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Design and performance evaluation of an RRA scheme for
voice-data channel access in outdoor microcellular
environments
Allan C. Cleary and Michael Paterakis
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In PCS networks, the multiple access problem is characterized by spatially dispersed mobile source terminals sharing a radio channel connected to a fixed base station.
Design and evaluation of a reservation random access (RRA) scheme that multiplexes voice traffic at the talkspurt level to efficiently integrate voice and data traffic in outdoor microcellular environments.
TDMA + Random Access Algorithm The time frame is divided into two request intervals (voice
and data) and an information interval. Voice and data terminals competition for channel access is eliminated.
Three random access algorithms were exploit for the transmission of voice request packets and one for the transmission of data request packets. Simulations were used to investigate the steady state voice packet dropping distribution per talkspurt, and to illustrate preliminary voice-data Integration considerations.
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A well designed multiple A well designed multiple access scheme will provide :access scheme will provide :
Maximization of system capacityMaximization of system capacitySatisfy QoS requirements (voice packet dropping probability and access Satisfy QoS requirements (voice packet dropping probability and access delay) delay)
Integration different classes of traffic (voice-data)Integration different classes of traffic (voice-data)
VsVsBandwidth limitationsBandwidth limitationsContradictory requirements of voice and data trafficContradictory requirements of voice and data traffic
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Usual proposals for Usual proposals for Multiple access schemes proposed for PCS usually involve FDMA,TDMA,CDMA or
combinations thereof.
RRA operates over a time-slotted channel RRA operates over a time-slotted channel combining a random access algorithm (e.g. combining a random access algorithm (e.g. Slotted Aloha) with TDMA.Slotted Aloha) with TDMA.
Slots Slots Information or Request Information or Request Information Slots Information Slots Reserved or Available Reserved or Available
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Basic idea of RRABasic idea of RRA Contending terminals (those with packets and
without a reservation) use a random access algorithm to compete for channel resources
After successfully transmitting a request, the terminal receives a reservation for an information slot (or slots)
A terminal with a reservation, transmits freely during its reserved slot(s) and the reservation is held for as long as it continues to transmit packets in successive frames
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Different versions RRA schemesDifferent versions RRA schemes
RRARRA Every slot is information slot.Every slot is information slot. TThe contending terminals attempt to
transmit their voice (or data) packet into the available information slots.
A voice terminal that successfully transmits its packet during an available slot receives a reservation for the corresponding slot in successive frames, until it exits talkspurt.
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PRMAPRMA
The contending voice and data terminals both use a generalized slotted Aloha algorithm to access the channel.
To ensure that voice terminals have greater access to the available slots, the retransmission probabilities are weighted to favor voice terminals.
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IPRMAIPRMA
PRMA with a priority mechanism to ensure that voice packets have greater access to the available slots
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A promising alternative to PRMA…
Combination of a random access algorithm that identifies the end of the voice contention with a policy to resolve the voice traffic first. Thus, every terminal within the microcell can differentiate between available voice and available data slots and the voice and data random access transmissions can be separated.
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PRMA++PRMA++
Frame is divided into Request and Information Frame is divided into Request and Information slots of equal sizeslots of equal size
Contending voice terminals follow a generalized slotted Aloha algorithm to transmit their reservation request packets into the request slots. On successful receipt of a request packet, the base station either provides a reservation for an information slot (if available) or it queues the request. In the latter case, the terminal monitors the base-to-mobile channel until it is granted a reservation.
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Different approach of RRADifferent approach of RRA
A portion of the frame is partitioned into mini-slots. The contending terminals use slotted Aloha to transmit reservation request packets into the mini-slots. The base station provides acknowledgments and allocates channel resources.
Voice-data integration is achieved by partitioning the information slots into two intervals, one designated for voice and the other for data traffic.
