Bài giảng Mạng máy tính - Chương 2: Physical Layer and Media - Nguyễn Trung Dũng

07-Apr-09 1 Chapter 2 Physical Layer and Media Content • Overview • Data Communications Summary • Network Media • Actual Networks 07-Apr-09 2 Layer 1 overview • Bottom-most layer • Interact with layer 2 & pasive entity (media) • Move data in the form of electronics signals across transmission medium Digital communication feature Advantages: • Relatively inexpensive ditital circuits may be used • Privacy is preserved by using data encryption • Greater dynamic range is possible. • Data from voice, video, and data sources may be merged and transmitted over a common digital transmission system. • In long-distance systems, noise does not accumulate from repeater to repeater. • Errors in detected data may be small, even when there is a large amount of noise on the received signal. • Errors may often be corrected by the use of coding. 07-Apr-09 3 Digital communication feature (cont) Disadvantages: • Generally, more bandwidth is required than that for analog systems. • Synchronization is required. A communication system m(t) m’(t) Signal processing Carrier circuits Carrier circuits Transmission medium Signal processing r(t)s(t) noise n(t) TRANSMITTER RECEIVER 07-Apr-09 4 Frequency allocations International Telecommunications Union (ITU) Frequency band Designation Propagation Characteristics Typical Uses 3-30 kHz Very low frequency (VLF) Ground wave, low attenuation day and night; high atmospheric noise level Long-range navigation submarine communication 30-300 kHz Low frequency (LF) Similar to VLF, slightly less reliable; absorption in day time Long-range navigation and marine communication radio becons 300-3000 kHz Medium frequency (MF) Ground wave and night sky wave, attenuation low at night and high in day, atmospheric noise Maritime radio, direction finding and AM broadcasting 3-30 MHz High frequency (HF) Ionospheric reflection varies with time of day Amateur radio, military comm, telephone, telegraph Frequency band Designation Propagation Characteristics Typical Uses 30-300 MHz Very high frequency (VHF) Nearly LOS propagation VHF television, FM radio, AM aircraft communicaiton 0.3-3 GHz Ultrahigh frequency (UHF) LOS propagation UHF television, cellular telephone, navigational aids, radar, GPS, microwave links, personal communication systems 1.0 – 2.0 2.0 – 4.0 3 – 30 GHz L S Superhigh frequency (SHF) LOS propagation Satellite communication, radar microwave links 2.0 – 4.0 4.0 – 8.0 8.0 – 12.0 12.0 – 18.0 18.0 – 27.0 27.0 – 40.0 26.5 – 40.0 30 – 300 GHz S C X Ku K Ka R (EHF) 07-Apr-09 5 Frequency band Designation Propagation Characteristics Typical Uses 27.0 – 40.0 26.5 – 40.0 33.0 – 50.0 40.0 – 75.0 75.0 – 110.0 110 – 300 103 – 107 Ghz Ka R Q V W mm Infrared, visible light, and ultraviolet LOS Optical communications Information measure The information sent from a digital source when the jth message is transmitted is given by Ij = log2 bits where Pj is the probability of transmitting the j th message       j 1 P 07-Apr-09 6 Average information H = = bits m is the number of possible different source messages Pj is the probability of sending the jth message The average information is called entropy ∑ = m j jjIP 1 ∑ =          m j j j P P 1 2 1log Source rate R = T is the time required to send a message. The definitions previously given apply to digital sources. T H 07-Apr-09 7 Channel capacity • Claude Shannon (1948): If R < C C = B log2 C is channel capacity (bits/s) B is the channel bandwidth (Hz)       + N S1 Network Media 07-Apr-09 8 15 Copper Media • Describe the specifications and performances of different types of cable. • Describe coaxial cable and its advantages and disadvantages over other types of cable. • Describe shielded twisted-pair (STP) cable and its uses. • Describe unshielded twisted-pair cable (UTP) and its uses. • Discuss the characteristics of straight-through, crossover, and rollover cables and where each is used. 16 Cable Specifications 07-Apr-09 9 17 Cable Specifications Cables have different specifications and expectations pertaining to performance: • What speeds for data transmission can be achieved using a particular type of cable? The speed of bit transmission through the cable is extremely important. The speed of transmission is affected by the kind of conduit used. • What kind of transmission is being considered? Will the transmissions be digital or will they be analog-based? Digital or baseband transmission and analog-based or broadband transmission are the two choices. • How far can a signal travel through a particular type of cable before attenuation of that signal becomes a concern? In other words, will the signal become so degraded that the recipient device might not be able to accurately receive and interpret the signal by the time the signal reaches that device? The distance the signal travels through the cable directly affects attenuation of the signal. Degradation of the signal is directly related to the distance the signal travels and the type of cable used. 18 Cable Specifications • 