PAPER PRESENTATION

ON

BY

M.SUBHASHINI D.PAVANI 07L11A0547 07L11A0520 III B.Tech(CSE) III B.Tech(CSE)

Email:subbu.maddu547@gmail.com Email:hannu.subbu1@gmail.com

VRS &YRN COLLEGE OF ENGG & TECHChirala

ABSTRACT

Technology is no longer judged by its technical brilliance, but by the return on investment (both tangible and intangible). This in turn, is dictated by the killer application for that technology. Wireless Networks fit into this because the technology has been around long enough and can provide enough benefits to be seriously considered for deployment.

At the enterprise, it provides communication support for mobile computing. It overcomes and, in fact, annihilates the physical limitation of wired networks in terms of adaptability to a variation in demand. Network connectivity in a companys meeting room is a classic example. The number of users using that room would vary for different meetings. So, it would be difficult to decide how many wired network ports to put there. With wireless access, the number of users is mostly constrained by the bandwidth available on the wireless network.

Mobility is another feature by wireless. Mobile users can be truly mobile, in that hey dont need to be bound to their seats when connecting to the network. Mobility, however is not only associated with users, its also associated with the infrastructure itself. You can have a wireless network up and running in no time, a boon for people who need to do it for exhibitions, events, etc.

This leads to other provision of wireless, that of scalability. It really helps in extending your network. It also becomes important if an enterprise has a rented office and needs to shift to a new place. At home, the need for wireless is more to do with ubiquitous computing. Wi-Fi, or wireless fidelity, is freedom: it allows you to connect to the Internet from your couch at home, a bed in a hotel room, or a conference room at work without wires.

Wireless technology, therefore is really happening, and should be seriously considered. The following presentation explains introduction to Wi-Fi ,functionality, wireless LANs, their basic operations, topologies, security features, technical features , and answers some of the questions evaluating WLAN technology.

Introduction to Wi-Fi

Wireless Fidelity, better known as Wi-Fi, is a term

used to describe the underlying technology of

wireless local area networks (WLAN) based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards. Initially intended to connect mobile computing devices in local area networks (LAN), Wi-Fi applications have grown to include various data, voice, and video services such as Internet access and Voice over Internet Protocol (VoIP) [2, 3]. With the growing popularity of small portable devices, a wireless network connection proves more beneficial by minimizing expensive deployment costs, providing user mobility, and supporting high bandwidth and quality of service (QoS) needs compared with traditional wired LANs. In addition, various IEEE 802.11 standards now in development are aimed to increase the performance of Wi-Fi networks and to provide users with greater flexibility in wireless communications.

Wi-Fi technology platform is based on single-carrier direct-sequence spread spectrum (DSSS) and multi-carrier Orthogonal Frequency Division Multiplexing (OFDM) radio technologies to transmit and receive signals. The original version of the standard, now referred to as 802.11legacy, was released in 1997 and had specified a maximum data rate of 2 Megabits per second (Mbps), which was considered too slow to support many high-data rate applications (e.g., video telephony) [4, 5]. The IEEE 802.11 standards family consists of many amendments and service enhancements to the original standard, with the most popular being the a, b, and g standards. Although the 802.11a standard was the first standard created in the 802.11 family, the 802.11b standard became the first widely accepted wireless networking standard, later followed by 802.11a and 802.11g.

Wi-Fi is more commonly used in point-to-multipoint (PMP) environments to allow extended network connectivity (e.g., private/backbone network, Internet) of multiple portable devices such as laptops, telephones, or PDAs. Wi-Fi also allows connectivity in point-to-point (P2P) mode, which enables devices to directly connect and communicate to each other. A region covered by one or more APs is considered a hotspot. Home networks commonly deploy one AP that is typically connected directly to an Internet service provider (ISP), whereas larger networks (e.g., enterprises, small businesses) may require at least several APs positioned in strategic locations to provide flexibility of service to a large number of users [2]. Because of range constraints, Wi-Fi networks are used in localized regions. Figure 1 illustrates the components of a common Wi-Fi network.

Figure 1. Common Wi-Fi Network [6]

Wi-Fi can be used in conjunction with other emerging wireless technologies, such as Worldwide Interoperability for Microwave Access (WiMAX) and Wireless Mesh Networking, to extend the coverage area of terrestrial networks and to provide high-speed mobile data and telecommunications services. For example, WiMAX can be used as a backhaul technology to connect multiple Wi-Fi hotspots with each other and to other parts of the Internet [7]. WiMAX describes the technology behind wireless networks based on the IEEE 802.16 standards.

