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Bandwidth allocation based on priority and excess-bandwidth-utilized algorithm in WDM/TDM PON Cuiping Ni, Chaoqin Gan ∗ , Wei Li, Haibin Chen Key Laboratory of Specialty Fiber Optics and Optical Access Networks Shanghai University, 200072 Shanghai, China
a r t i c l e
i n f o
Article history: Received 29 March 2014 Accepted 31 July 2015 Keywords: Excess bandwidth allocation Load Priority Wavelength domain WDM/TDM PON
a b s t r a c t WDM/TDM passive optical network (PON) is being viewed as a promising solution for delivering multiple services and applications, such as high-definition video, video conference and data traffic. This paper introduces a novel bandwidth allocation scheme to efficiently utilize the bandwidth and support QoS (Quality of Service) guarantees in WDM/TDM PON. We develop the priority-based transmission windows and investigate the fair bandwidth allocation by using linearly proportional algorithm. Specially, the proposed scheme can increase the bandwidth-utilized rate (BUR) by collecting and allocating the excess bandwidths from both light-loaded ONUs (time domain) and wavelength domain. Besides, the performance of delay-sensitive services is greatly improved without degrading QoS guarantees for other services due to our priority-based scheme. Finally, we conduct detailed simulation experiments to research the performance and demonstrate the effectiveness of the proposed scheme. © 2015 Published by Elsevier GmbH.
1. Introduction The passive optical network (PON) for broadband access is now established as a promising technology to open the first-mile bottleneck. Given the steadily increasing number of users and emerging bandwidth intensive applications, current single-channel TDM (time-division multiplexing) PONs are likely to be upgraded to satisfy the growing traffic demands in the future. WDM/TDM PON [1] appears to be an attractive solution for the next generation optical access networks. It combines both technologies of TDM and WDM (wavelength-division multiplexing) [2]. WDM/TDM PON features large coverage area, high-effective cost, more flexibility, scalability, and service convergence, easy upgrade, and the higher utilization efficiency of network resources. In this paper, we consider the architecture of WDM/TDM PON as tree topology (as shown in Fig. 1). For downstream, there are w wavelengths (1 , 2 , . . ., w ) in the network and each wavelength is divided into several different timeslots (denoted by A1, A2, B1, B2, C1, C2, etc.) to avoid data collision among ONUs. For upstream, different timeslots are also divided on a wavelength to transmit data. Thus, this architecture allows for simultaneous time/wavelength-sharing of WDM/TDM PON resources among all ONUs (optical network units). Here, each ONU has the ability to transmit their data in any wavelength by using the power splitter
∗ Corresponding author. Tel.: +86 021 56332344; fax: +86 021 56332344. E-mail address: [email protected] (C. Gan).
located in the RN (remote node). A fast tunable laser with a tuning speed in the range of micrometers over seconds and a tuning range of 60 nm [3] is installed in every ONU to enable the ONU to instantly switch from one wavelength to another. In downstream transmission, it has the function that the specific wavelength can be routed to the corresponding ONU. However, less bandwidth is usually needed in upstream transmission than in downstream transmission. Thus, every ONU can share a specific wavelength channel with a few others. Hence, data packets from different ONUs may contend with each other and happen to collide when they are transmitted simultaneously over the same wavelength. To avoid data collision among ONUs, the data transmissions from different ONUs to the OLT (optical line terminal) are arbitrated through the multipoint-control protocol (MPCP, IEEE 802.3ah) [4]. The protocol is based on two control messages: the REPORT message sent from ONUs to OLT is used for reporting bandwidth request considering its buffer space. The other one is the GATE message. Using it, the OLT collects queue requests, makes bandwidth allocation decisions, and then notifies ONUs when and on which channel to transmit packets. Such a request–grant-based transmission mechanism, we believe, is highly likely to be adopted in hybrid WDM/TDM PONs for consistency [5]. The DBA algorithm is carried out at the OLT (not the ONU) and it follows an off-line approach to get full enough information to allocate the bandwidth more efficiently. The off-line approach can also facilitate the excess bandwidth collection from both time and wavelength domain. The DBA in OLT is discussed here according to the traditional multipoint-control protocol (MPCP, IEEE 802.3ah). Moreover, we enhanced the MPCP
http://dx.doi.org/10.1016/j.aeue.2015.07.020 1434-8411/© 2015 Published by Elsevier GmbH.
Please cite this article in press as: Ni C, et al. Bandwidth allocation based on priority and excess-bandwidth-utilized algorithm in WDM/TDM PON. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.07.020
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Fig. 1. Architecture of WDM/TDM PON.
protocol by adding the wavelength field and timestamp of each wavelength in the control message. The wavelength field is used to mark different wavelength in the network and the timestamp of each wavelength record the time moment of the traffic transmission on each wavelength. Thus, the management can be centralized in the OLT and achieve the synchronization. Under the framework of MPCP, there have been many discussions pertaining to dynamic bandwidth allocation. Provision of Quality-of-Service (QoS) guarantees is an important and challenging issue in the efficient bandwidth allocation. Some previous schemes for controlling QoS [6–8] have been proposed, such as IPACT-GS [8]. Ohayon [9] presented a novel approach to guarantee the performance of real-time applications such as voice and video. It distributed reservations of time slots between stations. QoS-based DBA (dynamic bandwidth allocation) schemes that can guarantee jitter [10] and fairness [11] have been proposed, but still remain the idle period [12] problem. An adaptive call admission control (CAC) scheme is proposed in [13] to avoid performance deterioration. The proposed adaptive CAC scheme utilizes a measurement-based online monitoring approach of the system performance, and a prediction model to determine the amount of bandwidth to be borrowed from calls, or the amount of bandwidth to be returned to calls. Reference [14] considered the problem of bandwidth reservation for both dedicated channels and future time slots, and introduced a time-wavelength co-allocation (TWCA) scheme to effectively improve the overall system throughput and to minimize the transfer latency for data aggregation. In multi-wavelength optical network, the packet loss probability and average delay are analyzed [15]. A QoS-constraint multicast restoration scheme in WDM mesh networks is proposed to meet the “delay-constraint” [16]. Reference [17] has provided an end-to-end bandwidth allocation scheme for delay-sensitive traffic (i.e. voice traffic). To further minimize the delay and delay variation, a fitting report position (FRP) scheme [18] and hybrid granting protocol (HGP) [19] have been proposed. FRP adaptively adjust the position of the report message within the transmission window in accordance with the current network load. HGP combines two granting mechanism: granting before REPORT and granting after REPORT, while two GATE messages lead twice the amount of guard times per polling cycle compared to other DBA schemes. We investigate the wavelengths assignment and time-slot allocation to provide fine QoS support and efficient bandwidth utilization under the transmission mechanism (i.e. MPCP). In this paper, a priority-based scheme combined with excess
bandwidth allocation is proposed to guarantee bandwidth and minimize the average packet delay for different types of applications. Two priority-based transmission windows are arranged and corresponding time slots for control messages and transmission information (including the bandwidth prediction for delay-sensitive services) are allocated in these two transmission windows. Here, for each ONU in one transmission cycle, the OLT sends one GATE message to grant the size of two transmission windows. In our scheme, bandwidths for high-priority services are pre-allocated with bandwidth prediction in idle period and REPORT message is transmitted ahead of the low-priority services’ data transmission, that greatly compensates the idle period and improve the average packet delay. Meanwhile, we further research the excess bandwidth allocation in our scheme. The excess bandwidths are especially collected from the wavelength domain besides the light-loaded ONUs (time domain). Then they are allocated to heavy-loaded ONUs according to linearly proportional algorithm to achieve fairness among ONUs. Such excess-bandwidth-utilized algorithm from two domains greatly increases the bandwidthutilized rate (BUR) in WDM/TDM PON. The rest of the paper is organized as follows. In Section 2, priority queue management and scheduling discipline are presented. The bandwidth allocation based on priority and excess-bandwidthutilized algorithm is proposed in Section 3. Section 4 presents the simulation results and performance evaluation of the proposed scheme. Finally, we conclude our paper in Section 5. 2. Priority queue management and scheduling discipline To improve the general performance of both bandwidth utilization and average packet delay, the guaranteed QoS for different applications in optical access networks should be taken into account. Namely, not only the high BUR but also the packet delay minimization and the better jitter performance for delay-sensitive applications must be considered. Fig. 2 shows that queue management tasks are carried out by each ONU. In the access network, the users asked for bandwidths to transmit data of all kinds of services. To guarantee the QoS, the service should be classified first. Different classes have different QoS criterion. The services are usually divided into three classes according to the previous researches [12,20]. The highest-priority services can be mapped to expedited-forwarding (EF) services. Here, the characteristics of EF services are time-critical, low-loss and bandwidth-guaranteed. The services are typically constant
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Fig. 2. Queue management in the proposed scheme.
