Efficient use of protocol stacks for LAN/MAN-ATM interworking

1160 IEEE JOURNAL ON SELECTED AREAS 1N COMMUNICATIONS, VOL. 1 I , NO. 8, OCTOBER 1993

Efficient Use of Protocol Stacks for LAN/MAN-ATM Interworking

Iakovos S. Venierk, Member, IEEE, John D. Angelopoulos, Member, IEEE, and George I. Stassinopoulos, Member, IEEE,

Abstract- Connectionless (CL) services as already developed in LAN’s have to interwork with emerging ATM connection- oriented (CO) networks. MAN’S are a significant evolutionary step and CO MAN’S, in particular, constitute a technically ap- pealing method to integrate both environments and provide shared medium access for all services. The functions required for enhancing DQDB MAN’S with CO services in a way compatible with ATM are described. Within the integrating framework of B-ISDN, the practically useful scenarios for interworking LAN’s, CO and CL MAN’s, and ATM zre investigated and classified in order to exploit the advantages of an efficient protocol relay utilizing the resources of the lowest possible layer. This is aimed at assisting interworking unit (IWU) designers and implementors to decide on key options in this versatile and evolving environment. The CO DQDB case emerges as the most advantageous development in terms of interworking simplicity and robustness.

I. INTRODUCTION HE evolution towards high-speed networks can not be T satisfied by a straightforward projection of solutions

adopted for low-speed networks and requires qualitative changes in network architecture, protocols, and hardware. In the protocol stack, higher-speed inevitably demands the hardware implementation in lower layers of functions traditionally resident in higher layers. Small fixed-length packets are chosen (e.g., ATM cell, DQDB slot) to facilitate fast hardware implementation of switching, routing and multiplexing. In effect, the role of ATM is central to the broadband development [18], [21], [36], 1371. ATM gives at a low-layer switching of virtual circuits (and, hence, is connection oriented by nature) and point-to-point transmission facilities. It provides an excellent and very efficient multiplexing capability on a link (flexible bandwidth, synchronization simplicity) but does not go as far as allowing common access (see, however, special developments in passive optical access networks [38]).

An important feature of data and information handling is the necessity of labeling its units (PDU’s, cells, slots, frames). Labels are used to identify the source and/or destination as well as for internal network functions. For distributive applications, labeling conveys only the source. Units may carry a label in

Manuscript received June 1992; revised Deceember 1992. This work was supported in part by the CEC RACE Project, R1044 “Broadband User-Network Interface-BUNI” and Esprit Project 5 193 “Metropolitan Area Communication System-Integration Phase-MAXI.”

The authors are with the National Technical University of Athens, Athens 157-73, Greece.

IEEE Log Number 92 1 1668.

the form of a source identifier or belong to a dedicated physical or logical channel originating at a source. For point-to-point communications where broadcasting occurs at the lower layers, i.e., LAN’s, the destination also has to be identified. The assignment of units to logical channels is again accomplished by appropriate labels. Moreover, labeling may be used for information routing, thus enabling functions which are of no direct value either at the source or at the destination.

In digital packetized information handling, labeling maps into the concepts of connection-oriented (CO) or connection- less (CL) transfer. The latter labels information units indi- vidually, the former labels connections; i.e., it is prearranged that information units belonging to the same source and same destination(s) all carry the same label. This “prearrangement” involves the setting up and releasing of connections. Labeling of PDU’s takes place at different layers, and CO/CL transfer is always understood with respect to a specific layer. The labeling function and labels themselves have to be regarded as a resource. Label generation, validation, checking, and identi- fication involves real time processing. Label values are limited and their assignment and availability has to be managed [23]. Finally, their width is an important part of transmission overhead.

In this paper, we consider the use of labels with reference to the interworking of LAN/MAN’s supporting either CL or/and CO transfer through an ATM-based WAN. Internetworking strongly depends on real-time label processing for addressing, switching, and routing of information in the interconnected networks and within the interworking units (IWU’s). This dependence is an essential factor for the IWU performance in terms of throughput, delay, and traffic control. The differ- ence in IWU protocol processing functionality will be clearly pointed out for each investigated case and classified in order of IWU complexity. Since it is not possible to present numerical results for all the seven interworking cases assessed in this study, it is our objective to reach performance conclusions from a qualitative evaluation of relay protocol complexity.

Although the B-ISDN (broadband ISDN) will be mainly based on ATM, in all evolution scenarios MAN’s play an important role. The reason for this is their simplicity and maturity compared to ATM, a fact that is reflected in the advanced state of the relevant standards [ 101. However neither LAN’s nor MAN’S can presently provide essential broadband services involving connection establishment combined with flexible use of bandwidth. Therefore, the ultimate solution will be a public ATM network which will interconnect the initially

0733-8716/93$03.00 0 1993 IEEE

1161 VENIERIS et al.: EFFICIENT USE OF PROTOCOL STACKS

Indirect CL data service support Direct CL data service support

Fig. 1. Intemetworking with E-ISDN.

isolated LANMAN islands and at the same time introduce a host of new broadband services, mainly of CO type. This situation is depicted in Fig. 1, where ATM-based customer premises networks (CPN’s) and LAN’s are either connected directly to the public ATM or via MAN access networks.

The introduction of a CO service to DQDB MAN’s al- lows the early provision of essential broadband services, by providing a cheaper shared medium access as well as a local switching functionality equivalent to an ATM local exchange (LEX). The lower cost could attract small business and even residential customers to B-ISDN at an earlier stage. Thus, the CO MAN’s will become the precursor of the broadband era, providing to their users an introduction to the B-ISDN world and, in some sense, a first taste of high- quality broadband services stimulating further demand. This constitutes an evolution strategy analogous to the one observed presently for CL services. Additionally, since these high-speed MAN’S represent an appreciable investment, it is essential to guarantee their long-term survival as gathering networks [40] by integrating them into B-ISDN [39]. A MAN possessing both CO and CL service capabilities in the way which will be described in this paper will be compatible with the ATM- based B-ISDN to the most possible extent, simplifying the interworking between the two techniques which is a major objective of this study.

The introduction of a CO service to DQDB MAN’s is cru- cial to this study. Dually, we cannot overlook the introduction of a CL data service to the inherently CO environment of

LAN user -.