Wi-Fi also can be used to create a wireless mesh network, which is a decentralized, reliable, resilientand relatively inexpensive solution that can support areas of lacking or destroyed network infrastructure (e.g., connectivity for a field office or emergency command center). Wireless mesh networks consist of several nodes that act as repeaters to transmit data from nearby nodes to users located far away, resulting in networks that span large distances [8]. Table 1 provides a high-level comparison between Wi-Fi, WiMAX, and Wireless Mesh technologies.

Table 1. High-level Comparison of Emerging Wireless Broadband Technologies [3, 7, 8]

TechnologyWi-FiWiMAXWireless Mesh

Features WLANs (e.g., indoor, office, campus environment)

PMP mode, with each client connected to an AP; P2P mode, with each mobile user connected directly to each other

Can operate in line-of-sight and non line-of-sight situations

Supports fixed, portable and mobile communications Metropolitan area networks (MAN)

PMP and P2P capabilities

Can operate in line-of-sight and non line-of-sight situations

Supports fixed, portable, and mobile communications

Typically used as a backhaul to connect multiple Wi-Fi hotspots to external networks Peer-to-peer communications, with each mobile user acting as a client and AP

Self-organizing, self-healing, and auto-configuring

Typically uses wireless technologies in the unlicensed band, including Wi-Fi

Functionality

Access

Wi-Fi networks typically consist of one or more APs and one or more clients. An AP will broadcast its Network Name, also referred to as the Service Set Identifier (SSID), through data packets, called beacons, every 100 milliseconds (ms). Beacons are transmitted at a rate of 1 Mbps to ensure that the connected user receiving the beacon is actually provided an expected data rate of at least 1 Mbps [3, 5, 9].

The Wi-Fi standard enables users to decide whether to connect to an AP. The firmware installed in the Wi-Fi client adapter card also has an influence during the connection process. If two APs of the same SSID are in range of the client, the firmware may automatically decide which of the two APs it will connect to based on signal strength.

Media Access Control and Physical Layers

The 802.11a, 802.11b, and 802.11g Wi-Fi amendments all use the same Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) signaling method in the media access control (MAC) layer as defined in the original 802.11 standard. The difference between the three standards lies in the physical (PHY) layer and with the modulation technique used to transmit a signal. The 802.11b amendment employs complementary code keying (CCK), which is a direct extension of the DSSS modulation technique [10, 11]. Conversely, 802.11a uses OFDM technology, which results in an increase in channel availability and data rate performance [10]. 802.11g utilizes both techniques depending on the application need for a particular situation [12]. IEEE 802.11b Wireless Networking Overview

Approval of the IEEE 802.11 standard for wireless local area data rates have put the promise of truly mobile computing within reach. While wired LANs have been a mainstream technology for at least fifteen years, WLANs are uncharted territory for most networking professionals. In September of 1999, the Institute of Electrical and Electronic Engineers (IEEE) ratified the specification for IEEE 802.11b, also known as Wi-Fi. IEEE 802.11b defines the physical layer and media access control (MAC) sub layer for communications across a shared, wireless local area network (WLAN).

At the physical layer, IEEE 802.11b operates at the radio frequency of 2.45-gigahertz (GHz) with a maximum bit rate of 11 Mbps. It uses the direct sequence spread spectrum (DSSS) transmission technique. At the MAC sub layer of the Data Link layer, 802.11b uses the carrier sense multiple access with collision avoidance (CSMA/CA) media access control (MAC) protocol. A wireless station with a frame to transmit first listens on the wireless medium to determine if another station is currently transmitting (this is the carrier sense portion of CSMA/CA). If the medium is being used, the wireless station calculates a random back off delay. Only after the random back off delay elapses can the wireless station again listen for a transmitting station. By instituting a random back off delay, multiple stations that are waiting to transmit do not end up trying to transmit at the same time (this is the collision avoidance portion of CSMA/CA). Collisions can occur and, unlike with Ethernet, they might not be detected by the transmitting nodes. Therefore, 802.11b uses a Request to Send (RTS)/Clear to Send (CTS) protocol with an Acknowledgment (ACK) signal to ensure that a frame is successfully transmitted and received.

Wireless Networking Components

IEEE 802.11b wireless networking consists of the following components:

Stations: A station (STA) is a network node that is equipped with a wireless network device. A personal computer with a wireless network adapter is known as a wireless client. Wireless clients can communicate directly with each other or through a wireless access point (AP). Wireless clients are mobile.