bit rate (CBR), such as voice transmission services. Then, the medium-priority services can be mapped to assured-forwarding (AF) services. AF services are not delay sensitive, but require bandwidth guarantees. They are typically variable bit rate (VBR) services, such as video stream. And the low-priority services can be mapped to best-effort (BE) services. BE services are neither delay sensitive nor bandwidth guaranteed, such as web browsing, file transfer and e-mail application. Then the traffic of the three classes will be polled and some packets will be dropped according to their priority and the polling mechanism of the DBA algorithm. After traffic polling, the traffic will be stored in the shared-memory buffers. Here, each ONU maintains three separate priority queues sharing the same buffering space. Here, priority-queue scheduling discipline includes two functions: firstly, it decides the order serving requests. Secondly, it manages the priority queues according to their bandwidth requests. In the traditional queues management, packets are queued by the first-in-first-out (FIFO) structure [12]. The order delivering these packets is determined by their priorities. However, the drawback is that the high-priority services are always transmitted firstly. This results in the lower-priority services’ starvation. The proposed scheme can minimize the delay and improve the jitter performance of the delay sensitive services (EF) without degrading QoS guarantees for other services (AF and BE). To achieve
better QoS guarantee and facilitate the priority-based transmission windows in our scheme, we specially grouped these three types of services into two priorities with respect to the bit rate [21]: delaysensitive services (EF) are defined as high priority (HP) services and non-delay-sensitive services (including AF and BE) are defined as low priority (LP) services. HP services enjoy constant bit rate and tend to be a narrow-band nature. So a properly-sized window pre-allocated for HP services can provide both delay and jitter guarantees. LP services are more delay tolerant and generally have a wide-band nature. Thus, the bandwidths for LP services can be allocated according to the REPORT message. 3. The proposed bandwidth allocation scheme As illustrated in Fig. 1, our PON comprises one OLT, one RN and multiple ONUs. Every ONU are connected with the OLT through the RN. In our scheme, data transmission of different ONUs in the shared upstream channels is arbitrated by the MPCP. Here, according to the definition of above HP and LP services, we group the services into two priorities (as illustrated in Fig. 2) in order to further satisfy the requirements of both packet delay and delay jitter of the different services. In Fig. 3, the proposed scheme carries out the following operations: the OLT sends the GATE message to each ONU. This control
Fig. 3. Priority-based bandwidth allocation.
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message grants bandwidths in both HP window and LP window for upstream transmission. To make the statements of the DBA mechanism based on TDM technology more clearly, we take one wavelength for example. In other words, we discussed the situation of one of the wavelengths in the WDM/TDM PON, and this wavelength attaches part of the ONUs in the network. For the other wavelengths in the network, the value of Tcycle will vary and the DBA mechanism will be the same.
3.1. Bandwidth allocation in HP window The amount of HP (EF) services is deterministic and can be calculated accurately due to the CBR characteristic (i.e. it is not necessarily based on the REPORT message). This permits that the OLT can grant the appropriate bandwidth window for HP services beforehand, which can considerably reduce the waiting time of the delay-sensitive services. So, bandwidth allocation for HP services comes first. Then the REPORT message for the required bandwidth of the LP (AF and BE) services in the next cycle follows. By sending the REPORT message before the transmission of LP services, OLT can receive the LP services’ bandwidth requests before the start of data transmission in LP window. In this way, the dynamic bandwidth allocation can be carried out simultaneously when the data is transmitting. Thus, the DBA computing time is decreased, which will greatly compensate the idle period and improve the average packet delay. The bandwidth allocation for the REPORT message is fixed to 64 bytes. Meanwhile, considering the delay-sensitive characteristic of the HP services, some bandwidths in HP window are reserved for the incoming HP packets during the idle period. As mentioned above, HP bandwidth allocation is not necessarily based upon the REPORT message of every ONU. Instead, the OLT determines and grants the prospective bandwidths for the HP services of each ONU before allocating bandwidths for other services. To allocate the bandwidth for HP services, it is necessary for the OLT to accurately predict the prospective transmission start time of each ONU in the next cycle. The amount of HP services arriving in the cycle could be calculated accordingly. In one cycle, HP bandwidth allocation of one ONU can be achieved sequentially and transmission start time of the next ONU can be also determined correspondingly. The management of the proposed DBA algorithm is centralized in the OLT, so the time synchronization can be achieved here. We have set up the timestamps during the data transmission. Thus, we can get the transmission start time and end time for the HP services of the ith ONU. The start and end time are included in i i the control message (REPORT). Let tstar (ms) and tend (ms) be HP HP respectively the transmission start time and end time for the HP services of the ith ONU and Lrep be the length of REPORT message i (bytes) for the (64 bytes). We give the bandwidth prediction Bpre HP incoming HP packets during the idle period according to the actual amount arrived in the previous cycle [6]. RN (Gbps) is the transmission speed of the wavelength. So the bandwidths granted in HP window for ONUi can be determined as follow: i Bgran
HP
=
i RN × (tend
HP
8
i − tstar
) HP
i + Lrep + Bpre
HP
3.2. Bandwidth allocation in LP window After calculating and allocating the bandwidths for HP services, the OLT grants the bandwidth for LP services according to their REPORT message. To allocate the bandwidths for LP services fairly and effectively, we develop a novel excess-bandwidth-utilized algorithm by considering the excess bandwidths from both the light-loaded ONUs (time domain) and the wavelength domain. The proposed bandwidth allocation based on priority and excessbandwidth-utilized algorithm is specified as follows: Suppose the hybrid WDM/TDM PON with W wavelengths and N ONUs (W < N). RN (Gbps) is the transmission speed of the wavelength. The transmission cycle Tcycle (ms) is the sum of transmission time and guard time Tguard (ms) for all ONUs. In one transmission cycle, the initialized value of the available bandwidth Btotal (bytes) is expressed as follow: Btotal =
RN × (Tcycle − N × Tguard ) × W
i (bytes) as the minimum guaranteed bandwidth for Denote Bmin
i ONU. Then, Bmin is calculated as follow [22]: i Bmin = wi × Btotal
(3)
where wi is the weight assigned to each ONU based on its SLA (service-level agreement). Under the condition that there is no SLA classification for ONUs, N w = 1. Then: i.e. wi = w = 1/N, ∀i and i=1 i i Bmin =
RN × (Tcycle − N × Tguard ) × W Btotal = N 8×N
(4)
Our excess-bandwidth-utilized algorithm is based on two policies.
3.3. Excess-bandwidth allocation from the light-loaded ONUs (time domain) Here, the time domain refers to ONUs transmit in each wavelength in WDM/TDM PON. Assume that X is the set of the i i i + Breq ≤ Bmin ) in the system light-loaded ONUs (where Bgran HP LP i and Y is the set of the heavy-loaded ONUs (where Bgran i Breq LP
i Bmin ).
HP
+
i Breq LP
> is the request bandwidths of the LP services. Obviously, it is reported in the previous cycle in the HP window. Then, the total excess bandwidths Bexcess (bytes) can be calculated: Bexcess =
i i (Bmin − Bgran
HP
i − Breq
LP )
(5)
i∈X
The total extra bandwidths demands of the heavy-loaded ONUs Bdemand (bytes) can be also calculated:
(1)
Here the HP services are transmitted firstly in a transmission window to minimize their average packet delay and support the QoS of the HP services. The reserved bandwidth for the incoming HP packets during the idle period further improves the QoS of the HP services. The REPORT message is arranged to be transmitted before the predicted incoming packets. This is because the OLT can get the REPORT message as soon as possible and obtain more time to carry out DBA for LP services. Thus, the LP services’ waiting time can be reduced and the QoS of the LP services can be guaranteed.
(2)
8
Bdemand =
i (Bgran
HP
i + Breq
LP
i − Bmin )
(6)
i∈Y
The bandwidths allocation for the LP services (i.e. ONUi ’s LP window in Fig. 3) is computed as follows:
i Bgran
LP
=
i Breq
LP
,
i (Bdemand ≤ Bexcess or Breq
i i Bmin − Bgran
HP
i + Bexcess ,
LP
i i ≤ Bmin − Bgran
HP
)
(otherwise) (7)
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equals to the transmission start time plus the cycle time (Tcycle ) on jth wavelength.)
