Persistent reservation protocol for variable-length messages in WDM-based local networks

J.H.Lee

Abstract: The author proposes a persistent reservation protocol in a wavelength division multiplexing (WDM)-based local network using a passive star topology. With this protocol, once a node reserves a data channel, the node persistently uses the channel until its message is completely transmitted. Therefore, the protocol needs only one tuning time for a node to begin to transmit its message, which reduces deterioration of throughput due to tuning overhead. The proposed protocol can efficiently support variable-length messages. The network can accommodate a variable number of nodes and can operate independently of any change of the number of nodes. Hence, any ‘new’ node can join the network at anytime without network re-initialisation. Moreover, with the protocol, data channel and destination conflicts can be avoided. Throughputdelay characteristics are analysed with a finite population model, and are verified by simulation results.

1 Introduction

Optical fibre has superior characteristics, such as large bandwidth and very low error rates [l]. Single-mode optical fibre has a bandwidth of about 20THz in the low-loss region of 1 . 2 ~ to 1 . 6 ~ [2]. However, maximum capacity of the fibre is limited to the electronic processing speed of a few Gbit/s. Wavelength division multiplexing (WDM) tech- nology is one method to exploit huge bandwidth [3, 41. In WDM, the bandwidth of the fibre is divided into a set of parallel channels, each operating with a different wave- length. Therefore, a network using WDM technology becomes a multichannel network. In this case, how to share the enormous bandwidth of optical medium among all nodes of the network is the major issue of the lightwave network.

In [5-71, a number of random-access protocols have been introduced and analysed for very hgh-speed optical LANs using a passive star topology. Most of the protocols are based on the transmission of fmed-length data packets. However, when there is a need to accommodate circuit- switched traffic or traffic with long holding times (e.g. file transfers), it is necessary for a protocol to efhiently sup- port variable-length messages. In [8, 91, a buffered reserva- tion protocol with variable-length messages has been proposed, when a system is asynchronous. In this protocol, when a message arrives at a node, the node transmits its control packet through the control channel by the slotted ALOHA protocol. If the control packet is transmitted without collision, the information is queued into the desti- nation buffer for transmission. And then, an identical and distributed scheduling algorithm is invoked by the node (as well as any other nodes) to determine the data channel

0 IEE, 2001 ZEE Proceechgs online no. 20010212 DOL lO.l@49/ipcom:20010212 Paper fxst received 26th July 1999 and in revised form 12th June 2000 The author is with the Department of Information and Communications Engi- neering, Dongguk University, 26,3 Pddong, Chung-gu, Seoul 100-715, Korea

number and time duration to transmit message, which requires global information. The need of global informa- tion, however, has a serious drawback when a ‘new’ node is added to the network. With the protocol using global infor- mation, all nodes including the new node must know the time of transmission and the data channel that will be used between source-destination pairs. This will require re-ini- tialisation of the network whenever any new node tries to join the network. Therefore, it is necessary for a protocol to support variable-length messages without requiring global information.

In [IO], dynamic scheduling protocol has been proposed in order to support variable-length messages efficiently, without requiring global information. With the dynamic scheme, a transmitting node is likely to use different data channels in every slot. When the tuning times of the trans- ceiver are nonzero, the overhead due to the tuning time is not negligible, whch deteriorates the performance.

In this paper, a persistent reservation protocol is proposed. This protocol can support variable-length messages without requiring global information. Therefore, any new node can join the network at any time without requiring network re-initialisation. Destination and data channel conflicts can be avoided by using this protocol. Moreover, once a node acquires a data channel for trans- mitting a message, it persistently exploits the channel until the message is completely transmitted. Therefore, deteriora- tion of throughput due to tuning overhead can be reduced by using the proposed protocol.

