d-rma: a dynamic reservation multiple access protocol for third generation cellular systems

D-RMA: A Dynamic Reservation Multiple Access D-RMA: A Dynamic Reservation Multiple Access ProtocolProtocol

for Third Generation Cellular Systemsfor Third Generation Cellular Systems

Published in IEEE Transactions On Vehicular Published in IEEE Transactions On Vehicular Technology,Vol. 49, NO.5, September 2000 by Antonio Technology,Vol. 49, NO.5, September 2000 by Antonio

Iera and Salvatore Marano.Iera and Salvatore Marano.

Presented by Antonis Stefanogiannis on 16Presented by Antonis Stefanogiannis on 16thth November November 2004, as partial fulfilment of the postgraduate course 2004, as partial fulfilment of the postgraduate course ‘Special Topics in Communication Networks’ with the ‘Special Topics in Communication Networks’ with the

School of E.C.E., T.U.C.School of E.C.E., T.U.C.

16th Nov. 200416th Nov. 2004 A.I.StefanogiannisA.I.Stefanogiannis 22

INTRODUCTION INTRODUCTION (Towards 3G Wireless Broadband Personal Communication (Towards 3G Wireless Broadband Personal Communication

Systems)Systems)

Paper was published on September 2000……Paper was published on September 2000…… Personal Communication Systems (PCS),Personal Communication Systems (PCS), based on wireless technologies, have based on wireless technologies, have

evolved towards the support of a wider range of applications including evolved towards the support of a wider range of applications including voice, video, voice, video, datadata and and multimediamultimedia..

Ideal scenarioIdeal scenario : several Mobile Audio Visual Terminals (MAVTs) by which users, via a : several Mobile Audio Visual Terminals (MAVTs) by which users, via a Base Station (BT) connecting wired and wireless networks, have access to multimedia Base Station (BT) connecting wired and wireless networks, have access to multimedia services scattered all over a high-speed communication backbone.services scattered all over a high-speed communication backbone.

Attention focused on the Attention focused on the integrationintegration between enhanced wireless systems and between enhanced wireless systems and broadband networks, such as B-ISDN using ATM protocol. Enable wireless PCSs to broadband networks, such as B-ISDN using ATM protocol. Enable wireless PCSs to embrace a embrace a wider range of communication capabilitieswider range of communication capabilities, besides those offered , besides those offered (substantially voice services when this paper was published). (substantially voice services when this paper was published).

This integration process requires the design of This integration process requires the design of Multiple Access protocolsMultiple Access protocols showing showing high high flexibilityflexibility and and efficiencyefficiency to support future integrated services. to support future integrated services.


MULTIPLE ACCESS PROTOCOLSMULTIPLE ACCESS PROTOCOLS (Overview) (Overview)

Multiple Access Protocols proposed for satellite systems or wireless LANs Multiple Access Protocols proposed for satellite systems or wireless LANs (start point) reconsidered for cellular mobile radio systems.(start point) reconsidered for cellular mobile radio systems.

An interesting output of this activity was the An interesting output of this activity was the Packet Reservation Multiple Packet Reservation Multiple Access (PRMA)Access (PRMA) protocol. protocol.

In PRMA the frame rate coincides with the arrival rate of voice packets so In PRMA the frame rate coincides with the arrival rate of voice packets so that that a voice terminal requires exactly one slot per framea voice terminal requires exactly one slot per frame. Within the frame . Within the frame the terminal detects an the terminal detects an availableavailable or or reservedreserved slot according to the slot according to the feedback information stream broadcasted on the down-link from the BT.feedback information stream broadcasted on the down-link from the BT.

An active terminal An active terminal contendscontends in order to access the channel on the in order to access the channel on the availableavailable slots. If the attempt is successful, the transmission begins and the terminal slots. If the attempt is successful, the transmission begins and the terminal holds a reservation for a slot in subsequent frames until the end of its holds a reservation for a slot in subsequent frames until the end of its talkspurt.talkspurt.

Every up-link slot can be used for reservationEvery up-link slot can be used for reservation, thus an , thus an acknowledgement at acknowledgement at the end of each slotthe end of each slot is required on the down-link. is required on the down-link.

Main inconveniences : An increase in the traffic load causes a decrease in Main inconveniences : An increase in the traffic load causes a decrease in the probability of finding free slots in the frame and an uncontrolled the probability of finding free slots in the frame and an uncontrolled increase in the access delay. Large number of acknowledgement slots are increase in the access delay. Large number of acknowledgement slots are required on the down-link.required on the down-link.


MULTIPLE ACCESS PROTOCOLSMULTIPLE ACCESS PROTOCOLS (Overview, cont.) (Overview, cont.)

An An adaptive protocoladaptive protocol, named , named Multiple Access (MA)Multiple Access (MA) protocol attempts to solve the protocol attempts to solve the previous problem. This protocol allows a reservation only on the so-called previous problem. This protocol allows a reservation only on the so-called R-slotsR-slots, and , and the transport of information on the the transport of information on the I-slotsI-slots..

In this protocol, a In this protocol, a minimum number of R-slots is fixedminimum number of R-slots is fixed to guarantee good performance to guarantee good performance under high loading conditions. When the traffic is low every other under high loading conditions. When the traffic is low every other free I-slot can be used free I-slot can be used for reservationfor reservation..

Main inconveniences : partial separation between control and information channels. Main inconveniences : partial separation between control and information channels. While, reservation packets transmitted over the R-slots are strongly While, reservation packets transmitted over the R-slots are strongly protectedprotected against against interference, this interference, this protection is absentprotection is absent on I-slots and the reservation packets can be on I-slots and the reservation packets can be corrupted by channel impairments.corrupted by channel impairments.

A new version of PRMA, named A new version of PRMA, named PRMA++PRMA++, has been proposed and analyzed., has been proposed and analyzed. Separation of control and information slots permits guarantees of different degrees of Separation of control and information slots permits guarantees of different degrees of

quality to quality to controlcontrol and and informationinformation channels. channels. The number of R-slots as well as their positions within the frame are The number of R-slots as well as their positions within the frame are fixedfixed. . Unlike PRMA, a lower number of Unlike PRMA, a lower number of acknowledgement slotsacknowledgement slots are required on the down-link. are required on the down-link. Main inconveniences : fixed number of R-slots has the effect of causing Mobile Terminals Main inconveniences : fixed number of R-slots has the effect of causing Mobile Terminals

(MTs) to experience access (MTs) to experience access delay even at low loaddelay even at low load. In the presence of multiple slot . In the presence of multiple slot assignment (i.e. multimedia traffic), the experienced assignment (i.e. multimedia traffic), the experienced access delayaccess delay may become a critical may become a critical factor.factor.


