2.1.

Satellite Network Topology

The satellite optical switching network includes two nodes, namely, a boundary-accessing node and an internal switching node. The function of photoelectric aggregation and de-aggregation processing is completed at the accessing node of the satellite optical switching network.^1,24,25 The optical switching function is processed at the internal nodes of the network. As shown in Fig. 1(a), in a six-GEO-satellite ring network each GEO satellite completes the accessing and switching functions. The relationship between the number of GEO satellites and coverage can be seen in Ref. 26. As the number of GEO satellites increases, the global coverage gap decreases until multi-satellite coverage of key regions is achieved. In this study, the same six-GEO-satellite ring network as in Ref. 5 is used for analysis and latency performance comparison, and the proposed network topology can achieve a better two-GEO-satellite coverage. The satellite optical switching network domain is independent of the electrical network domain. Independent addressing and routing are performed in an optical switching network, and the accessing node of the optical switching network includes the gateway function.

Fig. 1

A typical ring network composed of six GEO satellites: (a) network structure and (b) network topology and ports connection relationship.

2.2.

Data Format and Switching Type

BDP and BCP are transmitted in the optical switching system. BDP is transparently transmitted by the aggregation of the boundary nodes of the network. BCPs include burst registration packets (BRPs), burst registration response packets (BRPas), and switching conflict detection packets (BCoLs). BRPs are generated at the boundary source node of satellite optical networks, BCoLs are generated at optical switching nodes, and BRPa are generated at all switching nodes passing through BDPs. Detailed descriptions of the various types of packets are presented in Tables 1Table 2Table 3Table 4Table 5–6.

Table 1

Data types of the optical switching network.

Table 2

Format of the i-th BCP.

Table 3

Format of the i’th BRP.

Table 4

Format of the i-th BRPa.

Table 5

Format of the i-th BCoL.

Table 6

Format of the first collision unit of the i-th BCOL.

Each optical link is composed of a control channel and data channel, which transmit BCPs and BDPs, respectively. The accessing node of the satellite optical network completes the generation, transmission, and reception of BCPs and BDPs. The switching node can obtain the arrival time, type, destination address, and other information of each BDP through a corresponding BRP (one type of BCP) and send the switching collision result, BCoLs, to other nodes.

To satisfy the transmission requirements of various types of services, the proposed satellite optical switching method supports both random collision switching and link reservation switching, as shown in Fig. 2. The boundary node of the satellite optical network sends the corresponding BRP before sending the BDP. The time interval between BRPs and BDPs is reserved according to the total processing time of various types of BCPs in the optical network. The arrival time and switching information of BDPs can be obtained after receiving BRPs. After receiving the transmission failure of a BDP, the source node resends the BDP after randomly backing off a time value in a scheduling cycle until it is successfully sent.

Fig. 2

Two switching types supported by DC-SOBS: (a) competitive switching method and (b) reservation switching method.

2.3.

DC-SOBS Mechanism

At the boundary source node of the optical switching network, the electrical packets sent to the same destination node through the optical network are aggregated according to the BDP format, which is temporarily stored in the high-speed BDP buffer for transmission. Because all-optical transparent transmission and switching is used for the BDP in the data channel of the network, no specific packet format is required when generating a BDP. A functional diagram of the accessing node is shown in Fig. 3. The length of a BDP is specified as a fixed value $T_{\mathrm{BDP}}$, and $T_G$ is the protection time of the optical switching operation at the beginning and end of a BDP. The effective length of a BDP is $T_{Bl}$, which is related to $T_G$ as shown in Eq. (1). If the remaining space is not sufficient to encapsulate the last electrical frame, this BDP encapsulation is completed and the electrical frame is encapsulated into the next BDP. When the current BDP transmission time is reached, regardless of whether the BDP is encapsulated, the electric packets are transformed to a fixed-length BDP and sent to the optical network. The BDP de-aggregation converge at the destination boundary accessing the node of the optical network. Each BRP and BDP are a group, and the transmissions of each group are independent of each other. Because the processing time of each optical switching node is reserved, the BRP sends first at time $t_{\mathrm{Sbr}}$, and then sends the corresponding BDP at time $t_{\mathrm{Sbd}}$. The offset interval between each BRP and BDP is $T_{\text{offset}}$, which satisfies Eq. (2) (Fig. 4)

Fig. 3

Functional diagram of the accessing node.

