Chapter 8: Wireless Services Part 2: Wireless LAN Computer Data Communications

Introduction Overview of Wireless LANs Key Application Areas Wireless LAN requirements IEEE 802.11 Architecture IEEE 802.11 Terminology IEEE 802.11 Services IEEE 802.11 Medium Access Control (MAC) IEEE 802.11 Protocol Architecture IEEE 802.11 Physical Layers Wireless LAN Access Point Wireless LAN Access Point/ Client Features Wireless LAN Applications

Overview of Wireless LANs A wireless local area network (LAN) is a flexible data communications system implemented as an extension to, or an alternative for, a wired LAN Using radio frequency (RF) technology, wireless LANs transmit and receive data over the air, minimizing the need for wired connections. Issues of high prices, low data rates, occupational safety concerns, & licensing requirements now addressed Key application areas: LAN extension cross-building interconnect nomadic access ad hoc networking

LAN extension Originally targeted to reduce cost of wiring, but new buildings now have sufficient wiring in place Still useful in buildings where wiring is problematic buildings with large open areas, historical buildings with insufficient twisted pair small offices wired LANs are not economical Typically, a wireless LAN will be linked into a wired LAN on the same premises

Single-Cell LAN extension In addition, there is a control module (CM) that acts as an interface to a wireless LAN. The control module (CM) includes either bridge or router functionality to link the wireless LAN to the backbone. It includes some sort of access control logic, such as a polling or token-passing scheme, to regulate the access from the end systems. Figure 1 illustrates the Single-Cell LAN extension

Single-Cell LAN extension Hubs or other user modules (UMs) that control a number of stations off a wired LAN may also be part of the wireless LAN configuration. This configuration can be referred to as a single-cell wireless LAN; all of the wireless end systems are within range of a single control module.

Single-Cell LAN Extension Figure 1: Single-Cell LAN Extension

Multi-Cell LAN extension A multiple-cell wireless LAN, there are multiple control modules interconnected by a wired LAN. Each control module supports a number of wireless end systems within its transmission range. For example, with an infrared LAN, transmission is limited to a single room; therefore, one cell is needed for each room in an office building that requires wireless support. Figure 2 illustrates the Multi-Cell LAN extension

Multi-Cell LAN Extension Figure 2: Multi-Cell LAN Extension

Cross-building interconnect Connect LANs in nearby buildings, be they wired or wireless LANs Point-to-point wireless link is used between two buildings (e.g. two microwave or infrared transmitter/receiver units can be placed on the rooftops of two buildings within the line of sight of each other) Devices are typically bridges or routers.

Nomadic Access Provides a wireless link between a LAN hub and a mobile data terminal (e.g. laptop computer) Figure 3 illustrates the Infrastructure Wireless LAN Examples Enable an employee returning from a trip to transfer data from a personal portable computer to a server in the office. Access in an extended environment such as a campus or a business operating out of a cluster of buildings. In both of these cases, users may wish access to the servers on a wired LAN from various locations.

Infrastructure Wireless LAN Nomadic Access :- Infrastructure Wireless LAN Figure 3: Infrastructure Wireless LAN

Ad hoc networks A peer-to-peer network (no centralized server) set up temporarily to meet some immediate need For example, a group of employees, each with a laptop or palmtop computer, may convene in a conference room for a business or classroom meeting. The employees link their computers in a temporary network just for the duration of the meeting. Figure 4 illustrates the Adhoc Networks

Ad hoc networks Figure 4: Adhoc Networks

Wireless LAN Requirements Efficient throughput Support for multiple nodes Connection to backbone LAN Broad service area (~ 100-300m) Allows for reduced power consumption while not using the network (e.g. sleep mode) Transmission robustness and security Co-located network operation License-free operation Handoff/roaming Dynamic and automated addition, deletion, and relocation

Wireless LAN Technology Infrared (IR) LANs Individual cells are limited to a single room, because infrared light does not penetrate opaque walls Spread spectrum LANs In most cases, these LANs operate in the ISM (Industrial, Scientific, and Medical) bands so that no FCC licensing is required for their use in the U.S. Narrowband microwave Do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands

IEEE 802.11 Architecture Figure 5: IEEE 802.11 Architecture

IEEE 802.11 Architecture IEEE has defined the specifications for a wireless LAN, called IEEE 802.11, which covers the physical and data link layers. The standard defines two kinds of services; the basic set (BSS) and the extended service set (ESS) Figure 5 illustrates the IEEE 802.11 Architecture.

IEEE 802.11 Architecture - BSS IEEE 802.11 defines the basic service set (BSS) as the building block of a wireless LAN. A basic service set is made of stationary or mobile wireless stations and optional central base station, known as the access point (AP). The BSS without an AP is a stand-alone network and cannot send data to other BSSs. It is called an ad hoc architecture. A BSS with an AP is sometimes referred to an infrastructure network. Figure 6 illustrates the BSS.

IEEE 802.11 Architecture - BSS Figure 6: Basic service sets (BSSs)

IEEE 802.11 Architecture - ESS An extended service set (ESS) is made up of two or more BSSs with APs. In this case, the BSSs are connected through a distribution system, which is usually a wired LAN. The distribution system connects the APs in the BSSs. IEEE 802.11 does not restrict the distribution system; it can be any IEE LAN such as an Ethernet.

