Collision resolution for buffered users

A stack protocol of CTM type with quartet feedback

The throughput characteristics of contention-based random-access systems (RAS's) which use <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -ary tree algorithms (where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q \geq 2</tex> is the number of groups into which contending users are split) of the Capetanakis-Tsybakov-Mikhailov-Vvedenskaya type are analyzed for an infinite population of identical users generating packets according to a Poisson process. Both free and blocked channel-access protocols are considered in combination with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> -ary collision resolution algorithms that exploit either binary ("collision/no collision") or ternary ("collision/ success / idle") feedback. For the resulting RAS's, functional equations for transformed generating functions of the first two moments of the collision resolution interval length are obtained and solved. The maximum stable throughput as a function of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> is given. The results of a packet-delay analysis are also given, and the analyzed RAS's are compared among themselves and with the slotted ALOHA system in terms of both system throughput and packet delay. It is concluded that the "practical optimum" RAS (in terms of ease of implementation combined with good performance) uses free (i.e., immediate) channel access and ternary splitting (i.e., <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q = 3</tex> ) with binary feedback.

The IP address lookup in the communication link technologies is becoming a major bottleneck by exploring the exact matching and the parallel hashing in each prefix length. In this paper, we present an algorithm for IP address lookup and collision resolution to reduce memory space and solve collision for high speed routing based on multiple table indexing structure. Our scheme makes possible direct access in a designated IP prefix without replicating the forwarding table entries, and provides efficient collision resolution for fast route lookup with only small memory. Simulation results indicate that our algorithm provides a faster lookup than the conventional lookup schemes.

This paper considers the collision resolution algorithm when multiple packets transmit simultaneously in each slot in a non-orthogonal multiple access (NOMA) systems. Packets are divided into two groups and two different received powers can be selected. Packets in each group can select one power level as their received power. As more than one packet may transmit in a time slot in each group, collision will happen and throughput may decrease at the same time. We introduce the collision resolution period (CRP) algorithm and the collided packets will transmit in the CRP. By controlling the transmission probability in each group, we can make a big different comparing to the system without a CRP. This paper will show the optimal transmission probability and the maximum throughput. Numerical results show that our proposed algorithm yields the maximum throughput of 0.6489.

LTE random access procedure performs satisfactorily in case of asynchronous, uncorrelated traffic arrivals. However, when the arrivals are correlated and arrive synchronously, the performance of the random access channel (RACH) is drastically reduced, causing a large number of devices to experience outage. In this work we propose a LTE RACH scheme tailored for delay-sensitive M2M services with synchronous traffic arrivals. The key idea is, upon detection of a RACH overload, to apply a collision resolution algorithm based on splitting trees. The solution is implemented on top of the existing LTE RACH mechanism, requiring only minor modifications of the protocol operation and not incurring any changes to the physical layer. The results are very promising, outperforming the related solutions by a wide margin. As an illustration, the proposed scheme can resolve 30k devices with an average of 5 preamble transmissions and delay of 1.2 seconds, under a realistic probability of transmissions error both in the downlink and in the uplink.

Routers need to store temporarily a large number of packets in response to congestion. DRAM is typically needed to implement large packet buffers, but DRAM devices have worst-case random access latencies that are too slow to match the bandwidth requirements of high-performance routers. Existing DRAM-based architectures for supporting linespeed queue operations can be classified into three categories: prefetching-based, randomization-based, and reservation-based. They are all based on interleaving memory accesses across multiple parallel DRAM banks for achieving higher memory bandwidths, but they differ in their packet placement and memory operation scheduling mechanisms. In this paper, we present an efficient reservation-based packet buffer architecture based on the concept of blocks. The proposed block-based solution achieves an order of magnitude reduction in the total SRAM size. It is scalable to growing packet storage requirements in routers while matching increasing line rates.

