The architectural impact on guaranteeing synchronous message deadlines in FDDI (Fiber Distributed Data Interface) token ring networks is examined. The FDDI network does not have facility to support (global) priority arbitration which is a useful facility for scheduling hard real time activities. As a result, it was found that the worst case utilization of synchronous traffic in an FDDI network can be far less than that in a centralized single processor system. Nevertheless, it is proposed and analyzed that a scheduling method can guarantee deadlines of synchronous messages having traffic utilization up to 33 pct., the highest to date

Agrawal, Gopal

Chen, Baio

Davari, Sadegh

Zhao, Wei

English

NASA Technical Reports Server

Architectural Impact of FDDI Network
on Scheduling Hard Real- Time Traffic
https://ntrs.nasa.gov/search.jsp?R=19920014340 2020-03-17T12:23:41+00:00Z

Preface
This research was conducted by Gopal Agrawal, Baio Chen and Wei Zhao of
the Department of Computer Science at Texas A&M University and Sadegh Davari
of the Department of Computer Science at the University of Houston - Clear Lake.
The research was supported by a grant from RICIS and in part by an Engineering
Excellence grant from Texas A&M University; it demonstrates the benefits of a
Memorandum of Understanding between the two Unversities and is an example of
fulfilment of the RICIS mission statement.
RICIS funding was provided through a service contract between the University
of Houston - Clear Lake and the Texas Engineering Experiment Station at Texas
A&M University. RICIS Research Support funds are derived from Cooperative
Agreement NCC 9-16 between the NASA Johnson Space Center and the University
of Houston-Clear Lake.
The views and conclusions contained in this report are those of the authors
and should not be interpreted as representative of the official policies, either express
or implied, of UHCL, RICIS, Texas A&M, TEES, NASA or the United States
Government.

