Bridge Channel Access Algorithms for Integrated Services Ethernets

Transcription

1 Bridge Channel Access Algorithms for Integrated Services Ethernets Jim M. Ng, Edward Chan Dept. of Computer Science City Polytechnic of Hong Kong Tat Chee Ave., Kowloon Hong Kong Abstract: To provide integrated data/voice services on an inter-connected CSMA/CD type network, a proper channel access algorithm must be used by the bridge in order to minimize theend-to-end delay. T wo algorithms, a static aggressive mechanism and a load adaptive dynamic mechanism, are presented and compared with the algorithm specified in the IEEE standard. The performance of each algorithm is analyzed, and its suitability for the integrated service environment is evaluated. 1. INTRODUCTION Ethernet, a CSMA/CD based local area network (LAN), has been widely used in many offices because of its simplicity in structure, its expandability, and its ease of installation. In order to adapt the need of the modern office and to enhance the performance of the network, it is advisable to have a subnetwork for each department and interconnect the subnetworks together using bridges. Bridges are intelligent devices that can filter off local traffic and only pass on the inter-network messages. This can not only reduce the overall network load, but also provide some security to each subnetwork. Although Ethernet has been used for carrying data information for most of its installations, itsfeasibility in carrying voice messages has been proven [1],[10]. However, most studies werebased on having voice messages on a single LAN segment, and the performance of a bridged network has not been studied extensively. As Ethernets increase in popularity and complexity, the performance of the integrated services bridged-ethernets becomes an issue of considerable practical importance. 2. THE BRIDGED CSMA/CD NETWORK Ethernet, which is based on IEEE standard [2], uses the CSMA/CD access mechanism where stations contend for the transmission medium. Any one station transmitting will have its message broadcasted on its own network. When the bridge receives a message, it will forward the message if its destination is in the neighboring network. The message is buffered at the bridge until the bridge captures the transmission channel in the destination network. Hence, the bridge performs filtering of messages for the two networks. The bridge contains two large buffers, one for each side of the network as shown in Figure 1. Each buffer uses First In First Out (FIFO) scheduling mechanism.

2 To study the performance of bridged network, two main issues will be investigated. First of all, when a voice station or the bridge has successfully captured the channel, there is the question of whether sending only one packet (non-exhaustive service) or sending all packets in the queue (exhaustive service) has a better performance. Secondly, as voice messages cannot tolerate long delay, it is critical to send the packets out within a tight time constraint. Since the bridge is responsible for transferring all cross network traffic to the other network, it generates a relatively high traffic load compared to normal network stations. Thus a more aggressive approach to capture the channel is desirable. We will look at two channel access mechanisms, a static and a dynamic adaptive algorithm, and compare their relative merits. Simulation will be used in our study of the integrated service bridge network. 3. SIMULATION MODEL Shoch[3] has shown that the data load on most local networks occupies less than 10% of the network capacity. However, his measurements are more than a decade old and most LANs are populated with machines much faster than those used in his experiment. Hence, in our simulation, 20 data stations are used to generate a background loading of 3 Mbit/sec (i.e. 30% of the channel capacity) on each network. The data messages have a bi-modal distributation, with 70% small messages of 100 bytes (excluding headers/trailers) each and 30% long messages of 1200 bytes each. This should be closer to traffic pattern observed in many LANs than the fixed 40 byte message size used in Shoch s original work. Data packets arrivals at each station are modeled by a Poisson process. In order to ensure that the data stations need no changes in its communication software and hardware, the data stations will follow the IEEE standard. Voice input is sampled at a rate of 8K samples per second, and each analogue sample is digitized into an 8- bit code. Hence, the resulting digitized voice signal data rate is 64K bit/sec. Since each conversation is composed of silent intervals and talkspurts[4], we assume that the durations of talkspurts and silent intervals to be exponentially distributed with means 0.17s and 0.41s respectively. Furthermore, as noise may appear on the channel, any talkspurts which are less than 0.015s will be ignored; also,any silent intervals less than 0.2s will be treated as part of a talkspurt as they may probably be minor breaks in continuous speech. The packetizing interval is 20ms, so a 160-byte voice message (excluding header/trailer) will be formed during this period. Since voice messages can tolerate certain amount of packet loss, the network performance can be enhanced by reducing the number of retransmissions for the voice stations. Some studies [7,8] suggested that with maximum number of transmission attempts set to eight, an acceptable packetloss rate can still be maintained. However, for the data stations and the bridge, a maximum of 16 attempts will remain. As the buffer size required by the bridge is not known, we will assume the bridge to have two infinite size buffers. Statistics will be captured for the buffer queue length in order to help determine the appropriate buffer size. In our model, a 10 Mbit/s channel is used and each network is assumed to be 1 km long. For simplicity, only two Ethernets are used. For each network, there are 20 data stations and 120 voice stations, and a bridge is used to interconnect the two networks. Since station clustering has only slight effects on the system performance during normal operation [5,6], our model has all the stations uniformly distributed on each network. The simulation attempts to follow IEEE exactly. To simulate the 9.6 µs interframe gap, 12 bytes are added to each frame in addition to the 26 bytes of header/trailer. Whenever there is a collision, the channel is jammed for 3.2 µs.

