a new scheme for dynamic management of isochronous channels in integrated rings

Computer Networks and ISDN Systems 24(1992)131-144

131North-Holland

A new scheme for dynamic managementof isochronous channels in integrated rings

Reuven CohenIBM Ti Watson Research Center, P.O. Box 704, Yorktown Heights, NY 10598, USA

Adrian SegallDepartment of Computer Science, Technion IIT, Haifa 32000, Israel

Abstract

Cohen, R. and A. Segall, A new scheme for dynamic management of isochronous channels in integrated rings, ComputerNetworks and ISDN Systems 24 (1992) 131-144 .

The present paper proposes a new scheme that allows dynamic maintenance, allocation and release of the isochronousbandwidth in hybrid rings . The main properties of the new scheme are : (1) it permits the stations to bundle together severallow-speed isochronous channels to form one high-speed channel ; (2) the acquiring of isochronous channels by ring stations,as well as their release, are cheap, fast and simple processes, since there is no need to communicate with other stations forthis purpose; (3) the acquiring and release processes are decentralized ; (4) the overhead bandwidth required by the schemeis very small; (5) the allocation rate is not affected by the number of stations in the ring or by the number of stations thatrequire isochronous channels at the same time .

The main principle behind the new scheme is to divide the isochronous bandwidth into fixed length slots, and to maintainthe available channels in each slot as a linked list . Since the linked-lists use only octets of available channels, the requiredoverhead is negligible . The paper presents also a mechanism for recovery from transmission errors that may disrupt thelinked-lists and interfere with the proper operation of the new scheme . The main concepts of the paper are applicable to theFDDI-II networks .

Keywords : isochronous communication, integrated ring, dynamic channels allocation .

1 . Introduction

High-speed Local Area Networks (LANs) andMetropolitan Area Networks (MANS) have beenproved as an attractive approach for transmissionof data, voice, video, facsimile, graphics and othervisual information . Such networks are referred toas integrated LANs .

Different types of transmitted data are classi-fied into isochronous and non-isochronous(asynchronous) categories, according to the fol-lowing service requirements [9] :

* This work was done while this author was with the Depart-ment of Computer Science, Technion IIT, Haifa 3200,Israel .

0169-7552/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

Bandwidth-Isochronous traffic is uniform intime and recurring at regular equal intervals .Voice waveform is sampled at 8 kHz (onceevery 125 µs) and encoded as an 8-bit PCMsignal . Therefore, a voice call requires band-width of 64 Kbit/s. Conversely, non-isochro-nous traffic is bursty, not uniform, with peakbandwidth demands typically limited only bymemory speed .Duration-The average duration of non-iso-chronous connection is much shorter than theaverage duration of isochronous connection .Delay-The end-to-end delay of isochronoustraffic, like voice and video, is tightly limited .Therefore, voice and video samples cannot besubstantially buffered before transmission andshould be transmitted as soon as available .

132

R. Cohen, A. Segall / Management of isochronous channels

• Transmission quality-Isochronous traffic neednot be protected by a frame check sequence,since for such a traffic a bit-error-rate of 10 -6is sufficient, whereas the error rate in fiber-optic cables is in the range of 10 -12 . Non-iso-chronous traffic, on the other hand, requiresan error protection .

Integrated LANs can be classified into three cat-egories according to their handling the iso-chronous traffic [10]. The approach considered inthis paper is the so-called Synchronous UniframeApproach: time is divided into 125 µs cycles, andisochronous data samples are transmitted one pera cycle, using an assigned TDM channel . In theother two approaches [2,10], isochronous data istransmitted in the form of packets, each consist-ing of several isochronous samples . The advan-tage of the first approach is that the isochronoussamples are transmitted as they are produced,that is one 8-bit sample every 125 µs . This pre-vents the packetization and queuing delays andleaves as much time as possible for propagationdelay .

Adrian Segall was born in Rumania in 1944. He received the B .Sc. and M .Sc. degrees in electricalengineering from the Technion, Israel Institute of Technology in 1965 and 1971, respectively, and thePh.D. degree in electrical engineering with a minor in statistics from Stanford University in 1973 .

After serving active duty in the Israel Defence Army, he joined in 1968 the Scientific Department ofIsrael's Ministry of Defence. From 1973 to 1974 he was a Research Engineer at System Control Inc ., PaloAlto, CA and a Lecturer at Stanford University . From 1974 to 1976 he was an Assistant Professor ofElectrical Engineering and Computer Science at the Massachusetts Institute of Technology . From 1976 to1987 he was on the faculty of the Department of Electrical Engineering at the Technion . He is presentlyBenjamin Professor of Computer Communication Networ s in the Department of Computer Science,Technion, Israel Institute of Technology. From 1982 to 1984 he was on leave with the IBM T .J. WatsonResearch Center, Yor town Heights, NY. His current research interest is in the area of computercommunication networ s .

Dr. Segall is a Fellow of IEEE and has served in the past as Editor for Computer Communication Theory of the IEEETransactions on Communications and Editor for the IEEE Information Theory Society Newsletter . He was selected as an IEEEdelegate to the 1975 IEEE-USSR Information Theory Wor shop, and is the recipient of the 1981 Miriam and Ray Klein Awardfor Outstanding Research and of the 1990 Taub Award in Computer Science .

A popular approach to integrated LANs isbased on ring topology with high-speed fiber op-tic. This paper considers such a ring, that com-bined isochronous and non-isochronous traffic us-ing the Synchronous Uniframe Approach [1,4-6,8,9,11]. In such a network, the stations arelocated on a directed ring, and one or morecycles of fixed length circulate around the ring.The bandwidth of the transmission medium isdivided into two parts . The first is for packet-switched communication, used for non-isochro-nous purposes as data and control messages . Thesecond is for circuit-switched communication,used for isochronous purposes like voice andvideo. The bandwidth for the circuit-switchedtraffic is divided into several channels of 64Kbit/s. A single channel can carry a single phonecall, while N such channels provide a bandwidthof N 64 Kbit/s, to satisfy video transmissions orother isochronous data requirements .

