Performance Analysis of FDDI By Raj Jain This paper is a modified version of "Performance Analysis of FDDI Token Ring Networks: Effect of Parameters and Guidelines for Setting TTRT," by Raj Jain, published in the Proceedings of the SIGCOMM '90, September 1990. Copyright 1990, Association for Computing Machinery, Inc.] Abstract Institute (ANSI).[1,2] This The performance of an standard allows as many as FDDI LAN depends upon 500 stations to communicate configuration and workload by means of fiber-optic parameters such as the cables using a timed- extent of the ring, the token access protocol. number of stations on Normal data traffic and the ring, the number of time-constrained traffic, stations that are waiting such as voice, video, and to transmit, and the frame real-time applications, size. In addition, one are supported. All major key parameter that network computer and communications managers can control to vendors and integrated improve performance is the circuit manufacturers offer target token rotation time products that comply with (TTRT). Analytical modeling this standard. and simulation methods Unlike the token access were used to investigate protocol defined by the the effect of the TTRT IEEE 802.5 standard, on various performance the timed-token access metrics for different protocol used by FDDI ring configurations. This allows synchronous and analysis demonstrated asynchronous traffic that setting the TTRT at 8 simultaneously.[3] The milliseconds provides good maximum access delay, i.e., performance over a wide the time between successive range of configurations and transmission opportunities workloads. for a station, is Fiber distributed data bounded for both types interface (FDDI) is a 100- of traffic. Although megabit-per-second (Mb this delay is short for /s) local area network synchronous traffic, (LAN) defined by the that for asynchronous American National Standards traffic varies with the Digital Technical Journal Vol. 3 No. 3 Summer 1991 1 Performance Analysis of FDDI network configuration the token by a station is and load and can be as called the token rotation long as 165 seconds. Long time (TRT). Using this maximum access delays are scheme, a station on an n- undesirable and can be station ring may have to avoided by properly setting wait as long as n times the the network parameters and THT interval to receive a configurations. token. This maximum access This paper begins with a delay may be unacceptable description of the timed- for some applications if token access method used the value of either n or by FDDI stations and THT is large. For example, then proceeds to discuss voice traffic and real-time how various parameters applications may require affect the performance that this delay be limited of these systems. The to 10 to 20 milliseconds target token rotation time (ms). Consequently, using (TTRT) is one of the key the token access method parameters. We investigated severely restricts the the effect of the TTRT on number of stations on a FDDI LAN performance and ring. developed guidelines for The timed-token access setting the value of this method, invented by Grow, parameter. The results solves this problem by of our investigation ensuring that all stations constitute a significant on a ring agree to a portion of this paper. target token rotation time (TTRT) and limit Timed-Token Access Method their transmissions to meet this target.[4] The token access method There are two modes of for network communication, transmission: synchronous as defined by the IEEE and asynchronous. Time- 802.5 standard, operates constrained applications in the following manner. A such as voice and real-time token circulates around traffic use the synchronous the ring network. A mode. Traffic that does station that wants to not have time constraints transmit information waits uses the asynchronous mode. for the arrival of the A station can transmit token. Upon receiving the synchronous traffic token, the station can whenever it receives a transmit for a fixed time token; however, the total interval called the token transmission time for each holding time (THT). The opportunity is short. station releases the token This time is allocated either immediately after at the ring initialization. completing transmission or A station can transmit after the arrival of all asynchronous traffic only the transmitted frames. The if the TRT is less than the time interval between two TTRT. successive receptions of 2 Digital Technical Journal Vol. 3 No. 3 Summer 1991 Performance Analysis of FDDI The basic algorithm for asynchronous traffic is as follows. All stations on a ring agree on a target Performance Parameters token rotation time. The performance of any Each station measures the system depends upon both time elapsed since last system parameters and receiving the token, i.e., workload parameters as the TRT. On token arrival, shown in Figure 1. There a station that wants to are two kinds of system transmit computes a token parameters: fixed and user- holding time using the settable. Fixed parameters following formula: cannot be controlled by THT = TTRT - TRT