THE D-RMA PROTOCOLTHE D-RMA PROTOCOL( A protocol for 3G Cellular Mobile Radio Systems)( A protocol for 3G Cellular Mobile Radio Systems)

A protocol which is A protocol which is explicitly explicitly designed to support multimedia traffic.designed to support multimedia traffic. Based on PRMA++Based on PRMA++ but, it introduces a but, it introduces a flexible dynamic approachflexible dynamic approach in the choice of the percentage in the choice of the percentage

of bandwidth to be used for reservation.of bandwidth to be used for reservation. It It couplescouples the advantage deriving from a complete separation between reservation and the advantage deriving from a complete separation between reservation and

information channels , along with dynamic adaptation of the percentage of reservation bandwidth information channels , along with dynamic adaptation of the percentage of reservation bandwidth within a frame to traffic condition. It demonstrates within a frame to traffic condition. It demonstrates better performancebetter performance when compared to ‘non- when compared to ‘non-dynamic’ protocols in terms of both the offereddynamic’ protocols in terms of both the offered QoS QoS and and number of connectionsnumber of connections which can be which can be activated in a microcell at one time.activated in a microcell at one time.

Belongs to the family of protocols, which have been explicitly conceived with the aim of Belongs to the family of protocols, which have been explicitly conceived with the aim of dynamically adapting the traditional Reservation Multiple Access protocols to multimedia traffic dynamically adapting the traditional Reservation Multiple Access protocols to multimedia traffic needs and sharing of buffer resources and channel bandwidth.needs and sharing of buffer resources and channel bandwidth.

Up-link frame format constituted by a sequence of Up-link frame format constituted by a sequence of information slots (I-slots) information slots (I-slots) and and Reservation slots Reservation slots (R-slots)(R-slots), whose total number is set to N., whose total number is set to N.

The choice of maintaining a complete separation between I-slots and R-slots, and dynamically The choice of maintaining a complete separation between I-slots and R-slots, and dynamically adapting the percentage of reservation bandwidth (number of R-slots) within a frame to traffic adapting the percentage of reservation bandwidth (number of R-slots) within a frame to traffic conditions coupled with a suitable conditions coupled with a suitable bandwidth allocation strategy, isbandwidth allocation strategy, is preferred in order to meet the preferred in order to meet the QoS requirements of multimedia (integrated video/voice) services.QoS requirements of multimedia (integrated video/voice) services.

This means that the number of R-slots, This means that the number of R-slots, NrNr varies dynamically. The condition varies dynamically. The condition NrNrminmin≤ Nr ≤ Nr ≤ Nr≤ Nrmaxmax< N< N holds, where holds, where NrNrmin min ((NrNrmax max )) is the is the optimum optimum number of the R-slots for the number of the R-slots for the highest (lowest) load. highest (lowest) load.

WhyWhy??????


THE D-RMA PROTOCOL THE D-RMA PROTOCOL (Transmission Frame Format)(Transmission Frame Format)

The The transmission frame formattransmission frame format of the proposed MAC protocol is shown in the figure of the proposed MAC protocol is shown in the figure below.below.

The positions for The positions for NrNrmax max R-slots within the frame are R-slots within the frame are fixedfixed. They are scattered in a . They are scattered in a homogeneous mannerhomogeneous manner along the frame, so that a new burst will not have to wait a along the frame, so that a new burst will not have to wait a long time to intercept an available R-slot.long time to intercept an available R-slot.

All non-activated R-slots, are used for transporting information, like I-slots.All non-activated R-slots, are used for transporting information, like I-slots.


THE D-RMA PROTOCOLTHE D-RMA PROTOCOL (Access Mechanism) (Access Mechanism)

When a terminal has packets to transmit (i.e. start of burst/spurt is detected by VAD), When a terminal has packets to transmit (i.e. start of burst/spurt is detected by VAD), with probability with probability pp (permission probability), it sends a reservation packet on the first (permission probability), it sends a reservation packet on the first available R-slot.available R-slot.

If the reservation packet collides or is corrupted by channel impairments, a negative If the reservation packet collides or is corrupted by channel impairments, a negative acknowledgement is transmitted from the BT (Base Station) on the down-link. acknowledgement is transmitted from the BT (Base Station) on the down-link. Unsuccessful terminals retry to transmit their reservation packets on the next free R-Unsuccessful terminals retry to transmit their reservation packets on the next free R-slot, with probability slot, with probability p (generally p (generally pptt ≥ p ≥ pr r ).).

On the contrary, if a success occurs, the BT sends an acknowledgement on the down-On the contrary, if a success occurs, the BT sends an acknowledgement on the down-link and assigns the requested number of I-slots to the calling terminal. If that link and assigns the requested number of I-slots to the calling terminal. If that number is not available, the reservation packet is buffered in a number is not available, the reservation packet is buffered in a reservation queue reservation queue within the BT, and waits for I-slots to become available.within the BT, and waits for I-slots to become available.

The described contention mechanism is similar to other reservation protocols. The described contention mechanism is similar to other reservation protocols.

What really makes the D-RMA protocol really different from other What really makes the D-RMA protocol really different from other reservation protocols???reservation protocols???


THE D-RMA PROTOCOL THE D-RMA PROTOCOL (Access Mechanism, cont.)(Access Mechanism, cont.)

What really makes the D-RMA protocol really different from other reservation What really makes the D-RMA protocol really different from other reservation protocols, is the protocols, is the dynamic management of reservation bandwidth.dynamic management of reservation bandwidth.

In response to In response to traffic load fluctuationstraffic load fluctuations (in terms of number of transmitting terminals (in terms of number of transmitting terminals i.e. originating/terminating calls or hand-off events), the BT is allowed to i.e. originating/terminating calls or hand-off events), the BT is allowed to modify modify the the number of active R-slots within the transmission frame.number of active R-slots within the transmission frame.