W −1
ij
Bexcess =
j=0
j
[(Tcycle − Tend ) × RN ] 8
×
i (Breq
N−1 i=0
LP
i − Bgran
i (Breq
LP
LP
)
i − Bgran
LP
) (11)
Now, more excess bandwidth can be allocated for the heavyloaded ONUs by collecting the excess bandwidths from the wavelength domain. Accordingly, to the bandwidth granted for LP services, Formula (7) needs to be updated as follow:
i Bgran LP
Fig. 4. The assignment of wavelength and ONUs.
i (Bgran
HP
i + Breq
LP
i − Bmin )
Bdemand
(8)
i where the total excess Bexcess is updated every time Bexcess is assigned: i Bexcess = Bexcess − Bexcess
(9)
3.4. Excess-bandwidth allocation in the wavelength domain We adopt the earliest-channel-available-first rule to enjoy the benefits of the multi-channel in WDM/TDM PON, and assume that each ONU can support all wavelengths (as illustrated in Fig. 2) [23]. When we collect the excess bandwidth in the network, the bandwidths which have already been allocated for HP services should be removed. Namely, after all the HP services’ bandwidth allocation, we can start to collect the excess bandwidth in both wavelength and time domain for LP services. The grant time for the first available wavelength j for ith ONU to transmit data in the next cycle can be given:
W −1 N−1 ij
Tgrant =
j=0
i=0
i (Bgran
HP
i + Bgran
LP
)×8
RN + Tguard × (N − 1) × W
i Breq
LP
,
i (Bdemand ≤ Bexcess or Breq
ij i i i Bmin − Bgran + Bexcess + Bexcess , HP
LP
i i ≤ Bmin − Bgran
HP
)
(otherwise) (12)
To guarantee the fairness of the bandwidth allocation among ONUs, the linearly proportional algorithm is used to compute the i ): excess bandwidths of the ith light-loaded ONU (Bexcess i Bexcess = Bexcess ×
=
(10)
The assignment of wavelength and ONUs is shown in Fig. 4. For the multi-wavelength characteristic in the WDM/TDM PON, the transmission cycle Tcycle is determined by the data transmission of the last ONU in one cycle (i.e. when the transmission of the last bit of all the ONUs in one cycle terminates, Tcycle is determined accordingly.). Thus, except for the wavelength which holds the last ONU in one cycle, the other wavelength still remains excess bandwidths after excess-bandwidth allocation from the light-loaded ONUs (time domain). In the scheme, we utilize these excess bandwidths on each wavelength to transmit more services. These excess wavelength bandwidths are allocated to the heavyloaded ONUs who need more bandwidths after the allocation above (Formula (7)) and also distributed in a linearly proportional manner to further ensure the fairness among the ONUs: ij (Define: Bexcess (bytes) is the excess bandwidths collect on jth j wavelength for ith heavy-loaded ONU and Tend (ms) is the end j
time of the jth wavelength’s transmission in each cycle. And Tend
As the bandwidth allocation specified above, the main characteristics of this priority-based bandwidth allocation are: 1) The packets delay minimized, the jitter performance optimized: it is because of arranging the HP window and LP window and granting firstly the bandwidths for HP services in a properly sized window. 2) The reserved bandwidth further guarantees the QoS of the HP services. 3) Reduce the idle period: the dynamic bandwidth allocation can be carried out simultaneously when the data is transmitting in LP window by sending the REPORT message earlier in HP window. Thus, the DBA computing time is decreased. 4) Ensure fairness: the fairness among different ONUs is ensured by carrying out the bandwidth allocation in a linearly proportional manner. 5) Full utilization of the excess bandwidth: the re-allocation of the excess bandwidths from both light-loaded ONUs (time domain) and wavelength domain realizes the full utilization of the bandwidth resources. 4. Performance evaluation Simulation researches are conducted to demonstrate the performance of the proposed scheme. The simulation is developed by using C++. Here, the distance from one ONU to the OLT was assumed to be 10–20 km. The total number of ONUs is: N = 32, the number of wavelength channels is: W = 4, and the wavelength speed is: RN = 1 Gbps. The guard time is: Tguard = 1 s. The maximum cycle time is: Tcycle = 2 ms and the ONU buffering queue size is: BZ = 10 megabytes. For the service model considered here, an extensive study shows that most network services (i.e., http, ftp, variable bit rate (VBR) video applications, etc.) can be characterized by self-similarity and long-range dependence (LRD) [24]. In this paper, we group these services to LP services and they are generated according to a Pareto distribution with a Hurst parameter (H = 0.8). The packet sizes are uniformly distributed between 64 and 1518 bytes. On the other hand, HP services (e.g., voice applications (CBR)) are modeled by using a Poisson distribution and the packet size is fixed to 70 bytes. In order to evaluate the effect of different traffic demands on the performance of scheduling methods, we carry out the simulator under different offered load, which is calculated by the following formula [25]: Offered load = number of ONUs × (ONU load) × user-to-ONU rate/wavelength speed. The general bandwidth allocation is usually the combination of both DWBA-2 in [3] and M-DWRR (modified deficient weighted round robin) scheduling discipline in [24]. To facilitate the description, we name it the General QoS-DWBA Scheme. In the General
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Fig. 5. Bandwidth-utilized rate (BUR) under different priority queue weight sets. (a) Set 1: WHP = 30%, WLP = 70%. (b) Set 2: WHP = 10%, WLP = 90%.
QoS-DWBA Scheme, light-loaded ONUs can be allocated bandwidths immediately on the particular channel without waiting for the rest of the ONUs to send REPORTs, which can guarantee the bandwidths and improve the delay performance. Excess bandwidths from light-loaded ONUs are allocated to heavy-loaded ONUs, which increases the BUR. M-DWRR mechanism is adopted in the General QoS-DWBA Scheme to achieve fairness during intra-ONU scheduling. In our scheme, bandwidths and delay performance can be also guaranteed and improved by arranging proper transmission window based on priority. Besides, idle period is reduced and fine QoS is guaranteed as mentioned above in section III. To further increase the BUR, we re-allocate the excess bandwidths from both light-loaded ONUs (time domain) and wavelength domain. The fairness among different ONUs in our scheme is ensured by carrying out the bandwidth allocation in a linearly proportional manner. In this paper, we mainly research two kinds of network performance under different traffic load from all ONUs: BUR and average packet delay. And we compared them of our scheme with those of the General QoS-DWBA Scheme. Content detailed is as follows: Two different service groups are employed to evaluate the performances of the proposed scheme and compare it with that of the General QoS-DWBA Scheme. The following two sets of priority queues weights are considered as follows: • Set 1: WHP = 30%, WLP = 70%; • Set 2: WHP = 10%, WLP = 90%; Fig. 5 illustrates the BUR keeps increasing in a linear manner before an inflection point appears in both two sets. In Fig. 5(a), the inflection point (i.e. the maximum value) appears when load = 1.1. In other words, the BUR keeps increasing when load ≤ 1.1. When load > 1.1, the BUR value maintains constant. Correspondingly, the inflection point in Fig. 5(b) also appears when load = 1.1. The increase of the traffic load leads to the increase of the bandwidth demands of the ONUs. Thus, more excess bandwidths are allocated and the bandwidth resources are made full use of. However, after the excess bandwidth in one cycle is used up, the BUR maintains constant. The constant value of BUR is 0.915 in set 1 and 0.954 in set 2. One reason to account for this difference is that the HP proportion in set 2 is only 10% among the total services and the pre-allocated window size for HP services is smaller. Thus the bandwidth waste for HP services may be prevented. Another reason is less HP proportion leads to more excess bandwidths for LP services, which makes
excess-bandwidth allocation more flexible and effective. Thus, set 2 can get a higher BUR. Compared with the General QoS-DWBA Scheme, Fig. 5 shows the proposed scheme can get the better performance of the BUR under two priority weight sets. It mainly owes to the excess bandwidth collection from both light-loaded ONUs (time domain) and wavelength domain and relative allocation. However, when the offered load increases from 0.4 to 0.9 in set 1 (or from 0.5 to 0.9 in set 2), the General QoS-DWBA Scheme shows better performance. It is mainly because the increased traffic load reduces the flexibility of the bandwidth allocation for the LP services. This deficiency has little effect on the overall network performance. On the other hand, we can observe that in set 1 the BUR performance (before the inflection point) in our scheme is more similar to the General QoS-DWBA Scheme than in set 2. It is because the less proportion HP services accounts for, the more chances are for LP services to allocate the bandwidth based on their REPORT message. And such REPORT-based bandwidth allocation is as same as the General QoSDWBA Scheme. So, this similarity, to some extent, verifies that our scheme is in line with the actual result. Furthermore, the simulation result also presents: the BUR is lower when the HP services account for a higher proportion (BUR in Fig. 5(a) is lower than that in Fig. 5(b)). Namely, the sacrifice of the bandwidth utilization is inevitable for satisfying the high quality services transmission. Fig. 6 indicates the average packet delay of the HP and LP services under the two different sets. It is apparent that HP services achieve the better delay performance than that of the LP services. One observation is that the average packet delay of the both two sets increases when the offered load gets higher. This is in agreement with the results obtained in practice. Another observation is the increasing speed of LP services is much higher than HP services. It complies with the general situation. The comparison of average packet delays of the proposed scheme and the General QoS-DWBA Scheme is presented in Fig. 6. Obviously, the proposed scheme exhibits lower delay than that of the general scheme, especially for HP services. This is because our scheme has introduced the pre-allocated bandwidth for HP services in order to finely satisfy the QoS requirements of the delay-sensitive services. Moreover as expected, our scheme achieves the minimum average delay for HP services because HP services arriving during the waiting period are considered and made prediction according to the actual amounts of the previous cycle. Meanwhile, we note that the average packet delay for LP services performs differently under
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Fig. 6. Average packet delay under different priority queue weight sets. (a) Set 1: WHP = 30%, WLP = 70%.