Architectural Impact of FDDI Network
on Scheduling Hard Real-Time Traffic
Gopal Agrawal, Baio Chen, and Wei Zhao
Department of Computer Science
Texas A&M University
College Station, TX 77843
Sadegh Davari
Department of Computer Science
University of Houston - Clear Lake
Houston, TX 77058
v
Abstract
We examine the architectural impact on guaranteeing synchronous message deadlines in FDDI
token ring networks. The FDD! network does not have facility to support (global) priority
arbitration which is a useful facility for scheduling hard real-time activities. As a result, we found
that the worst case utilization of synchronous traffic in an FDDI network can be far leas than
that in a centralized single processor system. Nevertheless, we propose and analyze a scheduling
method which can guarantee deadlines of synchronous messages having traffic utilization up to
33% -- the highest to date.
1 Introduction
High speed networks are vital for the support of distributed hard real-time applications. In hard real-time
systems tasks must satisfy explicit time constraints; otherwise, grave consequences may result. Consequently,
the messages transmitted in the network are also time constrained. In this paper, we are concerned with
architectural support and its impact in scheduling messages with hard real time constraints in an FDDI
network.
The basic design requirements of scheduling real-time tasks in a centralized environment (e.g., a single
processor system) and scheduling real-time traffic in a communication network (e.g., a token ring network)
are similar: Both are constrained by time to allocate a serially used resource to • set of processes. For
scheduling synchronous (i.e., periodic) computation tasks in a single CPU environment, the objective of the
scheduler is to guarantee that the deadlines of all the tasks are met. Liu and Layland [10] snallnred • fixed
priority preemptive algorithm, called rate monotonic algorithm which assigns priorities in the reverse order
of task's periods. They showed that the Worst Case Achievable Utilization of the algorithm is 89_. That
*This work is supported in part by an Engineering Excellence grant from Texu A&M University _ by • grant from the
Research Institute for Computing and Information Systems of the University of Houston - Clear Lake.
is, as long as the utilization of the task set is no more than 69% the task deadlines are guaranteed to be
satisfied - even in the worst circumstances.
FDDI networks do not have facility to support (global) priority arbitration. As a remdt, the rate moQo-
tonic algorithm cannot be directly implemented. However, the methodology for analyzing rate monotonic
scheduling algorithm has a more profound significance than merely its relevance to the algorithm. This
methodology stresses the fundamental requirement of predictabilily and stability in hard real-time environ-
ments and is therefore also befitting to other hard rea/-time scheduling problems. In this methodology the
worst case achievable utzlizalion is used as a metric for evaluating the predictability of a scheduling algo-
rithm. That is, as long as the CPU utilization of all tasks is within the bounds, specified by the metric, all
tasks will meet their deadlines. This metric also gives a measure of the stability of the scheduling algorithm
in the sense that the tasks can be freely modified as long as their total utilization is held within the limit.
We should adopt this methodology in our study of guaranteeing deadlines of periodic messages in an FDDI
network. In particular, we should pay attention to the architectural impact on the worst case achievable
utilization of FDDI networks.
The remainder of the paper is organized as follows: Section 2 will outline the characteristics of the system
under consideration, i.e., the message and network models. In Section 3, we will study the architectural
impact on guaranteeing periodic (synchronous) message deadlines. Section 4 contains the concluding remarks
and suggestions for future work.
2 System Characteristics
Wc consider Ihe network topology as consisting of n nodes connected by point-to-point links forming a circle
i e . Ih,. Ir)k,-,} ring The overhead caused by the ring latency, which is the token walk time when none of the
nodcs disturb it, is denoted by r. The usable ring utilization would therefore be (1 - a) [20].
.Messages generated in the system at run time may be classified a.s either synchronous messages or
asynchronous messages. We assume that there are n streams of synchronous messages, S1, $2,..-,S,_, one
for each node in the system j. These n streams of synchronous messages form the synchronous message set,
M, i.e.,
M = {ShS2 ..... S_}. (i)
The characteristics of messages are as follows:
1. Synchronous messages are periodic, i.e., messages i_ a synchronous message stream have a constant
inter-arrival time. We denote Pi to be 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, ira message
in stream S, arrives at time t, then its deadline is at time t + Pi.
3_ Messages are independent in that message arrivals do not depend on the initiation or the eomphtioa
of transmission requests for other messages.
l This assumption can be relaxed. In {3], we show that this message modal is logically and tlrnmingly eq_v_alL to that d
the systems where a node may have zero, one, or more than one sync_ messag_ streams.
4. The length of each message in stream Si is Ci which is the maximum amount of time needed to transmit
this message.
The Utilization factor of a synchronous message set, U(M) is defined as the fraction of time spent by
the network in the transmission of the synchronous messages. That is,
n
,2,
where n is the number of synchronous message streams.
3 Impact on Guaranteeing Periodic Messages
Scheduling (i.e., control of message transmission) in an FDDI network is implemented in its MAC layer
by a timed token protocol which was proposed by Grow in [5]. In this protocol, meaaagea are segregated
into separate classes: the synchronot_s class and the as_-nchronous class. The idea behind the timed token
protocol is to control the token rotation time. At network initialization time, a protocol parameter called
Target Token Rotation Time (TTRT) is determined which indicates the expected token rotation time. Each
station is assigned a fraction of the TTRT, known as synchronous capacity 2 which is the maximum time
a station is permitted to transmit its synchronous messages every time it receives the token. Once a node
receives the token, it transmits its synchronous message, if any, for a time no more than its allocated
synchronous capacity It can then transmit its asynchronous messages only if the time elapsed since the
previous token departure from the same node is less than the value of TTRT, i.e., only if the token arrived
earlier than expected In this way, the time elapsed between any two consecutive token visits at a particular
node (ie.. tile toke,1 rotation time) is bounded In [7, 16], Johnson and Sevcik formally proved that when
the network operates normally, the token rotation time between two consecutive visits to a node is bounded
hv 2 . TTRT.
Although the bounded token rotation time property provides a necessary condition for guaranteeing all
the periodic message to meet their deadlines, it is inadequate. A node may be unable to complete the
transmission of a synchronous message before its deadline since insufficient synchronous capacity may be
allocated to the node. Thus, for a given network model 3, whether or not a set of synchronous message can be
guaranteed is dependent upon the allocation of the synchronous capacities to the nodes. We have analyzed
the performance of FDDI networks with several synchronous capacity allocation schemes. The results can
be briefly summarized as follows:
Full Length Allocation Scheme. With this scheme, the synchronous capacity allocated to a node is
equal to its total time required for transmitting its synchronous messages, i.e.,
Hi = C,. (3)
This scheme attempts to transmit a synchronous message arriving at a node in a single turn rather
ihan splitting it into chunks and distributing its transmission evenly over its period P_. Although the
2Some other synonymous terms that researchers use are: Ba_dt_Itll alloca_iol [20], Sy_c&eomo_a ailocasfiom [6], Sl_c/Lromola
band.irk aJtigamc_tta [7], Higl_ Priorit_ toke*_ koldial time {13], etc.
3 Ln terms of parameter values of n - the number of nodes, ¢ - the token walk time, and TTFLT - the target, token ro_ation
time.
synchronous capacity allocated is sufficient, we have found [3] that in an FDDI network, the worst case
achievable utilization of synchronous traffic can asymptotically approach 0% if this allocation scheme
is used.
That is, for any given e > 0, there exists a message set M such that U(M) _< • and the deadline of
some message in M cannot be met when the synchronous capacities are allocated using the Full Length
Scheme.
Equal Partition Allocation Scheme. In this scheme, the usable portion of TTRT is divided equally
among the n nodes for allocating their synchronous capacities, i.e.,
TTRT- r
H, - , (4)
71
where n is the number of nodes in the system.
For this scheme, we also showed that the worst case achievable utilization of synchronous traffic can
asymptotically approach 0% [3].
Normalized Proportional Allocation Scheme. With this scheme, the synchronous capar.ity is allo-
cated according to the normalized load of the synchronous message on a node, i.e.,
H, - C,/P, (TTRT - r), (5)
U
where U = _,=1 CjP,.
Let a be the ratio of the ring latency r to the target token rotation time (TTRT). We obtained in
{3] that the worst case achievable utilization of an FDDI network is ½(1 - a) when the Normalized
Proportional Allocation Scheme is utilized.
From the results of our analysis, the architectural impact of the FDDI network on guaranteeing syn-
chronous message deadlines is evident. Implementing the optimal rate monotonic scheduling algorithm re-
quires the support of global priority arbitration. Without such facility, the worst case achievable utilization
cannot be as high as that achieved on centralized systems.
Nevertheless, the FDDI network does provide means to control message transmissions through appropriate
settings of the protocol parameters like synchronous capacities of the nodes. Improper settings of these
parameters can result in worst case achievable utilization of 0% while appropriate settinss can guarantee the
deadlines of synchronous messages with utilization up to 33%. To the best of our knowledge, no other method
has been reported to achieve a better utilization. Hence, this allocation scheme should be recommended for
use in hard real-time FDDI token ring networks.
4 Future Work
Currently we are investigating issues in the design and implementation of an optimal synchronous capacity
allocation scheme. An optimal scheme should always guarantee a message set if there exists another allocation
scheme which can do so. Clearly, the optimal scheme has the highest worst case achievable utilLsstiou. We
are also working on multi-link ring networks where more than one link can connect two neighboring nodes.
With this architecture, we would like to study the network performance in the context of the worst case
achievable utilization.
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Architectural impact of FDDI network on scheduling hard real-time traffic

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