数据通信：传输媒介

1 .Chapter 3 The Media: Conducted and Wireless

2 .Media The 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) media The message flows through a physical media Wireless (unguided) media The message is broadcast through space

3 .Conducted media Twisted pair wire (UTP): Insulated pairs of wires, twisted to minimize electromagnetic interference between wires Coaxial cable: Wire with a copper core and an outer cylindrical shell for insulation Fiber optic cable: High speed streams of light pulses from lasers or LEDs carried inside hair-thin strands of glass or plastic

4 .Twisted Pair Wire One or more pairs of single conductor wires that have been twisted around each other Twisted pair wire is classified by category Category 1 through Category 7 NOTE: Categories 2 and 4 are obsolete Twisting the wires helps to eliminate electromagnetic interference between the two wires Shielding can further help to eliminate interference

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6 .Coaxial Cable A single wire wrapped in a foam insulation surrounded by a braided metal shield, then covered in a plastic jacket. Cable can be thick or thin Baseband coaxial technology uses digital signaling (DC) in which the cable carries only one channel of digital data Broadband 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 cable A 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.

9 .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.

10 .Mixing Media

11 .Wireless Media Radio, 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.

12 .Wireless Media Radio, 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 transmission Approximately 20-30 miles between towers Transmits data at hundreds of millions of bits per second Signals will not pass through solid objects Popular with telephone companies and business to business transmissions

14 .Satellite microwave Similar to terrestrial microwave except the signal travels from a ground station on earth to a satellite and back to another ground station Can also transmit signals from one satellite to another Satellites can be classified by how far out into orbit each one is (LEO, MEO, GEO, and HEO)

15 .Uses LEO (Low-Earth-Orbit) – 100 to 1000 miles out Used for wireless e-mail, special mobile telephones, pagers, spying, videoconferencing MEO (Middle-Earth-Orbit) – 1000 to 22,300 miles Used for GPS (global positioning systems) and government GEO (Geosynchronous-Earth-Orbit) – 22,300 miles Always over the same position on earth (and always over the equator) Used for weather, television, government operations HEO (Highly Elliptical Earth orbit) – satellite follows an elliptical orbit Used by the military for spying and by scientific organizations for photographing celestial bodies When satellite is far out into space, it takes photos When satellite is close to earth, it transmits data

16 .Satellite Microwave Satellite microwave can also be classified by its configuration: Bulk carrier configuration Multiplexed configuration Single-user earth station configuration (e.g. VSAT)

17 .Cellular Telephones Wireless telephone service Also called mobile telephone, cell phone, and PCS To support multiple users in a metropolitan area (market), the market is broken into cells Each cell has its own transmission tower and set of assignable channels

18 .Types of service (I) 1st Generation AMPS (Advanced Mobile Phone Service) - first popular mobile phone service Uses analog signals and dynamically assigned channels D-AMPS (Digital Advanced Mobile Phone Service) - applies digital multiplexing techniques on top of AMPS analog channels 2nd Generation PCS (Personal Communication Systems) - all-digital mobile phone service 2nd generation PCS phones came in three technologies: TDMA - Time division multiple access CDMA - Code division multiple access GSM - 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 networks Can transmit data at 30 kbps to 40 kbps CDMA2000 1xRTT (one carrier radio - transmission technology) – used by Verizon Wireless, Alltel, U.S. Cellular, and Sprint PCS 50 kbps to 75 kbps IDEN technology – used by Nextel 3rd Generation UMTS (Universal Mobile Telecommunications System) – also called Wideband CDMA The 3G version of GPRS UMTS not backward compatible with GSM (thus requires phones with multiple decoders) 1XEV (1 x Enhanced Version) –3G replacement for 1xRTT Will come in two forms: 1xEV-DO for data only 1xEV-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 multipoint distribution service (MMDS) and local multipoint distribution service (LMDS) looked promising a few years ago but died off. Now companies are eyeing Wi-Max, an IEEE 802.16 standard; initially 300 kbps to 2 Mbps over a range of as much as 30 miles; forthcoming standard (802.16e) will allow for moving devices.

21 .Bluetooth Radio 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 objects Typical link range is from 10 cm to 10 m, but can be extended to 100 m by increasing the power Will enable users to connect to a wide range of computing and telecommunication devices without the need of connecting cables Typical uses include phones and pagers, modems, LAN access devices, headsets, notebooks, desktop computers, and PDAs

22 .Wireless LAN (IEEE 802.11) Transmits data between workstations and local area networks using high speed radio frequencies More on this in Chapter 7 (LANs) IEEE 802.11 (older 2 Mbps) IEEE 802.11b (11 Mbps, 2.4 GHz) IEEE 802.11a (54 Mbps, 5 GHz, in 2002) IEEE 802.11g (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 range Very common with remote control devices Can also be used for device-to-device transfers, such as PDA to computer

24 .Free Space Optics Uses lasers, or more economically, infrared transmitting devices Line of sight between buildings Typically short distances, such as across the street Newer auto-tracking systems keep lasers aligned when buildings shake from wind and traffic Current speeds go from T-3 (45 Mbps) to OC-48 (2.5 Gbps) with faster systems in development Major weakness is transmission thru fog A typical FSO has a link margin of about 20 dB Under perfect conditions, air reduces a system’s power by approximately 1 dB/km Scintillation is also a problem (especially in hot weather)

25 .Ultra-wideband Not limited to fixed bandwidth Broadcasts over wide range of frequencies simultaneously Many of these frequencies are used by other sources Uses such low power that it “should not” interfere with these other sources Can achieve speeds up to 100 Mbps (unshared) but for small distances such as wireless LANs Proponents say UWB gets something for nothing since it shares frequencies with other sources Opponents say too much interference Cell phone industry very against UWB because CDMA most susceptible to interference GPS may also be affected One solution may be have two types of systems Indoor (stronger) Outdoor (1/10 the power)

26 .ZigBee Short distance and low transfer rates (20-250 Kbps) – home automation, automatic meter reading, medical sensing & monitoring. Mash communications – communicates to other ZigBee devices Low 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 cost ROI (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 medium Depends upon the medium Airwaves and fiber are speed of light Copper wire is two thirds the speed of light Data transfer speed: the time to transmit the remaining bits in the message Measured in bits per second

28 .Media Selection Criteria (II) Distance and expandability Can 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 issue Environment: 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