A Novel Access Protocol for Collision-Free and Low ...

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Email: [email protected] ... each node can send its burst on S wavelengths, the scalability ... collision-free scheme without virtually occupying wavelength.
A Novel Access Protocol for Collision-Free and Low-Latency OBS-Ring Networks Hieu Bui-Trung and Ngoc T. Dang

Vuong V. Mai and Anh T. Pham

Posts and Telecommunications Institute of Technology Hanoi city, Vietnam Email: {hieubt,ngocdt}@ptit.edu.vn

Computer Communications Lab., The University of Aizu Aizu-Wakamatsu city, Fukushima, Japan Email: [email protected]

researchers have concentrated on two solutions. For the first solution, the OBS-ring architecture is extended in order to use bandwidth efficiently [4]. For the second one, authors propose to employ access protocols in OBS-ring networks to avoid burst collision [3]-[8]. The network extension has been done by dividing the total number of wavelengths that are provided by an optical fiber into S sets of wavelength [4]. The number of wavelengths in each set is equal to the number of nodes in an OBS ring. As each node can send its burst on S wavelengths, the scalability of the network is therefore expanded S times. However, this proposal has the disadvantage in increasing the overall cost of the network because it requires to employ S couples of highcost tunable transmitters/tunable receivers at each node. Thus, this method has weaknesses in term of competitive prize and practicability. With the second solution, some approaches called collisionfree access protocols have been applied. Firstly, a single tokenbased access protocol has been employed [3]-[5]. In that access protocol, if node A tends to transmit bursts to node B, it has to keep the token of node B. In the ring, due to the fact that there is only one token of B, there is not more than one node transmitting bursts to node B at a given time. Consequently, no collisions and no burst losses occur on the network. Similar to that, multi-token-based access protocol was also used in [6][8]. The different here is that the multiple tokens are employed instead of single one to support a flexible burst transmission on different wavelengths. Although token-based access protocols can prevent collisions, the mean queuing delay is still large when the number of nodes in OBS-Ring networks is large. This is because it takes a long time for tokens to rotate. The high mean queuing delay makes the networks inefficiency and not suitable for some real-time applications. In this paper, we therefore propose a novel efficient access protocol referred as locating idle time slot (LIT)-based access protocol. It deals with collisions by locating idle time slots on wavelengths to transmit bursts. Comparing with token schemes, LIT-based access protocol provides a collision-free scheme without virtually occupying wavelength resource. Thus, the mean queuing delay is improved significantly. Additionally, for further performance improvement, we apply multiple fixed transmitters/fixed receivers(FTs/FRs)

Abstract—In this paper, we propose a novel access protocol for optical burst switching (OBS)-ring networks. The proposed access protocol is based on the scheme of locating idle time slot at all wavelengths before sending bursts. This would help to avoid burst collision and efficiently use wavelength resource. To support this access protocol, a low-cost node architecture that provides burst transmission on multiple wavelengths is also proposed. The numerical results show that OBS-ring network using proposed access protocol outperforms the ones using wavelength-based and multi-token-based access protocols in terms of mean queuing delay. In addition, it can support a larger number of nodes and high burst arrival rate compared to that of the conventional OBR-ring networks.

I. I NTRODUCTION In the past several years, the amount of research being done in the area of optical burst switching (OBS) has increased tremendously. OBS is designed to achieve a balance between optical circuit switching (OCS) and optical packet switching (OPS). In comparison with OCS and OPS, OBS has several advantages including high bandwidth utilization, low setup latency, low overhead, and high traffic adaption [1]. Following the survey of Battestelli and Perros about OBS and its versions [2], there are two popular types of OBS network topology, which are the mesh and the ring. In recent years, various studies have focused on ring topologies [3][8]. Those studies are motivated due to the fact that the ring is simpler than mesh and extensively used on current optical networks such as backbone and metro area networks (MANs). In [3], the authors proposed an OBS-ring network using wavelength-based access protocol. Each node of the network is assigned a unique wavelength called home wavelength, which is used for transmitting its burst. Thus, the number of wavelengths used for carrying bursts equal to the number of nodes. However, the total number of wavelengths that can be provided by an optical fiber is much larger than the number of nodes in a ring network. As a result, there are many unused wavelengths hence the network is not scalable and bandwidthefficient. Moreover, because there is only one receiver at a node, the collisions may occur and the burst loss may appear in this kind of OBS-ring network when multiple nodes transmit their bursts to a node at the same time. To overcome the drawbacks of OBR-ring network proposed in [3], including bandwidth-inefficiency and burst collision,

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Fig. 1.

