high service demands and to deal with a high number of connected users. ... domain of multi-channel networks to prove that multi-channel solutions make a ...
A comparison between single and multi-channel CSMA/CD protocols of equivalent capacity D. Rodellar, C. Bungarzeanu, H. Garcia Laboratoire de Télécommunications, Ecole Polytechnique Fédérale de Lausanne C. Brisson, A. Küng, Ph. Robert Laboratoire de Métrologie, Ecole Polytechnique Fédérale de Lausanne Abstract This paper presents the comparison between single channel and multi-channel Medium Access Control (MAC) protocols for broadband optical Local Area Networks. The multi-channel protocol exploits the Wavelength Division Multiplexing (WDM) techniques (each channel corresponds to a different optical wavelength allocated dynamically) and it is based on the Ethernet standard, using a Carrier Sense Multiple Access / Collision Detection (CSMA/CD) scheme. Average delay and efficiency are taken as performance criteria for the simulation of the protocols. Comparison of single channel and multi-channel protocols with different total capacity has been carried out using a MAC protocol simulator. We show that multi-channel protocols make a more efficient use of the available capacity than single channel protocols. This paper illustrates the advantage of distributing total capacity over several channels with an appropriate protocol.
1.
Introduction
Local Area Networks (LAN) are evolving towards higher channel capacities to solve the high service demands and to deal with a high number of connected users. Connecting terminals together for high capacity networks opens up a new field of Medium Access Control (MAC) protocols adapted to high-speed network architectures. The current trend in Ethernet evolution is to provide a high bit rate on a single channel. Fast Ethernet (100 Mbit/s) and Gigabit Ethernet (1 Gbit/s) are present-day solutions to Ethernet (10 Mbit/s) problems. A great effort has been made to keep being compatible with this existing protocol, using just one single channel. However, some research[2] has been done in the domain of multi-channel networks to prove that multi-channel solutions make a more efficient use of the available capacity than single channel ones[10]. This paper illustrates the advantage of distributing total capacity over several channels with an appropriate protocol. Comparison of single channel and multi-channel protocols at different bit rate has been carried on using a MAC protocol simulator. This programs assists in the protocol design process by evaluating the performance and testing the protocol for correctness without resorting to an actual implementation.
The following section outlines an architecture proposal to efficiently establish communications between hundreds of stations on a LAN. Then, the protocol and its parameters are described. Finally, performance measures will be given to compare single channel and multi-channel protocols for a given range of total channel capacities, using a MAC protocol simulator. 2.
Network architecture proposal: multi-channel LAN
Computer communications have benefited greatly from the introduction of optical fiber as the transmission medium[7][8]. Commonly, optical communication networks are divided into two separate levels: the definition of the protocols (communication) and the data transmission (physical layer). Both levels depend on each other and interact to obtain the final performance of the network. LOCAL OSCILATOR STATION #N
Nx(N+1) STAR COUPLER
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STATION #3 STATION #2
Fig. 1: Physical network architecture consisting of a shared local oscillator, a passive star coupler, and N different stations
A network architecture particularly well adapted to multi-channel protocols has been proposed[9]. A physical architecture consisting of an optical passive star that broadcasts all the packets to all the receivers and a common local oscillator that achieves a multi-channel reception using Wavelength Division Multiplexing (WDM), is shown in fig. 1.
TRANSMITTER
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External modulator
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COHERENT RECEIVER DATA recovery
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Fig. 2: Detailed description of a station with both transmitting and receiving schemes.
E/O
A coherent[5][6] receiver at each station allows multiple reception of all channels. The different channels correspond to different optical wavelengths. A detailed description of the transmitter and the receiver of each station is presented in fig. 2. 3.
The multi-channel Medium Access Control protocol
The multi-channel Ethernet[1] protocol[4] is based on the commercial Ethernet protocol standard according to the following criteria: 1.
The network consists of a number of stations distributed along a bus in its logical topology. However, it can be physically realized by a star topology.
2.
All stations can sense the total bus activity. All stations can sense whether the bus is busy. If the bus has been idle for an interframe time (96 bits) and there is a packet to be sent, the station starts transmitting the packet. If the bus is busy the station waits until the bus becomes idle. While transmitting, the station keeps sensing for a collision caused by some other activity on the bus. If a collision is detected, the transmission is aborted and a jamming signal (48 bits) is sent. Then the station reschedules the packet transmission according to the backoff algorithm (a randomized delay taking into account the number of consecutive collisions). Any message longer than the maximum packet size (12208 bits) is split into several packets. Each packet has a fixed length header and trailer (frame information). There is also a minimum packet size (512 bits) to make sure that no packet is shorter than two times the round-trip delay, and all collisions are detectable during the transmission time. All stations obey the same protocol and transmit at the same speed. There is no control channel, neither there is a common clock.
3.
