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Allan C. Cleary and Michael Paterakis

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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Allan C. Cleary and Michael Paterakis

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  1. Design and performance evaluation of an RRA scheme forvoice-data channel access in outdoor microcellular environments Allan C. Cleary and Michael Paterakis Χήρας Θεόδωρος- Special Topics in Communication Networks

  2. 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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  3. A well designed multiple access scheme will provide : Maximization of system capacity Satisfy QoS requirements (voice packet dropping probability and access delay) Integration different classes of traffic (voice-data) Vs Bandwidth limitations Contradictory requirements of voice and data traffic Χήρας Θεόδωρος- Special Topics in Communication Networks

  4. Usual proposals for Multiple access schemes proposed for PCS usually involve FDMA,TDMA,CDMA or combinations thereof. • RRA operates over a time-slotted channel combining a random access algorithm (e.g. Slotted Aloha) with TDMA. • Slots  Information or Request • Information Slots  Reserved or Available Χήρας Θεόδωρος- Special Topics in Communication Networks

  5. Basic 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 Χήρας Θεόδωρος- Special Topics in Communication Networks

  6. Different versions RRA schemes RRA • Every slot is information slot. • The 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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  7. PRMA • 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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  8. IPRMA • PRMA with a priority mechanism to ensure that voice packets have greater access to the available slots Χήρας Θεόδωρος- Special Topics in Communication Networks

  9. 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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  10. PRMA++ • Frame is divided into Request and Information slots 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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  11. Different 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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  12. Problems • 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 • 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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  13. 2. RRA Scheme Scenario : • Microcell with mobile source terminals generating traffic • 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) Χήρας Θεόδωρος- Special Topics in Communication Networks

  14. RRA Protocol • The frame duration is selected such that a voice terminal in talkspurt generates exactly one packet per frame

  15. 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 Χήρας Θεόδωρος- Special Topics in Communication Networks

  16. Voice 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 Χήρας Θεόδωρος- Special Topics in Communication Networks

  17. Channel Resource Allocation Strategy Dynamic table of the active terminals containing : • Terminal identifier • Virtual circuit identifier • Channel Resources Allocated • 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.

  18. 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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  19. 2.2. Random access algorithms for voice terminals • Ideal • 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) Χήρας Θεόδωρος- Special Topics in Communication Networks

  20. Slotted Aloha • Each contending terminal transmits its request with probability p (p=1/3 for the simulations) • For p=0.5 bistability occurred for more than 75 terminals in the system. Χήρας Θεόδωρος- Special Topics in Communication Networks

  21. 2-Cell Stack • At the start of every request interval the contending terminals initialize their counter, r, to 0 or 1 with probability 1/2. • 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 with probability 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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  22. 2-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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  23. 2.2. Random access algorithms for data terminals 2 Cell Stack • Simplicity. • Stability. • High throughput. (λMAX = 0.429 packets per slot) Χήρας Θεόδωρος- Special Topics in Communication Networks

  24. Blocked access mechanism First time transmission rule for newly generated datamessages 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

  25. 3. Voice traffic analysis 3.1 Assumptions Steady state probabilities 2-state discrete time Markov chain Χήρας Θεόδωρος- Special Topics in Communication Networks

  26. N = The number of active voice terminals = steady Changes 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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  27. 3.2. System state transitions Χήρας Θεόδωρος- Special Topics in Communication Networks

  28. 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 Dcis the mean random access delay (DC MIN = 1 Slot) Dqis the mean queuing delay Dris 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 Χήρας Θεόδωρος- Special Topics in Communication Networks

  29. Performance Evaluation1)Distribution of the voice packets dropped per talkspurt2)Explore preliminary voice-dataintegration issues Χήρας Θεόδωρος- Special Topics in Communication Networks

  30. Performance Evaluation • Packet size 53 Bytes (compatibility with ATM networks) • Speech codec AD-PCM • Ratio Talkspurt / Silence = 44% ( e.g. 1.0/1.35 s or 1.41/1.74 s ) • Voice delay limit = 24 ms = 2 frames • Mini slot duration = 70 bits Χήρας Θεόδωρος- Special Topics in Communication Networks

  31. Simulation 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. Χήρας Θεόδωρος- Special Topics in Communication Networks

  32. Results • Multiplexing gain = ratio of the voice capacity to the number of slots per frame • Voice Capacity = N (PDROP 1%) Analytical Results Χήρας Θεόδωρος- Special Topics in Communication Networks

  33. Results • Voice packet probability (%) /Active Voice terminals Χήρας Θεόδωρος- Special Topics in Communication Networks

  34. Fraction of voice packets dropped from the contender and queued states Slotted Aloha.

  35. Fraction of voice packets dropped from the contender and queued states 2 Cell Stack

  36. Comments • 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 Pdropand Mean access delay. Χήρας Θεόδωρος- Special Topics in Communication Networks

  37. Operation at Voice Capacity (N=97)SLIDE 33 If N=98 PDROP > 1 Χήρας Θεόδωρος- Special Topics in Communication Networks

  38. Simulation results for the steady state packet dropping distribution per talkspurt for each access protocol operating at voice capacity. Χήρας Θεόδωρος- Special Topics in Communication Networks

  39. Steady state mean voice access delay Χήρας Θεόδωρος- Special Topics in Communication Networks

  40. Aloha Vs 2 Cell Stack • Aloha is slightly worst at low values of N because when there is 1 contending terminal it successfully transmits its request 90 percent of the time, due to the probabilistic first time transmission rule. • However when the terminal follows 2 Cell Stack it always succeds. Χήρας Θεόδωρος- Special Topics in Communication Networks

  41. Analytical results for voice packet throughput vs. N. Linear behaviour almost while N<98

  42. 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. • 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. Throughput The proportion of time slots that successfully carry data information packets Χήρας Θεόδωρος- Special Topics in Communication Networks

  43. Data 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 Χήρας Θεόδωρος- Special Topics in Communication Networks

  44. Data wait delay and the Datamessage delay Vs the Data message arrival rate,λ,forthe 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

  45. Steady state mean data delays N = 86. Steady state voice Pdrop = 0.07, mean access delay = 18 ms and throughput = 38 packets/frame

  46. Steady state mean data delays, N =90. steady state voice Pdrop = 0.2, mean access delay = 20 ms and throughput = 40 packets/frame Χήρας Θεόδωρος- Special Topics in Communication Networks

  47. Conclusion The proposed RRA scheme is a promising scheme for providing voice-data integration in outdoor microcellular environments. Our Suggestions for future work 1)3 kinds of traffic with different priorities (+Low quality live video) 2)Simulations to investigate data delays when N=80-90 (PDROP < 1%) ,using Aloha and 2 Cell stack. Χήρας Θεόδωρος- Special Topics in Communication Networks

  48. Discussion Χήρας Θεόδωρος- Special Topics in Communication Networks

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