2 Persistent reservation protocol

The network architecture considered in this paper is shown in Fig. 1. The bandwidth of the fibre is divided into (N + 1) WDM channels, each operating with a different wavelength from the set {&, ..., A N } . There are M(M > If) nodes in the network, each of which has a sending buffer that can store at most one message at a time. Each message is composed of one or more fixed-length data packets such as 53-byte ATM cells. Nodes are connected to input and output ports of a central passive star coupler, where the incident light energy

81 IEE Proc.-Conminim.. Vol. 148, No. 2, April 2001

from any input is equally divided among all output ports. Thus, the star coupler acts as a broadcast medium. A receiv- ing node needs to be dormed of which channel is used to receive a message from a transmitting node, i.e. a pre-trans- mission co-ordination is required between the nodes. The channel with wavelength & is used as a control channel for co-ordination of the access among the nodes. Those with wavelengths {A,, ..., AN} are used as data channels for actual message transmission. Each node is equipped with a fned transmitter and a fuced receiver, both of whch are tuned to the control channel. Each node also has one tuna- ble transmitter and one tunable receiver which can operate in any one of data channels independently.

transmitter star coupler receiver

@ tunable laser @ tunable filter

0 fixed laser 0 fixed filter

Fig. 1 Nehrark architecture with a pussive star topology

Control packets are transmitted through the control channel, which are used for signalling between nodes and for reserving data channels during message transmission time. A control packet may contain source node address, destination node address and the wavelength to be used for the transmission of the actual message. Before a control packet is transmitted, the source node has to determine which data channel will be used for message transmission, and ths information should be included in the control packet, in order to notify the intended destination node. Random and idle schemes are considered as data channel selection schemes in this paper [l 1, 121. In the random data channel selection scheme, a node with a ready message ran- domly chooses a data channel before monitoring the con- trol channel. In the idle data channel selection scheme, a node first monitors the control channel, and then chooses a data channel among those whch are sensed idle. In either case, once a node reserves a data channel, it persistently uses the data channel in order to transmit its message. AU channels are slotted with the size of the transmission

time of a fuced-length data packet, as shown in Fig. 2. Slots on the data channels are called data slots and contain the actual data messages. Slots on the control channel are called control slots. The end-to-end propagation delay is defined as the time it takes traffic sent by a node to reach all nodes, which is denoted as z. One slot of the control channel is further divided into z seconds and x minislots, each having the size of the transmission time of a control

82

packet. Then, all control packets transmitted in one control slot can be propagated to all receivers within this slot. The control channel is shared by all nodes on a contention basis using the slotted ALOHA protocol.

data channel 2

data channel 3

data channel 4

control channe

Fig. 2 Mersuge t r m u s l o n procedure

Data channel conflict occurs when more than one of the source nodes try to use the same data channel at the same time. Moreover, when two or more nodes are successful in obtaining data channels, and wish to communicate with the same destination node in a slot, destination conilict occurs [13]. Hence, any source node should employ the following protocol in order to transmit a message without data chan- nel and destination conflicts.

2. I Message transmission procedure

2. I. I Local control channel monitoring (LCCM): Assume that a message (new or retransmitted) is generated at node i to be destined to node j , at the beginning of slot t. In the case of the random selection scheme, node i chooses one of the data channels randomly, say n. The node moni- tors the control channel during control slot t. Node i then checks the following conditions:

No successful control packet to nodej is observed during control slot t;

No successful control packet with nth data channel number is observed during control slot t in the case of the random data channel selection scheme. In this case, a successful control packet means that it does not collide with others, and does not have the same desti- nation address and/or data channel number among those of earlier successful control packets within each slot. If any of the above conditions is not satisfied, the transmission procedure at node i will be restarted after a random back- off period. Otherwise, let k be the number of successful control packets in control slot t. Then node i chooses one of (x - k) available minislots randomly, say yth minislot. Moreover, in the case of the idle selection scheme, the node randomly chooses one, say m, among data channels that are sensed idle. Then, the node continues to the next state.

