A TDMA based access control scheme for APON's - IEEE Xplore

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 11, NO. 516, MAYiJUNE 1993

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A TDMA Based Access Control Scheme for APON’s John D. Angelopoulos, Member, IEEE, Iakovos S. Venieris, Member, IEEE, and George I. Stassinopoulos, Member, IEEE

Abstruct- The cost of a dedicated fiber access to the broadband integrated services digital network (B-ISDN) is too high for small business and residential customers necessitating some form of resource sharing. Combining the asynchronous transfer mode (ATM) over a passive optical network (APON) with a suitable medium access control (MAC) protocol can provide significant cost savings and a reasonable bandwidth. In this way the customer line section can support broad-band services at an early stage. The MAC protocol presented in this paper places emphasis on service transparency aspects with an aim to incur minimal changes to the Local Exchange for APON connections. Sharing is effected through a reservation based time division multiple access method. The proposed MAC protocol is characterized by dynamic bandwidth allocation and multiple cell transmissions from each network termination in the upstream direction. This reduces waste due to the unavoidable synchronization preambles and other overhead.

in ATM PON’s (APON’s) rising as exemplified by the RACE

I1 broad-band access facilities (BAF) project currently in

progress which addresses the subject of providing low cost shared access of residential and small business customers in B-ISDN. The geographical lay-out of the residential and small business customers in the suburban areas lends itself to a double star or a tree-and-branch network topology [l]. In densely populated areas the ring can be considered as a candidate as well. However the inheritance of the existing ducting from telephony strongly favors star or treelike topologies. The extend of underground ducting (in the U.K. for example, 450000 km [2]) represents an investment difficult to ignore. It is not surprising that numerous techno-economical studies find an advantage of these topologies over the ring for the broadband local access [111, [121. I. INTRODUCTION The maximum distance from the head-end to the farthest 0 far most research regarding the access to broad-band situated broad-band network termination (B-NT1) [19], alternetworks addresses the needs of big business customers in natively referred to as the termination, is an important design traffic intense metropolitan areas, as is natural since their needs parameter in a PON since if affects the frame size and the are more urgent and they can afford the cost. Here the use of access delay as will become apparent below. More than 97% a dedicated fiber is assumed. The solutions offered however of the current local loops lie within a 10 km distance from the cannot be justified for residential and small business customers Central Office [2]. Adopting a maximum distance of 10 km who neither need nor can afford the full bandwidth offered. would cover most local access subnetworks except for a few What is needed in this case is a flexible way of sharing access rural cases. For these cases it is either possible to adapt the resources with an aim to concentrate traffic and reduce costs. present protocol using longer frames, or to abandon the fully An access network possessing the sought after features is the passive local access and introduce an active component at a passive optical network (PON), particularly in combination suitable distribution point. with the statistical multiplexing properties of ATM. To provide a reasonable average bandwidth per customer The PON presents definite advantages in terms of reliability a 622.080 Mbfs transmission rate was chosen. Power budget and cost effectiveness as has been demonstrated by several limitations restrict the economical split ratio to 1 : 32 [7], projects concentrating on providing either Telephony Service [15] which means serving only 32 terminations in the case of or N-ISDN or even B-ISDN access [3], [7]-[9], [14], [25], bringing the fiber to the home (FTTH). To allow serving more following developments in optical technology which provided than 32 terminations per APON, two approaches are possible: the technological tool to propose a deployment without active either use more expensive optical components which allow for electronics “in the street.” ATM on the other hand offers to greater power budget, or terminate the fiber at Curb Units and the PON the advantage that, although the available bandwidth extend the final drop to the customer terminations by copper is still shared, this is done in a dynamic way. So momentarily [7], [15] or even radio. The second solution, which does not larger bandwidth can be provided to those customers who need avoid active components in street vaults, although the optical it, at the expense of the bandwidth of customers experiencing network part is still passive, is referred to as fiber to the curb silent periods. It is not surprising then to see research interest (FTTC). The number of supported terminations is then mainly limited by the average bandwidth requirements per customer. In the BAF project [7], a number of 100 terminations was Manuscript received July 3, 1992; November 3, 1992. This work was partially funded by CEC RACE (R&D in Advanced Communications techselected providing an average of about 6 Mb/s per customer. nologies in Europe) 2024 BAF (Broadband Access Facilities) Project. The Since the copper drops are dedicated to each termination, there views expressed in this paper are those of the authors and not necessarily is no contention up to the queues at the curb units and the MAC those of the other BAF consortium members. The authors are with the National Technical University of Athens, Deprotocol is not significantly affected in either case. Although partment of Electrical & Computer Engineering, Computer Science Division, in this work we concentrate the presentation to the FTTH case Heroon Polytechnion 9, 157 73 Athens, Greece. with 32 B-NTls, the protocol we propose can easily be adapted IEEE Log Number 9207728.

