Two Media Access Control Protocols (MAC) for
Wireless Networks
The Packet Reservation Multiple Access (PRMA)
protocol
• PRMA is essentially a slotted ALOHA protocol.
• Designed under the assumption that only speech users are
present.
• If a speech user has a packet to transmit and there is an idle
slot he transmits his packet with probability Ps
• If he succeeds(no collision), he reserves this slot for the
corresponding frames until he enters silence(vertical
reservation).
• If not, he repeats until successful.
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The Integrated PRMA (IPRMA) protocol
• IPRMA is a variant of PRMA trying to integrate speech
and data users.
• In IPRMA, data users can reserve multiple slots in a
frame(horizontal reservation) but not more than k-M slots
(k = free slots)
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Graphical Representation of IPRMA
Data user A reserves 2 horizontal slots
Frame i
Rs A
Rs Rs Rd Rd
Data user B reserves 3 horizontal slots
Frame i+1
Rs Rd Rs Rs Rd Rd
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Pros and Cons of IPRMA
• Under low loading conditions significant improvements
because a data user can transmit many packets, thus
making more efficient use of the medium.
• But the speech users are hampered by this scheme.
• Setting M  0 is a mechanism to protect the speech users.
• On the other hand, when k = M and there are no speech
users but data users exist, we are wasting our resources.
• Also M should be dependent on the speech traffic present,
but a fixed value(M = 2) is assumed in all experiments.
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Determining Ps and Pd
• Authors determine optimal Ps by plotting max number of
speech users supported under the condition Pdrop<0.01 for
various values of Ps. They found Ps=0.35 for 36 speech
users.
• Pd is determined by making a plot of max data rate per user
assuming 20 data users in the system and so they find
Pd=0.08.
• But the optimal values of Ps and Pd will be different when
mixed load is applied in the channel.
• But now we have to make more calculations to find them.
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Throughput of IPRMA and PRMA
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Average speech packet delay of IPRMA and PRMA
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Average data packet delay of IPRMA and PRMA
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The PRMA with Dynamic Allocation of available
slots (PRMA/DA) protocol
• This variant of PRMA is more suitable when we have
speech, video and data users on the same channel
(multimedia wireless networks).
• Each frame is divided into available and reservation
slots(unlike IPRMA).
• Available slots are used to make declarations about the
traffic characteristics of each source.
• Reservation slots are used to transmit the actual
information.
• Available slots enable Call Admission Control policy.
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Frame Format of PRMA/DA protocol
variable
1 2
Available
slots
Na
1 2
variable
Rc
CBR
reservation
slots
1 2
variable
Rv
VBR
reservation
slots
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1 2
Rd
ABR
reservation
slots
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More on available and reservation slots
• In PRMA/FA(Fixed Allocation) each MS takes the
reservation slots it needs and the rest are available slots.
• In PRMA/DA we first allocate available slots and then
distribute reservation slots in each MS.
• Number of available slots depends on the intensity of
demand to access the network.
• Number of reservation slots assigned to each MS is
dependent on its statistical properties of traffic.
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Allocating available slots
• Number of available slots should be equal to the number of
contending stations, unless it affects the QoS of reserving
stations.
• In practice, we cannot have perfect knowledge of number
of contending stations so an algorithm to estimate them is
needed.
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Dynamic allocation of available slots
min(max( N ( k )  N ( k ) ,2 N ( k ) ),
a
s
c

 N f  N r ,CBR   jSVBR Rm, j )
N a( k 1)  
for N a( k )  N s( k )



1
for N a( k )  N s(k )
• Second term is there so as not to degrade the QoS of
reserving stations.
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Dynamic allocation of reservation slots
• Once the number of available slots has been determined we
allocate the reservation slots dynamically for each MS.
• If ABR users don’t need all their slots they grant them to
VBR users.
N r , ABR  min( N r  N r ,CBR
 min( DVBR , iS
VBR
Rm,i ), D ABR )
N r ,VBR  min( DVBR , N r  N r ,CBR  N r , ABR )
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Throughput of PRMA/DA and PRMA/FA
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CBR access delay of PRMA/DA and PRMA/FA
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VBR cell transmission delay of PRMA/DA and
PRMA/FA
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