In order to meet the isochronous traffic delayrestrictions, especially those of voice traffic, thatrequire transmission of voice samples as they are

Reuven Cohen was born in Israel in 1961 . He received the B.Sc., M.Sc. and Ph .D degrees in ComputerScience from the Technion, Israel Institute of Technology in 1986, 1988 and 1991, respectively . During1991, he was a lecturer in the Faculty of Computer Science at the Technion . His research interests includedistributed networks and MAC protocols . His most recent work has been in MAC protocols for ringnetworks .Dr. Cohen is currently at the IBM T .J. Watson Research Center, Yorktown Heights, NY .


133

cycle = 125µs

isochronous

octet (8 bits)

CYCLE

HEAD1 2 M non- iso-

chronous

produced, ring stations must receive a cycle every125 µs . Therefore, the cycle length is L = (125µs • T Mbit/s), where T is the transmission rate .For example, in a ring operating at 40 Mbit/s,the cycle length L is 5000 bits .

One of the ring stations is selected as a moni-tor and performs some special functions in addi-tion to its regular operation as a ring station . Themonitor is the cycle master, namely it is responsi-ble for adjusting the total ring delay to an exactmultiple of cycle period, 125 µs, so that an inte-gral number of cycles continually circulate on thering. The total ring delay is the sum of the roundtrip propagation delay over the fiber optic cable,the monitor's delay and the delay of each non-monitor station multiplied by the number of suchstations . For example, consider a 50 Km ringoperating at 40 Mbit/s, containing 200 stations,each with 8 bits latency. Assuming propagationdelay of 2.5 • 10 8 m/s and with no delay at themonitor, such a ring would have a round tripdelay of :

so-lo 200 . 8

D 2 .5 . 108+ 40 . 10 6

=2.0 . 10_4s+4.0 . 10

-5s=240 µs

ring delay

CYCLE

HEAD1 2 M non-iso-

chronous

a 64 Kbitlsec isochronous channelFig . 1 . 2-Cycle ring .

Fig . 2. Non-isochronous data-frame structure .

cycle = 125µs

4 isochronous -~

In this networ , the monitor adds a local delayof 10 µs in order to adjust the ring latency to 250µs, enabling 2 cycles to circulate the ring . Such adelay requires the monitor a serial register of :

(40 . 10 6 ) -(b- 10-6 ) = 400 bits

Recall that a cycle is received by a station every125 µs. Therefore, each station that uses an octet(8 bits) in every cycle has a 64 Kbit/s channel,and can use it for a single phone call. In order tosatisfy requirements of higher bandwidth trans-missions, a station can occupy more than oneoctet in each cycle, say N, thus having a channelof bandwidth N 64 Kbit/s. Figure 1 gives apicture of the abovementioned situation in a ring :2 cycles, of 125 µs each, circulate in the ring .Both cycles are divided in the same manner toisochronous and non-isochronous parts. Theisochronous part is sub-divided into octets. Anoctet in both cycles represents a 64 Kbit/s chan-nel.

The access to the non-isochronous bandwidthis controlled by means of a to en, as in a to en-ring networ [2,3,8]. A station wishing to sendnon-isochronous data to another station waits for

starting FRAME FCSending

delimiter CONTROL DEST SOURCE DATAdelimiter

1 34


a to en-frame in the non-isochronous bandwidthand seizes it . Then, the station transmits its data-frame in the non-isochronous bandwidth, and fi-nally it issues a new to en-frame. Figure 2 showsa typical structure of a non-isochronous data-frame.

A station with isochronous data must acquire apart of the isochronous bandwidth for the entireduration of the specific service. This is in contrastto the non-isochronous bandwidth which can beseized for a limited duration before a new to en-frame is issued. The present paper deals with themaintenance, allocation and release of theisochronous bandwidth .

The ring model considered in this paper issimilar to the one introduced in [5] and [6] . How-ever, the new scheme is applicable also to FDDI-11 [8] and to the networ s introduced in [1,9,11],although they divide the entire bandwidth intoisochronous and non-isochronous parts slightlydifferently .

The new scheme proposed in the present pa-per has the following properties :

(1) The scheme enables complete integrationof all inds of isochronous traffic with differentbandwidth requirements . The isochronous band-width is divided into low-speed (64 Kbit/s) chan-nels, and stations can dynamically bundle severallow-speed channels to form one high-speed chan-nel. This ind of bit-rate-on-demand approach,where the bandwidth of isochronous channels isdetermined according to the actual demand, im-plies better utilization of the isochronous band-width, rather than the preassign method [1], wherechannels have preassigned bandwidth. In the lat-ter approach, there is a possibility that somestation would fail to acquire a channel for someservice in spite of the fact that there is enoughavailable isochronous bandwidth, because theavailable bandwidth is pre-assigned for other,non-active, services .

(2) The process of acquiring isochronouschannels by the ring stations is very cheap, simpleand fast. There is no need to communicate withother stations, as with the isochronous manage-ment entity in [5], and therefore this processwaists no non-isochronous bandwidth . Thus, astation that is unsuccessful in acquiring the nec-essary bandwidth can repeat its attempts until itsucceeds, without overloading the ring . More-over, the allocation does not depend on the avail-

ability of the non-isochronous capacity of a spe-cial management entity that must be reached byring stations for allocation or release . A stationcan acquire the required bandwidth, if available,at the latest within one round trip delay of thering .

(3) The acquiring and releasing processes aredecentralized . Each station can acquire or releasechannels by local actions. This prevents the prob-lems occurring in protocols where a managementstation is required .

(4) The allocation rate is not affected by thenumber of stations in the ring or by the numberof stations that simultaneously require isochron-ous channels . This is an important property inhigh speed local area networ s with a dynamicnumber of stations, up to several thousands .