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0733-8724/93$03.00 0 1993 IEEE

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A. Rationale for-the Protocol Choice

B NTI =Broadband Network Termination 1 B LT = Broadband Line Termination

Fig. 1. APON topology setup

to more than 32 terminations and accommodate the FTTC case. The sharing of the fiber in the APON environment calls for a new class of protocols which take into account the specific restrictions imposed by the broadband local loop requirements. In this work a suitable MAC protocol intended for star-bus, double star or tree in general broad-band PON’s using ATM as the transfer technique is presented. This protocol is based on time division multiple access (TDMA) using a reservation scheme which has proven to be very effective in satellite networks [4], [27]. The simplified setup on which we will apply our access method is depicted in Fig. 1. At the head-end the fiber terminates at the broad-band line termination (BLT). The broad-band head end controller (BHEC) alternatively referred to as the “Controller” which resides at the B-LT is responsible for the execution of the bandwidth allocation algorithm resulting in the distribution of the allocated “boarding tickets” to be included in the downstream frame. Each termination can include in its upstream transmission as many cells as the granted tickets. At the customer side the protocol is executed at the B-NTls. At the splitter/combiners [6], the downstream frame is split into (up to) 32 identical copies. In the other direction carefully timed access units (AU’s) from each termination converge to form the upstream frame. Separate fibers are assumed for the upstream and the downstream direction. Also for the feeder section we assume a separate fiber for each APON cluster. However the use of wavelength division multiplexing (WDM) or other techniques [lo] for fiber sharing is not precluded and it does not affect the MAC protocol. Regarding distributive services we again assume use of a different fiber or wavelength so they are not considered in this discussion. Finally no local switching by the MAC is envisaged. The paper is organized as follows. In Section 11-A we present the rationale for the protocol whose frame formats appear in Section 11-B. The MAC protocol operation is described in Section 111 where performance and protocol variation issues are also assessed. The results of our study are recapitulated in Section IV.

The setup of the APON imposes its peculiar constraints on the access protocol rendering unsuitable most classes of known protocols. Contention methods based on carrier sensing andlor collision sensing are not practically sensible since the Terminations cannot directly detect what other B-NTls transmit. (However we can utilize a limited form of such a contention scheme at low offered load conditions as we will present below.) Protocols based on tokens are equally unsuitable also because of the lack of direct communications between terminations which would introduce great waste for just token passing. Pure time division multiplexing (TDM) schemes are not efficient because of the significant portion of bursty traffic expected in the B-ISDN. MAC protocols studied for satellite networks fit somehow better the situation in the APON. However they do not exploit the extended processing and storage opportunities that the head-end presents since in the satellite the weight, power and cost limitations restrict the functionality of the head-end to that of a mere transponder with perhaps some buffer memory. The opportunity of a naturally centralized controller presents a strong incentive for a reservation multiple access scheme with a central queue management. The mean design tragets for the MAC for are listed in Table I: TABLE 1 MAC DESIGN TARGETS

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The first obvious technological problems arising from the APON setup presented in the introduction are: TABLE I1 TKIINOLOGICAL PROBLEMS

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11. THE PROPOSEDMAC SCHEME

Before describing the proposed MAC protocol we consider the requirements as well as the design objectives which led to the chosen solution.

Service transparency. This target aims at introducing as little change and complexity as possible to the rest of the network when an APON is attached to it, although absolute transparency is not possible because of the varying Quality of Service requirements. High efficiency; i.e., high perccntage of available bandwidth must actually transport useful end-user information. Low access delay and access delay variation. This is important particularly for constant bit rate services. Fairness. We will aim at an equal access delay among all terminations. No disadvantage in fast acquisition of bandwidth proportional to the negotiated parameters because of position or belated access must be exhibited.

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The avoidance of collisions at the optical combiner of upstream traffic originating from different Terminations located at varying distances and experiencing varying propagation delays. The bit timing extraction and dc-wandering cancellation at the BHEC as it successively receives traffic originating from different sources with inevitable phase differences. Output power level control at the termination transmitters, since thc optical signals from each source encounter varying attenuation until thev reach the head-end.

ANGELOPOULOS et al.: A TDMA BASED ACCESS CONTROL SCHEME FOR APON’S

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The problems in Table I1 have been successfully tackled to an accuracy of one bit at least for speeds up to 155 Mbls [5], PI by: TABLE 111 PROPOSED SOLUTION

A ranging protocol which determines the propagation delay

differences between Terminations and imposes a waiting obligation for closer B-NTls making them all appear as equidistant. The inclusion of a suitable for the line code used synchronization preamble of a few octets length (plus a one octet delimiter for byte synchronization) giving the chance to the head-end receivers for bit timing extraction and dc-wandering cancellation. The use of special automatic power control methods.

The waste of the mandatory penalty in the form of synchronization preambles, delimiters and possibly guardbands on the other hand must be kept to a minimum. Therefore allowing the transfer of more than one cell once the penalty for access has been paid, is considered essential for high network utilization. The medium access protocol resides in a new APON MAC sublayer which is introduced between the ATM layer and the transmission convergence sublayer of the B-ISDN protocol reference model [IS]. The allocation method is based on requests for cell access made by the terminations on the special MAC frame header inserted in front of each train of cells which they are permitted to transmit on each frame. The BHEC responds to the following downstream frame by sending “boarding tickets” equal to the number of cells each termination is permitted to insert the upstream frame. The order of transmissions is determined by a B-NT1 Identifier (BNID) value which is allocated by management procedures at the time of activation and updated as terminations are added or taken out. The way this value is assigned is described in Section 11-B.3 (Activation and Ranging). The basic version of the proposed method of access makes the APON transparent to all services. In the upstream direction user, signalling and ATM management cells which successfully arrive at the head-end are delivered unaccessed to the local exchange for switching towards their destination. In the downstream direction cells arriving at all Terminations are extracted with the help of look-up tables based on usual ATM addressing methods; i.e., virtual channel path identifiers (VCI’s/vPI’s) [20]. B. The Frame Structure In the reservation method we will adopt, each upldownstream frame needs information included in the control field of a preceding downlupstream frame. Choosing a frame length which is just longer than the round trip propagation delay to the Termination situated farthest from the controller plus the required protocol processing time, allows each frame to use just received control information from the beginning of the previous opposite direction frame. Thus delay is minimized. A frame repetition period of 125 ps is adopted in order to facilitate the handling of isochronous services as well as a possible mapping to synchronous digital hierarchy (SDH)

32

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Fig. 2. Downstream frame format.

