SKEDSOFT

Rate-Constrained Communication
In rate-constrained communication, a minimum guaranteed bandwidth is established for each channel. For this minimum bandwidth, the maximum transport
latency and the maximum jitter are guaranteed to be smaller than an upper bound. If a sender (an end-system) sends more messages than the minimum guaranteed
bandwidth, the communication system will try to transport the messages according to a best-effort strategy. If it cannot handle the traffic, it will exercise backpressure
flow control on the sender in order to protect the communication system from overload generated by a misbehaving sender (e.g., a babbling end system).

In order to be able to provide the guarantees, the communication system must contain information about the guaranteed bandwidth for each sender. This information can be contained in static protocol parameters that are pre-configured into the communication controller a priori or can be loaded into the communication controller dynamically during run-time. Rate constrained communication protocols provide temporal error detection and protection of the communication system from babbling idiots.

Rate-constrained protocols provide a guaranteed maximum transport latency. The actual transport latency will normally be significantly better, since under normal conditions the global traffic pattern is much smaller than the assumed peak.

Token Protocol: One of the early rate-constrained protocols is the token protocol controlling the access to a multi-access local area networks (LAN). In a token system, the right to transmit is contained in a special control message, the token. Whoever has the token is allowed to transmit. Two time parameters determine the response time of a token system, the token-hold time THT, denoting the longest time a node may hold the token, and the token-rotation time TRT, denoting the longest time for a full rotation of the token. The maximum TRT is the product of the number of nodes and the THT of each node, which determines the guaranteed bandwidth allocated to a node.
A serious failure in any token system is the loss of the token, e.g., if the station that possesses the token fails. In such a situation, the network traffic is disrupted until
some other node detects the silence by monitoring a time-out, and generates a new token. A token system can be structured as a bus or a ring. Token rings were standardized by IEEE standard 802.5. They have been in wide use some years ago.

Mini-slotting Protocol ARINC 629: In the mini-slotting protocol ARINC 629, the access to a shared bus is controlled by two time-out parameters, the synchronization gap SG controlling the entrance to a distributed waiting room, and the terminal gap TG controlling the access from the waiting room to the bus. The synchronization gap SG is identical for all nodes, whereas the terminal gap TG, the personality timer, is different for each node and must be a multiple of the propagation delay (called a mini-slot). The following relation holds between these time-outs: SG > Max{TGi} for all nodes i. ARINC 629 is thus a waiting-room protocol similar to the bakery algorithm of Lamport. In the first phase, the set of nodes that wants to transmit a message is admitted to the distributed waiting room if there is silence on the bus for a longer duration than the synchronization gap SG. A node that has entered the waiting room and senses silence on the bus for a duration that surpasses its personal terminal gap TG starts transmitting its message. This protocol logic guarantees that a node cannot monopolize the bus, since even the node with the shortest terminal gap TG (the highest priority node) is allowed to send a second message only after all other nodes that are in the waiting room have finished their message transmission. Typical values for the time-out parameters on a 2 Mbit/s channel are: terminal gap (determined by the propagation delay): 4–128 ms, synchronization gap SG longer than the longest terminal gap. The ARINC 629 protocol is used on the Boeing 777 airplane.

Avionics Full Duplex Switched Ethernet: Avionics Full Duplex Switched Ethernet (AFDX) is a rate-constrained protocol based on switched Ethernet. Whereas the message format and the physical layer of AFDX is in agreement with the Ethernet standard IEEE 802.3, the protocol allocates a statically defined bandwidth to each sender on a virtual link basis. A virtual link connects a sender with a specified number of receivers. An AFDX switch guarantees that
1. The delivery order of messages traveling on a virtual link is the same as the send order.
2. A minimal bandwidth and a maximum transmission latency and jitter is guaranteed on a virtual link basis.
3. There is no data loss due to buffer over-subscription in the switch.

The configuration table of the switch contains state information for every virtual link and enables the switch to protect the network from nodes that try to overload
the network. The system integrator establishes the virtual links and sets the connection parameters. AFDX has been standardized under ARINC 664 and used in the Airbus A 380 and in the Boeing Dreamliner B787.

Audio Video Bus: Physical connections in multimedia systems are predominantly unidirectional and point-to-point, resulting in substantial wiring harnesses. In order to simplify the wiring harnesses, special multi-media protocols have been developed. Some of these special protocols are incompatible with standard IT protocols such as Ethernet. On the other side, standard switched Ethernet does not provide the temporal quality of service needed in multi-media applications.

A communication system for audio-video streaming must support the following temporal quality of service requirements:

1. It must be possible to precisely synchronize multiple audio video streams that are generated at different physical locations, e.g., for lip synchronization or mixing content. The required precision of synchronization is in the microsecond range.
2. The worst-case transport delay of a multimedia data stream, including buffering delays at source and destination, must be bounded. The duration for the dynamic switching from one video stream to another video stream must be in the millisecond range.
3. The communication resources that are dynamically allocated to a multi-media stream must remain available for the duration of a session. This requires a dynamic resource reservation schema.

The IEEE 802.1 audio/video bridging (AVB) task force develops a set of protocols based on the Ethernet standard that meet the aforementioned requirements.