the two sets. In Fig. 6(a), the LP’s delay performance of the proposed scheme doesn’t get better performance until the offered load is higher (load > 0.5). It is because the LP services are transmitted after the HP service transmission and the bandwidth pre-allocation for HP services may probably over-estimated under low traffic load. This results in more delay for LP services. Such delay is tolerant for LP services and makes little effect on the actual transmission. But in Fig. 6(b), when the proportion is WHP :WLP = 1:9, the scheme can always perform better than the general scheme as the offered load increases from 0.1 to 1.0. One reason is: the less proportion the HP is, the more accuracy the bandwidth pre-allocation computation is. This results in the decrease of the LP services delay. Another is that the REPORT message is sent before the transmission of LP services. Consequently, it allocates the bandwidth for LP services beforehand and further decreases the idle time. We also evaluate our scheme in terms of fairness, which is of great concern nowadays. In order to measure how fairly the excess distribution approaches were performed, we adopted the fairness index f2 [26].
ij Bexcess
2
bandwidth resources. As the network load increases, the fairness value decreases in both demand-oriented DBA and equal DBA approach. And demand-oriented DBA approach performs worse than equal DBA approach. In demand-oriented DBA approach, excess bandwidth is distributed based on the ONUs’ real-time bandwidth requirement while ONUs’ weights specified by SLA are not taken into account. Besides, when the load increases, excess bandwidth distribution becomes more frequently and the fairness value decreases more. The weight-oriented distribution approach is carried out based on the SLA, so the fairness of ONUs can be ensured. Another observation from Fig. 7 is that the proposed approach achieves a stable high fairness value. The fairness index keeps almost constant and is close to 1. There are two reasons for this situation: (1) the proposed bandwidth allocation is carried out in a linearly proportional manner, which ensure the stable fairness no matter how the load varies; (2) since the excess bandwidth is allocated in both time and wavelength domains, the redundant bandwidth problem is solved and fairly distributed. Thus, our bandwidth allocation scheme can not only achieve the high bandwidth utilization, but also ensure the fine fairness among all ONUs.
wij
f2 = N
ij
2
(13)
Bexcess wij
ij
Here Bexcess is the excess bandwidths collect on the jth wavelength for the ith heavy-loaded ONU, wij is the weight of the ith ONU on the jth wavelength and N is the number of ONUs. We compared our scheme with (1) the equal excess bandwidth allocation, (2) the demand-oriented excess bandwidth allocation and (3) the weight-oriented excess bandwidth allocation. The equal bandwidth allocation divides excess bandwidth equally among the ONUs. The second approach involves dividing the excess bandwidth according to demand, that is, divides the bandwidth according to relative request size. And the third approach uses ONU priority weights to divide the bandwidth. The total excess is divided among ONUs according to their weights (wij ). In our scheme, the fairness among different ONUs is ensured by carrying out the bandwidth allocation in a linearly proportional manner and the excess bandwidth allocation in both time and wavelength domains further enhance the fairness. According to Eq. (13), the four curves in Fig. 7 illustrate how fairly the four schemes are. When the network load is low, the fairness value is high in four approaches. It is because most ONUs can be well satisfied by the available
Fig. 7. Fairness index versus offered load under different DBA schemes.
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5. Conclusion A bandwidth allocation based on priority and excessbandwidth-utilized algorithm has been proposed and researched. HP window and LP window are arranged in this paper to transmit the corresponding services and control messages to improve the packet delay and guarantee the better QoS for the services. Specially, it is more concerned of providing an optimal delay variation for delay-sensitive services without degrading the QoS of other services. The bandwidths are allocated fairly among ONUs by using linearly proportional algorithm. By re-allocating the remaining wavelength and the light-loaded ONUs’ excess bandwidth (time domain), the BUR is greatly increased. Compared with the General QoS-DWBA scheme, it has the advantages of the higher BUR and the lower average packet delay. Acknowledgments This work is supported by Programs of Natural Science Foundation of China (Nos. 61132004, 61275073 and 61420106011), Shanghai Science and Technology Development Funds (Nos. 13JC1402600 and 14511100100), Shanghai Leading Academic Discipline Project (No. S30108) and Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Shanghai University (No. SKLSFO2012-05). References [1] Mahloo M, Jiajia Chen, Wosinska L, Dixit A, Lannoo B, Colle D, et al. Toward reliable hybrid WDM/TDM passive optical networks. Opt Commun 2014:S14–23. [2] Gong Y, Gan C, Wu C. Highly reliable wavelength-reuse wavelength-division multiplexing semipassive optical access network architecture with double cover area and high network capacity. Int J Commun Syst 2014, http://dx.doi.org/10.1002/dac.2740. [3] Dhaini AR, Assi CM, Shami A. Dynamic bandwidth allocation schemes in hybrid TDM/WDM passive optical networks. In: 3rd IEEE Proceedings of Consumer Communications and Networking Conference (CCNC), vol. 1. 2006. p. 30–4. [4] Ye X, Assi CM, Ali MA. Integrated bandwidth allocation and wavelength assignment in WDM-PON networks. In: Sarnoff Symposium. 2008. p. 1–5. [5] Dhaini AR, Assi CM, Maier M, Shami A. Dynamic wavelength and bandwidth allocation in hybrid TDM/WDM EPON networks. J Lightw Technol 2007;25(1):277–86. [6] Garcia AE, Rodriguez L, Hackbarth KD. Cost models for QoS-differentiated interconnecting and wholesale access services in future generation networks. J Telecommun Syst 2012;51:221–31.
[7] Delicado J, Delicado FM, Orozco-Barbosa L. RPSF: a new QoS bandwidth request mechanism for the IEEE 802.16. J Telecommun Syst 2012;50: 97–111. [8] Ngo MT, Gravey A, Bhadauria D. Controlling QoS in EPON-based FTTX access networks. J Telecommun Syst 2011;48:203–17. [9] Ohayon R. Virtual reservation scheme for supporting CBR multimedia services with strict QoS performance over WLAN and wireless mesh. Int J Commun Syst 2012;25(5):571–84. [10] Berisa T, Ilic Z, Bazant A. Absolute delay variation guarantees in passive optical networks. J Lightw Technol 2011;29(9):1383–93. [11] Okumura Y. Traffic control algorithm offering multi-class fairness in PON based access networks. IEICE Trans Commun 2010;93(3):712–5. [12] Hwang IS, Lee JY, Lee CH, Shyu ZD. A novel distributed QoS-based DBA scheduling mechanism for star-ring architecture on ethernet passive optical access networks. In: 13th International Conference on Network-Based Information Systems (NBiS). 2010. p. 103–10. [13] Sharifa Al K, Bassel A, Khaled D, Nasser A. Bandwidth borrowing-based QoS approach for adaptive call admission control in multiclass traffic wireless cellular networks. Int J Commun Syst 2013;26(7):811–31. [14] Zhu M, Guo W, Xiao S, Dong Y, Sun W. Design and performance evaluation of dynamic wavelength scheduled hybrid WDM/TDM PON for distributed computing applications. Opt Express 2009;17(2):1023–32. [15] Zhang T, Wang J. Performance of fiber delay-line buffers in asynchronous packet-based optical switching networks with wavelength conversion. Int J Commun Syst 2014, http://dx.doi.org/10.1002/dac.2732. [16] Hwang IS, Shyu ZD, Yang CZ, Chen KP. QoS-constraint multicast restoration with 2-tuple domination core nodes selection in DWDM mesh networks. J Telecommun Syst 2012;49:287–98. [17] Boudour G, Mammeri Z, Teyssié C. End-to-end bandwidth allocation scheme for voice traffic support over MANETs. J Telecommun Syst 2011;48:261–76. [18] Sue CC, Cheng HW. A fitting report position scheme for the gated IPACT dynamic bandwidth algorithm in EPONs. IEEE/ACM Trans Netw 2010;18(2): 624–37. [19] Shami A, Bai X, Assi C, Ghani N. Jitter performance in Ethernet passive optical networks. J Lightw Technol 2005;23(4):1745–53. [20] Blake S, Black D, Carlson M, Davies E, Wang Z, Weiss W. An architecture for differentiated services. IETF RFC 2475 1998. [21] Mancas C, Mocanu M. QoS optimization in congested multimedia networks. Telecommun Signal Proc 2013:38–42. [22] Buttaboni A, De Andrade M, Tornatore M. A multi-threaded dynamic bandwidth and wavelength allocation scheme with void filling for long reach WDM/TDM PONs. J Lightw Technol 2013;31(8):1149–57. [23] Kanonakis K, Tomkos I. Improving the efficiency of online upstream scheduling and wavelength assignment in hybrid WDM/TDMA EPON networks. IEEE J Sel Areas Commun 2010;28(6):838–48. [24] Dhaini AR, Assi CM, Shami A, Ghani N. Adaptive fairness through intra-ONU scheduling in Ethernet passive optical networks. IEEE Int Commun Conf (ICC) 2006;6:2687–92. [25] Sherif SR, Hadjiantonis A, Ellinas G, Assi C. A novel decentralized ethernet-based PON access architecture for provisioning differentiated QoS. J Lightw Technol 2004;22(11):2483–97. [26] Choi SY, Lee S, Lee TJ, Chung MY. Double-phase polling algorithm based on partitioned ONU Subgroups for high utilization in EPONs. J Opt Commun Netw 2009;1(5):484–97.