frames. Taking out 100 ps for the propagation delay on the 20 km fiber (at 5 pslkm), leaves 25 ps for protocol processing. This is adequate even for the main processing task which is the ticket allocation at the head-end, with a hardware based implementation such as the one proposed below in the protocol operation section. The 125 ps frame can contain the payload of synchronous transport module (STM4): i.e., 4 x 2340 = 9360 bytes, giving a useful bandwidth of 599040 Mb/s for payload. 1) The Downstream Frame: Each downstream frame as illustrated in Fig. 2 starts with a frame synchronization pattern and the delimiter. The MAC frame header is organized in a fixed length format consisting of one byte how many APON management cells are contained in the frame (immediately after the APON control fields) and as many local access control fields (LACF’s) as the number of active terminations. There follow a number of unused LACF’s corresponding to the unactivated terminations up to number 32. The 10 bit frame header error correction field (FHEC) protects the bytes of the frame header. The spare bits if unused can remain as padding to give to the MAC frame header the size of 48 octets. This is the size of the segmentation and reassembly protocol data unit as recommended for the ATM Adaptation Layer type 314 in [21]. Hence, the same generator polynomial as well as common hardware can be used for error protection. The order of LACF’s is the order of increasing BNIDs SO no extra identifier is required. The LACF’s contain a bit with the acknowledgment of successful reception of the cells in the last frame, or an indication to retransmit. It also contains a 6 bit number of “boarding tickets” which determine how many cells the corresponding termination can include in the next upstream frame. The number of announced APON management cells come after the MAC frame header followed by as many ATM information cells as can fit the 125 p s frame. Since the APON management cells never cross the APON boundaries their format and coding is a local APON matter. Their identification inside APON is based on APON control functions. 2) The Upstream Frame: The upstream frame illustrated in Fig. 3 consists of three areas namely the request access units (MU’S), the ticket-allocated access units (TAU’S) and the unbooked surplus access units (SAU’s). The number of M U ’ S is equal to the number of active terminations and are transmitted by each B-NT1 in the order of increasing BNID value. Each RAU consists of 7 bytes out of which only 7 bits are useful control information. To avoid using for every termination an expensive ultrahigh frequency clock, the first 4 bytes are dedicated to a preamble for bit timing extraction and one byte for a delimiter [SI. Of the seven

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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 1 1 , NO. 516, MAYIJUNE 1993

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Fig. 3.

Upstream frame format. ailing.

bits one bit says whether the first cell is APON management or not and 6 bits are used for requesting boarding tickets. The corresponding termination has placed in this field the length of queue (LOQ), that is, the number of outstanding cells in its queue the moment the RAU was formed. The implicit request is for a reservation of at least at many boarding tickets. The 9 last bits are for a simple error correction (possibly Hamming). Since the frame phase alignment procedure cannot guarantee more than half a bit accuracy, there is a danger that the preamble of the next access unit may interfere by half a bit and destroy the sampling of the last useful bit. For this reason the first byte of the preamble has only 7 timing bits. The first is a guardband bit during which there is no transmission. After the N M U ’ S we have a number of TAU’s equal to the number of terminations which in the previous frame received a nonzero number of boarding tickets and therefore participate in the TAU’s. Each B-NT1 transmits its information in turn according again to increasing BNID value order skipping over those who received zero tickets. Each TAU starts also with 4 byte preamble plus a one byte delimiter followed by as many cells as are the boarding tickets. Each Termination has recorded, after the reception of the control information of the previous downstream frame, how many tickets were allocated to B-NTls before it (i.e., with a lower BNID), so as to be able to determine its turn. The third area is the area of unbooked SAU’s. It exists only under conditions of low offered load. It consists of a number of slots less than the number of active terminations offered on a limited contention basis. Each of the remaining B-NTls which were not allocated any TAU’s (had an empty queue at RAU transmission time but had “late arrivals”), has a chance to transmit at most a cell on a SAU preceded as always by a preamble and a delimiter. The allocation scheme will be described in the operation of the protocol. The target is obviously to reduce the access delay at low offered load. 3) Activation and Ranging: Before any termination is allowed to transmit it must be activated and ranged. We consider for the following that we have A active B-NTls in the APON already, and each of them has been allocated a 6 bit BNID value. The 5 bits of the BNID cover the 32 possible terminations. The numbers above 32 are reserved for special uses. The value 111111 identifies a broadcast APON management address. The BNID value is used implicitly to identify the addressing of the control fields in the downstream frame and the order of transmission in the upstream frame. Also it is used explicitly in APON management cells to address the source and destination. The ranging sequence is

mtim

Fig. 4. The “moving ceiling” allocation scheme.

periodically repeated in order to prevent errors due to several factors mainly temperature drifts. Because the ranging of all Terminations represents an appreciable overhead, it is repeated as rarely as possible. As a starting point the ranging of one B-NT1 every 1 s is proposed. Ranging is effected by a special ranging frame addressed to only one B-NT1 at a time by specifying its BNID. This signals to all other B-NTls to keep silent during the next upstream frame and only the addressed B-NT1 sends a frame phase response signal with a reply pulse timed with reference to the delimiter. This pulse is directed to special circuits at the receiver of the controller which range the termination (measure the delay relative the end of the delimiter of the downstream frame) and prepare a management cell for the next downstream frame with the new corrected value which the B-NT1 will have to use from now on. Newly powered terminations can signal their presence and need for activation by forcing a collision using a special pattern during the periodic ranging of one already active B-NT1. Thus the next periodic ranging is addressed to the newcomer. Once a new termination has been ranged, it is given the next unused BNID value with a broadcast management cell which informs all B-NTls that the next upstream frame will have an increased number of request units. The frames do not include explicit but instead each unit is implicitly considered to carry the BNID value corresponding to its position; i.e., first unit is for (or from) the termination with BNID 0, second for 1, etc. 111. PROTOCOLOPERATION DESCRIPTION