ATM-based B-ISDN as suggested in Recommendations 1.2 1 1, 1.327, and 1.364 of CCITT [l], [2], [7]. Two mechanisms are envisaged there for the “provision of CL data service in B-ISDN.” The first referred as indirect CL data service support makes use of dedicated ATM connections for CL data transfer allowing CL protocols to terminate outside the B- ISDN. The second referred as direct employs the use of CLSF (CL service functions), provided in special nodes of the ATM- based B-ISDN where CL protocols are terminated. These two mechanisms are illustrated in Fig. 1. The possibility of a frame relay service operating over the ATM network [24], [32] is not considered in this study. This is because our focus is mainly placed on providing relay to a nonreassembled ATM cell/QA (Queued-Arbitrated) slot PDU, keeping in line with the target of limiting interworking functions to the lowest possible layer.

The purpose of this paper is twofold: First, to clearly identify the relay protocol functions in terms of label processing for all cases of LANMAN-ATM interworking and second to introduce signaling, bandwidth management, and VCI (vir- tual channel identification) allocation mechanisms into DQDB enabling the provision of CO services. Mechanisms for “CL data service provision in B-ISDN” are considered in Section I1 along with the emerging addressing, multiplexing, and routing functions of the IWU’s which lead to the protocol stack presen- tation of Sections 11-A and 11-B. The CO functions of DQDB appearing in Section I11 are developed with an aim to ascertain a high degree of compatibility with ATM. A comparative evaluation of the results of the relay protocol complexity of all

1162 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 11, NO. 8, OCTOBER 1993

considered interworking scenarios is summarized in tabulated form in Section IV.

11. INTERWORKING OF CL LAN/MAN AND ATM

The ATM is assumed as the adopted switching and multi- plexing method for the WAN illustrated in Fig. 1. LAN users are of not enough bandwidth to justify a direct semipermanent connection to the WAN and, at any rate, the presence of LANMAN’s stipulates that most of the traffic generated is largely local not involving the WAN. Before turning our attention to the transit traffic, we highlight the salient char- acteristics of the two environments. The physical architecture imposes different protocol stacks and different roles assigned to each layer. In the multiple access and broadcast environ- ment of LAN/MAN’s, the underlying MAC (medium access control) layer in effect solves the addressing and routing problem through packet labeling and packet label identification functions. CL services are provided by the LLC (logical link control) dwelling on top of the MAC. Connections and CO data traffic are not technically excluded, but are largely unnecessary for local traffic. On the other hand, for WAN’s following the ATM principle, routing is prominent since traffic is provided through concatenation of appropriate point-to-point physical connections. The technological advances entailed in ATM, require and allow to implement former Layer 3 functions (e.g., X.25 packet networks) at the ATM layer, i.e., directly on top of the physical layer. Hence, the ATM layer provides end- to-end virtual connections. Moreover, routing does not include addressing. Functions for the latter reside at higher layers and ensure, through the intervention of a control plane, the setting up of connections and associated tables required by routing.

For the transit traffic, i.e., traffic between LAN/MAN’s, we now have to distinguish different cases. These are based mainly on the distribution of potential destinations rather than statistics of individual traffic sources or aggregates of sources attached to the same LANMAN.

i) Interconnection is required for a small bounded number of LANMAN’s. Enterprise networks are examples here.

ii) The set of LAN/MAN’s potentially requiring intercon- nection is dynamically changing or, seen from each individual LANMAN, traffic is potentially generated for a large and not a priori-specified set of other LANMAN’s.

To bring i) and ii) into a common denominator regarding the total intensity of WAN transit traffic, we assume that i) is represented by a small-sized traffic matrix with large components and ii) by a large-sized traffic matrix with small components. So, the total transit traffic generated at each LAN/MAN is the same for both cases.

For case i), one is lead to a two-level approach. A small number of n LAN/MAN’s can be interconnected by n(n - 1) unidirectional connections provided permanently by the WAN functionalities. Within each connection, either information units are individually labeled or further connections are set up between individual source destination pairs. The latter is possible for ATM WAN’s, where virtual channel connec- tions (VCC’s) are provided within virtual paths connections (VPC’s). For ii), the setting up of LAN/MAN-to-LAN/MAN

connections seems not to be appealing. These connections are large in number andlor of short life expectation, due to the small volume and large burstiness of transit traffic between two individual LAN/MAN’s. Here, a more direct approach seems appropriate; information units obtain labels individually and ideally should be individually routed to their destination.

However, the ATM principle is inherently CO and can forward packets (cells) only along predefined paths (at ATM layer). Cells cannot be individually routed. Hence, to avoid the setting up of n(n - 1) connections of potentially extremely short duration, the following solution is promoted in standard- ization bodies. Forward all LAN/MAN traffic through suitably designed IWU’s to a common connectionless server (CLS) by n - 1 ATM unidirectional connections, and switch the information units according to their label and forward these either to n - 1 outgoing connections leading to the destination IWU’s or to another CLS.

The nodes providing CLSF terminate the CL protocols and relay of information is performed on the basis of an address field included in the CLNAP (CL network access protocol) PDU [7]. To enable the recognition of the address field along all interconnected networks (e.g., LAN, MAN, ATM), an address translation function should be provided by the IWU’s. Such methods are examined in the next paragraph. To achieve faster implementations that do not require reassembly of the entire CLNAP PDU, a new mechanism borrowed from the DQDB standard [IO] was adopted [171, [271, [311, [331, [36]. The address information is coded in a fixed position at the beginning of the CLNAP PDU and it is possible to access this information without reassembly. Due to its limited length, the address information is carried in the first segment (ATM cell) of this PDU, which is marked as BOM (begin of message). BOM is coded in the ST (segment type) field of the segmentation and reassembly (SAR)-type 314 protocol [3] performing the segmentation process. The remaining segments of the CLNAP PDU are routed by virtue of a multiplexing identifier (MID) label, which is present in all segments whose unique value has been matched to the address information of the BOM segment’. Thus, the CLNAP PDU is never assembled/disassembled within the CLSF node but is always recognizable as an entity since the whole cell stream constituting it carries the same MID, and ATM preserves the cell sequence within individual VPC’s/VCC’s. This operation, although it constitutes a violation of the OS1 layering concept, is adopted due to the significant performance enhancements it entails. The protocol stack of the CLSF node appearing in Fig. 2 follows Recommendation 1.364 [7] and is analogous to the levels of the SMDS protocol [30] which, in turn, complies to the DQDB PDU’s of the MCF (MAC convergence functions) block (see, also, the proposals appearing in [311). In fact, the protocol stack of the CLSF extends up to ATM since no reassembly takes place. However, the AAL and CLNAP layer is also shown to highlight the distinction between pure ATM switching and CLS switching where higher-than-ATM layer functionality is also realized.