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Bandwidth allocation based on priority and excess-bandwidth-utilized algorithm in WDM/TDM PON Cuiping Ni, Chaoqin Gan ∗ , Wei Li, Haibin Chen Key Laboratory of Specialty Fiber Optics and Optical Access Networks Shanghai University, 200072 Shanghai, China
a r t i c l e
i n f o
Article history: Received 29 March 2014 Accepted 31 July 2015 Keywords: Excess bandwidth allocation Load Priority Wavelength domain WDM/TDM PON
a b s t r a c t WDM/TDM passive optical network (PON) is being viewed as a promising solution for delivering multiple services and applications, such as high-definition video, video conference and data traffic. This paper introduces a novel bandwidth allocation scheme to efficiently utilize the bandwidth and support QoS (Quality of Service) guarantees in WDM/TDM PON. We develop the priority-based transmission windows and investigate the fair bandwidth allocation by using linearly proportional algorithm. Specially, the proposed scheme can increase the bandwidth-utilized rate (BUR) by collecting and allocating the excess bandwidths from both light-loaded ONUs (time domain) and wavelength domain. Besides, the performance of delay-sensitive services is greatly improved without degrading QoS guarantees for other services due to our priority-based scheme. Finally, we conduct detailed simulation experiments to research the performance and demonstrate the effectiveness of the proposed scheme. © 2015 Published by Elsevier GmbH.
1. Introduction The passive optical network (PON) for broadband access is now established as a promising technology to open the first-mile bottleneck. Given the steadily increasing number of users and emerging bandwidth intensive applications, current single-channel TDM (time-division multiplexing) PONs are likely to be upgraded to satisfy the growing traffic demands in the future. WDM/TDM PON [1] appears to be an attractive solution for the next generation optical access networks. It combines both technologies of TDM and WDM (wavelength-division multiplexing) [2]. WDM/TDM PON features large coverage area, high-effective cost, more flexibility, scalability, and service convergence, easy upgrade, and the higher utilization efficiency of network resources. In this paper, we consider the architecture of WDM/TDM PON as tree topology (as shown in Fig. 1). For downstream, there are w wavelengths (1 , 2 , . . ., w ) in the network and each wavelength is divided into several different timeslots (denoted by A1, A2, B1, B2, C1, C2, etc.) to avoid data collision among ONUs. For upstream, different timeslots are also divided on a wavelength to transmit data. Thus, this architecture allows for simultaneous time/wavelength-sharing of WDM/TDM PON resources among all ONUs (optical network units). Here, each ONU has the ability to transmit their data in any wavelength by using the power splitter
∗ Corresponding author. Tel.: +86 021 56332344; fax: +86 021 56332344. E-mail address: [email protected] (C. Gan).
located in the RN (remote node). A fast tunable laser with a tuning speed in the range of micrometers over seconds and a tuning range of 60 nm [3] is installed in every ONU to enable the ONU to instantly switch from one wavelength to another. In downstream transmission, it has the function that the specific wavelength can be routed to the corresponding ONU. However, less bandwidth is usually needed in upstream transmission than in downstream transmission. Thus, every ONU can share a specific wavelength channel with a few others. Hence, data packets from different ONUs may contend with each other and happen to collide when they are transmitted simultaneously over the same wavelength. To avoid data collision among ONUs, the data transmissions from different ONUs to the OLT (optical line terminal) are arbitrated through the multipoint-control protocol (MPCP, IEEE 802.3ah) [4]. The protocol is based on two control messages: the REPORT message sent from ONUs to OLT is used for reporting bandwidth request considering its buffer space. The other one is the GATE message. Using it, the OLT collects queue requests, makes bandwidth allocation decisions, and then notifies ONUs when and on which channel to transmit packets. Such a request–grant-based transmission mechanism, we believe, is highly likely to be adopted in hybrid WDM/TDM PONs for consistency [5]. The DBA algorithm is carried out at the OLT (not the ONU) and it follows an off-line approach to get full enough information to allocate the bandwidth more efficiently. The off-line approach can also facilitate the excess bandwidth collection from both time and wavelength domain. The DBA in OLT is discussed here according to the traditional multipoint-control protocol (MPCP, IEEE 802.3ah). Moreover, we enhanced the MPCP
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Fig. 1. Architecture of WDM/TDM PON.
protocol by adding the wavelength field and timestamp of each wavelength in the control message. The wavelength field is used to mark different wavelength in the network and the timestamp of each wavelength record the time moment of the traffic transmission on each wavelength. Thus, the management can be centralized in the OLT and achieve the synchronization. Under the framework of MPCP, there have been many discussions pertaining to dynamic bandwidth allocation. Provision of Quality-of-Service (QoS) guarantees is an important and challenging issue in the efficient bandwidth allocation. Some previous schemes for controlling QoS [6–8] have been proposed, such as IPACT-GS [8]. Ohayon [9] presented a novel approach to guarantee the performance of real-time applications such as voice and video. It distributed reservations of time slots between stations. QoS-based DBA (dynamic bandwidth allocation) schemes that can guarantee jitter [10] and fairness [11] have been proposed, but still remain the idle period [12] problem. An adaptive call admission control (CAC) scheme is proposed in [13] to avoid performance deterioration. The proposed adaptive CAC scheme utilizes a measurement-based online monitoring approach of the system performance, and a prediction model to determine the amount of bandwidth to be borrowed from calls, or the amount of bandwidth to be returned to calls. Reference [14] considered the problem of bandwidth reservation for both dedicated channels and future time slots, and introduced a time-wavelength co-allocation (TWCA) scheme to effectively improve the overall system throughput and to minimize the transfer latency for data aggregation. In multi-wavelength optical network, the packet loss probability and average delay are analyzed [15]. A QoS-constraint multicast restoration scheme in WDM mesh networks is proposed to meet the “delay-constraint” [16]. Reference [17] has provided an end-to-end bandwidth allocation scheme for delay-sensitive traffic (i.e. voice traffic). To further minimize the delay and delay variation, a fitting report position (FRP) scheme [18] and hybrid granting protocol (HGP) [19] have been proposed. FRP adaptively adjust the position of the report message within the transmission window in accordance with the current network load. HGP combines two granting mechanism: granting before REPORT and granting after REPORT, while two GATE messages lead twice the amount of guard times per polling cycle compared to other DBA schemes. We investigate the wavelengths assignment and time-slot allocation to provide fine QoS support and efficient bandwidth utilization under the transmission mechanism (i.e. MPCP). In this paper, a priority-based scheme combined with excess
bandwidth allocation is proposed to guarantee bandwidth and minimize the average packet delay for different types of applications. Two priority-based transmission windows are arranged and corresponding time slots for control messages and transmission information (including the bandwidth prediction for delay-sensitive services) are allocated in these two transmission windows. Here, for each ONU in one transmission cycle, the OLT sends one GATE message to grant the size of two transmission windows. In our scheme, bandwidths for high-priority services are pre-allocated with bandwidth prediction in idle period and REPORT message is transmitted ahead of the low-priority services’ data transmission, that greatly compensates the idle period and improve the average packet delay. Meanwhile, we further research the excess bandwidth allocation in our scheme. The excess bandwidths are especially collected from the wavelength domain besides the light-loaded ONUs (time domain). Then they are allocated to heavy-loaded ONUs according to linearly proportional algorithm to achieve fairness among ONUs. Such excess-bandwidth-utilized algorithm from two domains greatly increases the bandwidthutilized rate (BUR) in WDM/TDM PON. The rest of the paper is organized as follows. In Section 2, priority queue management and scheduling discipline are presented. The bandwidth allocation based on priority and excess-bandwidthutilized algorithm is proposed in Section 3. Section 4 presents the simulation results and performance evaluation of the proposed scheme. Finally, we conclude our paper in Section 5. 2. Priority queue management and scheduling discipline To improve the general performance of both bandwidth utilization and average packet delay, the guaranteed QoS for different applications in optical access networks should be taken into account. Namely, not only the high BUR but also the packet delay minimization and the better jitter performance for delay-sensitive applications must be considered. Fig. 2 shows that queue management tasks are carried out by each ONU. In the access network, the users asked for bandwidths to transmit data of all kinds of services. To guarantee the QoS, the service should be classified first. Different classes have different QoS criterion. The services are usually divided into three classes according to the previous researches [12,20]. The highest-priority services can be mapped to expedited-forwarding (EF) services. Here, the characteristics of EF services are time-critical, low-loss and bandwidth-guaranteed. The services are typically constant
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Fig. 2. Queue management in the proposed scheme.