It will be demonstrated, later on, that better performances compared to fixed number It will be demonstrated, later on, that better performances compared to fixed number of active R-slots (‘fixed Nr’ policy) can be obtained by properly varying Nr. It will also of active R-slots (‘fixed Nr’ policy) can be obtained by properly varying Nr. It will also be shown that a be shown that a reduction of Nr is preferred when traffic load increasesreduction of Nr is preferred when traffic load increases and an and an increase in Nr is desired when the load decreasesincrease in Nr is desired when the load decreases..

The The optimum numberoptimum number of active R-slots for a given channel load value, has to be of active R-slots for a given channel load value, has to be chosen according to a chosen according to a quality indexquality index, which is an ad-hoc defined QoS metric, and will , which is an ad-hoc defined QoS metric, and will be presented later on.be presented later on.


THE D-RMA PROTOCOL THE D-RMA PROTOCOL (Access Mechanism, cont.)(Access Mechanism, cont.)

A block diagram for D-RMA protocol (model of the access mechanism) is given in the A block diagram for D-RMA protocol (model of the access mechanism) is given in the figure below.figure below.


THE D-RMA PROTOCOLTHE D-RMA PROTOCOL (Access Mechanism, cont.) (Access Mechanism, cont.)

As mentioned before, the proposed protocol is able to dynamically adapt itself, on a As mentioned before, the proposed protocol is able to dynamically adapt itself, on a frame basisframe basis, when traffic load variations occur. Particularly, , when traffic load variations occur. Particularly, at the end of each frameat the end of each frame the BT runs an the BT runs an algorithmalgorithm which examines the channel load (by controlling the which examines the channel load (by controlling the number of terminals within the microcell, which have a connection established) and number of terminals within the microcell, which have a connection established) and determines a suitable Nr value. This algorithm is computed by the determines a suitable Nr value. This algorithm is computed by the Reservation Reservation Bandwidth ControllerBandwidth Controller, as shown in the previous figure., as shown in the previous figure.

Bandwidth requests which cannot be served are buffered and handled according to a Bandwidth requests which cannot be served are buffered and handled according to a specific rule (specific rule (FIFO, Scan & ServeFIFO, Scan & Serve) implemented by the ) implemented by the Access Controller. TheAccess Controller. The latter latter also allocates the requested also allocates the requested Basic Bandwidth UnitsBasic Bandwidth Units ( (BBUsBBUs), I-slots, to each terminal ), I-slots, to each terminal according to suitable strategies, such as according to suitable strategies, such as Complete SharingComplete Sharing ( (CSCS), ), Complete Complete PartitioningPartitioning ( (CPCP), or ), or Mutually Restricted AccessMutually Restricted Access ( (MRAMRA). Therefore, the Access ). Therefore, the Access Controller performs both Controller performs both reservation queue managementreservation queue management and and information bandwidth information bandwidth allocation managementallocation management, following specific policies for each one. Both of these types , following specific policies for each one. Both of these types of management have a strong effect on the performance of D-RMA protocol.of management have a strong effect on the performance of D-RMA protocol.

An overview of FIFO and Scan & Serve reservation queue (buffer) management An overview of FIFO and Scan & Serve reservation queue (buffer) management policies and CS,CP,MRA information bandwidth allocation management strategies will policies and CS,CP,MRA information bandwidth allocation management strategies will be given later on.be given later on.


THE D-RMA PROTOCOLTHE D-RMA PROTOCOL (Access Mechanism, cont.) (Access Mechanism, cont.)

An efficient and simple technique to manage the variation of Nr by traffic load An efficient and simple technique to manage the variation of Nr by traffic load fluctuations, is a ‘fluctuations, is a ‘threshold-based strategy'. threshold-based strategy'. According to this strategy, the BT According to this strategy, the BT (Reservation Bandwidth Controller) decides to change Nr when traffic load exceeds (Reservation Bandwidth Controller) decides to change Nr when traffic load exceeds one of a set of one of a set of fixed threshold values. fixed threshold values. Specifically Specifically loadto-threshold mappingloadto-threshold mapping is is available as a available as a look-up tablelook-up table whose figures are whose figures are a-priori evaluated,a-priori evaluated, according to a according to a quality index which will be defined later on, in this presentation.quality index which will be defined later on, in this presentation.

Is only a threshold value enough???Is only a threshold value enough??? With the above approach, a With the above approach, a persistent need to vary the active R-slot numberpersistent need to vary the active R-slot number, Nr, in , Nr, in

subsequent frames will arise, when the load subsequent frames will arise, when the load rapidly fluctuatesrapidly fluctuates around the threshold around the threshold value. This phenomenon would create an value. This phenomenon would create an undesired overload of signalling trafficundesired overload of signalling traffic on on the downlink. In order to overcome this problem an the downlink. In order to overcome this problem an hysteresis control mechanismhysteresis control mechanism is is introduced. Precisely, each threshold value is provided with an introduced. Precisely, each threshold value is provided with an hysteresis marginhysteresis margin, as , as illustrated in the figure below (optimum number of active R-slots, Nr, vs. traffic load), illustrated in the figure below (optimum number of active R-slots, Nr, vs. traffic load), which permits a change in Nr only when offered load shows a which permits a change in Nr only when offered load shows a convincing (actual) convincing (actual) trendtrend of rising (when crossing L of rising (when crossing Lrr) or falling (when crossing L) or falling (when crossing Lff).).


THE D-RMA PROTOCOL THE D-RMA PROTOCOL (Simulation Study, Parameters Definition)(Simulation Study, Parameters Definition)

A A flexible discrete event simulatorflexible discrete event simulator has been implemented and used to study the D-RMA has been implemented and used to study the D-RMA protocol behaviour.protocol behaviour.

The achieved performance has been analyzed and compared by evaluating the following The achieved performance has been analyzed and compared by evaluating the following parameters :parameters :

DDHvo Hvo : the : the voice holding timevoice holding time - the time spent from the instant when a talkspurt is - the time spent from the instant when a talkspurt is generated until its first packet is transmitted on the channel (upper bound generated until its first packet is transmitted on the channel (upper bound 32ms32ms).). D DHvo Hvo

consists of two components, consists of two components, DDvovo , , DDPROCvoPROCvo and it holds that D and it holds that DHvoHvo= D= Dvovo+ D+ DPROCvo , PROCvo , where the first where the first term is the term is the access delayaccess delay (i.e. the time required to gain a channel reservation from the BT), (i.e. the time required to gain a channel reservation from the BT), and the second term is the and the second term is the time required to process the datatime required to process the data before the transmission over before the transmission over the air interface.the air interface.