If the value of THT is the network manager and positive, the station can vary from one ring to transmit for this time another. Cable length and interval. After completing the number of stations transmission, the station on a ring are examples of releases the token. If a fixed parameters. It is station does not use its important to study network entire THT, other stations performance with respect on the ring can use the to these parameters; if remaining time through performance is sensitive successive applications of to them, each set of fixed the algorithm. parameters may require a different guideline. Note that even though the System parameters that stations attempt to keep can be set by the network the TRT below the target, manager or the individual they do not always achieve station manager include this goal. The TRT can various timer values. Most exceed the target by as of these timers influence much as the sum of all the reliability of the ring synchronous-transmission and the time it takes to time allocations; however, detect a malfunction. The these allocations are key settable parameters limited so that their sum that affect system is less than the TTRT. As a performance are the TTRT result, the TRT is always and the synchronous time less than twice the TTRT. allocations. In this paper, the The workload also has performance was studied a significant impact on under asynchronous traffic performance. A guideline conditions only. The or recommendation may be presence of synchronous suitable for one workload traffic will further but not for another. The restrict the choice of key workload parameters TTRT, as discussed later in are the number of active the section Guidelines for stations and the load per Setting the Target Token station. By active we mean Rotation Time. stations on a ring that are either transmitting Digital Technical Journal Vol. 3 No. 3 Summer 1991 3 Performance Analysis of FDDI or waiting to transmit. A key productivity metric is ring may contain a large not the throughput under number of stations, but low load but the maximum generally only a few are obtainable throughput under active at any given time. high load. This latter Active stations include the quantity is also known as currently transmitting the usable bandwidth of the station, if any, and network. And the ratio of stations that have frames the usable bandwidth to the to transmit and are waiting nominal bandwidth (e.g., for the access right, 100 Mb/s for an FDDI LAN) i.e., for a usable token is called the efficiency of to arrive. The load per the network. For instance, station varies with the if we consider a set of number of stations, the configuration and workload interval between bursts, parameters with a usable the number of frames per FDDI bandwidth of at most burst, and the frame size. 90 Mb/s, the efficiency is 90 percent. Performance Metrics The response time is the The quality of service a time interval between the system provides is measured arrival of a frame and by its productivity and the completion of its responsiveness.[5] For an transmission, including FDDI LAN, productivity is queuing delay, i.e., from measured by its throughput, the first bit in to the and responsiveness is last bit out. This metric measured by the response is meaningful only if a time and maximum access ring is not saturated, delay. because at loads near or above capacity the The productivity metric of response time approaches concern to the network infinity. With such loads, manager is the total the maximum access delay throughput measured in for a station, i.e., the megabits per second. time interval between Over any reasonable time wanting to transmit and interval and for most receiving a token, has more loads, the throughput is significance. equal to the load. For Another metric that is example, if the load on a of interest for a shared ring is 40 Mb/s, then the resource such as FDDI is throughput is also 40 Mb the fairness with which /s. But this does not hold the resource is allocated. if the load is high. For Fairness is particularly example, if there are three important under a heavy stations on a ring, each load. Given such a load, with a 100-Mb/s load, the the asynchronous bandwidth total arrival rate is 300 is allocated equally to all Mb/s and the throughput active stations. However, is considerably less-close the FDDI protocols are to 100 Mb/s. Thus, the fair only if the priority 4 Digital Technical Journal Vol. 3 No. 3 Summer 1991 Performance Analysis of FDDI levels are not implemented. For an FDDI network with In the case of multiple a ring latency D (i.e., priority implementation, the time it takes a bit it is possible for two to travel around the stations with the same ring) and a TTRT value priority and the same of T, the efficiency load to have different and maximum access delay throughput depending are computed using the upon their location.