Real-Time Schedulability of Two Token Ring Protocols
Sanjay Kamat and Wei Zhao
Department of Computer Science
Texas A & M University
College Station, TX 77843-3112
ABSTRACT
When designing real-time communication protocols,
the primary objective is to guarantee the deadlines of
synchronous messages while sustaining a high aggregate
throughput. In this study, we compare two token ring
protocols for their suitability in hard-real-time systems.
A priority driven protocol (eg. IEEE 802.5) allows implementation of a priority based real-time scheduling
discipline like the rate monotonic algorithm. A timed
token protocol (eg. FDDI) provides guaranteed bandwidth and bounded access time for synchronous messages. We study these two protocols by deriving their
schedulability criteria - i.e., the conditions which determine whether a given message set can be guaranteed.
Using these criteria, we evaluate the average performance of these protocols under dierent operating conditions. We observe that neither protocol dominates the
other for the entire range of system parameter space.
We conclude that the priority driven protocol performs
better at low bandwidths (1-10 Mbps) while the timed token protocol has a superior performance at higher bandwidths.
1 Introduction
In this paper, we study two solutions that deal with the
problem of guaranteeing the deadlines of synchronous
messages on a token ring network. Synchronous messages are the most common time-critical messages in
distributed real-time systems. A real-time synchronous
message stream is a sequence of messages arriving periodically at a network node with each message having a
deadline for transmission. The most important goal in
building a real-time network is to be able to guarantee
the deadlines of these messages, that is, to ensure that
they are always transmitted before their deadlines.
Ring based networks have become popular in recent
years because of their simplicity and the potential use
of a high speed medium. For example, a token ring
network based on the Fiber Distributed Data Interface
(FDDI) standard [1] has been selected as the backbone
network for NASA's Space Station Freedom [26].
Two variants of token based Medium Access Control
(MAC) protocol have emerged as design alternatives
for scheduling the transmission of messages over a ring
network. These protocols are:
Priority driven protocol (PDP) .
This work was supported in part by the National Science
Foundation under Grant NCR-9210583, the Oce of Naval Research under Grant N00014-92-J-4031, and an Engineering Excellence Grant from Texas A&M University.
The individual messages are assigned priorities
and a token with a priority eld is employed to
regulate the access to the transmission medium
in a manner that allows the node with the highest priority message to transmit rst. This access
control scheme has been employed in the IEEE
802.5 standard [7].
Timed token protocol (TTP) .
In this approach, a priorityless token is passed by
one station to its neighbor as a right to transmit. A priori limits are imposed for all the stations on how long they can transmit after receiving the token. This protocol has been incorporated in network standards such as FDDI, the
High-Speed Data Bus and the High-Speed Ring
Bus (HSDB/HSRB) [17, 18, 24] and the Survivable Adaptable Fiber Optic Embedded Network
(SAFENET) [5, 14].
This paper deals with the applicability of these protocols in real-time environments. In Section 2, we dene the objectives of this study and review the previous relevant work. In Section 3, we describe the system model and the notations employed in this paper.
Section 4 deals with the priority driven protocol. The
basic protocol is described, followed by a discussion of
an implementation of the rate monotonic scheduling algorithm. The schedulability conditions for this scheme
are then derived. Section 5 parallels Section 4, but
deals with the timed token protocol. In Section 6, we
formally dene the average breakdown utilization metric, and present the results of comparison between these
two protocols based on this metric. We present our conclusions in the nal section.
2 Background
Our work complements the recent research in the study
of communication protocols for real-time applications.
The worst case performance for asynchronous real-time
messages in a token ring environment has been investigated in [11, 27], while the average case performance
is studied in [20]. The implementation of the optimal
rate monotonic policy on the IEEE 802.5 token ring
was rst proposed by Strosnider, Lehoczky and Sha
[22]. However, no analysis of such a scheme has been
reported so far. The timed token protocol was rst proposed by Grow in [6]. This protocol has the important
property of bounded access time for the nodes on the
ring. The real-time performance of this protocol has
been recently studied in [2, 3, 4, 9]. It was shown that
with a proper choice of protocol parameters, this protocol can guarantee a synchronous trac of up to 33%
in the worst case. However, the issue of average case
performance of this protocol has not been addressed so
far.