’ The still-under-discussion ATM adaptation layer (AAL) type 5 [8] provides no ST and MID labels. This means that an AAL type 5 in the CLSF protocol stack would be forced to reassemble the entire CL PDU for routing purposes.

~ _ _ _ _ _ _

VENIERIS er al.: EFFICIENT USE OF PROTOCOL STACKS 1163

._ .- CLNAP

SAR type 314 ---I - .-

I ATM I AAL: ATM Adaptation Layer ATM: Asynchronous Transfer Mode CLNAP: ConnectionLess Network Access Protocol CPCS: Common Part Convergence Sublayer CS: Convergence Sublayer DM: Derived MAC DQDB: Distributed Queue Dual Bus IM: Initial MAC MCF: MAC Convergence Function QA: Queued Arbitrated SAR: Segmentation and Reassembly

Fig. 2. Correspondence of sublayers for CL data service.

The address label of the BOM segment should uniquely identify the equipment across all interconnected domains, as is the case when 64-bit E164 [4] addresses are used. Moreover, with E 164 addresses which incorporate geographical informa- tion as well, identification of the route to the IWU’s across the domains is possible. However, in IEEE 802.3-5 LAN’s [ 111 only 16 bit addresses may be employed; even in DQDB MAN’s, the use of E164 addressing is not mandatory. One can overcome this issue of diversity with address mapping [29] of the local addresses to “proxy” E164 addresses at each IWU, as part of the interconnection of the private to the interdomain network (see Fig. l), i.e., LAN’s to MAN’s or ATM. Extensive discussions on this methodology can be found in [9]. The IWU must possess tables mapping all local addresses to proxy E164 addresses so it can recognize and receive the PDU’s. It will then replace the proxy address with the local address and inject the PDU into the local domain. However, in the opposite direction, i.e., when receiving a PDU from the local domain, it cannot practically possess a global table with all possible mappings. However, with the use of self-learning procedures like the ones used for LAN bridges [ 121, it can cache addresses recently used. In case, therefore, that the entry does not exist in its mapping tables, it will communicate with a central or (preferably) hierarchically organized directory service to obtain the proxy E164 address which also identifies the remote IWU. It can, then, use encapsulation so that the destination address (DA) can be recognized and routed (via CLSF nodes and possibly other intermediate IWU’s, e.g., from ATM to a MAN) eventually to the remote IWU. The latter will replace the DA with the local address, which is the only one to identify the TE (terminal equipment) in the local domain. Finally, the local routing methods will be employed across its possibly bridged subnetworks.

Protocol stacks for the interworking of CL LAN/MAN and ATM are now discussed for cases i) and ii). These correspond to the CCITT mechanisms for “CL data service provision in B-ISDN.” We consider two general application areas of interconnection through the ATM network: interworking of non-ATM compatible LAN’s, i.e., LAN’s following the IEEE 802.3-5 Series standards [ l l ] ; and interworking of ATM-

compatible CL MAN’s, i.e., MAN’S following the IEEE 802.6 DQDB standard [lo].

A . Protocol Stacks for Interworking of LAN’s Two main scenarios are considered. In the first, LAN’s

are connected to the ATM indirectly; that is, LAN’s will be primarily connected to MAN’s which communicate via the ATM network according to Fig. 3 and/or Fig. 4. For example, the CL service of the MAN’s is considered. The protocol stack for this case of interworking is illustrated in Fig. 3. Depending on the specific application, the problem of IEEE 802.3-5 LAN and DQDB MAN interworking can be accomplished by means of the IEEE 802.ld Standard [12] referring to local bridging (the case illustrated in Fig. 3, where relaying denoted as R is performed on top of MCF) or by using a remote bridge architecture as exemplified in [34] and [35]. The remaining part of interworking, i.e., MAN-ATM is considered in Section

The second scenario refers to the direct interworking of LAN’s and ATM, i.e., without the intervention of MAN’s. It is reasonable to expect that the direct interconnection of enterprise LAN’s will constitute an important application served by the public ATM network. Remote LAN’s of the same administration domain will then form a private network superposed to the backbone ATM network.

The principle of interworking is to convey the union of all generic MAC parameters affecting the MAC operation of the receiving side in the payload of ATM cells. This is a common requirement for interconnecting local or remote nonhomogeneous LAN’s [ 191. Along this line, a relay protocol for interconnecting LAN’s through ATM is proposed in [28]. With an objective to avoid encapsulation of the entire source MAC frame, only the generic MAC parameters required by the receiving LAN are coded in the relay PDU. To further assure that the relay PDU structure respects bandwidth utilization, we avoid duplications of the functions provided by the lower-layer protocols of the IWU. Given that error detection is present in all AAL protocols, the encapsulated MAC frame need not be error protected and the corresponding FCS (frame check sequence) field can be eliminated. The general case involves the communication of terminals attached to LAN’s which are bridged by the ATM network. Some functions are redundant or appear simplified when one terminal is attached to a LAN and another directly connected to the ATM network. In the case of LAN and ATM terminal interworking, only the addressing part of the original MAC frame is required. Nevertheless, for the sake of standards compliance but at the expense of bandwidth utilization, the relay protocol can simply encapsulate MAC frames to CLNAP PDU’s according to the principles given in [27] leading to a remote bridge architecture. This is illustrated in Fig. 5.

When the first CCITT mechanism for “CL data service provision in B-ISDN’ is applied, then routing within ATM is performed by virtue of the ATM VPI (virtual path identification). The CLNAP address field and the MID mechanism of SAR type 3/4 is not strictly required, which allows us to use a more simple SAR protocol providing only

II-B .

1 I 6 4 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 1 1 , NO. 8, OCTOBER 1993

I I I

IWU IWU IWU

I

I I I

I

HlGHE

802.3-5

I HIGHEF~ ltAYER I

HlGHE

eo2 3-5

Fig. 3. Protocol stack for the interworking of CL MAN’s through provisioned ATM connections.

DQDB MAN IWU CLS ATM c LS IWU

DQDB MAN

HIGHER kl CLNAP

type 314

HIGHER LAYER

802 3-5

HIGHER 1 LAYER I

Fig. 4. Protocol stack for the interworking of CL MAN’s through a CL-ISDN data service.