DDHvi Hvi :: the the video holding timevideo holding time – the analogous definition of D – the analogous definition of DHvo Hvo for video (upper bound of for video (upper bound of 100ms 100ms for time-critical traffic, like video).for time-critical traffic, like video).

PdropPdropVO VO : the : the average packet dropping probabilityaverage packet dropping probability for a voice terminal, computed as for a voice terminal, computed as dropped packets/arrived packets dropped packets/arrived packets (upper bound (upper bound 0.010.01).).

PdropPdropVI VI : the average packet dropping probability: the average packet dropping probability for a video terminal (upper bound for a video terminal (upper bound 0.0010.001).).


THE D-RMA PROTOCOL THE D-RMA PROTOCOL (Simulation Study, Quality Index Definition)(Simulation Study, Quality Index Definition)

The following The following quality index, Q,quality index, Q, is defined : is defined :

Where Where PdropPdropVOVO, Pdrop, PdropVI VI , D, Dvovo , D , Dvivi , , were defined before, were defined before, αα is the percentage of voice is the percentage of voice terminals over the total number of transmitting terminals and terminals over the total number of transmitting terminals and TTONON is the average is the average talkspurt duration.talkspurt duration.

Actually, quality index, Q, is an Actually, quality index, Q, is an ad-hoc defined QoS metricad-hoc defined QoS metric. The quality index is the sum . The quality index is the sum of two terms, one concerning of two terms, one concerning video trafficvideo traffic and the other one concerning and the other one concerning voice trafficvoice traffic. . Both the access delays and the packet dropping probabilities of the two examined types Both the access delays and the packet dropping probabilities of the two examined types of traffic are considered to define the quality index, Q.of traffic are considered to define the quality index, Q.

Previewing the sum of these two terms, we obtain that Previewing the sum of these two terms, we obtain that QQ is a dimensionless number is a dimensionless number taking values in the interval taking values in the interval [0,1[0,1].The ].The maximum admissible valuemaximum admissible value of of QQ (i.e. (i.e. Q=1Q=1) is ) is obtained in obtained in ideal conditionsideal conditions when both dropping probabilities and access delays are when both dropping probabilities and access delays are zero.zero.

For a given load, the optimum number of R-slots which can be activated, is given by the For a given load, the optimum number of R-slots which can be activated, is given by the following criterion :following criterion :

‘ ‘Optimum Nr is the one which maximizes Q’Optimum Nr is the one which maximizes Q’

viONON

vi

VOONON

vo

DTT

Pdropa

DTT

PdropaQ

11

)1(11


THE D-RMA PROTOCOL THE D-RMA PROTOCOL (Simulation Study, Quality Index Definition, cont.)(Simulation Study, Quality Index Definition, cont.)

Performance index, Q, considers even Performance index, Q, considers even small additional access delays undesirablesmall additional access delays undesirable. . The rationale of this can be easily understood by focusing on the concept of The rationale of this can be easily understood by focusing on the concept of holding holding timetime..

Holding time takes into account the time spent within the system to Holding time takes into account the time spent within the system to cross thecross the whole whole transmitting chaintransmitting chain (contention, queuing, data processing : channel coding, (contention, queuing, data processing : channel coding, interleaving, modulation, etc.). For some services the time required for data interleaving, modulation, etc.). For some services the time required for data processing can be high , this reducing the value of the allowable access delay. For processing can be high , this reducing the value of the allowable access delay. For this reason it becomes very important to minimize this reason it becomes very important to minimize DDvovo (D (Dvivi ) as much as possible to ) as much as possible to leave more time available for performing other transport layer functions. The greater leave more time available for performing other transport layer functions. The greater the margin, the more effective the techniques can be (i.e. deeper interleaving). the margin, the more effective the techniques can be (i.e. deeper interleaving).


THE D-RMA PROTOCOL THE D-RMA PROTOCOL (Simulation Study, Speech Traffic Model)(Simulation Study, Speech Traffic Model)

The speech traffic model used during the simulation tests considers a voice source as The speech traffic model used during the simulation tests considers a voice source as a sequence of a sequence of talkspurts talkspurts and and gaps. Bygaps. By assuming that a Voice Activity Detector (VAD) assuming that a Voice Activity Detector (VAD) can be used to differentiate between principle talkspurts (ON) and principle gaps can be used to differentiate between principle talkspurts (ON) and principle gaps (OFF), voice traffic can be characterized by the two-state Markov chain model (OFF), voice traffic can be characterized by the two-state Markov chain model displayed in the figure below.displayed in the figure below.

In the ON state, voice packets are generated at a constant rate. No packets are In the ON state, voice packets are generated at a constant rate. No packets are generated in the OFF state. Time spent in each state is exponentially distributed with generated in the OFF state. Time spent in each state is exponentially distributed with means means αα-1-1 ( (TTOFFOFF ) for the OFF state ) for the OFF state and and ββ-1-1 ((TTONON ) for the ON state.) for the ON state.

Reservation required when in the ON state and released when in the OFF state (no Reservation required when in the ON state and released when in the OFF state (no packets available for transmission).packets available for transmission).

If a packet is queued in a terminal buffer for a time longer than DmaxIf a packet is queued in a terminal buffer for a time longer than Dmaxvovo it is dropped. it is dropped.


THE D-RMA PROTOCOLTHE D-RMA PROTOCOL (Simulation Study, Video/Data Traffic Model) (Simulation Study, Video/Data Traffic Model)

Video or data terminals are modelled as Video or data terminals are modelled as bursty sourcesbursty sources, which generate frames of , which generate frames of variable length, exponentially distributed with mean variable length, exponentially distributed with mean LL (Kb). The arrival rate of a new (Kb). The arrival rate of a new burst is assumed equal to burst is assumed equal to λλmm..

Since a video service is a ‘time critical service’, it is also assumed that video packets Since a video service is a ‘time critical service’, it is also assumed that video packets are dropped if their waiting time within the terminal queue exceeds the maximum are dropped if their waiting time within the terminal queue exceeds the maximum delay value Dmaxdelay value Dmaxvivi. .


