Int. J. Communications, Network and System Sciences, 2011, 4, 417-423
doi:10.4236/ijcns.2011.47049 Published Online July 2011 (
Copyright © 2011 SciRes. IJCNS
Performance of Random Contention PRMA: A Protocol
for Fixed Wireless Access
Salman Ali AlQahtani1, Ahed Alshanyour2, Ashraf Mahmoud2
1Department of Information and Communications Technology, King Fahad Security College, Riyadh, Saudi Arabia
2Computer Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia
E-mail:, {ahed, ashrafs}
Received May 21, 2011; revised June 24, 2011; accepted July 2, 2011
This paper introduces a packet reservation multiple access (PRMA) with random contention for fixed wire-
less access communications. The performance of PRMA scheme with random contention is compared with
the performances of traditional PRMA under permission contention scheme. The proposed scheme is a sim-
pler contention mechanism that does not depend on a pre-determined permission probability as PRMA under
permission contention scheme. In this new method, terminals select the contention slot uniformly from the
pool of remaining free slots in the current frame. We evaluate the performance of the new contention mecha-
nism in terms of various metrics including maximum number of carried voice calls and packet delays for a
given acceptable drop rate of voice packets. We show that the new mechanism is superior to that of PRMA
under permission contention scheme for loaded systems and is expected to be insensitive for traffic source
Keywords: Contention Scheme, Fixed Wireless, Performance Evaluation, PRMA
1. Introduction
Packet reservation multiple access (PRMA) is designed
for a two-way wireless communications network with a
star topology. It enables dispersed terminals to transmit
packetized information over a shred channel to a base
station. While PRMA controls the upstream traffic, the
base station broadcasts continuous binary feedback mes-
sages to the contended stations on each slot to indicate
availability of these slots for contention for the subse-
quent frames. Due to this low control and feedback mes-
sages, PRMA is designed for indoor or short range mi-
crocell where the transmission delay are negligible and
the feedback assumed to be available instantaneously.
In this paper we focus on voice transmission while
PRMA could be used for integrated voice and data trans-
mission. PRMA uses TDMA technology where trans-
mission frame is divided into a set of slots. Terminals
have packets for transmission contend on these slots.
Successful contention leads to slot reservation but suc-
cessful contention occurs only if one terminal contends
on a given slot. Successful reservation for a slot is ac-
knowledged by the base station. Collision occurs if more
than one terminal contends for the same slot. Terminal
that success in reserving a slot keeps on this reservation
for transmitting subsequent packets, slot reservation is
terminated as terminal buffer has no packets to transmit.
To increase system efficiency PRMA uses speech activ-
ity detection. Speech activity detection is used at voice
terminals to determine when the speaker is silent or talk-
Contention scheme in PRMA is based on permission
probability, terminal that has packets to transmit can con-
tend on the slot only if it has a permission to contend,
this type of contention is used to avoid collision occur-
rence. For maximum system capacity, permission proba-
bility should be small because increasing this permission
probability leads to excessive number of collisions. Also
reducing the permission probability beyond a certain
value leads to a poor utilization for free slots. For maxi-
mum system utilization, Nada and Goodman [1] use
equilibrium point analysis to evaluate system behavior.
For a particular example their numerical calculations
show that the maximum capacity could be achieved with
permission probability rages between 0.25 and 0.35 is 37
simultaneous conversations, these simultaneous conver-
sations share a PRMA channel with 20 slots per frame
Copyright © 2011 SciRes. IJCNS
All terminals mentor the binary feedback messages
that are broadcasted by the base station. Based on these
feedback messages terminals can know in advance the
available slots within the transmission frame. Using
permission scheme as proposed in [2], all active stations
that have permission to transmit are contending on the
first incoming available slot. This simultaneous conten-
tion will increase the probability of collision occurrence.
In this work, we propose a new contention scheme that
reduces the number of simultaneous contention on a spe-
cific available slot. The new proposed scheme, random
contention scheme, is based on the fact that available
slots are known in advance. Therefore, rather than al-
lowing all ready active stations to contend on the incom-
ing available slot, we limit this number of simultaneous
contention by allowing each active terminal to select its
expected available slot randomly from the pool of re-
maining free slots in the current frame. The new scheme
still has the maximum capacity that is reached by the
permission contention scheme.
