Cycle-time properties of the timed token medium access control protocol

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Zhang, S., Burns, A. orcid.org/0000-0001-5621-8816 and Cheng, T.H. (2002) Cycle-time

properties of the timed token medium access control protocol. IEEE Transactions on

Computers. pp. 1362-1367. ISSN 0018-9340

https://doi.org/10.1109/TC.2002.1047759

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Brief Contributions

________________________________________________________________________________

Cycle-Time Properties of the Timed Token

Medium Access Control Protocol

Sijing Zhang, Alan Burns, and Tee-Hiang Cheng

Abstract—We investigate the timing properties of the timed token protocol that are necessary to guarantee synchronous message deadlines. A tighter upper bound on the elapse time between the token’sltharrival at any nodeiand its ðlþvÞtharrival at any nodekis found. A formal proof to this generalized bound is presented.

Index Terms—Protocol timing properties, timed token medium access control (MAC) protocol, timed token networks, FDDI networks, real-time communications.

æ

1

INTRODUCTION

GUARANTEEINGmessage deadlines is a key issue in distributed real-time applications. The timed token medium access control (MAC) protocol [1], [2] is suitable for real-time applications due to its special timing property of bounded token rotation time. With this protocol [2], messages are distinguished into two types: synchronousandasynchronous. Synchronous messages are periodic with delivery time constraints. Asynchronous messages are nonperiodic with no delivery time constraints. During network initialization time, all nodes negotiate a common value for the Target Token Rotation Time (T T RT) which should be small enough to meet responsiveness requirements of all nodes. Each node iis assigned a fraction of theT T RT, denoted as Hi, as its

synchronous bandwidth, which is the maximum time the node is allowed to transmit its synchronous messages every time it receives the token [3], [4]. Whenever nodeireceives the token, it transmits its synchronous messages (if any) first for a time up to

Hi. If the time elapsed between the previous and the current token

arrivals at the same nodeiis less thanT T RT(i.e., the token arrived earlier than expected), node i can then send asynchronous messages to make up the difference.

Johnson [5] first proved that the token rotation time cannot exceed2T T RT. Chen et al. [3], [4] generalized the bound derived by Johnson on the maximum token rotation time, extending it from between any two to between any v (integer v2) successive token’s arrivals at a node. This result was widely used in studying various synchronous bandwidth allocation (SBA) schemes [3], [4], [6], [7], [8], [9], [10], [11]. Unfortunately, their generalized bound may not keep tight when v grows large and, consequently, the worst-case performance of SBA schemes, which is derived based on this bound, could be too pessimistic and, hence, much poorer than actually achievable. Han et al. [12] derived a more generalized bound between any two token visits to any two nodes, which makes the previous results by Johnson and by Chen et al. a special

case. However, for the bound on successive token arrivals to a particular node, the bound by Han et al. is no tighter than that derived by Chen et al.. Zhang and Burns [13], [14] improved the bound of Chen et al. by deriving another generalized upper bound which may become tighter when the number of successive token rotations grows large.

This paper derives a tighter upper bound on the elapsed time between the token’s lth arrival at any node i and the token’s ðlþvÞtharrival at any nodek(where integerv0). This result generalizes all the previous findings on the cycle-time properties and is better than any of them in the sense that it is more general and/or tighter. For the rest of this paper, Section 2 describes the network model, Section 3 presents a formal proof to our generalized bound, and Section 4 shows how our result generalizes and why it is better than any published result on cycle-time properties. An example, given in Section 5, shows the superiority of our derived result for guaranteeing hard real-time messages with the timed token protocol. Finally, the paper concludes with Section 6.

2

NETWORK

MODEL

The network is assumed to consist ofnnodes forming a logical ring. The message transmission is controlled by the timed token MAC protocol [2]. Let i be the maximum amount of overhead

incurred from the token’s arrival at node i till its immediately subsequent arrival at node ðiþ1Þ, then the maximum time unavailable for message transmission during one complete token rotation, denoted as, can be expressed by ¼Pn

i¼1i. Since

forms part of the token rotation time and synchronous sion with guaranteed bandwidth precedes asynchronous transmis-sion, clearly, as a protocol constraint on the allocation of synchronous bandwidth,Pn

j¼1HjT T RTÿ must be met. The

protocol constraint is assumed to hold for the rest of this paper.

3

THE

FORMAL

PROOF

Before formally proving a better result on the protocol cycle-time property, we need to define some terms and to show a lemma.

Let “c; i” (a pair of integers) denote the token’s cth visit to nodei. Visitc; iis followed byc; iþ1if1i < nor bycþ1;1if

i¼n. Similarly, ifi¼1, thenc; iÿ1(the visit immediately before

c; i) should be taken to becÿ1; n. The sum total of some quantity, say,q, for all the visits fromj; ktow; zinclusive can be expressed as

Pw;z

x;y¼j;kqx;y¼Pny¼kqj;yþPwx¼ÿj1þ1

Pn

y¼1qx;yþPzy¼1qw;y.

