is nontrivial even in moderately sized networks and requires a fully managed solution.
Credit-based, time-aware and cyclic (peristaltic) shapers require network-wide coordinated time and utilize network bandwidth inefficiently, as they enforce packet transmission at periodic cycles.
ATS employs the urgency-based scheduler (UBS) which prioritizes urgent traffic using per-class queuing and per-stream reshaping.
The UBS is an improvement on Rate-Controlled Service Disciplines (RCSDs) to control selection and transmission of each individual frame at each hop, decoupling stream bandwidth from the delay bound by separation of rate control and packet scheduling, and using static priorities and First Come - First Serve and Earliest Due - Date First queuing.
Worst case clock sync inaccuracy does not decrease link utilization, contrary to time-triggered approaches such as TAS (Qbv) and CQF (Qch).
With additional traffic streams and larger networks, the size of the database proportionally increases and MRP updates between bridge neighbors significantly slow down.
The Link-Local Registration Protocol (LRP) is optimized for a larger database size of about 1 Mbyte with efficient replication that allows incremental updates.
SRP and MSRP are primarily designed for AV applications - their distributed configuration model is limited to Stream Reservation (SR) Classes A and B defined by the Credit-Based Shaper (CBS), whereas IEEE 802.1Qcc includes a more centralized CNC configuration model supporting all new TSN features such as additional shapers, frame preemption, and path redundancy.
RAP will improve scalability and provide dynamic reservation for a larger number of streams with support for redundant transmission over multiple paths in 802.1CB FRER, and autoconfiguration of sequence recovery.
RAP supports the 'topology-independent per-hop latency calculation' capability of TSN shapers such as 802.1Qch Cyclic Queuing and Forwarding (CQF) and P802.1Qcr Asynchronous Traffic Shaping (ATS).
It will also improve performance under high load and support proxying and enhanced diagnostics, all while maintaining backward compatibility and interoperability with MSRP.
To achieve these goals, DetNet uses resource allocation to manage buffer sizes and transmission rates in order to satisfy end-to-end latency requirements.
Service protection against failures with redundancy over multiple paths and explicit routes to reduce packet loss and reordering.
[13] Traffic Engineering (TE) routing protocols translate DetNet flow specification to AVB/TSN controls for queuing, shaping, and scheduling algorithms, such as IEEE 802.1Qav credit-based shaper, IEEE802.1Qbv time-triggered shaper with a rotating time scheduler, IEEE802.1Qch synchronized double buffering, 802.1Qbu/802.3br Ethernet packet pre-emption, and 802.1CB frame replication and elimination for reliability.
Also protocol interworking defined by IEEE 802.1CB is used to advertise TSN sub-network capabilities to DetNet flows via the Active Destination MAC and VLAN Stream identification functions.
