Operational experience with DQDB MANs

Operational experience with DQDB MANs Alessandro Fenyves and Alessandro Lazzari

In most developed countries, Metropolitan Area Networks (MAN) are being field tested (or it is planned to do so) as an intermediate, evolutionary step toward Broadband Integrated Services Digital Networks (B-ISDN). This paper deals with operational experience gained with the field introduction of MANs in Italy. Specifically,a DQDB-based MAN is described which is the basis of various implementations planned in major Italian cities. A field trial with the system started in October 1991 in Torino, the first example ofa DQDB MAN installed for public service in Italy, and one of the first in Europe. The experience gained so far with this field trial is reported, together with the most significant test results obtained on running the system. Considerations on system evolution and a description of other similar MAN installations planned in the short-term in Italy are also discussed. Keywords: metropolitan area networks, DQDB

A MAN is a set of interconnected subnetworks, each of which shares a communication medium much the same as a LAN. A MAN can cover large areas, typically but not limited to a large city, and connects many users to a variety of services. For a MAN subnetwork, the IEEE 802.6 Committee has defined a protocol stack for the MAC and physical layers. The MAC access method is referred to as DQDB (Distributed Queue Dual Bus). Here, data units are segmented into fixed length slots formatted as ATM cells. A DQDB MAN allows one to allocate the total channel capacity to any mix of packet and isochronous data transfer, it is independent of the physical medium, and can operate over existing PCM line systems. A complete specification of the standard and an introductory overview can be found elsewhere I-4. Siemens Telecomunicazioni SpA, 1-20060 Cassina de" Pecchi, Milano, Italy 0140-3664/93/010019-08 © 1993 Butterworth-Heinemann Ltd computer communications volume 16 number 1 january 1993

The IEEE 802.6 standard has been specified by Bellcore in tile access network of SMDS (Switched Multimegabit Data Service), which is being offered in the United States and which is presently under consideration by the European SMDS Interest Group (ESIG). In addition, the European Telecommunications Standards Institute (ETSI) has reached an advanced stage in defining a public MAN in Europe based on a user/network interface similar to that of SMDS.

FIELD TRIAL SYSTEM Since October 1991, the first DQDB MAN installed in Italy for experimental public service has been in operation in Torino. This field trial system is manufactured by Siemens and provided to the national network operator SIP. The system stems from an original design from the Australian company QPSX, a pioneer of the DQDB concept. A short description of the system architecture and its features is given in the following sections.

System architecture Only four types of functional groups, or Network Elements, set up the MAN: the Customer Gateway (CGW), which interfaces user equipment to a userdedicated subnetwork or CAN (Customer Access Network); tile Edge Gateway (EGW), which connects a C A N to the MAN backbone, called the MSS (MAN Switching System); the Subnetwork Router (SR), for linking subnetworks of the same MSS; and the Customer Network Interface Unit (CNIU), which merges C G W and EGW functions, and which is normally used when the MAN backbone crosses the customer's premises.

19

O p e r a t i o n a l e x p e r i e n c e with DQDB MANs: A Fenyves and A Lazzari

Fi~tre 1 shows an example MAN. The Network Management Centre (NMC) performs operation and maintenance functions; it is a commercial computer that communicates with the MAN equipment via dedicated data links (tnvo are shown in Figure 1). The dual bus of each subnetwork propagates slotted frames of 125 ps in the two opposite directions; the number o f traffic carrying slots d e p e n d s on the transmission bit rate. Figure 2 shows the 27-slot frame o f a 140 Mbit/s subnetwork. Slots in a frame may be assigned in any mix either to packet or to isochronous data. Figure 3 shows the layered protocol architecture of the MAN with the additional proprietary layers running on top o f the 802.6 MAC. For connectionless service, user applications are linked to LAN 'ports' supporting bridge or router services. For single or multi 64 kbit/s channel isochronous services, provided by setting up semi-permanent paths under the N M C control, physical access is through standard 2 Mbit/s ports.

Network equipment The basic system building block is the Cluster, a set o f interfaces and Access Units (AU), or DQDB nodes,

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connected to the same subnetwork, sharing c o m m o n equipment and controlled by a Cluster Management Processor (CMP). A cluster consists o f four different hardware units: the Overhead Unit (OHU), incorporating the CMP and the transmission line terminals; the Extension Unit (EXU), for equipping extra Access Units; the LAN Bridge Unit (LBU), for equipping extra bridge type ports; and the Mtdtibus Unit (MBU), for connecting router type ports. Figure 4 shows the front image of a C G W cluster equipped with an OHU, an EXU and an MBU 5.