Our objective here is to derive precise criteria for ascertaining whether a given synchronous message set can
be guaranteed using each of these two protocols. The
second objective of this study is to carry out a quantitative comparison between these protocols for real-time
communications. Through this study, we want to identify the conditions under which one protocol has a superior performance compared to the other and hence
should be recommended in practice.
The schedulability of a synchronous message set depends on several message and network parameters. The
schedulability conditions we derive in this paper provide simple and ecient means for checking whether a
given message set is schedulable by a protocol for specied network parameters.
However, the schedulability conditions by themselves do not reect the overall capacity of the protocols
to schedule real-time messages over the entire population of message sets. For a meaningful comparison between two protocols, we need an aggregate measure to
quantify the overall performance of a protocol. Recent
research in real-time systems has focussed on utilization
based metrics. In [2, 3], Minimum Breakdown Utilization 1 has been used to quantify the performance of a
protocol. Minimum Breakdown Utilization of a protocol is dened as the threshold utilization below which
all the real-time messages are always guaranteed.
The minimum breakdown utilization indicates the
worst case real-time performance of the protocol.
Hence knowledge of this bound simplies run-time
network administration - schedulability tests are not
needed as long as the oered load is below this bound.
However, at the design stage, when faced with a choice
between alternative protocols, and in the absence of
a detailed knowledge of the message sets, it is more
appropriate to base the selection on the average case
performance of a protocol. We have chosen Average
Breakdown Utilization as our performance metric. Average Breakdown Utilization of a protocol reects how
high the message set utilization can be on average without missing the message deadlines. In other words, a
protocol breakdown with respect to missing the deadlines of real-time messages is unlikely for message sets
having utilization lower than the Average Breakdown
Utilization.
Lehoczky, Sha and Ding [10] were the rst to use average breakdown utilization as a performance metric for
evaluating a scheduling algorithm. They rst derived
the precise schedulability criteria for the rate monotonic algorithm [12] in a cpu scheduling environment
consisting of independent tasks and having negligible
scheduling overheads. They used these schedulability
criteria to analyze the average case performance of the
algorithm and showed that the average breakdown utilization of the rate monotonic algorithm under these
ideal conditions can be as high as 88%. They also described a systematic procedure for estimating the value
of the average breakdown utilization. Subsequently,
Strosnider and Katcher[23] have used this approach to
evaluate the performance of the rate monotonic algorithm under non-ideal conditions by accounting for the
preemption overheads for various operating system imalso referred as Worst Case Achievable Utilization in literature [2, 3, 4, 12].
1
plementation policies. They authors have reported that
the average breakdown utilization of the rate monotonic
algorithm can be signicantly lower than the ideal due
to preemption overheads and possibilities of blocking or
priority inversion. Our analysis of the two protocols extends the methodology adopted in [23, 10] to the token
ring environment.
Our study shows that neither protocol outperforms
the other under all operating conditions. The priority
driven protocol supports global priority arbitration and
can be used to approximately implement the optimal
rate monotonic scheduling policy. This implementation
has a better performance at low transmission speeds.
However, at high transmission speeds, the overheads incurred in priority arbitration become predominant lowering the performance of the priority driven protocol.
It is seen that the timed token protocol, which does
not support priority arbitration, has a superior performance for high-speed real-time networks.
3 System model
3.1 Network model
We consider a network of n nodes connected using a
ring topology. A special bit pattern called the token
circulates around the ring regulating the right to transmit for individual stations. Notations relevant to our
model are as follows.
BW = Bandwidth of the transmission medium.
WT = Token walk time around the ring.
WT consists of ring and buer latency and propagation delay around the ring.
= WT + Token transmission time.
3.2 Message model
Messages in a distributed system may be classied as
synchronous messages or asynchronous messages. We
assume that there are n streams of synchronous messages, S1 ; S2 ; : : : ; Sn in the system which form a synchronous message set, M, i.e.,
M = fS1; S2 ; : : : ; Sng:
(1)
In our model, there is precisely one synchronous stream
arriving at a given node on the ring. The characteristics
of messages are as follows:
1. Synchronous messages are periodic, i.e., they have
a constant inter-arrival time. We denote by Pi the
period length of stream Si (i = 1; 2; : : : ; n).
2. The deadline of a synchronous message is the end
of the period in which it arrives; that is, if a message in stream Si arrives at time t, then its deadline is at time t + Pi.
