With the advent of various multimedia streaming applications requiring reliability and high bandwidth, multicasting in wireless LANs has been gaining more attentions as the supported bit rate increases. However, according to the specification of the IEEE 802.11 standard, broadcast/multicast frames are transmitted at a fixed and low bit rate due to the absence of a feedback mechanism such as ACK. This simple broadcasting technique with no feedback signal raises some issues; reliability, efficiency and fairness. In this paper, hence, we propose a framework for multicasting termed Leader-Based Multicast Service (LBMS) to alleviate those limitations. LBMS consists of a leader-based transmission and feedback mechanism for multicasting by extending the IEEE 802.11v standard, which is the next-generation standard for network management. Also, we try to support legacy 802.11 stations can still participate in multicasting. Simulation exhibits that the gain of the proposed multicasting scheme increases as (i) more stations compete for the channel, and (ii) the wireless channel condition becomes poorer.

The IEEE 802.11 standards support multiple bit rates at the physical layer so that stations can maximize the system performance by controlling the transmission rate responding to the conditions of underlying time-varying channels. This paper proposes a novel scheme, termed as rate adaptation using rate-adaptive acknowledgment (RARA). The key idea of RARA is to regulate the ACK transmission rate as a means to dictate the sender to adjust the data transmission rate. Compared with previous works, the proposed scheme can react quickly to the prevailing conditions by the receiver-based decision while inducing marginal overhead. Simulation results show that the RARA consistently outperforms other rate adaption schemes.

In this paper, we discuss some problems related to direct transmissions between QSTAs in an infrastructure 802.11 WLAN with a legacy access point (AP). In the infrastructure mode, collisions of data frames caused by hidden terminals interference can be avoided by the RTS/CTS exchange anticipating the data frames, if and only if all data communications between stations are performed via the AP. The interference problem is solved because all terminals are within the transmission range (TX range) of the AP. However, the RTS/CTS exchange is not enough to protect direct transmissions between QSTAs. In this paper, we analyze the state-of-the-art direct transmissions protection approaches, show their limitations, propose and simulate our own protection schemes.

Multicasting is useful for various applications such as multimedia broadcasting. In current 802.11, multicast frames are sent as broadcast frames at a low transmission rate without any acknowledgement or binary exponential backoff. This naive multicasting mechanism degrades the performance of not only multicast flows but also unicast flows. In this paper, we propose a new multicasting mechanism based on the leader- based approach to improve the legacy multicast transmissions, maintaining coexistence with legacy 802.11 devices. Simulations show that our protocol achieves well-balanced performance in terms of reliability, latency, goodput, and transmission fairness in comprehensive environments.

The IEEE 802.11e standard supports a direct-link setup (DLS) mechanism optionally to improve the throughput. Using this mechanism, IEEE 802.11e stations in proximity can directly exchange frames with no intervention of an access point. However, extensive simulations reveal the severe unfairness between external and local TCP connections in the IEEE 802.11e DLS mode due to the characteristics of TCP: a window-based flow control mechanism. This paper first analyzes why a fairness problem happens between external and local TCP connections in the IEEE 802.11e DLS mode. Then, we seeks to achieve fairness between them by introducing a novel mechanism dubbed half direct-link setup plus (H-DLS) which differentiates the paths for TCP DATA and ACK packets of local TCP connections. Simulation results reveal that H-DLS achieves the fairness between external and local TCP connections while keeping the aggregate throughput higher than the original IEEE 802.11 infrastructure.

In this paper, we propose a hybrid query tree protocol that combines a tree based query protocol with a slotted backoff mechanism. The proposed protocol decreases the average identification delay by reducing collisions and idle time. To reduce collisions, we use a 4-ary query tree instead of a binary query tree. To reduce idle time, we introduce a slotted backoff mechanism to reduce the number of unnecessary query commands. For static scenarios of tags, we extended the proposed protocol by adopting two phases. First, inleafqueryphasefor existing tags, the interrogator queriesleaf-nodes directly to reuse query strings in the previous session. Second, inrootqueryphasefor new arriving tags, the interrogator starts the query process from theroof-node. Simulation reveals that the proposed protocol achieves lower identification delay than existing tag collision arbitration protocols regardless of whether tags are mobile or not.

