Hybrid Routing and Adaptive Spectrum Allocation for Flex-Grid Optical Interconnects

Yuanyuan Hong; Xuezhi Hong; Sailing He; Jiajia Chen

doi:10.1364/JOCN.10.000506

Journal of Optical Communications and Networking
Vol. 10,
Issue 5,
pp. 506-514
(2018)
•https://doi.org/10.1364/JOCN.10.000506

Hybrid Routing and Adaptive Spectrum Allocation for Flex-Grid Optical Interconnects

Yuanyuan Hong, Xuezhi Hong, Sailing He, and Jiajia Chen

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Yuanyuan Hong, Xuezhi Hong, Sailing He, and Jiajia Chen, "Hybrid Routing and Adaptive Spectrum Allocation for Flex-Grid Optical Interconnects," J. Opt. Commun. Netw. 10, 506-514 (2018)

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Abstract

A hybrid routing scheme with an adaptive spectrum assignment is proposed for flex-grid all-optical core switch supporting multihop transparent paths in data center networks. Compared with conventional spectrum assignment algorithms (RSA) developed for a multihop network with optical-electric-optical (OEO) conversion in every hop (i.e., RSA for EO) and that devised for an all-optical multihop network (i.e., RSA for AO), the present RSA algorithm provides better utilization of network resources. Being aware of the all-optical bypass path in hopping, the proposed RSA reduces the blocking probability due to lack of bandwidth-tunable transceivers, which is the major reason for blocking for an RSA for the EO. Similar to the RSA for the AO, the proposed RSA is compatible with the number-of-hops adaptive spectrum assignment, which improves spectrum efficiency. On the other hand, the new algorithm enhances connectivity by eliminating the number-of-hops limitation, which severely constrains the performance of RSA for the AO. Simulations for the system are carried out to investigate the performance of the new algorithm. The impacts of various parameters, such as traffic load, ratio of connection requests with different data rates, and resource configuration on the link cost, are studied in terms of network blocking probability (BP). The achievable traffic load of the proposed RSA under varied connection degrees (i.e., the maximum number of ports that one rack has in order to connect to the core switch) and number of racks is also assessed to keep BP no more than 0.1. The results show that the proposed RSA with appropriate cost functions outperforms the EO and AO, which implies that it has the highest scalability.

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Symbol	Definition
$N N$	The set of nodes. Each node refers to a rack. Nodes $i, j, s, t, x \in N N$ .
$E E$	The set of links. Each link refers to a direct physical link connecting the nodes. Link $e \in E E$ .
$G (N N, E E)$	The physical topology constituted by the node set $N N$ and link set $E E$ . Each node in $N N$ is connected to up to $D$ other nodes. $D$ is defined as the node degree. In the simulation, the switch configuration is randomly generated, with the probability of $D / (N - 1)$ to establish a direct physical link between any pair of nodes.
$S S$	The set of spectrum slots in each link with the total number of spectrum slots $S$ . Slot $s s \in S S$ .
$State (s s, e)$	The state of spectrum slot $s s$ in link $e$ . It equals 0 if slot $s s$ is idle or 1 if slot $s s$ is occupied in link $e$ .
$NumBVT (i)$	Number of available BVTs in node $i$
$Traf (s, t, cap)$	Connection request, which starts from node $s$ to node $t$ and demands the capacity cap.
$Translink (i, j)$	Optical transparent link from node $i$ to $j$ , also referred to as an all-optical connection in the case of AO, without any OEO conversions in any intermediate nodes.
$Numlink (i, j)$	The number of hops in $Translink (i, j)$ . According to [12], the number of hops should not exceed to a threshold in order to keep the transmission performance in an acceptable level.
$Transtopo (N N, Translink)$	The topology constituted by the node set $N N$ and the optical transparent link set Translink. The cost of $Translink (i, j)$ is determined by the cost functions, i.e., Eq. (1) or (2) in the proposed RSA.
$pathpro (s, t)$	A path from node $s$ to $t$ , which is calculated based on the proposed RSA and employs OEO conversions only between two consecutive optical transparent links and hence not necessarily at each intermediate node.
${pathpro}_{k} (s, t)$	By running $K$ -shortest path (KSP) function, up to $K$ paths are generated as the candidates for $pathpro (s, t)$ , where $k = 1, 2, \dots, K$ .
$M_{k} (s, t)$	The number of optical transparent links in ${pathpro}_{k} (s, t)$ . The destination node of the $m$ -th optical transparent link in ${pathpro}_{k}$ is indicated as $x_{k} (m)$ , $m = 1, 2, \dots, M_{k} (s, t)$ . $x_{k} (0) = s$ .
$S S index (s, t, m)$	The index of spectrum slot, which represents the starting slot of the $m$ -th optical transparent link in $pathpro (s, t)$ . All the hops in the same optical transparent link are allocated with the same number of spectrum slots, i.e., $Numss (s, t, m)$ starting from spectrum slot index $S S index (s, t, m)$ .
$S S {index}_{k} (s, t, m)$	Index of spectrum slot, which represents the starting slot of the $m$ -th optical transparent link in ${pathpro}_{k} (s, t)$