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c o m p u t e r c o m m u n i c a t i o n s v o l u m e 16 n u m b e r 1 j a n u a r y 1993

Operational experience with DQDB MANs: A Fenyves and A Lazzari

Network management The Network Management System (NMS) is a combination of hardware and software providing both an interface to the operator and facilities for controlling, configuring and monitoring the MAN. It comprises a centralized N M C and the peripheral CMPs of the clusters. The NMS encompasses five functional management areas according to ISO 7498-4, i.e. Configuration, Fault, Performance, Account and Security Management. For organizational reasons, the NMC is usually located where Operation and Maintenance Centres for other networks are already installed.

existing telephone exchange buildings, or offices. As a general rule, a C G W should be connected to the closest SIP exchange; this implies that distinct sites of the same customer might be connected to different SIP offices, as for Sites 3 and 5, both pertaining to the university. The MAN spans over three SIP offices, from where a total of six CANs depart. Office No. 1 (Lancia) also hosts the NMC. The MSS is a looped bus subnetwork, while all CANs are point-to-point open busses. All subnet transmission links are fibre optic cables operating at 34 Mbit/s. CGWs have been equipped in a standard way with three 802.3 bridge ports, one 802.3 router port, and one 802.5 router port with IP, DECnet and XNS routing protocols.

TORINO PLANT EXPERIENCE The DQDB MAN of Torino is a part of the SIP strategic plan to gra~lually introduce high-speed data services on a national basis. The Torino D Q D B network links together LANs located at sites spread over the entire metropolitan area pertaining to different selected organizations or customers. The system allows data communication both among distinct customers sharing common traffic interests, and among distinct sites of the same organization located in different city areas. Network

topology

Figure 5 shows the MAN configuration as of the end of March 1992 ('Phase-2'). To isolate public traffic from customer's own traffic, a CAN architecture was chosen. Edge Gateways (EGW) are located in safe places, i.e. in

Phased deployment and on-line growth The MAN topology described above was reached in a two-step sequence. On October 28 1991 a reduced 'Phase-l' configuration was cut in service comprising three out of the six customer's sites. Figure 6 shows this first configuration, which remained in service for about four months, until the full configuration, known as 'Phase-2', was reached. Figure 7 shows the total data traffic flowing in the Torino MAN 'Phase-l' during February 1992. Growing from 'Phase-l' to 'Phase-2' took place gradually, with on-line procedures applied to the live network, so gaining experience on how to modify a

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MAN topology with minor service disruption. Part of this experience is summarized below. Adding new nodes to a working cluster This procedure was frequently needed, e.g. when two new EGWs were added in Office 2. As nodes were physically already in place, only NMC commands were given to initialize and activate them. On tile other hand, if a new physical module had to be inserted to implement a new node, we had to power down the cluster, causing a cluster bypass to happen, plug the new module into the cluster and power up the cluster again. This procedure does not impact the rest of the subnetwork, but disrupts service to the other AUs in the cluster. This drawback will be eliminated in the next system release (1.1.A) by the so-called 'hot swap' feature, so enabling such an operation on a powered active cluster. Inserting new clusters in a looped bus subnetwork This operation was needed at Central Office No. 1 (Lancia), where only transmission equipment existed in 'Phase-l'. Two directly connected OLTEs (Optical

22

Line Terminal Equipment) were already in place. Removing the interconnecting by-pass coax cables caused the subnetwork to open and reconfigure. This typically lasts a few seconds, during which time all traffic across the subnet is blocked. When the dual bus restarted in the open mode, traffic resumed automatically. The equipment was then connected to the OLTEs; being in its by-pass state, it behaved as transparently as with the previous by-pass coax cables. As the adjacent clusters sensed that full connectivity had been restored, a reconfiguration of the subnet from open into looped bus mode took place, and powering on the new cluster caused its loading and initialization from the NMC. Adding a new CAN This operation implies two consecutive steps: put new nodes/network elements in service and activate the reconfigured MAN. During growth to 'Phase-T, this sequence was repeated for any CAN added. Before newly defined network elements can be activated, an. updated configuration database has to be loaded. This is done on-line through NMC commands without disturbing network traffic at all. After its physical installatign, the new equipment is initialized using the updated database. At this point the MAN is not fully operational, since not all the configuration tables in the network are updated. In particular, while new CGWs of a CAN can send data to any destination, they cannot be reached from the other CANs. To complete the reconfiguration, all 'old' clusters in the MAN must be re-initialized one at a time by loading updated routing tables from the NMC. During the updating sequence, some of the clusters know the new configuration, while others still work with the old one, so not all traffic relations are possible. This transient state is very short, however: as more clusters restart with the updated configuration, the more C G W combinations are brought on line, until the last cluster restarts, making the MAN completely operational under the new configuration.