3. The payload length of each message in stream Si is
Ci, which is the amount of time needed to transmit this message.
The length in terms of the information bits to be
transmitted is denoted by Cib. Hence we have,
Ci
b
Ci
:
= BW
(2)
The actual amount of time needed to complete
the transmission of a message from stream Si
when all the protocol overheads
are taken into
account will be denoted by Ci0.
The utilization of a synchronous message set, U (M ),
is dened as the fraction of time spent by the network
in the transmission of synchronous messages. That is,
( )=
U M
n C
X
i
i=1 Pi
:
(3)
We will use the term active interval of a message to
denote the interval of time beginning the arrival of the
message till the completion of its transmission.
We consider all asynchronous messages to be ones
without real-time constraints.
4 Schedulability of PDP
In this section, we describe the priority driven token
ring protocol, which has been incorporated in the IEEE
802.5 standard [7], and discuss an implementation of
the rate monotonic algorithm using this protocol. We
also consider a modied version of the IEEE 802.5 standard protocol which can provide a more ecient implementation of priority arbitration. The schedulability
conditions for both these implementations are then derived.
4.1 Protocol description
In this protocol, the token contains a priority eld
which regulates node's access to the ring. The value
of this priority eld indicates the service priority level.
Every frame (token and data) also contain a priority reservation eld. Each node examines the priority
reservation eld in frame header as a frame passes by.
A node overwrites the contents of the reservation eld
by inserting the highest priority of its pending messages (if any) if and only if this priority is higher than
the one currently present in this eld. This enables
the reservation eld to represent the highest priority
of messages waiting in the system. The station holding the token can transmit till its token holding timer
expires after which it releases a free token having a priority equal to the known highest priority of pending
messages. A node is allowed to capture the token and
transmit its messages only when it has higher priority
messages pending compared to the token priority. The
reader is referred to [7, 21] for further details on the
protocol operation.
4.2 Protocol parameter settings
The eectiveness of this protocol in guaranteeing the
deadlines of synchronous messages depends on the
choice of proper values for three protocol parameters:
message priorities, message frame length and token
holding timer value at each station.
Assignment of Message Priorities
In order to guarantee their deadlines, the messages must be assigned priorities in a proper manner. This problem is similar to the problem of assigning priorities to periodic tasks in the context
of cpu scheduling. It is well known that the rate
monotonic algorithm [12], which assigns higher
priorities to tasks with shorter periods, is the optimal static priority, preemptive cpu scheduling
algorithm for this problem. Further, as was rst
proposed by Strosnider in [22], this algorithm can
be applied to token rings. Thus the synchronous
messages are assigned priorities in the inverse order of their periods.
Choice of Frame Size
The rate monotonic algorithm requires that a
node with a high priority message be able to preempt the transmission by a node with a lower priority message. Since the transmission of a lower
priority message cannot be arbitrarily preempted
by the arrival of a higher priority message, preemption is only approximated by dividing messages into frames.
The individual stations contend for the right to
transmit a frame by using the reservation eld
of the frame header. Hence preemption can take
place only after the transmitting station receives
back the header of the transmitted frame and
knows the highest priority claim by other stations
on the ring.
A small frame size provides a better approximation of the preemption requirement. However, there are xed overheads associated with the
transmission of each frame. Thus smaller frames
also imply higher overhead. Hence the choice of
frame size is a trade-o between the enhanced
responsiveness associated with ner granularity
provided by small frames and the high overhead
incurred.
We assume that the synchronous and asynchronous message frames have the same length.
b and F b
We use the notations Finfo
ovhd for the
length in bits of the information part and the
overhead part of a frame. F b denotes the total
length of a frame in bits and F = F b=BW denotes the time needed to transmit it. We also
introduce the following notations to simplify the
expressions to be derived later.
$
Li
=
Cib
b
Finfo
%
&
and
Ki
=
Cib
'
b
Finfo
Note that Ki denotes the total number of frames
into which a message from stream Si will be split.
If Ki = Li , then all the frames are of the maximum length, while Ki = Li + 1 implies that the
last frame is of less than full length.
Token Holding Timer
As per the implementation proposed in [22], the
token holding timers at all the stations are set so
that at most one frame can be transmitted by a
station on capturing a token.
