# Time Shared Optical Network (TSON): A Programmable Network Edge Solution for Multi-Access Support
## 2 TSON, the Past
### Evolution Stage 1: TSON Baseline  
The TSON idea was initiated by designing a multi-wavelength fully bi-directional synchronous and innovative flexible frame-based system [1; 2]. It offers a flexible and multiplexed optical network infrastructure and on-demand guaranteed time-shared multi-granular services. This solution is a contentionless [5] solution through the deployment of a central resource allocation engine of route, wavelength, and time assignment, in charge of setting up the sub-wavelength paths. TSON offers two type of nodes, edge and core nodes, offering different functionality and levels of complexity. The TSON edge nodes provide the interfaces between different domains, such as wireless, passive optical network (PON), and DC, to the optical domain and vice versa. Figure 4 presents the baseline of the TSON edge node data plane architecture. The architecture consists of one 10 Gb Ethernet client to send and receive data, and two 10 Gb wavelengths (lambdas). In this architecture, the ingress part of the edge node provides the aggregation and mapping, whereas the egress edge nodes include the reverse functionality. The TSON can support elastic time and optical bandwidth allocation. The ingress part waits to finish the processing of the current frame, and then starts to transmit time-slices on the basis of the optical bandwidth. The optical bandwidth allocated to the different services can be elastically defined on the basis of the requirements of each service. The functionality of the ingress part of the edge node in detail is in receiving and passing the Ethernet frames to the 10 GE medium access control (MAC), then MAC discards the preambles and frame check sequence (FCS), transmits the data to the receiver (Rx) in a first in first out (FIFO) manner, and indicates whether the packet is status of the frame, that is, the frame is good or bad. The Rx FIFO receives the data, waits for the frame status indication from the MAC, and then sends it to demultiplexer (DEMUX) block when the data is valid. The role of DEMUX is to analyse the Ethernet  
Figure 3: The generic convergence-node architecture. LTE-A: long term evolution advanced, PON: passive optical network, WiFi: wireless fidelity, LiFi: light fidelity, BVT: bandwidth variable transponder. frame information, that is, destination MAC address, source MAC address, and others, and dispatch them into different FIFOs. The FIFOs wait for the aggregation unit to grant access to send data. The aggregation unit grants access to the FIFO when the burst-length Ethernet frames are ready in the FIFO and the time-slice allocation is available, then it transmits the bursts with a suitable wavelength through the transmitter (Tx) FIFO. The egress part of the edge node receives a burst (time-slice) and passes it to the Rx FIFO wavelength (Lambda0/Lambda1). The segregation unit segregates the burst to Ethernet frames when the burst is completely received in Rx FIFO, and then transmits them to a Tx FIFO. Finally, Tx FIFO sends the Ethernet frames to the MAC, where the MAC passes the data to the transmitter before finally transmitting them out. The TSON core node does not carry out any data processing. Instead, it needs to control the fast-optical switch in order to set up a path with a client's request to switch the traffic optically. The core node demands one switch per wavelength to direct the incoming optical time-slice signals towards the output ports on the basis of the control plane definition. In addition, the dimension of the space switch is defined by the number of fibres that are interconnected through the node. TSON supports two different functional modes: Ethernet and elastic bandwidth allocation mode. The Ethernet mode supports Ethernet-based packets without any time-slicing and elastic bandwidth allocation is based on time-slicing. The TSON control plane is presented in Section 4. It should be noted, however, that TSON is fully software defined networking (SDN) enabled.