This work is organized as follows. Section two is a
survey of related work to PRMA. In Section three we
described the PRMA technique as well as the new pro-
posed contention scheme. In Section four, we present the
simulation model used for performance analysis, and we
discuss workload model and parameters used in the
simulation. Also, the results of the performance evalua-
tion are presented and discussed. Finally, conclusions are
given in Section five.
2. Related Works
Contention techniques such as ALOHA and carrier sense
multiple access (CSMA) are not suitable for serving
large number of terminals. These techniques make ineffi-
cient use of the shared transmission medium. Moreover,
with increasing number of terminals throughput goes
down and transmission delay increases. [2] explored
PRMA as a technique for transmitting over short range
radio channels. The system is proposed as a solution for
inefficient contention techniques.
There have been many proposals for enhancing the
performance of PRMA which concentrated on improving
channel efficiency more and providing some kind of fair-
ness for data applications, neglecting the variable chan-
nel access delay problem [3-10]. Goodman et al. previ-
ous work extended in [3] by examining the influence of a
large number of system variables on PRMA performance.
Through computer simulation they found that with 32
kbps speech coding and 720 kbps transmission medium
PRMA can support up to 37 simultaneous conversations
under dropping probability less than 0.01. In [1], their
previous work are extended by using equilibrium point
analysis to evaluate PRMA system behavior mathemati-
cally. They derive the probability of packet dropping
given the number of simultaneous conversations; they
also establish conditions for system stability and effici-
ency. Their numerical calculation show close agreement
with computer simulation results. In [4], it has been
shown that PRMA gracefully accept low-rate data termi-
nals with moderate data packet delays and extended the
equilibrium point analysis by including voice and data
traffic in the numerical model.
To prevent slots from being wasted in the event of a
collision during access contention, [5] proposed a minis-
lotted PRMA protocol. The proposed protocol is shown
to yield improvements over PRMA for both types of traf-
fic, voice and data, at the expense of introducing small
amounts of additional clipping to some talk spurts. In an
ideal condition with only speech traffic, the proposed
system supports 41 speech terminals with less than 1%
packets dropped. To overcome the stability problem un-
der heavy load conditions, a modified version, which is
referred to as non-collision PRMA, NC-PRMA, is pro-
posed in [6]. The protocol is based on assigning a pair of
dedicated control minislots in the call set-up phase for
each terminal, the contention mechanism required for
reservation in the PRMA protocol is replaced by an indi-
vidual reservation procedure. As expected, packet colli-
sion never occurs. The study results also show that the
overall performance of the NC-PRMA system is superior
to that of the PRMA system.
Authors in [7] proposed a hybrid access protocol
known, as contention time-division multiple access (C-
TDMA). C-TDMA shows some features of contention-
based (slotted-ALOHA) and reservation-based (PRMA)
protocols. It has been recommended for use in the uplink
of future European multimedia distribution systems. A
simple Markov model is proposed to describe C-TDMA
behavior. Their results in terms of throughput and delay
under variable traffic conditions indicate that C-TDMA
is able to grant optimum throughput/delay figures for
typical multiuser systems. Moreover, for a digital speech
scenario, a performance comparison with PRMA demon-
strates that C-TDMA yields equivalent performance to
PRMA in terms of number of users supported by the
system with a limited packet dropping rate.
In this work, which is an extension of [9], we propose
the use of a simple contention resolution method for
voice terminals that does not depend on a pre-determined
permission probability figure as in the original PRMA. In
this method, the user selects a slot uniformly from the
pool of remaining free slots in the frame. In the results
section we show that the proposed method while pro-
duces the same capacity figures as the original method,
provides lower packet delays for loaded systems. Finally,
Copyright © 2011 SciRes. IJCNS
we expect the new method to be insensitive with respect
to traffic burstiness degree.
3. PRMA System Descriptions
PRMA is a technique for transmitting, over short range
radio channels, a mixture of voice and data packets. The
PRMA protocol is organized around time frames with
duration matched to the periodic rate of voice packets. In
each frame, active terminals try to reserve free time slot
dynamically. PRMA is closely related to reservation
ALOHA, R-ALOHA. It is distinguished from R-ALOHA
by its response to network congestion and by its short
round-trip transmission time. In R-ALOHA, congestion
causes excessive packet delays. In PRMA, information
packets are discarded if they remain in terminal trans-
mission buffer beyond a certain time limit. For indoor
application the round-trip propagation time between ter-
minals and base stations is less than one microsecond.