It should be noted that the standard IEEE 802.5
protocol does not automatically provide for priority arbitration at individual frame level if the
token holding timer value exceeds the time to
transmit one frame. Such ne grain priority arbitration is achieved in the above implementation
by setting the token holding timer to allow transmission of only one frame by the station holding a token. The standard also requires that a
free token be issued after the token holding timer
expires. Hence an implementation of the rate
monotonic scheduling algorithm using the standard IEEE 802.5 protocol incurs the overhead of
circulating a token for transmission of every message frame.
Besides analyzing the standard protocol, we also
evaluate a slightly more ecient version of the
protocol which directly supports frame level contention. In this modied version, after transmitting a frame, the transmitting station can transmit another data frame, instead of issuing a free
token, if it is still the highest priority active station on the ring. Hence this version does not need
a token holding timer.
frame. Thus, for this special case, the eective
transmission time for the last frame is given by
max(Ci ? Li Finfo + Fovhd ; ).
The following lemma helps us to bound the total
blocking period that needs to be considered in testing
the schedulability of a particular message. (Please refer
[9] for the proof).
LEMMA 4.1 Consider
the i highest priority synchronous message streams S1 ; S2 ; : : : ; Si, (ordered in
decreasing priority sequence). During the entire active
period of one message from the stream Si , the total duration of blocking of any of the messages in streams
S1 ; S2; : : : ; Si by messages having priority lower than
that of Si is bounded by 2 max(F; ).
Now the schedulability equations for this protocol
are given by the following theorem.
THEOREM 4.1 The criteria for schedulability of a
synchronous message set M on a token ring network
(for all phasings) when the rate monotonic scheduling
policy is employed, are given by,
8i21 i n;
3
i?1
0
X
B
lP
C
1
k
5 1 (4)
+ i +
min 4 C 0
4.3 Schedulability analysis
Our objective is to derive precise criteria to test
whether a given message set is schedulable by the protocol. In deriving these conditions, we focus on the
message lengths and periods as well as the important
network parameters which inuence the schedulability.
We assume the worst possible impact of asynchronous
disturbances and message arrival phasings.
The schedulability conditions for the priority driven
protocol account for the dierent overheads such as
transmission and priority arbitration overheads. We
also take into account the possibility of blocking - a
form of priority inversion where a low priority message
frame is transmitted even though a higher priority message is ready for transmission. Blocking occurs in this
protocol due to the approximate nature of preemption
as well as due to the distributed nature of priority arbitration.
We need to consider the following two cases to derive the eective frame transmission time.
1. F .
In this case, the transmitting node receives back
the header of the transmitted frame after it has
nished sending the last bit of the frame. Since
priority arbitration at this node requires it to examine the reservation elds in the header of the
transmitted frame, the transmission medium is
not available to any other node till the header
returns to the transmitting node.
Hence, in this case, the eective time to complete
the transmission of each frame is .
2. F > .
In this case, for all but the last frame, the transmitting station always receives back the frame
header even before it completes the transmission
of the frame. Thus, the eective transmission
time for the each of the rst Li frames is F .
However, the last frame can be smaller than the
maximum frame size. As discussed earlier, this
happens when Ki = Li + 1. In that case, after transmitting the last frame, the transmitting station may have to wait till it receives
the header back. The eective medium requirement by the last frame will then be the maximum of and the time to transmit the last
(k;l)2Ri j =1 j lPk
Pj
lPk
lPk
where B = 2 max(F; ), and Ri is dened as
Ri
= f(k; l)j1 k i; l = 1; bPi =Pkcg
and Ci0, the augmented message length which accounts
for all the overheads, is obtained as follows:
1. For implementation using IEEE 802.5 protocol
Ci0
8
Ki + K i >
>
<
=>
>
:
if F 2
otherwise
Li F + Ki 2
+(Ki ? Li ) max (Ci ? Li Finfo + Fovhd ; )
2. For implementation using the modied version
Ci0
8
Ki +
>
>
<
=>
>
:
if F 2
Li F + otherwise
2
+(Ki ? Li ) max (Ci ? Li Finfo + Fovhd ; )
In the above expressions, the token circulating overhead has been assumed to be =2 on the average.
While this overhead is incurred for every transmitted
frame in the implementation using the IEEE 802.5 protocol, it can be shown that this overhead is incurred
only once for entire message transmission in case of the
modied version.
