QoS Supported Dynamic Traffic Scheduling in WDM/TDM Networks with Arbitrary Tuning Latencies Nen-Fu Huang, Te-Lung Liu, and Ching-Fang Hsu Department of Computer Science, National Tsing Hua University, Taiwan 300, Republic of China E-mail :
[email protected]
Abstract-This paper proposes a dynamic traffic scheduling algorithm in single-hop WDM/TDM networks with arbitrary tuning latencies to support guaranteed QoS. To furnish different levels of QoS, two classes of traffc are considered constant bit rate (CBR) and available bit rate (ABR). An effective bandwidth normalizationscheme for ABR traffc is also derived. By applying distinct normalization schemes to CBR and ABR traffic individually, the bandwidth can be allocated more accurately and meanwhile the QoS is also guaranteed. Two slot allocation policies are also suggested to allocate the time slots to connections. The performance of proposed algorithm is evaluated and compared by simulations under different system parameters, such as wavelength number, traffic load, and tuning latency. 1. Introductioin With the rapid growth of development of multimedia applications, the trend of accommodating a large variety of traffic with various quality-of-service (QoS) requirement has emerged. Optical networks based on wavelength division multiplexing (WDM) technology are capable of providing several terabits per second bandwidth; such attractive characteristics also makes WDM be considered as the most promising approach to achieve this goal. Among several proposed physical topologies for WDM network, the star topology with a star coupler operating is the most widely discussed one under a LAN environment. In this topology, each end station is equipped with one or more transmitter(s) and receiver(s). The tunability of transceivers mainly depends on tuning range and tuning delay. With current technology, ideal transceivers that may tune across all wavelengths in a negligible time are still under design [l], [3]. For provisioning higher flexibility, the following discussions are based on a WDM star-coupled network with TT-TR (tunable transmitters and tunable receivers) technique. With the popularity of multimedia applications, more and more researches set provisioning different QoS levels as one of their main goals [4]-[7]. In [5], both isochronous and asynchronous traffic are considered under a WDM star network with multiple transceivers while only isochronous traffic is scheduled in [4]. A scheduling mechanism for the assignment of guaranteed bandwidth (GBW) is proposed in [6] and two classes of traffic, real-time and nonreal-time, are processed in [7]. Because of the limitation of hardware components, the tuning latency is still considerable. But it was not taken into consideration in most literatures although such simplification is not practical [4]-[6]. In [4], the tuning latency is assumed to be incorporated into a time slot. Following this assumption, the bandwidth utilization will drop rapidly with the increasing of the latency time. In recent years, more and more discussions
broke up this assumption and revealed the impacts that the tuning latency may bring [1]-[3]. This paper proposes a dynamic traffic scheduling algorithm in single-hop WDMA'DM networks with arbitrary tuning latencies to support guaranteed QoS. Both constant bit rate (CBR) and available bit rate (ABR) are considered. An effective bandwidth normalization scheme for ABR traffic is also derived for more accurate allocation. Two slot allocation policies are also suggested to allocate the time slots to connections and evaluated by simulations. The rest of this paper is organized as follows. System assumptions and formal problem definition are presented in Section 2. In Section 3, the proposed dynamic traffic scheduling algorithm is introduced. The simulation model and results are shown and discussed in Section 4 and Section 5 concludes this paper. 2. Assumptions and Problem Definition 2.1 Assumptions A. Network Model WDM star topology using a broadcast-and-select star coupler is considered. Assume there are W+1 wavelengths on each fiber link, A,,OVSW, where ,?L, is dedicated to the control channel and other W wavelengths are data channels. Transmission in the network operates in a time-slotted fashion. As to the control channel A,,, every time slot is further divided into N minislots, where N is the number of stations. Each minislot is pre-assigned to one station and each station exactly possesses only one minislot. Assume each station is equipped with a pair of fixed transceivers (control channel) and a pair of tunable transceivers (data channel). For simplicity, the normalized tuning delay 6, expressed in units of cell duration instead of units of time, is used [ 11, [3]. All transceivers are tunable over all wavelengths with the same 6. A schedule is feasible if the following constraints are all satisfied: (1) For each station i, at most one transmission is allowed within the same time slot. (2) For each station i, at most one reception is allowed within the same time slot. (3) For all transmitters, the time period between any two consecutive transmissions on different wavelengths can not be shorter than 6 time slots. (4) For all receivers, the time period between any two consecutive receptions on different wavelengths can not be shorter than 6 time slots. B. QoS Parameters With QoS provisioning, both CBR and ABR traffic types are considered. For simplicity, traffic streams are referred to
0-7803-6451-1/00/$10.00Q 2000 IEEE
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virtual circuits (VC's). As periodically recurring is the most significant characteristic of CBR VC's, the period of a traffic schedule containing CBR traffic is called as cycle length, denoted by L which will be explained later. The behavior of a CBR VC is usually characterized by the peak cell rate (PCR). In addition to PCR, the minimum cell rate (MCR) is also used to characterize the behavior of an ABR VC. We use a 2-tuple as described in Section 2.1. Step 2. Affordability Check This step checks whether the adding of normalized rate c,' into the pi and pi violates the constraints of p i l l and
derive the normalized rate =. It is apparent that a lot of bandwidth is saved in this way. The proposed transformation for ABR traffic can be described by the following mathematical expressions: d,' = 2r10g2d m 1
c,'
c,
dm'
dm
=
[~x2rioa2d.i]
dm'
p:".