THE D-RMA PROTOCOL THE D-RMA PROTOCOL (Simulation Study, List of Nominal Values Utilized in (Simulation Study, List of Nominal Values Utilized in

Simulations)Simulations)

Reported simulations have been run under the conditions listed in the following table.Reported simulations have been run under the conditions listed in the following table.

A voice source generates data @ 8Kbps thus requiring one BBU (I-slot) per frame. On A voice source generates data @ 8Kbps thus requiring one BBU (I-slot) per frame. On the contrary, considered video source generation rates are 32Kbps and 64Kbps, the contrary, considered video source generation rates are 32Kbps and 64Kbps, which require four and eight BBUs (I-slots) respectively.which require four and eight BBUs (I-slots) respectively.


THE D-RMA PROTOCOL PERFORMANCE RESULTSTHE D-RMA PROTOCOL PERFORMANCE RESULTS(Voice-Only Traffic Performance)(Voice-Only Traffic Performance)

First, we examine the D-RMA performance results when in the presence of First, we examine the D-RMA performance results when in the presence of voice-only voice-only traffictraffic (homogeneous traffic). (homogeneous traffic).

The illustrated figure (optimum Nr vs. voice channel load The illustrated figure (optimum Nr vs. voice channel load ρ)ρ) shows that the optimum shows that the optimum number of active R-slots, Nr, has a number of active R-slots, Nr, has a linearly decreasing relationshiplinearly decreasing relationship with the channel with the channel

load, load, ρρ, and consequently with the number of active terminals. , and consequently with the number of active terminals. Why???Why???


THE D-RMA PROTOCOL PERFORMANCE RESULTSTHE D-RMA PROTOCOL PERFORMANCE RESULTS(Voice-Only Traffic Performance, cont.)(Voice-Only Traffic Performance, cont.)

In Reservation Multiple Access (RMA) protocols two contributions affect access delay. In Reservation Multiple Access (RMA) protocols two contributions affect access delay. The first is due to the access contention process, referred as The first is due to the access contention process, referred as contention delay Dcontention delay Dconcon . . The second is due to the waiting time of the reservation packet within the BT The second is due to the waiting time of the reservation packet within the BT

reservation queue, referred as reservation queue, referred as queue delay, Dqueue delay, Dqueque . . The access delay The access delay DDvovo= D= Dcon con + D+ Dqueque .. Under low traffic load conditions, the Under low traffic load conditions, the access contention delay is dominantaccess contention delay is dominant. Therefore, . Therefore,

the presence in the frame of a greater number of available R-slots contributes to the presence in the frame of a greater number of available R-slots contributes to reduce the access delay.reduce the access delay.

Conversely, for great values of traffic load, the Conversely, for great values of traffic load, the queue delayqueue delay dominates. Thus, it is dominates. Thus, it is rational to reduce the number of active R-slots in order to increase the information rational to reduce the number of active R-slots in order to increase the information channel bandwidth.channel bandwidth.



The figure below (average voice access delayThe figure below (average voice access delay, , DDvovo , vs. voice offered channel load , vs. voice offered channel load ρ)ρ), , shows the access delay Dshows the access delay Dvovo as a function of the offered channel load as a function of the offered channel load ρρ, for both D-, for both D-RMA RMA and a protocol which exploits fixed number of R-slots, ‘Nr-fixed’ protocol.and a protocol which exploits fixed number of R-slots, ‘Nr-fixed’ protocol.

The lower the access delay, the higher the number of terminals that can be The lower the access delay, the higher the number of terminals that can be simultaneously activated.simultaneously activated.



The figure below, shows the average packet dropping probability Pdrop for voice The figure below, shows the average packet dropping probability Pdrop for voice terminal as a function of the offered channel load terminal as a function of the offered channel load ρρ, for both D-RMA , for both D-RMA and a protocol and a protocol which exploits fixed number of R-slots, ‘Nr-fixed’ protocol.which exploits fixed number of R-slots, ‘Nr-fixed’ protocol.

The lower the packet dropping probability, the higher the number of terminals that The lower the packet dropping probability, the higher the number of terminals that can be simultaneously activated.can be simultaneously activated.



In order to better understand the improvement deriving from the adoption of D-RMA In order to better understand the improvement deriving from the adoption of D-RMA instead of a ‘Nr-fixed’ solution, we refer to the previous figure (instead of a ‘Nr-fixed’ solution, we refer to the previous figure (which is illustrated which is illustrated again below for convenienceagain below for convenience) and focus on the worst case of Pdrop=10) and focus on the worst case of Pdrop=10-2-2..

If Pdrop has to be kept below this upper bound, then the maximum value of If Pdrop has to be kept below this upper bound, then the maximum value of admissible load when D-RMA is adopted is about 0.89.In the same condition, the admissible load when D-RMA is adopted is about 0.89.In the same condition, the values obtainable when Nr=6 and Nr=8 are adopted, are 0.84 and 0.80 respectively. values obtainable when Nr=6 and Nr=8 are adopted, are 0.84 and 0.80 respectively.

Thus, the achieved gains are about 5% and 10%.Thus, the achieved gains are about 5% and 10%.Are these achieved gains Are these achieved gains low??? low???



Even Even greater marginsgreater margins are obtainable under different, not shown, simulated network are obtainable under different, not shown, simulated network conditions.conditions.

These percentages refer to one carrier only! As each microcell contains a set of These percentages refer to one carrier only! As each microcell contains a set of carriers, a considerable carriers, a considerable increment increment in the number of simultaneously active terminals in the number of simultaneously active terminals can be obtained.can be obtained.


THE D-RMA PROTOCOL PERFORMANCE RESULTSTHE D-RMA PROTOCOL PERFORMANCE RESULTS(Multimedia Traffic Performance, cont.)(Multimedia Traffic Performance, cont.)

The protocol behaviour analysis is extended to the case of heterogeneous traffic. The protocol behaviour analysis is extended to the case of heterogeneous traffic. Reported simulations have been run under the nominal conditions listed in the table Reported simulations have been run under the nominal conditions listed in the table given before. Furthermore, it is assumed that the nature of the heterogeneous traffic given before. Furthermore, it is assumed that the nature of the heterogeneous traffic is 75% voice and 25% video, the bandwidth allocation strategy applied is Complete is 75% voice and 25% video, the bandwidth allocation strategy applied is Complete Sharing (CS) and the reservation buffer policy is FIFO.Sharing (CS) and the reservation buffer policy is FIFO.



