Sharif Digital Repository

Wireless LANs are becoming widespread in enterprises due to extensive usage of mobile devices and appearance of bring-your-own-device policies. However, mechanisms common in home WLANs are not sufficient for managing lots of access point and clients in an enterprise WLAN and WLAN controllers which are used in enterprises nowadays are limited to a fixed set of commands operated from a CLI and the source code is not changeable. Also, their effectiveness can be harvested alongside access points from the same vendor. Thus, dynamic high level programming for whole network configuration is an absolute need in this changing business environment and Software-defined Networking (SDN) will empower us... 

In this thesis, we propose an analytical model to evaluate the maximum stable throughput of a cognitive IEEE 802.11-based WLAN overlaid on a cellular network. In the considered scenario, the secondary nodes operate in uplink frequency band of primary network and transmit on their own idle detected primary channels using OFDM technique. The activity of primary nodes is modeled independently by ON-OFF alternating states with negative exponential distributed durations. The main feature of the proposed analytical model is to include different status of the secondary users, i.e., different spectrum opportunities for different cognitive users, as the result of dynamic nature of primary users as... 

In this thesis all the information about different techniques and methods used in Local Area Network (LAN) are described and organized as follows:
Chapter 1: The network properties are described including Network definitions and different types of LAN and their relative applications
Chapter 2 :Different techniques and protocols of data transfer and switching methods are being described and compared.
Chapter 3 : Network technology consists of different topologies for physical connections and various transfer Medias are covered.
Chapter 4 :All network protocols are described including different methods of DATA access in LOOP, RING and BUS topologies and the ISO and IEEE... 

On the demand of higher directivity, higher throughput and less interference, phased array systems and MIMO have become attractive to reach these goals. The advantages of phased array systems include the ability to perform electronic beam steering and arbitrary pattern synthesis. Phase shifters and variable gain amplifiers are used to reach these advantages. In this thesis, a ‘’ C‘’ band (4.8 GHz-6.2 GHz) vector-sum phase shifter using HMIC technology for realizing a phased array receiver is designed. This phase shifter can change the signal phase and amplitude. The main blocks of this phase shifter are low-insertion phase and low-noise variable gain amplifier, which consists of current... 

Fair allocation of bandwidth between stations of a shared medium, is an important and challenging problem. It is even more challenging when there is no coordinator to arbitrate access to the medium and therefore, a distributed algorithm is needed to provide fairness. IEEE 802.11 DCF is the dominant distributed medium access control (MAC) protocol in a wireless LAN. However, DCF is not in general fair and suffers from short-term unfairness. In addition to that, it is desired to provide differentiated service for the stations of a LAN based on their demands. Since priority based protocols may lead to starvation of the low priority traffic, we are more interested in fair queuing. On the other... 

Infrared wireless local area networks are a suitable choice for a fast and cheap wireless network, providing rates over 10 Mbps with access to unregulated bandwidth. Among multiple-access techniques for infrared wireless networks, CDMA1 has attracted the most interest, due to its simple structure and power ecency. However, spreading the spectrum in CDMA prohibits exploiting the low-rate strong channel codes. The other limiting factor in infrared networks is the channel dispersion. In this thesis, these two issues are considered and an internally coded scheme for time-hopping CDMA, suitable for wireless infrared communication is proposed and with accurate modelling of infrared dispersive... 

There has been a rapidly growing interest in deployment of Wireless LAN Networks (WLANs) in the past decade. The cost of WLAN devices has steadily decreased while their performance has increased. As a result, a wide range of new connectivity services are using WLAN standards for data exchange between nodes and networks through WLAN access points. Deployment of many WLAN access points in enterprise applications causes severe degradation of performance due to interference between access point channels. Therefore, it is wise to control the access point channel assignment procedure to reduce interference and improve network performance. This process can be done in using centralized or... 

According to the rapid growing in communication devices, users need higher rates for their usages. Fiber optics is one the major choices to support such rates. One of the simplest and cheapest networks is Local Area Network (LAN) as a Passive Optical Network (PON). In these networks, some computers are locally connected to a passive optical device and communicate through it. Multiple access to a shared channel is an important task in such a network. In this thesis, Code Division Multiple Access (CDMA) is used and a new algorithm for Multiple Access Control Sublayer (MAC) based on scheduling will be developed to improve Quality of Service (QoS) and stabilize the network.
In the scheduling... 

The increasing demand for high speed communications has made the optical fiber a favorite choice for different networks ranging from Local Area Networks (LANs) to Wide Area Networks (WANs). In this research we focus on the analysis and design of improved optical communication techniques with a focus on LAN applications.Noting the advantages of Optical Code Division Multiple Access (OCDMA), we have selected this method as the base of data transmission. Different modulation schemes can be implemented in an OCDMA system. Among these, Pulse Position Modulation (PPM) schemes can improve the system performance. We have evaluated Overlapping PPM (OPPM) in an OCDMA system employing Generalized... 

In this thesis, we evaluate the maximum stable throughput of a cognitive multi-hop IEEE 802.11-based WLAN overlaid on a cellular network. In the considered scenario, the secondary users operate in the downlink or uplink frequency band of primary network and transmit their data in free primary channels using OFDM technique. The activity of primary nodes is modeled independently by ON-OFF alternating states. To model the scenario, each secondary user is modeled with an open queueing network. The queueing network can model the transmission of data as well as the effect of primary users. By writing the traffic equations of the queueing network and applying the stability conditions, we are able... 

In this thesis the effect of frame aggregation mechanism in maximizing throughput in Wireless Local Area Networks (WLANs) have been investigated. Frame aggregation has become a part of the IEEE 802.11 standard as the amendment IEEE 802.11n. First, we have a quick review on access control layer in IEEE 802.11 standard then illustrate the necessity of frame aggregation. Afterwards, we propose an analytical model for the analysis of an IEEE 802.11n network comprised of an Access Point (AP) and several conventional nodes, all in the coverage area of each other. This analytical model focuses on downlink throughput; therefore only AP uses frame aggregation and the other nodes use basic IEEE 802.11... 

We study an LTE-WiFi-Aggregation (LWA) network where some of the users support the LWA function, first standardized in 3GPP Rel. 13, and can simultaneously receive data from a WiFi access point and a cellular base station. In LWA networks, a base station can communicate with one or more Wi-Fi access points and if need be, it can transmit some of the user packets through the access points and their Wi-Fi networks. We consider an LWA network where some of the users are in the coverage of a base station and an access point, and support the LWA function. We aim to provide a scheduling policy for routing the LWA packets such that their expected delay is minimized. Three methods are proposed for... 

The activity status of primary users in a cognitive network scenario is a key parameter in saturation throughput of the secondary users. In this respect, usually a simple on-off traffic model with exponentially distributed intervals is exploited. For time-scheduled users in modern common standards like WiMAX or LTE this traffic model is not valid anymore. In this thesis, we focus on the saturation throughput analysis of a cognitive network scenario, comprised of a WLAN overlaid on a network with time-scheduled primary users like WiMAX. In this scenario, the cognitive nodes (CNs) hear downlink map (DL-MAP) and thus, know the frequency and time locations of all allocated slots at each downlink...