3 4. BRIDGE CHANNEL ACCESS METHODS The standard backoff mechanism defined in IEEE specifies that a station which experiences a collision needs to back off for r slot time, where r is a random number between 0 and 2 k, k = min(n,10) and n is the number of attempt for transmission. In order tohandle the packets sent to the bridge efficiently and reduce their transmission delay, a more aggressive approach to capture the channel is necessary for the bridge. A simple approach to increase its aggressiveness is by reducing its backoff time. Two algorithms will be studied and compared with the standard algorithm: The "Static" Algorithm. In this aggressive algorithm, the bridge will backoff for 0 or 1 slot time before attempting for retransmission again; after the first ten attempts for transmission, the delay window will be doubled for each further collision. Hence, k = 0 when n <= 10 k = n - 10 when n > 10. The "Dynamic" Algorithm. This is a load sensitive approach where the backoff window will grow or shrink dynamically as it takes into account the bridge queue length as well as the number of retransmission; i.e. the aggressiveness of the bridge will change dynamically in response to the bridge and remote network loading condition. Similar to the standard algorithm, after each collision, the station will backoff for r slot time, with r being between 0 and 2 k ; however, in this algorithm k = min(n,10,t). Again, n is the number of attempt, and T = max(1,int[c / Q(t)]), where c is a constant and Q(t) is the bridge queue length at time t, and T is an integer with minimum value of one. When the queue in the bridge gets longer, T will have a smaller value, and hence reduce the backoff time. The constant c is used to control the aggressiveness of the algorithm. If c is chosen to be very large, the algorithm should behave like the standard algorithm; witha smaller c, the algorithm increases its aggressiveness even with a short queue. 5. EXHAUSTIVE AND NON-EXHAUSTIVE SERVICES When the bridge or a voice station has successfully captured the channel, it can use the exhaustive service to transmit all the packets in its buffer queue. On the other hand, it has been suggested that when a voice station has captured the channel, transmitting only the first packet in the buffer (non-exhaustive service) may give a small variance in delay [9]. However, if non-exhaustive service is used by the bridge, internetwork packets may experience a longer queuing delay. Both the bridge and the voice stations will be studied for both exhaustive and non-exhaustive services, resulting in four different combination; i.e., 1. Both the bridge and voice stations using exhaustive service; 2. The bridge uses exhaustive service while the voice stations use non-exhaustive service; 3. The bridge uses non-exhaustive service while the voice stations use exhaustive service; 4. Both the bridge and the voice stations use non-exhaustive service. The performance measurement of interest is the delay experienced by each voice packet. This delay includes the queuing delay, the channel access delay, the transmission delay and the propagation delay. For those packets that need to cross the bridge to go from network A to B, delay incurred from both networks A and B will be measured.