The general philosophy of the allocation scheme is to keep low the build up of queues at the terminations relying on policing for a guarantee that what is requested refers to compliant cells. For delay sensitive services additional measures are needed which will be presented in variations below. This allocation policy which we can assimilate to a “moving ceiling” is graphically depicted in Fig. 4, where the columns represent the LOQ requests. Each termination is granted the number of requests exceeding the ceiling. The ceiling moves up or down until the total number above it equals the available slots in the frame. The “moving ceiling” which is the heart of our scheme, is particularly suitable for a hardware implementation. For


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ti = n c L t s allocated UI ith B-NTI F = Total n m k r of slm availabk in a frame (F=176for 125 6 ) R = Sum of all ri A =Number of active B-NTls (range 0 to 32) P = Number of B-NTls which p-ted non-zero requests

Fig. 5. Ticket allocation procedure.

example a fast implementation which can fit in one programable gate array is outlined below: It consists of 32 six bit counters connected to adder circuits which can calculate the sum of the values of the counters in a few clock cycles. By virtue of the support circuitry the counters are incremented and decremented either in parallel or one at a time in a cyclic fashion. Initially they are loaded with the values of the LOQ requests as they arrive from the upstream frame. The sum R is calculated and compared to the available cell total in the frame F. If the value is exceeded, all the counters are decremented by one and the process repeated until the sum falls below F. Then the mode changes and the counters are decremented one at a time until the sum equals F. Counters with zero value are not allowed to roll over to 111111. Such a circuit can easily execture the algorithm in a few microseconds. We will describe how the whole scheme works according to the offered load with the help of the diagram shown in Fig. 5. The four vertical branches represent conditions of increasing offered load from left to right. Consider a start from very light load such that after satisfying all the few requests enough tickets D are left to provide at least one unsolicited ticket to all A active terminations. In such a situation the “ceiling” has to move down providing unsolicited tickets until all the slots in the next frame have been allocated. For the random starting point of the cyclic allocation of the last tickets which remain when the division of the surplus D by A leaves a remainder, we need a makeshift random number. We obtain such a number in the desired range 0 to A - 1, by dividing the binary number consisting of the last 6 bits of the frame header error correction (FHEC) field (see Fig. 3) of the last downstream frame by the number A . The remainder of this division is in the desired range and can serve our purpose. In this situation no SAU’s are left at all and only TAU’s will

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be used (no B-NT1 can have both TAU’s and SAU’s on the same frame). Thus the next downstream frame contains for each B-NT1 a number of unsolicited boarding tickets enough to divide equally (or almost equally) the available payload of the upstream frame among all active terminations. So the bandwidth allocation starts as almost perfect TDM. As the offered load increases however, the requests of some terminations pick up. The BHEC satifies all demand if possible by cutting down on unsolicited tickets. If the leftover D is not enough for all active Terminations, it is allocated by providing first an extra unsolicited ticket to all B-NTls which presented a request since they are likely to be at a burst and have high cell arrival probability. The rest are left as SAU’s. If however they do not suffice even for the P terminations which presented non zero requests, then they are all left as SAU’s. We see that SAU’s are always less than the contending terminations. Under heavy load the offered load exceeds the total available bandwidth. In this case the BHEC tries to allocate the tickets in a way leaving the B-NTls with an equal number of unsatisfied requests. So after temporarily assigning tickets are requested, the “ceiling” moves up until the sum is equal to the number F of slots available in the frame. Let us now examine the way SAU’s are allocated. Terminations know of the existence of SAU’s, only implicitly once the total number of boarding tickets does not tally with the number available in the frame. Only B-NTls with zero tickets can contend for SAU’s. The SAU’s are up for limited contention according to the following scheme which aims are reducing the chances of collisions. Each SAU slot can be used only by certain terminations according to a multiple allocation algorithm which can be repeated with deterministic results by all B-NTls. The scheme requires a reasonably random number between 0 and the maximum BNID value currently in use; i.e., A - 1. The makeshift random number we previously used as a starting BNID value of the cyclic allocation is used here too. So the first SAU is allocated to the termination with BNID value equal to this number or the first higher one. The next SAU is allocated to the B-NT1 with the next higher BNID value. If the last BNID is reached, we roll over to zero and continue. When the SAU’s are exhausted we reallocate them to more terminations as many times as needed to satisfy all BNTls. AS an example the multiple allocation of 3 SAU’s to 7 Terminations is depicted in Fig. 6. The BHEC which executes the same algorithm knows in the event of a collision which were the contending terminations and responds with LACF’s which have the retransmit bit set and an extra ticket. Since no B-NTl can have cells in both TAU’s and SAU’s at the same time no ambiguity of the retransmit bit exist. It is worth noting that this processing takes place concurrently with information transmission and does not burden the time critical processing tasks. The expendiency of SAU’s is to give the chance of a low delay at low offered load but without sacrificing a full place for the small possibility of a late arrival to a termination with zero outstanding cells as opposed to the higher possibilities for B-NTls at burst. However, sharing the possibility among more terminations gives better chances of use. The limited contention introduced by distributing SAU’s requires

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B-NTI = Bmadbaad Network Ternation TAU = Ticket-aUofated Access Unit SAU = SurplusAccess Umt