IEEE 802.11 Architecture - ESS The extended service set (ESS) uses two types of stations: mobile and stationary. The mobile stations are normal stations inside a BSS. The stationary stations are AP stations that are part of a wired LAN. When BSSs are connected, the stations within reach of one another can communicate without the use of an AP. However, communication between two stations in two different BSSs usually occurs via two APs. Figure 7 illustrates an ESS

IEEE 802.11 Architecture - ESS Figure 7: Extended service sets (ESSs)

IEEE 802.11 Terminology

IEEE 802.11 Services The IEEE 802.11 services are listed below: Association Reassociation Disassociation Authentication Deauthentication Privacy

IEEE 802.11 Services Association: Reassociation: Establishes an initial association between a station and an AP. Before a station can transmit or receive frames on a wireless LAN, its identity and address must be known. For this purpose, a station must establish an association within an AP within a particular BSS. The AP can then communicate this information to other Aps within the ESS to facilitate routing and delivery of frames. Reassociation: Enables an established association to be transferred from one AP to another, allowing a mobile station to move from one BSS to another.

IEEE 802.11 Services Disassociation: Authentication: A notification from either a station or an AP that an existing association is terminated. A station should give this notification before leaving an ESS or shutting down. Authentication: Used to establish the identity of a stations to each other. For a wireless LAN, in which connectivity is achieved simply by having an attached antenna that is properly tuned. The authentication service is used by stations to establish their identity with stations they wish to communicate with.

IEEE 802.11 Services Deauthentication: Privacy: This service is invoked whenever an existing authentication is to be terminated. Privacy: Used to prevent the contents of messages from being read by other than the intended recipient. The standard provides for the optional use of encryption to assure privacy.

IEEE 802.11 Medium Access Control (MAC) MAC (Medium Access Control) layer covers three functional areas: Reliable data delivery Access control Security

IEEE 802.11 Medium Access Control Reliable Data Delivery Basic data transfer mechanism involves an exchange of two or four frames (data, ACK, and optional CTS/RTS) Access Control Used DFWMAC (distributed foundation wireless MAC) that provides a distributed access control mechanism with an optional centralised control built on top of that. Figure 8 illustrates the IEEE 802.11 Protocol Architecture. Security The prevention of unauthorised access using wireless network.

IEEE 802.11 Protocol Architecture Figure 8: IEEE 802.11 Protocol Architecture

IEEE 802.11 Physical Layer Figure 9: IEEE 802.11 Physical Layer

IEEE 802.11 Physical Layer The physical layer for IEEE 802.11 has been issued in four stages: - 802.11 (1997) MAC layer and three physical layer specifications; two 2.4-GHz band, one infrared, all operating at 1 and 2 Mbps IEEE 802.11a (1999) operates in the 5-GHz band at up to 54 Mbps IEEE 802.11b (1999) operates in the 2.4-Ghz band at 5.5 and 11 Mbps. IEEE 802.g (2002) extends IEEE 802.11b to higher data rates

Original 802.11 Physical Media Definitions Direct-sequence spread spectrum (DSSS) operating in the 2.4 GHz ISM band, at data rates of 1 Mbps and 2 Mbps Frequency-hopping spread spectrum (FHSS) operating in the 2.4 GHz ISM band, at data rates of 1 Mbps and 2 Mbps Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nm All of the original 802.11 products were of limited utility because of the low data rates

IEEE 802.11b Extension of the IEEE 802.11 DSSS scheme, providing data rates of 5.5 and 11 Mbps (higher data rate is achieved with more complex modulation) Apple Computer was first, with AirPort wireless networking, followed by other vendors Wireless Ethernet Compatibility Alliance created to certify interoperability for 802.11b products

Problems with 802.11 and 802.11b Original 802.11 and 802.11b may interfere with other systems that operate in the 2.4-GHz band Bluetooth HomeRF other devices--including baby monitors and garage door openers Limited data rate results in limited appeal

Higher-Speed 802.11 Options 802.11a 802.11g Uses 5-GHz band. Uses orthogonal frequency division multiplexing (OFDM) rather than spread spectrum Possible data rates are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps 802.11g Higher-speed extension to IEEE 802.11b. Combines physical layer encoding techniques used in 802.11a and 802.11b to provide service at a variety of data rates

Wireless LAN Access Point Access Point is a transceiver device which connects to the wired network from a fixed location. It receives, buffers and transmit data between the WLAN and the wired network infrastructure. It supports 15-50 client devices and can function within a range of less than a hundred to several hundred feet End users access the wireless LAN through PC cards.

Wireless LAN Access Point /Client Features Roaming Provides mobility within a subnet, across subnets or across SSID subnets Protect against AP failure, client PCs will detect link loss and roam to alternate AP. Best AP selection PC Clients should scan all channels for the best AP and rescan periodically and move to a better AP if possible.

Wireless LAN Access Point /Client Features Load balancing Load balancing provides the capability for PC clients to improve network performance by switching to the least utilized Access Point PC client driver should periodically look for a better AP during network idle The AP needs to monitor utilization and signal clients to discourage association and encourage roaming during sustained periods of high utilization

Wireless LAN Access Point /Client Features Network traffic filtering Used in AP to eliminate unnecessary traffic to improve network performance. Management To provide capability to centrally monitor and manage an enterprise wide wireless network. (SNMP protocol support)

Wireless LAN – Applications Home Home networking of computers and internet applicances Small Office Home Office (SOHO) Ease of installations and re-location, eliminate wiring cost, scalability Public Hot-spots High speed wireless internet access at hot spots such as hotels, airports, conference centre and other areas while travelling

Wireless LAN - Applications Enterprise Extension to existing infrastructure network, provide constant connectivity to corporate facilities while supporting users mobility. E.g. University, wireless internet and intranet access across the campus.