4 6. PERFORMANCE ANALYSIS OF BRIDGE CHANNEL ACCESS METHOD Figures 2 to 13 show the performance measures of the integrated services bridged network. The characters besides each curve indicate the algorithm used by the bridge. "Std" is for the standard backoff algorithm specified in IEEE 802.3, "S" is for the static algorithm, and"d(c)" is for the dynamic algorithm, where c = 10, 15, 20 and 40 are used. Also, for figures 2 to 6, the number next to the curve indicates the service type (exhaustive or non-exhaustive) used by the bridge and voice stations as described in the previous section. Intuitively, when the bridge uses exhaustive service, inter-network voice packets will experience a shorter delay. On the other hand it may have a negative impact on the intra-network traffic of the remote (i.e. destination) network. Similarly, when all voice stations use the exhaustive service, the bridge will experience a longer delay. However, this impact on the bridge can be balanced out to a certain extent if the bridge employs one of the aggressive channel access strategies outlines in an earlier section. Figures 2 to 6 show the network performance when different combinations of service types are used. Figures 2 and 3 show the cross bridge voice packets delay, and figures 4 and 5 show the average queue length in the bridge. The dynamic algorithm with c = 20 is used in this comparative study. As shown from the figures, when the standard truncated binary exponential backoff mechanism is used by the bridge, the bridge queue length increases rapidly with the increase of inter-network load and the inter-network packets will experience a much higher delay, as compared with the other two aggressive approaches. Using the standard backoff mechanism by the bridge hence cannot providea satisfactory service to the inter-network traffic. Exhaustive service can increase the aggressiveness of the station implicitly as all the packets in the stations can be transmitted once the channel has been captured. As a result, packets in the station will have a shorter average queuing time. As shown in figure 3 and 5, when exhaustive service is used by the bridge, the inter-network packets experience a much shorter delay, and the bridge has a shorter queue, regardless of the type of service (exhaustive or non-exhaustive) used by the voice stations. This result agrees with our expectation. Figure 6 shows the interesting fact that when the bridge uses an aggressive channel access algorithm, regardless of which service types are used by the bridge and the voice stations, the intra-network voice delay is similar. This means that although using exhaustive service by the bridge can implicitly increase its aggressiveness in channel access, modifying the channel access algorithm can produce a much greater increase in aggressiveness. This increase in aggressivenessby the bridge is so dominant that the choice of service types by the voice stations has comparatively little impact. The key observation so far is that the choice of either exhaustive or non-exhaustive service by the bridge and the voice stations has no significant impact on the intra-network traffic, and using the exhaustive service by the bridge can reduce the inter-network delay time. This observation is true for the aggressive algorithms and not the standard algorithm, but as noted earlier, the standard algorithm does not meet the required performance requirements and will be used as a basis for comparison and not as a viable option. Hence exhaustive service will be used for both the bridge andthe voice stations for the rest of this study. The merit of the different aggressive channel access schemes will be studied in the next section. 7. PERFORMANCE OF THE AGGRESSIVE CHANNEL ACCESS METHODS We will now compare the suitability of different channel access methods used by the bridge. Again, figures 7, 8 and 9 show that the standard backoff algorithm is clearly not suitable for the bridge. When the standard algorithm is used, the cross network traffic experiences highdelay, high delay variance, and rapid increase in queue length. On the other hand, when the aggressive static algorithm is used, the system favors the bridge.