Degree and Number of Racks	$D = 6$ , $N = 32$	$D = 4$ , $N = 64$	$D = 6$ , $N = 64$	$D = 8$ , $N = 64$	$D = 6$ , $N = 96$
Possibility of blocking	0	0.45	0.05	$< 1 e - 4$	0.12

RSA Algorithms	Proposed RSA with ${cost}_{1}$		Proposed RSA with ${cost}_{2}$		EO		AO
$N = 64$ , traffic load $capacity = 14 Tbps$	$D = 4$	$D = 8$	$D = 4$	$D = 8$	$D = 4$	$D = 8$	$D = 4$	$D = 8$
Number of hops	3.10	2.20	3.37	2.21	3.42	2.21	2.44	2.21
$D = 6$ , traffic load $capacity = 14 Tbps$	$N = 32$	$N = 96$	$N = 32$	$N = 96$	$N = 32$	$N = 96$	$N = 32$	$N = 96$
Number of hops	2.03	2.72	2.12	2.74	2.13	2.73	2.02	2.54

Symbol	Definition
$N N$	The set of nodes. Each node refers to a rack. Nodes $i, j, s, t, x \in N N$ .
$E E$	The set of links. Each link refers to a direct physical link connecting the nodes. Link $e \in E E$ .
$G (N N, E E)$	The physical topology constituted by the node set $N N$ and link set $E E$ . Each node in $N N$ is connected to up to $D$ other nodes. $D$ is defined as the node degree. In the simulation, the switch configuration is randomly generated, with the probability of $D / (N - 1)$ to establish a direct physical link between any pair of nodes.
$S S$	The set of spectrum slots in each link with the total number of spectrum slots $S$ . Slot $s s \in S S$ .
$State (s s, e)$	The state of spectrum slot $s s$ in link $e$ . It equals 0 if slot $s s$ is idle or 1 if slot $s s$ is occupied in link $e$ .
$NumBVT (i)$	Number of available BVTs in node $i$
$Traf (s, t, cap)$	Connection request, which starts from node $s$ to node $t$ and demands the capacity cap.
$Translink (i, j)$	Optical transparent link from node $i$ to $j$ , also referred to as an all-optical connection in the case of AO, without any OEO conversions in any intermediate nodes.
$Numlink (i, j)$	The number of hops in $Translink (i, j)$ . According to [12], the number of hops should not exceed to a threshold in order to keep the transmission performance in an acceptable level.
$Transtopo (N N, Translink)$	The topology constituted by the node set $N N$ and the optical transparent link set Translink. The cost of $Translink (i, j)$ is determined by the cost functions, i.e., Eq. (1) or (2) in the proposed RSA.
$pathpro (s, t)$	A path from node $s$ to $t$ , which is calculated based on the proposed RSA and employs OEO conversions only between two consecutive optical transparent links and hence not necessarily at each intermediate node.
${pathpro}_{k} (s, t)$	By running $K$ -shortest path (KSP) function, up to $K$ paths are generated as the candidates for $pathpro (s, t)$ , where $k = 1, 2, \dots, K$ .
$M_{k} (s, t)$	The number of optical transparent links in ${pathpro}_{k} (s, t)$ . The destination node of the $m$ -th optical transparent link in ${pathpro}_{k}$ is indicated as $x_{k} (m)$ , $m = 1, 2, \dots, M_{k} (s, t)$ . $x_{k} (0) = s$ .
$S S index (s, t, m)$	The index of spectrum slot, which represents the starting slot of the $m$ -th optical transparent link in $pathpro (s, t)$ . All the hops in the same optical transparent link are allocated with the same number of spectrum slots, i.e., $Numss (s, t, m)$ starting from spectrum slot index $S S index (s, t, m)$ .
$S S {index}_{k} (s, t, m)$	Index of spectrum slot, which represents the starting slot of the $m$ -th optical transparent link in ${pathpro}_{k} (s, t)$

