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Inverse two-way signaling scheme for optical burst switched networks
Optical Switching and Networking 9 (2012) 214–223
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Optical Switching and Networking journal homepage: www.elsevier.com/locate/osn
Inverse two-way signaling scheme for optical burst switched networks✩ Junling Yuan a,∗ , Xianwei Zhou a , Jianping Wang a , Yongqi He b , Ke Wang a a
School of Computer and Telecommunication, University of Science and Technology Beijing, Beijing, China
b
State Key Laboratory of Advanced Optical Communication Systems and Networks, Peking University, Beijing, China
article
info
Article history: Received 15 July 2011 Received in revised form 17 December 2011 Accepted 8 February 2012 Available online 18 February 2012 Keywords: Optical burst switching OBS Signaling Reservation Inverse two-way
abstract Optical burst switching is a promising all-optical switching technology for the next generation optical networks and signaling is one of the key components of it. According to propagation direction of control packets, the existing signaling schemes for optical burst switched networks can be roughly divided into three categories: one-way, two-way, and hybrid signaling. Since one-way signaling launches bursts without the acknowledgment of successful reservation of resources, it has small end-to-end delay but high data loss rate; two-way signaling uses a control packet to gather the state of links and needs the successful acknowledgment before sending out a burst, so it has large end-to-end delay but low data loss rate. A hybrid signaling gets a tradeoff of performance between one-way and twoway signaling. In this paper, we propose another hybrid signaling named inverse two-way signaling. By introducing the process of link state collection into one-way signaling, the inverse two-way signaling can decrease the data loss rate without increasing the end-toend delay compared to one-way signaling. In other words, the inverse two-way signaling is an improved version of one-way signaling. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Due to the existence of electronic processing bottleneck, the traditional switching technologies based on O–E–O (optics–electronics–optics) conversion at all intermediate nodes cannot meet the demands of high speed data transmission in the next generation optical networks. For that reason, all-optical switching technologies are widely investigated in recent years [1]. The early studies are concentrated on optical circuit switching (OCS) technology. In an OCS network, once a connection is configured, it would be maintained for a rather long period, whether it is used by traffics or not. The OCS technology is easily realized, but it has many disadvantages, such as low channel utilization ratio, lack
✩ This work is supported by National Basic Research Program of China (973) (Nos. 2010CB328201 and 2010CB328202), National Science Foundation of China (Nos. 60902042 and 61003250). ∗ Corresponding author. E-mail address: [email protected] (J. Yuan).
1573-4277/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.osn.2012.02.002
of flexibility for connections, etc. To solve these problems, several new all-optical switching patterns are proposed, such as optical packet switching (OPS) and optical burst switching (OBS). Since optical buffers and optical logical devices are needed in OPS, the technology is impracticable for the foreseeable future. Comparatively, OBS technology is potentially realized in the near future [2]. In OBS networks, control packet and data burst are launched at different times and on different channels. A burst header packet (BHP) is sent out first through the control channel to reserve resources of the data channel; after a predetermined offset time, the data burst is transferred through the data channel using the reserved resources [3]. The offset time between the data burst and the BHP is specified by the signaling scheme. Signaling is one of the most important technologies for OBS networks. According to the propagation direction of control packets, the existing signaling schemes can be classified as one-way, two-way, or hybrid signaling [4]. We generally consider two performance of a signaling scheme: end-to-end delay and data loss rate. The
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end-to-end delay of one-way signaling is less than that of two-way signaling, but the data loss rate of one-way signaling is greatly higher than that of two-way signaling. Hybrid signaling gets a tradeoff between one-way signaling and two-way signaling. In this paper, we give another hybrid signaling scheme named inverse two-way signaling, which has a similar end-to-end delay to one-way signaling and a tradeoff of data loss rate between one-way signaling and two-way signaling. The rest of this paper is organized as follows. Section 2 presents the related works for OBS signaling schemes. In Section 3 we introduce the inverse two-way signaling in detail and compare its performance with one-way signaling and two-way signaling. Simulations are done in Section 4 to verify the analysis. In Section 5, a brief conclusion is drawn. 2. Related works Several signaling schemes have been designed for OBS networks in the last years. From different points of view, signaling schemes can be divided into different categories. The classifications of signaling schemes are comprehensively introduced in the book of Jue and Vokkarane [4]. We introduce three of them which are related to this paper. By the configuration occasion of optical switch, a signaling scheme may be immediately configured or delayconfigured. In an immediately configured signaling, once an intermediate node operated the BHP of a burst, it immediately configures the optical switch to the selected wavelength; although there is a section of idle time before the burst’s coming, the wavelength cannot be used by other bursts. In a delay-configured signaling, an intermediate node gets the arrival time of the burst from the BHP, but does not configure the optical switch until the arrival time of the burst; the time section between the arrival time of the burst and the BHP can be used by other bursts. A signaling scheme can be explicitly released or implicitly released, according to the release occasion of the occupied wavelength. In an explicitly released signaling, the release of occupied wavelength is by a separate release packet launched from the source; an intermediate node does not release the occupied wavelength until it receives the release packet. In an implicitly released signaling, the leaving time of a burst can be calculated by the information included in the BHP, an intermediate node gets the leaving time and releases the occupied wavelength at that moment. By arbitrarily combining the configuration style of optical switch and release style of occupied wavelength, four signaling schemes can be obtained [5]. Among them, the combination of delay-configured and implicitly released gets the highest channel efficiency, except a little complexity of control. According to the propagation direction of control packets, the signaling schemes can be classified as oneway, two-way, and hybrid signaling. 2.1. One-way signaling schemes In one-way signaling schemes, a BHP is sent out first to reserve selected resources for its burst; after a
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predetermined offset time the burst is launched, without waiting for the acknowledgment of successful reservation. There are two typical one-way OBS signaling schemes: just-in-time (JIT) [6] and just-enough-time (JET) [7]. JIT is an immediately configured and explicitly released signaling, whereas JET is a delay-configured and implicitly released signaling. JIT is easily realized, but its channel efficiency is lower than that of JET. Rodrigues et al. [8] proposed an improved version of JIT named enhanced just-in-time (E-JIT) signaling in 2007. Two improving operations are adopted in the E-JIT signaling: changing the releasing manner from explicit to implicit for a successful burst; adding an explicit releasing policy to release the previous reserved resource for a failed burst. The E-JIT signaling can improve the channel efficiency of the JIT signaling. To reduce the queuing latency of packets in JET signaling, two prediction-based signaling schemes are additionally proposed. In 2003, Liu et al. [9] proposed the first prediction-based JET signaling. In conventional JET signaling, since a BHP carries the length of the corresponding burst, its launching time has to be no earlier than the completion time of the burst. In the scheme of Liu et al., assembly of burst is timerbased; the length of current burst is predicted by a linear predictive filter; hence, BHP can be sent out before the completion of its burst by carrying a predicted length. In 2008, Seklou and Varvarigos [10] proposed another prediction-based JET signaling named fast reservation (FR) signaling. Different from in the scheme of Liu et al., assembly of burst in FR signaling is simultaneously timerbased and length-based; length of burst and duration time of assembly process are simultaneously predicted by two least mean squares filters. The prediction-based methods can substantially reduce queuing delay of packets. However, since the queuing delay is relatively small comparing to the propagation delay in core networks, the actual effect of the predictionbased methods is slight by reducing end-to-end delay. In contrast, since the actual length of burst may be shorter than the reserved (estimated) length of resource, it may lead to waste of the resource and then slight increase of data loss rate. One-way signaling schemes choose resources by only referencing the state of the first link on the source– destination path and launch bursts without waiting for the acknowledgment of successful reservation, so the bursts are easily to conflict with each other at the intermediate nodes. Hence, the data loss rate of one-way signaling is relatively high. 2.2. Two-way signaling schemes The reservation of resources in a two-way signaling can be either forward reservation or backward reservation. In a forward-reservation two-way signaling, the reservation order is from source to destination. A BHP is sent out to reserve the requested resources; when the destination receives the BHP, it sends back an ACK packet to notify the source whether the requested resources are successfully reserved. Forward-reservation two-way signaling is just like one-way signaling, except that it has an additional acknowledging process.