Fig. 4

Functional diagram of the switching node.

After the optical switching nodes receive BRPs and BCoLs, they complete the switching processing independently. Each optical switching node processes four algorithms, namely, BRP relay, BDP collision detection, BDP collision results send, and BDP switching scheduling. BDP collision detection should be performed at each current node, and the detection results are sent to the corresponding later nodes. After $s_i$ receives the detection result of the previous hop nodes, it executes the BDP switching scheduling iteration to obtain the scheduling results and configure the optical devices. A functional diagram of the optical switching node is shown in Fig. 3. When the timer is equal to the schedule time point, the switching node executes the scheduling of all the BDPs in a subset schedule window. Each egress port has a scheduling unit, and the unit obtains the results within a fixed time and configures the optical devices to complete switching. The time relationship of each egress port is shown in Fig. 5. For $\forall {\mathrm{BDP}}_i$, $\mathrm{BdArr}{T}_i$ is obtained when ${\mathrm{BRP}}_i$ arrives at the switching node. When the most recently arrived time of the BDP is equal to the reserved scheduling time $T_{\mathrm{SP}}$, the switching node starts the BDP switching scheduling. One switching scheduling completes the scheduling of all BRP in one scheduling window time $t_{\mathrm{WD}}$. All ${\mathrm{BRP}}_j$ with arrival times $\mathrm{BdArr}{T}_j$ satisfying Eq. (3) require scheduling in the current scheduling cycle and results. According to the scheduling result, performing the configuration operation before ${\mathrm{BDP}}_i$ arrives completes the switching function. The trigger time of the next scheduling cycle is determined by the arrival time of the first ${\mathrm{BDP}}_k$ outside the current scheduling window

Fig. 5

Temporal relationship between BRP11, BRP12, BRP22, and their respective associated BDPs in scheduling subsets 1 and 2.

Formally, the topology of a network in Fig. 1(b) is defined by an undirected graph $G(S,L)$, where the switching nodes in satellites are vertices $S$ and the communication links connecting vertices are edges $L$. $s_i$ represents the $i$’th switching node, $S=\{s_1,s_2,\dots ,s_N\}$, and the total number of switching nodes in the network is $N$. $l_i$ represents the $i$’th laser link, $L=\{l_1,l_2,\dots ,l_M\}$, and the total number of links in the network is $M$. $B$ represents the set of BDPs in the optical switching network, $b_i$ represents the $i$’th BDP, and $B=\{b_1,b_2,\dots \}$. Br represents the set of BRPs, ${\mathrm{br}}_i$ denotes the BRP that $b_i$ corresponds to. $\mathrm{Br}=\{{\mathrm{br}}_1,{\mathrm{br}}_2,\dots \}$. $V_i(S_i^{*},P_i,T_i)$ indicates a set of switching nodes, egress ports, and time slices; $b_i$ is occupied in the optical switching network, where $S_i^{*}$ represents the switching node set; $P_i$ represents the $b_i$ switching egress ports set; and $T_i$ is a set of $b_i$ arrival times. $h_i$ is the total number of switching nodes passed by $b_i$. $v_i(s_{ik}^{*},p_{ik},t_{ik})$ indicates that $b_i$ at $t_{ik}$ arrives at $s_{ik}^{*}$, and the output from the egress port $p_{ik}$. $S_i^{*}=\{s_{i1}^{*},s_{i2}^{*},\dots ,s_{ihi}^{*}\}$, $P=\{p_{i1},p_{i2},\dots ,p_{ihi}\}$, and $T=\{t_{i}1,t_{i}2,\dots ,t_{ihi}\}$.