(5) The overhead bandwidth required by thenew scheme is negligible : less than 1% .

A recovery mechanism from errors and fail-ures that may interfere with the proper operationof the scheme, is also presented . This mechanismuses only a negligible part of the non-isochronousbandwidth.

The organization of the rest of this paper is asfollows . Section 2 introduces previous approachesfor isochronous bandwidth management . Section3 presents the new scheme . Section 4 shows howtransmission errors affect the new scheme andpresents the recovery mechanism. Section 5 con-cludes the paper, and the Appendix presents theformal descriptions of the allocation and deallo-cation algorithms .

2. Previous approaches to isochronous bandwidthmanagement

One way to approach isochronous bandwidthmanagement is to use the same scheme as fornon-isochronous bandwidth management. Thatamounts to adding a full/empty status bit toeach isochronous channel [7] . A station that re-quires an isochronous channel tests the status bitof every channel, and when it finds an emptychannel, the station changes its status to full anduses it from then on for the specific isochronousservice . Upon service termination, the station re-leases the channel by changing its status bit bacto empty . This simple approach has several draw-bac s :

the overhead problem: The overhead is one bitper a channel . Since a voice channel has 8 bits,the overhead is 1/9 = 11 .1% .the gathering problem: A station that needs abandwidth of N 64 Kbit/s must find N empty64 Kbit/s channels and mar them full. How-ever, since the station does not now if thereare N available channels until it receives thelast full /empty bit, it may seize some chan-nels and then find out that the number ofavailable channels is not sufficient . Obviously,all acquired but not used channels are wasted .Moreover, holding these channels until thestation acquires sufficient bandwidth, may leadto a deadloc .

In order to overcome the gathering problem onemay consider a pre-assigning approach, where theisochronous bandwidth is divided into channelswith different bandwidth, to support differentservice requirements. However, this results in an-other problem :

the pre-assignment problem: The pre-assign-ment method is the opposite to the demand-assignment method. It requires an accuratenowledge about the expected isochronousbandwidth demand. Failure in acquiring achannel with a specific bandwidth, not becausesuch bandwidth is not available at that time,but because the available bandwidth is pre-as-signed for other requirements, results in band-width waste .

Two other, better, methods have been previouslysuggested. According to the first approach [5],one of the ring stations functions as anIsochronous Management enTity (IMT) . The IMTis responsible for dynamically allocating and as-signing isochronous channels for isochronouscommunications. The IMT maintains in its localmemory a table of all isochronous channels, andto each channel it associates an available /occupied indicator. A source station that needsisochronous bandwidth, sends a request messageto the IMT, using the non-isochronous band-width, and indicates the required bandwidth. TheIMT verifies whether there are enough availablechannels to fulfill the request . If enough channelsare available, it mar s them as occupied andsends bac to the source station, again using thenon-isochronous bandwidth, a message with theacquired channel numbers . The source stationuses these channels as long as the supported


1 35

service requires . Then, it releases them by send-ing the IMT a non-isochronous message, and theIMT mar s them as available again. Althoughvery simple in principle, this method has thefollowing deficiencies :

Each allocation requires the exchange of sev-eral non-isochronous messages between thesource and the IMT. Exchange of additionalmessages is required later for the deallocationprocess.The time required for the source and the IMTto exchange non-isochronous messages may besignificant for call establishment of someisochronous applications .When a request fails due to lac of availablechannels, more messages are necessary for re-peated requests and the time required for allo-cation increases as well .Since the IMT is involved in all the allocationand deallocation processes, the allocation ratedepends on the non-isochronous capacity ofthe IMT. Thus, when the number of stationsaround the ring increases, the ability of theIMT to seize a non-isochronous to en-framemay become a bottlenec . Moreover, even ifnumber of stations does not dramaticallychange, the time required to achieve anisochronous channel strongly depends on thenumber of stations that need the IMT servicesat a given time .As in every centralized scheme, serious prob-lems may ta e place when the IMT fails orleaves the networ , and additional distributedprotocols are needed for proper operation ofthe scheme . First, another station should beelected in order to function as an IMT, and itsidentity must be nown to all other stations .Then, the new IMT must somehow acquire theinformation about the situation of all channels,in order to build an up-to-date allocation table .

The second approach has been proposed in [1] .The isochronous bandwidth is divided into sev-eral channel categories, each of different band-width . Each category contains channels of thesame bandwidth and one additional channel,called the tic et channel, dedicated to allocationsand deallocations of channels from this category.For each channel category, the IMT has the tasof placing a tic et, that is an identifier of anavailable channel of this category, in the tic etchannel . A station that requires a channel from a

1 36

R. Cohen, A . Segall / Management of isochronous channels

certain category waits for a tic et in the tic etchannel of this category, and ta es it by replacingit with a nil symbol. To return the tic et, thestation waits for a nil symbol in the channel, andreplaces it with its free tic et. This tic et may beused immediately by the downstream neighbor ifthe latter requires a channel from this category .

Comparing to the previous approach, the lat-ter is also centralized, but it does not requireexchanging of non-isochronous messages . In thisapproach there is a trade-off between the over-head for the tic et channels and the time re-quired to seize a channel, when such a channel isavailable: enlarging the bandwidth for the tic etchannels increases the overhead but decreasesthe time required to seize a channel, since moretic ets are available in each round trip . More-over, this approach uses the pre-assigning method,and thus it suffers from the so-called pre-assign-ment problem .

