GB2396090A - Processing upstream messages at an Optical Line Terminal - Google Patents

Abstract

An optical line terminal (OLT) controls one or more optical network units (ONUs, not shown) connected thereto. An upstream cell termination unit (66) receives messages from the ONUs. A message code detector (52-<WC 1>1 n) extracts valid messages from among the messages received by the upstream cell termination unit, through a process of filtering out such messages that indicate that the se<WC 1>nding ONUs have no information to send. A cell extraction unit (51-1 n) identifies <WC 1>the sending ONUs which are sourcing the valid messages extracted by the message code detector. A PLOAM grant generator (6e-1 n) produces message transm<WC 1>iss<WC 1>ion permissions (i.e., PLOAM grants) which permit the sending ONUs identified by the cell extraction unit to transmit messages (i.e., PLOAM cells). The message transmission permissions <WC 1><WC 1>produced by the PLOAM grant generator are then transmitted to the ONU through a PLOAM cell transmitter. The proposed OLT can receive messages from a plurality of ONUs more reliably, because of its capability of allocating message transmission permissions to the ONUs depending on their individual conditions.

Description

GB 2396090 A continuation (72) Inventor(s): Kazuhiro Uchida Kazuya Ryu

Kanna Okamura Masashi Shibata Shinichi Fujiyoshi Katsuhiko Hirashima Toshinori Koyanagi Toshiyuki Sakai Setsuo Abiru (74) Agent and/or Address for Service: Haseltine Lake & Co Imperial House, 15-19 Kingsway, LONDON, WC2B BUD, United Kingdom

1 2396090

OPTICAL TERMINAL

5 The present ir-vention relates to an. opt-cal line terminal, and more particularly, to =n optical line terminal witch controls one or more optics' net-.'o-k urn's connected thereto.

'10 Today's tle_cmmunications meat needs call -or raped deplor.t as otcal access netwc's, n.-a^7y,-e Offer to the Come" (FTT3 systems, to provide more scpclsticated data communcstons services. In particular, cased passi-re opt-'cal netwo As (ATM-PON) zze c_ great 15 nt-rst as an enabling technology for high-ozn-ldth low-

cos data network systems.

FIG. '3 illustrates a typical -POT system, in which an optical line terminal (OLT) 5 is linked with z -> plurality of optical network. units 1-1 to 1-3 via fiber 20 optic cables 2-1, -A, 2-3, and-4 be-'ng joined together with a passive optical star coupler 3. ITU-T Recommendation G. 983.i describes how to control such z plurality or ONUs ram a single termination circuit or The CLT. This ImTJ_T recommendation defines dedicated Physical 25 Layer Operation and Management (PLOWS) calls to allow the OLT to mcn-tor and control its subordinate ONUs. Using PLCA cells, the OLT supples the Onus with control

-2 parameters Or hem, and the ONUs send their info=aticn back to the OLT. There are a plurality of prederined messages encoded to represent various kinds of information to be sent by the OLT and ONUs.

5 Each ONU needs permission from the GLT when it transmits PLOWS cells to the OLT. This permission is.7=noTn as the "PLOWS grants," whlan are mapped in a PLOAM cell sent from Q:T to ONUs (this direction is referred to hereafter as "downstream'). Mach single PLCA g ant 10 permits can specific Get- to sand a single PLOW call to the OLD (tools Erection is re. er=2d to hereafter as Upstream. Thle PLC.M grants have unique --e- assi=ned values, which vary from OU to ONU. This nature 10WS the CLT to control the traffic of upstream BLOOM cells sent 15 from Onus on the same ATM-PON network, on an individual basis by manipulating BLOOM grant values zppropri-ely.

F,G. 24 shows, in a simpler fled way, how OLT mud ONUs transmit PLOWS cells. It is assumed the there are a plr21ity (n) o. ONUs (ONU 41 to Cab In' being [irked to 20 their local OrT. Small boxes PG1 to PGn represent PLOAM grants corresponding to ONU "1 to 03 In, respectively. As seen from FIG. 24, the OLT sends downstream PLOAM cells to issue a sequence of PLOAM grants PG1 to PGn. After a predetermined period, the ONUs respond to them by sending 25 upstream messages in the order of ONU #1, ONU #2,... 0NU-$n.

When the respcnolng ONU has no information to retain, it replies so by sending a PLOAM cell with a what is known as

-3 Moo message n code. In this way, the ONUs return a PLOWS call of some kind in response to each PLOPS grant even it they have no substantial information to send.

Downstream messages from the OLT may be divided into the following three categories in terms of how frequently they are sent: (1) messages to be sent regularly (hereafte- Cyclic messaged), (2) messages that should be delivered within z predetermined time limit (hereafter " ime-criical messages), and (3) messages to 10 be se=. on demand (hereafter "noncrtrical message").

Smtlar2r, upstream messages from Cans can be categorized -to the following three g' ours: () messages.o De sent regularly, ho) messages that have to be -e_urned to the GLT in rescnse to some particular messages sent therefrom, 15 and (3) messages to he sent at the discretion of Ones.

While the OLT can send those messages at anti theme as Beguiled, ONUs need to obtain a PLOAM grant in advance o transmission of each message.

In addition to the messages described above, the À 2Q A-PON system transport ATM cells containing user data, which are referred to hereafter as data cells. n As with the above-noted message cells (i.e., PLOAM cells) , Onus need permission from their local OLT bet ore sending those data cells. This permission is Known as "data grants. n 2o Similarly to PLOWS grants, data grafts are mapped on a certain field of a PLOAM cell for 2eive to ONUs at the

beginning of each frame, where different codes of data

-4 c,-ants cre assianed ro individual ONUs. he or communicate- with Gnus by sending and receiving message cells and data cells; while issuing such PLOAM grants and data grants as required.

5 In the meantime, the upstream signals from ONUs in An ATM-PON system are transported over separate optical transmission lines initially and combined by an critical star coupler before reaching the OFT. Since the OLT-ON distances may var-y -rom CNU to OMIT, the upstem -O transmission signals reach the GET after deren. delay tomes. The=e:o-e, woodcut appropriate delay compersaLier mechanisms, upstream cells sent by the CNUs in response to GLOAM grants or data grants could co lide with each otha= at the point -he e the signals are combined (i e., Optical 15 star coupler), thus disrupting the data to De reei-:e1 cy the OFT.

To world around the problem of uneven signal delvers, each ONU employs on equalization mechanism which intentionally adds an appropriate time delay to its 20 outgoing cells. More specifically, this additional delay equals the difference between each ONU's specific delay time and the maximum delay time observed in the AM-PCN system, so that all ONUs' round trip delays will be totally eqlzed. Such 03-speciic delay times are 25 measured when each 0 is activated as a node in the AM PON system. When. z new ONU is connected, the OLT stops issuing anti giants to temporarily clear out the upstream

f traffic, And instead Issues unassigned grants to indicate that a delay measurement process is under way. This time period es.aolished for delay measurement is referred to as the nranging window,' or simply a window" in the G.983.1 5 terminology. When such a window is open, the OLT causes the target ONU, which is now in the process of activation, To return 2 cer+zin message for delay measurement.

Observing a subsequent response from the target ONE-, to CLT measures -he cell delay time specific to that ONU. The 10 delay time meshed In this way 11 be used to z'c Upstream cell collisions during normal communication Operations. while e specifications of such AM-PON s-,stems

are provided -is the ITU-T recommendations G.98-.l' the 15 lack of some date led dern'1ions actually causes several problems in the following areas.

(11 Discarding of upstream messages In OND-s As pravously noted, each ONU needs to set permission, or PLOWS grants, from its local OLT, when it 20 transmits a P LOAM cell to the OLT. Upstream messages from ONUs, on the other hand, are not uniform, but include (a) messages to be sent regularly, () messages to be sent in response to the OLT's requests, and (c) messages to be sent at each ONU's discretion. This means that the OLT has 25 to send PLOAM grants in a timely manner, taking into consideraticn the rreuencles of those upstream messages.

If it is ur.abl to do SO the OLT is likely to miss some

-6 messages frcm the ONUs One posslbe method to satisfy the above requirement is the use of buffer storage in An ONU to temporarily hold a certain amount of messages, so tha. the 5 ONU w ll transmit all its pending messages without d scard-ng them as long as the OfT's GLOAM grant delays a-e w_thn a tolerable range However, this requires the COT to issue PLOWS grants that outfaces the production of messages in the ONUs. Otherwise, the butcher in an CMJ 13 would be gradually r; lied with the pe-.dig messages, some o. which should ce discarded in the er.a. Although some prad_cta'c7e messages such as these OI types (se and (I) might not be discarded as long as the OLT can send PLOPS grants constantly for hem, this method Enact work well 75 for unfired Stable messages of type (c).

(2) Discard-ng of upstream messages in OFT In general, OLT has burfer storage for tmpcrarily storing messages (PLOAM cells) received from ONUs for ater parsing arid processing. Some messages require a tons 20 processing time more than one cell interval. Also, other higher-priority tasks may interrupt the message handling process, keeping it in a suspended state for a while Ir^ such conditions continue, the OLT would be f arced to discard some pending messages due to the message burr er 25 overflow.