Network ser~ces

The Torino MAN presently provides a pilot service. This means that connectivity is guaranteed during normal working hours and tile tariffpolicy is provisional. Apart from this, the system started from the outset as fully operational in all technical aspects. At the moment, only connectionless service is provided with LAN bridge and router ports that can terminate either Ethernet or Token Ring. Bridge ports are organized into predefined service groups which functionally are equivalent to geographically dispersed multiport MAC bridges with address learning and ageing capabilities. The router service corresponds to a layer-3 relay subsystem that at the time of writing can

computer communications volume 16 number 1 january 1993

Operational experience with DQDB MANs: A Fenyves and A Lazzari

support TCP/IP, DECnet IV, XNS, IPX and SNA protocols. Following agreement with customers, it was decided that initially only DECnet and IP routing service should be used. The three initial customer sites had one Ethernet LAN each to be connected through the MAN; these LANs were already internetted through routers and 2 Mbit/s leased lines. Thus, by removing the existing routers and attaching each LAN to the 802.3 router port of the relevant CGW, service could resume in a painless way through the MAN without any modification to LAN node addresses. The bridge service has been extensively used for experiments and performance evaluation tests. Its regular utilization is under study for the interconnection of limited size Ethernets, where the higher throughput characteristics of bridges could be better exploited.

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Future developments The Torino MAN is subject to a continuous evaluation regarding future growth and service upgrading. Shortterm expansion plans (around the end of 1992) imply an increase of about 50% in the number of customer sites. For service upgrading, new features made available by product evolution (see below) will be gradually introduced, on demand or when needed, into the existing customer service sets. Of particular interest is the solution proposed for expanding the Fiat network, namely a kind of dedicated, CNIU-based MSS subnetwork connected to the MAN backbone through subfietwork routers (SR). Among the advantages of this solution are a higher degree of security (all the traffic carried by the subnetwork pertains to Fiat), easier traffic monitoring and accounting operations, and less throughput limits for both Fiat internal traffic (less network elements crossed) and Fiat external traffic (capacity of SR is higher than that of EGW). In addition, a CGW should be installed downtown at the 'Provincia' palace and connected via a CAN to an additional EGW at SIP's Office No. 3. By the end of 1992, the Torino MAN should then assume the combined CAN/CNIU architecture shown in Figure 8 ('Phase-T).

Milano. System acceptance with the customer was based on these tests. Many of them were also repeated for field acceptance testing before cutting in service and in case of major network changes. The second test plan, having the specific purpose of evaluating the system's performance in the field, is being executed mainly by CSELT. Although not yet completed at the time of writing, this activity has already provided many interesting results.

Acceptance testing The test plan agreed with SIP and CSELT comprised five basic system areas. The main items for each area are listed in the following:

Basic connectivity (verifying singlecast and multicast communications, both for bridging and for various protocol routing services)

S Y S T E M AND P E R F O R M A N C E T E S T I N G Two kinds of test plans have been conceived for the MAN. Both have been substantially agreed upon with SIP and CSELT. The first test plan aimed to verify that all the required features were supported. It was carried out in detail during the summer of 1991 in the Milano laboratories on the 'Phase-l' equipment before it was installed at Torino. Figure 9 shows the MAN equipment being tested in the Siemens laboratories in

computer communications volume 16 number 1 january 1993

Figure9

MAN equipmentduring lab testing in Milano

23

Operational experience with DQDB MANs: A Fenyves and A Lazzari

• • • •

Configuration nlanagenlent (hardware configuration procedures, startup, faults, alarms, diagnostics) NMCoperation (shut-downand s t a r t u p o f N M C ; backup and restore of database) Security (operation with service groups; access to the N M C through passwords) Network admhlistration (gathering of accounting records, retrieval and backup; logfile management; performance monitoring).