[6] following formulas: Low-priority stations (1) that are close to high- priority stations may get n(T-D) better service than those Efficiency =_______ farther downstream. We nT+D assumed a single priority implementation to keep (2) the analysis simple. Since Maximum access delay = (n - such implementations have 1)T + 2D no fairness problem, this metric will be discussed no Equations (1) and (2) further in this paper. constitute the analytical model. Their derivation is We used two methods to simple and is presented analyze performance: in the next section. analytical modeling and Those readers who are not simulation. We first interested in the details present the analytical can proceed to the section model used to compute the Application of the Model. efficiency and maximum access delay of a network under a heavy load. Then we discuss the simulation model workload used to analyze the response time at loads below the usable bandwidth. A Simple Analytical Model The maximum access delay and efficiency are meaningful only under heavy load. Therefore, we assume that there are n active stations, each generating enough frames to saturate the FDDI network. Basic Equations Digital Technical Journal Vol. 3 No. 3 Summer 1991 5 Performance Analysis of FDDI Derivation 4. t = D. Station S1 receives the token. First consider a ring with Since S1 now has an three active stations, as infinite supply of shown in Figure 2. (Later, frames to transmit, we will consider the it captures the token general case of n active and determines that the stations.) The figure shows TRT is D. Thus, the time the space-time diagram interval during which S1 of various events on the can hold the token, the ring. The space is shown difference between TTRT horizontally, and the time and TRT, is T - D. t is shown vertically. The 5. t = T. The THT at token reception is denoted station S1 expires. by a thick horizontal line S1 releases the token. segment. The interval of time during which 6. t = T + t12. Station S2 transmission of frames receives the token. S2 takes place is indicated last received the token by a thick vertical line at t = t12; thus, the segment. value of TRT is T. S2 Assume that all stations cannot use the token at are idle until t = D, this time and releases when the three active it. stations suddenly get a 7. t = T + t13. Station S3 large (infinite) burst of receives the token. S3 frames to transmit. The last received the token sequence of events shown in at t = t13; thus, its Figure 2 is as follows: TRT is also T. S3 cannot 1. t = 0. Station S1 use the token at this receives the token and time and releases it. resets its own token 8. t = T + D. Station S1 rotation timer to zero. receives the token. S1 Since the station has no last received the token frames to transmit, the at t = D; its TRT is token proceeds to the also T. (Note that the next station S2. TRT is measured from 2. t = t12. Station S2 the instant the token receives the token arrives at a station's and resets its token receiver, i.e., event 4 rotation timer to zero. for station S1, and not t12 is equal to the from the time it leaves signal propagation delay a station's transmitter, from S1 to S2. i.e., event 5.) S1 cannot use the token 3. t = t13. Station S3 and releases it. receives the token and resets its token rotation timer to zero. t13 is equal to the signal propagation delay from S1 to S3. 6 Digital Technical Journal Vol. 3 No. 3 Summer 1991 Performance Analysis of FDDI 9. t = T + D + t12. Station The above discussion S2 receives the token. illustrates that the system Since TRT is only D, goes through a cycle of it sets the THT to the events and that the cycle remaining time, i.e., time is 3T + D. During T - D. S2 transmits every cycle, each of the for that interval and three stations transmits releases the token at t for a time interval = T + D + t12 + (T - D). equal to T - D; the total 10.t = 2T + t12. The THT at transmission time is 3(T station S2 expires. S2 - D). The number of bits releases the token. transmitted during this time is 3(T - D) x 108, and 11.t = 2T + t13. Station S3 the throughput equals 3(T receives the token. - D) x 108/(3T + D) bits Since TRT is T, S3 per second. The efficiency, releases the token. i.e., the ratio of the 12.t = 2T + D. Station S1 throughput to the FDDI receives the token. bandwidth of 100 Mb/s, is Since TRT is T, S1 3(T - D)/(3T + D). releases the token. During the cycle, each 13.t = 2T + D + t12. station waits for a time Station S2 receives the interval of 2T + 2D after token. Since TRT is T, releasing the token for S2 releases the token. another opportunity to transmit. This interval is 14.t = 2T + D + t13. the maximum access delay. Station S3 receives For lower loads, the access the token. Since TRT is delay is shorter. only D, it transmits for Thus, for a ring with three the time interval T - D active stations, and releases the token at t = 2T + D + t13 + (T 3(T-D) - D). Efficiency =_______ 3T+D 15.t = 3T + t13. The THT at station S3 expires. S3 Maximum access delay = (3 - releases the token. 