Architecture of an OBS-ring network.

Access network

Access network

Burst Assembler/ Analyzer

We consider an OBS-ring network with N nodes as shown in Fig. 1. Each node is located on the ring and connected to others by an optical fiber. This network might be a backbone network or MAN, where each node owns several access networks. OBS-ring network provides data transport services from an access network to others. The total number of wavelengths in the network is M +1, where M wavelengths denoted as W1 , W2 , ..., WM are used for transmitting data bursts and the remaining wavelength (W0 ) is dedicated for the transmission of control frame. To support transmitting and receiving bursts on M data wavelengths, each node use M fixed transmitters and M fixed receivers. The general structure of a node is presented in Fig. 2. Each node has three functions including (1) transmitting bursts: packets from access networks are assembled into bursts. Those bursts are then stored in queues based on their destination node address. At suitable time, bursts will be transmitted into the ring by FTs; (2) receiving bursts: at destination nodes, bursts will be received by FRs. They are then disassembled back to packets before being pushed to corresponding access networks; and (3) processing the control frame: the control frame contains information of all burst transmitted on the ring. By processing the control frame, a node collects necessary information for receiving burst or transmitting burst in suitable times. The structure of a control frame is shown in Figure 3. The control frame comprises of M time slots that are corresponding to M data wavelengths in the ring. Each time slot contains a flag and control information of bursts transmitted

(N-1) queues

Scheduler

General Controller General Controler

II. OBS- RING N ETWORK USING M -FT/M -FR

CF Processer

to each node. This allows a node to transmit bursts on multiple wavelengths. Compared to OBS-ring networks using wavelength-based access protocol proposed in [3], the number of wavelengths in our proposed OBS-ring networks is not limited by the number of nodes. Moreover, in comparison with tunable receivers, fixed receivers are much more popular and cheaper. Consequently, by applying LIT-based access protocol and multiple FTs/FRs to OBR-ring networks, the OBS-Ring network can achieve collision-free, low-latency, and efficient use of wavelengths. To show the advantages of our proposal, in this paper, we will theoretically analyze and compare the performance of OBS-ring network using LIT-based access protocol and that of the ones using wavelength-based and multi-token based access protocols. The comparison will be done in terms of mean queuing delay versus various network’s parameters such as burst arrival rate per nodes, the number of nodes, and the number of wavelengths. The rest of this paper is organized as follows. Section II briefly describes the OBS-ring network architecture using multiple FTs/FRs. Section III presents the multi-token-based and LIT-based access protocols. In Section IV, the mean queuing delay of OBS-ring networks is theoretically analyzed. Numerical results are presented in Section V, and Section VI concludes the paper.

M FTs

M FRs

Optical Add-Drop Multiplexer

Fig. 2.

Node structure in OBS-ring networks using M -FT/M -FR.

on that wavelength. The control information of a burst has the following fields: a) destination address; b) offset value; and c) burst length. The flag of each slot carries information which indicates the starting point of that slot. In addition, other purpose of flags related to token protocols will be explained later on. III. C OLLISION -F REE ACCESS P ROTOCOL In OBS-ring networks using M -FT/M -FR, collisions may occur due to the fact that N nodes share M data wavelengths. Figure 4 illustrates an example of this issue [7]. When node k wishes to transmit a burst and inspects that the current drop wavelength Wi of this burst is available, it transmits the burst towards on that wavelength (see Fig. 4(a)). However, since

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Control frame Slot 1

Slot 2

Idle time slot

Slot k

Busy time slot

Slot M

WM Burst 1

Burst 2

Destination address

Burst h

Offset

Burst t

Flags

WM-1

Burst length

W2

Fig. 3.