Packet durations typically are 500 - 1000 µs. The short
propagation times allow terminals to learn quickly the
results of transmission attempts. In many cases, an ac-
knowledgment message for the current time slot can ar-
rive at the terminals before the beginning of the next
time slot [1,2].
PRMA is a slotted protocol where frame duration, T,
is a design variable parameter divided into a set of slots,
N, number of these slots depends on both frame size and
transmitted packet size. Frame size is related to frame
duration and channel transmission rate, Rc. Packet size is
related to source bit rate, Rs. Terminal generates only
one speech packet during frame time. The amount of
source information per packet is RsT bits and the total
length is RsT + H bits. H represents the size of the header.
Header contains routing, synchronization and control in-
The speech activity detector is modeled as a two-state
Markov process as shown in Figure 1.
The probability of transmission from the talk, TLK,
state toward the silence state, SIL, is the probability that
the talkspurt with mean duration t1 ends during a time
Figure 1. Two-state model of a slow speech activity detector.
States are talking (TLK) and silence (SIL).
slot of duration
1exp rt
  (2)
While the probability of transmission from the SIL
state to the TLK state is the probability that silencespurt
with mean duration t2 ends during a time slot
1exp t
  (3)
Figure 2 represents a PRMA speech terminal with N +
2 states. Terminal could be in one of the N + 2 states,
speech activity detector control the transmission of the
terminal between SIL state and contending state, CON.
Detecting talkspurt moves the terminal toward CON state
and terminal starts contending on available time slots as
packets be ready for transmission in its transmitting
buffer. Detecting silencespurt by speech activity detector
moves the terminal into the silence state.
The terminal in CON state gets a reservation and start
transmit if the following three conditions are hold simul-
taneously: 1)The contended slot is unreserved; the prob-
ability that the slot is not reserved is (1 – r), where r is
the probability that the slot is reserved by some other
terminal. 2) The terminal has a permission to transmit,
transmission permission is p. 3) No other terminals have
a permission to transmit
 , where c repre-
sent the number of contended terminals.
Permission for transmission is generated according to
a binary random event generator independently at each
terminal. The permission probability, p, is a system de-
sign parameter. A feedback message from the central
base station is broadcasted to all terminals with the end
of each time slot to indicate whether this slot is reserved
or still available for reservation in the next frame. By the
starting of each slot, terminal that has packets and per-
mission start transmitting their first available packet.
Base station detects collision if it gets a disturbed signal
due to the transmission of two packets or more within the
Figure 2. Transition State Diagram for Voice Terminals, γf,
represents the probability of transition from state RES0 to
state SIL, this probability is equal the probability that the
talkspurt ended in the most recent frame.
Copyright © 2011 SciRes. IJCNS
same time slot. Terminals can detect whether they en-
counter a collision or no by detecting the feedback mes-
sage broadcasted by the base station. If some terminal
encounters a collision it keeps the transmitted packet in
its buffer and tries to contend on other available time slot.
Terminal that succeeded in reserving the contended time
slot keeps on this reservation for transmitting all subse-
quent speech packets it generates during the talking spurt.
In our new contention scheme, terminal with ready
packets to transmit select their slots by running a uniform
random generator on available free slots. Collision could
occur if more than one terminal contends randomly for
the same time slot. Terminal starts transmitting its speech
packets as the contended slot is coming. Collision detect-
ed by the terminal after receiving the feedback broad-
casted message from the base station. Collided terminals
start picking new available slots for transmission. This
scheme of contention could increase packet queuing de-
lay by deferring packet transmission until the availability
of the contended slot, but it reduces the number of colli-
sions compared with the permission contention scheme.
Reducing number of collisions will reduce packet queu-
ing delay for heavy traffic systems, because in these sys-
tems the probability of contending on the same slot by
more than one terminal is relatively high. This high pro-
bability induces successive collisions for incoming slots.
Reducing this probability requires reducing permission
probability, but reducing permission probability beyond
a certain value results in wasting many available time
Speech packets are sensitive to the delay; therefore,
speech packets cannot tolerate high queuing delay, a
maximum threshold delay, Dmax is considered for packets.
Packets with queuing delay exceeding this threshold de-
lay dropped from terminal transmitter buffer. Packets
transmission depends on slot reservation; therefore pack-
ets could encounter excessive queuing delay. Terminals
transmitter buffers start dropping the oldest packets if
there is no available space for recent generated packets.
Buffer size, B, depends on both the maximum threshold
delay and the frame duration.