HIGHER I LAYER I 1 0 0 2 3 5

Fig. 5 . Protocol stacks for the direct interworking of LAN’s through ATM.

segmentation and reassembly like the SAR type 5 [8] or the one proposed in [27] with some complementary frame delimitation functions. In both alternatives, CRC is provided by the CPCS (common part convergence sublayer) protocol before segmentation. Depending on the VPI allocation scheme, the VCI can be used to identify the MAC source address or the originating IWU [28]. In this alternative, the size of the header appended by the relay protocol is further reduced. However, additional tables are required in the IWU for mapping MAC addresses-to-ATM VCI’s.

If relay within ATM is performed by the use of CLSF (second CCITT mechanism), then the IWU should now replace MAC addresses with proxy E164 addresses and the IWU

should conform to the protocol stack of the CLSF. That is a CLNAP and AAL type 314 protocol should appear in the IWU where addresses and MID will be always coded in the same octets of a BOM segment to enable on-the-fly (without reassembly) routing of ATM cells in the ATM nodes providing CLSF. CRC is now provided on a segment basis by the SAR type 314 protocol.

B . Protocol Stacks for Interworking of CL MAN’s Interworking of CL MAN’s through the ATM network is an

excellent evolutionary step for providing interconnection of a variety of existing LAN’s. It is accepted by different forums as the way of introducing broadband services to the business

VENIERIS et al.: EFFICIENT USE OF PROTOCOL STACKS

4TM CO

0

0

1165

ATM CO

environment. The protocol stacks for this interworking are illustrated in two figures (the part of Fig. 3 surrounded by the dashed line and Fig. 4), each one corresponding to a CCITT mechanism for the “support of CL data service in B- ISDN.” The assumed compatibility between layers and PDU’s formats of the ATM protocol model for CL services and the DQDB MCF, as shown in Fig. 2, has a strong impact on interworking allowing relaying of CL information at a very low level (QA slot/ATM cell). All information that is important for interworking is always placed in fixed positions of the corresponding PDU’s and can be easily assessed and processed without reassembly. Conversion of specific subfields of the QA slot header to the correspondent subfields of the ATM header is performed by the IWU in all cases of MAN-ATM interworking, in the way described in [22]. To avoid confusion, it must be kept in mind that although the protocol stack of the IWU appears the same in Figs. 3 and 4, the relay protocols of the IWU’s for each CCITT mechanism are not identical.

In the first CCITT mechanism, the CL protocol is terminated outside the ATM network. The relay protocol of the IWU in Fig. 3 uses the address fields of an incoming BOM QA segment (either the source or destination address or even both with respect to the VPI/VCI allocation schemes exemplified in [17]), to determine the VPI and VCI values that should be used when transforming the QA slot to an ATM cell. This operation is performed according to information included in the IWU address-to-VPINCI correspondence tables. The VPI’s are used by the VP cross connects to route ATM cells to the appropriate destination IWU, where the reverse addressing operations take place. The allocation of bandwidth to these VPC’s is an issue assessed in depth in [33], where an ATM bandwidth advertising mechanism is proposed and its capability to dynamically adapt the requested bandwidth according to the current load conditions is demonstrated. Since no DQDB-to-ATM MID translation is required in the IWU, the QA segment encapsulated in the ATM cell payload is not modified but is transferred transparently to the receiving DQDB user. Hence, segment bit error correction is only provided end-to-end.

In the second CCITT mechanism, apart from the conversion of QA slots to ATM cells and their routing to the appropriate ATM node realizing CLSF, the IWU also replaces the DA field of the QA slot with an address recognizable by the CLSF node by consulting an additional address-to-address correspondence table, as discussed earlier in this section. This is because routing of ATM cells to the next ATM CLSF node is performed on the basis of the BOM, DA, and MID fields of each segment. The replacement of addresses, together with the translation of the DQDB MID to an ATM MID, necessitate the calculation of a new CRC on the payload of each cell. Recall that the cell payload is a SAR type 3/4 segment. This means increased protocol processing delay, but the information is now more protected. The allocation of VPI/VCI values among CLSF’s, together with the associated bandwidth, is either the responsibility of ATM network management or, in another option, can be provided by specific mechanisms of the CLSF conforming to the proposals of [33].

CL

0

.AYER

CO CL

0

0 . 0

0 . 0

0 .

0 . 0

0

0

Higher Layer

AAL

ATM

~

TABLE I IWU PROTOCOL FUNCTIONS FOR

MANLAN-ATM INTERWORKING (USER PLANE)

INTERNET-

a. DA-to-DA 1 0 1 b. DA-to-ATM

VPIIVCI I 0 I 0

c Accessof DA I 0 I 0

d. Segmentation & --c l o l o

e. CRC

I . I 1. MID-to-MID

g.AccessolST. 1 1 MID

h. QA slot-to-ATM cell header

D i rec t l y 1 CL I CO

. The access (all interworking cases) and replacement of

labels, addresses, and/or MID (when the CLSF is used) in the IWU’s result in increased interworking protocol complexity, require the maintenance and update of address correspondence tables, and hence decreases the IWU performance. To simplify interworking, there is an intentionally arranged compatibility between protocols of different networks as between ATM and DQDB, which allows us to dispense with the time- and buffer- consuming segmentation and reassembly present in all LAN interworking scenarios. However, the substitution of certain labels within PDU’s, e.g., MID, necessitate the provision of higher-than-ATM layer functions; namely, new CRC calcula- tions. The only exception is the use of semipermanent ATM connections for the interworking of CL MAN’S.

The IWU functions for each case considered are summa- rized in the first six columns of Table I, where the IWU complexity decreases from left to right. It is our intention in this paper to demonstrate cases where the processing of information in the IWU is kept to a minimum in the user plane, comprising only those functions that are performed by a normal high-speed ATM switch (e.g., VPI/VCI translation, header error control calculation, etc.). This implies that, instead of several labels (e.g., DA, ST, MID), only one label should enable routing to the destination. A formula for calculating the required user plane processing time at the IWU is provided in the Appendix for all seven cases. The impact of each IWU pro- tocol function on processing time and, hence, on performance can be directly deduced from Table 11. In analogy to ATM, we define in the next section the CO service of DQDB with an aim to achieve a simple, robust, and efficient interworking which fully exploits the ATM and DQDB similarities taking

1166 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 11, NO. 8, OCTOBER 1993

TABLE I1 mu PROCESSING TIME PER USER-PLANE PDU

t (xtzxl-4)t, t ( ( X + 2 X 1 ) / 4 8 ) t d

t ((XtXltX2)3/44 t (X2.Xl.4))t, t

advantage of the VPI/VCI label functionality. This simplicity is evident, at first glance, in the last column of Table I and first row of Table 11.