A new oncoming burst

The structure of control frame.

A transmitting burst

W1 t

Collision

Fig. 5. Fixed trasmitter (a)

Fig. 4.

t + TSP

ITSs and BTSs on all wavelengths in a signaling period.

Fixed trasmitter (b)

control frame coming back to node k, token i will be released and updated from zero to one. It can see that in the time period from t to t + TSP there is only node k transmitting bursts on wavelength Wi , so that there are no collisions occurring on this wavelength. Multi-token-based access protocol, however, has a weakness in efficiency of using wavelength. For example, we assume that node 1 captures token i to transmit bursts to node 2 on wavelength Wi . Although these bursts are completely received by node 2, other nodes from node 3 to node N still cannot use wavelength Wi until the next signaling period. This is because before coming back to node 1 the control frame has to go through node 3 to node N, at each that node, token i notifies that wavelength Wi is busy.

Collision issue in OBS-ring networks.

node k is unable to detect a new oncoming burst on wavelength Wi , collisions will occur if the new burst arrives at the node before the current burst transmission has been completed (see Fig. 4(b)). In this case, the OBS network may fail to make efficient use of the bandwidth since the transmissions are not collision-free and some of the bursts are lost or delayed. Therefore, it is necessary to employ an access protocol to solve the burst collision problem. In this section, for the purpose of comparison, two collisionfree access protocols will be presented. First, we briefly describe the multi-token-based access protocol, which has been proposed and used in previous works [6]-[8]. We then present a novel access protocol that is based on locating idle time slot before sending bursts.

B. LIT-based Access Protocol We note that collisions will occur when a node transmits its bursts and passes bursts (from other nodes) at the same time and on the same wavelength. Therefore, a simple idea to avoid collisions is the node must transmit bursts in the time when there are not any burst passing. Considering a specific wavelength at a node, we define that a busy time slot (BTS) is a time period for passing a burst through the node and an idle time slot (ITS) is a time period between two adjacent busy time slots. Let t be the arrival time at the control module of the control frame. A passing burst, which has length of h and offset time of δ, will occupy the node in a time period from t + δ to t + δ + h, i.e., a busy time slot. An example of BTSs and ITSs during one signaling period is illustrated in Fig. 5. Each ITS is characterized by three parameters including the wavelength (Wi ), the starting point (tIT S ) and the length of that ITS (LIT S ). At each wavelength, there may be more than one ITS. To transmit a burst with size L and offset time δ, the general controller must first consult the information from control frame to locate a suitable ITS that satisfies the condition tIT S ≤ t + δ + L ≤ tIT S + LIT S . For optimizing wavelength usage efficiency, if there is more than one ITS satisfying the condition, ITS with smallest value of LIT S will be chosen. It is possible that none of ITSs should be suitable or all data wavelengths are full of BTSs. In this case, the burst

A. Multi-Token-Based Access Protocol This protocol uses tokens to resolve collisions on data wavelengths. Unlike traditional token-based protocols proposed in convenient OBS-ring networks [7] that use N tokens assigned to N nodes, in OBS-ring networks using M -FT/M FR, M tokens are used for M data wavelengths, one for each wavelength. A token indicates the corresponding wavelength being in idle or busy state. In fact, the token of wavelength Wi is a binary bit denoted as Si and located in the flag field of the i-th slot. If wavelength Wi is idle, then Si is set to one. Otherwise, it is set to zero. When the control frame coming to node k at time t, by checking value of all tokens, the node has the state of all wavelengths and then can locate idle wavelengths to transmit bursts. To note that, if the node tends to transmit bursts on an idle wavelength such as wavelength Wi , it has to capture token i by setting Si from one to zero. When the control frame coming to next nodes, since these nodes see that Si is set at zero value, they will not transmit bursts on wavelength Wi . After a signaling period, i.e., the amount of time for the control frame completing a round on ring (TSP ), when the

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is delayed and waits to be transmitted in the next signaling period.