5 Schedulability of TTP
5.1 Protocol description
In this protocol, the access to the transmission medium
is controlled by passing a token, as in case of the priority driven protocol. However, the token has no priority
eld and the right to transmit is passed by one station
to its immediate neighbor. At the ring initialization
time, a global value called the Target Token Rotation
Time (TTRT) is determined by a process of bidding
among the stations on the ring. This protocol parameter indicates the expected token rotation time, i.e.,
the interval between two successive token arrivals at a
station. The token is usually expected to visit a station within TTRT period after its previous visit (unless its previous arrival itself was late, in which case,
it is expected to arrive early enough to compensate for
the previous delay). The transmission of asynchronous
messages can cause the token to get delayed at a station.
A fraction of TTRT (after subtracting certain protocol overheads - collectively denoted by ) is assigned
as the synchronous bandwidth2 (hi ) of an individual
station. After a station receives a token, it will transmit its synchronous messages, if any, for a maximum
period given by its synchronous bandwidth and then
it can transmit any asynchronous frames only if it has
received the token earlier than expected. Furthermore,
it can transmit the asynchronous frames only for a period of time by which the token has been early. Also
the total transmission by a station on receipt of token cannot exceed TTRT . However the transmission of
an asynchronous frame once begun will always be completed. This is known as the asynchronous overrun and
contributes towards the protocol overheads.
5.2 Protocol parameter settings
The performance of this protocol depends on the selection of appropriate values for the following protocol
parameters: T T RT , Synchronous capacities of individual stations, synchronous message frame length, and
asynchronous message frame length.
TTRT Selection
The synchronous bandwidth allocation schemes
proposed so far assume that the T T RT value
has been already independently chosen. Previous reported studies [8] on guidelines for choosing an appropriate TTRT value have considered
network models with purely asynchronous loading. The issue of selecting an appropriate value
for T T RT for a real-time environment has not
been addressed in the literature so far. As shown
by Johnson in [19], the time between two successive token visits to a station can be at most
twice T T RT . Hence the T T RT value should be
less than half of the minimum period. Our studies have shown that the performance of the timed
token protocol in a real-time environment is sensitive to the T T RT value and often a value much
lower than half of the minimum period produces
2 Also referred as synchronous bandwidth in literature [2, 3, 4].
We use the term synchronous bandwidth, in accordance with the
most recent version of the FDDI standard.
better results. In particular, we have shown that
for the special case of all message periods being
equal to P , the breakdown utilization ispmaximized by choosing a T T RT value close to P .
Furthermore,
the heuristic of choosing T T RT as
p
Pmin is found to give good results in the
more general case of unequal periods. Due to
space limitation, we omit the derivation of these
results here. The interested reader is referred to
[9] for further details.
Thus we assume thatpeach station bids for TTRT
by the rule given by Pi and the minimum is
selected.
Synchronous Bandwidth Allocation
In our study of the timed token protocol, we
assign the synchronous capacities to individual
stations according a local bandwidth allocation
scheme proposed in [3]. This scheme was shown
to have a minimum breakdown utilization of
33%. An algorithm to generate the optimal synchronous capacities for given T T RT value has
been proposed in [4]. We nd that the average
performance of the local scheme is comparable
to that of the optimal scheme, particularly when
T T RT is chosen as described above [9]. In a local scheme, the allocation of capacities is done
purely on the basis of local information at individual nodes. Hence local schemes are easy to
implement and are more exible.
The local scheme assigns synchronous capacities
to stations according to the following rule:
hi
0
= q C?i 1
where qi is dened as
qi
=
(5)
i
Pi
T T RT
(6)
and Ci0 is the augmented message length which
takes into account the transmission overheads.
Synchronous Message Frame Length
As in case of the priority driven protocol, the
synchronous messages are divided into frames.
Since a station can transmit synchronous messages for at most hi time after receiving a token,
the synchronous frame length for a station is simply the synchronous bandwidth allocated for that
station. Since overhead bits need to be transmitted along with the payload data in each frame,
the augmented message length is computed as follows.