Signs “¼;” “<,” “,” “>,” and “” can be used to link two visits such that “x; y¼c; i,” “x; y < c; i,” “x; yc; i,” “x; y > c; i,” and “x; yc; i” mean, respectively, visit x; y being the same as, before (earlier than), no later than, after (later than), and no earlier thanc; i.

Lettc;ibe the time when the token makes itsctharrival at node

i. Let hc;i, ac;i, and c;i, respectively, represent the times spent

transmitting synchronous and asynchronous traffic and the various overheads involved in visitc; i. Then, the duration of visit

c; i, denoted as vc;i, can be expressed as vc;i¼hc;iþac;iþc;i.

Further, letBc;i be the length of a complete token rotation ending

with visitc; i, i.e.,Bc;i¼Pc;ix;y¼cÿ1;iþ1vx;y.

According to the timed token protocol [2], each node i can transmit synchronous messages for a time up toHiand can then

send asynchronous messages (if the token arrived early) up to the amount of time by which the token arrived early. So, forc1and

1in, hc;iHi, ac;imaxð0; T T RTÿBc;iÿ1Þ. Also, with this

protocol, no nodes are allowed to hold the token for a time over

1362 IEEE TRANSACTIONS ON COMPUTERS, VOL. 51, NO. 11, NOVEMBER 2002

. S. Zhang is with the School of Computing and Technology, University of

Derby, Kedleston Road, Derby DE22 1GB, England, UK. E-mail: s.zhang@derby.ac.uk.

. A. Burns is with the Department of Computer Science, University of York,

Heslington, York YO1 5DD, England, UK. E-mail: burns@cs.york.ac.uk.

. T.-H. Cheng is with the School of Electrical and Electronic Engineering,

Nanyang Technological University, Nanyang Avenue, Singapore 639798, Republic of Singaport. E-mail: ethcheng@ntu.edu.sg.

Manuscript received 6 Nov. 1997; revised 17 Jan. 2001; accepted 24 Oct. 2001.

For information on obtaining reprints of this article, please send e-mail to: tc@computer.org, and reference IEEECS Log Number 105901.

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T T RT, i.e.,ac;iT T RTÿhc;i. Combining these two constraints on

ac;i, we get

ac;iminfT T RTÿhc;i;maxð0; T T RTÿBc;iÿ1Þ g: ð1Þ

BecauseBc;iÿ1¼, when no nodes have messages, either

synchro-nous or asynchrosynchro-nous, to send during the preceding token rotation ending with visitc; iÿ1, the bandwidth available for transmitting asynchronous messages is bounded by T T RTÿ (since

ac;imax½0; T T RTÿBc;iÿ1Š).

Let Pf

eQj be defined as follows (where eandf are positive

integers and “mod n” represents “modulo_n” operation):

X

f

e

Qj¼

Pf

j¼eQj if 1efnande6¼ ðf mod nÞ þ1

Pn

j¼eQjþPfj¼1Qj if 1f < enande6¼fþ1

0 ife¼ ðf mod nÞ þ1:

8 > <

> :

Then, the sum of overheads incurred during less than n

successive token visits (from node eto node f inclusive) can be expressed as Pf

ej.

We also need the following lemma for the proof of our generalized result.

Lemma 1 [14].If the token is early on visitc; i (i.e., early on its cth

arrival at nodei), then

tc;iþ1ÿtcÿ1;i¼

X

c;i

x;y¼cÿ1;i

vx;yT T RTþhc;iþc;iT T RTþHiþi:

Before formally proving our generalized upper bound given in Theorem 1, we briefly describe the simple idea behind the proof: For visits betweenl; iandlþv; kÿ1, examine each visit backward starting from lþv; kÿ1. If the current visit is a late visit (where only synchronous transmission is allowed), replace it by the allocated amount of synchronous bandwidth (allocated to the node which the late visit corresponds to) plus an upper-bounded amount of overheads involved in the visit and then move onto thenext visit immediately before this late visit. Otherwise, if the current visit is an early visit, replace ðnþ1Þ successive visits ending with this early visit by the bound specified in Lemma 1 and then move onto the next visit immediately before these ðnþ1Þ

visits. Check the new current visit in exactly the same way as above and continue the backward examining process until visitl; i

has been replaced by a bound formed either individually or jointly with other visits. Finally, by smartly concatenating and formulat-ing all produced component bounds (each for one or ðnþ1Þ

replaced visits), one can easily reach the proposed upper bound. Following this proof route, the upper bound is easy to derive though it looks quite complex.

