Performance Comparison of ATM LAN with FDDI and Fast ... - CiteSeerX

Performance Comparison of ATM LAN with FDDI and Fast Ethernet Mahbub Hassan and Leonard Oon School of Computing and Information Technology Monash University Gippsland Campus Switchback Road, Churchill 3842, Australia Email: mahbubh,[email protected]

Abstract

to contend with these high speed LANs, the performance in carrying bursty data trac has to match or exceed those of others. Previous work in the literature comparing ATM with Fast Ethernet and FDDI are very rare. In [7], the author has evaluated these three high speed LANs based on cost and functions; quantitative performance analysis is not provided. In this paper, we develop simulation model of a reference network to compare these three high speed LANs; the focus is to study the performance of identical data applications running over these three LANs. The diculty in de ning the performance criteria when comparing ATM with existing LAN technologies arises from the fact that ATM is a connection oriented, switch-based technology whereas both Fast Ethernet and FDDI are \best-eort", shared medium based. A MAC (medium access control) level performance (MAC frame delay, link utilisation, etc) comparison is, therefore, not very useful. We consider the application level throughput as our performance criteria for the comparison of the dierent LANs. The motivation is that application throughput will be aected dierently by dierent LANs. We compare the performance of TCP/IP based applications under hot-spot trac model. The hot spot model simulates the client-server environment in a local area where many stations are communicating with a single server. This model also oers the greatest challenge to the network for handling congestion. The organisation of the paper is as follows. In Section 2, the three LAN technologies are brie y described. The simulation model used to compare dierent LANs is explained in Section 3. Section 4 presents the results for performance comparison of the three high speed LANs. Finally, conclusion is drawn in Section 5.

Simulation model of a reference network is developed to compare the performance of three high speed Local Area Networks | Fast Ethernet, FDDI and ATM. TCP/IP based application throughput is considered as the performance criteria in comparing the three LANs. Our comparison is based on a hotspot trac model; many stations send bursty data trac to a single destination. We nd that for such trac scenario, FDDI performs the best. ATM performance, without any congestion control, is orders of magnitude lower than both Fast Ethernet and FDDI. We show that performance of ATM can be signi cantly increased by providing larger buer at the hot-spot output port of the ATM switch.

1 Introduction Although Asynchronous Transfer Mode (ATM) was originally proposed for the public B-ISDN, there has been much interest and activity in the last few years to realise ATM as a LAN technology [1, 2, 6]. It is expected that ATM will penetrate the LAN market before the telcos start deploying ATM for the public B-ISDN. ATM is capable of supporting all kinds of trac - data, voice, video. However, the initial use of ATM LANs will be dominated by existing data applications until multimedia applications are in place. Hence rigorous performance study of ATM LAN as a carrier for data trac is necessary before committing to this expensive and new technology in the local area. The demand for high bandwidth in the local area has led to some high speed LAN technologies in the recent years. Two such LANs are 100 Mbps FDDI, standardised in ANSI X 3.139-1987, and 100 Mbps Fast Ethernet, standardised in 802.3 as 100BaseT. These technologies are relatively much simpler than ATM and hence it is expected that ATM will be an expensive option because of the complexity of an ATM adapter or interface card [8]. If ATM is 1

2 LAN Protocols 2.1 Fast Ethernet

The new high speed (100 Mbps) version of popular 10 Mbps Ethernet is called Fast Ethernet. IEEE 802.3 has now de ned the standard for Fast Ethernet as 100Base-T. The MAC protocol is CSMA/CD, the same protocol used for other 802.3 standards, e.g, 10Base2. CSMA/CD is implemented in the network interface card (NIC) attached to every station. The mechanism of CSMA/CD is summarised below. Stations are connected to a common cable or bus. When a station wants to transmit, it rst checks the cable. If the cable is idle, it starts transmitting a frame immediately. If the cable is busy, it waits until the cable is free. After starting transmission, the station monitors the cable and aborts the transmission if a collision occurs; the station will try to retransmit the collided frame after a random period. The collision occurs when two or more station nds the cable idle and start transmitting within a short period of time. All stations share the bandwidth of the cable on a \best-eort" basis; no bandwidth is reserved prior to transmission. Ethernet was designed for the asynchronous bursty data trac; arrival of the next burst is not deterministic.

