The Fiber Distributed Data Interface (FDDI) is an ANSI draft proposed standard for a 100 megabit per second fiber optic token ring. FDDI supports two types of traffic, synchronous and asynchronous. Synchronous traffic is time critical traffic; stations are assigned guaranteed bandwidth to support their synchronous needs. Asynchronous traffic is lower priority and is sent only if time permits. It is proved analytically that the FDDI access protocol provides all stations on the ring with equal access to the channel to transmit asynchronous frames, regardless of the relative sizes of synchronous bandwidth allocations for individual stations. Analytic results are supported with data from simulation runs

Johnson, M. J.

English

NASA Technical Reports Server

Fairness of Channel Access for Non-Time-Critical
Traffic Using the FDDI Token Ring Protocol
Marjory J. Johnson
March 1986
Research Institute for Advanced Computer Science
NASA Ames Research Center
RIACS TR 86.9
(NASA-TM-89389) FAIBNESS OF CHANNEL ACCESS
FOB NON-TIME-CBITICAl TRAFFIC USING THE FDD!
TOKEN RING PROTOCOL- (NASA) 14 p CSCL 12B
N86-2S606
Onclas
63/66 43344
Research Institute for Advanced Computer Science
https://ntrs.nasa.gov/search.jsp?R=19860020134 2020-03-20T14:34:11+00:00Z
Fairness of Channel Access for Non-Time-Critical Traffic
Using the FDDI Token Ring Protocol
Marjory J. Johnson - - - - - - .
March, 1986
Research Institute for Advanced Computer Science
NASA Ames Research Center
The Fiber Distributed Data Interface (FDDI) is an ANSI draft proposed stan-
dard for a 100 megabit per second fiber-optic token ring. FDDI supports two
types of traffic, synchronous and asynchronous. Synchronous traffic is time-
critical traffic; stations are assigned guaranteed bandwidth to support their
synchronous needs. Asynchronous traffic is lower priority and is sent only if
time permits. The purpose of this paper is to prove analytically that the FDDI
access protocol provides all stations on the ring with equal access to the chan-
nel to transmit asynchronous frames, regardless of the relative sizes of synchro-
nous bandwidth allocations for individual stations. Analytic results are sup-
ported with data from simulation runs.
This work was supported by NASA Contract NAS2-11530 and by Cooperative
Agreement NCC 2-387 from the National Aeronautics and Space Administration
(NASA) to the Universities Space Research Association (USRA).
Fairness of Channel Access for Non-Time-Critical Traffic
Using the FDDI Token Ring Protocol
Marjory J. Johnson
Research Institute for Advanced Computer Science
NASA Ames Research Center
Moffett Field, California 940S5
USA
1. Introduction
NASA's requirements for the local area network (LAN) on the Space Sta-
tion include support for diverse types of traffic. A media access protocol for the
Space Station LAN must provide low delay and deterministic access to the chan-
nel for real-time control functions (e.g., navigation control and environmental
control), while at the same time ensuring eventual access to the channel for
traffic of a non-time-critical nature. That is, high priority frames must not
prevent low priority frames from eventually being transmitted.
A candidate media access control (MAC) protocol for use on the Space Sta-
tion is the Fiber Distributed Data Interface (FDDI), an ANSI draft proposed
standard for a 100 megabit per second fiber-optic token ring. FDDI supports
two types of traffic, synchronous and asynchronous. Synchronous traffic is
time-critical traffic, such as real-time control and possibly voice transmission,
that requires guaranteed access to the channel; all other traffic is called asyn-
chronous.
A notable feature of the FDDI protocol is its ability to guarantee
bandwidth for synchronous traffic. Even though asynchronous traffic is non-
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time-critical, it is nevertheless worthwhile to determine the quality of service pro-
vided for transmission of asynchronous frames (especially, in consideration of the
NASA requirement that even low priority traffic must eventually be serviced).
The purpose of this paper is to show that the FDDI access protocol provides all
stations on the ring with equal access to the channel to transmit asynchronous
* frames, regardless of the particular assignment of synchronous bandwidth. We
present an analytic proof, given some simplifying assumptions, and then support
the result with data from simulation runs.
2. FDDI Access Protocol
The media access control protocol for FDDI is a timed token rotation proto-
col. Station (i.e., network interface unit) timers cooperatively attempt to main-
tain a specified token rotation time by using the observed network load to regu-
late the amount of time that a station may transmit. As part of the ring initial-
ization process, stations negotiate a value for T Opr , a parameter which speci-
fies the expected token rotation time. Each station is assigned a percentage of
T Opr for its synchronous transmission. Asynchronous traffic is sent only if
the load on the ring is light enough to support it.