Because the DC-SOBS mechanism is adopted in each switching node, node $s_q$ is used to determine whether $b_i$ can successfully reach when performing $b_i$ switching scheduling. Therefore, obtaining all the BCoLs of $b_i$ is necessary before $s_q$, $\forall q\in (1,q-1)$, and the same scheduling algorithm is used to iteratively obtain the scheduling result of $s_q$. Because $b_j$, which conflicts with $b_i$ in one of $q-1$ nodes before, may also conflict with $b_k$ at any other node, a conflict chain is formed between $b_i$, $b_j$, and $b_k$ in the network. Each collision requires a scheduling iteration to obtain passable $b_i$, $b_j$, and $b_k$. The collision chain of BDPs is shown in Fig. 6. When scheduling BDP1 in $S7$, not only the conflict in $S7$, but also the scheduling results of BDP1 to BDP5 from $S1$ to $S6$ should be considered because a collision chain is formed from BDP1 to BDP5. The dashed BDP in Fig. 6 shows the BDP discarded at the current switching node.

Fig. 6

Collision chain diagram of BDP1 in switching node S7. A dashed background displays a discarded BDP in the current switching node.

To limit the number of scheduling iterations in each switching node within a certain range, limiting the range of the collision chain is necessary. Two BDP hop count constraints were added for each BDP: ${\mathrm{TTL}}_{bh}$, collision hops, and ${\mathrm{TTL}}_{bc}$, cumulative collision times. ${\mathrm{TTL}}_{bh}$ is used to control the length of each collision chain branch. One variable ${\mathrm{TTL}}_{\mathrm{bhi}}$ corresponds to one $b_i$, and the initial ${\mathrm{TTL}}_{\mathrm{bhi}}={\mathrm{TTL}}_{\mathrm{bh}}$. When a conflict exists in $b_i$, ${\mathrm{TTL}}_{\mathrm{bhi}}$ reduces by 1 and discards $b_i$ as the value of ${\mathrm{TTL}}_{\mathrm{bhi}}$ is 0. The function of ${\mathrm{TTL}}_{bc}$ is to control the number of branches in a collision chain. One variable ${\mathrm{TTL}}_{\mathrm{bci}}$ also corresponds to one $b_i$, and the initial ${\mathrm{TTL}}_{\mathrm{bci}}={\mathrm{TTL}}_{\mathrm{bc}}$. The counting rule takes the minimum ${\mathrm{TTL}}_{\mathrm{bci}}$ of all conflicting BDPs to $-1$ as the new ${\mathrm{TTL}}_{\mathrm{bcs}}$ of all collision $\mathrm{BDPs}$. Figure 7 shows a collision example of the set ${\mathrm{TTL}}_{bc}=4$. Because two collision constraints (${\mathrm{TTL}}_{\mathrm{bh}}$ and ${\mathrm{TTL}}_{\mathrm{bc}}$) are set, the current BDP is directly discarded when a collision occurs at the number of hops from ${\mathrm{TTL}}_{\mathrm{bh}}$ to $H_{\max }$. Therefore, when the conflict of the next BDP occurs from 1 to ${\mathrm{TTL}}_{\mathrm{bh}}-1$, the collision chain will continue to link to the next BDP, as displayed in Fig. 7. All BDPs in the collision chain are discarded in the next switching node when the ${\mathrm{TTL}}_{bc}$ constraint is satisfied. Thus, the maximum hop length of collision chain $L_{\mathrm{ColMax}}$ is given by Eq. (4). The iteration cycles parameter is set to $N_{\mathrm{sch}}=L_{\mathrm{ColMax}}$. $H_{\max }$ is the maximum number of hops in the BDPs in the satellite optical switching network.

Fig. 7

Schematic diagram of the maximum length of the collision chain when ${\mathrm{TTL}}_{bc}=4$.

The switching nodes execute the DC-SOBS mechanism proposed in this study, including BRP relay, BDP collision detection, BDP collision results distribution, and BDP switching scheduling in four steps. BRPs relay register the arrival time, forwarding address, remaining hops, and other information of BDPs. The BRP relay processing is presented in Algorithm 1.