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The multi-channel protocol behaves according to the more specific criteria: 7.
The bus is no more a single channel but a group of channels. Full connection of the stations to all the channels is assumed, both for transmission and for reception.
8.
There is a hopping time[4] (which is the laser tuning time) to jump from one channel to another.
9.
A new parameter is introduced for the number of consecutive collisions[4] encountered by the packet before jumping to another wavelength. This parameter can be adjusted such that slow tuning lasers will stay longer on the same wavelength. But for fast tuning lasers, ther’s no longer necessary to wait for changing the emitting wavelength, with virtually no influence on the delay measure.
Multimedia applications will expect transmission protocols with high bit rate and minimum delay. Ethernet provides the required fairness and the ability to easily add and remove stations from the network. Furthermore, the multichannel capabilities contribute by maximizing throughput and by minimizing delay. Finally, our architecture proposal also offers scalability. More network capacity is easily obtained by increasing the number of channels, however the receiver realisation should be modified to admit more channels.
WDM networks emerge as a viable alternative to provide Gbit/s communications[6][7]. The proposed multi-channel protocol improves the performance of CSMA/CD under heavy load, keeping its good characteristics for light traffic load conditions. The improvement is achieved by using multiple channels corresponding to different wavelengths. Channel hopping is introduced in the protocol as a new dimension. Hosts choose a wavelength to transmit data and if the chosen channel is busy or there is a collision, packets have the possibility to be transmitted on another channel. 4.
Simulation results
We optimize our protocol parameters[9] by simulating the entire network activity using real Ethernet traces, which is more judicious than a fixed unique packet length model. A finite number of hosts generates packets from a file containing several multiplexed Ethernet traces in an environment composed of workstations, typical of a research or software development network. This traffic has a long-range dependent statistic (also called self-similar[3] traffic). Self-similar traces provide simple, accurate, and realistic descriptions of Ethernet traffic. First, the performance of the Ethernet standard is evaluated varying the total capacity from 10 Mbit/s to 150 Mbit/s. For capacities higher than those, the protocol has to be modified to guarantee a minimum network range of several kilometers. In fact, the real implementation of those higher capacity protocols will also change the physical layer specifications in order to get always the same network range. In our simulator the propagation delay (network range divided by the propagation speed) is kept constant as the network range decreases by the same factor as the capacity increases. 4.1
Single channel MAC protocols performance
The following simulation results are obtained increasing the number of stations and emitting self-similar traffic from all stations. The mean bit rate of each station is about 2 Mbit/s. Performance measures are presented in the next figures, where four capacities are considered: 10, 50, 100, and 150 Mbit/s. We define the efficiency to be the total number of successfully transmitted packets divided by the offered packets. In fig. 3 this efficiency is represented versus the normalized offered traffic (offered bit rate divided by the capacity of the channel). 1
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Normalized offered traffic Fig. 3: Efficiency versus the normalized offered traffic, ranging the total capacity from 10 Mbit/s to 150 Mbit/s
As the total capacity increases the congestion appears for a lower normalized offered traffic. The efficiency of the Ethernet protocol decreases as the capacity of the channel increases. This is mainly due to the high number of collisions because the number of connected stations grows for higher capacities. A comparison of the average transfert delay (from the beginning of first attempt to transmit a packet to the end of its successful transmission) for the four given capacities is presented in fig. 4. We can see that the average delay rises as the capacity increases, for the same reasons given above. t/s
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Normalized offered traffic Fig. 4: Average delay versus the normalized offered traffic, ranging the total capacity from 10 Mbit/s to 150 Mbit/s
4.2
Multi-channel MAC protocols performance
The equivalent simulations are executed in the multi-channel case. Here, we have a different number of channels but each channel has a capacity of 10 Mbit/s. The results of efficiency and average delay are shown in fig. 5 and fig. 6, where the number of channels are 1, 5, 10, and 15 (the total capacity varies from 10 Mbit/s to 150 Mbit/s). 1
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Normalized offered traffic Fig. 5: Efficiency versus the normalized offered traffic, ranging the total capacity from 10 Mbit/s to 150 Mbit/s, in the multi-channel case
As a first conclusion we can notice that the four cases have similar performances and it is difficult to differentiate each different plot. This behavior is due to the fact that the multi-channel Ethernet divides the collision domains among the available channels. This effect is similar to the case of switched Ethernet, where we use special stations, namely switches, to minimize the collisions by separating the collisions of stations which have a great activity from those of more quiet stations. The simulations are run for a hopping time of 0.5 µs, that is the time the laser takes to jump from the wavelength where the packet has encountered multiple collisions to the selected one to transmit that packet again. That justifies the difference in average delay between the case of one channel and the other cases with 5, 10, and 15 channels, in fig. 6.