Each station has two internal timers which control access to the network, a
Token Rotation Timer (TRT) and a Token Holding Timer (THT). A station's
TRT is initially loaded with T Opr and it is reset to T Opr whenever it
expires. The token is said to arrive at a station on time if its TRT has not
expired since the station last received the token. Otherwise, the token is said to
be late. Specific rules governing a station's actions when it receives the token
are as follows.
1. If the token is on time, then the current value of TRT is placed in THT,
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and TRT is reset to T_Opr . Both asynchronous and synchronous frames
may be transmitted. We assume, reasonably, that a station's synchronous
transmission precedes its asynchronous transmission.
2. If the token is late, the TRT is not reset. In this case the TRT is said to
accumulate its lateness, and only synchronous frames may be transmitted.
3. TRT is enabled during transmission of all frames, synchronous and asyn-
chronous. THT is enabled only during transmission of asynchronous
frames.
4. The length of time an individual station may transmit synchronous frames
may not exceed its synchronous bandwidth allocation. The length of time
that a station may transmit asynchronous frames is governed by its THT;
no asynchronous frames may be transmitted after its expiration. No frames
of either class may be transmitted after expiration of the station's TRT.
5. If either the THT or the TRT expires while the station is transmitting, it
completes transmission of the current frame before issuing the token.
For more information about the protocol see [1,2,3,4,5,6,7,9].
3. Fairness of Access
The FDD! MAC protocol explicitly guarantees access to the communica-
tions channel for synchronous traffic. Asynchronous traffic has lower priority.
In fact, if all the stations' synchronous bandwidth allocations add up to 100 per-
cent of T__Opr , asynchronous traffic will be virtually blocked from the ring. We
assume for the remainder of this paper that synchronous assignments are suffi-
ciently less than 100 percent of T_Opr , so as to allow asynchronous transmis-
sion to occur. In addition we assume that all asynchronous frames have the
-4 -
same priority.
It is not difficult to show that the FDDI protocol provides equal access to
the communications channel if all traffic on the network is asynchronous, i.e., if
all synchronous bandwidth assignments are zero. The presence of synchronous
traffic clearly limits access to the channel for asynchronous transmission.
Surprisingly, traffic patterns are still predictable under some reasonable assump-
tions about the constancy of synchronous traffic, and access to the communica-
tions channel for asynchronous transmission is still fair.
3.1. Ideal Situation, Assuming No Overhead
Throughout this section, we assume zero overhead, i.e., propagation delay
and internal station delay are both zero, stations abruptly cease transmission
upon expiration of either the THT or the TRT, token transmission time is zero,
etc. To make our results independent of the pattern of generation of frames, we
assume that an infinite supply of asynchronous frames is available for transmis-
sion at each station on the ring. Finally, we assume that each station uses its
entire synchronous bandwidth allocation each time it receives the token. Due to
the time-critical nature of synchronous transmission, such constancy of synchro-
nous traffic seems to be a reasonable assumption. Let S be the total bandwidth
allotted for synchronous traffic from all stations on the ring.
We use the following terminology. A cycle is one complete rotation of the
token. We define an extended cycle to be a block of n +1 adjacent turns on the
ring, where n is the total number of stations. That is, an extended cycle begins
and ends with the same station; it is a cycle, extended to include the next
station's access to the channel.
- 5 -
Lemma 1: The maximum time available for asynchronous transmission dur-
ing any extended cycle is T Opr -S .
Proof: According to the FDDI protocol, when a station receives the token
on time, the amount of time it may transmit asynchronous frames is equal to the
amount of time which remains before its TRT expires. If the token arrived on
time the last time around, causing the station's TRT to be reset, this value is_
T_Opr — C , where C is the transmission time during the immediately preceding
cycle; otherwise, it is less. Since each station uses its entire synchronous
bandwidth allocation during each token rotation, C ~^S . Hence, the maximum
amount of time left on any station's TRT when it receives the token is
T_Opr —S . Since any asynchronous transmission which occurs during an
extended cycle causes a direct reduction in the amount of time available for
asynchronous transmission by downstream stations during the same extended
cycle, T Opr -S is the maximum amount of time available for asynchronous
transmission by all stations during a single extended cycle.
Corollary 2: Each cycle has length ^ T_Opr . That is, the token will never
arrive late at a station.