Theorem 1 (Generalized Johnson’s Theorem). For any integersl and vðl1; v0Þand any nodesi and k ð1in;1knÞ, if tlþv;k> tl;i, t h e n , u n d e r t h e p r o t o c o l c o n s t r a i n t ( i . e . ,

Pn

j¼1HjþT T RT),

tlþv;kÿtl;i

vnþkÿi nþ1

T T RTþX

kÿ1

iþ1

Hþþ

vnþkÿiÿ1 n

ÿ vnþkÿi

nþ1

þ1

X

n

j¼1

Hjþ

!

:

Proof.The time interval oftlþv;kÿtl;iexactly corresponds to visits

from l; i to lþv; kÿ1, inclusive. There are two cases to consider:

CASE 1:The token is late on all visits froml; itolþv; kÿ1

inclusive.

Since ax;y¼0 (l; ix; ylþv; kÿ1), the time elapsed

during any complete token rotation, if any, is bounded by

Pn

j¼1Hjþ. As there are, in total, (vnþkÿi) visits between

l; iandlþv; kÿ1, inclusive, and each token rotation consists of

nsuccessive visits, the number of complete token rotations is given byb¼ bvnþkÿi

n c. The elapsed time in the remaining visits

from node i to node (kÿ1) inclusive, if any, is bounded by

Pkÿ1

i HjþPkiÿ1j. Based on the above analysis, we have,

tlþv;kÿtl;i¼

X

lþv;kÿ1

x;y¼l;i

vx;y¼

Xbÿ1

j¼0

X

lþjþ1;iÿ1

x;y¼lþj;i

vx;y

!

þ X

lþv;kÿ1

x;y¼lþb;i

vx;y

¼X

bÿ1

j¼0

X

lþjþ1;iÿ1

x;y¼lþj;i

ðhx;yþx;yÞ

!

þ X

lþv;kÿ1

x;y¼lþb;i

ðhx;yþx;yÞ

b X

n

j¼1

Hjþ

!

þX

kÿ1

i

Hjþ

Xkÿ

1

i

j

ðsincex;yy; ¼

Xn

j¼1

i; hx;yHyÞ

¼ vnþkÿi

n

X

n

j¼1

Hjþ

!

þX

kÿ1

i

Hjþ

since X

kÿ1

i

j

!

X

kÿ1

iþ1

Hjþþ vnþkÿiÿ

1 n

þ1

X

n

j¼1

Hjþ

!

X

kÿ1

iþ1

Hjþþ vnþkÿiÿ

1 n

þ1

X

n

j¼1

Hjþ

!

þ vnþkÿi

nþ1

T T RTÿX

n

j¼1

Hjÿ

!

since X

n

j¼1

HjþT T RT

!

¼ vnþkÿi

nþ1

T T RTþ X

kÿ1

j¼iþ1

Hjþ

þ vnþkÿiÿ1

n

ÿ vnþkÿi

nþ1

þ1

X

n

j¼1

Hjþ

!

:

CASE 2:There is at least one early visit froml; itolþv; kÿ1

inclusive.

Letp1; q1; ; . . . ; pm; qm, where

l; ip1; q1< p2; q2< < pm; qm< lþv; k;

be allmearly visits betweenl; iandlþv; kÿ1inclusive such that, for 1sm (assuming lþv; k¼pðmþ1Þÿ1; qðmþ1Þ),

ps; qs< pðsþ1Þÿ1; qðsþ1Þ, whereps; qsis the last early visit before

pðsþ1Þÿ1; qðsþ1Þ. The following observations can be made from

the above definitions:

1. For1s < m, betweenps; qsexclusive andpðsþ1Þ; qðsþ1Þ

inclusive, there are at least ðnþ1Þ successive visits. Since there are, in total,vnþ ðkÿiÞvisits betweenl; i

andlþv; kÿ1inclusive, we havem dvnþkÿi nþ1 e.

2. Any visit between ps; qs and pðsþ1Þÿ1; qðsþ1Þ

exclusive (where 1sm and, as assumed,

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Thus,Ppðsþ1Þÿ1;qðsþ1Þÿ1

x;y¼ps;qsþ1 vx;y¼

Ppðsþ1Þÿ1;qðsþ1Þÿ1

x;y¼ps;qsþ1 ðhx;yþx;yÞ.

3. If p1ÿ1; q1> l; i, there are no early visits (thus no

asynchronous transmission) between l; i and p1ÿ

1; q1ÿ1inclusive. Hence,

X

p1ÿ1;q1ÿ1

x;y¼l;i

vx;y¼

X

p1ÿ1;q1ÿ1

x;y¼l;i

ðhx;yþx;yÞ:

4. According to Lemma 1, the time elapsed in anyðnþ1Þ

successive visits ending with an early visit is bounded by T T RT plus the synchronous bandwidth used and the amount of overheads incurred in this early visit. For simplicity of proof, suppose that this bound is formed by two imaginary parts:the firstnsuccessive visits(that form one complete token rotation) being bounded by

T T RT and the ðnþ1Þth visit of only synchronous transmission. Note that this imaginary (equivalent) situation, though not what happens in reality, leads to the same theoretically derived upper bound and, at the same time, simplifies the derivation making the proof easy to follow.