111. CONNECTION-ORIENTED DQDB SERVICE

The CL service currently provided by high-speed LAN’s and MAN’S cannot offer the required quality of service (QOS) to support new user needs extending beyond the traditional LAN data services. In contrast, the main advantage of the CO technique is the ability to allocate resources at call set up. This permits the network to keep track of resources and refuse con- nections, if it cannot guarantee acceptable performance. Thus, it provides the required performance predictability, which is essential for time-sensitive services. This is particularly important in broadband networks where the usual reactive, congestion control methods, i.e., based on backpressure to source, are inefficient or even impossible at high speeds. The reason is the high propagation delay-bandwidth product, which accumulates in long links, provides a large volume of transit information before feedback can reach the source [141, [21], [24]. CO services allow the employment of congestion control.

Of course, the medium sharing of DQDB imposes band- width restrictions, and large customers with gradually increas- ing traffic needs will be eventually provided with a dedicated ATM link to the LEX. In this respect, the introduction of a CO service to DQDB MAN’S is considered an important evolutionary step. Provisions for the CO service are included in the IEEE 802.6 standard [IO]. In this section, we consider the functional requirements which enable DQDB MAN’S to provide high QOS CO services and also address the required signaling and congestion control functions in a way compatible to B-ISDN. Efficient interworking then follows naturally, as described in Section 111-C.

class A class B Class c class D isochronous

t to ATM io DQDB

Fig. 6. Proposed information flow within the DQDB node.

A . The Connection-Oriented Convergence Functions of DQDB MAN

To achieve compatibility, we propose the adoption of the same AAL protocols of the ATM network [3] for the connection-oriented convergence functions (COCF) of the DQDB node (see Fig. 6). A similar approach is presented in [13]. Note that the SAR type 3/4 format intended for class D, i.e., CL, traffic of 1.362 [3] was borrowed from the DQDB DMPDU (derived MAC protocol data unit) so that relay of CL information can be easily accomplished by the use of ATM nodes supporting CLSF. Furthermore, there is an intentional functional correspondence between the DQDB IMPDU (initial MAC PDU) and the CLNAP + CS type 3/4 format as appears in Fig. 2. In consequence, the COCF block must include provisions for the other three classes of [3], i.e., A, B, C as depicted in Fig. 6. The services of the QAF (QA functions) block are offered to all three COCF’s as well as to the MCF dedicated to support class D traffic.

In the QA mode of access, slots are not preassigned as in the prearbitrated (PA) mode; instead, access is determined by means of a queueing mechanism for outstanding slots. This queue is logically unique and global, though maintained in a distributed manner. Unused QA slots are available to users who happen to be in an active state on a contention basis. Thus, the QA access mechanism is inherently affecting sta- tistical multiplexing of all user traffic, not unlike ATM where bursts are accommodated by exploiting possible silent periods occumng to other concurrent calls. Although in DQDB this occurs in a distributed manner, the QA method nevertheless lends itself to a smooth interworking of the two techniques. Hence, the functionality at ATM cell/QA slot level is largely equivalent. What we intend is to enhance the DQDB higher layers with mechanisms equivalent to signaling and bandwidth allocation as envisaged in ATM.

As shown in Fig. 6, the MAN information flow for any class of service destined for an ATM network is routed via the QAF block, utilizing the QA mode of access (dashed lines). This is true even for time-related traffic, for the reasons explained below.

L

VENIERIS er al.: EFFICIENT USE OF PROTOCOL STACKS 1167

In ATM, Class A dedicated to support constant bit rate services covers a wider variety of services than the isochronous DQDB service. The isochronous convergence functions (ICF) of DQDB were required to support terminals which do not packetize traffic or possess end-to-end synchronization mech- anism, but rely on a synchronous transfer mode (for example, todays’ telephone sets). In addition, they guarantee the re- quired QOS of isochronous services. When DQDB is enhanced with CO services, the ICF are not required and will only remain to support isochronous services offered to users of the same or directly interconnected DQDB MAN’S. However, in the case of a connection between a DQDB MAN user and an ATM user or between remote DQDB users interconnected via an ATM network (as depicted in Fig. 1). isochronous traffic should be packetized and injected into the COCF block where AAL type 1 protocols would be present and not into the ICF block. In ICF, PA slots are used to convey octets of isochronous information belonging to different connections. These are distinguished by the use of offsets. In ATM, cells carrying packetized class A traffic will be exclusively allocated to a specific connection. To preserve an easy interworking based on ATM cell/QA slot relay, we introduce a type 1 COCF service dedicated to support any class A traffic requiring routing via an ATM network. This service will use the same end-to-end synchronization functions utilized in ATM. The implications of this approach to the functional architecture of the IWU between DQDB and ATM are presented in Section 111-c.

B . Extension of B-ISDN Signaling Concepts to DQDB MAN

The provision of the CO service dictates the introduction of signaling. The extension of B-ISDN signaling principles into the QA functions of DQDB will assure full integration. To preserve uniformity of terminology with the guidelines presented in Appendix A of IEEE 802.6 standard [ 101 for the PA isochronous functions, we suggest the introduction of the following entities for the CO DQDB service. These correspond to the connection-related functions [5], and are:

A QA Signaling termination (QA-ST). This is the entity which executes the DQDB signaling protocols. A VCI server for QA access, responsible for the allocation of DQDB VCI’s. A QA bandwidth manager (BWM). This entity is respon- sible for monitoring bandwidth usage and allocation of bandwidth to new DQDB connections.

These QA functions are, in particular, those required for CO services. CO and CL services use the same medium access principle, namely QA, in contraposition to PA for isochronous services (see Fig. 6). For many aspects, most notably bandwidth allocation, CO and CL services have to be managed jointly. In the following, we will provide insight into the operation of these entities using an ATM approach. A further benefit of this approach is that current and future enhancements regarding ATM can easily be adapted to the CO DQDB case.

