PERFORMANCE OF INTERCONNECTING FDDI NETWORKS

THROUGH ATM WHEN MANAGING VIDEO TRAFFIC*

David Andrés, Joan Vila, Jordi Domingo and Josep Solé

Departament d’Arquitectura de Computadors, Universitat Politècnica deCatalunya, Campus Nord, Mòdul D6, 08071 Barcelona, Catalonia (Spain)

E-mail: {estasen|joanv|jordid|pareta}@ac.upc.es

Abstract

The interest in LAN interconnection through high speed networks is becomingincreasingly important because new applications with stronger timing andbandwidth requirements are arising. This paper studies the end-to-endperformance achieved when routing FDDI traffic through an ATM network. Trafficconsists of merged IP and coded, packetized video. A main contribution of this workis the use of real traffic traces obtained from video codecs and a LAN router to carryout the simulations. The delay variation introduced by the different networks isstudied, together with the loss of information due to overflows at the ATM accessbuffer and excessive delays. Results are shown for different sizes of ATM accessbuffer and virtual channel bandwidths. The proposed configuration has shownadequate performances, but it requires strict buffer and virtual channel sizing. Toimprove system robustness, a time-out policy for packets in the buffer has beenadded, with successful results.

Keyword Codes: C.2.1

Keywords: Internetworking, ATM (Asynchronous Transfer Mode), PerformanceEvaluation.

1. INTRODUCTION

In present days, the communications world points to integration of all existingnetworks into a world-wide, all-purpose network, B-ISDN (Broadband IntegratedServices Digital Network), which will be based on ATM (Asynchronous Transfermode) [1]. Therefore, interconnection of LANs through ATM is becoming a matterof interest [2]. This work intends to make a contribution in this direction.

FDDI (Fiber Distributed Data Interface) is currently becoming the technologyfor high speed Local Area Netwoks (LANs), because of the high reliability andperformances of fiber optics, the variety of available topologies and the wide rangeof types of traffic it can manage [3].

* This work has been supported by CICYT (Spanish Education Ministry) under contract TIC 92-

1289-PB.

Up to now, several studies have covered the interconnection of FDDI networksthrough ATM [4,5] by modeling traffic sources using analytical models. Acontribution of the present work is the use of real traffic to model the differentsources, in order to obtain more realistic results. Another contribution is the use ofa router as a gateway in order to achieve a global connectivity.

This paper is organized as follows: Section 2 describes the studied configuration.Section 3 explains how this configuration and its traffic sources have been modeled.Section 4 focuses on simulations and their results. Section 5 analyzes the effects ofadding a time-out feature. Finally, Section 6 is a summary with the conclusionsand further work to continue.

2. WORKING SCENARIO

The configuration evaluated in this work consists of an interconnectionenvironment of two FDDI LANs through an ATM network, as depicted in Figure 1.The elements forming this environment are described below. This system offers aconnectionless service among end users.

2.1. FDDI LANs

Both FDDIs support 35 stations, uniformly situated along the ring in such a waythat the distance between two neighboring stations is 200 m. Only FDDI-I mode isconsidered. There are two types of stations, characterized by the kind of traffic theymanage:

- Asynchronous traffic. It consists of data packets from a router which linksseveral LANs to the backbone. All these communications use the TCP/IP protocolsuite.

- Synchronous traffic. This kind of traffic corresponds to video sequences. Eachframe is packetized by using as many UDP/IP frames as needed when enteringFDDI.

Figure 1: Working Scenario

Packetization of video traffic in IP frames allows it to be transmitted over anykind of network whenever its bandwidth and delay jitter performances satisfy therequirements of the terminal equipments. It is assumed that video traffic istransmitted in real time, so retransmissions make no sense. Therefore, thesimplicity of UDP is preferred to the reliability of TCP [6].

2.2. ATM network

This network is based on Virtual Channels, and its performance corresponds tothe first expected features of B-ISDN: peak rate is the only policeable trafficdescriptor, and no change in the traffic descriptor is allowed during thecommunication [7,8]. This choice leads to the following consequences:

- Users are responsible for not exceeding the contracted peak rate.

- The technology giving support to this network (fiber optics) ensures a very lowbit error rate.

- Cell loss and delay variations within the network are very low, due to the strictconditions of access, which limit the peak rate of ATM bursts.