1)T + 2D = 2T + 2D 16.t = 3T + D. Station S1 To generalize the above receives the token, and analysis for n active the sequence of events stations, substitute n begins to repeat. The for 3. Equations (1) and token passes through (2) are the results; the stations S1, S2, and derivation is complete. S3, all of which find it unusable. Application of the Model 17.t = 3T + 2D. The cycle continues with S1 capturing the token as in event 4. Digital Technical Journal Vol. 3 No. 3 Summer 1991 7 Performance Analysis of FDDI Equations (1) and (2) can The key advantage of this be used to compute the model is its simplicity, maximum access delay and which allows us to see the efficiency for any immediately the effect FDDI ring configuration. of various parameters on For example, consider a network performance. With ring with 16 stations and only one active station, a total fiber length of which is usually the case, 20 kilometers (km). (Using equation (1) becomes a two-fiber cable, this corresponds to a cable Efficiency(n = 1) T-D_ length of 10 km.) Light T+D waves travel along the fiber at a speed of 5.085 As the number of active microseconds per kilometer stations increases, the (µs/km). The station delay efficiency increases. With between receiving and a very large number of transmitting a bit is stations, approximately 1 µs per station. The ring latency Maximum efficiency(n = #) = can be computed as follows: 1- (D) T Ring latency D = (20 km) This efficiency formula x (5.085 µs/km) + (16 is easy to remember and stations) x (1 µs/station) permits "back-of-the- = 0.12 milliseconds (ms) envelope" calculations of FDDI LAN performance. This Assuming a TTRT of 5 ms and special case of n = # has all 16 stations active, already been studied.[7] Similarly, we can use Efficiency = 16(5-0.12)_= 97.5percentquation (2) to calculate 16x5+0.12 the maximum access delay with one active station as Maximum access delay = (16 follows: - 1) x 5 + 2 x 0.12 = 75.24 Maximum access delay(n = ms 1) = 2D Thus, on this ring, the That is, a single active maximum possible throughput station may have to wait is 97.5 Mb/s. If the load as long as twice the ring is greater than this for latency between successive any substantial length transmissions because every of time, the queues will alternate token that it build up, the response receives would be unusable. time will increase, and the For n = #, the maximum stations may start to lose access delay approaches frames due to insufficient infinity. buffers. The maximum access delay is 75.24 ms; thus, asynchronous stations may have to wait as long as 75.24 ms to receive a usable token. 8 Digital Technical Journal Vol. 3 No. 3 Summer 1991 Performance Analysis of FDDI Simulation Workload Guidelines for Setting the One way to measure the Target Token Rotation Time responsiveness of a system This section presents the is to use simulation to rules specified by the ANSI analyze the response time. FDDI media access control This metric depends upon standard for setting the the frame arrival pattern value of the TTRT. We also of the workload and is discuss efficiency, maximum discussed further in the access delay, and response Response Time section. The time considerations, as workload we used in our well as reasons to limit simulations was based on the value of TTRT. an actual measurement of ANSI FDDI Standard traffic at a customer site. The chief application at According to the ANSI FDDI this site was the warehouse standard, the following and inventory control rules must be observed when (WIC). Hence, we named the setting the TTRT: workload WIC. 1. Since the TRT can be Previous network as long as twice the measurements show that TTRT, a synchronous when a station wants to station may have to transmit, it generally wait a time interval transmits not one frame, of up to 2T before but a burst of frames. receiving the token. The WIC workload has this Therefore, synchronous trait as well. Therefore, stations should request we used a bursty Poisson a TTRT value of one-half arrival pattern in our the required service simulation model with an interval. For example, a interburst time of 1 ms and voice station that wants five frames per burst. to receive a token every We limited the frames to 20 ms or less should two sizes: 65 percent of request a TTRT of 10 ms. the frames were small (100 2. The TTRT must allow bytes), and 35 percent transmission of at were large (512 bytes). least one maximum-size This workload constitutes a frame in addition to total load per station of the synchronous time 1.22 Mb/s. Forty stations, allocation, if any. That each executing this load, is, would utilize 50 percent TTRT > ring latency of the FDDI bandwidth. + token time Higher load levels can be + maximum frame time obtained either by reducing the interburst time or + synchronous increasing the number of time allocation stations on the ring. Digital Technical Journal Vol. 3 No. 3 Summer 1991 9 Performance Analysis of FDDI The maximum-size frame Efficiency and Maximum on FDDI is 4500 bytes Access Delay Considerations plus preamble and takes In addition to the rules approximately 0.361 specified by the standard, ms to transmit. The the TTRT values should maximum ring latency is be chosen to allow high- 1.773 ms. The token time performance operation (11 bytes including 8 of a ring. This section bytes of preamble) is discusses these performance 0.00088 ms. This rule, considerations. therefore, requires