Nah-Oak Song

Enhancement of IEEE 802.11 distributed coordination function with exponential increase exponential decrease backoff algorithm

A new backoff algorithm is proposed to enhance the performance of the IEEE 802.11 distributed coordination function (DCF), which employs binary exponential backoff (BEB) algorithm. The proposed algorithm, called the exponential increase exponential decrease (EIED) backoff algorithm, is quite simple to implement while significantly improving the network performance over BEB. Another backoff algorithm called multiple increase linear decrease (MILD) backoff algorithm is considered for performance comparison. The simulation results show that EIED outperforms BEB and MILD in terms of both throughput and delay. The performance gain of EIED comes from successfully balancing the two extreme backoff policies of BEB and MILD.

A Network-Assisted User-Centric WiFi-Offloading Model for Maximizing Per-User Throughput in a Heterogeneous Network

In this paper, we propose a novel network-assisted user-centric WiFi-offloading model for maximizing per-user throughput in a heterogeneous network. In the proposed WiFi-offloading model, the heterogeneous network collects network information, such as the number of users in WiFi networks and their traffic load. Then, the network decides the specific portion of traffic to be transmitted via WiFi networks, i.e., φ*, which maximizes the per-user throughput. After informing the φ* to users, users offload their traffic with the probability of φ* and achieve the maximum per-user throughput while offloading more traffic into WiFi networks. Through the proposed WiFi-offloading model, a user and the heterogeneous network achieve 20% higher throughput than the on-the-spot WiFi-offloading model in a dense traffic environment.

Analysis of the stability and performance of exponential backoff

New analytic results are given for the stability and performance of the exponential backoff (EB) algorithm. Previous studies on the stability of the (binary) EB have produced contradictory results instead of a consensus: some proved instability, others showed stability under certain conditions. In these studies, simplified and/or modified models of the backoff algorithm were often used to make analysis more tractable. In this paper, care is taken to use a model for backoff that reflects the actual behavior of backoff algorithms. We show that EB is stable under a throughput definition of stability; the throughput of the network converges to a non-zero constant as the offered load N goes to infinity. We also obtain the analytical expressions for the saturation throughput and the medium access delay of a packet for a given number of nodes, N. The analysis considers the general case of EB with backoff factor r, where BEB is the special case with r = 2. The accuracy of the analysis is checked against simulation results.

A mobility measure for mobile ad hoc networks

A mobility measure for mobile ad hoc networks is proposed that is flexible because one can customize the definition of mobility using a remoteness function. The proposed measure is consistent because it has a linear relationship to the rate at which links are established or broken for a wide range of mobility scenarios, where a scenario consists of the choice of mobility model, the physical dimensions of the network, the number of nodes. This consistency is the strength of the proposed mobility measure because the mobility measure reliably represents the link change rate regardless of network scenarios.

Security Considerations for Handover Schemes in Mobile WiMAX Networks

IEEE 802.16e uses EAP-based authentication and key management for link layer security. Due to the lack of ability to support mobility, however, EAP-based key management becomes a principal impediment to the achievement of an efficient and secure handover in IEEE 802.16e mobile WiMAX networks. In this paper, an overview of the EAP-based handover procedures of the latest IEEE 802.16e standard is given and their security flaws are analyzed. Possible solutions for secure handover in IEEE 802.16e networks are also proposed in this paper. The proposed handover protocol guarantees a backward/forward secrecy while gives little burden over the previous handover protocols.

On the Stability of Exponential Backoff

Random access schemes for packet networks featuring distributed control require algorithms and protocols for resolving packet collisions that occur as the uncoordinated terminals contend for the channel. A widely used collision resolution protocol is the exponential backoff (EB). New analytical results for the stability of the (binary) EB are given. Previous studies on the stability of the (binary) EB have produced contradictory results instead of a consensus: some proved instability, others showed stability under certain conditions. In these studies, simplified and/or modified models of the backoff algorithm were used. In this paper, care is taken to use a model that reflects the actual behavior of backoff algorithms. We show that EB is stable under a throughput definition of stability; the throughput of the network converges to a non-zero constant as the offered load N goes to infinity. We also obtain the analytical expressions for the saturation throughput for a given number of nodes, N. The analysis considers the general case of EB with backoff factor r, where BEB is the special case with r = 2. We show that r = 1/(1 − e−1) is the optimum backoff factor that maximizes the throughput. The accuracy of the analysis is checked against simulation results.

Analysis of EIED backoff algorithm for the IEEE 802.11 DCF

Exponential Increase Exponential Decrease (EIED) backoff algorithm, a flexible backoff algorithm with a number of adjustable parameters, was proposed by Song et al. to enhance the performance of the IEEE 802.11 DCF, where the performance benefit of EIED was shown by simulation. In this paper, we extend on the previous work, and provide an analysis of EIED backoff algorithm and an optimization methodology of the parameters of EIED based on the analysis.

Optimal Rate Selection for Persistent Scheduling with HARQ in Time-Correlated Nakagami-m Fading Channels

Orthogonal frequency division multiple access (OFDMA) systems, such as IEEE 802.16e/m and 3GPP Long Term Evolution (LTE), require significantly large signaling overhead for delivering small-sized delay-sensitive traffic such as voice over IP (VoIP) service. It may result in a significant decrease in the spectral efficiency. In order to overcome this drawback, recently, a persistent scheduling scheme has been standardized in standard bodies. In order to reduce the signaling overhead, the modulation and coding scheme (MCS) is set to be fixed during a burst period for the persistent scheduling procedure and an incomplete transmission, if it occurs, is recovered by a hybrid automatic repeat request (HARQ) scheme. In this paper, considering the HARQ retransmissions, we propose a rate selection scheme for the persistent scheduling satisfying given quality of service (QoS) requirements. In our proposed rate selection scheme, we consider two important factors which were not considered in previous HARQ-related work: time-correlations of the wireless channel in the HARQ retransmissions and the resource usage of signaling overhead. Numerical results show that the proposed rate selection scheme can efficiently enhance the utilization of radio resources compared to the conventional schemes.

On the scalability of ad hoc networks

We investigate the inherent scalability problem of ad hoc networks originated from the nature of multihop networks. First, the expected packet traffic at the center of a network is analyzed. The result shows that the expected packet traffic at the center of a network is linearly related with the network size, that is, the expected packet traffic at the center of a network is O(k), where k is the radius of a network. From the result, the upper bound of the diameter of a network D=2k, that guarantees the network is scalable, is obtained. The upper bound is given by C/r-1, where C is the channel capacity available to each node and r is the packet arrival rate at each node.

Performance Analysis of IEEE 802.11e EDCA With a Virtual Collision Handler

This paper presents an analytical model of IEEE 802.11e enhanced distributed channel access (EDCA) with a virtual collision handler (VCH). The proposed analytical model provides different access categories (ACs) for supporting the quality of service (QoS) of differentiated services by using different contention window (CW) sizes and arbitration interframe space (AIFS) values in EDCA. The proposed model considers a network of stations, each of which has multiple queues and a VCH for different ACs. Based on the proposed model, we analyze the throughput and delay performance of the EDCA with four different ACs and investigate the effect of the VCH by considering two VCH schemes. The analytical and simulation results show that the proposed analytical model is very accurate for various CW sizes and AIFS values.