(3) Timing contentions among downstream messages Dcrsream messages can be classified into the

f -. following g ours: (a) cvcic messages, (b) tme-cri'cal messages, and (c) spontaneous, non-critical messages. The OLT must send those messages in a timely manner to satisfy their temporal requirements. Suppose here that there is a spontaneous message of type (c] conflicting with a temporally restricted message or type (a) or (b). If the OLT serves the former message first, for instance, the temporal requirement of the latter messages mar not always be ensured.

10 (43 Allcca.ion of upstream b&ndwath Tile Oft is rasponslole for ailccating necessary upstream -cQr,dw'd2s to CNOs on the network by Issuing data grants in an apprc-ita manner. However, no practical methods hays been proposed In this technical area.

15 (a) Pending grants when ranging window is open The OLT opens a ranging window by issuing unassigned grants only when measuring tine cell delay tame (or distances) of an GNU. Th-s window period occupies Mime slots for data calls and PLOTS cells, meaning that some 23 data grants and OILCAN grants ONUs should be queued while the window is open. On the other hand, the service provider must guarantee a specific user bandwidth for conformance to he service contract with their customers, In other words, the OLT is obliged to continuously cutout 25 a prescribed number of data grants per unit time, and for this reason, i. is necessary to ensure the issuance of data g ants ashen If. is interrupted. To date, however, no

- p_actical methods have teen proposed to satins_ this requirement. Another related problem is that the issuance OI data grants may be postponed until P1CAM grants a_e processed. 5 (6) Handling of queued message during window period The OhT has a first-in first-out (FIFO) buffer to store incoming messages from ONUs before parsing and processir. c the.. Delay measurement messages received from ONUs during a window period Me also entered to the FIND lo buffer. The OPT has to test whether those delay measurement messages have been returned property wth_n the predetermined w-ndow period, as well as filtering cut other kinds of messages received during that period.

On the cthe_ hand, the message hand'' er in the OLT lo processes incoming messages in the order of reception, reading out each from the FIFO buffer. That is, the processing o_ upstream messages is per_o=ed without reference to the reception time of each upstream cell.

This leads to a lack of synchronization between the 20 incoming messages and window period indicator, thus m&king it difficult for the OLT to test whether each delay measurement message has been returned properly within the window period.

(7) Vat oddity checking for delay measurement messages 25 In z anglug process, the OrT measures upstream call delay times by analyzing the phase of each received delay measurement message. In parallel with the

- 9 - f" mecswemnt, the OLT has to check whether the rcei-ved delay measurement message s correct or not. Previously-c OLTs, however, are unable to maX" this test correctly, since they perform the delay measurement and message handling processes asynchronously with each other for the same reason as described in the preceding item (6.

According to the present invention there is provided 15 an optical line terminal which controls one or more optical network units connected thereto, Comprising: reception means for receiving messages from the optical network units; message extraction means for extracting -,-alid messages from among the messages received by said reception 15 means, by filtering out such messages that Vindicate that the sending optical network units have no information to send; destination identification means for identifying the sending optical network units which are sourcing the valid messages extracted by said message extraction means; 20 message transmission granting means for producing message transmission permissions wh-ch permit the sending optica' network units identified by said destination identification means to transmit messages; and transmission means for transmitting the message transmission permissions produced 25 by said message transmission granting means to the optical network unit.

-10 Refeence -vi'1 now be made, by way cf example, tc the accompanying drawings, in which: FIG. i is z block diagram which shows ad OLT embodying the present invention; FIC-. 2 is block diagram which sucks Abbe ceta-ls of a cyclic message monitor USA its related elements shown 10 in FIC. 1; FIG. 3 is a timing diagram -which e,?'ai.s the - operation of the elements shown in FI&. 2, FIG. is a I' ocX die gram. wh- -A shcUs another implementation of the cyclic message mcnlto- in detail, 15 together with its related elements; FIG-. 5 is a timing diagram wn-ch explains the operation or the elements shown in FIG. 4; FIC-. 6 is z block diagram which shows the Weevils of a downstream message monitor and its related elements 20 shown in FIG. 1; FIG. 7 is a timing diagram which explains the operation of the elements shown in FIG. 6; FIG. 8 is a block diagram which shows the details of an upstream message monitor and its related elements 25 shown FIG. 1 in accor ce wi h an exigent of the present invention' FIG. is a timing diagram which explains the operation of the embodiment shown in FIG. 8;

- FIG. 10 ls a blcok diagrm wh-ch shows the detali_ of a buffer monitor and its related elements shown in FIG-.

1; FIG-. 11 is a timing diagram which explains the 5 operation of the elements shown in FIST. 10; FIG-. 12 is a block diagram which shows the details of a buffer monitor and its related elements shown in FIG. 1; FIG. 13 is a Block diagram which shows the details lo o- a message p=-'ori.y controller And -i s related elements shorn in FIG. 1; FIG. ii is a block diagram -h-,ch shows mob= detal 7 S 0 the message p=icrity controller and As related elements shown in FIG. 1; 1 FIG. 15 is a block diagram which shows the de ails of a data grant gen_ ato' and its related elements shown in FIG. 1; FIG. 16 is a liming diagram which explains the operation of the elements shown in F_C-. 1; 20 FIG. 17 is a block diagram which shows another implementation of the data grant generator in detail, together with its related elements; FIG. 'S is a timing diagram -which explains the operation of the elements shown in FIG. 17; 25 FIG. 19 is a block diagram which shows the details of a window controlle- and its related elements shown in FIG. 1;

FIG. TO is a timing diagram which e=lal,s the operation of the elements shown in FIG. 19; FIG. 2i is a block diagram which shows another implementation of the window controlled in detail, 5 together with its redated elements; FIG. 22 is a timing diagram which explains thy operation of the elements shown in FIG. 21; FIG. 23 is a diagram which b imply shows as. A-

ON transmission system; acid 10 FIG. 24 Is diagram -inch b=-fly shows =rOv cells exchanged between OLD and Oft.

Preferred embodiments If the present invention 15 will be described below wits deference to Figures i, 8 and 9. FIG. 1 is a block diagram of an cptical line terminal (OLT) where the present invention is embodied. In this preferred embodiment of FIG. 1, upstream cells sent SO from ONUs (not shown) ark race_ved by a upstream cell termination unit 5a. It separates PLOWS cells from cthes and supplies them to an upstream message buffer 6b in a controller 6, while providing the other calls to an upstream ATM call processor 5. The upstream ARM cell 25 processor 5b passes upstream ATM cells (upstream user cells) to ARM equipment (not shown) after applying a predetermined process to them.

-13 The controller 6 extracts messages out or the received upstream cells And applies relevant processes to them. It- also handles downstream traffic including insertion Of PLODS cells, etc. The ARM equipment provides 5 downstream ARM cells (downstream user cells) to a downstream ATM cell processor 5c. The downstream ATM c21] processor 5c then passes them to a multiplexer (MUK) 5d tarter applying a predetermined process to them. The May 5d multiplexes downstream ATM cells and Provo cells Into a lo s ngle stream in ibe time domain so as to t_ansm-,t them to Ohms in _ prescribed sequence.

- The controller o comprises the following functional units: a window controller 6a, an upstream message buffer 6b, an upstream message monitor 6c, lS buffe monitor 6d, a PLOWS grant generator Be, a data Arrant generator 6f, a cyclic message monitor 6g a downstream message monitor 6h, a PrOAM cell transmitter 6i, a downstream message generator 6j, a cyclic message b-frer ok, a time-critical mes sage buffer em, a non-crltlcal 20 message buffer en, a message priority controller 6p, mud an upstream message processor 6q, The functions o' these units will be briefly described as follows.

The window controller 6a executes processes related to the ranging window to be opened when activating 25 an ONU. The upstream message buffer 6b temporarily stores upstream cells supplied from the upstream cell termination unit 5a. The upstream message monito' 6c detects a message

-14 f -deniflaztian code of Pro message" con.a_ned r a received upstream message, and if "no messages is round, it extracts its PON ID to identify which GNU is sourcing the message. The upstream message monitor 6c also triggers 5 generation of PLOWS grants directed to such ONE's tha. have sent upstream messages other than "no message.

The Suffer monitor 6d commands the PLOPS grand generator Be to temporarily step sending PLOPS crafts, i-

the number of pend-'na messages accumulated in the upstream G message Buffer 6b exceeds a -redet=:ned threshold. The upstream message monitor 6c also monitors the messages - accumulated lo the upstream message buffer 6b, and i- z "to message" message rece-'vd r am any part cular Olin -s found, it then directs the P70AM grant generator Ge to l5 temporarily stop sending PLOWS grants to that ONTO The PLOWS grant generator Se produces PLOWS grin.

signals indicating permission of PLOAM cell transmission granted to specific ONUs. The PLOAM grant generator ce supplies these signals to the PLOAM cell transmitter oi 20 The data grant generator 6f, on the other hand, prcdces data grants indicating permission for the transmission of ATM cell that is granted to specific ONUs. It supplies such data grants to the PLOAM cell transmitter 6i Bere, the data slant aeneator 6f controls the frequency or data 25 grants for each ONU in such a way that the resulting usage of upstream belt slots will yield a desired user bandwidth allocation

- 1 The cyclic message monitor 6g causes the- PLOAlM grant generator Be to produce PLOAM grants so that ONUs coon send a class of messages produced on a regular basis.