1) The Signaling Termination and VCI Server: The QA-ST is the entity responsible for the acceptance or rejection of

new calls. In that sense, the QA-ST should contain all those protocol functions related to the operation of the B-ISDN signaling protocols. In line with B-ISDN standardization work, we assume the conformance of the Layer 3 signaling protocol to the ISDN signaling control part (ISCP) principles, which incorporate the novel concepts implied by the multimedia, multiconnection, and multiparty nature of a B-ISDN call. Along this line, a modified LAP-D (link access procedure on the D-channel) protocol will provide all the Layer 2.2 functions necessary for signaling traffic. Extensive investigations of these issues can be found in [6], [16], [25], [26]. Regarding the lower layers of QA-ST, it is reasonable to expect that the MAC PDU’s will conform to the AAL type 314 or AAL type 5. These are the candidates for signaling standardization in

For accessing the QA-ST, we propose a procedure similar to that of ATM metasignaling, i.e., by the use of a predefined “metasignaling” DQDB VCI, on which terminals attached to any DQDB node. The IWU can send requests for allocation of signaling VCI’s (SVCI’s) to the QA-ST. A reference number can be used for identification of the QA-ST response. The QA- ST responds with a message sent on a predefined broadcast VCI (BVCI) on the opposite bus. The message incorporates the values of the allocated SVCI. Signaling messages inside DQDB may then be captured using the allocated SVCI’s. As an alternative to metasignaling and the setting up of signaling connections, it is possible to pass signaling messages as CL traffic. This solution, however, requires signaling messages to be formatted as IMPDU’s where the destination information is included in the DA field. If the same signaling message should be passed to an ATM signaling entity for interworking reasons, then the IMPDU should be transformed to the CS PDU type 3/4 or 5 (the AAL protocol for signaling information transfer in B-ISDN) in the IWU. At the same time, the DA field of the lMPDU should be accessed and matched to an ATM VCI but not coded anywhere in the CS PDU. In contrast to the metasignaling procedure, this alternative affects the simplicity of interworking since complex protocol conversion and reassembly operations are now part of the IWU functionality. In addition, the QOS required for signaling traffic is not guaranteed.

Notice that the metasignaling procedure for setting up DQDB SVCI’s within the CO DQDB is initiated either by terminals attached to a node or by the IWU on arrival of a call setup request from the ATM WAN. In addition, the IWU takes part in a parallel metasignaling procedure with the ATM WAN resulting in the setup of ATM SVCI’s between IWU and LEX.

User plane DQDB VCI’s will be allocated to new connec- tions, by the QA-ST, in consultation with the QA VCI server. By establishing a suitable DQDB VCI allocation scheme, the VCI label can also function as a service priority indicator apart from the usual connection identification. As will be explained, priorities have to be employed and CL traffic slots will be distinguishable as QA slots of lowest priority. Considering the interworking with ATM, one can notice that the DQDB VCI size is four bits longer than the ATM VCI. When the DQDB VCI is allocated by the QA-ST, which has access to

B-ISDN [26], [8].

1168 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 11, NO. 8, OCTOBER 1993

signaling information and hence to QOS requirements, this VCI value will indicate by using two out of the four extra bits the priority of the PDU’s belonging to that particular connection. The DQDB node now knows, without resorting to higher-layer functions, the priority of the QA slot. To that end, the VCI values in the QA VCI server are organized into three pools corresponding to the three access priorities. The lower-priority “pool” only includes the default CL VCI (all bits 1). The other two will be distinguished by the values 00 and 01 of the first two bits of the VCI field.

2) Bandwidth Management: The introduction of a CO ser- vice to MAN’S immediately raises the same bandwidth nego- tiation, allocation, and enforcement issues which are currently under intensive investigation for the ATM and constitute the main theoretical hurdle before deployment [ 181, [21]. These issues are:

A suitable set of source characterization parameters al- lowing a reasonably accurate estimation of the traffic profile statistics of the aggregate traffic. A suitable connection acceptance control (CAC) scheme based on negotiation at call setup, guaranteeing that the accepted connections will provide the agreed QOS. A usage parameter control (UPC) scheme guaranteeing the enforcement of the negotiated parameters.

The first issue is not any different than in ATM, since the same sources and services will be supported. The same parameters as in ATM can be used for traffic modeling (i.e., peak rate, average rate, burstiness, burst duration, etc.). It must be stressed that the usual approach of modeling arrivals to DQDB nodes as Poisson is not adequate anymore and the traffic environment of B-ISDN must be adopted [20].

The second issue is essentially the same as in ATM multi- plexing, and any CAC algorithm eventually adopted for ATM can be adapted to the CO DQDB notwithstanding certain differences arising from its distributed nature as the well as the contention scheme. Unfairness caused by the node’s position on the bus has no equivalent in ATM links; however, it does not appreciably affect the CO traffic because of the priority mechanism as will be explained.

Except for the requirement for a CAC scheme, a method is also needed to divide dynamically the available bandwidth among the supported service classes in a way satisfying the widely varying QOS. We assume availability of a CAC function which, on the basis of traffic descriptors at request, can estimate the aggregate “equivalent” bandwidth [ 2 11 of all accepted connections, i.e., a bandwidth value between the total average and peak values, which is an acceptable tradeoff between multiplexing gain and loss probability. Marking of the CO VCI’s by the head of bus (HOB) brings us essentially back to the PA method, where bandwidth is rigidly fixed. This is not desired even for class A packetized traffic, which consists of the superposition of many nonidentical constant bit rate sources. Instead, any DQDB node should be allowed to seize and mark empty slots with the allocated CO VCI values. This is necessary to allow variation of the total number of slots used for CO services according to bursts and, hence, exploitation of the statistical multiplexing potential of the QA mode of access.

It is not possible to guarantee the demanding and varying QOS of CO services if all traffic is subjected indiscriminately to contention for the common medium, particularly under heavy load. The employment of the access priority mechanism is necessary. The highest priority should be reserved for services with stringent jitter and delay requirements such as service class A. Services with less tight jitter requirements, mainly variable bit rate (VBR) CO classes B and C, will access at the second priority level. Finally, CL class D services (for which no quality contract was agreed) are at the lowest priority level. Different queues for the three priority groups are employed. The priority each segment belongs to is identified in the node by the VCI label allocated by the QA VCI server as previously described.