3. The new scheme

The main principle of the new scheme is tomaintain in the isochronous part of the cycle alin ed-list that indicates the available isochronouschannels . In order to maintain such a list, only

cycle = 125µs

isochronous

(3 slots)

slot header

ring delay

slot (257 bytes) structure

the unoccupied isochronous bandwidth is used .The only required overhead is two octets forevery slot of 257 octets, namely 2/257 = 0 .778% .Figure 3 demonstrates the situation in a ring witha round trip delay of 250 µs according to the newscheme. Two 125 µs cycles circulate the ring .Each cycle is divided into 3 basic parts : (1) sev-eral bits of CYCLE HEAD field, (2) the isochronouspart and (3) the non-isochronous part. Theisochronous part of the cycle is divided into slots(3 in our example) . Each slot consists of up to 257octets . The first two octets in each slot are over-head . Therefore, a minimum overhead is achievedwhen the slot size is exactly 257 octets, and fromnow on it is assumed that this is the slot size .Adapting the new scheme to FDDI-II [8] resultsin overhead of 2/98 = 2 .04% since the slot size inthis networ is 96 octets (the slot in FDDI-II iscalled a wideband channel (WBC)).

An isochronous channel consists of a givenoctet in a given slot in all cycles. The same slot inall cycles represents one high-speed channel of255 64 Kbit/s = 163 .2 Mbit/s, so-called super-channel . The structure of each slot is also shownin Fig. 3 . The first two bytes are the slot header,consisting of two fields : counter and head . Thenwe have 255 octets, where each octet is a part ofa 64 Kbit/s channel . A station that requires a 64

cycle = 125µs

.r isochronous -a-

CYCLEHEAD 1 2 3

non-iso-

chronousCYCLEHEAD 1 2 3

non- iso-chronous

Fig. 3 . 2-Cycle ring according to the new scheme .

super - channel

counter headoctet

0octet

1octet254

Kbit/s channel can occupy, for example, octetnumber 101 in slot number 2 in all cycles . 64Kbit/s is the minimal isochronous bandwidth thatcan be dedicated to a station in the scheme .However, a stations may divide such a channelbetween several different local services .

The monitor selects one of the cycles as theleading-cycle . The leading-cycle has the samelength and structure as all other cycles have, butit is distinguished by one bit in the CYCLE HEAD

field . By default, if the ring delay allows only onecycle, this cycle is the leading-cycle . The leading-cycle is special in that it holds in the octets ofunoccupied isochronous channels informationabout the identities of the unoccupied isochron-ous channels .

Except for error recovery, as explained in Sec-tion 4, selecting the leading-cycle and initializingit is the only operation performed by the monitorregarding isochronous bandwidth maintenance,allocation and release . All other actions are per-formed by the stations themselves, while receiv-ing and forwarding the leading-cycle . By testingthe counter field of a given slot (8 bits long) inthe leading-cycle, a station finds out whether therequired bandwidth is available in the super-channel associated with this slot. If not, it doesnothing. If the bandwidth is available, it acquiresthe corresponding number of channels in thatsuper-channel by changing the counter and cer-tain octets in the slot, and uses the octets of theacquired channels from then on, in the leading-cycle as well as in all other cycles . The release isdone in the leading-cycle, again by changing thecounter and some octets of unoccupied channelsin the slot. The detailed operation is describedbelow.

All actions performed by a non-monitor sta-tion upon allocation and deallocation ofisochronous bandwidth require a delay of 8 bits .The delay of the monitor adjusts the total ringdelay to an exact multiple of 125 µs, as previouslyexplained .

In all cycles, including the leading-cycle, octetsof occupied channels contain isochronous data .The contents of the octets of unoccupied chan-nels in the non-leading-cycles is not relevant, andfrom now on we shall refer only to the leading-cycle . Octets of unoccupied channels and thehead field in each slot in the leading-cycle form alin ed-list. In such a list, each octet of an unoccu-


1 37

pied channel is a pointer to the next availablechannel in the super-channel . The last unoccu-pied octet in the slot contains the number 255,whose meaning is the end of the lin ed-list (nil) .Since 255 is the largest 8-bit number, the size ofthe slot is limited to 255 data octets, whose num-bers are 0, . . . , 254. In order for a station to nowhow many channels are available before startingthe acquiring process, the counter field, that indi-cates the number of available channels in thesuper-channel associated with the lin ed-list, isused. The counter field is placed before the headfield and a station leaves the slot unaltered if theavailable bandwidth is not sufficient for its pur-pose . The important fact to note is that the newscheme uses the octets of available channels in theleading-cycle in order to maintain informationabout the available channels . Thus, the only over-head is the two octets, counter and head, at thebeginning of each slot .

At initialization, all channels of the super-channel are available . The monitor creates anappropriate structure in each slot of the leading-cycle, as shown in Fig . 4(a) . Suppose that a sta-tion S; needs an isochronous channel with abandwidth of 100 Kbit/s . Therefore, it shouldseize two 64 Kbit/s channels . When station S;receives the leading-cycle, it tests the counter ofeach slot and loo s for a slot with counter > 2.Following initialization, the first slot fulfils thisrequirement . Recall that each non-monitor sta-tion has a delay of 8 bits . Therefore, when stationS; completes receiving the counter field it shouldstart transmitting it . The transmitted value of thecounter field is 253, since this is the number ofavailable channels in this super-channel after sta-tion S; seizes two channels . Generally, a stationthat needs N channels, where N z counter, seizesthe last N available channels in the super-chan-nel . Therefore, station S; traverses 253 hops inthe lin ed-list in order to disconnect the last twoelements of the list. It does so upon receiving andtransmitting the cycle octets : when it receives thehead field it nows where to start . In our casehead = 0, so the first available channel is 0. Sta-tion S; then waits to receive octet 0 that repre-sents channel 0 in this super-channel, and findsthat the next available channel is 1 . Upon receiv-ing octet 1, station S . finds that the next availablechannel is 2, and so on . This process is repeated253 times, until S; receives octet 252 . Station S,

1 38


nows that this octet represents the 253rd avail-able channel and thus from then on this will bethe last available channel of the super-channel .Therefore, the value of this octet should indicatenil, i.e. 255 . Thus, station S; transmits this octetas 255, instead of the received value 252 . By thisaction, station S; disconnects the last two ele-ments from the lin ed-list . This is the only changemade by Si in the slot, in addition to changingthe counter field . However, S; still does not nowwhat are the sequential numbers of its new chan-

counter head octet 0 octet 1 octet 2

counter head

counter head

counter head

counter head

counter head

0

0

250 0 1255(nil)

occupied

by S,

occupied

by S,

occupied

by S,

occupied

by S;

occupied

by S ;