- The connectionless service is offered over semi-permanent virtual paths andchannels.

2.3. FDDI-ATM adaptor

This adaptor behaves as a router since it transforms IP packets from FDDI intoan ATM call sequence, and reconstructs IP packets from an incoming cell stream,as shown in Figure 2. While the use of remote MAC bridges is an appropriatestrategy for providing communications within a single organization, thehierarchical addressing facility of routing allows communication between LANs ofdifferent organizations, which may use different MAC layer protocols [7]. Then, theadaptor does not intend to extend the FDDI networks but to incorporate them intothe Internet community over the ATM network, so the use of a router is fullyjustified.

The ATM adaptation level provides several access points, which are appropriatefor different kinds of services: AAL1 for circuit emulation; AAL2 for variable-rateconnections; AAL3/4 for reliable data transfers; and AAL5 for simple and efficient

Figure 2: Protocol stack

data communications. For LAN traffic both AAL3/4 and AAL5 are suitable [9].AAL5 has been adopted in this work because AAL3/4 uses several overhead fieldswhich do not have a clear application in this environment, while AAL5performances satisfy the requirements of the work.

FDDI includes a LLC/SNAP layer because it is based on medium sharing [10].ATM does not include such a layer since in this work each service is assigned anexclusive virtual channel. The functional structure of the FDDI-ATM adaptor isshown in Figure 3.

- FDDI Interface: It selects FDDI frames addressed to the adaptor and removestheir MAC overheads.

- Router: It removes LLC/SNAP headers in order to obtain the IP PDU. It finds outwhich virtual channel the IP packet is to be transmitted over by means of a look-up table indexed by the destination address field.

- ATM Interface: It adds an AAL5 trailer to the IP PDU and generates a cellstream over the corresponding virtual channel at the contracted rate.

The interface between the router and the ATM module consists of a memory areawhere the router stores packets. The router indicates, for each packet, its positionin the interface memory and the virtual channel over which ATM transmissionshould occur. The ATM module segments one packet into a cell stream, whose peakrate is contracted by the user. then it indicates to the router the end of packettransmission [11].

3. SYSTEM MODELING

3.1. Traffic modeling

One of the key aspects of this work is the use of real traffic traces. This lets theresults be based on real conditions rather than on suppositions led by theoreticalmodels [12].

Asynchronous data come from traces recorded at the connecting point of thedepartment router to the campus network of the university. This router is fed bythe aggregate traffic from several LANs and interconnection devices. The traceshave been obtained at different dates and times and correspond exclusively to IPtraffic.

Figure 4 displays the average load from each of the 31 stations devoted to

Figure 3: Functional structure of the FDDI-ATM adaptor

asynchronous traffic, in order to show the volume of traffic they manage. HerePayload stands for the whole IP frame, and Overhead includes MAC FDDIoverheads. The total load from these stations is 1.02 Mbps. Overheads take 118.87Kbps, so 905.66 Kbps are left for payloads.

The shortest recorded IP packet length is 35 bytes (which corresponds to 1 ATMcell) and the longest is 1500 bytes (32 ATM cells). This is due to the fact that allLANs in the campus are based on Ethernet, and IP packet size is set to fit Ethernetframe. This allows a further study based on Ethernet instead of FDDI withoutsignificant changes in the simulation algorithms.

In the case of synchronous traffic, the traces have been recorded directly at theoutput of video codecs using DCT-based variable bit-rate coding algorithms. Theirframe frequency ranges from 10 Hz to 50 Hz, in order to avoid correlated results.Progressive sequences (1 video frame per picture) and interlaced sequences (twoframes per picture) have been considered, in order to achieve a generalenvironment. The traces are read in a circular way, i.e. when the end is reached, itrestarts reading from the beginning. Table 1 shows the most important parametersof these traces.

The values in the “Average Bit-Rate” column are computed by dividing the totalamount of bits in the sequence by the time needed to transmit them. As thesequence bit-rate is variable, each frame may be transmitted at different bit-rates.