Tests carried out by Siemens for their own verification purposes were basically the same as the acceptance tests, as were the instruments used.The instrumentation comprised portable PCs equipped with Ethernet and Token Ring adapter cards running various routing protocols. Bridge connectivity was mainly tested through IP echoes ('Ping' program). Specific checks required the use o f a LAN protocol analyser. Of course, extensive u s e . o f the N M C and its terminals was necessary for mhny tests.

Test methodology The acquisition campaign of performance parameters being carded out by CSELT and SIP aims to get a comprehensive picture of the quality of service offered to users. Parameters of interest are, among others, transit delay, throughput, accuracy and service availability. Tests are performed on the live network using dedicated CANs (i.e. CANs loaded only with test traffic) which are made available in dedicated time windows. Tests are carded out separately for bridges and for routers. In both cases, delays and maximum throughput (stress test) are evaluated; only Ethernet communications have been tested so far. The delay is measured by sending a single IP packet from a specifically programmed LAN analyser at one location (CGW) to a PC at another location. The PC, running idle waiting for the packet, immediately sends back a reply packet to the analyser, which reads the round trip time. By subtracting the previously measured answering time of the PC from this value yields the network delay. This delay is the sum of two components: delay across the CGWs, and delay across the rest of the network (EGWs and CANs plus MSS). If the PC and the analyser are connected on the same C G W cluster, the second component is eliminated and it is thus possible to evaluate the C G W delay time. Throughput measurements make use in principle of the same CGW-to-CGW communication. In this case one has to find out where, in the chain of crossed network elements, the bottleneck is located, and arrange the test to evaluate its throttling effect. Previous tests had shown that the bottleneck was not the EGW, so particular attention was paid to peripheral elements. The stress traffic is generated by one or more high

24

performance analysers, capable of overloading the Ethernet bus. Stress loading of the receiving end may require more than one traffic source. Incoming traffic is measured with another LAiN analyser.

Test results Additional data are necessary for a definitive judgement on each aspect of interest, although some stable conclusions can already be drawn from the statistics collected (reported elsewhere 6, with details on test arrangements). The results obtained, which in some cases show better values than those measured during lab testing, can in general be considered satisfactory 7. Some highlights are given in what follows. Table I shows throughput and delay values for the 10 Mbit/s Ethernet bridge service. It can be noted that the bridge port throughput is slightly higher in the receiving than in the transmitting state. In either case, throughput values correspond to the steady state condition that cannot be overriden without loosing packets. Length values are total bytes of Ethernet frames. Even if presently lower than expected due to known hardware reasons, throughput is higher than that attainable through commercial remote bridges, against which the MAN service should compete. Table 2 shows throughput and delay values for the router service using the IP protocol. No throughput difference is seen between sending and receiving router ports. As for bridges, throughput values correspond to the condition just prior to packet losses. It has to be pointed out, however, that the indicated throughput values cannot always be sustained indefinitely, since they might cause a C P U crash in the peripheral routers. A router would automatically exit from this transient situation as soon as the offered traffic goes down; nevertheless, the maximum allowable figures must be Table 1

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computer communications volume 16 number 1 january 1993

Operational experience with DQDB MANs: A Fenyves and A Lazzari

decreased accordingly. Further measurements are under way to determine safe values. The CAN-to-CAN delay figures are of course the same as in Table 1.

ation of requests from private service providers planning to offer broadband services to their own customers within a private area.

PLANNING OTHER MANs

FEATURES AND NETWORK EVOLUTION

Other DQDB MANs from Siemens are planned in Italy in the next few years. Of particular interest in the scientific field is a MAN which has to be installed by the fourth quarter of 1992 in Pisa under the patronage of the National Research Council (CNR). This network, shown in Figure 10, will connect a number of universities and research institutes of the town of Galileo. SIP will be the ne~vork operator and C N R the customer. Plans exist for connecting the Pisa MAN via an MSS router to a similar MAN to be installed in Firenze in the same timeframe. The two MANs should provide the basic infrastructure for the test of scientific applications requiring high-speed data traffic, such as an advanced service of high-definition radiological image exchange. Even though mainly devised for public service, the system described readily adapts to private environments, so that private or single customer, multi-site dedicated networks can easily be implemented. An example of this type is a DQDB MAN planned for service launch by the end of 1992 in Roma in its 'Phase-l' development. This network features a single customer, multi-site topology spanning a very large metropolitan area; a CNIU instead of a CAN-architecture; and a 140 MBit/s line speed to cope with the bandwidth to be assigned to planned future isochronous services. In addition, the growing interest in MAN systems for campus-like applications could soon lead to a formaliz-

