high-performance protocols for ring lan and man computer networks

NORTH- HOLLAND

H i g h - P e r f o r m a n c e P r o t o c o l s for R i n g L A N a n d M A N C o m p u t e r N e t w o r k s

KHALID KHALIL Bell Communication Research, ~4~ Hoes lane, Piscataway, NJ 08854

and

M. S. OBAIDAT* The City College of The City University of New York, Department of Electrical Engineering, New York, NY 10031

ABSTRACT

This paper presents an adaptive token release mechanism that can be used in both basic token ring protocols (e.g., IEEE 802.5) and timed token protocols t (e.g., FDDI). The essence of the mechanism is to allow stations to share current state information that enables the token-holding station to decide whether to release the token or to keep it for another token-holding period. Simulation results show that using the adaptive token release mechanism, token ring protocols are able to support a large spectrum of real-time applications without sacrificing efficiency.

1. I N T R O D U C T I O N

A well-known disadvantage of current token ring protocols is the unnecessary token release [1-3]. The unnecessary token release occurs when the token-holding station releases the token due to the expiration of its token- holding time while no other station is waiting for the token, This results in one round trip waiting delay before the station gets the token back to resume the transmission. Depending on the size and the speed of the ring, the overhead resulting from the frequent releases of the token may con- siderably limit the ring performance [3]. In multiple token protocols (e.g., FDDI) the token-holding station releases the token immediately after it completes a transmission. As a result, for high speed ring, multiple token

*To whom correspondence should be addressed. ~U.S. Patent No. 5, 155, 725

INFORMATION SCIENCES 83, 37-47 (1995) (~) Elsevier Science Inc., 1995 655 Avenue of the Americas, New York, NY 10010

0020-0255/95/$9.50 SSDI 0020-0255(94)00067-L

38 K. KHALIL AND M. S. OBAIDAT

protocols are more efficient than single token protocols [4, 5]. References [6-11] present some of the previous related work.

In this paper, we introduce a high-performance adaptive token release mechanism that can be implemented in both the traditional token ring LANs and FDDI networks. This mechanism improves the efficiency of the token ring protocols by allowing stations to share up-to-date status information with the other stations on the ring. Using this information, a station can determine whether to release the token or to retain it for another token-holding period.

2. PROPOSED PROTOCOLS

We introduce here a possible way to improve ring efficiency by allowing stations to exchange their status information among themselves. Status information may include queue size, traffic type, traffic priority, etc. This information can be exchanged either through a control frame or through specific bits in the frame header; however, in the following discussion, unless otherwise specified, we will assume that this information is exchanged through specific bits in the frame header. Using this information, a station can determine whether there is any other acti'¢e station in the ring. If the token-holding station realizes that it is the only active station and no other station in the ring is waiting for the token, then it keeps transmitting; otherwise, it releases the token. As a result, the token-holding station will not release the token until it transmits all the queued frames or when another station requests the token. In other words, the problem of unnecessary token release will no longer exist.

The technique we propose to exchange status information is to have the token-holding station, upon the expiration of its Token-Holding Timer, THT, send a Request for Permission (RFP) message to start a new access cycle. If there is another station(s) waiting for the token, then the waiting station(s) replies by a Permission Denied (PD) message. Meanwhile, the token-holding station keeps sending its frames until it receives its RFP or a PD message (the PD message can be viewed as a modification to the contents of the RFP message). If it receives a PD, it completes transmission of the frame currently being transmitted and then releases the token. This technique solves the request removal problem since the RFP and PD will be removed from the ring by the transmitt ing station after one round. Also, this technique is simpler and adds a minimal complexity at each station.

Relative priority, absolute priority, and hybrid priority schemes can be supported using the adaptive token release mechanism. In relative priority, each type of traffic is assigned a different priority level with a different token holding time. Using the relative priority scheme, all traffic types are

H I G H - P E R F O R M A N C E P R O T O C O L S FOR RING N E T W O R K S 39

serviced in the same token cycle, but the amount of t ime for which a stat ion can hold the token will depend on the traffic type. In absolute priority, only the stat ion with the highest priority traffic is allowed to t ransmit . Absolute priority is useful to handle synchronous traffic in which it will be assigned the highest priority. A hybrid priority scheme may also be used to satisfy the performance objectives of a mix of synchronous and asynchronous traffic with different communication requirements.