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Problems Problems 1. An entire time slot is wasted when a collision is caused
by terminals simultaneously contending for channel access. The amount of degradation depends on the packet size (time slot duration) and it increases with the traffic load
2. The other approach (RRA) wastes a part of the frame for control signaling
The base station controls the allocation of the channel resources. This centralized control can be exploited to implement access control policies, dynamic channel assignment and/or the integration of different priority traffic classes.
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2. RRA Scheme2. RRA SchemeScenarioScenario : :
Microcell with mobile source terminals generating Microcell with mobile source terminals generating traffictraffic
Base station allocates channel resources, delivers feedback information and serves as an interface to the mobile switching center
Mobile switching center provides access to the fixed network infrastructure
Focus on the mobile-to-base (many-to-one) channel
Each voice terminal is equipped with a voice activity detector (VAD) that generates packets during periods of vocal activity (talkspurt), thus multiplexing occurs at the talkspurt level)
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RRA ProtocolRRA Protocol
The frame duration is selected such that a voice terminal in talkspurt generates exactly one packet per frame
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Both of the request intervals are subdivided into minislots and each mini-slot accommodates exactly one, fixed length, request packet
For both voice and data traffic, the request must include a source identifier. For data traffic, the request might also include message length and quality of service parameters such as priority and required slots/frame
Both of the request intervals contain an equal number of mini-slots and the data terminals are given at most one information slot per frame
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Voice terminalsVoice terminals Voice (data) terminals with packets, and no reservation, contend
for channel resources using a random access protocol to transmit their request packets only during the voice (data) request interval
The base station broadcasts a short binary feedback packet (collision (C) versus non-collision (NC)) at the end of each mini-slot
Assumption : As the feedback packet is small (several bits) and the transmission delay within a microcell is negligible, the feedback information is immediately available to
the terminals ( before the next mini-slot). If there is a successful transmition of a request packet, the
terminal waits until the end of the frame to learn of its reservation slot. If unsuccessful within the request interval, the terminal attempts again in the request interval of the next frame
A terminal with a reservation transmits freely during its reserved slot
Voice packets that age beyond Dmax are dropped
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Channel Resource Allocation StrategyChannel Resource Allocation StrategyDynamic table of the active terminals containing : Dynamic table of the active terminals containing :
1. Terminal identifier2. Virtual circuit identifier3. Channel Resources Allocated4. Quality of service parameters
Upon successful receipt of a request packet, the base station provides an acknowledgment and queues the request.
The base station allocates channel resources at the end of the frame, if available.– If the resources needed to satisfy the request are unavailable, the
request remains queued. Voice terminals with queued requests and data terminals
with packets must continuously monitor the base-to-mobile channel.
Upon call completion, or when an active terminal exits the microcell (handover), the base station will delete the table entry after some prescribed period of time.
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Priorities…Priorities… Base station services every outstanding voice
request before servicing any data requests.– Within each priority class, the queuing discipline is
assumed to be FIFO.
Whenever new voice requests are received and every slot within the frame is reserved, the base station attempts to service the voice requests by canceling the appropriate number of reservations belonging to data terminals (if any).– BS notifies affected data terminal and places a request
at the front of the data request queue.
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2.2. Random access algorithms for voice terminals
1.1. IdealIdeal Every request packet present at the start of the
reservation request interval is correctly received by the base station within the duration of the request interval.
Provides an upper bound for the voice system capacity and a lower bound for the voice access delay (the time between the start of a talkspurt and the end of the first voice packet transmission into a reserved slot)
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Slotted AlohaSlotted Aloha
Each contending terminal transmits its Each contending terminal transmits its request with probability p (p=1/3 for the request with probability p (p=1/3 for the simulations)simulations)
For p=0.5 bistability occurred for more than For p=0.5 bistability occurred for more than 75 terminals in the system. 75 terminals in the system.
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2-Cell Stack2-Cell Stack1. At the start of every request interval the contending
terminals initialize their counter, r, to 0 or 1 with probability 1/2.
