Class Index • Wireless LAN’s • Wireless LAN applications • Nomadic Access and Ad Hoc Networking • Wireless LAN requirements • Wireless LAN Technology • Infrared LAN’s • Strengths & Weaknesses
Class Index • Transmission Techniques • Spread Spectrum LAN’s • Configuration • Transmission Issues • Narrowband Microwave LAN’s • Licensed Narrowband RF • Unlicensed Narrowband RF
Wireless LANs • Wireless LANs are an indispensable adjunct to traditional wired LANs, • They satisfy requirements for: • mobility, • relocation, • ad hoc networking, and • coverage of locations difficult to wire.
Wireless LAN applications • Early wireless LAN products, were marketed as substitutes for traditional wired LANs • In a number of environments, there is a role for the wireless LAN as an alternative to a wired LAN.
Wireless LAN applications • Buildings with large open areas (manufacturing plants, stock exchange trading floors, warehouses). • Historical buildings with insufficient twisted pair and where drilling holes for new wiring is prohibited. • Small offices where installation and maintenance of wired LANs are not economical.
Wireless LAN applications • In all of these cases, a wireless LAN provides an effective and more attractive alternative. • In most of these cases, an organization will also have a wired LAN to support servers and some stationary workstations.
Typical Wireless LAN Configuration – Single Cell Wireless LAN • There is a backbone wired LAN, such as Ethernet, that supports servers, workstations and one or more bridges or routers to link with other networks. • There is a control module (CM) that acts as an interface to a wireless LAN
Typical Wireless LAN Configuration – Single Cell Wireless LAN • Some of the end systems are standalone devices, such as a workstation or a server. • 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.
Typical Wireless LAN Configuration – Multiple Cell Wireless LAN
Nomadic Access and Ad Hoc Network • Nomadic access provides a wireless link between a LAN hub and a mobile data terminal equipped with an antenna. • Users may move around with their portable computers and may wish access to the servers on a wired LAN from various locations. • An ad hoc network is a peer-to-peer network (not centralized server) set up temporarily to need some immediate need.
Nomadic Access and Ad Hoc Network Nomadic Access Ad Hoc Network
Wireless LAN requirements • Wireless LAN must meet the same sort of requirements typical of any LAN, including: • High capacity. • Ability to cover short distances. • Full connectivity among attached stations. • Broadcast capacity
Wireless LAN specific requirements • Throughput: The MAC protocol should make as efficient use as possible of the wireless medium to maximize capacity. • Number of nodes: Wireless LANs may need to support hundreds of nodes across multiple cells.
Wireless LAN specific requirements • Connection to backbone LAN: In most cases, interconnection with stations on a wired backbone LAN is required. • Service area: A typical coverage area for a wireless LAN has a diameter of 100 to 300 m. • Battery power consumption: Mobile workers use battery-powered workstations that need to have a long battery life when used with wireless adapters. • Typical wireless LAN implementations have features to reduce power consumption while not using the network, such as a sleep mode.
Wireless LAN specific requirements • Transmission robustness and security: Unless properly designed, a wireless LAN may be interference prone and easily eavesdropped. The design of a wireless LAN must permit reliable transmission even in a noisy environment and should provide some level of security from eavesdropping.
Wireless LAN specific requirements • Collocated network operations: As wireless LANs become more popular, it is quite likely for two or more wireless LANs to operate in the same area or in some area where interference between the LANs is possible. • License-free operation: Users would prefer to buy an operate wireless LAN products without having to secure a license for the frequency band used by the LAN.
Wireless LAN specific requirements • Handoff/roaming: The MAC protocol used in the wireless LAN should enable mobile stations to move from one cell to another. • Dynamic configurations: The MAC addressing and network management aspects of the LAN should permit dynamic and automated addition, deletion and relocation of end systems without disruption to other users.
Wireless LAN – Mobile Data Network – Wired LAN comparison Kiviat Graphs provides a pictorial means of comparing systems among multiple variables.
Wireless LAN Technology • Wireless LANs are generally categorized according to the transmission technique that is used. • All current wireless LAN products fall into one of the following categories: • Infrared (IR) LANs • Spread spectrum LANs • Narrowband microwave LANs
Wireless LAN Technologies – Key Characteristics  CSMA: Carrier Sense Multiple Access. Check IEEE 802.11 Class 11
Infrared LANs – Strengths • The two competing transmission media for wireless LANs are microwave radio, and infrared (IR).