Note that removing any n successive visits (say, replaced by the imaginary upper bound of T T RT) does not break the neighboring relationship between nodes because any n successive visits make up one complete token rotation. That is, the node corre-sponding to the visit immediately before these n

visits and the node corresponding to the visit immediately after these n visits neighbor each other (i.e., the latter is the immediately subsequent node of the former), although these two corresponding visits are one-token-rotation apart. For example, the node corresponding to visit psÿ1; qsÿ1(i.e., node (qsÿ1))

and the node corresponding to visitps; qs(i.e., nodeqs)

neighbor each other, with the removed n visits (replaced by T T RT) between psÿ1; qs and ps; qsÿ1

inclusive.

5. Based upon how far the visitp1; q1 is froml; i, several

cases are considered below:

a. Ifp1ÿ1; q1l; i (i.e.,p1; q1lþ1; i).

Allmearly visits, each corresponding toðnþ1Þ

successive visits, fall within the visits froml; ito

lþv; kÿ1, inclusive. By 4, we can replace the first

nsuccessive visits of eachðnþ1Þsuccessive visits by the imaginary upper bound of T T RT. So, the final derived upper bound (on the elapse time from l; i to lþv; kÿ1 inclusive), for this case, should include “mT T RT.”

After removing m sets of token rotations (replaced bymT T RT), the number of remaining visits will be the total number of all visits minus the number of removed visits, i.e.,ðv:nþkÿiÞ ÿmn. Note that we should only consider transmission of synchronous messages in each of these remaining visits because it is either a late visit x; y (if

x; y6¼ps; qs;1sm) or has been supposed so

(ifx; y¼ps; qs) in our imaginary equivalent scenario

stated in 4. Due to the unbroken feature of neighboring relationships between nodes (when-ever the removal of any n successive nodes happens), the number of the imaginary equivalent

token rotations within these remaining visits is given by q¼ bðvnþkÿniÞÿmnc ¼ bvnþkÿi

n c ÿm. The

elapsed time in theqequivalent token rotations is bounded by

q X

n

j¼1

Hjþ

! ¼

vnþkÿi n

ÿm

X

n

j¼1

Hjþ

!

;

which should also be a component of the final derived upper-bound expression.

Also, with the unbroken neighboring feature between nodes, it is easy to check that the elapsed time in the leftover visits (after taking out the q

rotations) is bounded byPkÿ1

i HjþPkiÿ1j, which

should also appear in the final expression. b. Iflÿ1; ip1ÿ1; q1< l; i, i.e.,

l; ip1; q1< lþ1; i:

Divide all ðnvþkÿiÞ visits (from l; ito lþ

v; kÿ1inclusive) into two groups:

Group1¼ fx; yjp1; q1þ1x; ylþv; kÿ1g

Group2¼ fx; yjl; ix; yp1; q1g:

We now discuss the upper bounds for visits in these two groups, respectively.

For visits in Group 1, we can do exactly the same analysis as that adopted in (5.a) forðmÿ1Þ

early visits (i.e., “p2; q2”;“p3; q3”; ... ; “pm; qm”). So,

the final upper-bound expression (for Group 1) should include “ðmÿ1Þ T T RT” and

q¼ ðlþvÿp1Þ nþ ðkÿq1Þ ÿ1

n

ÿ ðmÿ1Þ:

Similarly, the time elapsed in theqrotations is upper bounded by

q ðX

n

j¼1

HjþÞ ¼

ðlþvÿp1Þ nþ ðkÿq1Þ ÿ1

n

ÿ ðmÿ1ÞÞ

X

n

j¼1

Hjþ

!

and the leftover visits can never exceed

Pkÿ1

q1þ1ðHjþjÞ ¼ Pkÿ1

q1þ1Hjþ Pkÿ1

q1þ1j. Also, both

of these two bounds, together with “ðmÿ1Þ T T RT,” form an upper bound for Group 1.

For Group 2, by Lemma 1, we have

X

p1;q1

x;y¼l;i

vx;y<

X

p1;q1

x;y¼p1ÿ1;q1

vx;yT T RTþhp1;q1þp1;q1

T T RTþHq1þq1:

Note thatl; i becomes the only visit in Group 2 whenp1; q1¼l; i. By (1), whenp1; q1¼l; i, we have

vl;iT T RTþl;iT T RTþi.