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Tell-and-wait (TAW) [11] is the most classic two-way forward-reservation signaling. Since it is an immediately configured and explicitly released signaling, its channel efficiency is low. In 2005, Vu et al. [12] proposed an improved version of TAW named dynamic two-way reservation (DTWR). It is a delay-configured and implicitly released signaling. Moreover, it can dynamically adjust the length of requested resource in the course of reservation: if only a portion (or several portions) of the requested time section is available, the reservation packet reserves the portion (correspondingly the longest portion); after the process of reservation, the source can at least send a part of the burst to the destination. The DTWR signaling can improve the channel efficiency of forward-reservation two-way signaling. The reservation order in a backward-reservation twoway signaling is from destination to source. A BHP is sent out first to gather the state of links along the source–destination path; when the destination receives the BHP, it chooses some available resources according to the gathered information and sends back an ACK packet to reserve the chosen resources. Since there is an additional process to gather the state of links, the success ratio of reservation in a backward-reservation two-way signaling is generally higher than that in a forward-reservation one. In 2006, Kong and Phillips [13] proposed a backwardreservation two-way signaling named pre-booking reservation mechanism. The scheme is delay-configured and implicitly released; hence, it has lower data loss rate and higher channel efficiency than forward-reservation twoway signaling schemes. Prediction-based two-way signaling is also considered. In 2009, Vlachos and Monoyios [14] proposed a scheme named virtual one-way signaling. Although ‘‘one-way’’ occurs in its name, it is actually a prediction-based twoway signaling. When the first IP packet arrives at the buffer, the source predicts the burst’s length in a roundtrip time (twice the source–destination delay) and sends out a BHP to reserve the corresponding length of resources for the burst generated next. When an acknowledgment of successful reservation arrives at the source, the source launches the burst (if its length is not longer than the reserved length). The virtual one-way signaling can decrease the end-to-end delay of the two-way signaling. But the predicted length would always have a gap with the actual length of burst, which leads to the waste or deficiency of reserved resources and results that its data loss rate is a little higher than that of the pre-booking reservation mechanism. 2.3. Hybrid signaling Both one-way signaling and two-way signaling has advantages and disadvantages. In 2007, Vokkarane [15] proposed a hybrid signaling named intermediate node initiated (INI) signaling. It can get a tradeoff between oneway signaling and two-way signaling. The reservation in one-way signaling is initiated by the source and that in backward-reservation two-way signaling is initiated by the destination. As a hybrid, the reservation in the INI signaling is initiated by some an intermediate node on the
source–destination path. In other words, in the INI signaling, the reservation from the source to the intermediate node is similar to two-way signaling and from the intermediate node to the destination is just as one-way signaling. The performance of the INI signaling can be adjusted by choosing the position of the reservation-initiating node on the source–destination path. 3. Inverse two-way signaling In this section, we first introduce the procedure of inverse two-way signaling; next script the method to gather the state of links. Later, we provide the way to send bursts in inverse two-way signaling. Finally, we analyze its performance. 3.1. Procedure of inverse two-way signaling The process of resource reservation is distinct in different signaling schemes: one-way signaling directly reserves resource without gathering the state of links, whereas (backward-reservation) two-way signaling collects the links’ state before the reservation of resource. Inverse twoway signaling collects the links’ state first, just like twoway signaling. Except that, the propagation direction of control packet for collection (collection packet) and control packet for reservation (reservation packet) in inverse twoway signaling is different from that in two-way signaling. In two-way signaling, collection packet is transmitted from source to destination and reservation packet is transmitted on the opposite direction. In inverse two-way signaling, collection packet is transmitted from destination to source and transmission of reservation packet is on the opposite way. In other words, the propagation direction of control packets in inverse two-way signaling is just ‘‘inverse’’ to that in two-way signaling. It is why we name it as inverse two-way signaling. Unlike two-way signaling in which the collection of links’ state is after the completion of burst assembly, inverse two-way signaling do link state collection and burst assembly in parallel. The procedure of inverse twoway signaling is as follows: (1) The destination periodically sends ACK to the source to collect state of links on the source–destination path; (2) Meanwhile, data coming from the edge network reaches the source and queues in the corresponding buffers. (3) When the source receives the ACK, it encapsulates the data aggregated during the last period into one or several burst(s), according to the links’ state getting from the ACK. (4) The source sends out a BHP for each burst to reserve the corresponding resource. (5) After the offset time (it may be diverse for different bursts), the source launches the burst. Fig. 1 is the timing diagram for the inverse two-way signaling. Where S is the configuration time of optical switch, ∆ is the processing time of control packet, and P is the collection period of links’ state.
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1 2
3
4
Fig. 2. Example to show the concerned time interval on different links.
Fig. 1. Timing diagram for the inverse two-way signaling.
Since we do not reroute for each burst but use the same source–destination path for a quite long time, it is reasonable to periodically send control packets from the destination to the source. The collection period has two functions: first, it affects the length and number of bursts generated (just like the time threshold of other signaling schemes); and second it impacts the freshness of the links’ state. 3.2. Link state collection In connection-oriented technology (such as OCS) based networks, each wavelength of a link has only two possible states: busy or idle. OBS technology is not connectionoriented but statistical multiplexing, so the state of a wavelength cannot be simply described as busy or idle. We can use busy section or idle section of the ingress port of a wavelength to describe its state:
• If the ingress port has been reserved to a burst during a time section, the wavelength is busy in this section.
• If the ingress port is not reserved to any burst during the time section, the wavelength is idle in the section. There may be several busy sections and idle sections in a long time interval. Since a burst does not simultaneously but sequentially use the links on the source–destination path, the concerned time interval for different links should also be sequential. Additionally, the starting point of the concerned time interval ought to equal the earliest time when the burst may arrive at the ingress port of the link. Fig. 2 is an example to show the concerned time interval on different links. Nodes 4 and 1 are the destination and the source, respectively. At time t0 node 4 sends out a
collection packet to node 1. The packet collects a time interval with length 1T of each link on the route. Suppose that T is the sum of the propagation time of collection packet, the processing time of it, and the offset time of the burst. The starting point of the concerned interval of link (1, 2) is t0 + T and the concerned time interval of it is (t0 + T , t0 + T + 1T ). Moreover, the concerned time intervals for links (2, 3) and (3, 4) are (t0 + T + t12 , t0 + T + t12 + 1T ) and (t0 + T + t13 , t0 + T + t13 + 1T ), respectively, where t12 (t23 ) is the propagation delay from node 1 to node 2 (node 3). Since the technology of both wavelength converter and fiber delay line (FDL) is not enough mature, we suppose that neither wavelength converter nor fiber delay line is used in the network. Then, a burst must be transferred on the same wavelength of the links and directly transmits to the destination without any stop at intermediate nodes. It means that, if the time section of one link for a burst is confirmed, the time sections for other links are uniquely determined. For that reason, it is unnecessary to record the state of all links separately. We only record the idle sections which are ‘‘jointly’’ idle for all the links. Here ‘‘jointly’’ means that the idle sections on different links can be sequentially used by a burst. Fig. 3 is an example to show the jointly idle section of all the links. The rectangles express the concerned intervals for links. The shadowed sections mean that the parts have been reserved to other bursts. S(i,j) and T(i,j) represent the starting point and terminal point of the idle section of link (i, j), respectively. To get the jointly idle section of all links, we map the idle section of all the links to the time scale of link (1, 2) by subtracting the corresponding propagation delay. By comparison we can see that the only jointly idle section of all the links is (S(1,2) , T(2,3) − t12 ) in this example. Generally, there may be not only one but several idle sections on one wavelength and more idle sections on all the wavelengths. A channel scheduling scheme can choose one or several of them to launch data. 3.3. Burst encapsulation In usual signaling schemes, the operations of burst assembly and reservation run separately; in inverse twoway signaling, these operations are cooperating with each other. Generally, a well-performed assembly algorithm has not only a time threshold but also a length threshold. The collection period P in inverse two-way signaling is
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1 2
3
4
Fig. 3. Example to show the jointly idle section on different links.