Algorithm 1

BRPs relay

After receiving a BRP, the switching node opens the RRP relay wait time window, as shown in Fig. 5, and starts the BDP collision detection when the window timer satisfies the constraint. The relay wait time window of $b{r}_i$ is $t_{\mathrm{cwi}}$, as given by Eq. (5). BDP collision detection detects whether $b{r}_i$ has a conflict and then generates BCoLs according to the collision detection results. The BDP collision detection process is shown in Algorithm 2. When the accessing node of the network generates BDPs, the transmission interval between each BRP and BDP is fixed, and the processing time overhead is introduced when it passes through a switching node. The BRP processing delay is reserved according to the maximum BRP processing delay for all switching nodes. Therefore, the time interval between the BRP and BDP after each switching node is reduced by $t_{\mathrm{BRmax}}$. If a switching node receives a $b{r}_i$ of $b_i$, then $b{r}_j$ of other $b_j$ in conflict with $b_i$ will arrive within $t_{\mathrm{cwi}}$. When the wait timer ${\text{Timer}}_{\mathrm{cwi}}=t_{\mathrm{cwi}}$, it executes the steps in Algorithm 2 to search ArriInfTable to find conflict BDPs and generate a BCoL frame. When the BCoL processing timer ${\text{Timer}}_{B\mathrm{coli}}=t_{\mathrm{BCmax}}$ the switching node sends the BCoL frame

Algorithm 2

BDPs collision detection

After $s_{ij}^{*}$ collision detection is complete, the node starts the BDP collision results distribution, as shown in Algorithm 3. The collision result distribution ensures that the BCoLs related to $b_i$ generated by $s_{ik}^{*}$, $\forall k\in (1,j-1)$ in the network are received before $s_{ij}^{*}$ starts switching scheduling of $b_i$. Because collision detection is performed independently in each switching node according to the received $b{r}_i$ to obtain the collision results, the BCoLs are sent according to the destination address of all relevant $b_i$ in the conflict results. All switching nodes, $\forall s_{il}^{*}$, $l\in (j+1,hi)$, in the $b_i$ path $V_i(S_i^{*})$ can obtain the collision information of $s_{ij}^{*}$. However, hidden conflicts may occur in the subsequent switching nodes; that is, the previous switching node does not send the corresponding BCoLs to the subsequent switching nodes. The collision hiding and information replications are shown in Fig. 8. For example, the partial switching paths of ${\mathrm{BDP}}_1$ are $s_{i-1}^{*}$, $s_i$, and $s_j$. The partial switching paths of ${\mathrm{BDP}}_2$ are $s_{i-1}$, $s_i$, and $s_{i+1}$. In egress port A of the switching node $s_{i-1}$, ${\mathrm{BDP}}_1$, and ${\mathrm{BDP}}_2$ conflict, ${\mathrm{BDP}}_2$ forward from egress port A of switching node $s_{i-1}$ to the switching node $s_i$ with scheduling, and ${\mathrm{BDP}}_1$ is discarded. In switching node $s_i$ egress port A, ${\mathrm{BDP}}_2$ and ${\mathrm{BDP}}_3$ conflict, and ${\mathrm{BDP}}_3$ forward to the switching node $s_{i+1}$ from egress port A of $s_i$ with scheduling, and ${\mathrm{BDP}}_2$ is discarded. If there is no conflict at switching node $s_{i+1}$, ${\mathrm{BDP}}_3$ passes through switching node $s_{i+1}$ to switching node $s_{i+2}$. ${\mathrm{BDP}}_4$ and ${\mathrm{BDP}}_3$ conflict at switching node $s_{i+2}$. The scheduling result discards $b_3$ and allows $b_4$ to pass. Because the collision information of ${\mathrm{BDP}}_1$ and ${\mathrm{BDP}}_2$ can only be obtained at the switching node in their switching paths, the switching node $s_{i+2}$ cannot obtain the collision information of ${\mathrm{BDP}}_1$. However, when scheduling ${\mathrm{BDP}}_3$ and ${\mathrm{BDP}}_4$, the switching node $s_{i+2}$ considers the collision between ${\mathrm{BDP}}_2$ and ${\mathrm{BDP}}_3$ in the switching node $s_i$, and ${\mathrm{BDP}}_1$ and ${\mathrm{BDP}}_2$ in $s_{i-1}$, and determines whether ${\mathrm{BDP}}_3$ is dropped in the previous nodes. Here, ${\mathrm{BDP}}_1$ is the hidden conflict BDP of switching node $s_{i+2}$. Therefore, it is necessary to copy ${\mathrm{BCoL}}_{12}$, the collision detection results of ${\mathrm{BDP}}_1$ to switching node $s_{i+2}$, and related subsequent switching nodes. Each egress port performs BCoL processing independently.