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With the above observations 1-5, the upper bound of Theorem 1 can be derived as follows:

tlþv;kÿtl;i

¼ ðtlþv;kÿtpm;qmþ1Þ þ X

mÿ2

j¼0

½ ðtpmÿj;qmÿjþ1ÿtpmÿjÿ1;qmÿjÞ þ ðtpmÿjÿ1;qmÿjÿtpmÿjÿ1;qmÿjÿ1þ1Þ Š

þ

ðtp1;q1þ1ÿtp1ÿ1;q1Þ þ ðtp1ÿ1;q1ÿtl;iÞ ifp1ÿ1; q1> l; i

ðtp1;q1þ1ÿtl;iÞ iflÿ1; i < p1ÿ1; q1l; i

ðtl;iþ1ÿtl;iÞ ifp1ÿ1; q1¼lÿ1; i

8 > < > : ¼ X

lþv;kÿ1

x;y¼pm;qmþ1

vx;y þ

X

mÿ2

j¼0

X

pmÿj;qmÿj

x;y¼pmÿjÿ1;qmÿj

vx;yþ

X

pmÿjÿ1;qmÿjÿ1

x;y¼pmÿjÿ1;qmÿjÿ1þ1

vx;y 2 4 3 5 þ Pp1;q1

x;y¼p1ÿ1;q1vx;yþ

Pp1ÿ1;q1ÿ1

x;y¼l;i vx;y ifp1ÿ1; q1> l; i

Pp1;q1

x;y¼l;ivx;y iflÿ1; i < p1ÿ1; q1l; i

vl;i ifp1ÿ1; q1¼lÿ1; i

8 > < > : X

lþv;kÿ1

x;y¼pm;qmþ1

ðhx;yþx;yÞþ

X

mÿ2

j¼0

X

pmÿj;qmÿj

x;y¼pmÿjÿ1;qmÿj

vx;y 2 4 3 5 þX

mÿ2

j¼0

X

pmÿjÿ1;qmÿjÿ1

x;y¼pmÿjÿ1;qmÿjÿ1þ1

vx;y 2 4 3 5 þ Pp1;q1

x;y¼p1ÿ1;q1vx;yþ

Pp1ÿ1;q1ÿ1

x;y¼l;i ðhx;yþx;yÞ

ðbyðCÞÞ ifp1ÿ1; q1> l; i Pp1;q1

x;y¼p1ÿ1;q1vx;y ðsince Pp1;q1

x;y¼l;ivx;y<Px;yp1;q¼1p1ÿ1;q1vx;yÞ

iflÿ1; i < p1ÿ1;

q1l; i

hl;iþal;iþl;i ifp1ÿ1; q1¼lÿ1; i

8 > > > > > > < > > > > > > :

ðby observation 2 aboveÞ

X

lþv;kÿ1

x;y¼pm;qmþ1

ðhx;yþx;yÞ þ

X

mÿ2

j¼0

ðT T RTþhpmÿj;qmÿjþpmÿj;qmÿjÞ

þX

mÿ2

j¼0

X

pmÿjÿ1;qmÿjÿ1

x;y¼pmÿjÿ1;qmÿjÿ1þ1

ðhx;yþx;yÞ

2

4

3

ðT T RTþhp1;q1þp1;q1Þ þPp1ÿ1;q1ÿ1

x;y¼l;i ðhx;yþx;yÞ

ifp1ÿ1; q1> l; i

T T RTþhp1;q1þp1;q1 iflÿ1; i < p1ÿ1; q1l; i

hl;iþl;iþminfT T RTÿhl;i;

maxð0; T T RTÿBl;iÿ1Þgðbyð1ÞÞ

ifp1ÿ1; q1¼lÿ1; i

8 > > > > > > < > > > > > > :

ðby Lemma 1Þ

mT T RTþ ð vnþkÿi n

ÿmÞðPn

j¼1HjþÞ

þPkÿ1

i HjþPkiÿ1j

ðby observation 5:a aboveÞ

ifp1ÿ1; q1l; i

ðmÿ1Þ T T RTþ ðbðlþvÿp1Þnþðkÿq1Þÿ1

n c ÿ ðmÿ1ÞÞ

ðPn

j¼1HjþÞþPqkÿ1þ11Hjþ Pkÿ1

q1þ1j þðT T RTþHq1þq1Þ

ðby observation 5:b aboveÞ

iflÿ1; i < p1ÿ1;

q1< l; i

ðmÿ1ÞT T RTþ

ðbvnþðknÿiÞÿ1c ÿ ðmÿ1ÞÞðPn

j¼1HjþÞ

þPkÿ1

iþ1HjþPikþÿ11jþ ðT T RTþiÞ

ðsinceal;iT T RTÿhl;iÞ

ðby observation 5:b aboveÞ

ifp1ÿ1;

q1¼lÿ1; i

8 > > > > > > > > > > > > > > > > > > > > > > > > > > > > < > > > > > > > > > > > > > > > > > > > > > > > > > > > > :

ðbecausex;yy; ¼

Xn

j¼1

iandhx;yHyÞ

mT T RTþ bvnþ ðkÿiÞ ÿ1

n c ÿ ðmÿ1Þ

X

n

j¼1

Hjþ

!