equivalent to the time threshold in the assembly algorithm for other signaling schemes. In order to limit the length of bursts, we give a length threshold Lmax in inverse two-way signaling too. Furthermore, a low threshold of data division Lmin is given to avoid dividing data into several very short bursts. Since there are always several idle sections, the channel scheduling scheme checks the idle sections one by one and decides whether use the idle section to transmit a burst, until all data have been transmitted or all idle sections have been checked. Suppose the length of the current idle section is Lidle and the remainder length of the data is Ldata . Assuming the configuration time of an optical switch is Tswitch , the detailed rule of burst encapsulation is as follows: (1) If Lidle ≤ Lmin + Tswitch : – if Ldata + Tswitch ≤ Lidle , encapsulates the data into one burst and stops; – else, does not generate a burst and checks next idle section. (2) If Lmin + Tswitch < Lidle ≤ Lmax + Tswitch : – if Ldata + Tswitch ≤ Lidle , encapsulates the data into one burst and stops; – else, generates a burst with length Lidle − Tswitch and checks next idle section. (3) If Lidle > Lmax + Tswitch : – if Ldata ≤ Lmax , encapsulates the data into one burst and stops; – else, generate a burst with length Lmax and check next idle section. The assembly algorithm for other signaling schemes only generates one burst when one of the thresholds is triggered; in inverse two-way signaling, several bursts may be encapsulated at a time. Besides, the bursts generated by the above rule is always no longer than Lmax , and at most one of them is shorter than Lmin . 3.4. Performance analysis The JET signaling is a well-performed one-way signaling and the pre-booking reservation mechanism is a wellperformed two-way signaling. In this subsection, we compare the end-to-end delay and data loss rate of inverse two-way signaling to that of the JET signaling and the pre-booking reservation mechanism. For the sake of convenience, we call JET as one-way and pre-booking reservation as two-way in the next of this paper.
3.4.1. End-to-end delay We first introduce several notations.
• Ttrans : transmission delay of a burst. A burst is aggregated by several packets and the bit rate of the packets may be different from each other. We use the transmission delay in the core network of the same quantity of data to denote the length of a packet. For example, if the size of a packet is 10 Kbits and the transmission rate of each wavelength in the core network is 10 Gbits/s, Kbits then the length of this packet is 1010Gbits = 1 µs. Sup/s posing a burst is composed by K packets with lengths lk (k = 1, . . . , K ), the transmission delay of this burst equals total length of the packets, i.e., Ttrans =
K
lk .
(1)
k=1
• Tagg : aggregation delay of a burst. The arrival times of the packets which compose the burst may be different. To be fairness, we calculate the average aggregation delay of all packets. Suppose the arrival time of the k-th packet is tk and suppose the encapsulation time of the burst is tencap . Then the aggregation delay of the burst is K
Tagg =
lk · (tencap − tk )
k=1 K
.
(2)
lk
k=1
• Tprop : propagation delay from the source to the destination. Denote the source–destination path of a burst as Pburst which is a set of links; suppose the propagation delay of link e is Te , then the propagation delay of the path is Tprop =
Te .
(3)
e∈Pburst
• TBHP : total operation time of the control packet of a burst from the source to the destination.
• Tswitch : configuration time of each optical switch. • 1t: gap between the actual beginning time and the earliest possible beginning time of a burst. In one-way signaling and inverse two-way signaling, the earliest possible beginning time of a burst is tencap +TBHP +Tswitch , whereas, in two-way signaling, it is tACK + Tswitch (tACK is the time at which the source receives the acknowledgment of successful reservation). If the selected wavelength is busy at the earliest possible beginning time, the actual beginning time of the burst would be latter than that moment, and thus a gap occurs. • Toffset : offset time between a burst and its BHP. In all the signaling schemes, the end-to-end delay can be expressed by a common formula D = Tagg + Toffset + Ttrans + Tprop .
(4)
Except that the calculation of the offset time is distinct in different signaling schemes. In one-way and inverse twoway signaling, the offset time is TBHP + 1t + Tswitch ; in twoway signaling, the offset time equals 2 · Tprop + 2 · TBHP + 1t +Tswitch . Substituting offset time into the common formula, we get the end-to-end delay of each signaling scheme:
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Doneway = Tagg + TBHP + Ttrans + 1t
+ Tswitch + Tprop
(5)
Dinverse = Tagg + TBHP + Ttrans + 1t
+ Tswitch + Tprop
(6)
Dtwoway = Tagg + 2TBHP + Ttrans + 1t
+ Tswitch + 3Tprop .
(7)
We can see that, the end-to-end delay of inverse twoway signaling is similar to that of one-way signaling and is far less than that of two-way signaling. 3.4.2. Data loss rate In each signaling scheme, data loss rate equals the ratio from the number of dropped data to the number of total data. However, the manner of data dropping in two-way signaling is different to that in one-way and inverse twoway signaling. In one-way and inverse two-way signaling schemes, if the requested resource of a burst has been reserved to another burst, a contention occurs and one of the bursts would be dropped. In two-way signaling, a burst is queuing at the source until the requested resources on all the links are successfully reversed; however, every data packet has its validity time, if the queuing time of a data packet excesses the deadline of its validity time, it would be dropped by the source. Since there is not a collection process in one-way signaling, the choice of time section for each burst is according to the state of the first link on the source–destination path. Hence, contention is very easy to occur on the following links. In inverse two-way signaling, there is a collection process. Supposing the source does not use the gathered information, inverse two-way signaling would be similar to one-way signaling. However, the application of the gathered information can decrease the probability of contention on the following links. Hence, the data loss rate in inverse two-way signaling is less than that in one-way signaling. In inverse two-way and two-way signaling schemes, the selection of time section is based on the gathered information of the whole path. The difference is that, in two-way signaling the burst would not be launched until the source receives the acknowledgment of successful reservation, but in inverse two-way signaling the burst is sent out without an acknowledgment. Two-way signaling which uses a guaranteed transmission mode can get a less data loss rate than inverse two-way signaling which adopts an unguaranteed transmission mode. From the above analysis we know that, the relationship of the data loss rate of one-way, two-way, and inverse twoway signaling schemes is that Rtwoway < Rinverse < Roneway .
(8)
In other words, the data loss rate of inverse two-way signaling is a tradeoff between one-way and two-way signaling schemes. 4. Simulations In this section, we do simulations to examine the performance of inverse two-way signaling. We first
Fig. 4. Topology of NSF-net.
investigate the impact of collection period to inverse twoway signaling; second compare the performance of inverse two-way scheme to one-way signaling (JET) and two-way signaling (pre-booking reservation mechanism). We do all experiments upon the NSF-net, which topology is shown in Fig. 4. The number marked on each edge is the length of fiber which connects two adjacent nodes [4]. The propagation delay coefficient of fiber is typically 5 µs/km. The configuration time of optical switch is supposed as 10 µs. The processing time of control packets is usually several hundred nanoseconds and is ignored in all the experiments. Each link has 16 wavelengths; and neither fiber delay line nor wavelength converter is used in the network. At each edge node, packets arrive under the Poisson rule and the length of packets is under the exponential distribution. The route for bursts is chosen by the Dijkstra’s shortest path algorithm [16]. In each signaling scheme, the wavelength is chosen by random and the length of concerned interval on each wavelength is chosen as 20 ms. 4.1. Impact of collection period to inverse two-way signaling For inverse two-way signaling, the length threshold Lmax is selected as 1 ms and the low threshold is 50 µs. Three different collection periods are investigated: 5, 10 and 15 ms. We consider four metrics of inverse twoway signaling: number of collection packets, number of reservation packets, end-to-end delay, and data loss rate. Since collection packets are sent out periodically, for a given network, the number collection packets generated per unit time (for example, one second) is determined by the collection period P. It equals N · (N − 1)/P, where N is the number of nodes in the network. Fig. 5 shows the number of collection packets generated per second under different collection periods in the NSF-net. We can see that the number of collection packets varies inversely with the collection period. The number of reservation packets equals the number of bursts. It is related to both the collection period and the traffic density. Since the source generates burst(s) when it receives a collection packet, a short period is likely to generate more reservation packets than a long one. On the other hand, the limit of Lmax results that data may be split into several bursts (correspondingly several reservation packets) for a long period. Hence, the number of reservation packets increases slightly along with the decrease of collection period. Fig. 6 shows the number of reservation packets generated per second under different collection periods.