The figure shown below illustrates the average voice access delay The figure shown below illustrates the average voice access delay DDvo vo as a function of as a function of the total (voice+video) offered channel load the total (voice+video) offered channel load ρρ, for both D-RMA and a ‘Nr-fixed’ , for both D-RMA and a ‘Nr-fixed’ protocol. See nominal parameter values in previous table (Rprotocol. See nominal parameter values in previous table (Rvivi=32Kbps, L=5.12Kb, =32Kbps, L=5.12Kb, ρ=75%ρ=75%voice + 25%video, bandwidth allocation strategy=CS).voice + 25%video, bandwidth allocation strategy=CS).

Curves in the figure above, show that D-RMA can always guarantee a Curves in the figure above, show that D-RMA can always guarantee a lower voice lower voice access delayaccess delay when compared to ‘fixed-Nr’ algorithms. D-RMA achieves a when compared to ‘fixed-Nr’ algorithms. D-RMA achieves a greater greater delay performancedelay performance if it is compared to algorithms exploiting a lower Nr. if it is compared to algorithms exploiting a lower Nr.



The figure shown below illustrates the average video packet dropping probability The figure shown below illustrates the average video packet dropping probability PdropPdropvi vi as a function of the total (voice+video) offered channel load as a function of the total (voice+video) offered channel load ρρ, for both D-RMA , for both D-RMA and a ‘Nr-fixed’ protocol. See nominal parameter values in previous table and a ‘Nr-fixed’ protocol. See nominal parameter values in previous table (R(Rvivi=32Kbps,L=5.12Kb,=32Kbps,L=5.12Kb,ρ=75%ρ=75%voice + 25%video, bandwidth allocation strategy=CS).voice + 25%video, bandwidth allocation strategy=CS).

If we focus on the above figure, for Pdrop=10If we focus on the above figure, for Pdrop=10-3-3 (upper bound value for the considered (upper bound value for the considered type of traffic), we notice a type of traffic), we notice a gain in the acceptable loadgain in the acceptable load which is about 6.7% when which is about 6.7% when compared to Nr=6, about 11-12% when compared to Nr=8, and about 17% when compared to Nr=6, about 11-12% when compared to Nr=8, and about 17% when compared to Nr=10.compared to Nr=10.

These are greater margins than those achievable in the voice-only traffic case and These are greater margins than those achievable in the voice-only traffic case and they prove that D-RMA is particularly adequate for supporting multimedia application they prove that D-RMA is particularly adequate for supporting multimedia application traffic in a heterogeneous mobile environment.traffic in a heterogeneous mobile environment.



This last figure illustrates the quality index Q as a function of the total (voice + video) This last figure illustrates the quality index Q as a function of the total (voice + video) offered channel traffic load offered channel traffic load ρ. ρ. See nominal parameter values in previous table See nominal parameter values in previous table (R(Rvivi=32Kbps,L=5.12Kb,=32Kbps,L=5.12Kb,ρ=75%ρ=75%voice + 25%video, bandwidth allocation strategy=CS).voice + 25%video, bandwidth allocation strategy=CS).

The improvement in the quality index Q, when optimizing an ‘optimum Nr based’ The improvement in the quality index Q, when optimizing an ‘optimum Nr based’ instead of a ‘fixed Nr based’ strategy, is obvious. An increase in the maximum instead of a ‘fixed Nr based’ strategy, is obvious. An increase in the maximum number of simultaneously activated terminals with respect to the ‘Nr-fixed’ algorithm number of simultaneously activated terminals with respect to the ‘Nr-fixed’ algorithm is always present.is always present.

But, the above figure reveals good Q measure performance under low traffic But, the above figure reveals good Q measure performance under low traffic conditions, irrespective of the value of Nr. conditions, irrespective of the value of Nr. Why??? Is D-RMA protocol Why??? Is D-RMA protocol ineffective at low loading???ineffective at low loading???



At low loading, the adoption of a constant small Nr value would correspond to an At low loading, the adoption of a constant small Nr value would correspond to an increaseincrease in the voice access delay, as observed in the figures Dvo vs. in the voice access delay, as observed in the figures Dvo vs. ρ ρ for both types for both types of traffic (voice-only and multimedia).of traffic (voice-only and multimedia).

Focusing on the figure Dvo vs. Focusing on the figure Dvo vs. ρ ρ for multimedia traffic type, it is obtained that a for multimedia traffic type, it is obtained that a gaingain of about 10ms is achievable by substituting the Nr=4 technique with D-RMA. This of about 10ms is achievable by substituting the Nr=4 technique with D-RMA. This amount of time can be amount of time can be differently exploiteddifferently exploited (to perform a deeper interleaving for (to perform a deeper interleaving for example), during the transport on the air interface, while remaining within the same example), during the transport on the air interface, while remaining within the same overall allowed transmission delay. overall allowed transmission delay.

This is an advantage which This is an advantage which cannot be evaluated through the Q index (see Q index cannot be evaluated through the Q index (see Q index definition)definition), and it consists in a , and it consists in a greater robustnessgreater robustness of the successfully transmitted of the successfully transmitted information against channel errors.information against channel errors.


THE D-RMA PROTOCOL PERFORMANCE RESULTS THE D-RMA PROTOCOL PERFORMANCE RESULTS (Coupling D-RMA Protocol with different Bandwidth Allocation (Coupling D-RMA Protocol with different Bandwidth Allocation

Strategies and different Reservation Buffer Management Strategies and different Reservation Buffer Management Policies)Policies)

A detailed test campaign has been also performed by coupling D-RMA with A detailed test campaign has been also performed by coupling D-RMA with different different bandwidth allocation strategiesbandwidth allocation strategies (CS,CP,MRA) and (CS,CP,MRA) and different reservation buffer different reservation buffer management policiesmanagement policies (FIFO, Scan & Serve). As mentioned before, both of these types (FIFO, Scan & Serve). As mentioned before, both of these types of management have a strong effect on the performance of D-RMA protocol.of management have a strong effect on the performance of D-RMA protocol.