The downstream message monitor 6h watches outgoing 5 downstream messages to detect a certain message requesting specific ONU to send back a response, as well as its relevant ONU ID which specifies the destination Out. When suck message is detected, the downstream message monitor 6h causes the GLOAM grant generator 6e to produce a PLOWS iO grant signal.

The PLOWS cell- transmitter 6i receives PLOWS - grants from the PLOAM grant genertcr 6e, Dada Fran s from the data g cant generator 6-, and outgo-ng messages from the priority controller Gp. It assembles PLOWS cells 15 ccn-aining those pieces of= norzLion, control 1 ing their sequence not to make them conflict with each other. The PLOAM cells produced as such are supplied to the MAX Ed So_ transmission.

The downstream message generator 6j produces 20 downstream messages addressed to individual ONUs. Those produced messages include cyclic messages, time-ritical messages, and noncritical messages, which are supplied to the cyclic message burfer 6k, time-critical message buS-e-

6m, And non-critcal message buffer en, respectively. The 25 cyclic message buffer dX accumulates cyclic messages selectively from among those produced by the downstream message generator 6j. The tme-cr-,tical message buffer em

-16 f accmulate_ me-citcal messages selectively from Gmor'S those produced bit the downstream message generator 6j. ha non-critic1 message buffer 6n accumulates non-critical messages selectively from among those produced by the 5 downstream message generator 6j.

The message priority controlled 6p reads cut messages from the cyclic message Purrer 6k, tme-cri ical message buffe- em, or non-critical message buffer En, depending On predetermined priority levels assigned to 0 them. The messages reed ou' in this way are then supplied the PtOM call transmitter 6i. When a plurality of - messages addressed to different ONUs have the same priority level, the message priority controller 5p coordinates the transmission of those messages so that al' 15 their recipients w-1- be treated evenly. Although not shown in PI&. 1, a p1=rality (n) of Onus #1 to in are assumed to be coupled to the OLT in this embodiment of the present invention.

The elements of the proposed OLT have beer briefly 20 described above, and the following section will now present their more detailed structures and operations.

Figures 2-7 and 10-22 and the related description

thereof present features o' OLTs which do not directly embody the invention. Nonetheless, the description and

figures concerned may be useful for an understanding of the invention described with reference to Figures 8 and 9 25 Referring first to FIG 2, the cyclic message monitor 6g and its related elements shown in FIG 1 will be described in more detail below.

FIG 2 is a detailed block diagram showing the cyclic message monitor 6g, PLOAM grant generator Be, and PLOAM cell transmitter 6i The cyclic message monitor 5g

-17 f comprises a p'u_zllty c' cyclic tr gger signal generators 20-l to 20-n. The PLOMM grant generator 6e comprises a plurality or PLOAM grant generators Se-l to 6e-n, which are coupled to the cyclic trisser signal generators 20-i 5 to 20-n, respectively.

The cyclic trigger signal gene' ators 20-I to 20-n produce trigger signals Tl to Tn. keeping pace with the -crsm-ssion of cyclic messages by the ONUs fl to In, r o Sped hi very, and the produced trigger signals are 1 suppled to the P;iM wart generators Se-l to 6e-n. This is possible because the OLT has prior information about at - 'chat in murals each OM Al to in produces cyclic messages.

--th such a knowledge, the OLD determines trigge- signal inter-ls of TV cyclic trigger signal generators 20-1 to 1 20-n. The PLOWS grant generators 6e-l o 6e-n produce BLOOM grant signals PC-l to PGn upon receipt of the trigger signals Tl to In from the cyclic trigger signal generators 20-l to 20-n. Their output signals PGl to POn are then supplied to the PLOWS cell transmitter 6i. The PLOAM cell TOO transmitter 6i sends those PLOAM grants to the multiplexer (MUX) 5.

Referring next to FIG. 3, the operation of the elements of FIG. 2 will be described below. FIG. 3 is a timing diagram which explains how the circuit of FIG. 2 25 work=. As seen from the trigger signal waveforms (A), (C), and (ad, the cyclic trigger signal generators 20-l to 20-n produce trigger signals Tl to Tn at the same integrals as

f --,8 tle Outs 1 to In produce heir cyclic messages. Note that the trigger signals Ti to Tn have difference phases so that they will not coincide with each other.

The cyclic trigger signal generator 20-1, o-

5 instance, produces trigger pulses T! as shown in (A) or FIG. 3. In response to each or them, the PLUM scant generator oe-1 supplies the PLOWS cell transmitter 6i widen a co_rescording PLOWS grant PC-1 as shown in ha) oF FIG. 3.

The other cyclic trigger signal generators 20-2 to 20-n 10 will germinate trigger pu_ses T2 to TO in the same way, causing their cor=spordlug PLOWS grant generators 6e-- to 6e-n to produce PLASM grants PC-2 to PGn accordingly. The PLOAM cell transmitter Si inserts those PLOWS grants PG1 t., PC-n Into PLOAM cells and sends them out to the ATM-PON 15 network.

n Figure 2, as - described above, =he OLm issues PLCAM grants in harmonization with cyclic messages, a class of messages that ONUs produce on a regular basis. The above-descriDed feature ensures tilt 20 the OLT can collect messages of this class completely.

Referring next to FIG. 4, another implementation or the cyclic message monitor 6g and its related elements will be explained in detail below. FIG. 4 is a detailed block diagram showing the cyclic message monitor 6g, PLOAM 25 grant generator 6e, and GLOAM cell transmitter 6i shown in FIG. 1. In contrast to the instance shown in FIG. I, the cyclic message monitor 6g has a di=rernt structure,

f - 1 9 comprlsing.-me,- 30 and z shirt register 31, instead of having plurality of cyclic trigger signal generators. In this alternate configuration, the tamer 30 generates trigger pulses at the same intervals as the Onus produces 5 cyclic messages. The shaft register 31 has multiple stages each or nic.h gives z phase shift equivalent to one message internal. Thus the shift registe= 31 successively shifts the trigger pulses supplied by the tamer 30, thereby p educing trigger signals T2 to Tn for the use in 10 the PLOWS grant generators 6e-1 to 6e- n. In response to the trigger signals T1 to In supplied from the shirt register 31, the PLODS grant generators on - 1 to 6e-n produce P7OAM grants EG1 to Fan, respectively. The produced PkOM grants PG1 to PC-n z=e then supplied o the S P,O cell -r=nsmitter 6i, which sends the grants to the mult pleaser (MUX) 5d.

The operation of the aboveescribed elements of Figure 4 will now be explained below. FIG. 5 is a timing diagram which explains the operation of the embodiment of FI&. 4.

20 The timer 30 generates trigger pulses T1 as shown in (A) of FIG. 5. The shift register 31 sucessi-vely shifts this trigger signal T1 'oy one message interval for each, thereby yielding delayed trigger signals T2 to Tn (see (B) to (by of FIG. 5). Those trigger signals T1 JO Tn are 25 directed to the PLOAM grant generators 6e-i to 6e-, respectively. That is, the tr-gger signal T2 is Relayed from the trigged signal T1 oy one message Interval, and

-20 - likwise, the other trigged signal T3 to Tn follow the original trigger signal Tl with the delay of two to (n-1) intervals, respectively. The PLOAM grant generators 6e-1 to 6e-n produce PLOWS grants PG1 to POn corresponding to 5 the trigger signals T1 to Tn supplied from the shift register 31, and feeds them to the PLOPS cell transmlLter 6i. The PECAN cell transmitte_ 6i inserts those PLOAM grants PO1 to PGn into PLOAM cells and sends them out to the ATM-PON network.

10 As An the pre-,ous example a- FIG. 2, the above-descrbed example of Figure 4 ensures that the OLT thoroughly collects cyclic messages produced by the ONUs on a regular basis. Also, it prevents BLOOM grants from causing congestion when they are loaded to lo PLOAM cells, because it can control the tine inerzls of trigger signals T1 to Tn more reliably.

Referring next to FIN-. 6, the following section will describe the details or the downstream message monitor 6h and other related elements shown in FIG. 1.

20 FIG. 6 is a block diagram showing the details or the downstream message monitor 6h, PLOAM grant generator Be, and PLOAM cell transmitter 6i explained in FIG. 1. As shown _n FIG. 6, the downstream message monitor 6h comprises the following functional blocks: cell 25 extractions units 41-1 to 41-n, message code detectors 42 1 to 42-n, and trigger signal generators 43-1 to 43-n.

As previously noted, the message priority -91 f ccutroller 6p reads out doT.ns' cam messages from the cyclic message

buffer 6X, time-cr tlc21 message buffer em, aid r;cn-critical message butler 6n. The downstream message monitor 6h checks those messages, And if any of them require the destlna.ion ONUs to return a response, it then generates relevant trigger signals T1 to Tn to trigger their correspcuding FLOAT Brat generators 6e-1 to 6e-n.

More spec-=ically, the cell extraction units 41-1 to 41-r' associated with the Opus 41 to in respectively, check the 10 OND ID field c' each outgoing downst_eem message so IS to

ax race 0 cells addressed to their associated ONUs.