bit rate (CBR), such as voice transmission services. Then, the medium-priority services can be mapped to assured-forwarding (AF) services. AF services are not delay sensitive, but require bandwidth guarantees. They are typically variable bit rate (VBR) services, such as video stream. And the low-priority services can be mapped to best-effort (BE) services. BE services are neither delay sensitive nor bandwidth guaranteed, such as web browsing, file transfer and e-mail application. Then the traffic of the three classes will be polled and some packets will be dropped according to their priority and the polling mechanism of the DBA algorithm. After traffic polling, the traffic will be stored in the shared-memory buffers. Here, each ONU maintains three separate priority queues sharing the same buffering space. Here, priority-queue scheduling discipline includes two functions: firstly, it decides the order serving requests. Secondly, it manages the priority queues according to their bandwidth requests. In the traditional queues management, packets are queued by the first-in-first-out (FIFO) structure [12]. The order delivering these packets is determined by their priorities. However, the drawback is that the high-priority services are always transmitted firstly. This results in the lower-priority services’ starvation. The proposed scheme can minimize the delay and improve the jitter performance of the delay sensitive services (EF) without degrading QoS guarantees for other services (AF and BE). To achieve
better QoS guarantee and facilitate the priority-based transmission windows in our scheme, we specially grouped these three types of services into two priorities with respect to the bit rate [21]: delaysensitive services (EF) are defined as high priority (HP) services and non-delay-sensitive services (including AF and BE) are defined as low priority (LP) services. HP services enjoy constant bit rate and tend to be a narrow-band nature. So a properly-sized window pre-allocated for HP services can provide both delay and jitter guarantees. LP services are more delay tolerant and generally have a wide-band nature. Thus, the bandwidths for LP services can be allocated according to the REPORT message. 3. The proposed bandwidth allocation scheme As illustrated in Fig. 1, our PON comprises one OLT, one RN and multiple ONUs. Every ONU are connected with the OLT through the RN. In our scheme, data transmission of different ONUs in the shared upstream channels is arbitrated by the MPCP. Here, according to the definition of above HP and LP services, we group the services into two priorities (as illustrated in Fig. 2) in order to further satisfy the requirements of both packet delay and delay jitter of the different services. In Fig. 3, the proposed scheme carries out the following operations: the OLT sends the GATE message to each ONU. This control
Fig. 3. Priority-based bandwidth allocation.
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message grants bandwidths in both HP window and LP window for upstream transmission. To make the statements of the DBA mechanism based on TDM technology more clearly, we take one wavelength for example. In other words, we discussed the situation of one of the wavelengths in the WDM/TDM PON, and this wavelength attaches part of the ONUs in the network. For the other wavelengths in the network, the value of Tcycle will vary and the DBA mechanism will be the same.
3.1. Bandwidth allocation in HP window The amount of HP (EF) services is deterministic and can be calculated accurately due to the CBR characteristic (i.e. it is not necessarily based on the REPORT message). This permits that the OLT can grant the appropriate bandwidth window for HP services beforehand, which can considerably reduce the waiting time of the delay-sensitive services. So, bandwidth allocation for HP services comes first. Then the REPORT message for the required bandwidth of the LP (AF and BE) services in the next cycle follows. By sending the REPORT message before the transmission of LP services, OLT can receive the LP services’ bandwidth requests before the start of data transmission in LP window. In this way, the dynamic bandwidth allocation can be carried out simultaneously when the data is transmitting. Thus, the DBA computing time is decreased, which will greatly compensate the idle period and improve the average packet delay. The bandwidth allocation for the REPORT message is fixed to 64 bytes. Meanwhile, considering the delay-sensitive characteristic of the HP services, some bandwidths in HP window are reserved for the incoming HP packets during the idle period. As mentioned above, HP bandwidth allocation is not necessarily based upon the REPORT message of every ONU. Instead, the OLT determines and grants the prospective bandwidths for the HP services of each ONU before allocating bandwidths for other services. To allocate the bandwidth for HP services, it is necessary for the OLT to accurately predict the prospective transmission start time of each ONU in the next cycle. The amount of HP services arriving in the cycle could be calculated accordingly. In one cycle, HP bandwidth allocation of one ONU can be achieved sequentially and transmission start time of the next ONU can be also determined correspondingly. The management of the proposed DBA algorithm is centralized in the OLT, so the time synchronization can be achieved here. We have set up the timestamps during the data transmission. Thus, we can get the transmission start time and end time for the HP services of the ith ONU. The start and end time are included in i i the control message (REPORT). Let tstar (ms) and tend (ms) be HP HP respectively the transmission start time and end time for the HP services of the ith ONU and Lrep be the length of REPORT message i (bytes) for the (64 bytes). We give the bandwidth prediction Bpre HP incoming HP packets during the idle period according to the actual amount arrived in the previous cycle [6]. RN (Gbps) is the transmission speed of the wavelength. So the bandwidths granted in HP window for ONUi can be determined as follow: i Bgran
HP
=
i RN × (tend
HP
8
i − tstar
) HP
i + Lrep + Bpre
HP
3.2. Bandwidth allocation in LP window After calculating and allocating the bandwidths for HP services, the OLT grants the bandwidth for LP services according to their REPORT message. To allocate the bandwidths for LP services fairly and effectively, we develop a novel excess-bandwidth-utilized algorithm by considering the excess bandwidths from both the light-loaded ONUs (time domain) and the wavelength domain. The proposed bandwidth allocation based on priority and excessbandwidth-utilized algorithm is specified as follows: Suppose the hybrid WDM/TDM PON with W wavelengths and N ONUs (W < N). RN (Gbps) is the transmission speed of the wavelength. The transmission cycle Tcycle (ms) is the sum of transmission time and guard time Tguard (ms) for all ONUs. In one transmission cycle, the initialized value of the available bandwidth Btotal (bytes) is expressed as follow: Btotal =
RN × (Tcycle − N × Tguard ) × W
i (bytes) as the minimum guaranteed bandwidth for Denote Bmin
i ONU. Then, Bmin is calculated as follow [22]: i Bmin = wi × Btotal
(3)
where wi is the weight assigned to each ONU based on its SLA (service-level agreement). Under the condition that there is no SLA classification for ONUs, N w = 1. Then: i.e. wi = w = 1/N, ∀i and i=1 i i Bmin =
RN × (Tcycle − N × Tguard ) × W Btotal = N 8×N
(4)
Our excess-bandwidth-utilized algorithm is based on two policies.
3.3. Excess-bandwidth allocation from the light-loaded ONUs (time domain) Here, the time domain refers to ONUs transmit in each wavelength in WDM/TDM PON. Assume that X is the set of the i i i + Breq ≤ Bmin ) in the system light-loaded ONUs (where Bgran HP LP i and Y is the set of the heavy-loaded ONUs (where Bgran i Breq LP
i Bmin ).