5 The dynamic load adaptive algorithm is meant to balance the above two algorithms, i.e. the bridge will increase/decrease its aggressiveness dynamically. We can hence expect that the cross network voice delay will be between that of the standard and static algorithms. Our predictions are confirmed by figures 7 and 8. It is clear that when using the dynamic algorithm, the crossnetwork traffic will experience a higher delay and delay variance than the static algorithm; however, the delay and delay variance are still well within the tight constraint set for voice messages. Furthermore, as shown in figure 9 the mean bridge queue length is less than 1.5 packets. The major advantage of the dynamic algorithm is that it results in a lower delay for intra-network traffic. This is shown in figures 10 and 11. While the dynamic algorithm is expected tooutperform the standard algorithm, it is interesting to note that the dynamic algorithm performs better than even the static algorithm. The reason behind this may properly be due to the fact that by regulating the aggressiveness of the bridge, the local traffic can made use of the channel more effectively. As mentioned in the previous section, the value of c in the dynamic algorithm can be used to control the aggressiveness of the channel access mechanism. Obviously, with a large c value, the inter-network traffic will experience a longer delay; however, note that different c values produce similar performance for intranetwork traffic (figures 7 and 10). The intra-network delay is not affected much by different c values because the two opposing factors of accessing the channel less frequently and using the channel for a longer time once the channel has been captured (exhaustive service) tendto balance out. So far, the impact of the various channel access mechanisms on inter-network traffic and intra-network delay has been studied separately. In reality, the two types of delay should be considered together in order to arrive at some sort of balance between the two. We define the fairness index to be the ratio between the delay experienced by a packet after joining the bridge untilits successfully transmission and the delay experienced by the intra-network voice traffic. For a fair algorithm, it should not bias to either the bridge or the local stations. The fairness index can be used to tune the value of c in the dynamic algorithm: c should be chosen such that the fairness index will have a value close to 1. This is an additional advantage of using the dynamic algorithm. Figure 13 shows that using the standard or static algorithms result in either a fairness index that is either too large or too small, meaning that either intra-network or inter-network traffic is favored. On the other hand, the dynamic algorithm with c appropriately chosen (e.g. c=10) results in a much "fairer" service to both the bridge and the local stations. Figure 12 illustrates the fact that, for both the static and dynamic algorithms, packet loss in the local network traffic increases as cross network traffic increases. Although most systems can only tolerate packet loss of less than 3%, over 60 cross network conversations can still be supported simultaneously when each network is carrying 30% data traffic and another 120 intra-network calls. For a system with this packet loss rate, a dynamic algorithm with c = 10 will be a good choice, since it results in a index close to 1.

6 8. CONCLUSION Under normal loading condition for data traffic, the choice for the bridge channel access mechanism may not be crucial; however, in order to support voice traffic, the choice of channel access mechanismcan seriously affect the performance. We have looked at two main groups of access mechanisms, namely the static and dynamic algorithms. Although the static algorithm can provide a satisfactory service for the cross network traffic, it has adverse effects on the intra-network traffic. On the other hand the dynamic algorithm, by controlling the aggressiveness of the access mechanism, can alleviate this impact. Furthermore, by tuning the control variable c properly, we can provide a fair service to both the bridge and the local traffic. Regarding the choice of service to be used by the bridge and the voice stations, choosing exhaustive service for the bridge helps to increase the its aggressiveness and attain slightly better overall performance. On the other hand, the choice of service for the voice stations has littleimpact on overall performance. The system that we have looked at can support approximately 60 cross network conversation, an additional 120 intra-network active voice stations, and some data traffic. However, with rapid advance in communication technology, voice messages can be reconstructed from system with lower bit rate, and have greater tolerance to higher loss rate. As a result, it will be possible to set up a network which can support a greater number of voice conversations in the near future. Figure 1. Model of the bridge

7 REFERENCES 1. Dunlop, J "Techniques for the integration of packet voice and data on IEEE LANs," Comp. Comm., Vol 12, No 5, Oct 1989, pp IEEE Comp. Soc., Local Area Networks Standard CSMA/CD, Shoch, J F and Hupp, J A "Measured Performance of an Ethernet Local Network," CACM,V ol 23, No 12, Dec 1980, pp Brady, P T "A Model for Generating On-Off Speech Patterns in Two-Way Conversation," Bell Sys. Tech. Jour., Sept 1969, pp Gonsalves, T A and Tobagi, F A "On the Performance Effects of Station Locations and Access Protocol Parameters in Ethernet Networks," IEEE Trans. Comm., COM-36, No 4, April 1988, pp Chan, E and Ng, J M "Effect of Clustering in a Bridged Ethernet Environment," Proc. Intern. AMSE Conference on Signals & Systems, Brighton, U.K., July 1989, pp Chamtac, I and Eisenger, M "Voice/Data Integration on Ethernet - Backoff and Priority Considerations," Comp. Comm., Vol 6, No 5, Oct 1983, pp Ng, J M and Chan, E "Integrating Voice Service on a Large CSMA/CD Network," Proc. SEARCC Conference Suda, T and Bradley, T T "Packetized-Voice/Data Integrated Transmission on a Token Passing Ring Local Area Network," Proc. IEEE Infocom 87, pp Swinehart, D C "Telephone Management in the Etherphone System." Proc. IEEE Globecomm 87 pp