Wireless AP: wireless AP is a wireless network node that acts as a bridge between STAs and a wired network. A wireless AP contains:

1. At least one interface that connects the wireless AP to an existing wired network (such as an Ethernet backbone).

2. A wireless network device with which it creates wireless connections with STAs.

3. IEEE 802.1D bridging software, so that it can act as a transparent bridge between the wireless and wired networks.

The wireless AP is similar to a cellular phone network's base station. Wireless clients communicate with both the wired network and other wireless clients through the wireless AP. Wireless APs are not mobile and act as peripheral bridge devices that extend a wired network.

Ports:

A port is a channel of a device that can support a single point-to-point connection. For IEEE 802.11b, a port is an association, a logical entity over which a single wireless connection is made. A typical wireless client with a single wireless network adapter has one port and can support only one wireless connection. A typical wireless AP has multiple ports and can simultaneously support multiple wireless connections. The logical connection between a port on the wireless client and the port on a wireless AP is a point-to-point bridged LAN segmentsimilar to an Ethernet-based network client that is connected to an Ethernet switch.

IEEE 802.11b Operating Modes (network topology)

IEEE 802.11 defines two operating modes: Ad hoc mode and Infrastructure mode. The basic topology of an 802.11 network is shown in Figure 1. A Basic Service Set (BSS) consists of two or more wireless nodes, or stations (STAs), which have recognized each other and have established communications. In the most basic form, stations communicate directly with each other on a peer-to-peer level sharing a given cell coverage area. This type of network is often formed on a temporary basis, and is commonly referred to as an ad hoc network, or Independent Basic Service Set (IBSS) .

In most instances, the BSS contains an Access Point (AP). When an AP is present, stations do not communicate on a peer-to-peer basis. All communications between stations or between a station and a wired network client go through the AP. APs are not mobile, and form part of the wired network infrastructure. A BSS in this Configuration is said to be operating in infrastructure mode.

The Extended Service Set (ESS) shown in Figure 2 consists of a series of overlapping BSSs (each containing an AP) connected together by means of a Distribution System (DS). Although the DS could be any type of network, it is almost invariably an Ethernet LAN. Mobile nodes can roam between APs and seamless campus-wide coverage is possible.

IEEE 802.11b Operation Basics

When a wireless adapter is turned on, it begins to scan across the wireless frequencies for wireless APs and other wireless clients in ad hoc mode. Assuming the wireless client is configured to operate in infrastructure mode, the wireless adapter chooses a wireless AP with which to connect. This selection is made automatically by using SSID and signal strength and frame error rate information. Next, the wireless adapter switches to the assigned channel of the selected wireless AP and negotiates the use of a port. This is known as establishing an association.

If the signal strength of the wireless AP is too low, the error rate too high, or if instructed by the operating system (in the case of Windows XP), the wireless adapter scans for other wireless APs to determine whether a different wireless AP can provide a stronger signal or lower error rate. If such a wireless AP is located, the wireless adapter switches to the channel of that wireless AP and negotiates the use of a port. This is known as reassociation.

Reassociation with a different wireless AP can occur for several reasons. The signal can weaken as either the wireless adapter moves away from the wireless AP or the wireless AP becomes congested with too much traffic or interference. By switching to another wireless AP, the wireless adapter can distribute the load to other wireless APs, increasing the performance for other wireless clients.

Radio Technology in 802.11

IEEE 802.11 provides for two variations of the PHY. These include two (2) RF technologies namely Direct Sequence Spread Spectrum (DSSS), and Frequency Hopped Spread Spectrum (FHSS). The DSSS and FHSS PHY options were designed specifically to conform to FCC regulations (FCC 15.247) for operation in the 2.4 GHz ISM band, which has worldwide allocation for unlicensed operation.

DSSS systems use technology similar to GPS satellites and some types of cell phones. Each information bit is combined via an XOR function with a longer Pseudo-random Numerical (PN) sequence as shown in Figure 3. The result is a high speed digital stream which is then modulated onto a carrier frequency using Differential Phase Shift Keying (DPSK).

When receiving the DSSS signal, a matched filter correlator is used as shown in Figure 4.The correlator removes the PN sequence and recovers the original data stream. Tat the higher data rates of 5.5 and 11 Mbps, DSSS receivers employ different PN codes and a bank of correlators to recover the transmitted data stream. The high rate modulation method is called Complimentary Code Keying (CCK). The effects of using PN codes to generate the spread spectrum signal are shown in Figure 5.

As shown in Figure 5a, the PN sequence spreads the transmitted bandwidth of the resulting signal (thus the term, spread spectrum) and reduces peak power. Note however, that total power is unchanged. Upon reception, the signal is correlated with the same PN sequence to reject narrow band interference and recover the original binary data (Fig. 5b). Regardless of whether the data rate is 1, 2, 5.5, or 11 Mbps, the channel bandwidth is about 20 MHz for DSSS systems. Therefore, the ISM band will accommodate up to three non-overlapping channels.