2.1.2 Control packet transmission (CPT): Node i transmits its control packet over the control channel in yth minislot of control slot ( t + l), and continues to the next state. In this case, the control packet is called the control packet for signalling (S-CP).

2,1.3 Control packet success detection (CPSD): Control packets transmitted in slot (t + 1) are detected by all nodes at the end of slot ( t + 1). If the S C P of node i is the successful control packet, the node considers that it has successfully reserved n(m)th data channel, tunes its tunable transmitter to the data channel, and continues to next state. The destination node will also tune its tunable receiver to the intended wavelength. Otherwise, after a random back- off time, the transmission procedure restarts from LCCM state.

IEE Proc -Cornmun Vol 148. N o 2 April 2001

2.1.4 Data message transmission (DMT): Node i continues to transmit its control packet through the corre- sponding minislot for every control slot, until the message is completely transmitted. In this case, the control packet is called the control packet for reserving (RCP), and it is always a successful control packet, i.e. node i transmits one data packet among its message through the reserved data channel, and, at the same time, it transmits its R C P through yth minislots in every slot, until only one data packet remains in node i. If the length of the remaining message is 1 at the beginning of a slot, node i transmits its data packet through the data channel and returns to LCCM to await a new message.

3 Performance analysis , Each node can be in one of three states: idle, backlogged or transmitting. A node is said to be idle if it has no message. A node is in transmitting state if it has succeeded in reserv- ing a data channel and is transmitting its message through the channel. Otherwise, a node is in backlogged state, try- ing to reserve a data channel in order to transmit its mes- sage. It is assumed that an idle node gets a new message with probability A. Each message length is assumed to be geometrically distributed with mean Up (slots). The tuning time of the tunable transmitter is assumed to be included in the message. It is also assumed that the destination of an S-CP is assigned with equal probability among M destina- tions. And, whenever an S C P is retransmitted due to res- ervation failure, its destination is assumed to be reassigned with equal probability among A 4 destinations [14]. With this assumption, the system can be modelled by a discrete- time Markov chain, whch is obtained by observing the state of the system at the end of a slot. The system state at the embedded points is given by (k,, k2), where k , and k2 represent the numbers of transmitting nodes and back- logged nodes, respectively. Then, kl is equal to the number of successful control packets in a control slot.

Let PC(k31kl, k2) be the probability that k3 among kl transmitting nodes continue to transmit their messages in the next slot. PC(k3/kl, k2) is binomially distributed and can be calculated as follows:

Pc(k3 I k l , k2) k l c k 3 (1 - p ) k3pk1-k3 (1) Similarly, if it is assumed that PA(k#,, k2) is the probabil- ity that k5 among (M - k , - k2) idle nodes get new messages, it can be calculated by the following:

PA(k5 I k l , k2) = M-kl-kzCkaXk5(1-X)M--lc1-k2--lc5

(2) Also, let PS(k41k,, k2) be the probability that k4 among k2 backlogged nodes reserve data channels successfully. If PS(k41kl, k2) is calculated, the state of the network can be obtained. In order to calculate this probability, first define the following two probabilities. When j distinguishable balls are distributed among m cells, denote Q(m, i, 19 as the prob- ability that each of i cells has only one ball, and Pr(m, i, 1) as the probability that (m - i) cells are empty. Then Q(m, i, j ) and Pr(m, i, I] can be calculated, respectively, as follows:

Q(m,z , j ) =

mc2Jcz 2 ! inin(m-,j-)

( - 1) m-Lc,,,c, U ! (m - i - V)j+J ,,=n

mJ 2

mcm-2 (-1)"zC,(i - Y ) j

(3) y=o Pr(m, i, j ) =

mJ

Then, Q(x - k l , m, I , is equal to the probability that each of m among I S-CPs does not collide with others, when (x- k, ) minislots are available. Pr(M - kl, n, m) is the probability that n among m uncollided S C P s choose dif- ferent destination addresses, when (A4 - k,) destinations are ready to receive. Moreover, Pr(N - k l , n, m) is the proba- bility that n among m uncollided S-CPs choose different data channels, when ( N - k,) data channels are available.

