Computer Networks - part IV(Wireless: Introduction & topologies)

A Wireless Network

Definition: A Group of interconnected nodes that exchange information and share resources through a wireless transmission medium

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TYPES OF WIRELESS NETWORKS

• Wireless PAN • Wireless LAN• Wireless Broadband • Wireless WAN (satellite , Microwave ,..etc)• Cellular Networks

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Broadband Wireless Technology

• Higher data rates obtainable with broadband wireless technology

– Graphics, video, audio

• Shares same advantages of all wireless services: convenience and reduced cost

– Service can be deployed faster than fixed service

– No cost of cable plant

– Service is mobile, deployed almost anywhere

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Characteristics of Wireless Transmission

• Similarities with wired– Layer 3 and higher protocols– Signal origination

• From electrical current, travel along conductor

• Differences from wired– Signal transmission

• No fixed path, guidance

• Antenna– Signal transmission and reception– Same frequency required on each antenna

• Share same channel

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Characteristics of Wireless Transmission (cont’d.)

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The Wireless Spectrum

• Continuum of electromagnetic waves– Data, voice communication– Arranged by frequencies

• Lowest to highest– Spans 9 KHz and 300 GHz

• Wireless services associated with one area• FCC oversees United States frequencies• ITU oversees international frequencies– Air signals propagate across borders

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The Wireless Spectrum (cont’d.)

The wireless spectrum

Antennas• Radiation pattern– Relative strength over three-dimensional area

• All electromagnetic energy antenna sends, receives

• Directional antenna– Issues wireless signals along single direction

• Omnidirectional antenna– Issues, receives wireless signals

• Equal strength, clarity• All directions

• Range– Reachable geographical area

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Signal Propagation• LOS (line-of-sight)– Signal travels

• In straight line, directly from transmitter to receiver

• Obstacles affect signal travel– Pass through them– Absorb into them– Subject signal to three phenomena

• Reflection: bounce back to source• Diffraction: splits into secondary waves• Scattering: diffusion in multiple different directions

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Signal Propagation (cont’d.)

• Multipath signals– Wireless signals follow different paths to

destination– Caused by reflection, diffraction, scattering– Advantage• Better chance of reaching destination

– Disadvantage• Signal delay

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Multipath signal propagation

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Signal Degradation• Fading– Change in signal strength

• Electromagnetic energy scattered, reflected, diffracted• Attenuation– Signal weakens

• Moving away from transmission antenna– Correcting signal attenuation

• Amplify (analog), repeat (digital)• Noise– Significant problem

• No wireless conduit, shielding

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Narrowband, Broadband, and Spread Spectrum Signals

• Defines wireless spectrum use:– Narrowband• Transmitter concentrates signal energy at single

frequency, very small frequency range

– Broadband• Relatively wide wireless spectrum band• Higher throughputs than narrowband

– Spread-spectrum• Multiple frequencies used to transmit signal• Offers security

Narrowband, Broadband, and Spread Spectrum Signals (cont’d.)

• FHSS (frequency hopping spread spectrum)– Signal jumps between several different frequencies

within band– Synchronization pattern known only to channel’s

receiver, transmitter• DSSS (direct-sequence spread spectrum)– Signal’s bits distributed over entire frequency band at

once– Each bit coded

• Receiver reassembles original signal upon receiving bits

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Fixed versus Mobile

• Fixed communications wireless systems– Transmitter, receiver locations do not move– Transmitting antenna focuses energy directly toward

receiving antenna• Point-to-point link results

– Advantage• No wasted energy issuing signals• More energy used for signal itself

• Mobile communications wireless systems– Receiver located anywhere within transmitter’s range

• Receiver can roam16

The Need for Wireless MANs• Traditional point-to-point and switched network

techniques used in WANs are inadequate for growing needs of organizations

• Need for high capacity and low costs over large area• MAN provides:– Service to customers in metropolitan areas– Required capacity– Lower cost and greater efficiency than equivalent service

from telephone company • Standards for Wireless MANS is WI-MAX

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Overview of WLAN Topologies

• Three types of WLAN Topologies:– Independent Basic Service Sets (IBSS)– Basic Service Set (BSS)– Extended Service Set (ESS)

• Service Set – A logical grouping of devices.• WLANs provide network access by broadcasting a signal across a wireless radio

frequency (Beaconning)• Transmitter prefaces its transmissions with a Service Set Identifier (SSID)• A station may receive transmissions from transmitters with the same or different

SSIDs. 18

Independent Basic Service Sets (IBSS)

• IBSS consists of a group of 802.11 stations directly communicating with each other.

• No Access Point used• Also known as an ad-hoc network.• Usage: Few stations setup up for a specific purpose for a short period of

time. (ex. file transfers.)• We will have a an IBSS lab, but our main focus will be BSSs and ESSs.