10BASE-T – speed of transmission at 10 Mbps – type of transmission is baseband, or digitally interpreted – T stands for twisted pair 07-Apr-09 10 19 Cable Specifications • 10BASE5 – speed of transmission at 10 Mbps – type of transmission is baseband – 5 represents the capability of the cable to allow the signal to travel for approximately 500 meters before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. – often referred to as Thicknet 20 Cable Specifications • 10BASE2 – speed of transmission at 10 Mbps – type of transmission is baseband – The 2, in 10BASE2, represents the capability of the cable to allow the signal to travel for approximately 200 meters, before attenuation could disrupt the ability of the receiver to appropriately interpret the signal being received. 10BASE2 is often referred to as Thinnet. 07-Apr-09 11 21 Coaxial Cable 22 Coaxial Cable • Copper braid or metallic foil – Acts as the second wire in the circuit – Acts as a shield for the inner conductor. – Reduces the amount of outside electro-magnetic interference. – Comprises half the electric circuit – Special care must be taken to ensure a solid electrical connection a both ends resulting in proper grounding – Poor shield connection is one of the biggest sources of connection problems in the installation of coaxial cable. 07-Apr-09 12 23 Coaxial Cable • Advantages: – Requires fewer repeaters than twisted pair – Less expensive than fiber – It has been used for many years for many types of data communication, including cable television • Disadvantages: – More expensive and more difficult to install than twisted pair – Needs more room in wiring ducts than twisted pair 24 Unshielded Twisted Pair (UTP) • Unshielded twisted-pair cable (UTP) is a four-pair wire medium used in a variety of networks. • TIA/EIA-568-A contains specifications governing cable performance. • RJ-45 connector • When communication occurs, the signal that is transmitted by the source needs to be understood by the destination. • The transmitted signal needs to be properly received by the circuit connection designed to receive signals. • The transmit pin of the source needs to ultimately connect to the receiving pin of the destination. 07-Apr-09 13 25 Shielded Twisted Pair (STP and ScTP) • Shielded twisted-pair cable (STP) combines the techniques of shielding, cancellation, and twisting of wires. – Each pair of wires is wrapped in metallic foil. – The four pairs of wires are wrapped in an overall metallic braid or foil. • A new hybrid of UTP with traditional STP is Screened UTP (ScTP), also known as Foil Twisted Pair (FTP). – ScTP is essentially UTP wrapped in a metallic foil shield, or screen. STP – Shielded Twisted Pair ScTP – Screened Twisted Pair 26 Shielded Twisted Pair (STP and ScTP) • Greater protection from all types of external and internal interference than UTP. – Reduces electrical noise within the cable such as pair to pair coupling and crosstalk. – Reduces electronic noise from outside the cable, for example electromagnetic interference (EMI) and radio frequency interference (RFI). • More expensive and difficult to install than UTP. • Needs to be grounded at both ends 07-Apr-09 14 27 Unshielded Twisted Pair (UTP) Straight-through Cross-over Rollover 28 UTP Straight-through Cable • The cable that connects from the switch port to the computer NIC port is called a straight-through cable. Host or RouterHub or Switch 07-Apr-09 15 29 UTP Straight-through Cable Host or RouterHub or Switch 30 UTP Cross-over Cable • The cable that connects from one switch port to another switch port is called a crossover cable. 07-Apr-09 16 31 UTP Cross-over Cable 32 UTP Rollover Cable • The cable that connects the RJ-45 adapter on the com port of the computer to the console port of the router or switch is called a rollover cable. 07-Apr-09 17 33 UTP Rollover Cable Rollover cable Console port Com1 or Com2 serial port Terminal or a PC with terminal emulation software Router 34 Optical Media • Explain the basics of fiber-optic cable. • Describe how fibers can guide light for long distances. • Describe multimode and single-mode fiber. • Describe how fiber is installed. • Describe the type of connectors and equipment used with fiber-optic cable. • Explain how fiber is tested to ensure that it will function properly. • Discuss safety issues dealing with fiber-optics. 07-Apr-09 18 35 The electromagnetic spectrum • The light used in optical fiber networks is one type of electromagnetic energy. • When an electric charge moves back and forth, or accelerates, a type of energy called electromagnetic energy is produced. • An important property of any energy wave is the wavelength. • Radio, microwaves, radar, visible light, x-rays are all types of electromagnetic energy. 36 The electromagnetic spectrum 07-Apr-09 19 37 The electromagnetic spectrum • Wavelengths that are not visible to the human eye are used to transmit data over optical fiber. • These wavelengths are slightly longer than red light and are called infrared light. • Infrared light is used in TV remote controls. • These wavelengths were selected because they travel through optical fiber better than other wavelengths. 