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Fig. 5. Number of collection packets generated under different collection periods in inverse two-way signaling.
Fig. 6. Number of reservation packets generated under different collection periods and for different traffic densities in inverse two-way signaling.
When the period increases, data have to wait more time at the source; hence, the end-to-end delay increases with the increase of collection period. Fig. 7 shows the endto-end delay of inverse two-way signaling under different collection periods. We can see that the increment of endto-end delay is roughly equal to the increment of collection period. The shorter the period is, the fresher the information of link state is and the more possible successful reservation is. Hence, the data loss rate decreases with the decrease of collection period. Fig. 8 shows the data loss rate of inverse two-way signaling under different collection periods. 4.2. Comparison of three signaling schemes In this subsection, we compare the performance of inverse two-way signaling to that of one-way (JET) and two-way (pre-booking reservation) signaling schemes. In each signaling scheme, the length threshold is 1 ms and the time threshold (for inverse two-way signaling, the collection period) is 10 ms. Besides, the low threshold for inverse two-way signaling is 50 µs and the deadline for two-way signaling is selected as 100 ms.
Fig. 9 shows the end-to-end delay of the signaling schemes for different traffic densities at each node. The end-to-end delay of inverse two-way signaling is far less than that of two-way signaling and is similar to that of oneway signaling, just as expected. We have analyzed that the end-to-end delay in all three signaling schemes can be commonly expressed as the formula (4). For the route for each burst is fixed as the shortest source–destination path, the propagation delay Tprop never changes. Thus, the trend of the end-to-end delay is determined by the other three factors. When the traffic density increases, (a) the gap 1t and the length of bursts increase in all signaling schemes, that leads to the increase of offset time Toffset and transmission delay Ttrans , respectively; (b) more and more bursts are generated by triggering the length threshold in one-way and two-way signaling schemes, which reduces the aggregation delay Tagg ; (c) the waiting time of bursts increases in two-way signaling, which results in the increase of offset time Toffset .
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Fig. 7. End-to-end delay of inverse two-way signaling under different collection periods and for different traffic densities.
Fig. 8. Data loss rate of inverse two-way signaling under different collection periods and for different traffic densities.
Fig. 9. End-to-end delay of one-way, two-way and inverse two-way signaling for different traffic densities.
The effect of (a) leads to slow increase of the end-toend delay in inverse two-way signaling; the effect of (b) overwhelms the effect of (a), hence the end-to-end delay decreases in one-way signaling. The co-effect of (a)–(c)
leads to the increase of the end-to-end delay in two-way signaling. Fig. 10 shows the data loss rate of all the signaling schemes for different traffic densities at each node.
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Fig. 10. Data loss rate of one-way, two-way and inverse two-way signaling for different traffic densities.
Fig. 11. Number of reservation packets generated per second of one-way, two-way and inverse two-way signaling for different traffic densities.
When the traffic density increases, the data loss rate of each signaling scheme increases. Just as analyzed in Section 3.4.2, the data loss rate in inverse two-way signaling is always a tradeoff between one-way and twoway signaling. In each signaling scheme, the number of reservation packets increases with the growth of traffic density. Since data may be split into several bursts in inverse two-way signaling, the number of reservation packets is always slightly more than that in one-way signaling. In twoway signaling, since the source may try several times to reserve resource to a burst, one burst may generate several reservation packets; also, the chance increases with the growth of traffic density. Hence, the number of reservation packets in two-way signaling is less than that in inverse two-way signaling for low load case and more than that for high load case. Fig. 11 shows the number of reservation packets generated in each signaling scheme. The process of link state collection is only existed in two-way and inverse two-way signaling schemes. In inverse two-way signaling, the number of collection packets is a constant for a fixed collection period. In
two-way signaling, the number of collection packets equals the number of reservation packets and it increases with the growth of traffic density. When the traffic density is high, the number of collection packets in twoway signaling is far more than that in inverse two-way signaling. Fig. 12 shows the number of collection packets in two-way and inverse two-way signaling schemes. 5. Conclusions In this paper, we proposed the inverse two-way signaling for OBS networks. It can be regarded as an improved version of one-way signaling. By adding the process of link state collection to one-way signaling, it can decrease the data loss rate of one-way signaling without increasing the end-to-end delay. However, the data loss rate of inverse two-way signaling is still very high comparing to twoway signaling. In the next, we want to investigate channel scheduling algorithms and collision resolution schemes in inverse two-way signaling to further decrease its data loss rate.
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Fig. 12. Number of collection packets generated per second of two-way signaling and inverse two-way signaling for different traffic densities.
References [1] G.I. Papadimitriou, C. Papazoglou, A.S. Pomportsis, Optical switching: switch fabrics, techniques, and architectures, Journal of Lightwave Technology 21 (2003) 384–405. [2] Y. Chen, C.M. Qiao, X. Yu, Optical burst switching: a new area in optical networking research, IEEE Network 18 (2004) 16–23. [3] C.M. Qiao, M. Yoo, Optical burst switching (OBS) — a new paradigm for an optical internet, Journal of High Speed Networks 8 (1999) 69–84. [4] J.P. Jue, V.M. Vokkarane, Signaling, in: Optical Burst Switched Networks, Springer, Poston, 2005, pp. 37–56. [5] I. Baldine, G.N. Rouskas, H.G. Perros, D. Stevenson, Jumpstart: a justin-time signaling architecture for WDM burst-switched networks, IEEE Communications Magazine 40 (2) (2002) 82–89. [6] J.Y. Wei, R.I. McFarland, Just-in-time signaling for WDM optical burst switching networks, Journal of Lightwave Technology 18 (2000) 2019–2037. [7] M. Yoo, C.M. Qiao, Just-enough-time (JET): a high speed protocol for bursty traffic in optical networks, Vertical-Cavity Lasers, Technologies for a Global Information Infrastructure, WDM Components Technology, Advanced Semiconductor Lasers and Applications, Gallium Nitride Materials, Processing (1997) 11–15. [8] J.J.P.C. Rodrigues, M.M. Freire, N.M. Garcia, P.M.N.P. Monteiro, Enhanced just-in-time: a new resource reservation protocol for optical burst switching networks, in: Proceedings of IEEE International
[9]
[10]
[11] [12]
[13]
[14]
[15]
[16]
Symposium on Computers and Communications, ISC’07, 1–4 July 2007. J. Liu, N. Ansari, T.J. Ott, FRR for latency reduction and QoS provisioning in OBS networks, IEEE Journal on Selected Areas in Communications 21 (7) (2003) 1210–1219. K. Seklou, E. Varvarigos, Fast reservation protocols for latency reduction in optical burst-switched networks based on predictions, in: Seventh International Conference on Networking, ICN’2008, 13–18 April 2008. I. Widjaja, Performance analysis of burst admission-control protocols, Communications, IEE Proceedings 142 (1995) 7–14. H.L. Vu, A. Zalesky, M. Zukerman, Z. Rosberg, J. Guo, T.W. Um, Delay analysis of optical burst switching networks, in: IEEE International Conference on Communications, ICC’2005, 16–20 May 2005. H. Kong, C. Phillips, Probooking reservation mechanism for nextgeneratin optical networks, IEEE Journal of Selected Topics in Quantum Electronics 12 (2006) 645–652. K.G. Vlachos, D. Monoyios, A virtual one-way signaling protocol with aggressive resource reservation for improving burst transmission delay, Journal of Lightwave Technology 27 (2009) 2869–2875. V.M. Vokkarane, Intermediate-node-initiation (INI): a generalized signaling framework for optical burst-switched networks, Optical Switching and Networking 4 (2007) 20–32. E.W. Dijkstra, A note on two problems in connexion with graphs, Numerische Mathematik 1 (1959) 269–271.