that the TTRT be set at a Figure 3 is a plot of value greater than or efficiency as a function equal to 2.13 ms plus of the TTRT. Three the synchronous time configurations called allocation. Violating "Typical," "Big," and this rule, for example, "Largest" are shown. by overallocating the The Typical configuration synchronous bandwidth, consists of 20 single results in unfairness attachment stations (SASs) and starvation, i.e., on a 4-km fiber ring. The some stations are unable numbers used are based on to transmit. an intuitive feeling of 3. A station must request what a typical ring would a TTRT greater than or look like and not based equal to the station on any survey of actual parameter T_min. The installations. Twenty default maximum value offices located on a 50 of T_min is 4 ms. m by 50 m floor require a Generally, most stations 2-km cable or a 4-km fiber. do not request a TTRT less than 4 ms. 4. A station must request a TTRT less than or equal to the station parameter T_max. The default minimum value of T_max is 165 ms. Assuming that there is at least one station with T_max equal to 165 ms, the TTRT on a ring cannot exceed this value. (In practice, many stations will use a value of 222 x 40 ns = 167.77216 ms, which can be conveniently derived from the symbol clock using a 22-bit counter.) 10 Digital Technical Journal Vol. 3 No. 3 Summer 1991 Performance Analysis of FDDI The Big configuration the curve depends upon consists of 100 SASs the ring configuration. on a 200-km fiber. This For larger configurations, configuration represents a the knee occurs at larger reasonably large ring with TTRT values. Even for the acceptable reliability. Largest configuration, the Configuring a single ring knee occurs in the range of with considerably more than 6 to 10 ms. For the Typical this number of stations configuration, the TTRT has increases the probability little effect on efficiency of bit errors.[8] as long as the TTRT is in the allowed range of 4 to The Largest configuration 165 ms. consists of 500 dual Figure 4 shows the maximum attachment stations access delay as a function (DASs) and a ring that of the TTRT for the three has wrapped. A DAS can have configurations. To show one or two media access the complete range of controllers (MACs). In this possibilities, we used a configuration, each DAS semilogarithmic scale on has two MACs. Thus, the the graph. The vertical LAN consists of 1000 MACs scale is logarithmic, in a single logical ring. while the horizontal scale This is the largest number is linear. The figure of MACs allowed on an FDDI shows that increasing LAN. Exceeding this number the TTRT brings about a would require recomputation corresponding increase of all default parameters in the maximum access specified in the standard. delay for all three Figure 3 shows that for configurations. all three configurations, Table 1 presents the the efficiency increases performance metrics for as the TTRT increases. the maximum access delay The efficiency is very and the efficiency as low at TTRT values close functions of the TTRT. to the ring latency but As evidenced in the table, increases as the TTRT on the Largest ring, a TTRT increases. Thus, to ensure of 165 ms causes a maximum a minimal efficiency, the access delay as long as 165 minimum allowed TTRT on seconds. This means that FDDI is 4 ms. This direct in a worst-case situation, relationship between the a station may have to wait efficiency and the TTRT may several minutes to receive lead some to conclude that a usable token. For many the largest possible TTRT applications, this delay is be chosen. However, notice unacceptable; therefore, a also that the efficiency reduced number of stations gained by increasing the or a smaller TTRT may be TTRT, i.e., the slope preferable. of the efficiency curve, decreases as the TTRT Response Time increases. The "knee" of Digital Technical Journal Vol. 3 No. 3 Summer 1991 11 Performance Analysis of FDDI Figure 5 shows the average response time as a function of the TTRT for a relatively large configuration, i.e., 100 stations and 10 km of fiber. The WIC workload was simulated at three load levels: 28, 58, and 90 percent. Two of the three curves are horizontal straight lines indicating that TTRT has no effect on the response times at these loads. The TTRT only affects the response time at a heavy load. In fact, it is only near the usable bandwidth that the TTRT has any effect on the response time. 12 Digital Technical Journal Vol. 3 No. 3 Summer 1991 Performance Analysis of FDDI Table 1 Maximum Access Delay and Efficiency as Functions of the TTRT ___________________________________________________________________ TTRT Maximum Access Delay (seconds) Efficiency (percent) ___________________________________________________________________ Typical Big 100 Largest typical Big 100 Largest 20 SAS 4 SAS 200 500 DAS 20 SAS 4 SAS 20 km 50 km km 20 km km DAS 200 km 4 0.08 0.40 4.00 98.94 71.87 49.55 8 0.15 0.79 8.00 99.47 85.92 74.77 12 0.23 1.19 11.99 99.65 90.61 83.18 16 0.30 1.59 15.99 99.74 92.95 87.38 20 0.38 1.98 19.98 99.79 94.36 89.91 