With this arrangement, the bandwidth which will be left for the CL services is dependent on the DQDB CAC mechanism. The aggregate effective bandwidth value beyond which the CAC will block further calls defines the minimum bandwidth for CL traffic. This limit need not be static, however, and can vary with traffic situations. This can be achieved by means of management procedures using the results of performance measurements. Thus, by means of the priority mechanism when the total CO traffic is low, CL traffic is allowed to utilize all residual bandwidth. As the CO traffic increases, CL traffic is restricted to the use of the remaining bandwidth up to the limit mentioned previously. In effect, what we describe is a dynamic movable boundary scheme along the lines of the one presented in [ 151 employed by the CAC but realized by means of the priorities mechanism in the DQDB MAC. It is apparent that the effect of any node position-dependent unfairness will mainly exhibit itself on the low-priority CL traffic. Therefore, no special care should be exercised with respect to the CAC algorithm other than basing the calculations on the expected worst case.

Regarding the third issue; i.e., of contract enforcement by UPC action, the same considerations with ATM are also valid in a CO MAN. It is clear that the UPC must be located at the points of access to the public DQDB nodes to also protect other MAN users. Since the same traffic sources with the same characteristics as in ATM will be served by a CO DQDB all the mechanism currently under research for policing ATM traffic [41] can be equally considered for the CO MAN traffic. The enforcement parameters will be communicated to the policing units using management procedures. Regarding CL traffic the priority mechanism implementing the dynamic movable boundary scheme does not allow CL traffic to disturb the quality offered to CO services and in that sense can be considered as a form of policing exercised on CL traffic.

C. Protocol Stacks for Intenvorking of CO MAN’S The case of CO MAN interconnection is illustrated in Fig.

7. The interworking functionality is now presented in two planes (control and user plane). The IWU has to establish signaling VCC’s with the nearest LEX or QA-ST of the destination MAN by virtue of the metasignaling protocol described in Section 111-B. The metasignaling capability of the IWU allows the establishment of a concatenation of signaling

VENIERIS et al.: EFFICIENT USE OF PROTOCOL STACKS 1169

DQDB USER QA-ST IWU ATM IWU QA-ST DQDB USER

C-plane

MIAP-D

(CS 314)

(SAR 3/4)

U-plane

IM

(CS 12.3)

DM SAR 1.2,3)

C-plane C-plane

ISCP

MLAP-D

cs 314 C & U-plane

C-plane

MLAP-D

(CS-3)

(SAR-3)

C-plane

U-plane

MLAP-D

QA

Fig. 7. Protocol stack for the interworking of CO MAN’s through a CO ATM.

VCC’s between signaling entities (MAN terminal, QA-ST, LEX). The corresponding SVCI’s will then be used for the transfer of signaling messages from one network to the other, but in a way transparent to the IWU. As is customary in ATM, metasignaling procedures are embedded in the management plane [26]. With this convention, signaling related functions of the IWU, which are resident in the control plane, are in effect reduced to those of the user plane, i.e., the IWU handles the ATM cells/QA slots labeled with an SVCI as those of any user connection. When examining Fig. 7, one should not overlook the existence of metasignaling functions which are not shown. The IWU must respond to metasignaling messages and terminate metasignaling connections on both sides (DQDB MAN and ATM WAN). It must be able to recognize incoming call setup messages, which entail the need to produce further signaling VCC’s, again with metasignaling but on the opposite side. A call setup message can be easily identified by the IWU without reassembly of the entire message. The message- type label resides in a fixed position of the BOM segment encapsulated in a DQDB slot, marked as signaling slot by an SVCI. If an AAL type 5 is standardized for signaling, then reassembly of the entire message can be surpassed provided that the IWU will now access all first and intermediate segments marked with the value 0 in the PT (payload type) field as recommended by 1.363. This field is also present in the QA segment header [lo].

Signaling results in the establishment of an end-to-end VCC between communicating remote MAN users, which is the concatenation of DQDB and ATM VCC’s. Hence, once signaling has been completed, the communicating IWU’s possess entries in their hash tables mapping the ATM VCI’s to DQDB VCI’s and vice versa. Note that for the user information exchange, the IWU must only provide translation of DQDB VCI’s to ATM VCI’s and vice versa. Knowledge of addresses, ST, and MID access and translation is not required by the IWU as was the case in Sections 11-A and 11-B. Instead, the QA-ST entity is now responsible for the maintenance of tables mapping ATM to DQDB addresses which are only used during the call setup phase of the establishment of DQDB SVCI’s. The QA segments encapsulated in the ATM cell payload pass

transparently through the ATM network. Protocol processing delay in the IWU is kept minimal, and the IWU behavior is similar to that of a simple ATM switch. In general, one can observe that IWU functionali’ty remains extremely simple while signaling-related higher-layer functions are shifted to the QA-ST entity of the DQDB MAN.

Iv . SUMMARY AND CONCLUSIONS

By using the logical resource of labels, i.e., VPI, VCI, ST, MID, and MAC addresses, as a unifying criterion, we classify the requirements of interworking LAN/MAN’s with ATM. The IWU functions for each interworking scenario considered are recapitulated in Table I, where the complexity of IWU’s decreases from left to right. The factors contributing to the IWU processing time are also summarized in Table I1 of the Appendix.

Due to the incompatibility of MAC frames and ATM cells, the interworking of LAN’s unavoidably resorts to the use of multiple labels at several layers as well as successive seg- mentation and reassembly in the IWU. It is shown, however, that even in the LAN case, suitable association of MAC parameters to subfields of the ATM cell header through a carefully designed relay protocol can result into relatively simple interworking with minimum protocol The interworking of MAN’s and ATM, thanks to provisions for compatibility, can be achieved on-the-fly, i.e., without reassembly of PDU’s, leading to fast and low-cost interworking implementations. Interworking is even more simple when the ATM network supports CL service via semipermanent virtual connections. It is shown that, in this case, MAC addresses are accessed for the determination of the VPI/VCI without being modified. This is not the case in the more complicated interworking scenario where the ATM network supports CL functions.

The CO MAN as presented in this paper has been identified as the less demanding and robust case in terms of interworking requirements. The advantage of signaling results in end-to- end VP/VC concatenation across MAN and ATM. Thus, the use of one label at a low level (ATM/QA) is now possible. Furthermore, the introduction of CO services to the initially CL MAN’s allows low-cost shared medium access to high-quality

1170 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 1 1 , NO. 8, OCTOBER 1993

services-an introduction strategy parallel to the one currently in progress for CL service. Thus, MAN’S are fully integrated into the long-term B-ISDN for customers with relative lower bandwidth needs and telecommunication budgets.