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 1 I , NO. 516, MAYIJUNE 1993

independent of the MAC, particularly since our emphasis is on transparency. To preserve transparency the Controller should not need to recognize the identifier of each Virtual (Chosen randomly) Channel/Path Connection as well as the particular traffic B-NTI characteristics of each connection. A preferable action can be \ devised by observing that a frame is really a jumping window. Therefore just counting cells per BAF-NT according to the toSAUl SAU2 SAU3 tal negotiated aggregate sum of peak rates is enough. Of course the fixed frame is not amenable to proper dimensioning but this is not necessary since what we are really after is a way to reject enough violating cells to protect the APON from severe degradation. Subsequent actions can be taken by management Terninatinas not show are supped procedures once it is verified by the normal policing unit after Lo have bcen pantedtickm (TAUS) On each Termination. its order in SAU a l l d o n is also shovvn the APON that a violation exist. This action could consist of initiation of fault isolation procedures, penalizing malicious Fig. 6 . Example of SAU allocation. users by punitive shut-down of connections, extra charging etc. The concatenation of multiple cells from each termination buffering of cells transmitted using SAU’s until a positive results in the violation of the negotiated peak rate and may acknowledgement is received. An additional drawback of SAU cause problems in the rest of the network, or cause rejection of distribution is the increased cell delay variation. However, compliant cells in policing units. To avoid this, traffic shaping this buffering is not significant because there is never more [22] at the exit of the APON in the upstream direction will than one cell transmitted in a SAU from each termination. be necessary. Dimensioning UPC in the presence of cell delay Also, since the cell delay variation at light load is low, the variation introduced at multiplexing stages is already found to increase due to SAU’s can be tolerated. Therefore it seems lead to high sizes which seriously degrade the effectiveness that the implementation of SAU’s is worth the introduced of policing schemes [24], [26]. The delay variation expected complexity although it is not an indispensible part of the in a PON is particularly high because of its distributed protocol. Remaining tickets could very well be distributed at nature and the dominance of the round trip delay of the random without the need for implementing retransmissions but reservation process. Therefore subsequent respacing becomes then the chances of utilization would be lower. unavoidable. Such a spacer can be combined with the normal UPC function since both operate on a VCINPI basis and A. Control and Performance Issues hardware savings can be obtained through buffer sharing. Regarding the admission of new connections we again Although the aim is to minimize the impact of the APON on usage parameter control (UPC) and connection admission aim at maintaining APON independent of the specific CAC control (CAC) functions (see the CCITT definitions in [22]), mechanism implemented by the local exchange. Of course before a call is accepted, the resources must be available in the the presence of the shared medium must be considered. Policing mechanisms implementing UPC functions oper- APON as with all other links on the possible route. But this ating on Virtual ChannelsNirtual Paths may reside in the is not really a different problem than with any other link. If Local Exchange, but then violating cells are not prevented the already established connections leave enough bandwidth, from taking up APON bandwidth. Of course the deterrent of the call can be accepted with a certain loss probability. The later rejection would still work against sources seeking tariff quantification of what is considered “enough” bandwidth is of cheating but not against fully or vandalous sources. Location course an important issue receiving a lot of research attention. of UPC functions in the termination provide the benefit that However we must remark that the same problem regarding violating cells could be prevented from entering APON, which the choice of CAC algorithm and suitable traffic description can be considered as a remote statistical multiplexer in the parameters (average rate, peak rate, burstiness, peak duration spirit of [16]. Such a solution satisfies [22]. However the etc. [16], [22]) currently under study for use in the rest of traffic profile is altered inside the APON. So a rough policing the ATM network, are in general applicable to the statistical protecting the APON plus a normal policing after it protecting multiplexing effected in APON. Therefore whatever solutions the local exchange as well as the rest of the network seem to are eventually adopted, they can be adapted to the peculiarities of APON. be needed. The important observation regarding the performance is For the former one could consider the possibility to integrate a UPC function operating on the aggregate of offered traffic that the MAC is operating under the protective shield of from each termination within the MAC. In this case the BHEC CAC and UPC, which limit the admitted load and enforce checks whether the tickets it is ready to grant would result this limiting. Therefore any detailed performance evaluation in violation of negotiated parameters. However, this requires would require the concurrent study of suitable CAC and the Controller to calculate the aggregate of the negotiated UPC algorithms which is outside the scope of this study. An parameters for all Virtual ChannelsiPaths of each B-NTI. In indication however about the delay behavior can be drawn view of the restricted time available for the execution of the from the simple approximate approach presented in [4] for an allocation algorithm it is preferrable to keep UPC operations analogous satellite protocol.


Let y denote the fraction of the available upstream bandwidth set aside for the reservation requests. So each Termination can send a reservation in each frame period P ( P = 125 ps). A n arriving cell waits on average P / 2 time until the reservations can be sent, plus T time for the M U transmission plus, one frame time ( P ) until tickets have arrived and the RAU of the next frame is sent, by which time it can be considered to have joined the logical common queue and cell cluster service begins. So the average delay until reservations are made and the common queue is joined is: TR = 3P/2 r (about 188 ps). For the heterogeneous traffic mix of the integrated services supported by the APON, the expected traffic characteristics of the superposition of the arrival processes can only be approximated by a Poisson process over relatively short time intervals [23] corresponding to cell scale fluctuations. It is an acceptable assumption hence for dimensioning network buffers. This is the time scale of interest in the evaluation of our protocol as well. For example, the time over which the delay is still found to fit well the Poisson assumption in [23] corresponds to a few frame times in our protocol. The Poisson assumption leads to reasonable estimations for the first moments of the waiting time distribution, as we intend to do here, under the condition of negligible congestion [13]. Because of its simplicity it can not be ignored in any situation where it is a tolerable assumption. So to be able to reach an approximate evaluation of the mean delay we accept that the arrival process of cell with reservations already placed in the global logical queue for a lot of B-NTls can be considered Poisson. The assumption is reinforced by the fact that in a shared medium the supported connections tend to be, many with low average each. Once a cell is in the common queue it experiences a service time of X / ( 1 - y ) where X is the transmission time using the full bandwidth. The common queue can be considered an M / G / l with p = A / p ( 1- y ) where p is the utilization factor, A the packet arrival rate and l / p the packet transmission rate. The delay can be derived by the Pollaczek-Khinchin formula: W = AE{X2}/2(1 - p ) where E{X2} is the second moment of the packet length and p = A/p(l - y ) . The total delay is now given by

+

+

W = TR AE{X2}/2p(1 - 7 - A)(l

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7).