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Basic Service Set (BSS)

• BSS, also known as an Infrastructure BSS.• Requires an Access Point (AP)

– Converts 802.11 frames to Ethernet and visa versa– Known as a translation bridge

• Stations do not communicate directly, but via the AP• APs typically have an uplink port that connects the BSS to a wired network

(usually Ethernet), known as the Distribution System (DS).20

Extended Service Set (ESS)

• Multiple BSSs can be connected together with a layer 2 “backbone network” to form an Extended Service Set (ESS).

• 802.11 does not specify the backbone network• The backbone network is also known as the Distribution System (DS) and

could be wired or wireless.• Stations are “associated” with only one AP at a time.• The SSID is the same for all BSS areas in the ESS (unless creating multiple

BSSs, i.e. one for Marketing and another for Sales).21

• What if you want to be able to move between access points without the latency of re-association and re-authentication (these will be explained)?

• Roaming gives stations true mobility allowing them to move seamlessly between BSSs. (More later)

• APs need to be able to communicate between themselves since stations can only associate with one AP at a time.

• Currently, inter-access point communication can only be achieved with proprietary, non-standard technologies.

• IEEE 802.11 working group (Task Group F) is working on standardizing IAPP (Inter-Access Point Protocol)

Extended Service Set (ESS)

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Access Points

• Access Point (AP)– Translates (converts) 802.11 frames to Ethernet and visa versa– Known as a translation bridge– Typically provides wireless-to-wired bridging function– All BSS communications must go through the AP, even between two

wireless statsions

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Quick Preview: Station/AP Connectivity

SSID (Service Set Identity)• At a minimum a client station and

the access point must be configured to be using the same SSID.

• An SSID is:– Between 2 and 32 alphanumeric

characters– Spaces okay– Must match EXACTLY, including

upper and lower case– Sometimes called the ESSID– Not the same as BSSID (MAC

address of the AP)

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WLAN Interconnection.

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Association

• Packet exchanged between computer, access point– Gain Internet access

• Scanning– Surveying surroundings for access point– Active scanning transmits special frame• Probe

– Passive scanning listens for special signal• Beacon fame

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Association (cont’d.)

• SSID (service set identifier)– Unique character string identifying access point• In beacon fame information

– Configured in access point– Better security, easier network management

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Association (cont’d.)

• ESS with several authorized access points– Must allow station association with any access point

• While maintaining network connectivity

• Reassociation– Mobile user moves from one access point’s range into

another’s range– Occurs by simply moving, high error rate

• Stations’ scanning feature– Used to automatically balance transmission loads

• Between access points

Factors To Consider For Wireless LANSame as any LAN

High capacity, short distances, full connectivity, broadcast capabilityThroughput: efficient use wireless mediumNumber of nodes:Hundreds of nodes across multiple cellsConnection to backbone LAN: Use control modules to

connect to both types of LANsService area: 100 to 300 mLow power consumption:Need long battery life on mobile

stationsMustn't require nodes to monitor access points or frequent

handshakesTransmission robustness and security:Interference prone and

easily eavesdropped

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Factors To Consider For Wireless LAN

• Collocated network operation:Two or more wireless LANs in same area

• License-free operation• Handoff/roaming: Move from one cell to another• Dynamic configuration: Addition, deletion, and

relocation of end systems without disruption to users

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Wireless LAN Applications

• LAN Extension• Cross-Building Interconnect• Nomadic Access• Ad Hoc Networking

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Applications - LAN ExtensionSaves installation of LAN cablingEases relocation and other modifications to network

structureWireless LAN to replace wired LANs has not happenedIn some environments, role for the wireless LAN

Buildings with large open areasManufacturing plants, stock exchange trading floors,

warehousesHistorical buildingsSmall offices where wired LANs not economical

May also have wired LANServers and stationary workstations

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Applications – Cross-Building Interconnect

• Connect LANs in nearby buildings• Point-to-point wireless link• Connect bridges or routers• Not a LAN per se– Usual to include this application under heading of

wireless LAN

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Applications - Nomadic Access

• Link between LAN hub and mobile data terminal – Laptop or notepad computer– Enable employee returning from trip to transfer data

from portable computer to server• Also useful in extended environment such as

campus or cluster of buildings– Users move around with portable computers– May wish access to servers on wired LAN

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Applications – Ad Hoc Networking

• Peer-to-peer network• Set up temporarily to meet some immediate

need• E.g. group of employees, each with laptop or

palmtop, in business or classroom meeting• Network for duration of meeting

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Bluetooth Networks• Version 1.1

– Maximum theoretical throughput: 1 Mbps– Effective throughput: 723 Kbps– 10 meter node difference– Designed for PANs (personal area networks)

• Version 2.0 (2004)– Different encoding schemes

• 2.1-Mbps throughput– 30 meters node difference– Usage: cellular telephones, phone headsets, computer

peripherals, PDAs

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Bluetooth Networks• Ericson’s original goals– Wireless technology compatible with multiple devices– Require little power– Cover short ranges

• Aim of Bluetooth Special Interest Group (SIG) – Refine and standardize technology– Result: Bluetooth

• Mobile wireless networking standard using FHSS (frequency hopping spread spectrum) RF signaling in 2.4-GHz band