For example, the first cell extraction unit 41-1 extracts such cells whose destination ONU ID agrees with that o tna GNU I'. With reference to each extracted messages 1 ider,ti'-lcaion code, the message code detectors 42-1 to 42-n aete-=ina whe her there is any downstream message that requires its destination ONU to return a response. _f such messages are found, they signal their corresponding trigger signal generators 43-1 to 43-n. The trigger sonar 20 generators 43-1 to 43-n provide their corresponding PLOAM grant generators 6e-1 to 6e- with trigger signals T1 to Tn. when the message coda detc+ors 42-1 to 42-n have detected that particular type of message codes mentioned above. The PLCAM grant generators 6e-1 to 6e-n produce 25 PrOAM grants PG1 to PGn in response to the trigger signals T1 to Tn supplied from the trigged signal generators 43-1 to 43-n, respectively. The PLCAM cell transmitter 6i

^'1 - - f supplles th- ML-X Id with the PLOWS grants PC-1 to PGn, after inserting them into PLOWS cells.

The operation of tile acove-desc__ elements of Figure 6 will now be explained below. FIG. 7 is a t_ming diagram 5 which explains the operation of the elements shown in FIT. 6. Suppose here that the downstream message monitor 6h receives a series of downstream messages as sno-wn in (A) c. FI&. 7, where each message contains An Off ID that identifies a specific Off, as we l as code that 10 indcc.es z spec-=ic Unction c the message. Then the cell extraction units 41- to 41-.1 extract their relevant cells (i.e., PLOAM calls having OiJ ID #1 to In, respectl-ely). The extracted cells are suppl ed to their corresponding message code detectors a2-1 to 42-n. T'e 7 cell extraction unit 41-1, for instance, will axtrac s such cells having ID 1 (i.e., cells addressed to ONU X1) and supplies them to the message code detector 42-1.

Examining each extracted message, the message code detectors 42-1 to 42-n he enm_ne -whether there is any 20 downstream message that requires its destination GNU.o return a response. If such messages are found, they signal to the trigger signal generators 43-1 to 43-r.. Consider that, in the example of FIG. 7, the code "01" denotes that the receiving ONU should respond. when this code 01" -s 9 found in the ceils supplied from the cell extraction units 41-1 to 41-n, the message code detectors 2-1 to 42-n request their correspond ng trigger signal generators 13-1

c 43-n to produce a ruler signal. The trigger signal generators 3-1 to 43n then produce trigger signals T1 to In as shown in (B) to (D) of FIG. 7, thereby activating the PLOWS grant generators 6e-1 to 6e-n. The PLOAM grant 5 generators 6e-1 to 6e-n produce PLOPS grants PGi to Pan ccrespondig to the trigger signals T1 to In supplied cm the trigger signal generators 43-1 o 43-n, and feeds them to the Floras cell transmitter 6i. The PLOPS cell t ansmitter See inserts those PLOAM grants POLL to sign inc 10 OILCAN cells mud ser,ds them out to the ATM-PON ntwc-.

It should be noted the, the downstream messages read out by the message priority controller 6p are also suppl-.ed directly to the PLOAM call trar, smitte_ 6i fc-

=mediate transmission as PLOWS cells. While some of those 15 messages derive PLEAD grants as a result o_ the above-

desc-lced processing o' tk dcwnstram message monitor 6h, SUCH Purim grants hay" a certain amount of time delay because of that processing tame. Accora-=gly the PLOWS cell transmitter 6 always sends out a message first and 20 then transmits its associated PLOAM grant. This sequence enables the receiving ONU to return its response message, using an upstream time slot specified by the subsequent PLOAM grant.

According to the above-described ex le o-Figure 6, 25 - downstream messages requiring response from ONUs will inltiza transmisscn of PrOAM rants.

Advantageously, the proposed OrT can recei-re response

-24- :'\ messages From Cab_ more rel-ably, since it is ensured that.

every such message is mme.diately accompanied by a PLOWS grant. Referring next to FIG. 8, Awe structure of the 5 upstream message buffer 6'D, upstream message monitor 6C, PLOW grant generator 6e, and PLOWS cell transmitter 6i ire Em. 1 cú pres:t irk 1 ha kirk in Ail Rev.

The upstream message monitor 6c comprises the fcllow-ng elements: cell extraction units -' to _l-n, 10 message code detectors 52- to 52-n, and trigger s-grcl generators 53-1 to 53-n. Upstream messages received from Opus Hi to In are supplied to the upstream message monitor Ec via the upstream message buyer 6b. when upstream messages other than "no message" have arrived, the 5 upstream message monitor 6c provides the relevant PLOWS grant generators 6e-1 to 6e-n with tr-gger signals T1 to Tn. This function is provided by the following structure.

Being associated with different ONUs =1 JO In, the cell extraction units 51-1 to 51-n extract massages having 20 relevant PON IDs from among those supplied by the upstream message butter 6b. For example, the first ce1 7 extraction unit 51-1 extracts such cells whose destination ONU IN is 41. The message code detectors Z-1 to 52-n test the codes contained in the upstream messages extracted by the cell 25 extraction units 51-1 to 51-n. When they are other than "no message, n the message code detectors 52-1 to 52 indicate so to their associated t igge_ signal ger.ertors

-25 . 53-1 to 53-. In response to finis, the tricker slgal generators 531 Lo 53-n produce trigger signals T1 to Tn.

and supply them to the PLOAM grant generators 6e-1 to 6e-n, respectively. 5 The PLOWS grant generators 6e-1 to 6e-n produce PLCAM grants PG1 to PGn upon receipt of the trigger signals T1 to TO -rom the trigger signal generators c3-1 to 53-n. Their outcomes PG1 to PGn are then supplied,c the ?LOAlv call transmitter 6i. The PLOWS cell transmitter 1-i Si supplies:re MUg ad with the PLOWS grants PC-l to P'-n, a=^ter inserting them into Phone cells.

The oper=4lon of the above-=escribed embodiment is as follows. F-C-. 9 is a timing diagram which shows the operation of the embodiment of FIG. 8. Suppose here that 15 the upstream message buffer 6b supplies z series c: upstream messages as shown in (A) of FIG. 9 where each message has a PON ID and a message identification code. In this situation, the cell extraction units 51-1 to 51-n extract calls having relevant PON IDs from among those 90 supplied by the upstream message buffer 6b. For example, the cell extraction unit 51-1 extracts such cells whose PON ID is #1. The message code detectors 52-1 to 52-n test the codes contained in the upstream messages extracted by the cell extraction units 51-1 to 51-n. when they are 25 other than Dno message, n the message code detectors 521 to 52-n direct the trigger signal generators 53-1 to 53-n to produce trigger signals T1 to Tn. respectively.

-26 f SUPPGSe her 2 that the code "03n means "no message, For instance. Then the message code detectors 52-1 to 52-n detect codes other than nO3, " thus causing the trigger signals T1 to mu to be produced as shown in 5 (B) to (D3 of FIG. 9. The PLOW grant generators 6e-1 to 6e-n now generate PLOAM grants PG1 to POn accordingly and feed them to the PLOAM cell transmitter 6i. The PLY cell transmitter 5i supplies the MUX 5d with those PLY. Grants PG1 to Ion, afte- inserting them into PLOWS cells.

10 It should be noted hat the PLOWS grant generators 6e-1 to 6e-r. raceme trigger signals T1 to TO not only from the upstream message monitor 6c (FIG. S), but rcm the cyclic massage monitor 6g (FIG. 2) and downstream message mc-iitor Eh (FIG. 6) as well. The presence G those 1 mul iple trigger son ces, however, might cause some ccgested situations. TO work around thus problem' the PLOAM grant ger era ors 6e-1 to 6e-n are designed to resolve overlapping trigger signals, if occurred, by giving z higher priority to the trigger signals from the 20 cyclic message monitor 6g (FIG. 2j and downstream message monitor 6h (FIG. 6), while discarding those from the upstream message monitor 6c (FIG. 8. This prioritization can be implemented as a simple logic circuit, which supplies the norm grant generators 6e-1 to 6e-n with the 25 logical products of the trigger signals of FIG. 8 and the inverted sum of the trigger signals of FIGS. 2 and 6. Even if some Trigger slgnas shown in FIG. 8 are discarded,

-27 f they car. ce asserted again since the next upstream ALONE cells corvey messages other than one message. n According to the aboYadescribed embodiment o' the invention, the proposed OLT- issues a POAlM grant when it 5 receives an upstream message other than "no message." This Feature mamas it pcssicle for the OLT to receive response messages from ONUs in a more _elable manner. The proposed approach, in turn, means that no PLOAM grants are to be sent in the case oF one message." finis policy w- 11 allow 1G other CNU-s to hay" more chances to transmit their messages, thus making them less likely JO discard their local inicrmaton. Referring next to FIG. in, the Following section w11 describe the detailed structure e' the upstream cell 15 t_rmina-on unit 5a, upstream message buffer 6'D; upstream message monitor 6c, PLOAM grant ganera_or 6e, and PLOTS cell transmitter 6i shown in FIG. 1. For simplicity, FIC-.

10 omits the buffs- monitor 6d. Also, ITS G. 10 selectively shows a functional block 50-1 corresponding to ONU #1.