Average delay [ms]
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Normalized offered traffic Fig. 6: Average delay versus the normalized offered traffic, ranging the total capacity from 10 Mbit/s to 150 Mbit/s, in the multi-channel case.
4.3
MAC protocols performance comparison
To summarize this study we have compared the delay-throughput figures for both single channel and multi-channel protocols in fig. 7 and fig. 8.
Average delay [ms]
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Normalized throughput Fig. 8: Average delay versus the normalized throughput for multi-channel simulations
1
We have normalised the throughput by dividing it by the total capacity. It is clear that the multi-channel protocols have a better performance measures than single channel ones, and these measures are almost not dependent on the capacity. As an example, Fast Ethernet (100 Mbit/s) has been compared[9] to a protocol having 10 wavelengths of 10 Mbit/s each. We represent in fig. 9 the delay-throughput figure for both protocols. When increasing the capacity, the major goals are to obtain an adequate delay and throughput results in order to support real-time services and to satisfy the high demand of the large number of connected users. The difference between both protocols at congestion (which correspond to the maximum throughput) is about 0.2 times the total capacity which is 100 Mbit/s in this case. As mentioned above, the mean bit rate per station is about 2 Mbit/s, the difference corresponds to about 10 more stations connected for the multi-channel protocol. Single channel congestion for 100 Mbit/s occurs for a normalized throughput of 0.65, while for multi-channel this congestion is reached for 0.85. Delay comparison makes only sense before the single-channel congestion, and below a throughput of 60 Mbit/s, multi-channel delay is 0.1 ms below Fast Ethernet delay.
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Normalized throughput Fig. 9: Average delay versus normalised throughput for single channel and multi-channel protocol of 100 Mbit/s total capacity
5.
Conclusion
Future LANs will need higher capacity than those of today because of the new multimedia services that are projected and the high number of users that will be connected. In this paper we have pointed the lowering efficiency of single channel protocols when the total channel capacity is increased. On the contrary, efficiency remains unchanged for multi-channel protocols when the total channel capacity is increased. We have proposed a physical network architecture that is well-adapted to multi-channel protocols, making use of WDM capabilities and optical coherent techniques, and we have demonstrate the usefulness of distributing total capacity over several channels, by simulation of both single channel and multi-channel MAC protocols.
It is known that a fundamental limit reduces the efficiency of single channel Ethernet protocols when increasing the total capacity[10]. To overcome this limitation we propose a multi-channel protocol without sanction when increasing the total capacity. However, this scalability is not completely chargeless as the receiver realisation should be modified to admit more channels. Further research should be done to optimize the protocols towards the multimedia applications needs. Multi-channel Ethernet protocols will provide the required fairness, the ability to easily add and remove stations from the network, a high bit rate and a minimum delay. 6.
Acknowledgments
The authors gratefully acknowledge funding by the Swiss government (Fonds National Suisse de la Recherche Scientifique) under grant 21-42085.94 and thank Professor P.-G. Fontolliet for his useful comments. Special thanks are extended to J. Ehrensberger and L. Jaussi for their generously given constructive comments. 7.
References
[1] IEEE - “ISO/IEC 8802-3 Std 802.3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access method and physical layer specifications”, New York, 1992. [2] P. A. Humblet, R. Ramaswami, K. N. Sivarajan, ‘An efficient communication protocol for High-speed packet-switched multichannel networks’, IEEE Journal of Selected Areas in Communications, vol. 11, No 4, May 1993 [3] Will E. Leland, Murad S. Taqqu, Walter Willinger, Daniel V. Wilson, ‘On the self-similar nature of Ethernet traffic’, ACM SIGComm’93, San Francisco, CA, USA, September 1993. [4] D. Rodellar, C. Bungarzeanu, H. Garcia, C. Brisson, P.-A. Nicati, ‘Performance Analysis of a Multiwavelength Ethernet Optical Local Area Network’, Proceedings of the European Conference on Networks and Optical Communications 1997 (NOC’97), pp. 63-70, Antwerpen, June 1997. [5] Sadakuni Shimada, ‘Coherent lightwave communications Technology’, Capman & Hall, 1995. [6] Milorad Cvijetic, ‘Coherent and nonlinear lightwave communications’, Artech House, 1996. [7] Leonid Kazovsky, Sergio Benedetto, Alan Willner, ‘Optical fiber communication systems’, Artech House, 1996. [8] G. P. Agrawal, ‘Fiber-Optic Communication Systems’, Wiley, 1992. [9] D. Rodellar, C. Bungarzeanu, H. Garcia, C. Brisson, A. Küng, Ph. Robert, ’A multichannel Ethernet protocol for a WDM local area star network’, 9th IEEE Workshop on Local and Metropolitan Area Networks (LANMAN 98), Banff, Canada, May 1998 [10] Andrew S. Tanenbaum, ‘Computer Networks’, Prentice-Hall, 1989.