2. Contending terminals with r = 0 transmit into the first request slot. With x being the feedback for that transmission, the transitions in time of r are as follows:
a. if x = non-collision:if r = 0, the request packet was transmitted
successfully.if r = 1, then r = 0.
b. if x = collision:if r = 0, then reinitialize r to 0 or 1 each withprobability 1/2.if r = 1, then r = 1
3. Repeat step 2, until either two consecutive feedbacks indicating non-collision occur or the request interval ends.
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2-Cell Stack2-Cell Stack
The operation of this protocol can be depicted by a two cell stack, where in a given request mini-slot
Bottom cell contains the transmitting terminals (those with r = 0)
Top cell contains the withholding terminals (those with r = 1). – Although not exploited during voice access, an attractive
feature of this algorithm is that two consecutive “non-collisions” indicate that the stack is empty.
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2.2. Random access algorithms for data terminals
2 Cell Stack 2 Cell Stack Simplicity. Stability. High throughput. (λMAX = 0.429 packets per slot)
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Blocked access mechanism First time transmission rule for newly generated data
messages
Collission Resolution Period-CRP The interval of time that begins with an initial collision (if
any) and ends with the successful transmission of all data request packets involved in that collision (or, if no collision occurred, ends with that mini-slot)
In the first mini-slot following a CRP, all of the terminals whose message arrived within a prescribed allocation interval, of maximum length Δ, transmit with probability one.– For 2 Cell Stack Δ = 2.33 slots
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3. Voice traffic analysis
3.1 Assumptions3.1 Assumptions
Steady state probabilitiesSteady state probabilities
2-state discrete time Markov chain
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N = N = The number of active voice terminals = steadyChanges in the number of calls is usually on the order of tens of
seconds, while the frame duration is on the order of tens of milliseconds.
All of the voice transitions (e.g., talk to silence) occur at the frame boundaries.
The voice delay limit, Dmax, is equal to the duration of two frames.
Thus a contending voice terminal that fails to successfully transmit a request packet during the voice request interval will drop one voice packet.
The channel is error-free and without capture. Errors within the system only occur when two or more packets arrive
simultaneously (collide) at the base station during a request slot.
Reserved slots are deallocated immediately. This implies that a terminal holding a reservation signals the base station upon the completion of a talkspurt.
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3.2. System state transitions
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Steady State Packet dropping probability Ratio of average number of voice packets dropped per frame to the
average number of voice packets generated per frame
Voice Access Delay Time between the start of a talkspurt and the end of the first voice
packet transmission in its reserved slot. Here the mean access delay, D, can be expressed as
D = Dc + Dq + Dr where
Dc is the mean random access delay (DC MIN = 1 Slot)
Dq is the mean queuing delay
Dr is the mean time between the start of the frame in which the reservation is granted and the end of the transmission in its reserved slot
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Performance Evaluation1)Distribution of the voice packets dropped
per talkspurt2)Explore preliminary voice-data
integration issues
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Performance Evaluation Packet size 53 Bytes (compatibility with Packet size 53 Bytes (compatibility with
ATM networks)ATM networks) Speech codec AD-PCMSpeech codec AD-PCM
Ratio Talkspurt Ratio Talkspurt / / Silence = 44% Silence = 44%
( e.g. ( e.g. 1.0/1.35 s or 1.41/1.74 s ) ) Voice delay limit = 24 ms = 2 framesVoice delay limit = 24 ms = 2 frames Mini slot duration = 70 bitsMini slot duration = 70 bits
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Simulation IssuesSimulation Issues All simulations consist of 10 independent runs of
305,000 frames per simulation. The first 5000 frames serve as the warm up period
(reduce start up effects).
During each run: Constant number of terminals within the system. Terminals are initially silent. The steady state voice packet dropping probability
is obtained from the ratio of the total number of voice packets dropped to the total number of voice packets generated over the simulation run.