Ci0
= Ci +
Ci0
hi
Fovhd
(7)
Using (5), and the fact that qi is an integer (from
(6) above), the above equation can be simplied
to
Ci0 = Ci + (qi ? 1) Fovhd
(8)
Resubstituting this in (5), the bandwidth assignment rule can be stated as
hi
= q C?i 1 + Fovhd :
(9)
i
Asynchronous Frame Length
We have varied the asynchronous frame lengths
for conducting dierent sets of experiments. In
this paper we present the results of experiments
for payload lengths of asynchronous frames chosen as 64 bytes. The results obtained for other
frame lengths are similar and are not shown here
due to limitations of space. While hi 's denote the
synchronous frame lengths in this protocol, we
use the notation F for the asynchronous frame
length (measured in units of time).
5.3 Schedulability analysis
Any synchronous bandwidth allocation scheme for the
timed token protocol has to satisfy the following two
constraints to be acceptable as a deadline guarantee
scheme.
1. The Protocol Constraint
n
X
i=1
hi
T T RT ? (10)
As stated in the protocol description, denotes
sum of various protocol overheads which reduce
the actual time available for transmission of data
during one typical token rotation duration.
The value of in equation (10) is given by,
= +F
(11)
Note that decreases as bandwidth is increased.
2. The Deadline Constraint
The allocation of synchronous capacities to the
nodes should be such that the synchronous messages are always guaranteed to be transmitted before their deadlines. Thus if Xi is the minimum
amount of time available for node i to transmit
its synchronous message during the entire period
of the message, then this constraint can be expressed as
Xi Ci0 :
(12)
The schedulability condition for this model can now
be stated as follows.
THEOREM 5.1 The criterion for schedulability of a
synchronous message set M on a token ring network using the timed token protocol with synchronous capacities
allocated using the local scheme (specied in equation
(9)) is given by,
n
X
Ci
Pi ? 1 + n Fovhd T T RT
i=1 TTRT
? :
(13)
6 Performance comparison
6.1 Metric denition
To better appreciate the denition of average breakdown utilization, consider a classication of all message sets for a given protocol. We can partition synchronous message sets into three classes with respect to
their schedulability by a given protocol.
1. Unsaturated Schedulable Class : The message sets
in this class are schedulable and remain so even if
any of the message lengths is slightly increased.
2. Saturated Schedulable Class : These message sets
are schedulable but an increase in any of the message lengths will lead to the violation of schedulability. Hence these message sets represent the
breakdown loads on the system.
3. Unschedulable Class : This class consists of message sets that are not schedulable.
Formally, for the priority driven protocol, a message set belongs to the unsaturated schedulable class if
the schedulability conditions of (4) are satised, and for
each message in the set, these conditions are strict inequalities. The saturated schedulable set is characterized by the same inequalities with the condition that
at least one of the equalities holds. Similarly for the
timed token protocol, the unsaturated schedulable class
is characterized by a strict inequality in (13) and the
saturated schedulable class by an exact equality in (13).
The unschedulable class in both the cases consists of
messages which do not meet their respective schedulability criteria.
We dene the average breakdown utilization of a
protocol to be the expected value of the utilization of
message set belonging to the saturated class of that protocol. Average breakdown utilization is thus a measure
of the average case performance of a system. It reects
how high the synchronous utilization could typically be
while the deadlines of the synchronous messages are still
guaranteed. Monte Carlo methods can be employed to
estimate the value of the average breakdown utilization
for a given distribution of periods by generating random message sets in the saturated class. The reader is
referred to [9] for details of these methods.
6.2 Comparison
We now discuss the results of comparison tests carried
out for the two protocols under dierent operating conditions. These operating conditions are specied by the
following values for the various parameters.
n = The number of stations = 100.
d = Distance between neighboring stations = 100
meters.
Average bit delay per station was taken as 4 bits
for the priority driven protocol and 75 bits for the
timed token protocol. These values are characteristic of IEEE 802.5 and FDDI standards.
The message periods were assumed to have a uniform distribution with two parameters, the average period and the maximum to minimum period
Finfo
Finfo
=
P
b
Finfo
BW +
Q
b
? Finfo
b
Finfo
(14)
b =BW .
since Finfo = Finfo
3 or transmitting another frame, as in case of the modied
IEEE 802.5 version
1.00
Avg. Breakdown Utilization
ratio. The results reported here are for average
period of 100 msec and for the maximum to minimum period ratio of 10. The results obtained for
other values of these parameters were similar and
are not reported here due to space limitations.
Signal propagation speed through the medium
was assumed to be 75% of the speed of light.
b = 112 bits.
Fovhd
Figure 1 shows the variation in the average breakdown utilization of the priority driven protocol and the
timed token protocol as the bandwidth is varied. We
can make following observations from this representative plot.