Integrat ing the adaptive token release mechanism with the token ring protocols (e.g., IEEE 802.5) is straightforward. The R F P message can be implemented using a control bit in the frame header or as a separate medium access control frame. If control bits were to be used, three bits are required; one R F P bit and two PD bits. A PDa bit is used to indicate that a stat ion with synchronous traffic is waiting for the token, and a PDs bit to indicate tha t a stat ion with asynchronous traffic is waiting for the token. When the T H T expires, the token-holding station sets the RFP bit in the header of the next frame to be t ransmit ted and continues transmitt ing. When a station tha t does not possess the token receives a frame with the R F P bit set, it checks the s tate of its t ransmit queue. If the t ransmit queue is not empty and the PD bits are not set, then it sets either the PDa or PDs bit, depending on its traffic type. Otherwise, it forwards the frame without altering its contents. When the token-holding station receives its frame back with the RFP bit set, it checks the PD bits. If either PD bit is set, then the stat ion copies it to the free token and releases it as soon as it completes the transmission of the frame currently being t ransmit ted. If the PDs is set on the token, then stations with asynchronous traffic are not allowed to use it, and the first s tat ion with synchronous traffic will seize the token. The second implementat ion of the RFP message is by using a separate medium access control frame. When the T H T expires, the token-holding station, after completing the transmission of the current frame, sends an R F P frame indicating the priority of its queued traffic and resumes transmission. When a stat ion receives the RFP frmne, it checks the s tate of its t ransmit queue. If the t ransmit queue is not empty, then it sets either the PDa or PDs bit, depending on its traffic type. On the other hand, if the t ransmit queue is empty, then the station forwards the frame without altering its contents. The PDa and PDs bits may be placed in either the header or the railer of the RFP.

The integration of an adaptive token release mechanism with the t imed token ring protocols (e.g., FDDI) is not straightforward since under some circumstances (e.g., single active station), a station is allowed to s tar t a new cycle without releasing the token. Starting a new cycle without releasing the token may cause the Timer Rotat ion Timers, TRTs, in other stat ions to expire while their Late Counters, LCs, are not zero, in which


case the ring recovery procedure will, unnecessarily, be initiated. This is the main problem that results from applying the adaptive token release mechanism directly to the T T R protocol, and therefore, some modifications must be considered.

The unnecessary ring recovery initiation resulting from the expiration of TRTs may be explained by considering the way the T T R protocol op- erates. If the stat ion T R T expires while Late Counter (LC) = 1, the s tat ion assumes that the token is lost and it initiates the ring recovery procedure. Under normal T T R operations, when the T H T expires at the t ransmit t ing station, it cannot send any more asynchronous traffic; however, synchronous frames can still be transmitt ing. If the stat ion does not have synchronous frames, it releases the token. Let us assume tha t a station is t ransmit ted highest priority frames, i.e., its threshold is equal to TTRT. This implies that if its T H T expires, then all the TRTs at all other stations tha t will be visited later by the released token will be also expired, and hence all their LCs will be incremented by one. Consequently, each station, upon receiving the token, will clear its LC, t ransmit its synchronous frames if any, and then release the token. This scenario also happens in the t ransmit t ing station. We refer to this cycle as "Idle Token Cycle." The main purposes of the idle token cycle are to: 1) reset the LCs so tha t false initiations of ring recovery procedure are prevented, and 2) allow only stations with synchronous traffic to access the ring. To avoid the unnecessary initiation of the ring recovery procedure, the proposed protocol uses a reset (RST) message. The exact actions taken by a station when it receives a RST message depends on the reset mechanism. Different reset mechanisms can be used. The RST message is t ransmit ted by the token-holding stat ion at the beginning of a new access cycle (in the case when one station is active). It may also be t ransmit ted after the token-holding stat ion receives a P D a and before it releases the token. The station tha t issues the RST message is responsible for removing it from the ring. When an RST message is t ransmit ted in response to a PDa message, the LC will be reset in all stations. Therefore, any active station that receives the token (which will follow immediately after the RST message) will be able to use it. In this case, the idle token cycle will be avoided and, hence, the throughput will be improved. I t is important to note tha t when PDs is received, the RST message is not sent. The reason for this is to prevent stations with asynchronous traffic (if any) from using the ring before those with synchronous traffic.