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ResultsResults Multiplexing gain = ratio of the voice capacity to the
number of slots per frame Voice Capacity = N (PDROP 1%)
Analytical Results
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ResultsResults Voice packet probability (%) Voice packet probability (%) // Active Voice terminalsActive Voice terminals
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Fraction of voice packets dropped from the contender and queued states
Slotted Aloha.
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Fraction of voice packets dropped from the contender and queued states
2 Cell Stack
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CommentsComments Contender and queued lines intersect at about N =
88 and N = 84 for Aloha and the two-cell access algorithms, respectively. This indicates that packet dropping due to contention is more significant for Aloha than for the 2-cell algorithm.
Packet dropping depends on the random access algorithm at lower loads (N <82 or gain < 1:64).
At high loads where dropping from the queuing delay is predominant, although the choice of random access algorithm does not improve the voice capacity (or significantly improve the throughput) it does improve the Pdrop and Mean access delay.
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Operation at Voice Capacity (N=97)Operation at Voice Capacity (N=97)SLIDE 33SLIDE 33
AlohaAloha 2 Cell2 Cell IdealIdeal
PPDROPDROP0.970.97 0.9190.919 0.8630.863
If N=98 PDROP > 1
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Simulation results for the steady state packet dropping distribution per talkspurt for each access protocol operating at voice capacity.
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Steady state mean voice access delay
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Aloha Vs 2 Cell StackAloha Vs 2 Cell Stack
Aloha is slightly worst at low values of N Aloha is slightly worst at low values of N because when there is 1 contending because when there is 1 contending terminal it successfully transmits its request terminal it successfully transmits its request 90 percent of the time, due to the 90 percent of the time, due to the probabilistic first time transmission rule.probabilistic first time transmission rule.– However when the terminal follows 2 Cell However when the terminal follows 2 Cell
Stack it always succeds. Stack it always succeds.
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Analytical results for voice packet throughput vs. N.
Linear behaviour almost while N<98
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Voice-data integration A data terminal that successfully transmits a
request packet receives a reservation, but, since it transmits low priority traffic, its reservation may be preempted to service a voice terminal.
1. Wait Delay or Access Delay The time between the message arrival and the end of the first data
packet transmission into a reserved slot
2 . Message Delay The time between the message arrival and the end of the last data
packet transmission into a reserved slot
3 . ThroughputThe proportion of time slots that successfully carry data information packets
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Data AssumptionsData Assumptions
Data messages are generated by a large unknown number of data terminals (theoretically infinite). The aggregate message arrivals are Poisson distributed with mean messages per frame.
The messages vary in length according to a geometric distribution with parameter q and mean B = 1/q.
*Simulation parameters : q = 1/8, B=8 Avg data msg size = 3400bits
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Data wait delay and the Datamessage delay Vs the Data message arrival rate ,λ, for
the system with N = 0
λMAX = 0.429*6 = 2.575 data messages per frame
Max data packet throughput = 2.575*8 = 20.6 packets per frame
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Steady state mean data delays N = 86.
Steady state voice Pdrop = 0.07, mean access delay = 18 ms and throughput = 38 packets/frame
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Steady state mean data delays, N =90.
steady state voice Pdrop = 0.2, mean access delay = 20 ms and throughput = 40 packets/frame
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ConclusionConclusion
The proposed RRA scheme is a promising scheme The proposed RRA scheme is a promising scheme for providing voice-data integration in outdoor for providing voice-data integration in outdoor microcellular environments.microcellular environments.
Our Suggestions for future workOur Suggestions for future work
1)3 kinds of traffic with different priorities (+Low 1)3 kinds of traffic with different priorities (+Low quality live video)quality live video)
2)Simulations to investigate data delays when 2)Simulations to investigate data delays when
N=80-90 (PN=80-90 (PDROP DROP < 1%) ,using Aloha and 2 Cell stack.< 1%) ,using Aloha and 2 Cell stack.