165____3.14_______16.34______164.84_____99.97______99.32______98.78 To summarize the results the Largest ring is poor presented so far, if (50 percent). A very large the FDDI load is below value of TTRT, such as 165 saturation, the TTRT ms, is also undesirable, has little effect. At because it results in long saturation, a larger maximum access delays. value of TTRT gives a The 8-ms value is the larger usable bandwidth most desirable, since and therefore increased it yields 75 percent efficiency. But a longer or more efficiency on TTRT also results in longer all configurations and maximum access delays. results in a maximum access The selection of the delay of less than one TTRT requires a trade- second on Big rings. Eight off between these two milliseconds is, therefore, requirements. To facilitate the recommended default making this trade-off, the TTRT. two performance metrics for Problems with a Large TTRT the three configurations There are three additional are listed in Table 1. TTRT reasons for preferring an values in the allowable 8-ms TTRT over a large TTRT range of 4 to 165 ms are such as 165 ms. First, shown. The data shows a large TTRT allows a that a very small value station to receive a large of TTRT, such as 4 ms, is number of frames back- undesirable, because the to-back. To operate in resulting efficiency on such an environment, all Digital Technical Journal Vol. 3 No. 3 Summer 1991 13 Performance Analysis of FDDI adapters must be designed Parameters Other Than The TTRT with large receive buffers. That Affect Performance Although memory is not Many parameters other considered an expensive than the TTRT affect the part of a computer, its performance of a network. cost is significant for This section discusses four low-cost components such configuration and workload as adapters. The board parameters: the extent of space for the additional the ring, the total number memory required by of stations, the number of choosing a larger TTRT active stations, and the is considerable as are the frame size. bus holding times required for such large back-to-back Extent of the Ring transfers. The total length of Second, a very large TTRT the fiber is called the results in an exhaustive extent of the ring. The service discipline (i.e., maximum allowed extent all frames are transmitted on an FDDI LAN is 200 km. in one token capture), Figures 6 and 7 are graphs which has several known illustrating the efficiency drawbacks. For example, and maximum access delay as exhaustive service is functions of the extent. A unfair. Frames coming to star-shaped ring with all higher load stations have a stations at a fixed radius greater chance of finding from the wiring closet is the token during the same assumed. The total cable transmission opportunity, length, shown along the whereas frames arriving horizontal axis, is twice at low load stations may the radius times the number have to wait. Thus, the of stations. As is evident response time is inversely from the figures, rings dependent upon the load, with a larger extent have a i.e., higher-load stations slightly lower efficiency yield lower response times and a longer maximum access and vice versa.[9]. delay than those with Third, with exhaustive smaller extents. service, the response time Note that in Figure 7, of a station is dependent the increase in maximum upon station location with access delay for each respect to that of high- configuration is not load stations. The station apparent due to the immediately downstream from semilogarithmic scale. a high-load station may obtain better throughput than the one immediately upstream. 14 Digital Technical Journal Vol. 3 No. 3 Summer 1991 Performance Analysis of FDDI Total Number of Stations Frame Size The total number of Frame size does not appear stations on a ring includes in the simple models of active and inactive efficiency and maximum stations. In general, access delays, because increasing the number frame size has little of stations adds to the impact on FDDI performance. ring latency because In our analysis, we assumed of the additional fiber that transmission stops length and additional at the instant the THT station delays. Thus, expires; however, the the number of stations standard allows stations to affects the efficiency and complete the transmission maximum access delay in of the last frame. a way similar to that of the extent; a ring that contains a larger number of stations than another has a lower efficiency and a longer maximum access delay. In addition, a large number of stations on a ring increases the bit- error rate. Consequently, large rings are not desirable. Number of Active Stations As the number of active stations, i.e., MACs, increases, the total load on the ring increases. Figures 8 and 9 show the ring performance as a function of the number of active MACs on the ring. We considered a maximum-size ring with a TTRT value of 8 ms for the analysis. The figures show that increasing the number of active MACs has a slight positive effect on the efficiency, but considerably increases the maximum access delay. Therefore, it is preferable to keep a minimal number of