Performance curves for various combinations of Performance curves for various combinations of bandwidth allocation strategiesbandwidth allocation strategies and and reservation buffer management policiesreservation buffer management policies are skipped in the paper due to length are skipped in the paper due to length constraints. Nevertheless, the obtained results can be summarized by stating that constraints. Nevertheless, the obtained results can be summarized by stating that the adoption of both the the adoption of both the MRA strategy and the Scan & Serve policyMRA strategy and the Scan & Serve policy, allow the , allow the achievement of the best performance when compared to simple CS strategy or MRA achievement of the best performance when compared to simple CS strategy or MRA policy with FIFO queue management.policy with FIFO queue management.

We will try to provide an explanation for the above result by overviewing the We will try to provide an explanation for the above result by overviewing the aforementioned bandwidth allocation strategies along with the reservation buffer aforementioned bandwidth allocation strategies along with the reservation buffer management policies.management policies.


RESERVATION BUFFER MANAGEMENT POLICIES RESERVATION BUFFER MANAGEMENT POLICIES (FIFO, Scan & Serve)(FIFO, Scan & Serve)

Consider slot allocation procedure with two types of traffic – Consider slot allocation procedure with two types of traffic – wideband (WB)wideband (WB) traffic traffic (requiring multi-slot allocation, i.e. video or graphics) and (requiring multi-slot allocation, i.e. video or graphics) and narrowband (NB)narrowband (NB) traffic traffic (requiring single-slot allocation i.e. voice and data) – as a (requiring single-slot allocation i.e. voice and data) – as a multi-server queuing multi-server queuing systemsystem making use of a FIFO queue with reservation requests, with making use of a FIFO queue with reservation requests, with N identicalN identical, , parallel serversparallel servers where requests are served to completion. where requests are served to completion.

Arriving request which cannot be served immediately is ‘put on hold’ in the queue for Arriving request which cannot be served immediately is ‘put on hold’ in the queue for delayed service and can wait for a specific time and is dropped thereafter.delayed service and can wait for a specific time and is dropped thereafter.

A given request can enter service only when all its requested number of servers are A given request can enter service only when all its requested number of servers are available. This means that servers maybe idle while requests are waiting!available. This means that servers maybe idle while requests are waiting!

Need for efficient utilization of servers while providing equitable access to different Need for efficient utilization of servers while providing equitable access to different types of requests.types of requests.


RESERVATION BUFFER MANAGEMENT POLICIES RESERVATION BUFFER MANAGEMENT POLICIES (FIFO, Scan & Serve, cont.)(FIFO, Scan & Serve, cont.)

Different traffic types access the N-server channel in their order of arrival or FIFO Different traffic types access the N-server channel in their order of arrival or FIFO order.order.

A ‘lA ‘largearge’ request (WB call) waiting at the head-of-the-line (HOL), may adversely block ’ request (WB call) waiting at the head-of-the-line (HOL), may adversely block a large number of ‘a large number of ‘smallersmaller’ (NB calls) requests which could get service sooner! ’ (NB calls) requests which could get service sooner! Therefore, FIFO policy is inefficient as far as the servers’ utilization/throughput is Therefore, FIFO policy is inefficient as far as the servers’ utilization/throughput is concerned.concerned.

However, any attempt to allow NB requests to overtake WB requests will result in However, any attempt to allow NB requests to overtake WB requests will result in indefinitely postponingindefinitely postponing the WB requests from accessing the channel. the WB requests from accessing the channel.

Suppose we ignore this fairness issue. Suppose we ignore this fairness issue. How can we overcome the FIFO How can we overcome the FIFO disadvantage of having a large request wait at HOL while servers disadvantage of having a large request wait at HOL while servers are unutilized???are unutilized???

Scan the queue of waiting requests and insert into service those sets of ‘smaller’ Scan the queue of waiting requests and insert into service those sets of ‘smaller’ requests, if any, that best fit into the currently available portion of the channel. This requests, if any, that best fit into the currently available portion of the channel. This policy is known as policy is known as Scan & ServeScan & Serve policy. policy.


BANDWIDTH ALLOCATION STRATEGIESBANDWIDTH ALLOCATION STRATEGIES(CS, CP, MRA)(CS, CP, MRA)

We consider again the integration of WB and NB types of traffic with a multiserver queuing We consider again the integration of WB and NB types of traffic with a multiserver queuing system model.system model.

Among many strategies, the simplest one is the Among many strategies, the simplest one is the Complete Sharing (CS)Complete Sharing (CS), in which the total , in which the total servers are shared on the servers are shared on the first-come-first-service (FCFS)first-come-first-service (FCFS) basis whenever sufficient servers basis whenever sufficient servers exist.exist.

Another strategy is the Another strategy is the Complete Partitioning (CP)Complete Partitioning (CP), where the total servers are partitioned , where the total servers are partitioned into into distinct server groupsdistinct server groups, and they are assigned exclusively to each customer group., and they are assigned exclusively to each customer group.

Heuristic optimizationHeuristic optimization of the above two strategies provides other strategies. of the above two strategies provides other strategies.

Let’s investigate MRA strategy. Let’s investigate MRA strategy. Basic assumptions : Basic assumptions :

1. Consider broadband (BB) network serving two types of traffic, NB and WB. Assume that 1. Consider broadband (BB) network serving two types of traffic, NB and WB. Assume that a common transport capacity of this network is divided by a common transport capacity of this network is divided by mm BBUs. (In the wireless BBUs. (In the wireless context, each uplink frame can be seen us a channel in which each slot represents one context, each uplink frame can be seen us a channel in which each slot represents one BBU).BBU).

2. The NB traffic requires one BBU, the WB traffic requires 2. The NB traffic requires one BBU, the WB traffic requires n n BBUs BBUs (n≤m(n≤m), and all BBUs ), and all BBUs assigned to a WB call are occupied and released together. assigned to a WB call are occupied and released together.

3. The WB traffic is non-queueable, whereas the NB traffic may be queued in this system 3. The WB traffic is non-queueable, whereas the NB traffic may be queued in this system (infinite queue length).(infinite queue length).


BANDWIDTH ALLOCATION STRATEGIESBANDWIDTH ALLOCATION STRATEGIES(CS, CP, MRA, cont.)(CS, CP, MRA, cont.)