þX

kÿ1

iþ1

Hjþ

vnþkÿi

nþ1

T T RTþX

kÿ1

iþ1

Hjþþ

vnþkÿiÿ1 n

ÿ vnþkÿi

nþ1

þ1

X

n

j¼1

Hjþ

!

ðby observation 1 above and the fact of the above

upper bound being an increasing function ofmÞ:

t u

As shown in the above proof process, the derived upper bound is independent ofhx;y(l; i=leqx; y < lþv; k) as long as the protocol

constraint holds. So, the bound still works even whenhx;y¼0for

somex; y. Realizing this fact is important for real-time commu-nication with the timed token protocol.

4

COMPARISON WITH

PREVIOUS

RESULTS

Zhang and Burns [13] demonstrate how the previous findings by Johnson [5] become a special case of their generalized upper bound and why their bound is tighter than that derived by Chen et al. when the number of consecutive token rotations grows large enough under Pn

j¼1Hj< T T RTÿ. It is easy to check that

Theorem 1 becomes the generalized upper bound expression derived by Zhang and Burns [13] whenk¼i.

LetdiðlÞbe the time when the token makes its lth departure

from nodeiandb;iðl; cÞbe the time difference betweendbðlÞand

the time when the token departs from nodeithecthtime afterdbðlÞ

[12], i.e.,

b;iðl; cÞ ¼ ddiðlþcÿ1Þ ÿdbðlÞ if 1b < in iðlþcÞ ÿdbðlÞ if 1ibn:

ð2Þ

Han et al. [12] derived a generalized upper bound on b;iðl; cÞ(i.e.,

the elapses time between the token’slthdeparture from nodeband the token’s ðlþcÞth departure from node i) which makes the previous results by Johnson [5] and by Chen et al. [3] a special case of their result. The following theorem shows their generalized upper bound expression.

Theorem 2 (Generalized Johnson’s Theorem by Han et al. [12]).

For the timed-token MAC protocol, under the protocol constraint, for anyl1andc1,

b;iðl; cÞ cT T RTþ

Xi

j¼bþ1

Hjþ ðcþ1Þ T T RT ;

wherePi

j¼bþ1Hjis subject to the definition of Pfj¼eHj as shown

below:

Xf

j¼e

Hj¼

HeþHeþ1þ þHf if 1efn

HeþHeþ1þ þHnþH1þH2þ þHf if 1f < en:

(6)

tlþv;kÿtl;i

vnþkÿi

nþ1

T T RTþX

kÿ1

iþ1

Hjþþ

vnþkÿiÿ1 n

ÿ vnþkÿi

nþ1

þ1

X

n

j¼1

Hjþ

!

vnþkÿi

nþ1

T T RTþX

kÿ1

iþ1

Hjþþ

vnþkÿiÿ1 n

ÿ vnþkÿi

nþ1

þ1

T T RT

since vnþkÿiÿ1 n

ÿ vnþkÿi

nþ1

þ10

and X

n

j¼1

HjþT T RT

!

¼ vnþkÿiÿ1

n

þ1

T T RTþX

kÿ1

iþ1

Hjþ:

ð3Þ

As shown below, even the above relaxed upper bound (3) is still tighter than that derived by Han et al. To enable comparison, we need to represent b;iðl; cÞ using our notation

tc;i. Letbe the delay between the departure of the token from

any node i and its immediate arrival to node (iþ1), i.e.,

diðlÞ ¼ tlþbi=nc;ði mod nÞþ1 ÿ , then b;iðl; cÞ can be converted as

follows:

b;iðl; cÞ ¼

diðlþcÿ1Þ ÿdbðlÞ if 1b < in

diðlþcÞ ÿdbðlÞ if 1ibn

¼

ðtðlþcÿ1Þþbi=nc;ði mod nÞþ1ÿÞ ÿ ðtl;bþ1ÿÞ if 1b < in

ðtðlþcÞþbi=nc;ði mod nÞþ1ÿÞ

ÿðtlþbb=nc;ðb mod nÞþ1ÿÞ

if 1ibn

8 > < > : ¼

tlþc;1ÿtl;bþ1 if 1b < i¼n

tlþcÿ1;iþ1ÿtl;bþ1 if 1b < i < n

tðlþcÞþ1;1ÿtlþ1;1 ifi¼b¼n

tlþc;iþ1ÿtlþ1;1 if 1i < b¼n

tlþc;iþ1ÿtl;bþ1 if 1ib < n

8 > > > > > > < > > > > > > :