Table 1: Characteristics of the synchronous traffic traces

TraceFrame

FrequencyNumber of Frames

InterlacedAverageBit-Rate

Average Overhead

Load

MaximumBit-Rate

1 50 Hz 640 Yes 1.86×107 8.08×105 3.68×107

2 10 Hz 79 No 6.73×105 3.11×104 1.16×106

3 25 Hz 150 No 6.90×106 3.01×104 9.55×106

4 30 Hz 225 No 4.69×106 2.08×105 1.06×107

Figure 4: Average load for each asynchronous station

The “Maximum Bit-Rate” column contains the bit-rate of the most intensive framein each sequence.

Trace 1 has been provided by the Instituto Superior Tecnico in Lisbon, Portugal.Traces 2 to 4 have been obtained by Telefonica I+D in Madrid, Spain.

To improve the system performance by reducing the effect of overheads, themaximum packet length has been set to a greater value than the standard 576bytes. The chosen value is 1500 bytes in order to make the interface be compatiblewith Ethernet. It is low enough, compared to FDDI frame size (4500 bytes) andAAL5 maximum frame size (65536 bytes), which smooths the effects of packetlosses due to single bit errors.

The average load due to synchronous traffic is 32.21 Mbps. The load due toasynchronous traffic is 1.02 Mbps. Therefore, the total average load is 33.23 Mbpsincluding overheads, so FDDI works at a third of its capacity.

3.2. FDDI modeling

FDDI has been simulated by assuming that the bit-delay at every station is zero.Propagation time has been taken into account (lightwave speed). Several values ofTTRT (5, 10 and 20 ms) have been tested [13], without observing significantdifferences among their results. Initially, TRT is set to zero for all stations. Thepreamble length in all FDDI frame headers has been set to 16 symbols (8 bytes) [3].

Video frames are transmitted by means of UDP/IP packets whose maximumpayload length is 1500 bytes, as justified above.

In order to model the system real behavior without the availability of real ATMtraffic records, the simulations are carried out as follows:

a) FDDI stations send traffic to one another and the adaptor.

b) The adaptor generates a cell stream.

c) Step a) is repeated by replacing traffic coming from the reference station by atrace recorded at step b) containing information about the packets from thereference station which have crossed the adaptor.

3.3. ATM modeling

The ATM network is assumed to introduce a constant delay. This assumption isjustified by the fact that this work is based on the study of IP packet delayvariations, and the contribution of cell delay variations to the value of packet delayis very low.

The adaptors access the ATM network through a physical link of 150 Mbps,which is equivalent to 354 Kcells per second.

The cells are transmitted at most at the contracted peak rate. If a burst of packetarrivals takes place, the buffer is likely to overflow, so information may be lost.Synchronous and asynchronous connections use virtual channels which areassigned different peak rates. the ATM network is not aware of the service eachconnection manages.

3.4. Adaptor modeling

The critical points in modeling the adaptor rely on communication among theinternal stages. In order to simplify the problem, the following assumptions aremade:

- The FDDI interface does not require to buffer traffic coming from the FDDI ring,as it only passes to the router the frames addressed to it and discards the rest ofthem, so no delay is considered at this step.

- In the FDDI interface, buffering is considered only at the LAN access from theinterface. At this point, packets are waiting for the interface to get the token.This buffer is supposed to be large enough to produce no packet losses.

- The router processing time is always considered constant and no buffer isrequired for communications with the FDDI interface.

- The only buffer whose losses are considered is the one located at the access pointto ATM, where packets wait for their segmentation into cells, because it hasshown to be the most critical. The reason is that bursty traffic uses a constantrate connection, what requires an important buffering capability with associatedpacket losses and delay variations.

- The reassembling process is supposed to be faster than peak cell arrival, so thebuffer at this point only needs to be able to hold one IP packet. Once reassembled,this packet is forwarded to the router. No losses are thus expected if the bufferis correctly sized.

The buffer giving access to ATM from the adaptor is shared by all connections,and its implementation is based on dynamic memory management. It represents atrade-off between spatial and temporal optimization. The considered buffer sizesare: 64 KB, 128 KB and 256 KB.

As stated above, the ATM network offers a maximum stream of 150 Mbps (whichin turn is equivalent to 354 Kslots per second). Each virtual channel requests thenext available slot if there is a cell to transmit, at most at the contracted cell rate.The packet output time-stamp is considered as the instant of departure of the lastcell of the packet

In this work, only delay variations on whole IP packets are of interest. As statedabove, ATM is not considered to introduce any delay variations. Thus the time-stamp used at the remote FDDI interface is the same as the one at the local adaptoroutput.