3. S IMULATION MODELS, RESULTS, AND DISCUSSION

We used simulation to s tudy the performance of the proposed mechanism as applied to the token ring the FDDI networks. The simulation technique is faster than the measurement technique~ and more accurate and flexible than analytic modeling [9, 12]. In this analysis, a ring stat ion model is assumed to consist of a Logical Link Control (LLC) and a Medium Access Control (MAC) sublayer. Multiple instances of a station model are connected via delay lines to form a unidirectional ring (the s tandby ring is not modeled in this study). The delay lines sinmlate the ring latency. The ring latency consists of the signal propagation delay in the fiber and the sum of stat ion latencies. The propagation delay and the station latency are assumed to be 5.085 #s /k in and 0.6 #s/s ta t ion, respectively. Unless otherwise specified, it is assumed that the ring is 50 km in length and tha t 30 stations are connected to it. This corresponds to 272 #s ring latency. These values are reasonable for metropoli tan area ring networks. In most of the experiments, only eight stations are used to send frames. We consider a ring of up to 12 active stations. Each station is assumed to t ransmi t frames at a different priority level; traffic from station i is assumed to have a priority level i. The convention used in this paper is tha t level one is the highest priority level, and its token-holding t ime threshold value, T_Pri(1), is equal to TTRT. The time thresholds of levels two eight are assumed to be 90, 80, . . . , 30% of TTRT. Tile LLC sublayer at each stat ion is responsible for generating a statistical traffic. It generates fixed size frames of 1.6 kbytes (corresponding to the maximum Ethernet f rame size). At 100 Mb/s , the transmission t ime of each frame is equal to 128 #s. The interarrival t ime between generated frames is assumed to be exponentially distributed. The mean value of the interarrival t ime is selected based on the offered load that we want to generate. The offered load (or simply the load) designates the station-generated traffic normal- ized to the medium capacity. In our model, the transmission queue at each stat ion is able to buffer 250 frames. Those frames tha t arrive while tile transmission queue is full are dropped. The sinmlator does not keep track of these dropped frames. Furthernlore, we assume correct operation, i.e., neittler error situations nor ring recovery procedures are considered. To calculate the max imum achievable throughput of the FDDI and the token ring, all stat ions are assumed to transnfit tlighest priority frames, and it is also assumed tha t they always have frames to send. For a fair comparison, the T T R T of the HiPer_Ring (High-Performance Ring, which is a s tandard token ring network tha t implements the adaptive token release mechanism) is always R1 #s shorter than that of the FDDI, where R1 is the ring latency. This R1 t ime will compensate for the extra token-holding


time used by the token ring stations while they are waiting to receive their RFP messages. A HiPer_Ring station is assumed to send an RST message when it receives a PDa message. Figure 1 shows the throughput of FDDI as a function of T T R T when 1, 2, and 12 stations are simultane- ously active. As can be seen, the maximum throughput depends on the ratio between T T R T and the ring latency. The throughput also depends on the number of active stations. As the number of active stations increases, the throughput also increases. Similar results have been reported in [8]. Figure 2 shows the maximum throughput as a function of T T R T for the HiPer_Ring. Generally, similar to FDDI, the throughput increases as the ratio between T T R T and the ring latency increases. Unlike FDDI, when a single station is active, the throughput reaches 100% of the theoretical ring capacity. This is because a station never releases the token. When there is more than one active station, the token is passed among them and some overhead is encountered. This overhead limits the throughput , as observed in the figure. Figure 3 shows the FDDI total throughput , as well as the throughput of each priority level traffic under different loading. The ring is assumed to have eight active stations, and each station a t tempts to trans- mit frames at one of the eight asynchronous priority levels. The arrival rates of frames to be t ransmit ted are identical for each station. The T T R T is 1 ms, and the eight token-holding t ime thresholds are, from highest to lowest, 1 ,0 .9 , . . . , 0.3 ms. Again, the ring latency is assumed to be 272 #s.

At low arrival rates, frames of each priority level are t ransmit ted since all token-holding time thresholds are greater than the ring latency. As the arrival rate increases, the throughput of lower priority frames decreases, and eventually reduces to zero. In this example, only the four highest priority frames have throughput greater than zero. Note tha t the first two priority frames have the same throughput since the number of frames t ransmit ted each time a station receives the token is the same for both priority levels (0.1 ms MOD 0.128 ins equals 0.9 ms MOD 0.128).

Generally, as the load on FDDI ring increases, the throughput of low- priority frames decreases and eventually reaches zero. Specific values for the total ring throughput and the throughput of each asynchronous priority level under overload conditions may be achieved by tuning the FDDI parameters (e.g., TTRT, token-holding t ime thresholds).