2.2 FDDI

FDDI (Fiber Distributed Data Interface) closely follows the Token Ring protocol of IEEE 802.5. Neighbouring stations are connected by point-topoint link to form a ring topology; the transmission on the ring is unidirectional. When no stations are transmitting, a token is going around the ring endlessly. A station has to grab the token before it can transmit; this prevents two stations from transmitting at the same time and this avoids any collision on the ring. FDDI implements an early token release; the token is released on the ring as soon as the station nishes transmission. This token-based protocol is implemented in the NIC. The bitrate of FDDI is 100 Mbps. Token priority can be set to reserve higher bandwidth for some stations than others. If all stations have the same priority then everyone will have fair share of the cable bandwidth. With FDDI, it is possible to reserve some bandwidth for synchronous trac; the rest of the bandwidth is shared by the asynchronous trac. However, we consider only the performance of FDDI in carrying asynchronous trac. No bandwidth, therefore, is reserved for synchronous trac.

LANs

Interface

Phys. Structure

Fast Ethernet CSMA/CD passive cable FDDI Token-passing passive cable ATM AAL active switch Table 1: Structure of dierent LANs.

2.3 ATM

Unlike Fast Ethernet and FDDI, ATM (Asynchronous Transfer Mode) is a switch-based technology. A simple ATM LAN can be built by connecting a number of stations directly to an ATM switch. The LAN can be extended by adding more ports to the switch or by interconnecting multiple switches. Although the name suggests that ATM is to support asynchronous trac, it is actually capable of supporting all kinds of trac including constant bit rate circuits. However, in this paper we only compare the performance of ATM in carrying bursty LAN trac. ATM switches accept 53-octet cells from the input ports and transmit them through the proper output ports depending on the destination of the cell. Stations connected to the ATM switch has to break large higher layer data units into cells while transmitting and assemble multiple cells into a higher layer unit at the receiving end. This fragmentation and reassembly is a function of the ATM Adaptation Layer (AAL) and is implemented in NIC. There are several data rate standardised for the links between a station and an ATM switch including a 100 Mbps multimode ber link [3]. A station wishing to transmit data to another station connected to the ATM LAN has to set up a Virtual Connection (VC) before sending data. A VC number is carried in each cell header and the routing of the cell within the ATM switch is based on this VC. There are two kinds of VC | switched VC (SVC) and permanent VC (PVC). With SVC, a VC is opened and closed when needed whereas the PVC is established at the LAN setup and remains so forever until changed by later by the LAN administrator. Since SVC has the overhead of connection set up, it is expected that PVC will be popular at the early trial of ATM LANs.

3 Simulation Model We have modeled three LANs, one for each of the high speed LANs. The generic OSI model used in the simulation is shown in Figure 1. The three LAN

Application TCP IP Network Interface LAN

Figure 1: Generic model of a station connected to a LAN. models dier only in the Network Interface layer and the physical structure of the LAN as shown in Table 1. Since our objective is to compare the performance of dierent LANs, the rest of the layers | Application, TCP and IP | are modeled exactly the same for all LAN models. The details of our simulationmodel for each layer is described below.

3.1 Application Trac Model There are 11 stations connected to a LAN with an access link of 100 Mbps. 10 client stations send trac to one destination station, the server. This models a hot-spot trac. The trac to the LAN is generated only at the application layer as variable size les; the size of a le is exponentially distributed with a mean of 34Kbyte. This mean size is selected following the le size distribution measured in one LAN le-server of 7-Gigabytes in size [5]. There may be single or multiple user at the application layer at each client-station and generate requests for le transfer to the server-station with an exponential interarrival time. The mean of the interarrival time is adjusted to oer desired load on the network. For example, in order to oer a total of 20 Mbps load, the application layer at each of 10 stations has to oer 2 Mbps load; the mean interar8 = 0:139264sec: rival time is selected as 3421024 106 The actual load to the LAN will be slightly higher than 2 Mbps because of the header/ trailer of the protocol data units (PDU) of the intermediate layers.

Parameter

Value

Maximum Window Size 65536 octet Maximum Segment Size 1460 octet Maximum ACK delay 0 sec Initial retx timeout 1 sec Maximum retx timeout 10 sec Persistent timeout 1 sec IP processing speed 10,000 packets/s Table 2: Values of simulation parameters

3.2 TCP and IP

A TCP connection is set up with the destination (server) when an application requests for a le transfer. The slow-start and congestion avoidance features as described in [9] has been implemented in our model. The values set for dierent TCP/IP parameters in our simulation are shown in Table 2.