THE FIRST GROUP

Node 1 0

IV. P ERFORMANCE A NALYSIS In OBS networks, the mean burst loss probability and the mean queuing delay (TM QD ) are two most important measures for evaluating quality of service (QoS). However, by applying collision-free access protocols, the burst loss is completely prevented. Thus, in this section, we will present a model to calculate TM QD of OBS-ring networks using LIT-based and multi-token-based access protocols. We assume that burst arrival process to nodes i is Poisson process with rate λi (burst/second) that is identically distributed exponential random variable. Also, the burst arrival to node i destined to node j is a Poison process with rate λi,j . For sake of simplicity, we consider the case that the traffic on the OBS ring is uniform and symmetric, i.e., λi = λ, ∀i ∈ (0, N − 1) and each node sends equal traffic to all destinations. λi,j therefore can be expressed as λi,j

λ . = N −1

Node i-1

Node 1 2

Node i

Node i+1

Node N-1

RING

Fig. 6.

A model for evaluating the arrival rate of bursts passing via node i.

the first group includes nodes from node 1 to node (i − 1); the second group consists of nodes from node (i + 1) to node N . In the case that upstream nodes belong to the first group, the arrival rate of burst can be expressed as   k−1 N i−1 X X X  λk,j  λk,j + λ′1 = k=1

(1)

=

i−1 X

j=i+1

j=1

(N + k − i − 1)

k=1

Other assumptions are as follows. The bursts have random lengths determined at each node as independent, identically, and geometrically distributed random variables with mean E[B] (bits). The data wavelengths and the control wavelength have the same bit rate of R (bit/s). Before calculating TM QD , we firstly compute the probability that a burst is delayed (PBD ).

λ . N −1

(3)

When upstream nodes belong to the second group, the arrival rate of burst can be expressed as λ′2 =

N X

k−1 X

k=i+1 j=i+1

λk,j =

N X

k=i+1

(k − i − 1)

λ . N −1

(4)

Based on Eqs. (3) and (4), the total arrival rate of burst from all upstream nodes passing via node i, after simplifying the equation, can be computed as

A. PBD for LIT-based Access Protocol In OBS-ring networks using LIT-based access protocol, when node i tends to push a burst into the network at time t′ , it will locate ITSs at all M wavelengths for transmitting the burst. If ITSs are not apparent on all M data wavelengths at time t, the burst will be delayed. The event of burst delayed can be considered as the case that customers arrive at a queuing system having M servers but having zero waiting positions. For sake of simplicity, optimization in locating the most suitable ITS is ignored, we therefore can use Erlang-B queuing model to calculate PBD . Generally, the blocking probability of the multi-server system that had Poisson arrival process with arrival rate of λ′ , service time following an exponential distribution with mean size S, and M servers working in parallel, can be written as [9] AM /M ! , B (A, M ) = PM k k=0 A /k!

THE SECOND GROUP

(N − 2) λ . (5) 2 Finally, based on Eq. (2), burst delay probability is calculated as M  (N −2)λE[B] /M ! 2R , (6) PBD−LIT = B (Ai , M ) = k  PM (N −2)λE[B] /k! k=0 2R λ′LIT = λ′1 + λ′2 =

where Ai is the traffic intensity passing via node i. B. PBD for Multi-Token-based Access Protocol

Similar to LIT-based access protocol, when a node using multi-token-based access protocol tends to push a burst into the network at time t′ , it will check all M tokens in the control frame to find idle wavelengths. If all wavelengths are busy at time t′ , the burst will be delayed. Thus, PBD also can be obtained by employing Eq. (2) with the number of servers equals M . However, the traffic intensity (A) is calculated in a different way. According to the principle of multi-token-based access protocol, when an upstream node catches a token for a particular wavelength, other remaining nodes cannot send their bursts on that wavelength. It means that, besides the real traffic, there is virtual traffic passing node i, which is corresponding to the

(2)

where A is the traffic intensity and written as A = λ′ S. The service time is related to the mean length of burst by S = E[B]/R. We assume that node i plays a role in passing bursts, i.e., bursts sent by upstream nodes use node i as a bridge to reach their destinations as shown in Fig. 6. Hence, λ′ is the total arrival rate of burst from all upstream nodes passing via node i. To evaluate λ′ , we classifies upstream nodes into two groups:

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TABLE I S YSTEM PARAMETERS AND C ONSTANTS .