ðbcnþ1ÿðnbþ1Þÿ1c þ1Þ T T RT

þPn

½ðbþ1Þmod nŠþ1Hjþ

if 1b < i¼n

ðbðcÿ1Þnþðiþn1Þÿðbþ1Þÿ1c þ1Þ T T RTþ Pi

bþ2Hjþ

if 1b < i < n

ðbcnþ1ÿ1ÿ1

n c þ1Þ T T RTþ

Pn

2Hjþ ifi¼b¼n

ðbðcÿ1Þnþðniþ1Þÿ1ÿ1c þ1Þ T T RTþPi

2Hjþ if 1i < b¼n

ðbcnþðiþ1Þÿðn bþ1Þÿ1c þ1Þ T T RTþ Pi

½ðbþ1Þmod nŠþ1Hjþ

if 1ib < n

8 > > > > > > > > > > > > > > > > > < > > > > > > > > > > > > > > > > > :

ðbyð3ÞÞ

¼

cT T RTþHbþ2þ þHiþ if 1bnÿ2< i¼n

cT T RTþ if 1b¼nÿ1< i¼n cT T RTþHbþ2þ þHiþ if 2bþ1< i < n

cT T RTþ if 2bþ1¼i < n cT T RTþH2þ þHiþ ifi¼b¼n

cT T RTþH2þ þHiþ if 1< i < b¼n

cT T RTþ if 1¼i < b¼n cT T RTþH1þ þHiþ if 1ib¼nÿ1

cT T RTþHbþ2þ þHn

þH1þ þHiþ

if 1ibnÿ2:

8 > > > > > > > > > > > > > > > > > > < > > > > > > > > > > > > > > > > > > :

On the other hand, according to Theorem 2, we have,

b;iðl; cÞ

cT T RTþHbþ1þ þHiþ if 1b < in

cT T RTþH1þ þHiþ if 1ib¼n

cT T RTþHbþ1þ þHn

þH1þ þHiþ

if 1ib < n:

8 > > > < > > > :

Comparing each case of the bound obtained from Theorem 2 with those of its corresponding cases obtained from (3), clearly, Theorem 1 gives a tighter upper bound (for b;iðl; cÞ) than

Theorem 2.

5

AN

EXAMPLE

Consider a network with three nodes. Assume that each nodei

(i¼1;2;3) has a synchronous message streamSicharacterized by

a period Pi, a maximum transmission time Ci, and a relative

deadlineDi. Messages fromSiarrive regularly with periodPiand

have relative deadlineDi(i.e., if a message fromSiarrives at timet,

it must finish its transmission at nodeiby timetþDi). Table 1 lists

all network and message parameters, whereSNandDNrepresent Source NodeandDestination Node, respectively.

Clearly, the protocol constraint holds for the given network parameters. We now check if these parameters can ensure that all synchronous messages will arrive at their destination nodes before their deadlines by, respectively, using our proposed generalized upper bound (Theorem 1) and that derived by Han et al. (Theorem 2). LetRi be the message response time for a message

fromSi(i.e., the interval from arrival of the message till completion

of its transmission at node i) and Rw

i (Ri) be the worst-case

message response time (i.e., the longest possible interval). In the worst case, a message fromSibecomes available for transmission

immediately after sometl;i, thus it misses thefirstchance of being

transmitted on visit l; i [13], [14]. Because Ci units of time are

needed for transmission of a whole message fromSiand nodei

can use at most Hi time units for transmitting synchronous

messages whenever it receives the token, a total ofdCi=Hietimes’

token arrivals is required for transmitting the whole message, which is divided intodCi=Hieframes (to be transmitted separately

on each of the token arrivals). Since the message misses the first chance at timetl;iin the worst case, we have

Ri¼

ðtlþdCi=Hie;iÿtl;iÞ þCiÿ ðd

Ci

Hie ÿ1Þ Hi ðfor use with Theorem 1Þ

iÿ1;iÿ1ðl;dCi=HieÞ

þCiÿ ðdCHiie ÿ1Þ Hi

ðfor use with Theorem 2Þ 8

> <

> :

ð4Þ

With the above analysis, we can calculateR1based on Theorem 2

as follows:

1366 IEEE TRANSACTIONS ON COMPUTERS, VOL. 51, NO. 11, NOVEMBER 2002

TABLE 1

(7)

R1¼3;3 l;

C1

H1

þC1ÿ

C1

H1

ÿ1

H1 ðbyð4ÞÞ

C1

H1

T T RTþX

3

j¼1

HjþþC1ÿ

C1

H1

ÿ1

H1

ðby Theorem2Þ

¼ 3:1

1

8þ1þ2:16þ0:84þ1þ3:1ÿ 3:1

1

ÿ1

1¼37:1:

Thus,Rw

1 ¼37:1ms > D1¼36ms, i.e., the message ofS1misses its

deadline when judged with Theorem 2. However, as shown below, this is not the case. Based on Theorem 1, we have

R1¼ tlþdC1 H1e;1ÿtl;1

þC1ÿ

C1

H1

ÿ1

H1 ðbyð4ÞÞ

d

C1

H1e n

nþ1

& ’

T T RTþX

3

j¼2

Hjþþ

C1

H1

ÿ d

C1

H1e n

nþ1

& ’!

