# Time Shared Optical Network (TSON): A Programmable Network Edge Solution for Multi-Access Support
## 2 TSON, the Past
### Evolution Stage 2: TSON Multi-Protocol  
In the second evolution stage of the TSON solution, the TSON baseline technology has been extended to support multi-protocol, as well as to provide synchronisation capabilities. We have chosen xHaul as the multi-protocol scenario, which is joined Backhaul (BH) and Fronthaul (FH) services. The reason to choose xHaul scenario is that the TSON can support varying service-related requirements as its operational characteristics can be dynamically modified. Therefore, multiple Ethernet clients for BH as well as CPRI integration for FH services are considered. The TSON client ports extension are only limited to the number of available transceivers on FPGA boards, as well as the FPGA density to support extended architecture. Figure 5 illustrates the TSON edge node to support multi-protocol data plane architecture. This architecture follows the TSON baseline legacy with the following extra features:  
* Supporting xHaul--the TSON Ethernet client ports are extended to multi-ports supporting BH services and native CPRI is added to support FH for the xHaul solution. * Wavelength multiplexing--the clients can use a common wavelength (i.e., lambda) if the total bandwidth of the client is less than 10 Gb. In this scenario, the frames are delivered to the related clients at the egress nodes on the basis of their header information. * Synchronisation--IEEE-1588 v2 [9] protocol is integrated for accurate synchronisation. In this architecture, the CPRI transceiver receives the CPRI frames and passes them to the CPRI_TSON interface unit in the ingress side. The CPRI_TSON interface is responsible for the clock domain crossing (CDC) and the mapping of the CPRI data and control to TSON format. Once the CPRI frames reach the aggregation block, then, according to the look-up table (LUT) information of the time-slice allocation, the aggregation block calculates the frames needed to construct the burst, aggregates the frames, waits for the valid time-slice, and finally sends the burst into a different wavelength. The egress side has the reverse functionality. Figure 6 presents how a CPRI flow can be embedded into the TSON framing structure. Figure 5: TSON edge node multi-protocol data plane architecture. Rx: receiver, Tx: transmitter, WL: wavelength, CPRI: common public radio interface, TSON: time shared optical network. High synchronisation accuracy over the network requires an accurate time-stamping method. The TSON network requires global frame synchronisation among TSON nodes to achieve high synchronisation accuracy over the network. We have adopted IEEE-1588-v2 protocol for the TSON solution. Xilinx 10 Gb Ethernet Subsystem IP core [10] supports high accuracy IEEE-1588-v2 one-step and two-step time-stamping on a 10 GBASE-R network interface, but does not support multiple IEEE-1588-v2-enabled subsystem instances [11]. To resolve this issue, a new subsystem has been developed to support multiple IEEE-1588-v2 enabled subsystems. The Xilinx approach uses time stamper unit for the transmit-side and the receiver-side inside the MAC and the physical coding sublayer/ physical media attachment (PCS/PMA) sub intellectual property (IP) cores, respectively. Unlike the Xilinx approach, the developed subsystem uses a separate developed time stamper. Figure 7 shows the proposed subsystem architecture. The developed time stamper follows the same features as the Xilinx Subsystem. The time stamper unit is located between the MAC and PCS/PMA IP cores, uses the Timer Syns clock, and follows the IEEE-1588-v2 protocol. In addition, the time stamper considers the physical layer delay for stamping. Figure 6: Common public radio interface (CPRI) frame structure over TSON. C&M: control and management, L1: layer 1, Ctrl_AxC: control data stream for antenna carrier, BFN: node B frame number. Figure 7: Proposed subsystem for synchronisation. AXI4: advanced eXtensible interface 4, MAC: medium access control, PCS/PMA: physical coding sublayer/physical media attachment, XGMII: 10 gigabit media independent interface.