1

0

(a) the lin ed-list following initialization

(b) station S; acquires 2 channels

(c) station S~ acquires 3 channels

249 250 251 252 253 254

0

(d) station S; releases its 2 channels

(e) station S acquires 3 channels

(f) station S~ releases its 3 channelsFig. 4. Examples of allocations and deallocations .

nels. Therefore, it continues traversing the lead-ing-cycle and finds that the last two previouslyavailable channels are 253 and 254 . From now on,octets 253 and 254 of this slot in the leading-cycleare occupied by station S ; . Figure 4(b) shows thesituation of this slot after the above allocation,and Fig. 4(c) shows the situation after anotherstation S~ acquires three channels : 250, 251 and252.

After seizing the required bandwidth, stationS; can inform its partner/s to the specific service

253 254

249 250 251 252 253 254

249 250 251

252 253 254

248 249 250 251

252 253 254

249 0 1255(nil)

occupied

by S

occupied

by S,

occupied

by S,

occupied

by S,

occupied

by S

occupied

by S

248 249 250 251 252 253 254

252 0 1 250occupied

by S251 252 255

occupied

by S

occupied

by S

255 0 1 2 3 254255(nil)

253 0 . 250 251 252255(nil)

occupied

by S;

occupied

by S;

occupied occupied occupied 2 55252 0 1 253 254by S, by S; by S, (nil)

that the isochronous connection will ta e placeon channels 253 and 254 of super-channel 1 . Thisis done by a simple separate protocol of sendingand receiving non-isochronous messages . Then,S; can exchange data with its partner over the 128Kbit/s channel . Upon call termination, station S ;releases the channels by inserting octets 253 and254 to the lin ed-list of the appropriate slot inthe leading-cycle, as shown in Fig . 4(d). Thereleasing process is also done by S ; upon receiv-ing and transmitting the leading-cycle, with adelay of 8 bits . Figure 4(e) shows the lin ed-listafter a third station S that needs isochronousbandwidth of 192 Kbit/sec acquires 3 channels ofthe same super-channel, in octets 249, 253 and254. This example shows that the acquiredisochronous bandwidth may be non-contiguous .Figure 4(f) shows the lin ed-list after station S ireleases channels 250, 251 and 252 .

A formal description of the allocation anddeallocation algorithms is presented in the Ap-pendix. As illustrated above and seen from thealgorithms, the ey point of the new scheme isthe fact that a station can add and delete ele-ments from a lin ed-list while receiving andtransmitting a cycle, with a delay of only 8 bits .Namely, the transmitted value of each octet canbe determined from the values of the correspond-ing received octet and of previous octets .

4. Error and failure recovery

4.1. The effect of transmission errors

Transmission errors in the lin ed-lists may havea crucial effect on the allocation scheme . In mostcases transmission errors cause loss of bandwidth .Loss of bandwidth can also ta e place due to astation failure, since such a station will neverrelease the channels it has previously acquired .Moreover, in some infrequent cases, errors in thelin ed-list may cause the same channel to be

counter head 0


1 39

1 2 3 4

Fig. 5. Lin ed-list example .

occupied by two or more stations simultaneously,resulting in abnormal connections .

In order to see how transmission errors mayresult in loss of bandwidth and in multiple alloca-tion of the same channel, recall that when astation executes the allocation algorithm, it firstchec s the counter field . If counter > N, where Nis the required number of 64 Kbit/s channels, thestation decreases the counter by N and thentraverses the lin ed-list and disconnects the lastN elements of the lin ed-list. The algorithm dic-tates that if at any time during the traverse, thestation finds out that the lin ed-list is abnormal,it stops execution of the algorithm, returns to theupper layer an `error' indication (see Table A .1 inthe Appendix) and does not use any new channel .

Definition 1 . A lin ed-list is said to be normal ifthe following hold :

each pointer of the lin ed-list points forward .the number of elements in the lin ed listmatches the contents of the counter field ; thus,element number counter, and no precedingelement, has value 255 (nil) .

When the above requirements do not hold, thelin ed-list is considered abnormal and no alloca-tion is done. Note that in some cases, a stationmay find that the lin ed-list is abnormal afterhaving disconnected some elements of thelin ed-list. For example, this will be the situationif the last element in the list does not indicate nil.In such cases, the station does not in fact acquirethe disconnected elements, and their identitiesare not passed on to the upper layer .

Consider the lin ed-list of Fig . 5 . This lin ed-list indicates that channels 0, 3, 5, 6 and 7 areavailable, whereas all other channels are occu-pied . Suppose that a transmission error occurs inthe counter field and decreases its value . Thus,the second requirement does not hold . The samehappens if a transmission error increases the valueof the counter field or changes the value of octet

5 6 7 8 9254

5 0 occupied occupied 5 occupied 6 7 255 occupied occupied

1 40


number 7 from 255 (nil), or changes the value ofone of the octets 0, 3, 5, 6 to 255 (nil). A trans-mission error that decreases the value of octet 3to 0, for example, causes the first requirement tonot hold. The result of all these cases is loss ofbandwidth of all available channels representedby this lin ed-list, since no station can acquirechannels from an abnormal lin ed-list .

In some rare situations it may happen thattransmission errors alter the lin ed-list but thelatter remains normal. In such situations it mayhappen that a station acquires an already occu-pied channel . For example, consider the situationof Fig . 5 . Note that octet 8 is occupied by aconversation and thus its value depends on whatis being transmitted on this conversation . Sup-pose, for example, that there has been an error inthe contets of octet 6 that changes its value from7 ,to 8 . Suppose also that the contents of octet 8happens to be 255 at the time when some stationtries to acquire bandwidth. In this case, the sta-tion considers the lin ed-list as normal and occu-pies channel number 8 . Consequently, this chan-nel is simultaneously occupied by two stations .Note that in general if a transmission errorchanges the value of octet 6 to some v, thelin ed-list remains normal only if v > 6 and octetnumber v happens to have value 255 .