active stations on each ring by segregating small groups on separate rings. Digital Technical Journal Vol. 3 No. 3 Summer 1991 15 Performance Analysis of FDDI The extra time used by a o The time to process a station after THT expiry frame increases only is called asynchronous slightly with the size overflow. Assuming all of the frame. A larger frames are of fixed frame size results in size, let F denote the fewer frames and, hence, frame transmission time. in less processing at During every transmission the host. opportunity, an active Overall, we recommend station can transmit as using as large a frame many as k frames: size as the reliability T-D considerations allow. k =____ F Summary Here, is Although many parameters used to denote rounding up affect the performance to the next integer value. of an FDDI ring network, The transmission time is the target token rotation equal to k times F, which time (TTRT) is the key is slightly more than T parameter that network minus D. With asynchronous managers can control to overflow, the modified optimize this performance. efficiency and maximum We analyzed the effect of access delay formulas other parameters such as become the extent of the ring (the nkF length of the cable), the Efficiency =___________ total number of stations, n(kF+D)+D the number of active stations, and frame size. Notice that substituting From our data we concluded kF = T - D in the above the following: equations results in o Rings with a large Equations (1) and (2). extent and those with a Figures 10 and 11 show the large number of stations efficiency and the maximum are undesirable because access delay as functions they yield a longer of the frame size. Frame maximum access delay size has only a slight and have only a slight effect on these metrics. positive effect on the Larger frame sizes do have efficiency of the ring. the following effects: o It is preferable to o The probability of error minimize the number of is greater in a larger active stations on a frame. ring to avoid increasing o Since the size of the maximum access protocol headers and delay. trailers is fixed, larger frames require less protocol overhead. 16 Digital Technical Journal Vol. 3 No. 3 Summer 1991 Performance Analysis of FDDI o A large frame size 3. Token Ring Access Method is desirable, taking and Physical Layer into consideration the Specifications, ANSI acceptable probability /IEEE Standard 802.5- of error. 1985, ISO/DIS 8802/5 The value of TTRT does not (New York: The Institute significantly affect the of Electrical and response time unless the Electronics Engineers, load is near saturation. Inc., 1985). Under very heavy load, 4. R. Grow, "A Timed-token response time is not a Protocol for Local Area suitable metric. Instead, Networks," Proceedings maximum access delay, i.e., of the IEEE Electro the time between wanting to '82 Conference on Token transmit and being able to Access Protocols, Paper do so, is more meaningful. 17/3, Boston, MA (May A larger value of TTRT 1982). improves the efficiency, 5. R. Jain, The Art of but it also increases the Computer Systems maximum access delay. A Performance Analysis, good trade-off is provided ISBN 0471-50336-3 (New by setting TTRT at 8 ms. York: John Wiley & Sons, Since this value provides 1991). good performance for all 6. D. Dykeman and W. Bux, ranges of configurations, "Analysis and Tuning of we recommend that the the FDDI Media Access default value of TTRT be Control Protocol," IEEE set at 8 ms. Journal on Selected Areas in Communications, References vol. 6, no. 6 (July 1988): 997-1010. 1. F. Ross, "An Overview 7. J. Ulm, "A Timed-token of FDDI: The Fiber Ring Local Area Network Distributed Data and Its Performance Interface," IEEE Journal Characteristics," on Selected Areas in Proceedings of the Communications, vol. 7, Seventh IEEE Conference no. 7 (September 1989): on Local Computer 1043-51. Networks (February 2. Fiber Distributed 1982): 50-56. Data Interface (FDDI) 8. R. Jain, "Error - Token Ring Media Characteristics of Access Control (MAC), Fiber Distributed Data ANSI X3.139-1987 (New Interface (FDDI)," York: American National IEEE Transactions on Standards Institute, Communications, vol. 1987). 38, no. 8 (August 1990): 1244-1252. Digital Technical Journal Vol. 3 No. 3 Summer 1991 17 Performance Analysis of FDDI 9. W. Bux and H. Truong, "Mean-delay Approximation for Cyclic-service Queueing Systems," Performance Evaluation, vol. 3 (Amsterdam: North- Holland, 1983): 187-196. 18 Digital Technical Journal Vol. 3 No. 3 Summer 1991 ============================================================================= Copyright 1991 Digital Equipment Corporation. Forwarding and copying of this article is permitted for personal and educational purposes without fee provided that Digital Equipment Corporation's copyright is retained with the article and that the content is not modified. This article is not to be distributed for commercial advantage. Abstracting with credit of Digital Equipment Corporation's authorship is permitted. All rights reserved. =============================================================================