20 Although FIG. 10 does not provide the details or functions blocks SQ-2 to 50-n for 05 #2 to #n, it will be appreciated that they have the same structure as the functional block 50-1. Under this assumption, the following explanation will focus on Rho unctor.ax block 25 Se-1. In the example of FIG. 10, the runcticnal block 59-i comprises: an upstream message prccssor oq-1, In

-28 upst em message buffer 6b-1, z buffer occupancy detec_o' oGa-l, an excess detector 60b-1, an upstream message monitor 6c-1, a PLOAM Giant generator 6e-1, z ccc message monitor 6g-1, and a downstream message monito- 6h 5 1. The upstream cel7 termination unit 5a receives upstream calls sent from ONUs and supplies them to the Ostrich message buffers db1 to 6b-n (partly not shown). The upstream message buffer 5b-1 in the functional block =0-1 stores ups-ream cells sent from the C>NU #1. Those cells 0 have been _oliected from among those supplied from the upstream cell termination =-it Fez, with rereeca to each cell's 2 PON ID field that indicates which Oat transmitted

it. The ups tam message processor 6q-1 reads out messages From the upstream message buffer 6-1 On a sequential 15 fashion, and i' executes an appropriate process, whi 7 e parsing the message identification code contained in each message. The buffer occupancy detector SDa-1 comprises an up/down counter (not shown) which is Incremented by one 20 when the upstream message buffer ob-1 recesses a new message and decramented by one when a stored message is read cut of the buffer 6b-1. The former event is indicated by the assertion of the buffers write enable signal NS, while the latter event is indicated by the assertion of I; the read request signal ORE. Through the above up/down counting operations, the resulting count values of the buffer occupancy detector 6Oa-1 will show the amount OI

-29 messages ac==muled in te upstream messag_ ouf-e' 6-1.

The excess detector 60b-i determines whether the comet value of the ouffer occupancy detector 60a-1 exceeds a predetermined threshold. If it exceeds the threshold, the 5 excess detector oOb-1 notifies the PLOWS gran_ generator 6e-1 of the excess.

The PLOAM grant generator 6e-1 responds to the excess notification -tom the excess detecto- 60b-1 by generating a = grin-, based solely Or the Trigger 10 signal supplied from the cyclic message monitc,- 6g-. Ague here that it neglects the like signals f_ci the downstream message mono or 6h-1 or upstream message monitor Bc-1. The Prone cell transmitter 6i __ceives the produced pica= grants PG1 to Pan from the functional blocks 53-1 to 50-n.

15 It supplies the MUX Ed Smith those POAM 5 antsy aver inserting them into PLOAM cells.

In the above structure, the GLOAM g ant generator 6e-1 -would exceptionally accept and process trigger signals from the cyclic message monitor 6g-1, even -r =n 20 excessive buffer occupancy level is observed. This 1= because the OrT must send PLOAM grants at predetermined intervals to receive cyclic messages from ONUs. In addition, the threshold given to the excess detector 60b-1 should be below the maximum capacity or the upstream 25 message buyer 6b-1.

Ire operation of t,e above-described elements of Figure 10 well How be explained below, assuming that the threshold

-30 . - ( given to the excess detector 6Ob-1 is "2 for illus.a,ive purposes. FIG. 11 is a aiming diagram which epllus the operation of the elements of FIG. 10. Suppose that the 5 upstream call termination unit 5a receives a species of upstream message as shown in (A) of FIG. 11, each. of which comprises a PON ID and a message identification code. The upstream message buffers in the functions blocks 50-1 to 50-n extract mud accumulate message cells carrying their r 10 relevant POT Ds. Take the functional block O-1,c example. The upstream message buffer 6b-1 in _his block - extracts such cells whose PON ID field exhibits "41. n

Each time a relevant cell is saved into the upstream message buffer 6b-1, the write enable signal AS 1- becomes active, or high level state, as shown in (B) of FIG. '1. This leads to the count value Of n 1 as shown in (D3 of PI&. 11, since the buffer occupancy dtecto 60a-1 per ores an upcouning operation. On the other hand, the upstream message processor 6q-1 asserts the read reddest 20 signal RIMS as shown in (C) of FIG. 11. This causes the buffer occupancy detector 60a-1 to perform a down-counting operation, thus making the count value return to n 0.

FIG. '1 further shows that the buffer occupancy detector 60a-1 increases its count value up to.3 after 25 accepting for' sprite enable pulses AS. At this stage, the excess detector 60b-1 sets the excess indication signal to high as shown 'F) of FIG. 11, slice the count value

A - 1 -

erce_s tire..reshold 2." Consecuen_y, the prop= Stunt generator 6e-1 neglects the activated output signal of the upstream message monitor 6c-4 (see (E) of FIG. it). That is, it temporarily suppresses the generation OI a PrC'4 5 grant as shown in (G) of FIG. 11. Note again that this suppression of PLOAM grants, however, does not apply to the requests from the cyclic message monitor '5g-1. after that, the upstream message processor '5q-1 produces another tend request signal BUS, which directs the bu=er 19 ccupncy detector 50a-1 to decrease its count U27 ue to 2." Then the excess detector aOb-1 negates the excess ind2icn signal to low, enabling the PLOWS grant generator Ge-1 to escort issuing GLOAM grants..

According to the abce-described example of 1 Figure 10, tine proposed OLT has a buffer occupancy detector 60a-1 to oosere how many messages ar-

acumlated fir. the upstream message buffer b-1. Anew the buffer occupancy, level exceeds a predeter^-!ned threshold, the excess detector 60-1 notifies the PLODS arant 20 generator 6e-1 of the event. As z result, the PLOAM grant generator 6e-l suppresses further issuance o_ PLOWS grants, unless it is requested by the cyclic message manitor 6g-1.

Controll-ng the upstream message traf ic in this way, the present invention prevents the upstream message buffer 6b 25 1 from overfcwing, while stir' accepting acyclic messages from ONUs.

Referring next to FIG. 12, the following section

-32 f will p-ov-'de the details of the upstream cell terir,aton unit a, ups_*ea message buffer 6b, upstream message monitor 6c, PLOAM grant generator, 6e, and PLOWS cell tansmlLte, 6i shown in FIG, 1. Since the structure snows in this FIG. is similar to that of FIG. 10, the following e explanation will focus on its distinctive elements, while affixing like reference numerals to like elements.

The example of FIC. 12 differs from that of FIG. 10 in that a nreshold memory cOc-1 is newly employed, 1Q Virile other elements are the same as those explained in FIC-. 10. This threshold memory SOc-1 stores the threshold value that _he excess detector 60b-1 uses for comparison.

The threshold value "<s 'ceded to th- excess detector 60b-1 on aemmud. The value can also be updated =_om an ex-nal 15 sorbic=. The abcvedescrbed structural arrangement makes it pcssihl" to reconfigure the fur ctional block 50-1 with the mast appropriate threshold value depending on the maximum capacity of the upstream message buffer 6-1 and 20 other related system environments (e.g., the number of supported ONUs). That is, an optimal control can be achieved by tuning the threshold in accordance with the actual hardware configuration and system environment.

Referring next JO FIG. 13, the following section 25 will provide the detailed structure and operation of the cyclic message buf,e 6k, timecritical message buffer em, non-critical message buffer en, Lana message priority

-33 ' - onrolle- 6p shown in FIG. 1. As seen from FIG. 13, tore message priority controller 6p comprises a message buffer monitor 70 and a read operation controller 71.

The message buffers 6k, 6m, and en are random access memories (Proms) each cor.figred to function as FIFO storage with a write and reed address pointers (not shown).

The two pointers point at the same address while no messages are stored. Each message cuffs accepts new message en fries by culling earn message &5 advancing the 10 write ad-ess poir,t_r accordingly. other they cutout messages, the read address points,- -s arced in accrdanc_ with r,e relevant data sizes.

The message buffer monitor 70 figures our how much message data is accumulated i' each message buffer By lo calculating the distance 'between the sprite address and read address of each buffer. Such status information, referred JO as the buffer states info=zticn 41 to 3, is provided To the read operation controller 7i. Tile read operation controller 71 uses those buffed status 20 informalcr. 41 to #3 to supply the message buffers with read permissions Al to IS according to some predetermined algorithms. More specifically, the read operation . controller 71 revokes the read permissions $2 and $3 (i.e., disables reading operations), when the read permission $1 25 is active i. e., messages are available in the cyclic message buf er 6k). It also revokes the read permission $3 when the read permission t' is active. This means that the

3a messages in the Thee buffers are prioritized in the order of cycl-c messages (highest), time-crltic21 messages (medium), aid non-cri.lcal messages (lowest), and the read operation controller 71 reads them out of the buffers On 5 the basis of their reseti-v-e priority levels.

The operation of he abcve-described elements of Fi e 13 is as follows. Suppose here that the cyclic message buster 6k currently has two messages, and that the tme-crltcal message buffer 6m and non-_ritical message buster En have 10 one message in each. These three message buire-s 6X, Em, and Gr output address Or values #1 to =3 depending on Heir r_spectl-v-e storage conditions. The message buffed monitor 70 Slgu=es out Ha amount of messages accumula_ed in each message buffer t calculating the distance between 15 the write address and read address of each buffer. In the present example, the address pointer values 41 from the cyclic message buffer 6k indicate that the amount of messages is currently "2, n while the address pointer values 42 and 43 from the time-critical message buffer 6m 20 and non-citcal message buffer on indicate that the amount of messages is 1. The message buffer monitor 70 then supplies the read operation controller 71 with the above information, i.e., the buffer status information f1 to #3. The read operation controller 71 uses the buffer 25 status information 1 to #3 to produce read permissions #1 to 43 for-the respective message buffers.