Section 4.2 presents the refresh mechanism thatovercomes the abovementioned problems . Thismechanism enables the monitor to refresh thelin ed-lists with a certain frequency, once every Fround trips. The refresh mechanism includes aspecial protocol, called the declaration protocol .According to this protocol, each station informsthe monitor what channels it occupies in everysuper-channel. The monitor uses this informationin order to rebuild the lin ed-list of each super-channel . Thus, regardless of transmission errorsor station failures that have ta en place so far,after every refresh each lin ed-list represents thesituation of its super-channel accurately . The ap-propriate frequency F of the refresh depends onthe bit-error-rate in the networ , the permittedloss of bandwidth due to errors and the permittednumber of possible channel duplications. Section4.3 shows that for a ring as considered in Section1, that is a 50 Km ring with 200 stations operatingat 40 Mbit/s, a frequency of F = 4000 (one re-fresh per second) ensures a negligible loss ofbandwidth, less than 0 .08%. Section 4.4 shows

that under the same conditions the number ofchannel duplications is virtually zero .

4.2. The refresh mechanism

In order to refresh the lin ed-lists, the monitorfirst initiates the declaration protocol which re-quires the transmission of one non-isochronousdata-frame by the monitor . The monitor loo s fora to en-frame in the non-isochronous bandwidth,seizes it and transmits a special data-frame, calleda declaration frame . The fact that this is a decla-ration frame is indicated in the FRAME CONTROLfield (see Fig. 2). This frame is destined to allstations around the ring . In the DATA field of thisframe, the monitor transmits a vector of Os whosesize is 255 times the number of super-channels .Each bit represents one isochronous channel ac-cording to the channel and super-channel num-bers . For example, channel number 200 in super-channel 3 is represented by the 710th bit (since(3- 1Y255+200=710).

A station that receives a declaration frame,changes the bit representing each of its ownoccupied channels from 0 to 1 . The case wheresome of those bits received as 1 indicates a dupli-cation in the allocation of the associated channel,in which case the station considers this channel asoccupied by another station, and the upper layeris informed to drop usage of this channel . Theupper layer should notify the partner(s) to thecommunication that the connection on this chan-nel is canceled, and it may try later to acquireanother channel, in order to reestablish the con-nection.

When the monitor receives the declarationframe bac , it nows which channels of eachsuper-channel are actually occupied so it canreconstruct the lin ed-lists in its local memory . Inthe next time it receives the leading-cycle, themonitor transmits the reconstructed lin ed-listfor each superchannel, regardless of the receivedcontents of the lin ed-lists. However, in order toeep consistency of the new lin ed-lists according

to the last declaration, no station is allowed toperform any changes in the lin ed-lists, allocationor release, from the time it updates the declara-tion frame and until it receives the reconstructedlin ed-lists. Therefore, when the monitor receivesthe leading-cycle for the first time after transmit-ting a declaration frame, it indicates in the CYCLE

HEAD that changes in the lin ed-lists are notallowed in this cycle. When the monitor receivesthe leading-cycle for the first time after receivingthe declaration frame, it cancels the prohibitionand incorporates the new lin ed-lists into theisochronous slots of the leading-cycle .

Following each refresh, all octets in the lead-ing-cycle that are parts of unoccupied channelsare connected to the lin ed-lists, and channelsthat have previously been occupied by more thanone station are now occupied by one station only,the one that is closest to the monitor among allthese stations .

4.3. Loss of bandwidth

This subsection analyzes the relation betweenthe bit error rate in the networ , the refreshfrequency, F, and the loss of bandwidth due totransmission errors. Recall that the refresh mech-anism rebuilds the lin ed-lists according to theactual occupancy at the time it is performed .Namely, all occupied channels are not repre-sented in the lin ed-lists, whereas all availablechannels are represented . However, from the timewhen a channel is mista enly considered as occu-pied until the next refresh, the bandwidth of thischannel is lost. Obviously, if the refresh mecha-nism is invo ed too frequently, namely F is small,the loss of bandwidth is decreased . However,each execution of the declaration protocol re-quires a part of the non-isochronous bandwidth,so we have a trade-off. It will be shown now thatone refresh every several thousands of roundtrips is sufficient in order to obtain negligible lossof the isochronous bandwidth. Obviously, thenon-isochronous overhead is also negligible atthis rate of refresh .

Let e be the bit-error-rate in the line connect-ing two ring stations, S be the number of stationsin the ring and p be the probability that a givenlin ed-list in the leading-cycle ma es a round tripwith no error. Note that each lin ed-list has atmost 257 . 8 bits . When all channels of the super-channel are available, the lin ed-list has exactly257 8 bits . In general, if c channels are occu-pied, where 0 c c 255, the lin ed-list has only(257 - c) 8 bits . Thus,

2578Sp,p=(1-e)


141

(1)

where p is the probability that the lin ed-list ofmaximum length ma es a round trip with noerror .

The expected loss E of isochronous bandwidthin an interval of F round trips is :

F

E

p; (F - i) (super-channel bandwidth)i=2

( 2 )

where (F - i) is number of round trips until thenext refresh, namely the number of lost roundtrips if the lin ed-list is destroyed in the i thround trip, and p; is the probability that the firsttransmission error after the refresh occurs in thei th round trip. The sum starts from i = 2 sincechanges in the lin ed-list during the first roundtrip of the lin ed-lists after the refresh are ig-nored by the monitor when the latter rebuilds thelin ed-lists .