HP
+
i Breq LP
> is the request bandwidths of the LP services. Obviously, it is reported in the previous cycle in the HP window. Then, the total excess bandwidths Bexcess (bytes) can be calculated: Bexcess =
i i (Bmin − Bgran
HP
i − Breq
LP )
(5)
i∈X
The total extra bandwidths demands of the heavy-loaded ONUs Bdemand (bytes) can be also calculated:
(1)
Here the HP services are transmitted firstly in a transmission window to minimize their average packet delay and support the QoS of the HP services. The reserved bandwidth for the incoming HP packets during the idle period further improves the QoS of the HP services. The REPORT message is arranged to be transmitted before the predicted incoming packets. This is because the OLT can get the REPORT message as soon as possible and obtain more time to carry out DBA for LP services. Thus, the LP services’ waiting time can be reduced and the QoS of the LP services can be guaranteed.
(2)
8
Bdemand =
i (Bgran
HP
i + Breq
LP
i − Bmin )
(6)
i∈Y
The bandwidths allocation for the LP services (i.e. ONUi ’s LP window in Fig. 3) is computed as follows:
i Bgran
LP
=
i Breq
LP
,
i (Bdemand ≤ Bexcess or Breq
i i Bmin − Bgran
HP
i + Bexcess ,
LP
i i ≤ Bmin − Bgran
HP
)
(otherwise) (7)
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equals to the transmission start time plus the cycle time (Tcycle ) on jth wavelength.)
W −1
ij
Bexcess =
j=0
j
[(Tcycle − Tend ) × RN ] 8
×
i (Breq
N−1 i=0
LP
i − Bgran
i (Breq
LP
LP
)
i − Bgran
LP
) (11)
Now, more excess bandwidth can be allocated for the heavyloaded ONUs by collecting the excess bandwidths from the wavelength domain. Accordingly, to the bandwidth granted for LP services, Formula (7) needs to be updated as follow:
i Bgran LP
Fig. 4. The assignment of wavelength and ONUs.
i (Bgran
HP
i + Breq
LP
i − Bmin )
Bdemand
(8)
i where the total excess Bexcess is updated every time Bexcess is assigned: i Bexcess = Bexcess − Bexcess
(9)
3.4. Excess-bandwidth allocation in the wavelength domain We adopt the earliest-channel-available-first rule to enjoy the benefits of the multi-channel in WDM/TDM PON, and assume that each ONU can support all wavelengths (as illustrated in Fig. 2) [23]. When we collect the excess bandwidth in the network, the bandwidths which have already been allocated for HP services should be removed. Namely, after all the HP services’ bandwidth allocation, we can start to collect the excess bandwidth in both wavelength and time domain for LP services. The grant time for the first available wavelength j for ith ONU to transmit data in the next cycle can be given:
W −1 N−1 ij
Tgrant =
j=0
i=0
i (Bgran
HP
i + Bgran
LP
)×8
RN + Tguard × (N − 1) × W
i Breq
LP
,
i (Bdemand ≤ Bexcess or Breq
ij i i i Bmin − Bgran + Bexcess + Bexcess , HP
LP
i i ≤ Bmin − Bgran
HP
)
(otherwise) (12)
To guarantee the fairness of the bandwidth allocation among ONUs, the linearly proportional algorithm is used to compute the i ): excess bandwidths of the ith light-loaded ONU (Bexcess i Bexcess = Bexcess ×
=
(10)
The assignment of wavelength and ONUs is shown in Fig. 4. For the multi-wavelength characteristic in the WDM/TDM PON, the transmission cycle Tcycle is determined by the data transmission of the last ONU in one cycle (i.e. when the transmission of the last bit of all the ONUs in one cycle terminates, Tcycle is determined accordingly.). Thus, except for the wavelength which holds the last ONU in one cycle, the other wavelength still remains excess bandwidths after excess-bandwidth allocation from the light-loaded ONUs (time domain). In the scheme, we utilize these excess bandwidths on each wavelength to transmit more services. These excess wavelength bandwidths are allocated to the heavyloaded ONUs who need more bandwidths after the allocation above (Formula (7)) and also distributed in a linearly proportional manner to further ensure the fairness among the ONUs: ij (Define: Bexcess (bytes) is the excess bandwidths collect on jth j wavelength for ith heavy-loaded ONU and Tend (ms) is the end j
time of the jth wavelength’s transmission in each cycle. And Tend
As the bandwidth allocation specified above, the main characteristics of this priority-based bandwidth allocation are: 1) The packets delay minimized, the jitter performance optimized: it is because of arranging the HP window and LP window and granting firstly the bandwidths for HP services in a properly sized window. 2) The reserved bandwidth further guarantees the QoS of the HP services. 3) Reduce the idle period: the dynamic bandwidth allocation can be carried out simultaneously when the data is transmitting in LP window by sending the REPORT message earlier in HP window. Thus, the DBA computing time is decreased. 4) Ensure fairness: the fairness among different ONUs is ensured by carrying out the bandwidth allocation in a linearly proportional manner. 5) Full utilization of the excess bandwidth: the re-allocation of the excess bandwidths from both light-loaded ONUs (time domain) and wavelength domain realizes the full utilization of the bandwidth resources. 4. Performance evaluation Simulation researches are conducted to demonstrate the performance of the proposed scheme. The simulation is developed by using C++. Here, the distance from one ONU to the OLT was assumed to be 10–20 km. The total number of ONUs is: N = 32, the number of wavelength channels is: W = 4, and the wavelength speed is: RN = 1 Gbps. The guard time is: Tguard = 1 s. The maximum cycle time is: Tcycle = 2 ms and the ONU buffering queue size is: BZ = 10 megabytes. For the service model considered here, an extensive study shows that most network services (i.e., http, ftp, variable bit rate (VBR) video applications, etc.) can be characterized by self-similarity and long-range dependence (LRD) [24]. In this paper, we group these services to LP services and they are generated according to a Pareto distribution with a Hurst parameter (H = 0.8). The packet sizes are uniformly distributed between 64 and 1518 bytes. On the other hand, HP services (e.g., voice applications (CBR)) are modeled by using a Poisson distribution and the packet size is fixed to 70 bytes. In order to evaluate the effect of different traffic demands on the performance of scheduling methods, we carry out the simulator under different offered load, which is calculated by the following formula [25]: Offered load = number of ONUs × (ONU load) × user-to-ONU rate/wavelength speed. The general bandwidth allocation is usually the combination of both DWBA-2 in [3] and M-DWRR (modified deficient weighted round robin) scheduling discipline in [24]. To facilitate the description, we name it the General QoS-DWBA Scheme. In the General
Please cite this article in press as: Ni C, et al. Bandwidth allocation based on priority and excess-bandwidth-utilized algorithm in WDM/TDM PON. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.07.020
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Fig. 5. Bandwidth-utilized rate (BUR) under different priority queue weight sets. (a) Set 1: WHP = 30%, WLP = 70%. (b) Set 2: WHP = 10%, WLP = 90%.