This paper via analysis and simulation revisits the interaction between MAC contention and TCP congestion control over IEEE 802.11 WLANs, misled in the previous efforts. The results reveal that the effective number of contending wireless stations is not proportional to the number of wireless stations with an upstream TCP flow in IEEE 802.11 wireless LANs. Thus, we propose a new scheme called TCP ACK priority (TAP) in which, by allowing an access point to transmit TCP ACKs at the highest priority, the optimal number of competing stations are allowed to contend for media access to utilize link bandwidth efficiently. We use an ns-2 simulator to evaluate the performance of TAP with the IEEE 802.11 DCF. The results show that there is an improvement in network performance without the loss of fairness between upstream TCP flows. The extensions for IEEE 802.11e/n are also considered.

Multicast is an efficient paradigm for transmitting data from a sender to a group of receivers. According to the IEEE 802.11 standard, the multicast service is defined as an unreliable service, that is, it does not include the use of ACK frames. Furthermore, different to the unicast service, the multicast service makes use of a single rate out of the various rates included in the basic service set defined by the IEEE 802.11 standard. Even though various proposals have recently appeared in the literature addressing these issues, none of them has come out with a structured set of control mechanisms taking into account the varying conditions characterising the wireless channels as well as the requirements of various applications. A novel cross-layer auto rate selection multicast mechanism for multi-rate wireless LANs, namely auto rate selection for multicast, capable of adapting the data transmission to the varying conditions of the channel and taking into account the characteristics of various applications, is introduced. The simulation results show that our proposal outperforms the IEEE 802.11 standard and the mechanisms recently proposed in the literature.

Multicast video streaming over multirate wireless LANs imposes strong demands on video codecs and the underlying network. It is not sufficient that only the video codec or only the underlying protocols adapt to changes in the wireless link quality. Research efforts should be applied in both and in a synchronized way. Cross layer design is a new paradigm that addresses this challenge by optimizing communication network architectures across traditional layer boundaries. This paper presents cross-layer architecture for adaptive video multicast streaming over multirate wireless LANs where layer-specific information is passed in both directions, top-down and bottom-up. The authors jointly consider three layers of the protocol stack: the application, data link and physical layers. The authors analyze the performance of the proposed architecture and extensively evaluate it via simulations. The results show that the real-time video quality of the overall system can be greatly improved by cross-layer signaling.

Abstract — The IEEE 802.11 protocol has become the de facto standard in wireless LANs. However, it reveals the unfairness problem between unicast flows and multicast flows since multicast packets are not subject to binary exponential backoff. To prevent a multicast flow from overwhelming the wireless link bandwidth is a crucial issue. This paper seeks to achieve fairness between unicast and multicast flows by introducing Unicast-Friendly Multicast (UFM). The central idea behind the UFM algorithm is to dynamically change the contention window size for multicast packets to limit the bandwidth share of a multicast flow equal to that of a unicast flow. The proposed UFM algorithm adjusts the multicast contention window size depending on the number of competing stations. We present two versions of UFM: the first one calculates the multicast window size by inferring the average contention window size of unicast flows, while the second one maintains the mapping table between the number of competing stations and the corresponding multicast window size given by related analysis. Simulation reveals that both versions of UFM achieve the fairness by providing almost the fair share of bandwidth to each flow regardless of unicast or multicast under the saturated network conditions. I.

International audience

IEEE 802.11 DCF exhibits poor scalability due to the large contention overhead. Therefore, the more the number of stations, the less the aggregate throughput. We propose a two-phase collision avoidance scheme to reduce the collision probability and to enhance the throughput performance. In our proposed scheme, contention among stations is resolved in two phases: SuperSlots and SubSlots. Also, our truncated backoff mechanism increases the throughput by reducing the idle time slots. We analyze the performance of our proposed scheme based on the previous analysis of 802.11 DCF. Both the analysis and simulation results exhibit that our proposed scheme achieves higher throughput than the current IEEE 802.11 backoff mechanism, and this differential increases with the number of stations in the network.