The network oriented features of the present MAN product are mostly related to connectionless service, with built-in privacy and security checks obtained by means ofaddress validation and screening mechanisms. A plan of new releases scheduled for completion within the second quarter of 1993 will supply customers and network operators with new features and improved performance, leading to a substantially improved MAN system. A full support ofisochronous connections and a Tl-based SMDS access interface will be among the first new services available. Also, frame relay, E1 and higher bit rate (34 and 45 Mbit/s) SMDS/CBDS accesses will be made available at a later stage. Inter-MAN traffic will be supported, both through direct connection of separately managed MANs and, later, across ATM-based connections made possible by a specifically developed interworking unit, which will initially be based on the ICI (Inter-exchange Carrier Interface) interface specified by Bellcore. New packet processor modules in the AUs will improve the throughput in applications such as EGWs and SRs, so enhancing system performance. The present limitation of one E G W per CAN will be eliminated, allowing a CAN to be connected to the MSS through parallel edges for increased reliability and throughput. The spanning tree approach presently used to interconnect more MSS sub-networks via SRs will be replaced by more efficient load sharing algorithms. Also, management functionalities will be enhanced. In addition to the above-mentioned interconnectibility of MANs controlled by different NMCs, better diagnostics capability, intra-cluster interface monitoring, end-to-end echo tests, improved alarms, accounting and statistics facilities will make network management more powerful and easier. All the above features will gradually be introduced into the Torino MAN as well as in any other MANs installed in between. This will issue a newchallenge, i.e. that of installing a new system release without bringing down the network. How and to what degree this target will be met is still a matter of internal debate and speculation and might become the subject of a future report.

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computer communications volume 16 number 1 january 1993

The field trial started on October 1991 in Torino - the first example o f a DQDB MAN for public use in Italy,

25

Operational experience with DQDB MANs: A Fenyves and A Lazzari

and one o f the first in Europe - has provided m a n y encouraging results. Performance measurements carried out by SIP and C S E L T on the running system in some cases provided better values t h a n those more conservative ones obtained during lab testing. Positive c o m m e n t s o f the customers, reporting improvements in their data networks since connection to the MAN, are probably the best practical confirmation o f the satisfactory results reached with tiffs field trial. In effect, such improvements may also partially derive from the new fibre optic transport network and the higher stability o f the M A N equipment. Further expansion o f the Torino M A N is planned in the short-term. The n u m b e r o f c u s t o m e r sites should have increase for example by about 50% by the end o f 1992. T h e experience gained with on-line growth procedures will o f c o u r s e help in reaching this without significant service interruptions or disturbances. Additional, MAN system installations p l a n n e d in the short-term, one for public use in Pisa u n d e r the patronage o f the National Research Council and one for private use in R o m a for a large multi-site single

26

customer, are examples o f the successful stepwise introduction o f this b r o a d b a n d technology in Italy.

REFERENCES 1

2

Distributed Queue DuaIBus (DQDB) Subnetwork of a Metropolitan Area Network (MAN). IEEE Standards Board, USA (1991) Newman,R Met al. 'The QPSX MAN', IEEE Commun. Mag.

3

(April 1988)pp 20--28 Neer, B and Zitzen, W Schnellstrasse zum Breitband-ISDN. Metropolitan Area Network yon SiemensJ~r breitbandigen hlformationsaustausch mit tlochgeschwbldigkeit. SiemensTelcom Report

4

5

6 7

4 (1990) Coronaro,M et aL "Metropolitan area networks: standards, services and performances', Proc. CNR Semhzar on Broadband Communication Networks & Sen'ices, Rome, Italy (30-31 October 1990) Metropolitan Area Nenrorks. Product Description, Siemens AG; Ordering No. A30930-N2420-II-l-7618(1991) Cicczrdi,A and Ferrero, F "Systemand service evaluation on high-speed network field trials', Proc. EFOC/LAN '92, Paris, France (24-26 June 1992) Di Pino, D Le Reti Pubbliche Metropolitane (MAN) nel Contesto dell'Evoluzionedellehzfrastrutturedi T L ~ Convegno IRI (Milano 2-3 April 1992)(inclusive of the Case History 'I! Politecnico di Torino" by S Gai)

computer communications volume 16 number 1 january 1993