2.2 Problem Definition Based on assumptions described in previous section, the scheduling problem can be defined as follows: Given N stations, W available wavelengthsfor data transmission, L-slot global cycle and a slot-allocation matrix D; each station is equipped with a pair of tunable transmitter and tunable receiver and each transceiver needs Gslotsfor tuningfrom li, to hi, i#j. (1) For a setup request r,=, find a new feasible slot-allocation matrix D, with a new global cycle length L, such that r, is arranged into 0, subject to that all the QoS of accepted VC's in D can not be aflected. (2) For a termination request r, = L, we append D to the right of D' by (L'A)-l times. We then build a M' matrix A , called available slot matrix, for available slot scanning, which is defined as follows A= Caulw ai,€ (0, I} such that
3. Proposed Slot Allocation Algorithm Before we present our slot allocation algorithm, some data structures and system parameters used in the algorithm are introduced first. A database, named VC Table, is used to keep the information of each VC. This includes the VC's ID, source ID, destination ID, ,',c d,' c i , and d i . cvc-id, s, e , c,', d,,,', ci, d i , > A WxL matrix D,named slot-allocation matrix, is used to represent current traffic schedule. Each entry do=O in D indicates thejth slot of A, is free. Otherwise, it specifies the ID of a specific VC which occupies the slot. (e.g. fig.1 (b)) Since each station is equipped with only a pair of tunable transceivers, for each L-slot time, at most L slots can be allocated to a station for transmission (reception). To satisfy this constraint, it is necessary to record the bandwidth of allocated VC's for each transmitter (called the transmitter utilization, and denoted as p,!), and that for each receiver (called the receiver utilization, and denoted as p; ). We should have p,! 21 and p,: S1. Thus, .
p,! =c( I c,is the normalized MCR of V q emitting from i ) di,' d i ' p;
d:'
d;'
{
A is initialized with a,=l. For any VC assigned to d; in D', we set aij = 0 and for a VC assigned to d,' in D' whose source is s or destination is e, we set axj = 0 for all x, 1SxSW. (constraints (1) and (2)) aXQw= 0 for all x#i, OIyS6. (constraints ( 3 ) and (4)) Before explaining in detail how A is exploited to scan available slots, a decomposition method, named grouping is introduced to horizontally partition A in every d,,,' slots. Each derived Wxd,' matrix is called a group and we have d m'
groups. The k-th group, denoted as Gk,where Gk = { g i I g; = Q ~ ( ~ + ( ~ - ~ ) . ~1: ) I, i IW,and 1 I j Id,,,'}. Since each group represents a deadline d,,,', the same c,,,' slots should be allocated at each group. To further evaluate whether for each offset within d,,,' slots, at least one wavelength on this slot position is available, a candidate vector (denoted as K) is designed; where 1
=c(?& I c,is the normalized MCR of VCj destiningto i) i t
a, = 0 if either transmitter or receiver is busy at slot j on Ai 1 i f both transmitter and receiver are free at slot j on I ,
0
j i
j r
Note that p : > l ( p : > l ) means that the transmitter of
The proposed connection setup algorithm consists of four steps to accommodate a new connection request: normalization, affordability check, available slot scan, and slot assignment. We omit connection release algorithm due to space constraints. Steps of connection setup are explained as follows:
1310
3kVi,g$=O, V k 3 i, g,; = 1,
15 k I-, L' 1 Ii I W ,1Ij Id:
dm
Let 14 denote the number of ones in K. If 14 (Fig. 2(c)), Ilcl=O