TSP

Node N

Node h

TT T R

TP

Name Bit rate Mean burst size Length of control frame Average distance between to consicutive nodes Speed of light Refractive index of fiber core Processing time

TG

Node 2

CF

CF

Node 1

Fig. 7.

Cotrol frame on a signalling period.

Wavelength−based access protocol Multi−token−based access protocol LIT−based access protocol

300 (µs) MQD

Mean queuing delay, T

Accordingly, based on Eq. (2), burst delay probability is calculated as  M (N −1)λE[B] /M ! R PBD−M T = . (8)  k PM (N −1)λE[B] /k! k=0 R

250 200 150 100 50 0

C. Mean Queuing Delay

When a burst is delayed, it has to wait for an opportunity to be sent in the next signaling period. The queuing delay for that burst is at least TSP . The lower bound of mean queuing delay can be expressed as

0

1000 2000 3000 4000 Burst arrival rate per node, λ (burst/s)

5000

Fig. 8. Mean queuing delay versus the burst arrival rate per node with N = 8 nodes and M = 10 wavelengths.

(9) V. N UMERICAL R ESULTS

where PBD is obtained from Eq. (6) or (8). Figure 7 shows a trip of the control frame on the signaling round. Without loss of generality, we assume that control frame starts from node 1, it then goes via node 2, ..., and node N before coming back to node 1. TSP is given by TSP = (TT + TR + TP + TG ) N,

Value 2.5 Gbit/s 80 Kbytes 100 bytes 20 km 3 × 108 m/s 1.5 40 µs

350

busy state of wavelength. Therefore, the total arrival rate of bursts from all upstream nodes passing via node i equals the total arrival rate of all nodes in the network except node i and is calculated as λ′M T = (N − 1) λ. (7)

TM QD ≥ PBD TSP ,

Symbol R E [B] LCF DAV G c n0 Tp

In this section, we show the numerical results to have the comparison in the mean queuing delay of our proposed OBSring networks using LIT-based access protocol and that of the ones using wavelength-based and multi-token-based access protocols. The network parameters and constants used in the analysis are shown in Table I. In Fig. 8 the effect of the burst arrival rate per node on the mean queuing delay (TM QD ) is investigated with N = 8 nodes and M = 10 wavelengths. It is clearly shown that TM QD increases as λ increases. In addition, OBS-ring networks using wavelength-based access protocol have largest mean queuing delay. This is because this kind of networks cannot make use of all available wavelengths. In this case, only eight out of total ten wavelengths can be used. Figure 8 also shows that, in OBS-ring networks using LIT-based access protocol, TM QD is improved significantly compared to that of the networks using multi-token-based protocol. This is due to the fact that the virtual busy state of wavelengths cause the increase of total burst arrival rate in the networks using multi-token-based access protocol. As a result, the largest burst arrival rate that multi-token-based

(10)

where TT is the average transmission time for the control frame to go from node i to node i + 1. TR is the time for a node receiving the control frame. TP is processing time. TG is the time for a node generating the control frame. We assume that the time for a node receiving the control frame equals the time for a node generating the control frame, i.e., TR = TG . We also denote that LCF (bits), DAV G (m) and n0 are the length of control frame, the average distance between two consecutive nodes, and the refractive index of the core of fiber, respectively. Equation (10) can be re-written as   LCF DAV G (11) +2 + TP N, TSP = c/n0 R where c is the speed of light in vacuum (m/s).

97

800

120 Wavelength−based access protocol Multi−token−based access protocol LIT−based access protocol

MQD

80

Mean queuing delay, T

Mean queuing delay, T

MQD

(µs)

(µs)

100

Wavelength−based access protocol Multi−token−based access protocol LIT−based access protocol

700

60

40

600 500 400 300 200

20 100 0

2

4

6 8 Number of nodes, N

0

10

2

4

6 8 10 12 Number of wavelengths, M

14

16

Fig. 9. Mean queuing delay versus the number of nodes with λ = 3000 burst/s and M = 10 wavelengths.