In Ike present context, the buffer status

-35 information #1 exhibis the Yzlue o n2," whch s-ngges.s that some highest- icrity messages are available. The read operation controller 71 thus grants a read permission #1 to ths cyclic message buffer 6k, whirs giving no 5 permission JO the other message buffers 61m and 6n.

Consequently, one message is read out o' the cyclic message bus er 6k and sent to the PLOAM cell transmitter 6-. It is then transmitted as a do'nstrem message to its destination ONE thrc-gn the MUX Ad.

10 Now that one message has been sent Out, the address painter values =1 indicates a rew -eat address (i.e., incremented by one). This causes the message buffer mcnitc- 7C JO change the Of cr status in-crat-on #1 From D2 to "1. This 'cufer status lnomation forever, lo still indicates the presence of a message W-, Ah the highest-prio.i -I. Therefore, the read operation controller 71 continuously asserts the read permission =1, Chile keening the other read permissions #2 and 43 negated. The above process is thus repeat_ed, and the second message in 20 the cyclic message buffer 6k is sent out. since all the pending messages have gone out of the cyclic message buffer 6k, the read operation controller 71 now grants read permission #' to the time-critical message buffer 6m to initiate message transmission. When the message in the 25 time-critical message buffer Cm is transmitted, a read permission #3 is granted to the ron-crtical message buffer en, initiating transmission or lowest-level

-36 message s.

The above-desribed sequence may, however, be interrupted by -rite operations. n that case, the address poluter values should be updated after the ongoing reading operation is finished. The message buffer monitor 70 then should recalculate z new set of buffer statics information #1 to #3 according to the updated address pointer values #1 to 43, and the read operation controller 71 controls the buffer read operations wi h rafeerce to the latest 10 bursar status information #1 to 3.

According.o Ace bcYe-described example of Figure 13, the controller 6 gives successively ower priority levels to cyclic massages, ime-c_lical messages, and non-cr=ical messages, and the messages ma queued sand is transmitted to ONUs according to their priority iev_ls.

Therefore, the proposed OLT can t_zrsmit cyclic messages and time-critica messages in a more reliable fashion, even in a cor.cested condition where many transmission requests are waiting services.

20 Referring next to FIG.- 14, a more detailed configuration of the example or F G. 13 will be described below. FIG-. 14 shows the details of the non critical message buffer 6n, message buffer monitor 70 and reed operation controller 71 shown in FIG. 13. Her-, a 25 plurality of noncritical message bursars 6n-1 to 6n-n store non-critical messages addressed the Cars 41 to in, respectively. Supplied with address pointer values #1 to

- 37 in foam the non-c=-tisal message buffers 6n-i to 6n-, he message buffer monitc, 70 calculates the distance between the w mite and read harasses o- each buffer. This -yields buffer status information. #1 to An each representing the 5 number of messages stoker in the corresponding message buffer. With reverence to this information, the reed operation controller 71 arranges the orders of outgoing messages to be read out of the non-critical message suffers 6n-1 to nun, so that all Ores wilt receive 10 messages ecz',ly.

The operation of Thea zbove-desc-ibed example is as ollcs. ass-.e hare that the non-crltlcal message buffers Gn-1 r- 6-n a e 71 empty. In th-s situalon, consider that ho non-critical messages di acted to ONU #1 lo and ONU- #5 are sup lad. These messages are stored into the non-critca message buffer En-1 and non-critical message buffer 6n-5. The message buffer monitor 70 now outputs Ale as the value of buffer status intcrmation 41, because the address pointer values #' indicates that its 20 relevant pointer distant is "1, n The message buffer monitor 70 does the same for the address pointer values 45 and buffer status information #5.

The read operation controller 71 has an integral modulo-n counter to test whether the buffer status 25 information corresponding to its count value is zero. That is, the counter values is used as an Incremental index in scanning the buirer status information $1 to in,

-38 permitting the read operation controlle- 71 too reach z valid instance that exhibits a non-zero vet ue. The counter value is held f there is no valid (non-zero) buffer status information. If any valid buFfer status information 5 is found, the read operation controller 71 grants a read permission to the relevant non-critical message buffet, thereby ln-tiating a read access to that buffer. It also stops the counter until the next scheduled time slot comes.

The read operat7cn controller 7i repeats the above actions 7 3 at every message transmission timing, -nc7ement7o the mculo-n counter. The count value ranges from n 1 to nnn and it returns to "1 aver reaching en."

In -he preser,t example, the read opera ion controller 71 operates its integral counter since there 15 are valid instances of buffer status information. First, when the counter indicates 1, n it grants a reed permission #1 to the firs' non-critical message buffer 6n-

1. Consequently, the non-crirical message buffer 6n-1 outputs its entry to the PLCw cell transmitter 6i. A few 20 cycles later, the counter reaches the value of ''5 n and causes the non-critical message buffer 6n- 5 (not shown) to output another message to the PLOAM eel' transmitter 6i.

The counter stops at n since no messages are stored in other message buffers 6n-6 to 6-n. rAhen a new message 25 comes, the counter resumes From 6" to continue scanning the buffers.

According to the above-described example of

:_ -3S

Figure 14, the proposed read cperatlon controller 71 searches message buffs ers to find a next message to send.

This search is designed to begin with a non-criti-al message buffer which comes next to the previous one. It is therefore possible to evenly use all non-critical message buffers disposed in the OLD..

The above example of FIG. 14 has assumed that the non-c_itical messages include some- different Girds o-

messages, but nova the same pr-ority level. I- the case lo that those messages srou'd be ha-died WE th Dot f.=rent priority Ala, the rev-ous exa,mple exc1 dined n G - 13 would acre. That is, the non-c=itical message but e would be d vet Add into as many sub-buffers as the p-lo-ity levels. 15 Furthe', the above ensample o' FIG. 1. has illustrated a structure related to rcn-cri: - cal messages.

This structure, however, does not apply to cyclic messages or timecritical messages, because the messages of these classes require transmission at a predetermined time or 20 within a certain time limit. Instead, those messages are written into message buffers after making necessary arbitration. Referring next to FIG. 15, the following section ill now present the details of the data grant generator 25 6f mad GLOAM cell transmitter 6i shown in FIG. 1.

The data g ant generator 6+ comprises: a modulo-n counter 80, z grant table 81, a grant FIFO 82, and a

-do - buffer ''all detector 84. Being enabled when a -rime starts, the modulo-n counter 80 increases its count value at the rate of a given reference clock, and returns to the initial value "1 aster reaching the maximum count nO.

5 The mcdulo-n counter 80 is stopped by a buffs -full signal provided by the buffer full detector 84, and holds its currant value until the nextframe starts. The grant table 81 stores data grant values corresponding to addresses 1 to a. The data grants ha-'e unique valves that Are 1C previcus-y assigned to individual ONUs to idan -fly whici-

ON's are Emitted to send an upstream cell. The modlo-n - counter 80 provides the grant table 81 with -ts count value as the addressing information For retrieving a specific date grant value.

15The allocation or data grants are determined in accordance with the allocation of user bandwidth resources Which ace shared by the ONUs or a network. Thing' or an A-PON system serving tenures ONUs #1 to #3, for instance, and assume that they share the upstream bandwidth in the 20 ratio of 1:2:3. In this case, one sixth of data grant entries stored in the grant table 81 are allocated to the first ONU #1, two sixth of them to the second ONU #2, and three sixths of them to the third ONU #3.

Referring back to I&. 15, the grant FIFO 82 is 25 composed or two FIFO memories, each capable of storing up to 53 data entries line two memories are alternately used in reading and writing modes. That is, when one FIFO

fit\' memory accp Us write dam a, the other FIFO memory outputs read Dada. When all the entries are read or written, ohs two FISr'O memories alternate their roles with each other.

The buffer full detector 84 drives its output to high when 5 the grant FIFO 82 is filled up with 53 data entries, meaning to all grants for one frame are ready. Tan PLANAR cell trm.smitter 6i receives data grants from the grant STIFF S2, and inserts them into 2 PLODS c211. The rssultaT,.t PLOWS cell is supplied to the MAX 5d ( not shown.

10 Referring next to FIG. lo, the ol3mw-_g section will describe the operation of the acolyte elements G. Figure 15.

FIG. lo is a timing diagram which e"xplains the ope.zticn of the elements of via. 15. Frame Bite, signal (A) indlcat =s the beginning of each single -rime.

15 When this frame - nterval signal becomes high, it r_ti.es the pccunting operation of the modulo-n counter 80 on synchronization with the reference clock (B). The result&=" modio-n connte output (C3 is fed to the C_a?t table 81.

20 As previously described, the grant tabs- 81 stores data grant values, and they are reed out according to its address input being driven by the modulo-n counter 8C Bird supplied to the grant FIFO 82 (see (F) of FIG. 16. The grant FIFO 82 employs two FIFO memories. One accepts write 25 data from the grant table 81, while the other outputs data grants that have previously been written (see (G) of FIC-.