3.3 LAN protocols

Due to the high transmission speed of Fast Ethernet, the cable length is limited to about 200 meter. This limitation is required for eective collision detection. For such short distance, we have taken the signal propagation delay on the cable as zero in our simulation. In order to perform eective comparison, the propagation delay in the FDDI cable and the ATM access link are taken zero as well. The bit error rate for the Fast Ethernet bus, FDDI links and the station to ATM switch link is taken as zero. We assume there is no internal blocking at the ATM switches; buers are implemented only at the output ports. Finite buer size is considered for the output ports in the ATM switch. A cell destined for an ouput port is lost if the buer for this port is full. PVC is implemented without any UPC (usage parameter control). No bandwidth is reserved for the PVCs. The switch bandwidth is shared among all stations on a \best-eort" basis. When transmitting, stations send data to the switch at the link speed (100 Mbps). AAL5 is implemented as the ATM adaptation layer. This con guration of ATM LAN was chosen because (i) it is a simple and inexpensive implementation option and (ii) it suits bursty data trac. The MAC buers for Fast Ethernet and FDDI are considered large; there is no loss due to buer over ow. Token priority in FDDI is set the same for all the stations to perform eective comparison with

Figure 2: Application throughput of 3 high speed LANs for dierent loads.

Figure 3: ATM performance for dierent outport buer size.

Fast Ethernet and ATM model. No bandwidth is reserved for synchronous trac.

clients communicate with a single server. Once the hot-spot port is identi ed, the performance of ATM can be signi cantly improved by con guring more buer at the port. Figure 3 illustrates the eect of increasing the buer size at the hot spot port. It is clearly seen that the application throughput increases linearly upto a threshold as we increase the buer size. Increasing the buer size beyond this point does not achieve signi cant performance improvement. For example, in Figure 3, we can see that the application throughput increases rapidly for increasing the buer size up to 1200 cells; increasing the buer size to 1500 cells does not provide any further improvement. More advanced and complex solutions for ATM performance problem are in the process of being standardised. Two feedback based congestion control techniques | rate and credit | are the subject of intense debate at the ATM Forum [8]. Some early packet discard strategies have been suggested in the literature [4]; the authors have shown that by discarding the entire higher layer packet, instead of dropping a single or a few cells from a packet, increases ATM performance signi cantly. However, all these techniques do not come for free; there are signi cant overhead associated with each of them.

4 Performance Comparison Performance criteria used to compare the performance of dierent LANs is the throughput of the application layer (S) normalised to 100 Mbps. The oered load from the application ()is calculated as the bit rate of the trac generated normalised to 100 Mbps. Therefore, an aggregate oered load of 0.2 means a total of 20 Mbps with application(s) at each station oering data to TCP at a rate of 2 Mbps. Application throughput, S, is plotted against different loads for 3 LANs and shown in Figure 2. The ATM switch was con gured with a 300-cell buer for each output port and the token rotation time for FDDI was set to 30 ms. We see that for such a hot-spot trac scenario, ATM performance is in orders of magnitude less than FDDI and Fast Ethernet. FDDI performs the best. The poor performance of ATM is due to the cell loss at the hot-spot output port. If a single cell is dropped from a higher layer packet, the entire packet is considered lost. Upon detection of a packet loss TCP reduces its congestion window to one and switches to slow-start. This dramatically reduces the application throughput. There is a simple solution to enhance the ATM performance for hot-spot trac. The rst step is to identify the hot spot output port at the ATM switch. This is not dicult for a LAN as the server is often the destination of hot-spot trac as many

5 Conclusion We have developed simulation model of a reference network to compare the performance of Fast Ethernet, FDDI and ATM in carrying TCP/IP based application data. We have considered a hot-spot

trac scenario where many stations send trac to a single destination. Our results show that FDDI has the best performance compared to others. We have demonstrated that without any congestion control technique implemented, ATM performance can be orders of magnitude less than Fast Ethernet and FDDI. Implementing some form of congestion control, even in the local area, is, therefore, absolutely necessary if ATM is to compete with other alternatives for supporting bursty LAN trac. Implementation of any congestion control technique, however, is not immune to disadvantages. These techniques are costly to implement and have their own processing overheads. We have shown that providing larger buer at the hot-spot output port can signi cantly improve ATM performance without implementing any congestion control mechanism. However, this approach requires the knowledge of the trac load and the location of the hot-spot trac. The size of the buer will depend on the trac load to the port. Since gathering these knowledge for a local area network is relatively easier than wide area counterpart, the above simple approach of providing larger buer can be adopted for ATM LANs.

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