If no traffic access control is applied, random fluctuations in the NB traffic may cause If no traffic access control is applied, random fluctuations in the NB traffic may cause excessive blocking of the WB traffic while some channels are idle. This is severe when excessive blocking of the WB traffic while some channels are idle. This is severe when the bandwidth ratio the bandwidth ratio n n for the two types of traffic is large, because the NB traffic leaves for the two types of traffic is large, because the NB traffic leaves insufficient bandwidth for accepting a WB call request.insufficient bandwidth for accepting a WB call request.

Solution : Solution : restrictrestrict the maximum number of NB calls that can be in service the maximum number of NB calls that can be in service simultaneously in order to reserve bandwidth for the WB traffic. Similarly, excessive simultaneously in order to reserve bandwidth for the WB traffic. Similarly, excessive delay of the NB traffic caused by the overloaded WB traffic can be protected by setting delay of the NB traffic caused by the overloaded WB traffic can be protected by setting somesome restriction restriction on the WB traffic. on the WB traffic.

Denote these reference values of restrictions for the NB and WB traffics as the Denote these reference values of restrictions for the NB and WB traffics as the cutoff cutoff valuesvalues cc11 and and cc22 respectively.respectively.

Upon arrivals, an NB call is queued ifUpon arrivals, an NB call is queued if

1. the number of NB calls in service is equal to 1. the number of NB calls in service is equal to cc11

2. there is no free channel2. there is no free channel Upon arrivals, a WB call is blocked and cleared ifUpon arrivals, a WB call is blocked and cleared if

1. the number of WB calls in service is equal to 1. the number of WB calls in service is equal to cc22

2. there are not enough free channels2. there are not enough free channels


BANDWIDTH ALLOCATION STRATEGIESBANDWIDTH ALLOCATION STRATEGIES(CS, CP, MRA, cont.)(CS, CP, MRA, cont.)

The cutoff value The cutoff value cc11 , is chosen in a way that makes the reserved channels for the WB , is chosen in a way that makes the reserved channels for the WB traffic a multiple of the bandwidth requirement of a WB call.traffic a multiple of the bandwidth requirement of a WB call.

By varying By varying cc11 and and cc22 we achieve an we achieve an effective control to adapt to the varying traffic effective control to adapt to the varying traffic loadload..

Set Set cc11=s =s and and cc22=r =r (the maximum number of WB calls that can be in service at the (the maximum number of WB calls that can be in service at the same time if the WB traffic can use the whole bandwidth, i.e. same time if the WB traffic can use the whole bandwidth, i.e. m= rm= r∙ n +s, 0≤s<n), ∙ n +s, 0≤s<n), this MRA strategy becomes the pure CS strategy.this MRA strategy becomes the pure CS strategy.

When When cc11+ n+ n∙ ∙ cc22=m =m , it becomes the pure CP strategy., it becomes the pure CP strategy. Therefore, MRA strategy is a Therefore, MRA strategy is a generalizationgeneralization that includes CS and CP strategies. that includes CS and CP strategies. Other strategies can be obtained by varying the cutoff values Other strategies can be obtained by varying the cutoff values cc11 and and cc22.. Complete Sharing (CS) of bandwidth for non-multimedia, and Mutually Restricted Complete Sharing (CS) of bandwidth for non-multimedia, and Mutually Restricted

Access (MRA) for multimedia services seem to be promising strategies in a Access (MRA) for multimedia services seem to be promising strategies in a multimedia wireless environment.multimedia wireless environment.


CONCLUSIONCONCLUSION

Design of 3G cellular systems requires the adoption of radio channel access schemes, Design of 3G cellular systems requires the adoption of radio channel access schemes, efficient in supporting multimedia services.efficient in supporting multimedia services.

D-RMA both permits wireless networks to embrace a wider range of capabilities and D-RMA both permits wireless networks to embrace a wider range of capabilities and contributes to the research work toward the definition of the Personal contributes to the research work toward the definition of the Personal Communication Systems (PCS).Communication Systems (PCS).

Its behaviour has been observed under homogeneous and heterogeneous traffic Its behaviour has been observed under homogeneous and heterogeneous traffic conditions.conditions.

In order to compare D-RMA behaviour with that of other traditional access protocols, In order to compare D-RMA behaviour with that of other traditional access protocols, a new quality index has been defined.a new quality index has been defined.

Better performance resulted from the adoption of the dynamic approach when Better performance resulted from the adoption of the dynamic approach when compared to a ‘Nr fixed’ policy.compared to a ‘Nr fixed’ policy.

Trade-off : complexity vs. protocol performance (in terms of both the offered QoS to Trade-off : complexity vs. protocol performance (in terms of both the offered QoS to multimedia services and number of connections that can be activated simultaneously multimedia services and number of connections that can be activated simultaneously in a microcell at one time).in a microcell at one time).


REFERENCESREFERENCES

Antonio Iera, Salvatore Marano, ‘D-RMA : A Dynamic Reservation Multiple Access Antonio Iera, Salvatore Marano, ‘D-RMA : A Dynamic Reservation Multiple Access Protocol for Third Generation Cellular Systems’, IEEE TRANSACTIONS ON VEHICULAR Protocol for Third Generation Cellular Systems’, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 49, NO. 5, SEPTEMBER 2000.TECHNOLOGY, VOL. 49, NO. 5, SEPTEMBER 2000.

Pallapa Venkataram, Anirban Roy, A. Chockalingam, ‘Performance of a Link Control Pallapa Venkataram, Anirban Roy, A. Chockalingam, ‘Performance of a Link Control Protocol for Local Wireless Multimedia Communications’, DEPT. OF ELECTRICAL Protocol for Local Wireless Multimedia Communications’, DEPT. OF ELECTRICAL COMMUNICATION ENGINEERING, INDIAN INSTITUTE OF SCIENCE, BANGALORE COMMUNICATION ENGINEERING, INDIAN INSTITUTE OF SCIENCE, BANGALORE 560012,INDIA.560012,INDIA.

Young Han Kim, Chong Kwan Un, ‘Analysis of Bandwidth Allocation Strategies with Young Han Kim, Chong Kwan Un, ‘Analysis of Bandwidth Allocation Strategies with Access Restrictions in Broadband ISDN’, IEEE TRANSACTIONS ON COMMUNICATIONS, Access Restrictions in Broadband ISDN’, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 41, NO. 5, MAY 1993. VOL. 41, NO. 5, MAY 1993.

d-rma: a dynamic reservation multiple access protocol for third generation cellular systems

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