This paper introduces a new split-TCP approach for improving TCP performance over IEEE 802.11-based wireless LANs. TCP over wireless LANs is not aggressive, which is a fundamental reason for poor performance. We propose augmented split-TCP (AS-TCP) to mitigate this problem. Our scheme extends the split-connection approach that divides a connection into two different connections at a split point such as an access point. Using AS-TCP, a mobile host emulates TCP ACK packets using MAC ACK frames, instead of receiving real TCP ACK packets. We compared AS-TCP with both normal TCP and I-TCP (indirect TCP) by simulation. Results show that AS-TCP achieves higher throughput, fairer resource allocation and, in power-saving mode, shorter delays.

IEEE 802.11 MAC protocol in distributed coordination function (DCF) mode is not scalable as the number of contending users increases. In this paper, we propose two new contention resolution schemes to reduce the number of collisions due to a large number of contending users. The first scheme, hybrid DCF (H-DCF), splits the monolithic contention resolution into two phases to enhance scalability. The other scheme, enhanced hybrid DCF (EH-DCF), provides fairness with the current IEEE 802.11 MAC and yet improves the efficiency of contention resolution. The major contribution of our work is to provide inter-operability with the IEEE 802.11 DCF as well as an efficient channel access, unlike the previous works. We conduct comprehensive simulation experiments by using an ns-2 simulator to demonstrate the advantages of these two schemes. The results show that they improve network performance by up to 10~30%, compared to the original IEEE 802.11 DCF. Therefore, H-DCF and EH-DCF can be good candidates for MAC protocols for hotspots where numerous users access wireless LANs intensively

To join an IP network and communicate with others, a node needs to be configured either manually by an administrator or automatically through a DHCP server. However, the former method is impractical for large networks, while the latter is infeasible in the case of a mobile ad hoc network due to the mobility of the nodes. This paper introduces two distributed IP address auto-configuration mechanisms for mobile ad hoc networks, namely (a) RADA (random address allocation) and (b) LiA (linear allocation). RADA is based on random IP address selection, while LiA linearly assigns new addresses by utilizing the current maximum IP address value. We have also introduced an improved version of LiA, known as LiACR (linear allocation with collision resolution), which reduces control overhead. Then, we discuss extensions of these mechanisms capable of handling network partitioning and merging. Performance evaluations of RADA, LiA and LiACR were conducted through simulation. The results related to address allocation time and control overhead are presented and compared.

Security is very important during transmission of data in wireless networks. Bit errors frequently occur in wireless networks, which consequently create critical data losses as encrypted data is decrypted. The relation between bit errors and serviced quality after decryption about several encryption algorithms supported by IPv6 was researched. RC4 shows a higher PSNR than DES in CBC mode and AES in CBC mode when JPEG and MPEG multimedia data is transmitted.

The integration of 3G networks and WLAN as complementary has begun to attract much attention in industry as well as academia. This topic is becoming a burning issue, and one of the key questions which it raises is how to support a seamless vertical handoff. This paper introduces a new interface selection algorithm for energy-efficient vertical handoff in tightly coupled systems capable of supporting seamless handoff. Our proposed scheme, Wise Interface Selection (WISE) switches the active network interface, after taking into consideration the characteristics of the network interface cards and the current level of data traffic, with the cooperation of the mobile terminals and network. Interface switching operates independently on both the downlink and the uplink for the purpose of energy conservation. We show through simulation that less energy is consumed with WISE than when only a 3G network or WLAN interface is used, resulting in a longer lifetime for the mobile terminals. In the case of TCP connections, additional throughput gain can also be obtained.

In this paper, we propose a novel technique for reducing the energy consumption in idle mode. Our proposed technique called the network capacity adaptation derives an effective network capacity required by a given application and logically slows down the network capacity when the traffic load is low. By slowing down the logical network capacity, the neighboring mobile stations can stay longer in the power-off mode, significantly saving the energy consumed in idle mode.