1). The date grants read out of this grant FIFO 82 are

-4- f',' fed to to" PLCAM cell transmitter 6i. The buffer full detector 84 counts the data grant entries to be Written into the grant FIFO 82 (see (D) of FIG. 16). If this count reaches "53,' the buffer -full detector 84 drives its output to,high (sea (I) of FIG. 16), making the modulo-n counter 80 stop tempcr==ily (see (C) of FIG. 16. This sonata continues until the frame intermural signal becomes high to indicate the beginning of a new frame. The modulo-

n cotter 80 then resumes the up-counting operation. linen 10 the cdulo-n counter 80 teaches its maximum limit "n, n its CQrn t value will return to "1 n and repeats the abcve-

described operations. Each time 2 PLOAM cell is issued, the grant FIFE 82 outputs data grants 2nd supplies them to the FLOAT cell tra-.smitter 6' one by one (see (hi of FIG. -- 16). PLOPS cells zn carry 53 grants per frame, meaning that the PLOAM call transmitter 6- transmits to the ATM-

PON system up to 53 data grants For each frame.

As previously noted, the Dada grants stored in the cram table 81 have Unique -values assigned to different 20 OMUs for identification purposes. Preferably, each ONU's share or data grants is proportional to the allocation oF user Width to that ONU, That is, the proposed CLT issues data grants to each ONU in accordance with the user bandwidth that is previously allocated to it. Further, if 2o necessary, the user bandwid4_n allocation to ONUs may be charged by altering the entries of the grant table 81.

According to the above-described example of

Al - - Fi gore 15, the proposed OUT allocates the desired use, bandwidth to each 0M by arranging the contents of the grant table 81. It is also possible to change the allocation as required.

5 Reserving next to FIG. 17, the following section will explain another detailed structure o the data grant generator 6 shown in FIG. 1.

FIG. 17 first 5CWS '. hat a fast mud slow clock signals are supplied to a selector 90. It selects the east 10 clock signal when the output of an AND gate 100 is high, and the slow clock si goal when the same is low. The selected clocX signal is supplied to a modulo-n counter pi.

In synchronizaticn filth this supplied clocX signal, the mcdulo-n counter 91 performs up-courting Aerations And : sends its Resultant count value to a Gerard table 92. anon the modu'o-n counter 91 has reached its maximum lint "n,.

its count -ralue will return to its initial -value "1 and repeats the above-described operations.

The grant table g2 stores a plurality In) of 20 addressable entries, from 41 to In, each of which holds a single data grant. When this grant table 92 shows an invalid data grant, an. invalid grant identification unit 93 outputs a high. This could happen when, for example, the data grant is not associated with any existing Onus.

2o While the grant table 92 supplies data grant values to 2 grant VITO 9S, the write operation to this grant FIFO is disabled by an AND gate 9 hen both the

f invalid g ant -denlcalon unit C3 and sate loo output high level signals. The grant F'FO 95 is actually comprises two FIFO memories, each being capable o.

bu,ernc 53 data entries. The two FIFO memories alternately operate in read mode or in write mode.

liven the graft FI=O 95 in write mode has accepted 53 Dada grunt entries, a buffer full detector 96 indicates _h_s gaffe- Full status by cutputing a high. The output o- an OR gave 97 goes high in the case hat an interrupt _O signal s produced, or the bufar fall detects so luoicates a buffer full status. This causes the modu7 on - ccuntr 91 to step its up- counting coeration fo- a while.

3n AND gate 99 receives the output of= an inverse' 98 and the Interrupt signal, arid it produces a trig:. level 1_ signal when both outputs are high. This means that the AND gate 99 sets its output to;-iah when an. interrupt is active but the grant FIFO 95 still has some space. When the output or the -D gate 9g is nigh, an up/down counte-

112 counts up at the rate of the clock signal selected by 25 the selector 90. I' also counts down at the same rate when the output of the AND gate 100 is high. A not-empty status detector ill sets its output Lo high when the up/do'n ccuntr 112 exhibits non-zero count values. The aforementioned AND gate 100 outputs a high when the 25 Outputs cr the investor 110 and not-empty status detector 111 are both high. This means that the AND gate 100 asserts its output slsna 1l there is no interrupt and the

-45 , /dcr counter 1'2 exhibits a non-zeo count value.

The abcve-described example of Figure 17 will operate as follows. FIG. is is -a timing diagram which explains the operation of the elements shown in FIG. 17. As shown in 5 (F) or FIG. 18, the AND sate 100 outputs a low in normal conditions, where there is no interrupt. Therefore, "he selector 90 chooses a slow clack signal (or normal clock signal) for the use in the modulo-n counter 91 as showing in (Al a- Flu. 18. The modulo-n counter 91 counts up at the 10 -ate of this slow clock signal and looks up;- grant table 9 as shown in (3) fir- F'&. 18. The scant table 99 - thus rear-' eyes a data grant value from the spec-isd address and supples it to the invalid grant identification unit g3 and grant VITO 95. I- the ratrlevad 15 date grant value is found to be inv-alld, the invalid Bray ldenlfication Malt 93 outputs a high, And this makes the YD gate 94 assert its output if the output of the AND gate 100 -s also -_gh. -n the present example, the ADD sate o4 outputs a high because the AND sate 100 is 20 deactivated by a low output signal of the not-empty status detector 111. As a real of the above, all data grants read out of the grant table 92 are written into the grant FIFO 95. Those data grants are then supplied from the grani FIFO 95 to the MUSE 5d _ough the PLOAM cell 25 transmitter 6i, which inserts them into do'nsream PLOAM cells. rpcse that an interrupt has occurred in the

-46 abov situation, as shown iA the middle part o FG. 1S, where the interrupt signal (C) goes high. Circe this interrupt signal activates the OR gate 97, the modulo-n counter 51 should stop -'ts up-counting operation for a 5 moment as snown in (B3 of FIG. 18. The interrupt signal also causes the ^ gate 99 to output 2 high, which enable the up/down counter 112 to count up at the rate OI the clock signal supplied from the selector 90 as shown- in (D) o- FIG. 18. The up/cown counter 112 continues fits up 10 cot ope_2t7cn until the Interrupt signal is negated, thesis inica.ir.g 'row many data grants have been suspender by th_ interrupt. The no7--empty status detector 111 detects such ^=^or-zro count values of he up/down counter 112, and causes its output (E) to go high. This h- ah ie-e' 15 signal, together with tile high-level output of the inverted 110 (-.e., negation c- the interrupt signal), will then cause the END gate 100 to output a high as shown in (F3 of FIG. 13. "acacrdlugly, the selector 90 selects the east clock signal and supplies it to the modulo-A-^ 20 courter 91 as shown. in (a) of -FIG. 18. Tee moduo-n counter 91 now Begins to count U? at a faster rate, thus accelerating the write operation of data grants to the grant FIFO 95.

Oh_ output of the ADD gate 100 is also supplied to 25 the AND gate S4, meaning that any invalid data grant from the grant table 92 -could stop the Write operation to the slant FIFO 95. As a result, data grants can be written

-47 f into the grant VITO 9' more quickly, skipping invalid ones, if any.

filth the high state of the AMID Data lQG's output, the up/down counter 112 switches its operation mode from 5 up-countng to dcr.-counting, mad the count value now begins to decrease as the clock signal is entered. When the count value or the up/down counter 112 reaches zero, the notempty stat Is detector 111 recogr.zes this and changes its outpu to low. This makes the cutup of the 10 AND cat_ 100 co low, and thus ha seletcr 90 selects to_ slow clock ain. Consequently, the modulo-r courter al regains i s Norma' counting operation mode, and the grant FIFO 95 begins to receive any grants including invalid ones. 15 Acocrr7ng to the aDovedescrited example of Figure 17, the co.roler deals with data grants suspended by an -nterrupt, through the process on co7'nting the number of suspended data grants daring the interrupt, and writing them (except for invalid grants) at a nigher 20 rate after the interrupt is negated. This mechanism enables the OLT to send necessary data grants to ONGs in a more reliable manner. Although, in the above embodiment, the feast clock signal is used only within a 'limited tame period when the suspended data grants zze being 25 Transferred, this period may be extended so that more data grants will be sent out at a higher clock rate.

Referring next to FIG. 'g, the following section

-48 w-.ll explain The st_acure and operaticn O=- the ustremm cell termnzLion unit 'a, window controller 6a, upstream message buffer 6'D, and upstream message processor 6g shown in FIG. 1.

As seen from FIG. 19, the w-ndow controller 62 comprises a window cpenng message ger.arto- 120, and a window closing message generator ill. At the rising edge of a window signal, the window per.ing message gener--tor 120 produces a pseudo message h-S to open a window, and 10 then writes 1- to the upstream message buffer 6. The term pseudo messaged ruses to messages which are not - received Ercm Onus, Out Internally produced by the OLT.

The window closing message generator 191, or the other hand, produces and writes another pseudo message 'wee into 15 the upstream message buffer 6b, defecting the spelling edge of the w ndow s_gna. Are upstream message processor 6g reads out those messages sequentially from the upstream message buffer 6b and executes specified processes for them. 20 The operation c1 the above-described example of Figure 19 will now be explained below. FIG. 20 is a timing diagram which expla-,ns the operation of the example or FIG. 19.