Contents lists available at SciVerse ScienceDirect
Optical Switching and Networking journal homepage: www.elsevier.com/locate/osn
Inverse two-way signaling scheme for optical burst switched networks✩ Junling Yuan a,∗ , Xianwei Zhou a , Jianping Wang a , Yongqi He b , Ke Wang a a
School of Computer and Telecommunication, University of Science and Technology Beijing, Beijing, China
b
State Key Laboratory of Advanced Optical Communication Systems and Networks, Peking University, Beijing, China
article
info
Article history: Received 15 July 2011 Received in revised form 17 December 2011 Accepted 8 February 2012 Available online 18 February 2012 Keywords: Optical burst switching OBS Signaling Reservation Inverse two-way
abstract Optical burst switching is a promising all-optical switching technology for the next generation optical networks and signaling is one of the key components of it. According to propagation direction of control packets, the existing signaling schemes for optical burst switched networks can be roughly divided into three categories: one-way, two-way, and hybrid signaling. Since one-way signaling launches bursts without the acknowledgment of successful reservation of resources, it has small end-to-end delay but high data loss rate; two-way signaling uses a control packet to gather the state of links and needs the successful acknowledgment before sending out a burst, so it has large end-to-end delay but low data loss rate. A hybrid signaling gets a tradeoff of performance between one-way and twoway signaling. In this paper, we propose another hybrid signaling named inverse two-way signaling. By introducing the process of link state collection into one-way signaling, the inverse two-way signaling can decrease the data loss rate without increasing the end-toend delay compared to one-way signaling. In other words, the inverse two-way signaling is an improved version of one-way signaling. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Due to the existence of electronic processing bottleneck, the traditional switching technologies based on O–E–O (optics–electronics–optics) conversion at all intermediate nodes cannot meet the demands of high speed data transmission in the next generation optical networks. For that reason, all-optical switching technologies are widely investigated in recent years [1]. The early studies are concentrated on optical circuit switching (OCS) technology. In an OCS network, once a connection is configured, it would be maintained for a rather long period, whether it is used by traffics or not. The OCS technology is easily realized, but it has many disadvantages, such as low channel utilization ratio, lack
✩ This work is supported by National Basic Research Program of China (973) (Nos. 2010CB328201 and 2010CB328202), National Science Foundation of China (Nos. 60902042 and 61003250). ∗ Corresponding author. E-mail address: [email protected] (J. Yuan).
1573-4277/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.osn.2012.02.002
of flexibility for connections, etc. To solve these problems, several new all-optical switching patterns are proposed, such as optical packet switching (OPS) and optical burst switching (OBS). Since optical buffers and optical logical devices are needed in OPS, the technology is impracticable for the foreseeable future. Comparatively, OBS technology is potentially realized in the near future [2]. In OBS networks, control packet and data burst are launched at different times and on different channels. A burst header packet (BHP) is sent out first through the control channel to reserve resources of the data channel; after a predetermined offset time, the data burst is transferred through the data channel using the reserved resources [3]. The offset time between the data burst and the BHP is specified by the signaling scheme. Signaling is one of the most important technologies for OBS networks. According to the propagation direction of control packets, the existing signaling schemes can be classified as one-way, two-way, or hybrid signaling [4]. We generally consider two performance of a signaling scheme: end-to-end delay and data loss rate. The
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end-to-end delay of one-way signaling is less than that of two-way signaling, but the data loss rate of one-way signaling is greatly higher than that of two-way signaling. Hybrid signaling gets a tradeoff between one-way signaling and two-way signaling. In this paper, we give another hybrid signaling scheme named inverse two-way signaling, which has a similar end-to-end delay to one-way signaling and a tradeoff of data loss rate between one-way signaling and two-way signaling. The rest of this paper is organized as follows. Section 2 presents the related works for OBS signaling schemes. In Section 3 we introduce the inverse two-way signaling in detail and compare its performance with one-way signaling and two-way signaling. Simulations are done in Section 4 to verify the analysis. In Section 5, a brief conclusion is drawn. 2. Related works Several signaling schemes have been designed for OBS networks in the last years. From different points of view, signaling schemes can be divided into different categories. The classifications of signaling schemes are comprehensively introduced in the book of Jue and Vokkarane [4]. We introduce three of them which are related to this paper. By the configuration occasion of optical switch, a signaling scheme may be immediately configured or delayconfigured. In an immediately configured signaling, once an intermediate node operated the BHP of a burst, it immediately configures the optical switch to the selected wavelength; although there is a section of idle time before the burst’s coming, the wavelength cannot be used by other bursts. In a delay-configured signaling, an intermediate node gets the arrival time of the burst from the BHP, but does not configure the optical switch until the arrival time of the burst; the time section between the arrival time of the burst and the BHP can be used by other bursts. A signaling scheme can be explicitly released or implicitly released, according to the release occasion of the occupied wavelength. In an explicitly released signaling, the release of occupied wavelength is by a separate release packet launched from the source; an intermediate node does not release the occupied wavelength until it receives the release packet. In an implicitly released signaling, the leaving time of a burst can be calculated by the information included in the BHP, an intermediate node gets the leaving time and releases the occupied wavelength at that moment. By arbitrarily combining the configuration style of optical switch and release style of occupied wavelength, four signaling schemes can be obtained [5]. Among them, the combination of delay-configured and implicitly released gets the highest channel efficiency, except a little complexity of control. According to the propagation direction of control packets, the signaling schemes can be classified as oneway, two-way, and hybrid signaling. 2.1. One-way signaling schemes In one-way signaling schemes, a BHP is sent out first to reserve selected resources for its burst; after a
215
predetermined offset time the burst is launched, without waiting for the acknowledgment of successful reservation. There are two typical one-way OBS signaling schemes: just-in-time (JIT) [6] and just-enough-time (JET) [7]. JIT is an immediately configured and explicitly released signaling, whereas JET is a delay-configured and implicitly released signaling. JIT is easily realized, but its channel efficiency is lower than that of JET. Rodrigues et al. [8] proposed an improved version of JIT named enhanced just-in-time (E-JIT) signaling in 2007. Two improving operations are adopted in the E-JIT signaling: changing the releasing manner from explicit to implicit for a successful burst; adding an explicit releasing policy to release the previous reserved resource for a failed burst. The E-JIT signaling can improve the channel efficiency of the JIT signaling. To reduce the queuing latency of packets in JET signaling, two prediction-based signaling schemes are additionally proposed. In 2003, Liu et al. [9] proposed the first prediction-based JET signaling. In conventional JET signaling, since a BHP carries the length of the corresponding burst, its launching time has to be no earlier than the completion time of the burst. In the scheme of Liu et al., assembly of burst is timerbased; the length of current burst is predicted by a linear predictive filter; hence, BHP can be sent out before the completion of its burst by carrying a predicted length. In 2008, Seklou and Varvarigos [10] proposed another prediction-based JET signaling named fast reservation (FR) signaling. Different from in the scheme of Liu et al., assembly of burst in FR signaling is simultaneously timerbased and length-based; length of burst and duration time of assembly process are simultaneously predicted by two least mean squares filters. The prediction-based methods can substantially reduce queuing delay of packets. However, since the queuing delay is relatively small comparing to the propagation delay in core networks, the actual effect of the predictionbased methods is slight by reducing end-to-end delay. In contrast, since the actual length of burst may be shorter than the reserved (estimated) length of resource, it may lead to waste of the resource and then slight increase of data loss rate. One-way signaling schemes choose resources by only referencing the state of the first link on the source– destination path and launch bursts without waiting for the acknowledgment of successful reservation, so the bursts are easily to conflict with each other at the intermediate nodes. Hence, the data loss rate of one-way signaling is relatively high. 2.2. Two-way signaling schemes The reservation of resources in a two-way signaling can be either forward reservation or backward reservation. In a forward-reservation two-way signaling, the reservation order is from source to destination. A BHP is sent out to reserve the requested resources; when the destination receives the BHP, it sends back an ACK packet to notify the source whether the requested resources are successfully reserved. Forward-reservation two-way signaling is just like one-way signaling, except that it has an additional acknowledging process.
