# Analysis of 5G Channel Access for Collaboration with TSN Concluding at a 5G Scheduling Mechanism
## 4 Architecture
### Tsn  
Time-Sensitive Networking describes a collection of IEEE standards that aim to realize deterministic communication links based on Ethernet. The functionality of TSN is based on a configuration scheme applied by all participating bridges. Therefore, the synthesis of a traffic forwarding strategy that meets all application requirements and the exchange of control plane messages distributing the outcome thereof is necessary. The task sharing of this process can be realized by three different types of architectures, several of which can be combined [20]:  
1. **Centralized :** All communication partners (Talkers and Listeners) are connected to a Central User Configuration (CUC), which forwards their requirements to a Central Network Configuration (CNC). The CNC has information about the network topology and the bridge capabilities. On this basis, the CNC computes a configuration scheme for the ports of every bridge in the network and distributes it. 2. **Decentralized:** No central configuration nodes are deployed. Instead, all TSN bridges distribute the TSN stream requirements under each other and determine a traffic forwarding scheme based on predefined protocols and algorithms. 3. **Hybrid:** A hybrid TSN architecture deploys a CNC, omitting the use of a CUC. Talkers and Listeners only communicate with the next node of the TSN bridge network, which forwards the stream requirements to the CNC. Current TSN standards depict several technologies to support a common traffic forwarding configuration in all bridges of an Ethernet network:  
* Credit-Based Shaper (CBS) [21]
* Time-Aware Shaper (TAS) [22]
* Cyclic Queuing and Forwarding (CQF) [23]
* Asynchronous Traffic Shaping (ATS) [24]  
In [25], the authors describe the CBS as a transmission selection algorithm for multiple traffic classes, with each queue maintaining a credit value for a service. While packets are waiting in the queue to be sent, the credit value increases. When a packet is sent from the queue, the value decreases. The dependence of the actual credit value of these events is described per queue by two parameters, _send slope_ and _idle slope_. While this mechanism maintains fairness among all services using the network, it does not guarantee optimal delay bounds. With CQF, all network devices are synchronized to share a periodic transmission time frame, guaranteeing that a packet is transmitted over a single hop within each cycle. The forwarding schemes are rather simple to configure, although this is only suitable for applications that tolerate a maximum jitter of the duration of a whole cycle. The ATS differs from other alternatives by not relying on a common synchronization of clocks over all network devices. It achieves deterministic transmission times by deploying per-hop shaping for each traffic flow. According to [26], it is well suited for sporadic traffic, while for periodic traffic with moderate to high loads it is outperformed by TAS. In this work we mainly focus on TAS, as it allows the network to support multiple TSN streams while enforcing low delay and jitter for all applications. With TAS, each egress port of a bridge has up to eight parallel queues, on which data packets are distributed regarding their respective traffic class. Each of the egress queues contains a gate in either the _closed_ or _open_ state, which controls whether a queue at a certain point in time can be served or not for the sending of data. If several gates are open at the same point in time, an additional transmission selection algorithm decides how to prioritize the queues. The configuration of the TAS gating behavior of one egress port is described by a GCL. The GCL contains a sequence of state arrays, and their dimension corresponds to the amount of traffic classes. Each state array describes the states of all gates and is linked to a time parameter, which describes the duration for which the state array is active.