The next result establishes a relationship between the amount of time a sta-
tion transmits asynchronous frames during a given turn to the amount of time
its upstream neighbor transmitted asynchronous frames during its turn before
last. Suppose there are n stations on the ring, labeled 0,1,...,n — 1. To simplify
notation we denote station m 's upstream neighbor by station m — 1, rather than
the more correct terminology, (m -1) mod n . In addition, to simplify the pre-
cise statement of the result, we assume that for a given m , station turns are
numbered so that station (m -l)'s z' turn precedes station m 's x' turn.
- 6 -
Lemma 3: The amount of time that station m , m =0,...,n —1, transmits
asynchronous frames during its c th turn, c ^1, is exactly the amount of time
that station (m —1) transmitted asynchronous frames during its (c —l) 3 t turn.
Proof: Since the token never arrives late at a station, the amount of time
available for transmission of asynchronous frames during each extended cycle is
exactly T Opr -S .- Since we are assuming an infinite source of asynchronous
frames at each station, each station transmits the maximum amount of data
allowed. Hence, the length of time used for asynchronous transmission during
each extended cycle is exactly T_Opr -S . Since the extended cycle starting
with station m -1's c -1st turn and the overlapping extended cycle ending with
station m 's c ' turn differ only by station m —1's c —Is turn on one end and
station m 's c ' turn on the other, the amount of asynchronous transmission
during these two turns must be equal.
Theorem 4- All stations have equal access to the ring to transmit asynchro-
nous frames.
Proof: By Lemma 3, the amount of time an individual station transmits
asynchronous frames shifts cyclically around the ring. Hence, after n +1 token
rotations, where n is the number of stations on the ring, each station has
received exactly the same amount of time to transmit asynchronous frames.
It is important to note that the above theorem is independent of the partic-
ular assignment of synchronous bandwidth to the separate stations. That is,
regardless of the relative sizes of synchronous bandwidth allocations for indivi-
dual stations, if the ring is observed for a sufficiently long period of time (i.e., for
a period of time that is large compared to n +1 times the average cycle time,
where n is the number of stations on the ring), then each station receives an
- 7 -
equal share of the total time available for asynchronous transmission.
3.2. Realistic Situation, Including Overhead
The two major simplifying assumptions made in the previous section were
the abrupt ceasing of transmission upon timer expiration and constancy of syn-
chronous traffic. In this section we discuss the effect of each of these assump-
tions on the validity of our analytic results.
If a station is transmitting an asynchronous frame when either the THT or
the TRT expires, the station completes transmission of that frame before passing
the token to the next station. Such a transmission overrun would detract from
the predicted amount of time available for transmission of asynchronous frames
by downstream stations. Overruns occur with equal probability at all stations.
Thus, if asynchronous frame size is constant, all stations should be affected
equally by this phenomenon and the validity of our analytic results should not
be affected significantly.
A station's inconstant use of its synchronous bandwidth allocation clearly
affects the amount of time downstream stations have available for asynchronous
transmission, so that the pattern of asynchronous transmission described in
Lemma 3 would be only approximate. It is reasonable to expect that a station's
use of its synchronous bandwidth allocation will be relatively constant. Hence,
minor increases and decreases in the amount of time available for asynchronous
transmission caused by inconstant synchronous bandwidth usage should cancel
each other's effects in the long run. That is, inconstant use of synchronous allo-
cation should have a minimal effect on our analytic results.
Other overhead factors mentioned in the previous section are minimal and
- 8 -
should have no noticeable affect on our analytic results. See [5] for a detailed
discussion of how these factors limit the maximum amount of both asynchronous
and synchronous transmission allowed during each token rotation.
At NASA Ames Research Center we have developed a simulation which
models the FDDI MAC protocol. Simulation runs support our analytic results.
Each table below categorizes channel access during a single simulation run,
according to station and transmission type. The first two tables illustrate the
fact that fairness of access to transmit asynchronous frames is independent of
use of the channel for synchronous transmission. Table 1 categorizes channel
access when no synchronous traffic is present on the ring. Table 2 categorizes
channel access when individual stations have different synchronous bandwidth
allocations; each station uses its entire synchronous bandwidth allocation every
time it receives the token.
Percentages presented in these tables measure use of the channel as a per-
centage of the length of the observation period. Total channel access adds up to
less than 100% because of overhead. In both cases, the asynchronous load was
virtually infinite at each station. Also, asynchronous frame size was constant, so
as to minimize the effects of transmission overrun on channel usage. Careful
study of the detailed output from the simulations reveals the pattern of asyn-
chronous transmission described in Lemma 3.
- 9 -
Access to Channel
Station
1
2
3
4
5
6
Sync Transmission (%)
0
0
0
0
0
0
Async Transmission {%)
15.2
14.9
14.3
14.6
14.9
15.2
Table 1.