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Tell-and-wait (TAW) [11] is the most classic two-way forward-reservation signaling. Since it is an immediately configured and explicitly released signaling, its channel efficiency is low. In 2005, Vu et al. [12] proposed an improved version of TAW named dynamic two-way reservation (DTWR). It is a delay-configured and implicitly released signaling. Moreover, it can dynamically adjust the length of requested resource in the course of reservation: if only a portion (or several portions) of the requested time section is available, the reservation packet reserves the portion (correspondingly the longest portion); after the process of reservation, the source can at least send a part of the burst to the destination. The DTWR signaling can improve the channel efficiency of forward-reservation two-way signaling. The reservation order in a backward-reservation twoway signaling is from destination to source. A BHP is sent out first to gather the state of links along the source–destination path; when the destination receives the BHP, it chooses some available resources according to the gathered information and sends back an ACK packet to reserve the chosen resources. Since there is an additional process to gather the state of links, the success ratio of reservation in a backward-reservation two-way signaling is generally higher than that in a forward-reservation one. In 2006, Kong and Phillips [13] proposed a backwardreservation two-way signaling named pre-booking reservation mechanism. The scheme is delay-configured and implicitly released; hence, it has lower data loss rate and higher channel efficiency than forward-reservation twoway signaling schemes. Prediction-based two-way signaling is also considered. In 2009, Vlachos and Monoyios [14] proposed a scheme named virtual one-way signaling. Although ‘‘one-way’’ occurs in its name, it is actually a prediction-based twoway signaling. When the first IP packet arrives at the buffer, the source predicts the burst’s length in a roundtrip time (twice the source–destination delay) and sends out a BHP to reserve the corresponding length of resources for the burst generated next. When an acknowledgment of successful reservation arrives at the source, the source launches the burst (if its length is not longer than the reserved length). The virtual one-way signaling can decrease the end-to-end delay of the two-way signaling. But the predicted length would always have a gap with the actual length of burst, which leads to the waste or deficiency of reserved resources and results that its data loss rate is a little higher than that of the pre-booking reservation mechanism. 2.3. Hybrid signaling Both one-way signaling and two-way signaling has advantages and disadvantages. In 2007, Vokkarane [15] proposed a hybrid signaling named intermediate node initiated (INI) signaling. It can get a tradeoff between oneway signaling and two-way signaling. The reservation in one-way signaling is initiated by the source and that in backward-reservation two-way signaling is initiated by the destination. As a hybrid, the reservation in the INI signaling is initiated by some an intermediate node on the
source–destination path. In other words, in the INI signaling, the reservation from the source to the intermediate node is similar to two-way signaling and from the intermediate node to the destination is just as one-way signaling. The performance of the INI signaling can be adjusted by choosing the position of the reservation-initiating node on the source–destination path. 3. Inverse two-way signaling In this section, we first introduce the procedure of inverse two-way signaling; next script the method to gather the state of links. Later, we provide the way to send bursts in inverse two-way signaling. Finally, we analyze its performance. 3.1. Procedure of inverse two-way signaling The process of resource reservation is distinct in different signaling schemes: one-way signaling directly reserves resource without gathering the state of links, whereas (backward-reservation) two-way signaling collects the links’ state before the reservation of resource. Inverse twoway signaling collects the links’ state first, just like twoway signaling. Except that, the propagation direction of control packet for collection (collection packet) and control packet for reservation (reservation packet) in inverse twoway signaling is different from that in two-way signaling. In two-way signaling, collection packet is transmitted from source to destination and reservation packet is transmitted on the opposite direction. In inverse two-way signaling, collection packet is transmitted from destination to source and transmission of reservation packet is on the opposite way. In other words, the propagation direction of control packets in inverse two-way signaling is just ‘‘inverse’’ to that in two-way signaling. It is why we name it as inverse two-way signaling. Unlike two-way signaling in which the collection of links’ state is after the completion of burst assembly, inverse two-way signaling do link state collection and burst assembly in parallel. The procedure of inverse twoway signaling is as follows: (1) The destination periodically sends ACK to the source to collect state of links on the source–destination path; (2) Meanwhile, data coming from the edge network reaches the source and queues in the corresponding buffers. (3) When the source receives the ACK, it encapsulates the data aggregated during the last period into one or several burst(s), according to the links’ state getting from the ACK. (4) The source sends out a BHP for each burst to reserve the corresponding resource. (5) After the offset time (it may be diverse for different bursts), the source launches the burst. Fig. 1 is the timing diagram for the inverse two-way signaling. Where S is the configuration time of optical switch, ∆ is the processing time of control packet, and P is the collection period of links’ state.
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1 2
3
4
Fig. 2. Example to show the concerned time interval on different links.
Fig. 1. Timing diagram for the inverse two-way signaling.
Since we do not reroute for each burst but use the same source–destination path for a quite long time, it is reasonable to periodically send control packets from the destination to the source. The collection period has two functions: first, it affects the length and number of bursts generated (just like the time threshold of other signaling schemes); and second it impacts the freshness of the links’ state. 3.2. Link state collection In connection-oriented technology (such as OCS) based networks, each wavelength of a link has only two possible states: busy or idle. OBS technology is not connectionoriented but statistical multiplexing, so the state of a wavelength cannot be simply described as busy or idle. We can use busy section or idle section of the ingress port of a wavelength to describe its state:
• If the ingress port has been reserved to a burst during a time section, the wavelength is busy in this section.
• If the ingress port is not reserved to any burst during the time section, the wavelength is idle in the section. There may be several busy sections and idle sections in a long time interval. Since a burst does not simultaneously but sequentially use the links on the source–destination path, the concerned time interval for different links should also be sequential. Additionally, the starting point of the concerned time interval ought to equal the earliest time when the burst may arrive at the ingress port of the link. Fig. 2 is an example to show the concerned time interval on different links. Nodes 4 and 1 are the destination and the source, respectively. At time t0 node 4 sends out a
collection packet to node 1. The packet collects a time interval with length 1T of each link on the route. Suppose that T is the sum of the propagation time of collection packet, the processing time of it, and the offset time of the burst. The starting point of the concerned interval of link (1, 2) is t0 + T and the concerned time interval of it is (t0 + T , t0 + T + 1T ). Moreover, the concerned time intervals for links (2, 3) and (3, 4) are (t0 + T + t12 , t0 + T + t12 + 1T ) and (t0 + T + t13 , t0 + T + t13 + 1T ), respectively, where t12 (t23 ) is the propagation delay from node 1 to node 2 (node 3). Since the technology of both wavelength converter and fiber delay line (FDL) is not enough mature, we suppose that neither wavelength converter nor fiber delay line is used in the network. Then, a burst must be transferred on the same wavelength of the links and directly transmits to the destination without any stop at intermediate nodes. It means that, if the time section of one link for a burst is confirmed, the time sections for other links are uniquely determined. For that reason, it is unnecessary to record the state of all links separately. We only record the idle sections which are ‘‘jointly’’ idle for all the links. Here ‘‘jointly’’ means that the idle sections on different links can be sequentially used by a burst. Fig. 3 is an example to show the jointly idle section of all the links. The rectangles express the concerned intervals for links. The shadowed sections mean that the parts have been reserved to other bursts. S(i,j) and T(i,j) represent the starting point and terminal point of the idle section of link (i, j), respectively. To get the jointly idle section of all links, we map the idle section of all the links to the time scale of link (1, 2) by subtracting the corresponding propagation delay. By comparison we can see that the only jointly idle section of all the links is (S(1,2) , T(2,3) − t12 ) in this example. Generally, there may be not only one but several idle sections on one wavelength and more idle sections on all the wavelengths. A channel scheduling scheme can choose one or several of them to launch data. 3.3. Burst encapsulation In usual signaling schemes, the operations of burst assembly and reservation run separately; in inverse twoway signaling, these operations are cooperating with each other. Generally, a well-performed assembly algorithm has not only a time threshold but also a length threshold. The collection period P in inverse two-way signaling is
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1 2
3
4
Fig. 3. Example to show the jointly idle section on different links.