(1)

It should be noted here that in order to take advantage of the fit of the situation to the M / G / l case we view the cell clusters queue in each BNTl as a packet with length equal to the random number of cells queued in a Termination times the fixed cell length. The fact that not all of the cells may manage to depart within the same frame is accounted for by the factor (1 - y) which scales p. We note from (1) that this protocol achieves near perfect scheduling at the expense of the fixed delay TR for making the reservations. As A approaches p( 1- y ) the delay is not bound. However it should be borne in mind that the MAC operates under the protective action of CAC which does not allow A to increase to a degree compromising performance. Therefore any evaluation of performance is meaningless without the concurrent study of a suitable CAC algorithm.

1101

The above analysis does not reflect the fact that BNTls with high BNID value experience higher delay on average than terminations with low BNID because the time from request to transmission is higher. That is, the MAC places arrivals to stations with low BNID value ahead of the arrivals to station with higher BNID value (within the same frame time of cource). This is the only source of unfairness due to the MAC. This could be corrected by rotating the BNID’s by one on every frame or after every ranging. We could also incorporate a byte with the BNID in each LACFd and thus exercise any desired discipline in the order of upsteam transmissions (e.g., terminations with longer queues first etc.). However this increases the implementation complexity. At low load, formula (1) gives a higher delay because it completely ignores the effect of the unsolicited tickets and the SAU’s which are dominant at low load. In this case most cells depart in the first frame after their arrival. In the above approximation the beneficial effect of unsolicited tickets and the SAU’s, which are only active at low load, are ignored. Nevertheless, the extent of similarity of the resulting global queues in both cases in adequate to justify the claim that our scheme also approaches near perfect scheduling at the expense of the fixed delay of about 1.5 frame time for making the reservations. The maximum throughput a B-NT1 can enjoy when the other terminations offer little load, depends on the number of cells a B-NT1 is allowed to insert in a frame. The 6 bit LOQ can handle 64 cellslframe which means 64 x 53 x 8 = 27136 bits every 125 ps that is: 217.088 Mb/s. It is reasonable to limit that to the broad-band user network interface rate of 155 Mbls [19] (although even that would be very seldom allowed in the APON environment by the CAC) by limiting the maximum number of cells per frame to 44.

B. Variations for Improved Handling of Constant Bit Rate Traffic In the basic access protocol presented above, the emphasis is on transparency. This way the incorporation of the APON does not lead to a unified complicated total system requiring integrated redesign of all its components. Instead, each system component keeps its independent functionality with the necessary adaptations. This simplifies the task and maintains uniformity with the rest of the network. However satisfying the more demanding in terms of quality of service constant bit rate (CBR) services may necessitate restricting the admitted by CAC traffic to low levels resulting in low utilization. To avoid this two variations of the basic protocol are proposed for comparison with respect to complexity introduced versus performance improvement for CBR services. Unavoidably they trade off transparency for performance by allowing a limited one way information flow from the Local Exchange control functions to the APON via the management plane. Both provide priority to CBR services. The first one exploits the predictability of CBR cell arrivals to exclude requests for such services. The BHEC draws information from the local exchange signalling entity about CBR connections and the negotiated parameters. It can therefore

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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 11, NO. 516, MAYIJUNE 1993

P

D

M

LOQO

LWI

W

EC

P=Preamble

D=Delimiter M=O First cell is info (No management cells follow) M=l First cell is management cell LOQ= Length Of Queue EC=Error Control LACFu = Local Access Field Conbol (upstream)

Fig. 7. Format of RAU with three priority levels

provide automatically boarding tickets at the needed intervals without the need for requests. Only the rest of the variable bit rate (VBR) traffic, which is kept in a separate buffer, participates in the reservation process. Each B-NT1 receives more tickets than it requested when it is time for a CBR service cell (or as before if there are spare slots). The frame formats remain the same only the algorithms on the controller and terminations are different. The second alternative recognizes three service classes in terms of quality of service requirements and introduces an equal number of priorities each one corresponding to a separate buffer in the Terminations. The three service classes are based on considerations of the access performance requirements of Service Classes A, B, C, and D defined by CCITT in [17] (cell access delay and cell loss at the point of access). The first class includes services with stringent access delay requirements and as such it corresponds to Class A of [17) and is allocated the higher level priority. The second class which is allocated the medium level of priority covers variable bit rate services of either Class B or C of (171 which are not very sensitive in access delay but are really strict in cell loss (e.g., signalling). The third class (lower priority) includes all services with low performance requirements as for examples connectionless services. The LACFup field of the upstream MAC frame now contains three separate LOQ field (see Fig 7). The look-up tables at the Terminations also include service type information. The boarding tickets however will still be an aggregate number and it is the responsibility of the B-NT1 to preferentially board first the higher priority cells. That is when the tickets cannot satisfy all requests, they are implicitly assumed to be intended for the higher priority cells and only those left are intended for lower priority LOQ requests. The allocation algorithm at the BHEC is also modified. Three separate allocation processes are active, one for each priority level. Each allocation process allocates tickets leftover from the higher priority process.