Algorithm 3

BDP collision results distribution

Fig. 8

Hidden BCoL and its replication. ${\mathrm{BCoL}}_{12}$ is a hidden BCoL for switching nodes from $s_{i+2}$, replicated at switching node $s_{i+2}$, and forwarded to subsequent switching nodes.

As shown in Fig. 5, when the latest BDP arrival time satisfies the constraints in Eq. (6), the switching scheduling of all BDPs in the schedule window is performed. Here, $t_{\mathrm{SHmax}}$ is the scheduling time, and $t_{\mathrm{SCmax}}$ is the computation time. During BDP switching scheduling, BDP validity detection is performed first, and each egress port conduct BDPs validity detection independently. By iterating the received BCoLs of the previous switching node, the BDPs that can reach the current switching node are screened, and then these BDPs are scheduled to obtain the results. According to the results, the optical switching device is configured, and switching is completed when the BDPs reach the switching node. The BDP switching scheduling is displayed in Algorithm 3.

First, the identification numbers of all $b_i$ in this scheduling cycle are obtained, then the collision information of these $b_i$ is searched in ColTab to obtain the conflict status of these $b_i$ in the previous switching nodes. BDP validity detection and switching scheduling were performed according to the collision tree. Each node of the collision tree is a conflict-switching node of the BDP switching path, and the information in the node is the collision information related to the BDP at the conflict-switching node. The next-level conflict tree node is established according to the conflict information. Each upper-level switching node of the BDP from various ingress ports that conflicts in the current node correspond to a lower-level collision tree node. The root node is the current switching node, and the construction method for the collision tree is shown in Fig. 9. ${\mathrm{BDP}}_1$ and ${\mathrm{BDP}}_4$ establish a collision tree at switching node $s_i$. All nodes at the same level in the collision tree are simultaneously judged.

Fig. 9

Schematic diagram of conflict tree construction for ${\mathrm{BDP}}_1$ and ${\mathrm{BDP}}_2$ at root switch node $s_i$.

If the collision tree level of a BDP is greater than or equal to $N_{\mathrm{sch}}$, it indicates that the BDP exceeds the collision-restriction conditions, and the BDP is directly discarded. BDPs that are not directly discarded are judged iteratively from the leaf node to the root switching node. According to ${\mathrm{TTL}}_{\mathrm{bc}}$ and ${\mathrm{TTL}}_{\mathrm{bh}}$, the BDP validity detection is judged to determine whether the BDP in the current switching node is effective. If BDP is effective, the BDP is valid. If the BDP is discarded, the BDP is invalid. Then, the next BDP collision tree judge is performed. After completing all BDP judges of the current scheduling cycle, the judge results of the current node are obtained.

In this study, a simple first-come-first-service (FCFS) scheduling strategy is used to verify the realizability and performance of the switching method. After the BDP validity detection is performed in each scheduling cycle, the BDP valid at the current switching node is scheduled, and the reserved BDP is scheduled first. The reserved BDP can pre-empt competitive BDP. Because BDPs at the end of the current cycle may span the next scheduling cycle and affect the BDP scheduling of the next cycle, the BDP of the previous cycle should be considered when scheduling the next scheduling cycle. The specific switching scheduling algorithm is as follows:

Algorithm 4

BDP switching scheduling

2.4.

