# C-RAN Traffic Aggregation on Latency-Controlled Ethernet Links
## 3 Traffic Aggregation
### Mapping of CPRI Traffic in IhON  
CPRIoE (Common Public Radio Interface over Ethernet) traffic characterization has been analyzed in [27; 30]. A list of parameters used in this study is reported in Table 1, while an example of the IHN output line is provided in Figure 3. RRHs generate CPRI flows at different rates (\(R_{CPRI}\)) set by the standard [21]. Each flow is composed of CPRI basic frames with fixed duration \(T_{CPRI}=260\) ns, equal for all CPRI options. A certain number of CPRI basic frames (\(N_{F}\)) are encapsulated in an Ethernet frame forming the CPRIoE payload of length:  
\[L_{F}=N_{F}*R_{CPRI}*T_{CPRI}. \tag{1}\]  
CPRIoE frames are then sent by RRHs towards IHON switches, where they are delayed by \(\Delta\). In addition, conventional BH traffic reaches the switches, loading the output channels with parameter \(\rho_{B}\). In order to avoid collision between different frames on the output line, a guard time \(T_{guard}\) is applied during which the transmission of any data is not permitted. IHON switches have \(m\) output channels, each characterized by a rate \(R_{W}\), and accommodates CPRIoE frames of duration:  
\[T_{G}=\frac{L_{H}+L_{F}}{R_{W}}, \tag{2}\]  
where \(L_{H}\) is the header of CPRIoE frames assumed to be 44 Bytes [27].  
Depending on \(N_{F}\) and \(R_{CPRI}\), the gap duration \(T_{GAP}\) is selected according to:  
\[T_{GAP}=\frac{L_{F}}{R_{CPRI}}-T_{G}. \tag{3}\]  
By looking at Formula (3), it is possible to notice that, depending on \(R_{CPRI}\), different values for \(T_{GAP}\) can be obtained for the same length of CPRIoE packets \(L_{F}\). An example of \(T_{GAP}\) using CPRI opt. 1 and 6 for a line rate of \(R_{W}=10\) Gbps is depicted in Figure 4.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Parameter** & **Description** \\ \hline \(N_{F}\) & Number of CPRI basic frames forming a CPRIoE payload. \\ \(L_{F}\) & Payload length for CPRIoE frame. \\ \(R_{W}\) & Output channel rate. \\ \(T_{G}\) & CPRIoE duration. \\ \(T_{GAP}\) & Gap duration. \\ \(\Delta\) & Fixed delay to avoid collision. \\ \(\rho_{B}\) & Offered backhaul (BH) load per channel. \\ \(L_{B}\) & Average length of BH frames. \\ \(T_{guard}\) & Guard time. \\ \(T_{CPRI}\) & CPRI basic frame duration. \\ \(R_{CPRI}\) & CPRI flow generation rate. \\ \(L_{H}\) & Length of CPRIoE header. \\ \(m\) & Number of channels in the switch output interface. \\ \hline \hline \end{tabular}
\end{table}
Table 1: List of parameters used to describe CPRI over Ethernet (CPRIoE) and hybrid nodes.  
Figure 3: CPRI over Ethernet (CPRIoE) parameters applied to hybrid node.  
This time gap is used in the hybrid multiplexing scheme in order to aggregate BH traffic on the same transport channel. To this end, two different policies are considered here:  
* A BH packet is transmitted when a gap is available and it is possibly pre-empted upon arrival of a new GST burst, in case BH packet duration is longer than the gap itself. In case of pre-emption, the BH packet is lost. This policy is indicated as P policy, with insertion of an entire packet into the by-pass GST flow.
* A BH packet waits for a gap and, in case the BH packet is longer than the gap, it splits into segments that are transmitted by as many gaps as needed. This avoids the need of pre-emption but introduces some overhead to manage segmentation and additional functionalities. This policy is indicated as S policy, where packets are divided into \(N_{S}\) segments of suitable size for their insertion into the GST flow.