QoS-DWBA Scheme, light-loaded ONUs can be allocated bandwidths immediately on the particular channel without waiting for the rest of the ONUs to send REPORTs, which can guarantee the bandwidths and improve the delay performance. Excess bandwidths from light-loaded ONUs are allocated to heavy-loaded ONUs, which increases the BUR. M-DWRR mechanism is adopted in the General QoS-DWBA Scheme to achieve fairness during intra-ONU scheduling. In our scheme, bandwidths and delay performance can be also guaranteed and improved by arranging proper transmission window based on priority. Besides, idle period is reduced and fine QoS is guaranteed as mentioned above in section III. To further increase the BUR, we re-allocate the excess bandwidths from both light-loaded ONUs (time domain) and wavelength domain. The fairness among different ONUs in our scheme is ensured by carrying out the bandwidth allocation in a linearly proportional manner. In this paper, we mainly research two kinds of network performance under different traffic load from all ONUs: BUR and average packet delay. And we compared them of our scheme with those of the General QoS-DWBA Scheme. Content detailed is as follows: Two different service groups are employed to evaluate the performances of the proposed scheme and compare it with that of the General QoS-DWBA Scheme. The following two sets of priority queues weights are considered as follows: • Set 1: WHP = 30%, WLP = 70%; • Set 2: WHP = 10%, WLP = 90%; Fig. 5 illustrates the BUR keeps increasing in a linear manner before an inflection point appears in both two sets. In Fig. 5(a), the inflection point (i.e. the maximum value) appears when load = 1.1. In other words, the BUR keeps increasing when load ≤ 1.1. When load > 1.1, the BUR value maintains constant. Correspondingly, the inflection point in Fig. 5(b) also appears when load = 1.1. The increase of the traffic load leads to the increase of the bandwidth demands of the ONUs. Thus, more excess bandwidths are allocated and the bandwidth resources are made full use of. However, after the excess bandwidth in one cycle is used up, the BUR maintains constant. The constant value of BUR is 0.915 in set 1 and 0.954 in set 2. One reason to account for this difference is that the HP proportion in set 2 is only 10% among the total services and the pre-allocated window size for HP services is smaller. Thus the bandwidth waste for HP services may be prevented. Another reason is less HP proportion leads to more excess bandwidths for LP services, which makes
excess-bandwidth allocation more flexible and effective. Thus, set 2 can get a higher BUR. Compared with the General QoS-DWBA Scheme, Fig. 5 shows the proposed scheme can get the better performance of the BUR under two priority weight sets. It mainly owes to the excess bandwidth collection from both light-loaded ONUs (time domain) and wavelength domain and relative allocation. However, when the offered load increases from 0.4 to 0.9 in set 1 (or from 0.5 to 0.9 in set 2), the General QoS-DWBA Scheme shows better performance. It is mainly because the increased traffic load reduces the flexibility of the bandwidth allocation for the LP services. This deficiency has little effect on the overall network performance. On the other hand, we can observe that in set 1 the BUR performance (before the inflection point) in our scheme is more similar to the General QoS-DWBA Scheme than in set 2. It is because the less proportion HP services accounts for, the more chances are for LP services to allocate the bandwidth based on their REPORT message. And such REPORT-based bandwidth allocation is as same as the General QoSDWBA Scheme. So, this similarity, to some extent, verifies that our scheme is in line with the actual result. Furthermore, the simulation result also presents: the BUR is lower when the HP services account for a higher proportion (BUR in Fig. 5(a) is lower than that in Fig. 5(b)). Namely, the sacrifice of the bandwidth utilization is inevitable for satisfying the high quality services transmission. Fig. 6 indicates the average packet delay of the HP and LP services under the two different sets. It is apparent that HP services achieve the better delay performance than that of the LP services. One observation is that the average packet delay of the both two sets increases when the offered load gets higher. This is in agreement with the results obtained in practice. Another observation is the increasing speed of LP services is much higher than HP services. It complies with the general situation. The comparison of average packet delays of the proposed scheme and the General QoS-DWBA Scheme is presented in Fig. 6. Obviously, the proposed scheme exhibits lower delay than that of the general scheme, especially for HP services. This is because our scheme has introduced the pre-allocated bandwidth for HP services in order to finely satisfy the QoS requirements of the delay-sensitive services. Moreover as expected, our scheme achieves the minimum average delay for HP services because HP services arriving during the waiting period are considered and made prediction according to the actual amounts of the previous cycle. Meanwhile, we note that the average packet delay for LP services performs differently under
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Fig. 6. Average packet delay under different priority queue weight sets. (a) Set 1: WHP = 30%, WLP = 70%.
the two sets. In Fig. 6(a), the LP’s delay performance of the proposed scheme doesn’t get better performance until the offered load is higher (load > 0.5). It is because the LP services are transmitted after the HP service transmission and the bandwidth pre-allocation for HP services may probably over-estimated under low traffic load. This results in more delay for LP services. Such delay is tolerant for LP services and makes little effect on the actual transmission. But in Fig. 6(b), when the proportion is WHP :WLP = 1:9, the scheme can always perform better than the general scheme as the offered load increases from 0.1 to 1.0. One reason is: the less proportion the HP is, the more accuracy the bandwidth pre-allocation computation is. This results in the decrease of the LP services delay. Another is that the REPORT message is sent before the transmission of LP services. Consequently, it allocates the bandwidth for LP services beforehand and further decreases the idle time. We also evaluate our scheme in terms of fairness, which is of great concern nowadays. In order to measure how fairly the excess distribution approaches were performed, we adopted the fairness index f2 [26].
ij Bexcess
2
bandwidth resources. As the network load increases, the fairness value decreases in both demand-oriented DBA and equal DBA approach. And demand-oriented DBA approach performs worse than equal DBA approach. In demand-oriented DBA approach, excess bandwidth is distributed based on the ONUs’ real-time bandwidth requirement while ONUs’ weights specified by SLA are not taken into account. Besides, when the load increases, excess bandwidth distribution becomes more frequently and the fairness value decreases more. The weight-oriented distribution approach is carried out based on the SLA, so the fairness of ONUs can be ensured. Another observation from Fig. 7 is that the proposed approach achieves a stable high fairness value. The fairness index keeps almost constant and is close to 1. There are two reasons for this situation: (1) the proposed bandwidth allocation is carried out in a linearly proportional manner, which ensure the stable fairness no matter how the load varies; (2) since the excess bandwidth is allocated in both time and wavelength domains, the redundant bandwidth problem is solved and fairly distributed. Thus, our bandwidth allocation scheme can not only achieve the high bandwidth utilization, but also ensure the fine fairness among all ONUs.
wij
f2 = N
ij
2
(13)
Bexcess wij
ij
Here Bexcess is the excess bandwidths collect on the jth wavelength for the ith heavy-loaded ONU, wij is the weight of the ith ONU on the jth wavelength and N is the number of ONUs. We compared our scheme with (1) the equal excess bandwidth allocation, (2) the demand-oriented excess bandwidth allocation and (3) the weight-oriented excess bandwidth allocation. The equal bandwidth allocation divides excess bandwidth equally among the ONUs. The second approach involves dividing the excess bandwidth according to demand, that is, divides the bandwidth according to relative request size. And the third approach uses ONU priority weights to divide the bandwidth. The total excess is divided among ONUs according to their weights (wij ). In our scheme, the fairness among different ONUs is ensured by carrying out the bandwidth allocation in a linearly proportional manner and the excess bandwidth allocation in both time and wavelength domains further enhance the fairness. According to Eq. (13), the four curves in Fig. 7 illustrate how fairly the four schemes are. When the network load is low, the fairness value is high in four approaches. It is because most ONUs can be well satisfied by the available
Fig. 7. Fairness index versus offered load under different DBA schemes.
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5. Conclusion A bandwidth allocation based on priority and excessbandwidth-utilized algorithm has been proposed and researched. HP window and LP window are arranged in this paper to transmit the corresponding services and control messages to improve the packet delay and guarantee the better QoS for the services. Specially, it is more concerned of providing an optimal delay variation for delay-sensitive services without degrading the QoS of other services. The bandwidths are allocated fairly among ONUs by using linearly proportional algorithm. By re-allocating the remaining wavelength and the light-loaded ONUs’ excess bandwidth (time domain), the BUR is greatly increased. Compared with the General QoS-DWBA scheme, it has the advantages of the higher BUR and the lower average packet delay. Acknowledgments This work is supported by Programs of Natural Science Foundation of China (Nos. 61132004, 61275073 and 61420106011), Shanghai Science and Technology Development Funds (Nos. 13JC1402600 and 14511100100), Shanghai Leading Academic Discipline Project (No. S30108) and Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Shanghai University (No. SKLSFO2012-05). References [1] Mahloo M, Jiajia Chen, Wosinska L, Dixit A, Lannoo B, Colle D, et al. Toward reliable hybrid WDM/TDM passive optical networks. Opt Commun 2014:S14–23. [2] Gong Y, Gan C, Wu C. Highly reliable wavelength-reuse wavelength-division multiplexing semipassive optical access network architecture with double cover area and high network capacity. Int J Commun Syst 2014, http://dx.doi.org/10.1002/dac.2740. [3] Dhaini AR, Assi CM, Shami A. Dynamic bandwidth allocation schemes in hybrid TDM/WDM passive optical networks. In: 3rd IEEE Proceedings of Consumer Communications and Networking Conference (CCNC), vol. 1. 2006. p. 30–4. [4] Ye X, Assi CM, Ali MA. Integrated bandwidth allocation and wavelength assignment in WDM-PON networks. In: Sarnoff Symposium. 2008. p. 1–5. [5] Dhaini AR, Assi CM, Maier M, Shami A. Dynamic wavelength and bandwidth allocation in hybrid TDM/WDM EPON networks. J Lightw Technol 2007;25(1):277–86. [6] Garcia AE, Rodriguez L, Hackbarth KD. Cost models for QoS-differentiated interconnecting and wholesale access services in future generation networks. J Telecommun Syst 2012;51:221–31.
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Please cite this article in press as: Ni C, et al. Bandwidth allocation based on priority and excess-bandwidth-utilized algorithm in WDM/TDM PON. Int J Electron Commun (AEÜ) (2015), http://dx.doi.org/10.1016/j.aeue.2015.07.020