equivalent to the time threshold in the assembly algorithm for other signaling schemes. In order to limit the length of bursts, we give a length threshold Lmax in inverse two-way signaling too. Furthermore, a low threshold of data division Lmin is given to avoid dividing data into several very short bursts. Since there are always several idle sections, the channel scheduling scheme checks the idle sections one by one and decides whether use the idle section to transmit a burst, until all data have been transmitted or all idle sections have been checked. Suppose the length of the current idle section is Lidle and the remainder length of the data is Ldata . Assuming the configuration time of an optical switch is Tswitch , the detailed rule of burst encapsulation is as follows: (1) If Lidle ≤ Lmin + Tswitch : – if Ldata + Tswitch ≤ Lidle , encapsulates the data into one burst and stops; – else, does not generate a burst and checks next idle section. (2) If Lmin + Tswitch < Lidle ≤ Lmax + Tswitch : – if Ldata + Tswitch ≤ Lidle , encapsulates the data into one burst and stops; – else, generates a burst with length Lidle − Tswitch and checks next idle section. (3) If Lidle > Lmax + Tswitch : – if Ldata ≤ Lmax , encapsulates the data into one burst and stops; – else, generate a burst with length Lmax and check next idle section. The assembly algorithm for other signaling schemes only generates one burst when one of the thresholds is triggered; in inverse two-way signaling, several bursts may be encapsulated at a time. Besides, the bursts generated by the above rule is always no longer than Lmax , and at most one of them is shorter than Lmin . 3.4. Performance analysis The JET signaling is a well-performed one-way signaling and the pre-booking reservation mechanism is a wellperformed two-way signaling. In this subsection, we compare the end-to-end delay and data loss rate of inverse two-way signaling to that of the JET signaling and the pre-booking reservation mechanism. For the sake of convenience, we call JET as one-way and pre-booking reservation as two-way in the next of this paper.
3.4.1. End-to-end delay We first introduce several notations.
• Ttrans : transmission delay of a burst. A burst is aggregated by several packets and the bit rate of the packets may be different from each other. We use the transmission delay in the core network of the same quantity of data to denote the length of a packet. For example, if the size of a packet is 10 Kbits and the transmission rate of each wavelength in the core network is 10 Gbits/s, Kbits then the length of this packet is 1010Gbits = 1 µs. Sup/s posing a burst is composed by K packets with lengths lk (k = 1, . . . , K ), the transmission delay of this burst equals total length of the packets, i.e., Ttrans =
K
lk .
(1)
k=1
• Tagg : aggregation delay of a burst. The arrival times of the packets which compose the burst may be different. To be fairness, we calculate the average aggregation delay of all packets. Suppose the arrival time of the k-th packet is tk and suppose the encapsulation time of the burst is tencap . Then the aggregation delay of the burst is K
Tagg =
lk · (tencap − tk )
k=1 K
.
(2)
lk
k=1
• Tprop : propagation delay from the source to the destination. Denote the source–destination path of a burst as Pburst which is a set of links; suppose the propagation delay of link e is Te , then the propagation delay of the path is Tprop =
Te .
(3)
e∈Pburst
• TBHP : total operation time of the control packet of a burst from the source to the destination.
• Tswitch : configuration time of each optical switch. • 1t: gap between the actual beginning time and the earliest possible beginning time of a burst. In one-way signaling and inverse two-way signaling, the earliest possible beginning time of a burst is tencap +TBHP +Tswitch , whereas, in two-way signaling, it is tACK + Tswitch (tACK is the time at which the source receives the acknowledgment of successful reservation). If the selected wavelength is busy at the earliest possible beginning time, the actual beginning time of the burst would be latter than that moment, and thus a gap occurs. • Toffset : offset time between a burst and its BHP. In all the signaling schemes, the end-to-end delay can be expressed by a common formula D = Tagg + Toffset + Ttrans + Tprop .
(4)
Except that the calculation of the offset time is distinct in different signaling schemes. In one-way and inverse twoway signaling, the offset time is TBHP + 1t + Tswitch ; in twoway signaling, the offset time equals 2 · Tprop + 2 · TBHP + 1t +Tswitch . Substituting offset time into the common formula, we get the end-to-end delay of each signaling scheme:
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Doneway = Tagg + TBHP + Ttrans + 1t
+ Tswitch + Tprop
(5)
Dinverse = Tagg + TBHP + Ttrans + 1t
+ Tswitch + Tprop
(6)
Dtwoway = Tagg + 2TBHP + Ttrans + 1t
+ Tswitch + 3Tprop .
(7)
We can see that, the end-to-end delay of inverse twoway signaling is similar to that of one-way signaling and is far less than that of two-way signaling. 3.4.2. Data loss rate In each signaling scheme, data loss rate equals the ratio from the number of dropped data to the number of total data. However, the manner of data dropping in two-way signaling is different to that in one-way and inverse twoway signaling. In one-way and inverse two-way signaling schemes, if the requested resource of a burst has been reserved to another burst, a contention occurs and one of the bursts would be dropped. In two-way signaling, a burst is queuing at the source until the requested resources on all the links are successfully reversed; however, every data packet has its validity time, if the queuing time of a data packet excesses the deadline of its validity time, it would be dropped by the source. Since there is not a collection process in one-way signaling, the choice of time section for each burst is according to the state of the first link on the source–destination path. Hence, contention is very easy to occur on the following links. In inverse two-way signaling, there is a collection process. Supposing the source does not use the gathered information, inverse two-way signaling would be similar to one-way signaling. However, the application of the gathered information can decrease the probability of contention on the following links. Hence, the data loss rate in inverse two-way signaling is less than that in one-way signaling. In inverse two-way and two-way signaling schemes, the selection of time section is based on the gathered information of the whole path. The difference is that, in two-way signaling the burst would not be launched until the source receives the acknowledgment of successful reservation, but in inverse two-way signaling the burst is sent out without an acknowledgment. Two-way signaling which uses a guaranteed transmission mode can get a less data loss rate than inverse two-way signaling which adopts an unguaranteed transmission mode. From the above analysis we know that, the relationship of the data loss rate of one-way, two-way, and inverse twoway signaling schemes is that Rtwoway < Rinverse < Roneway .
(8)
In other words, the data loss rate of inverse two-way signaling is a tradeoff between one-way and two-way signaling schemes. 4. Simulations In this section, we do simulations to examine the performance of inverse two-way signaling. We first
Fig. 4. Topology of NSF-net.
investigate the impact of collection period to inverse twoway signaling; second compare the performance of inverse two-way scheme to one-way signaling (JET) and two-way signaling (pre-booking reservation mechanism). We do all experiments upon the NSF-net, which topology is shown in Fig. 4. The number marked on each edge is the length of fiber which connects two adjacent nodes [4]. The propagation delay coefficient of fiber is typically 5 µs/km. The configuration time of optical switch is supposed as 10 µs. The processing time of control packets is usually several hundred nanoseconds and is ignored in all the experiments. Each link has 16 wavelengths; and neither fiber delay line nor wavelength converter is used in the network. At each edge node, packets arrive under the Poisson rule and the length of packets is under the exponential distribution. The route for bursts is chosen by the Dijkstra’s shortest path algorithm [16]. In each signaling scheme, the wavelength is chosen by random and the length of concerned interval on each wavelength is chosen as 20 ms. 4.1. Impact of collection period to inverse two-way signaling For inverse two-way signaling, the length threshold Lmax is selected as 1 ms and the low threshold is 50 µs. Three different collection periods are investigated: 5, 10 and 15 ms. We consider four metrics of inverse twoway signaling: number of collection packets, number of reservation packets, end-to-end delay, and data loss rate. Since collection packets are sent out periodically, for a given network, the number collection packets generated per unit time (for example, one second) is determined by the collection period P. It equals N · (N − 1)/P, where N is the number of nodes in the network. Fig. 5 shows the number of collection packets generated per second under different collection periods in the NSF-net. We can see that the number of collection packets varies inversely with the collection period. The number of reservation packets equals the number of bursts. It is related to both the collection period and the traffic density. Since the source generates burst(s) when it receives a collection packet, a short period is likely to generate more reservation packets than a long one. On the other hand, the limit of Lmax results that data may be split into several bursts (correspondingly several reservation packets) for a long period. Hence, the number of reservation packets increases slightly along with the decrease of collection period. Fig. 6 shows the number of reservation packets generated per second under different collection periods.