The CTM protocol permits an unlimited number of users to share a single slotted communications channel. A collision occurs when two or more users transmit messages in the same slot. Users who collide re-transmit their messages by randomly splitting into subgroups until each subgroup contains at most one message. L(n), the number of slots required to resolve a collision among n users, is a random variable whose asymptotic properties are of considerable interest. Under fairly general conditions, the asymptotic distribution of L(n) is normal. We briefly review the early research on this problem, and then present some newer results.

최근 HFC(Hybrid Fiber Coax) CATV 망이 초고속망의 하부구조로 자리를 잡아가고 있다. HFC-CATV 망의 상향채널은 가입자에서 헤드엔드로 데이터 전송시 500가입자 이상이 공유하기 때문에 충돌이 발생하게 된다. 특히, 음성, 비디오 그리고 데이터와 같은 다양한 종류의 트래픽 QoS(Qualify of Service)를 만족시키기 위해서 서비스별로 우선순위를 고려한 알고리즘이 요구되어진다. IEEE 802.14에서 제안된 우선순위 기반 3진 트리(Ternary tree)알고리즘은 정적 PNA(Priority New Access) 슬롯 구조를 가진다. 이 방법은 실시간으로 변화하는 트래픽의 요구 지연(request delay)이 높게 유지되며 서로 다른 우선순위 트래픽의 요구 경쟁 확률이 동일하다는 문제점을 가지고 있다. 따라서, 본 논문에서는 동적 PNA(Priority New Access) 슬롯 구조를 제안하여 높은 우선순위 트래픽의 요구지연을 낮게 하고 새로운 스테이션에 대해 우선순위 별로 경쟁 확률을 다르게 하는 알고리즘을 제안한다. 동일한 조건하에 두 알고리즘에 대해 요구지연 값을 비교 분석하였다. 성능 분석 결과 동적 PNA 우선순위 충돌해결알고리즘이 정적 PNA 우선순위 충돌해결알고리즘보다 낮은 요구지연을 가진다는 것을 확인할 수 있었다. Recently, the HFC-CATV network stand in a substructure of superhighway information network. Because of sharing up to 500 of subscribes, the Collision Resolution Algorithm needs in the upstream channel of HFC-CATV network. In order to provide Quality of Service (QoS) to users with real-time data such as voice, video and interactive service, the research of Collision Resolution Algorithm must include an effective priority scheme. In IEEE 802.14, the Collision Resolution Algorithm has high request delay because of static PNA(Priority New Access) slots structure and different priority traffics with the same probability. In order to resolve this problem, this paper proposed dynamic priority collision resolution algorithm with ternary tree algorithm. It has low request delay according to an increase of traffic load because high priority traffic first resolve and new traffic content with different probability. In the result of the simulation, it demonstrated that the proposed algorithm needs lower request delay than that of ternary tree algorithm with static PNA slots structure.

In many communication systems, the stronger of two or more overlapping packets might capture the receiver and thus be received without error. The effect of capture on collision-resolution algorithms in a slotted ALOHA type broadcasting network is investigated here. Extensions to the algorithms are suggested for both the situations in which the receiver can or cannot distinguish between success slots and capture slots. In particular, we present a class of retransmission schemes for packets that have been transmitted during capture slots but have not been received correctly. The performance analysis is confined to a simplified model in which the nodes of the network are divided into two groups and only packets sent by the nodes of one of them might be captured. For this simplified model and for each extended algorithm, explicit recursive equations are given, from which the average conditional collision-resolution interval length, as well as the maximal throughput, can be determined. As expected, we show that in the presence of capture, the performance of the network is improved and the maximal attainable throughput is increased. Extensions of the simplified model such as dividing the nodes into K groups instead of two, or considering the situation that capture depends on relative distances and transmission powers, are also discussed. For the latter situation we give simulation results.

The performance of hybrid random-access systems (HRAS) which use a combination of multiuser codes (MUC) and collision resolution algorithms (CRA) to accommodate the bursty transmissions of many independent users on a single communication channel is analyzed. Besides the computation of the system throughput, another contribution of this paper is the determination of the properties which a MUC must possess such that resulting HRAS performs better than a system which is only based on CRA'S.