Wireless local area network (LAN) has become popular due to the capability of supporting high data rates in ubiquitous computing. These high data rates are possible through new modulation schemes that are optimized for channel conditions. Especially, IEEE 802.11b supports the various data rates, 11 Mbps, 5.5 Mbps, 2 Mbps and 1 Mbps according to the channel conditions between the access point (AP) and the mobile station. The coverage area supported by the high data rate such as 11 Mbps is very small. However, the lower data rate increases the coverage area, thus the change of data rate can overcome this coverage problem of the high data rate. There are many wireless packet scheduling and buffer management algorithms, but they assume a wireless channel supporting only single base-rate and hardly take advantage of high data rate when multiple data rates are possible. In this paper, we propose the new queue management algorithm in order to utilize high data rate efficiently in the current wireless LAN. Basically, the proposed queue management algorithm manages the priority queue of packets, where packet with shorter transmission time has higher priority, since the transmission time is highly dependent on the available data-rate to the mobile station. We show our suggested algorithm achieves high throughput through the simulation in based in wireless LAN environments based on the IEEE 802.11b.

A traffic engineering problem consists of setting up paths between the edge nodes of the network to meet traffic demands while optimizing the network performance. It is known that total traffic throughput in a network, hence the resource utilization, can be maximized if the traffic demand is split over multiple paths. However, the problem formulation and practical algorithms, which calculate the paths and the traffic split ratio taking the route constraints or policies into consideration, have not been much touched. This paper proposes practical algorithms that find near optimal paths satisfying the given traffic demand under constraints such as maximum hop count, and preferred or not-preferred node/link list. The mixed integer programming formulation also calculates the traffic split ratio for the multiple paths. The problems are solved with the split ratio of continuous or discrete values. However, the split ratio solved with discrete values (0.1, 0.2 etc.) are more suitable for easy implementation at the network nodes. The proposed algorithms are applied to the multiprotocol label switching (MPLS) that permits explicit path setup. The paths and split ratio are calculated off-line, and passed to MPLS edge routers for explicit label-switched path (LSP) setup. The proposed schemes are tested in a large-scale fictitious backbone network. The experiment results show that the proposed algorithms are fast and superior to the conventional shortest path algorithm in terms of maximum link utilization, total traffic volume, and number of required LSPs.

This paper presents a new traffic engineering technique for dynamic constrained multicast routing, where the routing request of traffic arrives one-by-one. The objective we adopted is to minimize the maximum of link utilization. Although this traffic engineering is useful to relax the most heavily congested link in the Internet backbone, the total network resources, i.e. sum of link bandwidth consumed, could be wasted when the acquired path is larger (in terms of number of hops) than the conventional shortest path. Accordingly we find a multicast tree for routing request that satisfies the hop-count constraint. We formulate this problem as a mixed-integer programming problem and propose a new heuristic algorithm to find a multicast tree for multicast routing request. The presented heuristic algorithm uses the link-state information, i.e. link utilization, for multicast tree selection and is amenable to distributed implementation. The extensive simulation results show that the proposed traffic engineering technique and heuristic algorithm efficiently minimize the maximum of link utilization better than the shortest path.

Multipath routing employs multiple parallel paths between a traffic source and destination in order to relax the most heavily congested link in the Internet backbone. A large bandwidth path can be easily set up too. Although multipath routing is useful, the total network resources, i.e. sum of link bandwidths consumed, could be wasted when the acquired path is larger(in terms of number of hops) than the conventional shortest path. In addition, even though we can accommodate more traffic by establishing more paths between the same node pair, it is advised to limit the number of paths for practical reasons such as manageability. This paper presents a heuristic algorithm for hop-count and path count constrained dynamic multipath routing. The objective we adopted in this paper is to minimize the maximum of link utilization. We also obtain the traffic split ratio among the paths, for routers based on traffic partitioning by hashing at the flow level. The extensive simulation results show that the proposed algorithm always minimizes the maximum of link utilization and reduces the number of blocked requests.