The right hand of (2) assumes that every errorin the lin ed-list results in a loss of allisochronous bandwidth of the super-channel rep-resented by the lin ed-list from the time whenthe error occurs until the time it is corrected inthe next refresh . Note that this is a worst caseanalysis, since in practice a part of the bandwidthof that super-channel had already been occupiedwhen the error happens, and this part is not lost .Now, note that

pi=pj

-1 ' (1 -P)

( 3)

and since i , 2 and p 5p < 1, the following holds

pE > p `-1 . ( 1 - p)

( 4)

From (2) and (4) we get that

F-p F+pF - 1

1-p

(super-channel bandwidth)

This means that the lost isochronous bandwidthis less than

F-p •F +p F -i 100

1 - p

F. %

where p = (1 - e )zsza s

For example, consider a ring with S = 200stations and suppose that e = 10 -12 . This is areasonable bit-error-rate in fiber optics . Let F =4000 be the frequency of the refresh (once a

1 42


second, in a 2-cycle ring) . In this case we obtain aloss of bandwidth of at most 0 .08%, which is ofcourse negligible .

4.4. Number of channel duplications

As indicated in Section 4 .1, in some rare situa-tions a transmission error may change thelin ed-list while leaving it normal. In such a casean occupied channel may be acquired again byanother station, thus establishing two interferingconnections on the same channel . Section 4.2 hasshown how does the refresh mechanism solvesuch a conflict : following the execution of theprotocol, one of the stations that have previouslyoccupied the channel releases it, and tries toreestablish the connection with its partner onanother channel . Thus, each duplication resultsin a connection brea , and in waste of non-iso-chronous bandwidth required for a station toinform its partner to stop transmitting on theirdedicated channel . However, simulations showthat the number of duplications is in fact insignif-icant. In particular, for bit-error-rates of 10 - orless, the number of duplications is virtually zero .

The simulation model considers a ring with200 stations . The duration of the isochronousconnections is assumed to be exponentially dis-tributed with a mean length of 90 minutes, andthe refresh mechanism is performed once everyF = 4000 round trips . This model has been testedunder several loads, and for several values of thebit-error-rate e. The results show that as the loadof the system increases, the number of duplica-tion increases too. For example, under light load,there have been encountered no duplications forall values of e c 10 -9 during 500 h of ring opera-tion . Therefore, the simulation under the heavy

Fig. 6. The results of the heavy load simulations .

load can be considered as the worst case situa-tion . Figure 6 illustrates the results of the heavyload simulation: number of duplications in onelin ed-list during 100 h of ring operation versusbit-error-rate . As mentioned before, the numberof duplications is virtually zero for bit-error-ratesof 10 - and less .

5 . Conclusions

This paper considers a fiber-optic ring networthat supports the integration of several types ofinformation. The ring bandwidth is divided intotwo parts : one for pac et switched communica-tion, used for non-isochronous purposes as com-puter data, and the second for circuit switchedcommunication, used for isochronous purposes asvoice, video and so on . The isochronous band-width is divided into several 64 Kbit/s channels,and the issue of dynamic maintenance, allocationand release of these channels was considered .

The new scheme proposed in the paper main-tains the identities of available isochronous chan-nels as several lin ed-lists, using the bandwidth ofthose same available isochronous channels . Thelin ed-lists circulate the ring, while stations ac-quire channels by disconnecting elements fromthe lists, and release channels by reconnectingthe elements to the lists .

The new scheme has several important proper-ties. It supports the integration of isochronousdata with different bandwidth requirements . Al-location and deallocation are simple, quic andcheap decentralized processes that are not af-fected by increase in the number of stations inthe ring .

The way of specifying available channels usedby the new scheme may cause problems whentransmission errors disrupt the lin ed-list. There-fore, a recovery mechanism that refreshes thelin ed-lists with a certain frequency is proposedas well . This mechanism reduces the damage oferrors in the lin ed-list to a negligible degree .

Applying the new scheme in the FDDI-II isstraightforward . In the FDDI-II the isochronousbandwidth is divided into 16 wideband channels(WBC), where each WBC consists of 96 channelsof 64 Kbit/s . Therefore, the available isochronousbandwidth in each WBC should be representedby a lin ed-list that consists up to 96 elements .The overhead in this case is 2/98 = 2.04% .

Appendix

This appendix presents the formal descriptionof the allocation and deallocation of isochronouschannels. Recall that these algorithms are per-formed only on the leading-cycle .

Both allocation and deallocation algorithmsare invo ed when the start of an isochronous slotin the leading-cycle is detected . The algorithmshave two parts. The first part should be executedupon receiving the counter field of the slot, andthe other part should be executed upon receivingthe head field and each of the octets. If X is anoctet, then X - denotes its received value andX+ denotes its transmitted value . Thus, y - X-is a receiving action, that copies the receivedvalue of X into a local variable y . Similarly,X+ - y is a transmission action, that causes thevalue y to be transmitted as the new value of thefield X.

We denote by octet[i] the value of octet num-ber i, where 0 i 254 . The head field is de-noted by octet[-1], and the counter field is de-noted by counter . Thus, octet[-1] - denotes thereceived value of head and octet[20V denotesthe transmitted value of octet 20. The variablenext_unoccupied denotes the next unoccupiedoctet in the cycle . The function exit denotes ter-mination of the algorithm . For example, if uponreceiving octet[95] the function exit is executed,the algorithm should not be executed upon re-ceiving octets 96, . . . , 254. Thus, for 96 i S 254,holds octet[i]+- octet[i] -.