APPENDIX IWU PROCESSING TIME

We avoid detailed analytical studies and instead qualita- tively address, in the spirit of [42], the relative difference in IWU real-time processing efforts for the interworking cases presented. In this respect, a general partitioning formula for analyzing the time (T ) spent by the IWU for real-time processing of an incoming user-plane PDU (either MAC frame or ATM cell1QA slot) is presented in Table 11. Parameter k l is the processing time per octet covering copies between memories, U is the size (in octets) of the incoming user PDU (z octets) plus the PCI octets appended by the IWU protocols, IC2 the processing time per octet for CRC calculation, w the number of CRC protected octets, IC3 the segmentation time per segment, n the number of segments a frame is translated into, k4 the processing time on a per-incoming PDU basis (e.g., communication between processes), and ks the time consumed for label access and translation functions. We denote as t , the time devoted for accomplishing function i ( a to h) listed in Table I. Then, becomes equal to t,, k 3 to t d and kg consists oft,, t b , t,, t f , t f and t h . The T for each interworking column of Table I is then expressed by (1)-(7) of Table 11.

To calculate T I , T2, and T3, we note that the IWU PDU is the QA slot (z = U = 53 octets), which means that n = 0 and k5 = t h is the time consumed to translate the QA slot into an ATM cell. In ( 2 ) , IC5 is increased by t b , t,, and t , which are the times required for accomplishing the routing and addressing function. In T3, IC5 also includes t , and t f used for the substitution of routing and addressing labels. This forces new CRC calculations on the entire cell payload except the CRC field, i.e., v = 47 as provisioned in SAR type 314. For T4, the IWU PDU is the MAC frame plus the CPCS type 5 PCI and U becomes z + 2x1 octets, where 21 is the size of the CPCS type 314 PCI [3], [SI. A CRC is calculated over w = z + 221 - 4 octets, i.e., the entire CPCS type 5 PDU minus 4 octets which is the size of the CRC field [SI. The entire CPCS PDU is then segmented by the SAR type 5 in n = (z + 2x1 )I48 segments, each one consuming t d time units. Terms t b and t , refer again to routing and addressing functions. The value of U in T5 increases by x2 - z1 octets where xz represents the size of the CLNAP PCI [7] resulting in bigger n. This happens (also) due to the smaller size of the SAR type 314 payload (44 instead of 48 in the SAR type 5). Since CRC is now provided after segmentation and n is increased, higher delays for CRC calculations are expected. Note that the segmentation time t d for SAR type 314 is actually higher than the one for SAR type 5 [43]. In (6), the LAN PDU should now be encapsulated in an IMPDU which is then segmented in DMPDU’s. Each DMPDU is CRC protected. Hence, the first component of TS is T5, notwithstanding the lack of address translation performed (t,). The second component of TG is T2, as given in (2) .

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Iakovos S. Venieris (S’88-M’92) was born in Naxos, Greece, on March 3, 1965. He received the Dip1.-Ing. degree from the University of Patras, Patras, Greece, in 1988 and the Ph.D. degree in electrical engineering from the National Technical University of Athens (NTUA), Athens, Greece, in 1990.

In January 1988, he joined the Telecommuni- cations Laboratory of NTUA, where he is now a Research Associate. His research interests are in the fields of B-ISDN, high-speed LAN’s and MAN’S,

all optical networks, internetworking, signaling, resource scheduling and allocation for network management, modeling, performance evaluation, and queueing theory. He has over twenty publications in these areas. He has received several national and international awards for academic achievement. He has been exposed to standardization body work and has contributed to NA5 of ETSI and SG XVIII of CCI’lT. He is participating in several RACE and ESPRIT projects dealing with B-ISDN protocols, ATM switching and MAC techniques. He is a reviewer for the IEEE TRANSAC~ONS ON COMMUNICATIONS and the IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS.

Dr. Venieris is a member of the Technical Chamber of Greece.

John D. Angelopoulos was born in Athens, Greece, on March 1, 1950. He received the Dip1.-Ing. degree in electrical engineering from the National Technical University of Athens (NTUA), Greece in 1973, the M.Sc. degree in electrical engineering from the Nottingham University, England, in 1977, and the Ph.D. degree in electrical engineering from NTUA in 1993.

From 1977 to 1989, he worked for the aerospace and telecommunications industries, first as an R&D engineer in control and communications and contin-

uing later at various engineering management levels. He was involved in the design and development of military switchboards, encryption equipment, fire control computers, etc. As Head of the Digital Systems section, he supervised the company participation in national and European research projects in the fields of control and communications. Since 1989, he has turned to an academic career, joining the Technological Institute of Pireus as Assistant Professor. He is in parallel participating in NTUA research activities in the area of broadband networks. He has been involved in RACE and ESPRIT projects relating to MAN’S and ATM. His research interests include shared medium access to B-ISDN, high-speed LAN’s, hardware and firmware for high-speed communications.

Dr. Angelopoulos is a member of the Technical Chamber of Greece and the IEE.

George I. Stassinopoulos (M’82) was born in Athens in 1951. He received the degree in electrical engineering from the Swiss Federal Institute of Technology (ETH Zurich) in 1974, and the Ph.D. degree in automatic control from the Imperial College, London, in 1977.

From 1977 to 1981, he gained acquired industrial experience in the design and manufacture of microprocessor-based industrial controllers in the cement industry (AGET General Cement Company) as well as in computer networking and industrial

process control. Since 1981, he has been a member of the staff of the National Technical University of Athens, where he currently a Professor in the Department of Computer Science. His current research interests are in the fields of data communication networks, LAN’s, MAN’S, packet, circuit and hybrid switching systems, analysis and synthesis of communication networks, routing, flow control, and queueing theory. He has over forty publications in these areas. He has participated in several national research programs dealing with data communications networks. These included development of modeling, simulation, analysis, and design packages for packet and circuit- switched data data networks. He has also participated in many Race projects. He is a reviewer for the IEEE TRANSACTIONS ON COMMUNICATIONS, the IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, and the Computer Networks and ISDN Systems Journal.

Dr. Stassinopoulos is a member of the Technical Chamber of Greece.