I v . CONCLUSIONS

In this paper a protocol suitable for ATM PON’s following the treehtar topology was presented. The key feature of the protocol is the high degree of transparency to all services. The connection admission control and usage parameter control functions are left to operate independently and the local exchange is minimally affected even when streamed traffic is provided privileged handling. Its variations can satisfy a wide spectrum of design targets.

Based on previous research which successfully tackled the problems of differing propagation delays, bit timing synchronization, and power level control, the proposed MAC can successfully transfer all the expected services in the residential and small business environment, under a unified scheme. The flexibility of this scheme allows its dynamic transformation from a TDM policy at low offered load to an allocation proportional to the outstanding cells at high load which aims at minimizing delay and loss. This policy which operates in combination with the protective mechanisms of usage parameter control and connection acceptance control can achieve excellent complementary operation, providing a very good approximation of a perfect scheduler. High utilization is achieved by allowing transmission of up to 44 cells at a time, minimizing overhead in the form of unavoidable preambles, delimiters and possibly guardbands. Fairness among the terminations is very good. The overall scheme is appreciably simple in its implementation since it keeps its control functions independent of those of the rest of the network, although advantage is taken of their functionality in a complementary fashion. REFERENCES Y.-K. M. Lin and D. R. Spears, “Passive optical subscriber loops with multiaccess,” J. Lightwave Technol., vol. 7, no. 11, pp. 1769-1777, Nov. 1989. T. R. Rowbotham, “Local loop developments in the U.K.,” IEEE Commun. Mag., Mar. 1991. D. W. Faulkner, D. B. Payne, J. R. Stern, and J. W. Ballance, “Optical networks for local loop applications,” J. Lightwave Technol., vol. 7, no. 11, pp. 1741-1751, Nov. 1989. D. Bertsekas and R. Gallager, Data Networks. New York: PrenticeHall, 1987, pp. 250-254. H. Uno and N. Aragaki, “Fiber-optic point-to-multipoint interface configuration for broad-band ISDN,” J . Lightwave Technol., vol. 7, no. 11, pp. 1849-1959, NOV. 1989. D. B. Keck, A. J. Morrow, D. A. Nolan, and D. A. Thompson, “Passive components in the subscriber loop,” J . Lightwave Technol., vol. 7, no. 11, pp. 1623-1633, Nov. 1989. P. H. van Heijningen, T. W. M. Mosch, and M. van Vaalen, “Optical network for broadband services in the subscriber loop,” Electron. Commun. Eng. J., Dec. 1992. 1. M. McGregor, G. J. Semple, and G. Nicholson, “Implementation of a TDM passive optical network for subscriber loop applications,” J . Lightwave Technol., vol. 7, no. 11, pp. 1752-1758, Nov. 1989. M. J. M. van Vaalen, “Asynchronous transfer mode transmission on a passive optical home network,” 17th International Television Symp. Tech. Exhibition, pp. 97-107, June 1991. Yoshitaka Takasaki, “Upgrading Strategies for B-ISDN Subscriber Loops,” J . Lightwave Technol., vol. 7, no. 11, pp. 1778-1787, Nov. 1989. K. Lu, M. Eiger, and H. Lemberg, “Economics and engineering comparisons of broadband local loop architectures,” IEEE Workshop on Passive Optical Networks for teh Local Loop, May 1990, London. A. Zaganiaris et al., “Fibre to the Home: Techno-economical evaluation within Europe by the RACE program,” International Symposium on Subscriber Loops and Services, Apr. 1991, Amsterdam. 1. Norros, J. W. Roberts, A. Simonian, and J. T. Virtamo, “The superposition of variable bit rate sources in an ATM multiplexer,” IEEE J . Select. Areas Commun., vol. 9, no. 3, Apr. 1991. J. R. Stern et al., “TPON - A passive optical network for telephony,” Proc. ECOC, Brighton, U.K., Sept. 1988. A. R. Beaumont, 1. R. Cade, P. K. Ffitch, P. D. Jenkins, and B. Payne, “Passive optical network for telecommunications,” EFOCILAN’91 (London, UK), June 1991. CClTT SG XVIII, “B-ISDN General Network Aspects - Rec. 1.311,” Geneva, June 1992. CCITT SG XVIII, “B-ISDN ATM Adaptation Layer (AAL) Functional Description - Rec. 1.362,” Geneva, June 1992.

[ 1x1 ('CITT SG XVIII, "B-ISDN Protocol Reference Modcl - Rcc. 1321.'' (kiievii. June 1902. 1191 CC'ITT SG XVIII. "B-ISDN U\er-Net\\orl\ Interfacc - Rcc. I.4l.i." Gcncva. June IYY?. I 201 CC'ITT SG XVIII. "R-ISDN ATM Functional Chorncteristic\ - Rec. I . 1.50~" Genev;i. June 1902. 121 I CClTT SG XVIII. "B-ISDN ATM Adaptation l,aqcr (AAI.) Specificatioii - Rcc. 1.363." Geneva. June I Y V (211 CCITT SG XVIII, "Traffic control and Congestion Control in R-ISDN - Rec. 1.371."Geneva. June I Y Y ? . 1231 K. Sririim :ind W. Whitt. "Characterizing supcrposition ;irri\;iI p r o c e s m in packet multiplexcra for w i c c and data." / E E E .I. .Sc/cc.r. .4rcw\ C ( i m n i i i t i . , vol. SAC-4. no. 6 . Sept. lY8h. 1241 F. (iuillemin. P. Roycr. A. Dupuis. and L. Ronioeuf. "Pcak rate cnforccmcnt i n A'TM networks." Proc. I Ital)). May lYY2. l?.i] M. Gerla. P. Caniarda. and G. Chiaretti. "Fault tolerant PON topologies." t'roc. /LEE.. Itifocotu "42 (Florencc, Italy), May 1992. [ X I P. Ca\lclli, A. Forciiia. a n d A. Tonletti, policing tunctions in ATM networks." PI-oc. Italy). May IYY?. I271 B. Jabhari and D. McDysan. "Performailcc of demand :issipnmeiit TDMA and multicarrier TDMA satellite networks." I Ar(w\ C ~ o t m i i u i . ,\ol. IO. n o . 2. Feb. IYO?.