The upstream cell termination unit 5a extracts upstream messages (Cl from upstream calls (By received from ONUs at 25 cell intervals (A). The extracted messages are supplied to the upstream message buffer 6. In the example of FIG. 20, the upstream Cells (B) include data Chablis "Do And message

_dC cells fly' JO "M6 n There upstream cell rminctcn unit 5a extracts the latter cells and supplies them to the upstream message buffer 6b as shown in (C) of FIG. 20.

It is assumed here that a new ONU is attempting to join the network and the controller 6 has set the window slink (D) to high, accordingly. This iow-to-high trans__ion of the window signal directs the window opening massage ger,erator 120 to produce a pseudo message n-Ns (E3 to open a -=n. ging window. This message "WS is then 10 wr_ten 'Ned O the upst__zm message u_fe= 6. when -he -window s_gral (D) goes low, the w-'ndow closing message - generator 11 produces and writes a pseudo message "TEE" (F) to the upstream message buffer 6b. One above write operation c pseudo. assages is performed curing an Idle 1^ period avall=Dl i? each cell lnte1 (A', not to con-l_c with he all lug OI other messages.

In this way, the received upstream messages "1' to nM6n and the produced pseudo messages BUST and I'm" zze put into the upstream message buffer 6b, Tk upstream 20 message p ocesscr 6q sequentially reads them out of the buffer 6b and executes an appropriate process for each of them. Here, the upstream message processor 6q makes access to the upstream message buffer 6b when it has processed the previous message and is now ready to accept the next 25 one. Since the required processing time may dirIer from message to message, the read access to the upstream message buffer fib is not e,-essarly synchronized with the

- JO -

- phase o- interval o upstream cells illustrated en JOG) of FIG. 20.

Again, the window opening and window closing pseudo messages "NS" and nape" indicate the beginning and 5 end of a window period. TO those pseudo messages are received From the upstream message buffer D, then the upstream message processor 5q determines whether the OLT has proerlreceived delay measurement messages from Ogres during the w_ndoT Period. uppcse, for example' that the l0 message n4 mmorg tee -upstream messages (C3 is not a delay measurement message, cut is brother X-nd o_ message that happened to be irnsmted erroneously Turing the window period. In this case, the upstream message processor 6 would recognize he next message noun as a 15 valid delay measurement messages while neglecting the message 1'Ma n According to the aboQ-described ensample of Figure l9, - the Tart ndow controller 6a p_od-ces window opening and window closing pseudo messages at he 20 rising and falling edges to explicitly indicate the beginning and end o. a.window period, and it puts those pseudo messages into the upstream message buffer 6b. As a result, the upstream message processor 6c can distinguish delay measurement messages clearly from other messages.

25 This feature - enables mere reliable delay measurement. Referring lastly to FIGS. 21 and Z2, t'ne Followlg

- c; -

f.' section will present nG her structure and its operation oF the upstream cell termination urn' 5c, window conrcl'er 6a, upstream message burred 6b, and upstream message p ocssor 6q previously explained in FIG. 1.

5 As seen from the block diagram G" FIG. 21, the window controller 6a comprises a window opening message generator 120, a window closing message generator 121, and a delay t_me measurement urn' 123. Further, the upstream message processor c- explained in FIG. 1 now appears in 10 FIN-. 21 as An stem message processor 122.

w-ndo-.; opening message generator 120 produces a pseudo message "WEn indicating the start of a window pe'-o:pcn detect On of assertion of the window signal.

This pseuc message nWS is put into the upstream message 15 buffer 6'D. The.v_=dow closing message generator 121, on the other hand, podsces and writes another pseudo message "-my to the upstrsm message buffer fib, detecting the railing edge o the window signal. The pseudo message "DEN indicates the end o- the window period. Further, the delay 20 time measurement unit,3 measures delay times of upstream cells, inc'udlg DOLL data and message cells, received and supplied by the upstream cell termination unit 5a. Are measured delay time of each message is Add to the upstream message buffer 6-D. The upstream message buffer 6b saves 25 this delay time data when an upstream message is received from the upstream call termination unit 5a. This write access is performed du_lug an idle period, not to ccn-lict

- - t, -

with the writing of messages.

The -:cst-eam message processor 122 reads out those messages sequentially from the upstream message buffer 6b &-d executes specified processes for them. Further, when 5 window opening pseudo message "WS n is encountered, the upstream message processor 122 retrieves a delay measurement message and extracts its measured delay time within the window period, which begins with the window opening pseudo massage BOWS" and ends with the 10 ccr-espording window closing pseudo message Owe 29 rarity t;-,at lncrma ion, the upstream message processor 122 - calculates the deity tame of the newly Joined Cat.

We cer-ticn c- tha above-descrbed example of FI'-. -1 -Hill now be explained below, with reference to a 15 timing dlag_m o- FIG. 22. The upstream cell termination unit 5z et-acts upstream messages (C) from upstream cells (B3 received From Coos at call intervals (A3. The extracted messages (C) are supplied to the upstream message buf_=r oD. The delay time measurement unit 123 20 measures delay t mes of these cells and sends the measured delay tame cats (G) to the upstream message buffer 6. In the example of FIG. 22, the delay tine data (G) include: measured delay times DL-D of data cells, and measured delay times Do and DLMS or messages M4 and M5. The 2c upstream message buffer 6 D saves such delay tame data when the corrs?ond'nc upstream messages are received from the upstream cell termination 3'n; t 5a.

-53 .' The upstream message processc. 17Z retrieves those messages from the upstream message buffer 6'D sequentially and executes appropriate processes for them. If a window opening pseudo message "WS" is encountered, the upstream 5 message processor 122 _nterprats it as the beginning o, a window- period and thus tries to detent a relevant delay measurement message. This message should be found among - tr,e subsequent messages befc=e detecting a window closing pseudo message no- which indicates the end of the winch 10 period.

Supposes for Unzip a, that the message "My among the upstream messages C) is an irrelevant message that happened Lo be transmitted erroneously during the window period. In he example cat FIG. 22, the upstream message 15 processor 12 neglects this message nM4, 7 and instead, -'t recognizes he next message l'M5" as a valid delay measurement message. Slice this message "Mo" is immediately followed b-y the measured delay time D1M5, the upst-aam message processor 122 tares it and calculates the 20 delay time value of the newly activated ONU.

To summarize the above-descibed example cf FIGS. 21 and 92, the window controller 6a writes measured delay times into the Upstream message buffer 6-D, together with received upstream messages. When a particular ONU 25 returns a delay measurement message, the upstream message processor 1 72 can locate this message between a window opening pseudo message and a WE ndow closing pseudo message.

-54 f, Alsc, the message is Comedy at_ly followed b its relevant delay time value, which was measured and stored in the upstream message suffer 6b. This allows the ups-r=m message processor 122 to calculate the upstream delay time 5 of the ONU in a more accurate fashion.

The 2Do-e sections have described several preferred emhodments of the present invention, ilustatig In Men optical line terminal that controls Gnus wit:- more reliable tc'=Tliques to address 10 various pros ems in conl-ollng ATM-ON systems. Whom tec.miues wil'' nc ce shad as Follows.

C- Rag to an em'ocdi,er.t of - e presort invention, there is provided con OLT which controls o'.la or more ONUs connected thereto, And advantageously, th-s OLT 15 f,nct_ors AS follows..3m U Scream cell te=minaton unit rece--es messages rcm the Onus. message code detector extracts valid messages Mom among the messages received by the upstream call termination unit, through a process c- filtering out such messages that indicate the_ the 20 sending Ones have no information to send. A cell extraction unit identifies the sending ONUs which are sourcing the valid messages extracted by the message code detector. A BLOOM grant generator produces message transmission permissions (i.., PLOAM grants) wh Ah permit 25 the sending ONUs identified by the cell extraction unit to transmit messages 'ii.e., PLEAT cells). The message

- transmission permissions p-oded t the ELOPE graft generator are then tranqitted to the OUT through z PLOWS cell transmitter. The proposed 0= can receive messages from a plural lty Go- ONUS mere reliably, because of its capability of allocating message transmission pe-m;ssions to the ONUs depending on their individual conditions.

The fcrego=,g Cents &-e corseed as illustrative only of the principles of the p_esen' prevention. Further, since Romeos modifications end changes will readily occur to -:se skilled in the a=, it is not ds-ea to Hi other etodL-er.ts or The invention. to the epic. construc_ion and applications shown mud described, and accord Ugly, all suitable mode fixations and eouivalen_s mat He regarded as f alllug within the scope of the inrentiQn in the appended claims and their equivalents.

Claims (2)

: = -:

1. A-, cptcal fin" terminal which canticle one or more optical network unfits connected thereto, 5 comprising: reception means for receiving messages from the optical network units; message ex.raciion mears for attracting valid messages Prom Emory the messages received by Ski' n reception means, by -l.erin- c-a such messages the indicate tz; the sending optical network units date Rio irfcrmation to send; dstlr.2tcn identi--cation means for identifying he sendlr.a conical Metro In _nis which are sourcing the valid messages etrac ed b-y said message extraction means; message transmission granting means for producing message transmlssicn permissions which permit the sending optical rt-ork un_ts identified by said destination -identification means to transmit messages; and transmission means for transmitting the message transmission pe-iss'ons produced by said message transmission granting means to the optical network unit.

2. An Utica line terminal substantially as 25 hereinbefore described with reference to and as illustrated in Figures 1, 8 and 9 of the accompanying drawings.