In the case of the random selection scheme, a backloggd node randomly chooses a destination node and a data channel in advance before transmitting its S-CP. And it can transmit its S C P in the chosen minislot of the next control slot, when no successful control packet exists with the same destination node address and/or data channel number of the S-CP in a slot. Let Pscp(lJkl, k2) be the probability that I among k2 backlogged nodes transmit their SCPs through the control channel. Then Pscp(l~kl, k2) can be calculated as follows:

k2

Pscp(llkl,k2) = ~ P l , ( i ( k l , X ^ 2 ) P l r ( l l i , h : 1 , k 2 ) 0

(4) where

Among S-CPs transmitted by the backlogged nodes, successful control packets are only successful in reserving the data channel. Therefore, PS(k41kl, k2) can be obtained by the following:

m i n ( M - k l , x - k l r k 2 )

psR(k4 I k l , k.2) = Pr (N-k l , k i , n ) n=k4

m=n

k z

x Q ( ~ - - k i , ~ ~ , l ) P s c p ( l I k i , k 2 ) ' ( 5 ) l=m

In the case of the idle selection scheme, each node among k2 backlogged nodes monitors the control channel during one slot and checks whether there is a successful control packet with the same destination node address with that of the node. If there is no such successful control packet, the node randomly chooses one of the (N - kl) idle data chan- nels and then transmits its S-CP. Therefore, PS(k41k,, k2) can be obtained as follows:

n i i n ( M - k l , z - k l , k 2 ) c

l=m

Define ?q&* as the probability that there are kl transmit- ting nodes and k2 backlogged nodes at the end of a slot when the system has reached the steady state. Also let P~ul,u21~k,,k2) denote the conditional probability that there are

83 IEE Proc-Coir"., Vol 148, No. 2, April 2001

ul transmitting nodes and u2 backlogged nodes at the end of a slot, given that there are k, transmitting nodes and k2 backlogged nodes at the end of the previous slot. Then, n&,k2 can be computed by the following:

N M - k l

ki k 2

The average delay of message with respect to the transmis- sion time of a data packet, DT, is

M 1 DT = - - - +1 PST (9)

4 Numerical results

In this Section, the performance of the proposed protocol is investigated using various system parameters. Fig. 3 shows delay against network throughput for various average mes- sage lengths. Average message delay includes the transmis- sion time of a message, and thus the average delay increases as the average message length increases. In the random selection scheme, a node randomly chooses one of the data channels, and then monitors the control channel to check whether there is a successful control packet with the same data channel number and/or destination address with the node. If such a successful control packet does not exist, the node transmits its S C P . Meanwhile, in the idle selection scheme, a node first monitors the control channel to check whether a Successful control packet with the same

50

40

z

U a, 0

2 30

5 20

10

n 0 2 4 6 8 10

network throughput

Fig. 3 M = 5 0 N = 1O;x = 20 ~ random

(i) p = 0.1; (ii) p = 0.2; (iii) p = 0.5

Delay against throughput for various average message length

idle . . . . . . . . . ..

destination number does not exist. When such a control packet does not exist, the node chooses one of the data channels sensed idle and then transmits its S-CP. There- fore, control packets are generated and transmitted more in the idle case than in the random case. In a low-load region, any node trying to transmit its message transmits its S-CP according to the message transmission procedure, and the control packet may be sent without collision. Hence, the idle scheme has superior characteristics in the low-load region where S-CPs transmitted are not likely to collide with others, while the random scheme is superior in the high-load region. In the proposed protocol, onc:el,a node reserves a data channel, the node exclusively uses the chan- nel until its message is completely transmitted. So, as the length of the message increases, the throughput also increases.

