2 MediaThe world of computer networks and data communications would not exist if there were no medium by which to transfer data.The “media” is the substance through which the signal passes.The two major categories of media are:Conducted (guided) mediaThe message flows through a physical mediaWireless (unguided) mediaThe message is broadcast through space
3 Conducted media Twisted pair wire (UTP): Coaxial cable: Insulated pairs of wires, twisted to minimize electromagnetic interference between wiresCoaxial cable:Wire with a copper core and an outer cylindrical shell for insulationFiber optic cable:High speed streams of light pulses from lasers or LEDs carried inside hair-thin strands of glass or plastic
4 Twisted Pair WireOne or more pairs of single conductor wires that have been twisted around each otherTwisted pair wire is classified by categoryCategory 1 through Category 7NOTE: Categories 2 and 4 are obsoleteTwisting the wires helps to eliminate electromagnetic interference between the two wiresShielding can further help to eliminate interference
6 Coaxial CableA single wire wrapped in a foam insulation surrounded by a braided metal shield, then covered in a plastic jacket. Cable can be thick or thinBaseband coaxial technology uses digital signaling (DC) in which the cable carries only one channel of digital dataBroadband coaxial technology transmits analog signals (RF) and is capable of supporting multiple channels of data
7 Fiber Optic Cable (I)A thin glass cable approximately a little thicker than a human hair surrounded by a plastic coating and packaged into an insulated cableA photo diode or laser generates pulses of light which travel down the fiber optic cable and are received by a photo receptor
8 Fiber-Optic Cable (II) Fiber-optic cable is capable of supporting millions of bits per second for 1000s of meters.Fiber-optic cable is susceptible to reflection (where the light source bounces around inside the cable) and refraction (where the light source passes out of the core and into the surrounding cladding).Thus, fiber-optic cable is not perfect either. Noise is still a potential problem.Thick cable (62.5/125 microns) causes more ray collisions, so you have to transmit slower. This is step index multimode fiber. Typically use LED for light source, shorter distance transmissions.Thin cable (8.3/125 microns) – very little reflection, fast transmission, typically uses a laser, longer transmission distances; known as single mode fiber.
11 Wireless MediaRadio, satellite transmissions, and infrared light are all different forms of electromagnetic waves used to transmit data.Radio:Uses same basic principles of standard radio transmission.Microwave:Extremely high frequency radio communication beam transmitted on direct line-of-sight path.Infrared:Low frequency light waves carry data through the air on direct line-of-sight path.
13 Terrestrial microwave Land-based, line-of-sight transmissionApproximately miles between towersTransmits data at hundreds of millions of bits per secondSignals will not pass through solid objectsPopular with telephone companies and business to business transmissions
14 Satellite microwaveSimilar to terrestrial microwave except the signal travels from a ground station on earth to a satellite and back to another ground stationCan also transmit signals from one satellite to anotherSatellites can beclassified by howfar out into orbiteach one is (LEO,MEO, GEO, andHEO)
15 Uses LEO (Low-Earth-Orbit) – 100 to 1000 miles out Used for wireless , special mobile telephones, pagers, spying, videoconferencingMEO (Middle-Earth-Orbit) – 1000 to 22,300 milesUsed for GPS (global positioning systems) and governmentGEO (Geosynchronous-Earth-Orbit) – 22,300 milesAlways over the same position on earth (and always over the equator)Used for weather, television, government operationsHEO (Highly Elliptical Earth orbit) – satellite follows an elliptical orbitUsed by the military for spying and by scientific organizations for photographing celestial bodiesWhen satellite is far out into space, it takes photosWhen satellite is close to earth, it transmits data
16 Satellite MicrowaveSatellite microwave can also be classified by its configuration:Bulk carrier configurationMultiplexed configurationSingle-user earth station configuration (e.g. VSAT)
17 Cellular Telephones Wireless telephone service Also called mobile telephone, cell phone, and PCSTo support multiple users in a metropolitan area (market), the market is broken into cellsEach cell has its owntransmission towerand set of assignablechannels
18 Types of service (I) 1st Generation AMPS (Advanced Mobile Phone Service) - first popular mobile phone serviceUses analog signals and dynamically assigned channelsD-AMPS (Digital Advanced Mobile Phone Service) - applies digital multiplexing techniques on top of AMPS analog channels2nd GenerationPCS (Personal Communication Systems) - all-digital mobile phone service2nd generation PCS phones came in three technologies:TDMA - Time division multiple accessCDMA - Code division multiple accessGSM - Global system for mobile communications
19 Types of service (II) 2.5 Generation GPRS (General Packet Radio Service) – used by AT&T Wireless, Cingular Wireless, and T-Mobile (formerly VoiceStream) in their GSM networksCan transmit data at 30 kbps to 40 kbpsCDMA2000 1xRTT (one carrier radio - transmission technology) – used by Verizon Wireless, Alltel, U.S. Cellular, and Sprint PCS50 kbps to 75 kbpsIDEN technology – used by Nextel3rd GenerationUMTS (Universal Mobile Telecommunications System) – also called Wideband CDMAThe 3G version of GPRSUMTS not backward compatible with GSM (thus requires phones with multiple decoders)1XEV (1 x Enhanced Version) –3G replacement for 1xRTTWill come in two forms:1xEV-DO for data only1xEV-DV for data and voice
20 Broadband Wireless Systems Delivers Internet services into homes and businesses.Designed to bypass the local loop telephone line.Transmits voice, data and video over high frequency radio signals.Two basic technologies:Multichannel multipointdistribution service (MMDS)and local multipoint distributionservice (LMDS) looked promisinga few years ago but died off.Now companies are eyeingWi-Max, an IEEE standard;initially 300 kbps to 2 Mbps overa range of as much as 30 miles;forthcoming standard (802.16e)will allow for moving devices.