4. Silulation Results
A simulation program is written to evaluate the effici-
ency of PRMA under the new contention scheme com-
pared against the permission contention scheme. Table 1
provides a summary of system parameters that are used
in our simulation program. These parameter values are
adopted in [2] study for evaluating PRMA efficiency
under permission contention scheme.
The number of simultaneous conversations has been
used as a running parameter in all experiments; the maxi-
mum number of simultaneous conversations that can take
place with pdrop 0.01 is adopted as a measure of system
capacity, M. pdrop is defined as the number of dropped
packets to total number of generated packets. The Effect
of different system parameters on PRMA with random
contention scheme are evaluated and compared with
PRMA under permission contention scheme.
With our new proposed contention scheme, terminals
always have permission for transmission. While in per-
mission contention scheme terminals are allowed to con-
tend on the available slots only if they have permission
to do that, this permission is generated in each terminal
independently from other terminals. Figure 3 shows that
with permission scheme the maximum number of simul-
taneous conversations is changed with changing permis-
sion probability.
Table 1. PRMA variables.
Parameter Notation Units Values
Talk Spurt Mean Duration t1 sec 1.0
Silent Spurt Mean Duration t2 sec 1.35
Channel Rate Rc b/s 720000
Source Rate Rs b/s 32000
Frame Duration T sec 0.016
Overhead H bit 64
Speech Activity Detector Slow
Maximum Delay Dmax sec 0.032
Dropping Probability pdrop 0.01
Permission Probability p 0.3
Conversations M Variable
Figure 3. Maximum number of simultaneous conversation
with pdrop 0.01 as a function of permission probability, p.
Copyright © 2011 SciRes. IJCNS
For both low and high permission probability, most of
available slots are wasted without reservation. With low
permission probability slots are wasted because terminals
have no permission for contention while with high per-
mission probability, slots are wasted due to collision
between contended stations, the maximum number of
simultaneous conversations reached is M = 37 when p =
0.3. Using our new approach give us the same maximum
number of simultaneous conversations. Our approach is
better than the permission approach in terms of the aver-
age packet delay under heavy load traffic and number of
collisions in light load traffic. Figure 4 and Figure 5
show these facts. Packet delay is considered as the
elapsed time between packet generation and the start of
packet transmission, taking in account the limitation on
this time it should not exceed the maximum packet delay,
Dmax. Figure 4 shows that for maximum number of si-
multaneous conversations the average packets delay is
less than that in permission contention scheme. The high
values for the average packet delay under light load traf-
fic are due to the nature of random contention scheme. In
random contention scheme terminals could contend on
any available slot from the pool of remaining free slots in
the current frame, this slot could be just the next one or
could be the last one in the frame, in average terminals
need to wait for 4 msec (frame duration, T = 16 ms). The
low average packet delay at high traffic load is good be-
cause the objective from exploring PRMA system is to
achieve high capacity with minimum packet delay.
Figure 5 shows the number of collisions as a func-
tion of the number of simultaneous conversations. The
number of collisions is increased with increasing the tra-
ffic load due to the possibility of contending on the same
slot by many terminals at the same time. Permission con-
tention scheme solved this problem by reducing the per-
mission probability. Our new proposed scheme reduces
the number of collisions by distributing slots contention
Figure 4. Packet average delay pdrop 0.01 as a function of
number of simultaneous conversations.
Figure 5. Number of collisions with pdrop 0.01 as a function
of simultaneous conversations.
among all available slots rather than contending on the
same slot. Still our scheme faces collisions due to the
possibility of selecting randomly the same available slot
by different terminals. But the occurrence number of
these collisions in low traffic case is less than those in
permission contention scheme. In case of high traffic
load the number of collisions starts increasing rapidly
because all terminals that have data to transmit try to
contend randomly on very little number of slots. A solu-
tion for this problem could be by adopting a dynamic
contention scheme that reduces the number of collisions
in case of low number of available slots.
The frame duration, T, is determined by the amount of
speech information, RsT bits, in each packet. There is an
optimum value for frame duration because reducing fra-
me duration means that overhead bits will be the domi-
nant, this overhead will consume excessive channel re-
sources. Also, reducing frame duration reduces number
of slots N per frame (1). This reduction in the number of
slots per frame increases number of collisions because all
terminals try to contend on small number of slots. In-
creasing frame duration also leads to some other prob-
lems, increasing frame duration decreases the buffer size
which in turn leads to excessive packet drooping (4).