4. TRAFFIC MEASUREMENTS

4.1. Simulation

The simulations model 60 seconds of real time system performance, what

represents 2.12×107 ATM slots.

Asynchronous traffic acts as background traffic for both FDDIs and the firstATM adaptor. According to the destination IP network address, traffic is eitherprocessed by the adaptor or discarded.

Concerning synchronous traffic, Trace 1 is taken as the reference and the effectsof the system on the traffic generated from it are studied from end to end. The otherthree traces are studied only until the first adaptor output.

As initial conditions, all buffers are empty and TRT is set to zero at all FDDIstations. A transient period of 2 seconds is allowed in order to ensure that onlysteady-state results are obtained.

To set peak rate for video connections, the average bit rate of each trace has beentaken into account. However, the direct assignment of this value to the peak rateis not adequate for two reasons:

- Video stations generate variable rate traffic, what may originate importantdelay variations.

- Each protocol involved in the transmission adds its own overheads, as shown inFigure 5. UDP/IP introduces 28 bytes [14]; AAL5 adds 8 bytes plus a pad to reachan AAL5 PDU length multiple of 48 bytes [9], and ATM adds 5 header bytes percell.

For each virtual channel, the cells are transmitted at most at a fixed rate oncethe packets are segmented. The packet arrivals which make this rate be exceededare buffered while possible. In this work, these fixed rates have been set tomultiples of the average rate of the video sequence assigned to that virtualchannel: 1, 1.5 and 2. In fact, factors of 1.5, 2 and 2.5 (or 150%, 200% and 250%)have been used instead, in order to consider the overheads introduced by theprotocols.

The average total load from asynchronous stations is 1.02 Mbps. As the mainfocus of the study is on video traffic, a high peak cell rate is assigned to each virtualchannel associated to asynchronous stations. In this work, 1 Mbps has beenassigned to each asynchronous station.

All the measures on delay jitter and losses are based on IP packet statistics. Theglobal system delay for the reference connection is also presented at a video framelevel.

4.2. Measured variables

The key parameters when determining quality degradation in a video serviceare: loss of information and delay jitter. In the environment studied in this work,losses of information take place at the buffers at the access to ATM, due to theabsence of flexibility on the virtual channels cell rate. Another source ofinformation losses is the reception of excessively delayed information. Theimportance of these losses depends on both the receiver specifications and theinternetwoking environment behavior, so their quantification is more subjective.

Figure 5: Frame formats at the most relevant system points

Delay variations are evaluated from three components: the adaptor access delaythrough FDDI; the delay imposed by the router-ATM interface; and delay on FDDIaccess from the remote router. The delay between the FDDI interface and therouter is assumed to be constant. These delays are considered at both frame andpacket levels. This intends to ease the quantification of their importance.

4.3. Results

The delay variation introduced by each of the elements in the studiedenvironment is shown in Figure 6.

The following facts are interesting to be remarked:

- The main source of delay variation is the ATM adaptor, because ATM has totransform a variable rate traffic sequence into a less flexible cell stream. Asuitable buffer is then required, what is the origin of delays. FDDI is bettersuited for variable rate traffic as it is based on medium sharing. All these factsconfirm the greater importance of the adaptor buffer, and hence the interest inits sizing.

- The destination FDDI introduces less delay than the local FDDI despite theirsimilar load. The source FDDI station is likely to deal with bursts of packets,which is the origin of queues and delays. These bursts are smoothed by the ATMnetwork before arriving to the remote adaptor, so buffer occupancy and thereforethe introduced delay are lower.

Figure 6 shows results corresponding to a medium buffer size (128 KB) and amedium peak virtual channel rate (200%). All the other possible graphs show asimilar pattern, but horizontally compressed or expanded according to buffer sizeand virtual channel rate values.

Global video frame delay variations for different buffer sizes and virtual channelcapacities are shown in Figure 7. This delay is computed as the difference betweenthe generation of the video frame and the reception of the last packet from theframe.