Extended Minkowski Portal Refinement Algorithm for Coherent Collision Resolution in Time-marching Dynamical Simulations

In automated warehouses, teams of mobile robots fulfill the packaging process by transferring inventory pods to designated workstations while navigating narrow aisles formed by tightly packed pods. This problem is typically modeled as a Multi-Agent Pickup and Delivery (MAPD) problem, which is then solved by repeatedly planning collision-free paths for agents on a fixed graph, as in the Rolling-Horizon Collision Resolution (RHCR) algorithm. However, existing approaches make the limiting assumption that agents are only allowed to move pods that correspond to their current task, while considering the other pods as stationary obstacles (even though all pods are movable). This behavior can result in unnecessarily long paths which could otherwise be avoided by opening additional corridors via pod manipulation. To this end, we explore the implications of allowing agents the flexibility of dynamically relocating pods. We call this new challenging problem Terraforming MAPD (tMAPD) and develop an RHCR-based approach to tackle it. As the extra flexibility of terraforming comes at a significant computational cost, we utilize this capability judiciously by identifying situations where it could make a significant impact on the solution quality. In particular, we invoke terraforming in response to disruptions that often occur in automated warehouses, e.g., when an item is dropped from a pod or when agents malfunction. Empirically, using our approach for tMAPD, where disruptions are modeled via a stochastic process, we improve throughput by over 10%, reduce the maximum service time (the difference between the drop-off time and the pickup time of a pod) by more than 50%, without drastically increasing the runtime, compared to the MAPD setting.

In Lifelong Multi-Agent Path Finding (L-MAPF) a team of agents performs a stream of tasks consisting of multiple locations to be visited by the agents on a shared graph while avoiding collisions with one another. L-MAPF is typically tackled by partitioning it into multiple consecutive, and hence similar, "one-shot" MAPF queries, as in the Rolling-Horizon Collision Resolution (RHCR) algorithm. Therefore, a solution to one query informs the next query, which leads to similarity with respect to the agents' start and goal positions, and how collisions need to be resolved from one query to the next. Thus, experience from solving one MAPF query can potentially be used to speedup solving the next one. Despite this intuition, current L-MAPF planners solve consecutive MAPF queries from scratch. In this paper, we introduce a new RHCR-inspired approach called exRHCR, which exploits experience in its constituent MAPF queries. In particular, exRHCR employs an extension of Priority-Based Search (PBS), a state-of-the-art MAPF solver. The extension, which we call exPBS, allows to warm-start the search with the priorities between agents used by PBS in the previous MAPF instances. We demonstrate empirically that exRHCR solves L-MAPF instances up to 39% faster than RHCR, and has the potential to increase system throughput for given task streams by increasing the number of agents a planner can cope with for a given time budget.

In this paper we propose and evaluate several protocols for a random access channel. The channel is divided into a control subchannel which is used to inform all users of the approximate times of arrival of new packets and a data subchannel which is used to transmit the data. Packets arriving at different users at approximately the same time may cause a collision on the control subchannel. Such collisions are resolved on the data subchannel by a collision resolution algorithm. Eight protocols that differ in the feedback available on both channels and the time that the feedback is available are described and evaluated. The expected delay of each protocol is determined by modeling the system as a slotted concentrator. Comparisons are made among the protocols described and analyzed in this paper and between these protocols and other protocols that have been studied previously.

We consider connectionless packet switched CDMA systems where all users have to share a common bandwidth and signature sequence on a contention basis. Multiple access is achieved by the simultaneous acquisition of different users' spread spectrum transmissions. The throughput of a pure random access system of this kind suffers from acquisition errors and timing mismatches of the active users as well as instability issues. We consider schemes that use information available at the physical layer, i.e., at the output of the multiuser access detector (MUAD) designed for this random access system, to improve and stabilize the throughput. We consider timing randomization, pseudo-Bayesian stabilization and collision resolution algorithms and report the corresponding numerical results.