Wireless networks are increasingly used to carry applications with QoS constraints. Two problems arise when dealing with traffic with QoS constraints. One is admission control, which consists of determining whether it is possible to fulfill the demands of a set of clients. The other is finding an optimal scheduling policy to meet the demands of all clients. In this paper, we propose a framework for jointly addressing three QoS criteria: delay, delivery ratio, and channel reliability. We analytically prove the necessary and sufficient condition for a set of clients to be feasible with respect to the above three criteria. We then establish an efficient algorithm for admission control to decide whether a set of clients is feasible. We further propose two scheduling policies and prove that they are feasibility optimal in the sense that they can meet the demands of every feasible set of clients. In addition, we show that these policies are easily implementable on the IEEE 802.11 mechanisms. We also present the results of simulation studies that appear to confirm the theoretical studies and suggest that the proposed policies outperform others tested under a variety of settings.

http://cesg.tamu.edu/wp-content/uploads/2012/02/qos.pdf

A Theory of QoS for Wireless

Modern communication technologies are steadily advancing the physical layer (PHY) data rate in wireless LANs, from hundreds of Mbps in current 802.11n to over Gbps in the near future. As PHY data rates increase, however, the overhead of media access control (MAC) progressively degrades data throughput efficiency. This trend reflects a fundamental aspect of the current MAC protocol, which allocates the channel as a single resource at a time.This paper argues that, in a high data rate WLAN, the channel should be divided into separate subchannels whose width is commensurate with PHY data rate and typical frame size. Multiple stations can then contend for and use subchannels simultaneously according to their traffic demands, thereby increasing overall efficiency. We introduce FICA, a fine-grained channel access method that embodies this approach to media access using two novel techniques. First, it proposes a new PHY architecture based on OFDM that retains orthogonality among subchannels while relying solely on the coordination mechanisms in existing WLAN, carrier-sensing and broadcasting. Second, FICA employs a frequency-domain contention method that uses physical layer RTS/CTS signaling and frequency domain backoff to efficiently coordinate subchannel access. We have implemented FICA, both MAC and PHY layers, using a software radio platform, and our experiments demonstrate the feasibility of the FICA design. Further, our simulation results suggest FICA can improve the efficiency ratio of WLANs by up to 400% compared to existing 802.11.

/pdf/fine-grained-channel-access-in-wireless-lan-44ajxduwv9.pdf

Fine-grained channel access in wireless LAN

Isolation techniques for multiple co-located radio modules are disclosed. For example, an apparatus may include an antenna, a first transceiver to communicate wirelessly across a first link, a second transceiver to communicate wirelessly across a second link, a shared antenna structure operative to allow the first transceiver and the second transceiver to share the antenna for simultaneous operations, and an active signal canceller operative to generate a cancellation signal to cancel an interference signal for a radio-frequency coupling channel between the first and second transceivers. Other embodiments are disclosed and claimed.

Isolation techniques for multiple co-located radio modules

Conventional WiFi networks perform channel contention in time domain. This is known to be wasteful because the channel is forced to remain idle while all contending nodes are backing off for multiple time slots. This paper proposes to break away from convention and recreate the backing off operation in the frequency domain. Our basic idea leverages the observation that OFDM subcarriers can be treated as integer numbers. Thus, instead of picking a random backoff duration in time, a contending node can signal on a randomly chosen subcarrier. By employing a second antenna to listen to all the subcarriers, each node can determine whether its chosen integer (or subcarrier) is the smallest among all others. In fact, each node can even determine the rank of its chosen subcarrier, enabling the feasibility of scheduled transmissions after every round of contention. We develop these ideas into a Back2F protocol that migrates WiFi backoff to the frequency domain. Experiments on a prototype of 10 USRPs confirm feasibility, along with consistent throughput gains over 802.11. at high bit rates. Trace based simulations affirm scalability to larger, real-world network topologies.