Buffer size, B, actually representing the number of time
slots, D, which the packet could live in the terminal
transmitting buffer. Each speech terminal contains a first-
in first-out buffer to store packets that waiting successful
transmission. Each packet in the buffer has a counter that
records its age in terms of number of slots; if the packet’s
age exceeds D slots then this packet must be dropped
from the buffer. Therefore, increasing packet duration
will decrease packet opportunity for transmission.
Copyright © 2011 SciRes. IJCNS
Figure 6 shows the effect of frame duration on maxi-
mum number of simultaneous conversations with pdrop
0.01. PRMA with random contention scheme is very
sensitive to frame duration. Increasing frame duration
decreases number of simultaneous conversations. With
large frame duration, number of time slots, D, that
needed for a packet to stay in buffer is decreased and be
less than the average packet delay, 4 msec, this reduction
in D causes the reduction in maximum number of simul-
taneous conversations. PRMA with permission conten-
tion face the same scenario but its sensitivity to frame
duration is less.
Normalized capacity, η, is used as a measure for com-
paring different PRMA configurations. Number of simul-
taneous channels in TDMA system is adopted as a nor-
malized measure [1]. In perfect TDMA system with no
overhead, the number of voice channels is Rc/Rs which is
equivalent to 22.5 (using the proposed parameter values).
Normalized capacity,
, is defined as:
Figure 7 shows the normalized capacity under diffe-
rent number of channels1, simulation run for different
values of transmission rates, Rc. Increasing the transmis-
sion rate increases the normalized capacity. But this in-
crease in normalized capacity is bounded to 1.64; PRMA
cannot reach its upper bound2, 2.09, due to the effects of
wasted time slots. Both, random and permission conten-
tion schemes, show the same results. But at low traffic
rates random contention scheme gives better capacity
than permission scheme. This improvement in capacity
at low traffic load is due to utilizing most of available
slots, number of collisions in random contention at low
traffic rates is smaller than that in permission scheme and
therefore, number of wasted slots is reduced.
In PRMA, packets that stay in terminals transmitting
buffers for a time greater than Dmax will be dropped be-
cause voice applications cannot tolerate transmission
delays. Figure 8 displays the relationship between delay
limit, Dmax, and the number of simultaneous con- versa-
tion. For low packet delay limits, most of generated
packets are dropped from the buffer. Both contention
schemes are sensitive to packet delay limit. In random
contention scheme most of the generated packets are
dropped from the terminal buffers with low Dmax, when
Dmax < 0.004 s all generated packets are dropped from the
terminal transmitting buffers because the average packet
delay is 0.004 s as shown in Figure 4, therefore, for good
Figure 6. Number of simultaneous conversation with pdrop
0.01 as a function of frame duration.
Figure 7. Number of simultaneous conversations per chan-
nel as a function of the number of channels.
Figure 8. Conversation per channel as a function of speech
packet delay limit.
system capacity, Dmax must be adjusted carefully to avoid
this excessive packet drooping. Permission contention
scheme also cannot deal with low packet delay limits, as
shown in the Figure 8 normalized system capacity under
permission contention scheme at low packet delay limit
1Number of channels is equivalent to TDMA capacity, Rc is changed
from 64 kbps to 960 kbps, and therefore, number of channels is ranged
between from 2 to 30.
2Upper bound could be reached in the ideal conditions when there are
no wasted slots. upper bound is
NRRt tt.
Copyright © 2011 SciRes. IJCNS
is less than 1.
5. Conclusions
In this work we propose a new contention scheme, ran-
dom contention scheme, for PRMA system and we ex-
amined the influence of large number of system variables
on PRMA performance. Our results are compared with
PRMA system under permission scheme.
Simulation results show that with our new scheme we
could reach the maximum number of simultaneous con-
versation per channel as in PRMA under permission con-
tention scheme. Our approach gives better results in some
cases. The main features of our approach are: reducing
average packet delay at high traffic rates and reducing
number of wasted slots for low traffic rates. Simulation
results show that for high traffic cases, number of colli-
sions increased rapidly, this rapid increase in number of
collisions is due to small number of available time slots
compared to the number of active terminals. These re-
sults show that the new contention scheme could be im-
proved by introducing some dynamic contention policy
for high traffic rate cases. Dynamic contention policy
will increase system capacity by reducing the number of
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