Infrared LANs – Strengths • IR offer a number of significant advantages over the microwave radio approaches: • The spectrum of IR is virtually unlimited, which presents the possibility of achieving extremely high data rates. • The IR spectrum is unregulated worldwide, which is not true of some portions of the microwave spectrum. • IR shares some properties of visible light that make it attractive for certain types of LAN configurations.
Infrared LANs – Strengths • IR light is diffusely reflected by light-coloured objects; thus it is possible to use ceiling reflections to achieve coverage of an entire room. • IR light does not penetrate walls or other opaque objects. This has two advantages: • IR communications can be more easily secured against eavesdropping than microwave • A separate IR installation can be operated in every room in buildings without interference, enabling the construction of very large IR LANs.
Infrared LANs – Strengths • Equipment is relatively inexpensive and simple. (IR data transmissions usually use intensity modulation).
Infrared LANs - Weaknesses • Many indoor environments experience rather intense IR background radiation, from sunlight and indoor lighting. • This ambient radiation appears as noise in an IR receiver, requiring the use of transmitters of higher power than would otherwise be required and also limiting the range. • However, increases in transmitter power are limited by concerns of eye safety and excessive power consumption.
IR – Transmission Techniques • There are three alternative transmission techniques commonly used for IR data transmissions: • Direct beam IR • Omnidirectional • Diffused
IR – Transmission Techniques • Directed-beam IR can be used to create point-to-point links. • In this mode, the range depends on the emitted power and on the degree of focusing. • A focused IR data link can have a range of kilometres. • An IR link can be used for cross-building interconnect between bridges or routers located in buildings within a LOS of each other
IR – Transmission Techniques • An omnidirectional configuration involves a single BS that is within LOS of all other stations on the LAN. • Typically, this station is mounted on the ceiling. And acts as a multi-port repeater. • The ceiling transmitter broadcasts an omnidirectional signal that can be received by all of the other IR transceivers in the area. • These other transceivers transmit a directional beam aimed at the ceiling base unit.
IR – Transmission Techniques • In a diffused network, all of the IR transmitters are focused and aimed at a point and a diffusely reflecting ceiling. • IR radiation striking the ceiling is reradiated omni-directionally and picked up by all of the receivers in the area.
Spread Spectrum LANs • A SS wireless LAN makes use of a multiple cell arrangement. • Adjacent cells make use of different centre frequencies within the same band to avoid interference.
Spread Spectrum LANs • Within a given cell, the topology can be either hub or peer to peer. • In the former, the hub is typically mounted on the ceiling and connected to a backbone wired LAN to provide connectivity to stations attached to the wired LAN and to stations that are part of wireless LANs in other cells. • A peer to peer topology is one in which there is no hub. A MAC algorithm such as CSMA is used to control access. This topology is appropriate for ad hoc LANs.
Transmission Issues in Wireless LANs A desirable, though not necessary, characteristic of a wireless LAN is that it be usable without having to go through a licensing procedure. The licensing regulations differ from one country to another, which complicates this objective.
Narrowband Microwave LANs • The term “narrowband microwave” refers to the use of a microwave RF band for signal transmission, with a relatively narrow bandwidth (just wide enough to accommodate the signal). • Until recently, all narrowband microwave LAN products have used a licensed microwave band. More recently, at least one vendor has produced a LAN product in the ISM band.
Licensed Narrowband RF • Microwave radio frequencies usable for voice, data and video transmissions are licensed and coordinated within specific geographic areas to avoid potential interference between systems. • A narrowband scheme typically makes use of the cell configuration. • Adjacent cells use non-overlapping frequency bands within the overall band.
Licensed Narrowband RF • Licensed narrowband LAN guarantees interference-free communication. • Licensed spectrum gives the license holder a legal right to an interference free data communications channel. • Users of an ISM-band LAN are at risk of interference disrupting their communications, for which they may not have a legal remedy.
Unlicensed Narrowband RF • In 1995, “RadioLAN” became the first vendor to introduce a narrowband wirelss LAN using the unlicensed ISM spectrum. • This spectrum can be used for narrowband transmission at low power (<0.5 W). • The product operates at 10 Mbps in the 5.8 GHz band and has a range of 50 m in a semi-open office and 100 m in an open office.
Unlicensed Narrowband RF • The product makes use of a peer-to-peer configuration with an interesting feature: • As a substitute for a stationary hub, it automatically elects one node as the Dynamic Master, based on parameters such as location, interference, and signal strength. • The identity of the master can change automatically as conditions change. • The LAN also includes a dynamic relay functions, which allows each station to act as a repeater to move data between stations that are out of range of each other.