ðX

3

j¼1

HjþÞ þ C1ÿ C1

H1

ÿ1

H1 ðby Theorem 1Þ

¼ d

3:1 1e 3

3þ1

8þ2:16þ0:84þ1þ 3:1

1

ÿ d

3:1 1e 3

3þ1

ð4þ1Þ þ3:1ÿ 3:1

1

ÿ1

1¼33:1:

Thus, Rw

1 ¼33:1ms < D1¼36ms, i.e., the deadline ofS1 is met

when judged with Theorem 1.

Similarly, calculatingR2andR3based on Theorem 1, we have:

R2¼20:98ms < D2¼21ms; R3¼28:68ms < D3¼30ms. So, no

messages miss their deadlines when judged with Theorem 1.

6

CONCLUSION

We have presented a formal proof to a generalized upper bound on the elapsed time from the token’sltharrival at any nodeitill its

ðlþvÞtharrival at any nodek(where integerv0). Our derived upper bound expression, which is particularly important for hard real-time communications in any timed token network, is better than any of the previous findings on the protocol cycle-time properties due to the fact that it is more general and may produce a tighter upper bound.

REFERENCES

[1] R.M. Grow, “A Timed Token Protocol for Local Area Networks,”Proc. Electro ’82, Token Access Protocols, Paper 17/3,May 1982.

[2] “Fibre Distributed Data Interface (FDDI)—Token Ring Media Access Control (MAC),” ANSI Standard X3.139, 1987.

[3] G. Agrawal, B. Chen, W. Zhao, and S. Davari, “Guaranteeing Synchronous Message Deadlines with the Timed Token Medium Access Control Protocol,”IEEE Trans. Computers,vol. 43, no. 3, pp. 327-339, Mar. 1994.

[4] B. Chen, G. Agrawal, and W. Zhao, “Optimal Synchronous Capacity Allocation for Hard Real-Time Communications with the Timed Token Protocol,”Proc. 13th IEEE Real-Time Systems Symp.,pp. 198-207, Dec. 1992.

[5] M.J. Johnson, “Proof that Timing Requirements of the FDDI Token Ring Protocol Are Satisfied,”IEEE Trans. Comm.,vol. 35, no. 6, pp. 620-625, June 1987.

[6] G. Agrawal, B. Chen, and W. Zhao, “Local Synchronous Capacity Allocation Schemes for Guaranteeing Message Deadlines with the Timed Token Pprotocol,”Proc. IEEE Infocom ’93,Mar. 1993.

[7] M. Hamdaoui and P. Ramanathan, “Selection of Timed Token Parameters to Guarantee Message Deadlines,”IEEE/ACM Trans. Networking,vol. 3, no. 3, pp. 340-351, June 1995.

[8] C.C. Lim, L. Yao, and W. Zhao, “Transmitting Time-Dependent Multi-media Data in FDDI Networks,”Proc. SPIE Int’l Symp. (OE/FIBERS ’92),

pp. 144-154, 1992.

[9] N. Malcolm and W. Zhao, “The Timed-Token Protocol for Real-Time Communications,”Computers,vol. 27, no. 1, pp. 35-41, Jan. 1994.

[10] Q. Zheng and K.G. Shin, “Synchronous Bandwidth Allocation in FDDI Networks,”Proc. ACM Multimedia ’93,pp. 31-38, 1993.

[11] B. Chen, N. Malcolm, and W. Zhao,The Telecommunication Handbook: FDDI/ CDDI and Real-Time Applications,K. Terplan and P. Marreale, eds. CRC Press and IEEE Press, 2000.

[12] C.-C. Han, K.G. Shin, and C. Hou, “On Non-Existence of Optimal Local Synchronous Bandwidth Allocation Schemes,”Proc. 14th IEEE Ann. Int’l Phoenix Conf. Computers and Comm.,pp. 191-197, Mar. 1995.

[13] S. Zhang and A. Burns, “An Optimal SBA Scheme for Guaranteeing Synchronous Message Deadlines with the Timed-Token MAC Protocol,”

IEEE/ACM Trans. Networking,vol. 3, no. 6, pp. 729-741, Dec. 1995.

Figure

TABLE 1

TABLE 1

p.6