Consider a transmitting node whose message is com- pletely transmitted by using data channel i in slot (t + 1). Then the node transmits its R-CP in slot t, and does not transmit the R-CP in slot ( t + 1). Another node that tries to transmit its message through the ith data channel is not used by others and transmits its S-CP in one of the availa- ble minislots of slot (t + 2). If the S-CP is successfully transmitted, the node begins to transmit its message from slot ( t + 3) . Consequently, one slot (slot ( t + 2)) of the ith data channel is wasted whenever the message transmission is finished. Hence, the maximum network throughput is limited by N(llp - l)/(l/p). One way to maximise network throughput is that a transmitting node stops transmitting its R-CP when two data packets (instead of 1) remain to be transmitted.

Fig. 4 presents average message delay against network throughput for various numbers of data channels. When N is small, the number of minislots reserved by transmitting nodes is also small. Therefore, when a backlogged node monitors the control channel to transmit its message, the number of available minislots within a control channel observed by the node increases as N becomes smaller. When the number of minislots gets larger, the probability of control packet collision is reduced. So, as the load increases, throughput reaches the maximum. Meanwhile, as N increases, performance degrades in high-load regions because of hgher probability of S C P collision. This phe- nomenon appears clearly in the idle, case, because the S-CPs in the idle case are transmitted more than those in the random case.

100

80

- $ 60 U a, a c $ 40

20

0 5 10 15

network throughput

Fig. 4 M = 50; I = 15; p = 0.1 ___ random .... .... idle (i) N = 5; (ii) N = 10; (iii) N = 15

Delay against throughput for variours numbers of data c h e k

IEE Proc.-Coinmun.. Vul. 148, NL'. 2, April 2001 84

Fig. 5 shows the network throughput aginst arrival rate for various numbers of nodes. When the number of nodes is small, control packets transmitted are not likely to collide with others, because the number of backlogged nodes is small. As A4 increases, the number of backlogged nodes also increases. Therefore, the performance difference between idle and random cases is reduced as load increases, and even reversed with high load.

0 ’ I I I

”’... ........_..

8 -

-3 -4 1 0

arrival rate 10 10 10 10

Fig. 5 N = 10; x = 20; p = 0.1

~ random . . . . . . . . . . . idle (i) M = 70; (ii) M = 50; (iii) M = 30

Network throughput against arrival rate for various itumbers of nodes

’ O r ..-.-...- .- .- 3, li ..=_‘.9 ..-:.-. 9 ‘.9 c 9 ,/ II .,-.. *.P...-.-. ’. . ’. . : . . . . . . . .:-;. . , . .

6 -

I 4 -

I I I

10 10 10 -2 1 0

O 10 -3

” -3

10 -2

10 1

10 0

10 arrival rate

Fi .6 Network throughput agabut arrival rate for various propagation

M = 5 0 ; N = 1O;p=0.1 (i) Idle; (ii) random

~ I = 10 x = 20 x = 30

de&S

. . . . . . . . . . .

Fig. 6 shows the network throughput against arrival rate for various end-to-end propagation delays. In the case of small x, the probability of control packet collision is higher for the idle than the random case. Therefore, the perform- ance of the idle scheme is worse for high offered load. However, the possibility of control packet collisions becomes less as x increases, thereby yielding better per- formance in the idle case.

5 Conclusion

In this paper, the author has presented and analysed a per- sistent reservation protocol for high-speed local area net- works using a passive star topology. Random and idle schemes have been considered as data channel selection strategies. Once a node reserves a data channel, the node persistently uses the channel until its message is completely transmitted. Therefore, the protocol requires only one tun- ing time for a node to transmit its message. The perform- ance of the protocol has been analysed for finite node population. The analytical results have been verified by simulation, and the effects of various system parameters have been investigated. The protocol is suitable for a net- work in which accommodation of variable-length messages (such as circuit-switched traffic or traffic with long holding times) is required. The protocol is operated synchronously, and enables any new node to join the network at anytime without network re-initialisation.

6 Acknowledgment

This work was supported by Dongguk University Research Fund.

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