Fig. 10. Mean queuing delay versus the number of wavelengths with λ = 3000 burst/s and N = 8 nodes.

access protocol can support without delay is only 1500 burst/s, which is nearly a half of the arrival rate that LIT-based access protocol can support (3500 burst/s). The mean queuing delay is investigated versus the number of nodes in Fig. 9 with the burst arrival rate of 3000 burst/s and the number of wavelengths is 10. As the traffic intensity increases with the number of nodes, high queuing delay will occur when the number of nodes increases. Compared to OBSring networks that limit the number of wavelength to the number of nodes (i.e., wavelength-based access protocol), the ones using M -FT/M -FR with M > N (i.e., multi-token-based access protocol) can send bursts with lower mean queuing delay. However, when M = N , the queuing delay is the same for two cases of using wavelength-based and multi-tokenbased access protocols. Also, the advantage of using LITbased access protocol is proven in this result. More specially, OBS-ring networks using LIT-based access protocol is able to support 8 nodes without delay while the number of supportable nodes without delay for multi-token-based protocol is only 5 nodes. Finally, Fig. 10 shows the mean queuing delay versus the number of wavelengths with the burst arrival rate of 3000 burst/s and the number of nodes is 8. For wavelength-based access protocol, the increase of the number of wavelength cannot help to reduce the queuing delay as it is limited by the number of nodes (8 nodes). To achieve no delay, multitoken-based protocol requires to use 13 wavelengths, which is 5 wavelengths larger than the number of wavelengths that LITbased access protocol needs. This is because LIT-based access protocol uses wavelength resource in an efficient manner.

protocol, both collision-free and low-latency can be achieved. To support this access protocol, a low-cost node architecture that provides burst transmission on multiple wavelengths was also proposed. The numerical results show that OBS-ring networks using proposed access protocol can support lower mean queuing delay, larger the number of nodes, higher burst arrival rate compared to that of the conventional OBR-ring networks. ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology (MOST) under the Vietnam–Japan protocol project named “Models for Next Generation of Robust Internet”, grant number 12/2012/HD-NDT. R EFERENCES [1] J. P. Jue and V. M. Vokkarane, Optical burst switched networks, Springer, 2005. [2] T. Battestilli and H. Perros, “An introduction to optical burst switching”, IEEE Commun. Mag., vol. 4, no. 8, pp. S10–S15, Aug. 2003. [3] L. Xu, H. G. Perros, and G. N. Rouskas, “A simulation study of optical burst switching and sccess protocols for WDM ring networks,” Computer Networks, vol. 41, pp.143–160, 2003. [4] V. S. Puttasubbappa and H. G. Perros, “Access protocols to support different service classes in an optical burst switching ring,” Proc. Third International IFIP-TC6 Networking Conference, pp. 878–889, 2004. [5] J. P. Park, “A low-latency collision-free optical burst switching ring network protocol,” Proc. 32nd Australian Conference on Optical Fibre Technology, pp. 1–3, 2007. [6] A. Fumagalli et al., “A low-latency and bandwidth-efficient distributed optical burst switching architecture for metro ring,” Proc. IEEE International Conference Commun., vol. 2, pp. 1340–1344, 2003. [7] H. T. Lin and W. R. Chang, “CORNet: a scalable and bandwidth-efficient optical burst switching ring architecture for metro area networks,” Proc. International Conference on Networking and Services, pp. 100, 2006. [8] Li-Mei Peng, Won-Hyuk Yang, and Young-Chon Kim, “On the design of node architectures and MAC protocols for optical burst-switched ring networks,” Photonic Netw. Commun., vol. 21, no. 3, 267–277, 2011. [9] D. G. John, F. Shortle, J. M. Thompson, and C. M. Harris, Fundamentals of Queuing Theory, Fourth Edition, Wiley, 2008.

VI. C ONCLUSION We have proposed a novel LIT-based access protocol for OBS-ring networks. Thanks to the use of LIT-based access

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