Delay variations grow with increasing buffer sizes. The lower the virtual

Figure 6: Delay variation measured at each analyzed point

channel capacities, the stronger the delay variations. If video frame reception relieson one frame buffer, packets arriving with a delay above one frame period (e.g. 20ms for 50 Hz sequences, as is the case of the reference trace) will have to bediscarded. If two buffers are available, delays of up to 40 ms are allowed, and so on.

If only one buffer is available, the number of discarded packets is very large,except for high virtual channel rates. If two frames are allowed, this numberbecomes nonsignificant at just a peak rate of 200% the average rate. For low virtualchannel rates and large buffer sizes, the buffer empties slowly, so buffer occupancyincreases and therefore delay becomes higher.

Table 2 shows packet losses from the reference sequence due to overflows in theATM access buffer and to excessive delays at reception (i.e. higher than 20 or 40ms).

Table 2: Packet losses due to overflows and excessive delays

Reason for packet losses 64 KB 128 KB 256 KB

150% Overflows 2.4% 0.87% 0.4%

Delay > 20 ms 0.8636% 3.538% 4.623%

Delay > 40 ms - 0.7407% 3.4968%

200% Overflows 0.29% - -

Delay > 20 ms 0.1144% 1.272% 1.272%

250% Overflows 0.00011% - -

Figure 7: End-to-end delay variation for the studied virtual channel bandwidths(150%, 200%, 250%)

All other cases do not show any packet loss. These results lead to a firstconclusion: A growing buffer size reduces its losses, but the use of a low rate virtualchannel may lead to transmission of excessively delayed information. Thus a longburst may reach the remote endpoint later than needed. Moreover, this obsoleteinformation may also delay next packets in the connection. Therefore, in somecases it is better to use short buffers when transmitting real time traffic.

Figure 8 shows buffer occupancy levels occurred at all studied situations. Whenthe bandwidth of the connections is suitably dimensioned, an average-sized bufferis rarely filled up. But if some connections have been allocated too little bandwidth,the trend is to use all the available buffer capacity. This case yields excessivepacket losses even for the rest of the connections sharing the buffer.

To avoid both problems, the effects of limiting the time a packet can be bufferedhave been studied. Next section shows the results of this study.

5. RESULTS WITH LIMITED BUFFERING TIME

This policy consists of removing packets which have been buffered longer than atime-out period. This is applied for all buffered packets when attempting to add anew one, and for the first packet in the queue when it is going to be segmented.

Figure 9 shows the global delay variation for a virtual channel peak rate of 150%the trace average rate, when using this policy with time-outs of 20 ms (one videoframe period) and 12 ms. The differences with Figure 7 are more outstanding forlarger buffers, so the behavior found with short buffers without time-out isachieved now with larger buffers: short delay variation and moderately high losses.

Figure 8: Buffer occupancy for each virtual channel at each buffer size

The behavior using time-out is similar to limiting buffer size. Nevertheless, thisapproach is better than reducing buffer size because all packets are given a chanceto enter the buffer so that only the obsolete ones are eliminated. Small-sizedbuffers discard packets regardless of their delay.

Figure 10 shows the results obtained in the worst case (virtual channel peak rateat 150% of average trace bit-rate). They are presented in three-column sets: Thefirst column in the set represents the case without time-out. The second and thirdcolumns represent the cases with a 20 ms (1 video frame) and 12 ms (≈ 2/3 of aframe) time-outs, respectively. Losses are shown for local buffers of 64, 128 and 256KB, and for remote buffer sizes in the end system of one and two video frames (20and 40 ms). Losses are classified according to its origin: overflow, time-out in theadaptor buffer, or excessively delayed information arrival in the end system.

Figure 9: Effect of applying time-outs (20 and 12 ms) over global delay variationsat a virtual channel rate of 150%

Figure 10: Packet losses as a function of buffer size and maximum accepted delay

Best results are obtained with larger buffer, time-outs equal to or a little longerthan one video frame and the buffer size at the end system of, at least, two frames.Larger buffers at the destination system could improve the problem of losses, butwith the drawback of increasing delay and occupying large amounts of memorywhen using high resolution coding algorithms.

The worst losses are caused by buffer overflow. In this case, all the connectionsmay suffer from damage due to information loss in an indiscriminate way.Effectively, losses due to time-out only affect the undersized connections and,usually, only obsolete information is eliminated, what reduces losses due toexcessive delay at the remote end. Excessively delayed packets have wasted systemresources along all the internetworking environment, lowering the overallperformance without any benefit on its communication quality.