.John D. Angelopoulos ( M ) horn in Athen\, Greece, on March I . 1950 He r c c m t d the Dip1 Ing dcgrcc from the Nationdl Technic'il Uniccr\ity of Athcn5 (NTUA) Greece, i n Julb 1977 the M Sc degree trom the Nottingh'im Univcrsit\ Engldnd. in 1Y77 m d the Ph D degree trom NTUA. in Jdnudry IYY7 all i n electricd engineering From 1077 to I089 hc worked t o r the RAD \ection of the nero\p,ice and te1econimunic~ition indu\tries , i t fir\t in product development m d Liter in engineering management He wds i n \ o l \ c d i n the d m g n ,ind decelopment of m i l i t n \witchbodrd\ encrqption equipment fire control computer\ etc As head ol the digitril \\stem\ wetion he \upervi\ed the comp,inc pdrticipdtion i n ndtinndl and European rewareh proleit\ i n the held\ of control dnd conimuiiiccition\ From l9XY he turncd to 'in ,ic,idemic c m c r Ioining t h t Technologicdl In\titute of Pireu\ U here h L I \ 'I\ A\\i\t,int Prote\sor i n the Automation Depdrtment He 15 in pardlcl pnrticipiting in NTUA rewareh acti\itie\ i n the areas ot brodd-bdnd nct\+orl\s Ilc ha\ been involced i n RACE m d ESPRIT projects rcl'iting to Metropolitm A r u Network\ dnd ATM Hi\ rewdrch interc\t\ include \h,ired medium 'icce\s t o R-I\DN. high\peed LAN \. h'irdw'ire m d hrmw'ire tor high-speed communic,itions etc Dr Angelopoulo\ 15 a member ot ILL m d the Technicdl ch,inibcr ot (Jreece

lakovos S. Verieris ( M ) w d s born i n Naxos. Greece, on March 7. 1905 He received the Dip1 -Ing degree trom the Univcr\ity of Pdtrr\\, Pdtrds, Greece i n 1088. and the Ph D degree trom the Ndiondl Technical lJniver\ity of Athens (NTUA), Athens. Greece, i n 1000 d l l in electrical engineering From Jdnunrv IYXY he joined the Telecommunicdtion\ Labor,itor\ of NTUA, where he I\ now d rese'irch a\\ociatc Hi\ re\earch interests drc in the fields ot B-ISDN, high-speed LAN's m d MAN \. d l optic,il network\ internetworking \igndlling. rewurce scheduling and dllocdion lor nctwork mm,igement, modelling pertormmce evaluation and queueing theorb He ha\ over twent) publicdtion\ i n the dhovc dred\ Dr Venicri\ I\ 'I member ot the Ttchnical Chdmber ot Greece He ha\ received \everdl ndtion,il m d interndtionril awdrd5 for dcddeniic achievement He ha\ been exposed to st,inddrdiration bodv work and ha\ contributed to N A i of ETSI and SG XVIlI ot CCITT He 15 participating in \e\eral RACE dnd ESPRIT projects dealing with R-ISDN protocols. ATM \witching and MAC techniques He I\ d reviewer tor the l E t E TRAY\,Xl i O N 5 Ow C O M M U N I C A I I O Y ~ and the IEEE Jo[ K Y Z I 01 Srtrc rFi) Aw4a I Y C O M M L ~ i c ~ r l o h \

George I. Stassinopoulos (M 82) was born i n Athen\, Greece, in 1951 He received the Degree in electricdl engineering trom the Swiss Federal Institute of Technology (ETH Zuerich) in 1974 dnd the Ph D degree in Automdtic Control from the Imperidl College, London. Depdrtmcnt of Computing and Control in 1977 In lY77-IY81 he gdined acquired Industrial experience i n the design and manufacture of microproce\sor bdwd industridi controllers in the cement indwtry (AGET, Gcnerdl Cement Compdny) d\ well 'I\ in computer networking and indu\tridl proces\ control Since 1 Y X 1 he ha\ been a member ot the staff 01 the Ndtiondl Tcchnical University of Athen\. Depdrtment of Computer Science where is currcntl) d professor His current revarch intere\ts die in the field\ ot data communication network\ LAN \, MAN'\, packet circuit and hkbrid \witching system\, malysis and \vnthcsi\ of communicdtion networks. routing, flow control and queueing theor\ He has over tortv publications i n the dbove arc& He h d \ pdrticipdted i n four nation,il rmearch progrdnis dealing with ddta conimunicdtion\ nct%ork\ These included development ot modeling, siniuldtion, anaI)\i\ m d design packages t o r packet for circuit \witched ddta networks Ilc ha\ d I \ o pdrticipdted in many Rdce prolects He is d reviewer tor the IEEE T I I \ L \ M 110\\ O Y c c l M M t l Y I ( ztiolu5 and the IEEE JOIIRYI\Io\ SI I ICTFD AKtR\ i Y CO\IVl Y I ( A T I O \ \ Protc\wr Stdssinopoulo\ I\ d member ot the Technical Chdmber of Greece