Degree and Number of Racks	$D = 6$ , $N = 32$	$D = 4$ , $N = 64$	$D = 6$ , $N = 64$	$D = 8$ , $N = 64$	$D = 6$ , $N = 96$
Possibility of blocking	0	0.45	0.05	$< 1 e - 4$	0.12

Initialization
1. Input: $G (N N, E E)$ , Output: $Transtopo (N N, Translink)$ .
2. for any two nodes $i$ and $j$ do
3. $Translink (i, j) \leftarrow Dijkstra (G, i, j)$
4. if there is no available $Translink (i, j)$ , or $i = j$ then
5. $Translink (i, j) \cdot \cos t \leftarrow \infty$
6. else $Translink (i, j) \cdot \cos t \leftarrow \cos t_{k} (i, j)$ , where $k = 1$ or 2
7. end if end for
Main
8. Input: $Transtopo (N N, Translink)$ ), $Traf (s, t, cap)$ , NumBVT, State.
9. Output: $pathpro (s, t)$ , SSindex, Numss.
10. if $NumBVT (s) = 0$ or $NumBVT (t) = 0$ return noBvt Blocking
11. else $KSP (Transtopo (N N, Translink), s, t, K, NumBVT)$
12. if there is no pathpro found then return noBvt Blocking
13. else
14. for $k = 1$ to $K$ do
15. for $m = 1$ to $M_{k} (s, t) - 1$ do
16. if for each link $e$ in $Translink (x_{k} (m - 1), x_{k} (m))$ there are no sufficient common available spectrum slots found to satisfy the capacity requirement
17. terminate for
18. else set $S S {index}_{k} (s, t, m)$ and $m \leftarrow m + 1$
19. end for
20. if $m = M_{k} (s, t)$
21. $pathpro (s, t) = {pathpro}_{k} (s, t)$ , $S S index = S S {index}_{k}$ ,
22. Update State and NumBVT,
23. return Success
24. else $k \leftarrow k + 1$
25. end for
26. if $k = K + 1$ return noSpec Blocking

Initialization
1. Input: $G (N N, E E)$ , Output: $Transtopo (N N, Translink)$ .
2. for any two nodes $i$ and $j$ do
3. $Translink (i, j) \leftarrow Dijkstra (G, i, j)$
4. if there is no available $Translink (i, j)$ , or $i = j$ then
5. $Translink (i, j) \cdot \cos t \leftarrow \infty$
6. else $Translink (i, j) \cdot \cos t \leftarrow \cos t_{k} (i, j)$ , where $k = 1$ or 2
7. end if end for
Main
8. Input: $Transtopo (N N, Translink)$ ), $Traf (s, t, cap)$ , NumBVT, State.
9. Output: $pathpro (s, t)$ , SSindex, Numss.
10. if $NumBVT (s) = 0$ or $NumBVT (t) = 0$ return noBvt Blocking
11. else $KSP (Transtopo (N N, Translink), s, t, K, NumBVT)$
12. if there is no pathpro found then return noBvt Blocking
13. else
14. for $k = 1$ to $K$ do
15. for $m = 1$ to $M_{k} (s, t) - 1$ do
16. if for each link $e$ in $Translink (x_{k} (m - 1), x_{k} (m))$ there are no sufficient common available spectrum slots found to satisfy the capacity requirement
17. terminate for
18. else set $S S {index}_{k} (s, t, m)$ and $m \leftarrow m + 1$
19. end for
20. if $m = M_{k} (s, t)$
21. $pathpro (s, t) = {pathpro}_{k} (s, t)$ , $S S index = S S {index}_{k}$ ,
22. Update State and NumBVT,
23. return Success
24. else $k \leftarrow k + 1$
25. end for
26. if $k = K + 1$ return noSpec Blocking

Abstract

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Journal of Optical Communications and Networking