Analysis of transmission interval $T_{\text{offset}}$

The reserved processing time in $T_{\mathrm{offset}}$ includes four parts: BRP relay wait time $T_{\mathrm{BR}}$ of the switching node, collision detection time $T_{\mathrm{CD}}$, BDPs collision results distribution time $T_{\mathrm{CR}}$, and switching scheduling time $T_{\mathrm{SP}}$. As shown in Eq. (8), with the DC-SOBS mechanism, BRP processing in the electrical domain introduces processing overhead time, whereas BDP does not introduce this time when passing through the switching node. Therefore, the time interval between BDP and BRP is shortened when passing through a network-switching node. When a network-accessing node sends BRP and BDP pairs, it is necessary to ensure that the time interval $T_{\text{offset}}$ between BDP and BRP can still satisfy the processing time of the last optical switching node when the BDP reaches the last switching node on the switching path. The reservation-time relationship of each part in $T_{\text{offset}}$ is shown in Fig. 10, and a quantitative analysis and description of $T_{\text{offset}}$ are as follows:

Fig. 10

Detailed description of Eq. (8) including obtaining the time values of the components of $T_{\text{offset}}$.

First, $T_{\mathrm{BR}}$ is analyzed. The BRP relay processing time of $b_i$ reserved in $T_{\text{offset}}$ is $T_{\mathrm{BR}}(b_i)$, and $T_{\mathrm{BR}}(b_i)$ satisfies the constraints of Eq. (9), where $t_{\mathrm{BR}j}$ represents the BRP processing time of $s_{ij}^{*}$, and $h_i$ represents the number of switching nodes passed by $b_i$

Second, the BDP collision detection time $T_{\mathrm{CD}}$ is analyzed. The time switching node $s_n$ performs BDP collision detection is $t_{\mathrm{CD}n}$, and each $s_n$ has a different $t_{\mathrm{CD}n}$, so it is difficult to obtain $t_{\mathrm{CD}1},t_{\mathrm{CD}2},\dots ,t_{\mathrm{CDN}}$ for any of the accessing nodes. Let $t_{\mathrm{CDmax}}$ be the upper maximum value of $t_{\mathrm{CD}n}$, as shown in Eq. (10). Thus, we take $T_{\mathrm{CD}}=t_{\mathrm{CDmax}}$

Third, the BDP collision results distribution time $T_{\mathrm{CR}}$ is analyzed. After completing the BDP collision detection, the detection results (BcoLs) should be sent to all relevant subsequent switching nodes, which should be received by all BCoLs before they perform switching scheduling. The maximum scheduling iteration period is $N_{\mathrm{sch}}$. The $T_{\mathrm{CR}}$ reserves time for receiving the BCoLs of $N_{\mathrm{sch}}$-level switching nodes. Therefore, the $T_{\mathrm{CR}}$ and $T_{\mathrm{BR}}$ time analyses are similar. We set $t_{\mathrm{CRmax}}$ as the upper limit maximum value of $t_{\mathrm{CR}n}$, as expressed in Eq. (11). $T_{\mathrm{CR}}$ satisfies the constraint in Eq. (12), where $T_{\mathrm{BCoL}}$ is the sending time of BCoL

Fourth, the BDP switching scheduling time $T_{\mathrm{SP}}$ is analyzed. $T_{\mathrm{SP}}$ includes a fixed schedule window time $t_{\mathrm{WD}}$, switching scheduling time $t_{\mathrm{SHmax}}$, and configuration time $t_{\mathrm{SCmax}}$. The fixed window time $t_{\mathrm{WD}}$ is a constant value for the user set. The switching schedule time for each $s_n$ is distinct. Here, $t_{\mathrm{SHmax}}$ is set as the maximum of $t_{\mathrm{SHn}}$, which is the switching schedule time of $S$, as displayed in Eqs. (13–15)

In summary, the time interval $T_{\text{offset}}$ is expressed by the following equation