Access to Channel
Station
1
2
3
4
5
6
Sync Transmission (%)
8.8
8.8
0.0
17.9
0.0
0.0
Async Transmission (%}
8.5
8.8
9.3
8.8
8.9
8.6
Table 2.
Additional issues which affect fairness of channel access have been identified
via the simulation. In addition to the FDDI MAC protocol, our simulation
models some buffer management, in both the transmitter and the receiver. At
the transmitter we model a link queue, of length ^ 0 (to be specified by the
user). and at the receiver we model management of buffers to accept frames
delivered to the station.
If the link queue is of length > 0, frames are placed into the queue when
they are ready for transmission, and they remain there until they are transmit-
ted on the channel. Since this queue is never purged, frames which are unsend-
able but which are at the head of the queue may block other frames which could
legitimately be transmitted according to the FDDI protocol: This phenomenon
is illustrated in Table 3. In this case link queues of length 2 were specified. The
- 10 -
apparent unfairness of access for station 1 is caused by synchronous frames fill-
ing up its link queue, thus preventing asynchronous frames from being transmit-
ted even though sufficient time remains on the Token Holding Timer. A similar
problem would arise if we modeled multiple priority levels for asynchronous
frames, with lower priority asynchronous frames blocking higher priority asyn-
chronous frames.
Access to Channel
Station
1
2
3
4
5
6
Sync Transmission (%)
17.1
0.0
0.0
0.0
0.0
0.0
Async Transmission (%)
3.4
13.1
13.6
13.6
13.7
13.4
Table 3.
In an attempt to model end-to-end network usage, our simulation models a
buffer at the receiving station which stores a frame until the end application
(layer 7 of the International Standards Organization's (ISO) Open Systems
Interconnection (OSI) network reference model) is able to process it. Frames are
processed one at a time. Both the size of this buffer and the amount of time it
takes the application to process a frame are user-specified parameters. During .
the simulation, if there is no room in the buffer to store an incoming frame, it is
refused. Simulation results show that buffer limitations in the receiver adversely
affect fairness of channel access; stations will have equal access to transmit
frames, but will not be able to successfully complete transmission on an equal
basis. This situation is similar to that described in [8].
Clearly, fairness of access encompasses more than just fairness of opportun-
ity to access the channel. It also must include fairness to complete the
-11 -
transmission once it has begun. We intend to investigate these issues more
thoroughly in the future.
4. Conclusion
We have proved analytically, given some simplifying assumptions, that the
FDDI MAC protocol provides equal access to the communications channel for
transmission of asynchronous frames, regardless of the relative sizes of individual
stations' synchronous bandwidth allocations. Data from simulation runs sup-
ports our analytic results. Finally, we identify other issues which affect the fair-
ness of any media access control protocol, and we illustrate the effect of these
issues via simulation runs.
- 12-
References
1. "FDDI Token Ring Media Access Control," Draft Proposed American
National Standard, X3T9.5/83-16, Rev 8, Mar. 1, 1985.
2 R. M. Grow, "A Timed Token Protocol for Local Area Networks," Elec-
tro/82, Token Access Protocols (17/3), May 1982.
3. V. Iyer and.S. P. Joshi, "Streamlining Protocols at High Data Rates,"
Proceedings of Wescon/84, Session 2, Paper 4.
4. V. Iyer and S. P. Joshi, "FDDFs 100 M-bps Protocol Improves on 802.5
Spec's 4-M-bps Limit," EDN, pp. 151-160, May 2, 1985.
5. M. J. Johnson, "Proof that Timing Requirements of the FDDI Token Ring
Protocol are Satisfied," RIACS Technical Report 85.8, August, 1985.
6. S. P. Joshi and V. Iyer, "New Standards for Local Networks Push Upper
Limits for Lightwave Data," Data Communications, pp. 127-138, Jul. 1984.
7. F. E. Ross, and R. K. Moulton, "FDDI Overview - A 100 Megabit per
Second Solution," Proceedings of Wescon/84, Session 2, Paper 1.
8. J. Sventek, W. Greiman, M. O'Dell, and A. Jansen, "Token Ring Local
Area Networks: A Comparison of Experimental and Theoretical Perfor-
mance," Computer Networks, Vol. 8, No. 4, Aug. 1984, pp. 301-309.
9. J. M. Ulm, "A Timed Token Ring Local Area Network and its Performance
Characteristics," IEEE, Feb. 1982.
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Fairness of channel access for non-time-critical traffic using the FDDI token ring protocol

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