Multiple Access

The basic access method for 802.11 is the Distributed Coordination Function (DCF) which uses Carrier Sense Multiple Access / Collision Avoidance (CSMA / CA). This requires each station to listen for other users. If the channel is idle, the station may transmit. However if it is busy, each station waits until transmission stops, and then enters into a random back off procedure. This prevents multiple stations from seizing the medium immediately after completion of the preceding transmission.

Packet reception in DCF requires acknowledgement as shown in Figure 7. The period between completion of packet transmission and start of the ACK frame is one Short Inter Frame Space (SIFS). ACK frames have a higher priority than other traffic. Fast acknowledgement is one of the salient features of the 802.11 standard, because it requires ACKs to be handled at the MAC sub layer.

The underlying assumption is that every station can hear all other stations. This is not always the case. Referring to Figure 8, the AP is within range of the STA-A, but STA-B is out of range. STA-B would not be able to detect transmissions from STA-A, and the probability of collision is greatly increased. This is known as the Hidden Node.To combat this problem, a second carrier sense mechanism is available. Virtual Carrier Sense enables a station to reserve the medium for a specified period of time through the use of RTS/CTS frames.

3.4Wi-Fi Technical Features

The primary standards of Wi-Fi technology have been developed from the IEEE standards committees to provide wireless capabilities that include various technical features.

The 802.11 a/b/g standards support some measure of security using mechanisms such as Wired Equivalent Privacy (WEP) and Wi-Fi Protected Access (WPA), as developed by the Wi-Fi Alliance. The 802.11i standard is designed to offer increased security on wireless interfaces with hardware assistance. The 802.11a standard supports wireless access in the 5 GHz frequency range, whereas the 802.11b/g standards support the unlicensed band 2.4 GHz. While the 802.11b/g standards can support wireless access up to about 300 feet indoors and 1000 feet outdoors, the 802.11a standard supports wireless access at lesser range of up to about 225 feet. Additionally, both the 802.11a and 802.11g standards can support high-speed data rates up to about 54 Mbps, while the 802.11b standards supports data speed up to 11 Mbps [4, 10, 16].

Wi-Fi was originally designed for best-effort services. To enable QoS techniques in Wi-Fi networks, the IEEE 802.11 committee passed the 802.11e specification in September 2005 to define changes to the operation of the 802.11 MAC layer that enables prioritization and classes of service [14]. Table 2 summarizes those technical features for the various Wi-Fi technologies.

Table 2. High-Level Comparison of Wi-Fi Technologies [3, 9]

Technical FeaturePrimary Wi-Fi Technology

802.11a802.11b802.11g

Security WEP, WPAWEP, WPAWEP, WPA

Frequency Band/

Channel Modulation5 GHz/

OFDM2.4 GHz /

11 Channels, DSSS with CCK2.4 GHz/

OFDM above 20Mbps, DSSS with CCK below 20Mbps

Range and Coverage

(commonly advertised)shorter range than 802.11b (~225 feet)300 feet indoors)

1000 feet (outdoors)300 feet (indoors)

1000 feet (outdoors)

Data Rate Up to 54MbpsUp to 11MbpsUp to 54Mbps

Quality of Service (QoS) Supports 802.11eSupports 802.11eSupports 802.11e

ScalableYesYesYes

CompatibilityNot interoperable with 802.11b/gInteroperable with 802.11g. Not interoperable with 802.11aInteroperable with 802.11b. Not interoperable with 802.11a

The features highlighted in Table 2 are common across all 802.11 wireless technologies. each 802.11 attribute mentioned above, along with additional focus on security and QoS. Conclusion:

The use of wireless LANs is expected to increase dramatically in the future as businesses discover the enhanced productivity and the increased mobility that wireless communications can provide in a society that is moving towards more connectionless connections.

In conclusion, the panelists felt that hurdles in deploying WLANs can be overcome. Cost of wireless services are already falling. The issue is now to lower the costs of the device that is needed to access the WLAN. Lar2__hop design companies can make use of this opportunity to get into the market place. And Wi-Fi cannot move ahead quickly without support form private and government sectors

Bibliography:

1. Data over wireless networks-Gilbert Held

2. Electronics for you (magazine) June 2003 & February 2003

3. Electronics today (magazine) March 2003

4. A Technical tutorial on the IEEE 802.11 protocol - Pablo Brenner