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Fig. 5. Number of collection packets generated under different collection periods in inverse two-way signaling.
Fig. 6. Number of reservation packets generated under different collection periods and for different traffic densities in inverse two-way signaling.
When the period increases, data have to wait more time at the source; hence, the end-to-end delay increases with the increase of collection period. Fig. 7 shows the endto-end delay of inverse two-way signaling under different collection periods. We can see that the increment of endto-end delay is roughly equal to the increment of collection period. The shorter the period is, the fresher the information of link state is and the more possible successful reservation is. Hence, the data loss rate decreases with the decrease of collection period. Fig. 8 shows the data loss rate of inverse two-way signaling under different collection periods. 4.2. Comparison of three signaling schemes In this subsection, we compare the performance of inverse two-way signaling to that of one-way (JET) and two-way (pre-booking reservation) signaling schemes. In each signaling scheme, the length threshold is 1 ms and the time threshold (for inverse two-way signaling, the collection period) is 10 ms. Besides, the low threshold for inverse two-way signaling is 50 µs and the deadline for two-way signaling is selected as 100 ms.
Fig. 9 shows the end-to-end delay of the signaling schemes for different traffic densities at each node. The end-to-end delay of inverse two-way signaling is far less than that of two-way signaling and is similar to that of oneway signaling, just as expected. We have analyzed that the end-to-end delay in all three signaling schemes can be commonly expressed as the formula (4). For the route for each burst is fixed as the shortest source–destination path, the propagation delay Tprop never changes. Thus, the trend of the end-to-end delay is determined by the other three factors. When the traffic density increases, (a) the gap 1t and the length of bursts increase in all signaling schemes, that leads to the increase of offset time Toffset and transmission delay Ttrans , respectively; (b) more and more bursts are generated by triggering the length threshold in one-way and two-way signaling schemes, which reduces the aggregation delay Tagg ; (c) the waiting time of bursts increases in two-way signaling, which results in the increase of offset time Toffset .
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Fig. 7. End-to-end delay of inverse two-way signaling under different collection periods and for different traffic densities.
Fig. 8. Data loss rate of inverse two-way signaling under different collection periods and for different traffic densities.
Fig. 9. End-to-end delay of one-way, two-way and inverse two-way signaling for different traffic densities.
The effect of (a) leads to slow increase of the end-toend delay in inverse two-way signaling; the effect of (b) overwhelms the effect of (a), hence the end-to-end delay decreases in one-way signaling. The co-effect of (a)–(c)
leads to the increase of the end-to-end delay in two-way signaling. Fig. 10 shows the data loss rate of all the signaling schemes for different traffic densities at each node.
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Fig. 10. Data loss rate of one-way, two-way and inverse two-way signaling for different traffic densities.
Fig. 11. Number of reservation packets generated per second of one-way, two-way and inverse two-way signaling for different traffic densities.
When the traffic density increases, the data loss rate of each signaling scheme increases. Just as analyzed in Section 3.4.2, the data loss rate in inverse two-way signaling is always a tradeoff between one-way and twoway signaling. In each signaling scheme, the number of reservation packets increases with the growth of traffic density. Since data may be split into several bursts in inverse two-way signaling, the number of reservation packets is always slightly more than that in one-way signaling. In twoway signaling, since the source may try several times to reserve resource to a burst, one burst may generate several reservation packets; also, the chance increases with the growth of traffic density. Hence, the number of reservation packets in two-way signaling is less than that in inverse two-way signaling for low load case and more than that for high load case. Fig. 11 shows the number of reservation packets generated in each signaling scheme. The process of link state collection is only existed in two-way and inverse two-way signaling schemes. In inverse two-way signaling, the number of collection packets is a constant for a fixed collection period. In
two-way signaling, the number of collection packets equals the number of reservation packets and it increases with the growth of traffic density. When the traffic density is high, the number of collection packets in twoway signaling is far more than that in inverse two-way signaling. Fig. 12 shows the number of collection packets in two-way and inverse two-way signaling schemes. 5. Conclusions In this paper, we proposed the inverse two-way signaling for OBS networks. It can be regarded as an improved version of one-way signaling. By adding the process of link state collection to one-way signaling, it can decrease the data loss rate of one-way signaling without increasing the end-to-end delay. However, the data loss rate of inverse two-way signaling is still very high comparing to twoway signaling. In the next, we want to investigate channel scheduling algorithms and collision resolution schemes in inverse two-way signaling to further decrease its data loss rate.
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Fig. 12. Number of collection packets generated per second of two-way signaling and inverse two-way signaling for different traffic densities.
References [1] G.I. Papadimitriou, C. Papazoglou, A.S. Pomportsis, Optical switching: switch fabrics, techniques, and architectures, Journal of Lightwave Technology 21 (2003) 384–405. [2] Y. Chen, C.M. Qiao, X. Yu, Optical burst switching: a new area in optical networking research, IEEE Network 18 (2004) 16–23. [3] C.M. Qiao, M. Yoo, Optical burst switching (OBS) — a new paradigm for an optical internet, Journal of High Speed Networks 8 (1999) 69–84. [4] J.P. Jue, V.M. Vokkarane, Signaling, in: Optical Burst Switched Networks, Springer, Poston, 2005, pp. 37–56. [5] I. Baldine, G.N. Rouskas, H.G. Perros, D. Stevenson, Jumpstart: a justin-time signaling architecture for WDM burst-switched networks, IEEE Communications Magazine 40 (2) (2002) 82–89. [6] J.Y. Wei, R.I. McFarland, Just-in-time signaling for WDM optical burst switching networks, Journal of Lightwave Technology 18 (2000) 2019–2037. [7] M. Yoo, C.M. Qiao, Just-enough-time (JET): a high speed protocol for bursty traffic in optical networks, Vertical-Cavity Lasers, Technologies for a Global Information Infrastructure, WDM Components Technology, Advanced Semiconductor Lasers and Applications, Gallium Nitride Materials, Processing (1997) 11–15. [8] J.J.P.C. Rodrigues, M.M. Freire, N.M. Garcia, P.M.N.P. Monteiro, Enhanced just-in-time: a new resource reservation protocol for optical burst switching networks, in: Proceedings of IEEE International
[9]
[10]
[11] [12]
[13]
[14]
[15]
[16]
Symposium on Computers and Communications, ISC’07, 1–4 July 2007. J. Liu, N. Ansari, T.J. Ott, FRR for latency reduction and QoS provisioning in OBS networks, IEEE Journal on Selected Areas in Communications 21 (7) (2003) 1210–1219. K. Seklou, E. Varvarigos, Fast reservation protocols for latency reduction in optical burst-switched networks based on predictions, in: Seventh International Conference on Networking, ICN’2008, 13–18 April 2008. I. Widjaja, Performance analysis of burst admission-control protocols, Communications, IEE Proceedings 142 (1995) 7–14. H.L. Vu, A. Zalesky, M. Zukerman, Z. Rosberg, J. Guo, T.W. Um, Delay analysis of optical burst switching networks, in: IEEE International Conference on Communications, ICC’2005, 16–20 May 2005. H. Kong, C. Phillips, Probooking reservation mechanism for nextgeneratin optical networks, IEEE Journal of Selected Topics in Quantum Electronics 12 (2006) 645–652. K.G. Vlachos, D. Monoyios, A virtual one-way signaling protocol with aggressive resource reservation for improving burst transmission delay, Journal of Lightwave Technology 27 (2009) 2869–2875. V.M. Vokkarane, Intermediate-node-initiation (INI): a generalized signaling framework for optical burst-switched networks, Optical Switching and Networking 4 (2007) 20–32. E.W. Dijkstra, A note on two problems in connexion with graphs, Numerische Mathematik 1 (1959) 269–271.