For the fairness analysis, one station is assumed to have priority level one (T_Pri(1) = TTRT) , and the other seven stations are assumed to have the same priority level six (T_Pri(5) = 0.5 TTRT. If the protocol is fair, then we expect to see equal throughput from all the stations tha t have the same priority. However, as can be seen from Figure 4, the throughput of a stat ion depends on the location of this station with respect to the high- priority station. The closer the station is to the high-priority stat ion in the


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HIGH-PERFORMANCE PROTOCOLS FOR RING NETWORKS 45

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46 K. K H A L I L A N D M. S. O B A I D A T

d i rec t ion of token ro t a t ion , t he higher i ts t h roughpu t . Th is is because low- p r io r i t y s t a t ions are served in a spa t i a l order whenever the token is re leased by the h igh -p r io r i t y s ta t ion . As the token p r o p a g a t e s t h r o u g h the s ta t ions , the p r o b a b i l i t y t h a t t he h igh-p r io r i ty s t a t i on becomes act ive increases, in which case it will seize t he token. Th is min imizes the poss ib i l i ty t h a t s t a t i ons far f rom the h igh-p r io r i ty s t a t i on will get the token and be able to use it.

F igu re s 15 shows the pe r fo rmance of the abso lu te p r io r i t y scheme wi th the rest mechan i sm. As can be seen, the HiPer_Ring is fair. Al l s t a t i ons wi th the same p r io r i t y level have the same t h r o u g h p u t regardless of the i r pos i t ions wi th respec t to the h igh-pr io r i ty s ta t ion . Th is fair behav io r is i n d e p e n d e n t of the T T R T , as shown in F igures 5 and 6.

4. C O N C L U S I O N S

In conclusion, t he HiPer_Ring p ro toco l is a h igh -pe r fo rmance p ro toco l t h a t has a low response t ime under l ight a n d / o r a s y m m e t r i c load and has a fair access under heavy load. Moreover , it can s u p p o r t a wide va r i e ty of r ea l - t ime traffic w i thou t a m a j o r impac t on the t h r o u g h p u t . T h e ad- van tages of HiPer_Ring over exis t ing token r ing pro toco ls increase wi th t he increas ing t r ansmis s ion r a t e and the geographica l ne twork coverage. Th is p ro toco l has a g rea t po t en t i a l use in fu ture h igh-speed M A N networks .

R E F E R E N C E S

1. W. Bux, Local-area subnetworks: A performance comparison, IEEE Trans. Com- mun., COM-29:1465-1473 (Oct. 1981).

2. A. Bondavalli, M. Conti, E. Gregori, L. Lenzini, and L. Strigini, MAC protocols for high-speed MANs: Performance comparison for a family of Fasnet-based protocols, Computer Networks and ISDN Systems 18:97 113 (1989/90).

3. The Institute of Electrical and Electronics Engineers, Token ring access method and physical layer specifications, American National Standard ANSI/IEEE Std. 802.5-1987.

4. FDDI token ring media access control (MAC), American National Standard, Draft Proposal X3.139-1987.

5. FDDI token ring physical layer medium dependent (PMD), American National Standard, Draft Propopsal X3T9.5/84-48, Rev-9, Mar. 1, 1989.

6. J .M. Ulm, A timed token ring local area network and its performance characteris- tics, in Proc. IEEE 7th Local Computing Networks Conference, Minneapolis, MN, Feb. 1982, pp. 50-56.

7. K .C . Sevick and M. J. Johnson, Cycle time properties of the FDDI token ring protocol, IEEE Trans. Software Eng. SE-13:376-385 (Mar. 1987).

8. D. Dykeman and W. Bux, Analysis and tuning of the FDDI media access control protocol, IEEE Journal of Selected Areas in Commun. 6:997-1010 (July 1988).

HIGH-PERFORMANCE PROTOCOLS FOR RING NETWORKS 47

9. M. S. Obaidat , Protocol for token ring local computer networks and its performance, IEEE Electronics Letters 27(25):2393-2394 (Dee. 1991).

10. K.M. Khalil, K. Q. Luc, and D. V. Wilson, LAN traffic analysis and workload char- acterization, in Proc. IEEE 15th Annual Conference on Local Computer Networks, Minneapolis, MN, Oct. 1-3, 1990.

l l . M . J . Johnson, Proof tha t t iming requirements of the FDDI token ring protocol are satisfied, IEEE Trans. Commun. COM-35:620-625 (June 1987),

12. M. S. Obaidat , Simulation of queueing models in computer systems, in Queue- ing Theory and Applications (S. Ozicici, Ed.), Hemisphere, New York, 1990, pp. 111-151.

Received 1 February 1993; revised 15 June 1994

high-performance protocols for ring lan and man computer networks

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