21 BluetoothRadio Frequency (2.45 GHz ISM) specification for short-range, point-to-point or point-to-multipoint voice and data transfer:Can transmit through solid, non-metal objectsTypical link range is from 10 cm to 10 m, but can be extended to 100 m by increasing the powerWill enable users to connect to a wide range of computing and telecommunication devices without the need of connecting cablesTypical uses include phones and pagers, modems, LAN access devices, headsets, notebooks, desktop computers, and PDAs
22 Wireless LAN (IEEE )Transmits data between workstations and local area networks using high speed radio frequenciesMore on this in Chapter 7 (LANs)IEEE (older 2 Mbps)IEEE b (11 Mbps, 2.4 GHz)IEEE a (54 Mbps, 5 GHz, in 2002)IEEE g (54 Mbps, 2.4 GHz, in 2002)HiperLAN/2 (European standard, 54 Mbps in 5 GHz band)
23 Infrared Transmissions Special transmissions that use a focused ray of light in the infrared frequency rangeVery common with remote control devicesCan also be used for device-to-device transfers, such as PDA to computer
24 Free Space OpticsUses lasers, or more economically, infrared transmitting devicesLine of sight between buildingsTypically short distances, such as across the streetNewer auto-tracking systems keep lasers aligned when buildings shake from wind and trafficCurrent speeds go from T-3 (45 Mbps) to OC-48 (2.5 Gbps) with faster systems in developmentMajor weakness is transmission thru fogA typical FSO has a link margin of about 20 dBUnder perfect conditions, air reduces a system’s power by approximately 1 dB/kmScintillation is also a problem (especially in hot weather)
25 Ultra-wideband Not limited to fixed bandwidth Broadcasts over wide range of frequencies simultaneouslyMany of these frequencies are used by other sourcesUses such low power that it “should not” interfere with these other sourcesCan achieve speeds up to 100 Mbps (unshared) but for small distances such as wireless LANsProponents say UWB gets something for nothing since it shares frequencies with other sourcesOpponents say too much interferenceCell phone industry very against UWB because CDMA most susceptible to interferenceGPS may also be affectedOne solution may be have two types of systemsIndoor (stronger)Outdoor (1/10 the power)
26 ZigBeeShort distance and low transfer rates ( Kbps) – home automation, automatic meter reading, medical sensing & monitoring.Mash communications – communicates to other ZigBee devicesLow power requirement – sleep & activate
27 Media Selection Criteria (I) Cost:Initial cost - What does a particular type of medium cost to purchase? To install?Maintenance/support costROI (return on investment) - If one medium is cheaper to purchase and install but is not cost effective, where is the savings?Speed:Propagation speed: time to send first bit across the mediumDepends upon the mediumAirwaves and fiber are speed of lightCopper wire is two thirds the speed of lightData transfer speed: the time to transmit the remaining bits in the messageMeasured in bits per second
28 Media Selection Criteria (II) Distance and expandabilityCan this choice of medium be expanded easily?What is needed to extend the distance? A repeater? An amplifier?How much noise is introduced with this expansion?Don’t forget right-of-way issueEnvironment:Is the intended environment electromagnetically noisy? If so, should you use shielding? Or fiber?If using wireless, are there other wireless signals that can interfere?Will the microwave or free space optics be affected by bad weather?Security:Is the medium going to be carrying secure data? Should you worry about wiretapping?Encryption of the signal/data can help, but may not be the perfect solution