From Figure 1, we observe that the performance
of the priority driven protocol initially improves
as the bandwidth is increased but starts to drop
beyond a certain point. This is against the intuition that performance of the protocols should
improve as bandwidth is increased. The explanation for this behavior lies in the impact of overheads associated with frame transmissions in the
protocol.
At low bandwidths (when F > ) the eective time to complete the transmission of one
frame using the priority driven protocol is F =
Finfo + Fovhd . Hence the fraction of the time
b
. This frac= FFovhd
wasted on overheads is FFovhd
b
info
info
tion being a constant, the performance of the priority driven protocol improves as the bandwidth
is increased as long as F < . When the bandwidth is increased beyond a certain value, the
decrease in the ring latency and token transmission time cause the token circulation time to
exceed the frame transmission time F . In this
case, before releasing a new token3 , the transmitting node has to wait for the token to return
even after the transmission of a frame is complete.
Thus the eective frame transmission time in this
case is and
hence the fraction of wasted bandFinfo
width is ?Finfo
. itself consists of the signal
propagation delay, the ring latency, and the token transmission time. The propagation delay is
independent of the bandwidth and hence can be
considered as a constant (denoted by P ). The
ring latency and the token transmission time decrease
bandwidth. Their sum can be written
Q with
, where Q is the sum of token length and
as BW
the ring latency in bits. Hence the fraction of the
bandwidth that is wasted becomes
? Finfo = P + Q=BW ? Finfo
Modified IEEE 802.5
IEEE 802.5
0.80
FDDI
0.60
Packet Length = 64 Bytes
Average Period = 100 ms
0.40
0.20
0.00
1
10
100
1000
Bandwidth (Mbps)
Figure 1: Performance Comparison
Thus the percentage of wasted bandwidth increases with increase in bandwidth thereby lowering the performance of the priority driven protocol.
The timed token protocol does not exhibit this
anomaly as it allows a station to immediately
release the token after transmitting its frames.
Thus the performance of the timed token protocol improves as bandwidth is increased.
It is also clear that the modied version of the
IEEE 802.5 protocol performs better than the
implementation based on the standard. This is
obviously because the token passing overhead is
incurred once for every packet transmitted in the
implementation based on the IEEE 802.5 standard, while it is incurred only once for every message in the modied version.
It is observed that the priority driven protocol
performs better than the timed token protocol
at low bandwidths (1-10 Mbps). For these operating conditions, the overheads inherent in the
implementation of the rate monotonic scheduling
are not predominant. Hence the superior performance is due to the optimality of the scheduling
strategy being used.
On the other hand, the timed token protocol performs better than the priority driven one at high
bandwidths. Though the scheduling strategy of
the time token protocol is not optimal, it invokes
little cost (token passing overhead). In a system
of high bandwidth, the heavy overheads inherent
in the priority driven protocol degrade its performance, resulting in the relative superiority of the
time token protocol.
Similar observations are also made for systems with
parameter values other than those described here. We
have not shown them here due to the space limitations.
7 Conclusions
The schedulability conditions for the timed token protocol and the priority driven protocol (implementing
a rate monotonic scheduling policy) were developed.
These criteria were then used to study the average case
behavior of the two protocols by estimating their average breakdown utilization values. Our studies have
shown that neither protocol is ideal for real-time communications over the entire range of network operating
conditions. The priority driven protocol can be used to
implement optimal scheduling policy such as the rate
monotonic algorithm. At low transmission speeds (110 Mbps), this implementation is very ecient. The
timed token protocol essentially implements a round
robin scheduling policy without any priority arbitration mechanism. At low transmission speeds, the priority inversions caused by such a round robin scheduling approach tend to adversely aect the messages with
short deadlines. Thus at low transmission speeds the
priority driven protocol is better suited for real-time
applications than the timed token protocol. The priority arbitration overheads incurred by the priority driven
protocol increase with network bandwidth thereby lowering the performance of the priority driven protocol for
high speed networks. The timed token protocol which
does not incur priority arbitration overheads and allows
multiple frames to be on the ring at the same time is
found to perform better at high bandwidths such as
100 Mbps and above. We thus conclude that bandwidth ranges for which the respective protocols have
been found suitable for non-real-time systems are also
appropriate for real-time applications.
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