The allocation algorithm

The allocation algorithm receives from thehigher layer an integer N, 1 , N 255, and isinvo ed upon detecting the start of an isochronousslot in the leading-cycle . The algorithm tries toacquire N channels from the super-channel . Ifthere are enough available channels, it succes-sively returns to an intermediate entity the identi-ties of the acquired channels by the functionreturn. Otherwise, it returns the indication`not _available' . Upon termination, it returns tothe intermediate entity either an èrror' or a`done' indication . If the indication is `done', theintermediate entity transfers the identities of theacquired channels to the higher layer . If theindication is èrror', no channels are actually ac-


Table A .1The allocation algorithm

•

upon receiving counter - :if counter - < N then begin

counter` E-counter -exit(`not available')

endelse begin

counter+ <- counter - - Nnext . _ unoccupied *- -1s ip F counter- - N

end•

upon receiving octet[i] -, -1 i 254 :if i * next_ unoccupied then octet[i] <-octet[iLelse begin

next _ unoccupied - octet[1] -if next unoccupied 5 1 then exit(èrror')else begin

case ofs ip >0: octet[iV

octet[i] -s ip =0: return (octet[i]-),

octet[i] t- 2550 > s ip > - N : return (octet [ i ]- )s ip = - N: if octet[i] - # 255

then exit (èrror')else exit(`done')

end cases ip F s ip - 1if i = 255 then exit(èrror')

endend

143

quired. An èrror' indication is returned when thelin ed-list is abnormal, due to transmission er-rors. As explained in Section 4.1, this happens inthe following cases :- When the value of octet number i that repre-

sents an available channel is

i.- When the value of the last element of the old

list is not 255 (nil) .- When an element in the middle of the old list

has the value 255 (nil) .In addition to the variables mentioned before,this algorithm uses a variable s ip, that ta esvalues 0 < s ip cN. If the required bandwidth isavailable, then counter + f -- counter - - N and s ipis also initialized to counter - -N to denote howmany octets in the lin ed-list should be s ippedbefore the last N octets are disconnected fromthe list .

The allocation algorithm is presented in TableA.1 .

144

Table A .2The deallocation algorithm

•

upon receiving counter - :counter F- counter + Nnext unoccupied f- - 1temp f- nilindex F- 1

•

upon receiving octet[i] - , -1515254:if i#next_unoccupied then octet[i] + F-octet[i]else begin

case of(temp = nil)A(octet[i]

R[index]) :next - unoccupied f- octet[i] -octet[i] -octet[i] -

(temp =nil)A(octet[i] - > R[index]) :temp - octet[i] -octet[iV <- R[index]next-unoccupied -R[indexIindex - index + 1

(temp # nil)A(temp 5 R[index]) :octet[iV 4- tempnext _ unoccupied e temptemp <- nilif R[index]=255 then exit(`done')

(temp # nil)A(temp > R[index]) :octet[iV R[index]next unoccupied <-R[index]index index + 1

end caseend

The deallocation algorithm

The deallocation algorithm receives from thehigher layer the number N of 64 Kbit/s channelsto be released in a super-channel, 1 <N 255,and a sorted array R of N + 1 numbers repre-senting these channels. The last entry in R hasthe value 255 . For example, consider again Fig .4(f) and suppose that station S releases channels249, 253 and 254 . Therefore, R[1] = 249, R[2] _253, R[3] = 254 and R[4] = 255 . This algorithm isinvo ed upon detecting the start of the specificisochronous slot in the leading-cycle, and it in-serts the octets whose sequence number appearin R to the lin ed-list in the slot . The algorithmuses a pointer to the array R, called index, thatindicates the next number of a channel to bereleased. Following our example, if index = 2,


then octet 253 is the next to be connected to thelin ed-list . Another variable used by the algo-rithm is temp . This variable holds the number ofan octet that is temporarily being disconnectedfrom the lin ed-list during the deallocation pro-cess . For example, when station S releases chan-nel 249 to the lin ed-list of Fig. 4(f), it finds thatoctet[248] - = 250 and therefore it transmitsoctet[248] + - 249 . The value 250 is stored in temp,and when octet[249] is received, station S trans-mits octet[249F <- temp .

The deallocation algorithm is presented inTable A.2.

References

[1] J .R. Brandsma, A.A.M.L. Brue ers and J .L.W. Kessels,Philan : a fiber-optic ring for voice and data, IEEE Comm.Mag. 24 (12) (1986) 16-22 .

[2] W. Bux, F.H . Closs, K. Kuemmerle, H.J. Kelley and H .R.Mueller, Architecture and design of a reliable to en-ringnetwor , IEEE J. Selected Areas Comm . 1 (5) (1983)756-765 .

[3] Institute of Electrical and Engineers Inc ., IEEE Stan-dards 802.5-1985 (ISO DIS 8802/5), To en Ring AccessMethod and Physical Layer Specification .

[4] R.W. Klessig, Overview of metropolitan area networ s,IEEE Comm. Mag. 24 (1) (1986) 9-15 .

[5] R. Kositpaiboon and N .D. Georganas, Isochronous com-munication in an integrated circuit/pac et metropolitanarea networ , IEEE Trans. Comm. 36 (5) (1988) 621-620 .

[6] M. Liu and D.G . Messerschmit, S ew time slot switchingand slotted ring in a metropolitan area networ , Proc.IEEE INFOCOM '88, New Orleans, Louisiana, pp . 568-575 .

[7] T. Minami, K. Yamaguchi, T. Na agami, H . Ta anashi,N. Fujino, H. Hamano, M. Suyama and I. Yamada, 200Mbit/s Synchronous TDM loop optical LAN suitable formultiservice integration, IEEE J. Selected Areas Comm . 3(6) (1985) 849-858 .

[8] F .E. Ross, An overview of FDDI: the fiber distributeddata interface, IEEE J. Selected Areas Comm . 7 (7) (1989)1043-1051 .

[9] D.T.W. Sze ., A metropolitan area networ , IEEE J.Selected Areas Comm . 3 (6) (1985) 815-824 .

[10] P. Wong and T.P. Yum, An integrated services to en-controlled ring networ , IEEE J Selected Areas Comm . 7(5) (1989) 670-679 .

[11] M . Zu erman, Bandwidth allocation for bursty isochron-ous traffic in a hybrid switching systems, IEEE Trans.Comm. 37 (12) (1989) 1367-1371 .

a new scheme for dynamic management of isochronous channels in integrated rings

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