A situation to be avoided anyway is congestion in the shared buffer. It can beoriginated by an incorrectly-sized buffer or connection or both. Figure 11 showsbuffer behavior for different sizes and time-outs when the virtual channel peakrate is 150% the average trace rate (again, the worst case). As shown in Figure 7,without time-out congestion still occurs even with large buffers. However, whenusing a well-dimensioned time-out, the buffer occupancy level decreases quickly, soaverage and large buffers never become completely congested. This ensures thannew packets will always have room in the buffer, since they are only removed whenthey become obsolete.

6. CONCLUSIONS

In this work, the performance of an environment of two interconnected FDDIthrough an ATM network has been evaluated when combining asynchronoustraffic (TCP/IP packets) and synchronous traffic (real time video). A contribution isthe use of real traffic traces for modeling traffic for simulations.

It has been shown than FDDI LANs are suitable for real time connectionswithout needing the use of Hybrid Ring Control when load falls into reasonablelevels (in the work, one third of the maximum capacity). The ATM network hasbeen considered to be based on semipermanent virtual paths, where only peak bit

Figure 11: Buffer occupancy behavior at a virtual channel rate of 150%

rate is policed. This limitation makes access to ATM become the hardest bottleneckin the system. If connections are slightly oversized, the overall performance of thesystem is quite good. Connection undersizing degrades its quality and seriouslydamage all other connections performance.

Introducing a time-out for the packets prevents the buffer from being congestedby excessively delayed packets and reduces delay variations for synchronousconnections. This behavior improves performance of undersized connections andprovides more robustness to the whole system.

The work presented in this paper may be continued by extending the study tocover a full B-ISDN network, with more complex access policies and policefunctions. This forces to take into account new aspects such as delay jitter and celllosses.

Acknowledgments

The authors would like to acknowledge Telefonica I+D and the InstitutoSuperior Tecnico for their samples of video traffic.

REFERENCES

[1] CCITT, Recommendation I.121, “Broadband Aspects of ISDN”, 1991.

[2] S. L. Sutherland, J. Burgin, “B-ISDN Internetworking”. IEEECommunications Magazine, vol. 31, no. 8, 1993.

[3] Several authors, “FDDI Networking. Concepts, Standards and Practice”. IBCTechnical Services Ltd, 1992

[4] L. Mongiovì, M. Farrell, V. Trecordi, “A Proposal for Interconnecting FDDINetworks through B-ISDN”. Procedures of IEEE INFOCOM, 1991, pp. 1160-1167.

[5] J. A. Manan, D. A. Harle, “Analysis of an FDDI/ATM Network Gateway”. IEETenth U.K. Teletraffic Symposium, 1993, pp. 13/1-13/9

[6] T. Sakatani, T. Tajiri, “A Video/Audio Transmission System over LANs”. 5thInternational Workshop on Packet Video, 1993, pp. H3.1-H3.6.

[7] M. Gerla, T.-Y. C. Tai, G. Gallassi, “Internetting LANs and MANs to B-ISDNs for Connectionless Traffic support”. IEEE Journal on Selected Areasin Communications, October 1993.

[8] P. E. Boyer, D. P. Tranchier, “A reservation principle with applications to theATM traffic control”. Computer Networks and ISDN systems, no. 24, 1992, pp.321-324.

[9] J. D. Cavanaugh, T. J. Salo, “Internetworking with ATM WANs”. MinnesotaSupercomputer Center, Inc., 1992.

[10] D. Katz, “Transmission of IP and ARP over FDDI Networks”. RFC 1390, ciscosystems, Inc., 1993.

[11] SARA Chipset Technical Manual. Transwitch Corporation, 1992.

[12] R. Cáceres, “Measurements of Wide Area Internet Traffic”. Computer ScienceDivision, UCB, 1989.

[13] R. Jain, “Performance Analysis of FDDI Token Ring Networks: Effects ofParameters and Guidelines for Setting TTRT”. SIGCOMM’90, 1990, pp. 264-275.

[14] U. Black, “TCP/IP and Related Protocols”. McGraw-Hill, Inc., 1992.