/pdf/no-time-to-countdown-migrating-backoff-to-the-frequency-23a6e80ync.pdf

No time to countdown: migrating backoff to the frequency domain

With the advance of wireless local area network (WLAN) technology, handoff support has become one of the most important issues in IEEE 802.11 WLANs. However, the current IEEE 802.11 specification does not provide the fast handoff required for real-time multimedia applications. To support fast handoff in IEEE 802.11 networks, a number of fast-handoff schemes have been proposed in the literature. In this article we review these fast-handoff schemes and analyze their advantages and disadvantages qualitatively. After that, important design considerations for mobility support in future IEEE 802.11 networks are suggested. Also, we introduce a fast-handoff framework which adaptively meets different application requirements via a cross-layer approach.

Fast-handoff support in IEEE 802.11 wireless networks

With the growth of IEEE 802.11-based wireless LANs, VoIP and similar applications are now commonly used over wireless networks. Mobile station performs a handoff whenever it moves out of the range of one access point (AP) and tries to connect to a different one. This takes a few hundred milliseconds, causing interruptions in VoIP sessions. We developed a new handoff procedure which reduces the MAC layer handoff latency, in most cases, to a level where VoIP communication becomes seamless. This new handoff procedure reduces the discovery phase using a selective scanning algorithm and a caching mechanism.

/pdf/reducing-mac-layer-handoff-latency-in-ieee-802-11-wireless-3mt56xurpz.pdf

Reducing MAC layer handoff latency in IEEE 802.11 wireless LANs

In accordance with the present invention, computer implemented methods and systems are provided for reducing handoff latency in a wireless network. In response to detecting that a handoff is necessary, the present invention uses a selective scanning algorithm that includes the use of a channel mask and/or a caching algorithm for detecting one or more new access points.

Methods and systems for reducing MAC layer handoff latency in wireless networks

We measured the capacity for VoIP traffic in an 802.11b test-bed and compared it with the theoretical capacity and our simulation results. We identified factors that have been commonly overlooked in past studies but affect experiments and simulations. We found that in many papers the capacity for VoIP traffic has been measured via simulations or experiments without considering those factors, showing different capacity in each paper. After these corrections, simulations and experiments yielded a capacity estimate of 15 calls for 64 kb/s CBR VoIP traffic with 20 ms packetization interval and 34 calls to 36 calls for VBR VoIP traffic with 0.39 activity ratio.

Experimental Measurement of the Capacity for VoIP Traffic in IEEE 802.11 WLANs

In IEEE 802.11 wireless networks, the downlink delay rises as the number of VoIP nodes increases while the uplink delay remains small due to the same chance of media access between nodes and the Access Point (AP). This degrades the capacity and QoS of VoIP significantly. Therefore, we introduce Adaptive Priority Control (APC) to balance the downlink and uplink delay of VoIP traffic at the MAC layer, by giving to the AP a higher transmission priority, which is adaptively decided according to the uplink and downlink traffic volume. And, we verify through theoretical analysis that APC is an optimal method to balance the uplink and downlink delay.

/pdf/balancing-uplink-and-downlink-delay-of-voip-traffic-in-wlans-153kpcaoym.pdf

Balancing uplink and downlink delay of VoIP traffic in WLANs using Adaptive Priority Control (APC)

With the deployment of IEEE 802.11 networks, supporting real-time traffic with stringent quality of service (QoS) requirements on these networks becomes a critical issue. We propose two new media access schemes, namely dynamic point coordination function (DPCF) and modified DPCF (DPCF2). These can improve the capacity for voice-over-IP (VoIP) traffic by up to 20% in IEEE 802.11b networks. We show how we can also achieve a drastic improvement in the end-to-end delay with mixed VoIP and data traffic. Delay is kept around 100 ms in heavily loaded traffic conditions with an average value under 60 ms in normal traffic conditions.

/pdf/using-dynamic-pcf-to-improve-the-capacity-for-voip-traffic-rp3ckonxm3.pdf

Sangho Shin

Papers

Reducing MAC layer handoff latency in IEEE 802.11 wireless LANs

Methods and systems for reducing MAC layer handoff latency in wireless networks

Experimental Measurement of the Capacity for VoIP Traffic in IEEE 802.11 WLANs

Balancing uplink and downlink delay of VoIP traffic in WLANs using Adaptive Priority Control (APC)

Using dynamic PCF to improve the capacity for VoIP traffic in IEEE 802.11 networks