38 Ray model of light • When electromagnetic waves travel out from a source, they travel in straight lines. • These straight lines pointing out from the source are called rays. • However, light travels at different, slower speeds through other materials like air, water, and glass. • When a light ray called the incident ray, crosses the boundary from one material to another, some of the light energy in the ray will be reflected back. • The light that is reflected back is called the reflected ray. 07-Apr-09 20 39 Reflection • When a ray of light (the incident ray) strikes the shiny surface of a flat piece of glass, some of the light energy in the ray is reflected. • The angle between the incident ray and a line perpendicular to the surface of the glass at the point where the incident ray strikes the glass is called the angle of incidence. 40 Refraction • When a light strikes the interface between two transparent materials, the light divides into two parts. • Part of the light ray is reflected back into the first substance, with the angle of reflection equaling the angle of incidence. • The remaining energy in the light ray crosses the interface and enters into the second substance. 07-Apr-09 21 41 Refraction • If the incident ray strikes the glass surface at an exact 90-degree angle, the ray goes straight into the glass. • The ray is not bent. However, if the incident ray is not at an exact 90-degree angle to the surface, then the transmitted ray that enters the glass is bent. • The bending of the entering ray is called refraction. • How much the ray is refracted depends on the index of refraction of the two transparent materials. 42 Total internal reflection • The following two conditions must be met for the light rays in a fiber to be reflected back into the fiber without any loss due to refraction: – The core of the optical fiber has to have a larger index of refraction (n) than the material that surrounds it. The material that surrounds the core of the fiber is called the cladding. – The angle of incidence of the light ray is greater than the critical angle for the core and its cladding. 07-Apr-09 22 43 Total internal reflection • Restricting the following two factors controls the angle of incidence: – The numerical aperture of the fiber – The numerical aperture of a core is the range of angles of incident light rays entering the fiber that will be completely reflected. – Modes – The paths which a light ray can follow when traveling down a fiber. • By controlling both conditions, the fiber run will have total internal reflection. This gives a light wave guide that can be used for data communications. 44 Fiber Optic Cabling • The core is the light transmission element at the center of the optical fiber. All the light signals travel through the core. • Cladding is also made of silica but with a lower index of refraction than the core. Light rays traveling through the fiber core reflect off this core-to-cladding interface as they move through the fiber by total internal reflection. • Surrounding the cladding is a buffer material that is usually plastic. The buffer material helps shield the core and cladding from damage. • The strength material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar, the same material used to produce bulletproof vests. • The outer jacket surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants. 07-Apr-09 23 45 Fiber Optic Cabling Higher bandwidth. 46 Fiber Optic Cabling • If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called “multimode” fiber. • Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber. 07-Apr-09 24 47 Fiber Optic Cabling • Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. • One fiber carries transmitted data from device A to device B. • The second fiber carries data from device B to device A. • This provides a full-duplex communication link. • Typically, these two fiber cables will be in a single outer jacket until they reach the point at which connectors are attached. 48 Fiber Optic Cabling • Warning: The laser light used with single-mode has a longer wavelength than can be seen. • The laser is so strong that it can seriously damage eyes. • Never look at the near end of a fiber that is connected to a device at the far end. • Never look into the transmit port on a NIC, switch, or router. • Remember to keep protective covers over the ends of fiber and inserted into the fiber-optic ports of switches and routers. Core - Cladding 07-Apr-09 25 49 ST and SC Connectors • The type of connector most commonly used with multimode fiber is the Subscriber Connector (SC connector). • On single-mode fiber, the Straight Tip (ST) connector is frequently used 50 Signals and noise in optical fibers • Fiber-optic cable is not affected by the sources of external noise that cause problems on copper media because external light cannot enter the fiber except at the transmitter end. • Although fiber is the best of all the transmission media at carrying large amounts of data over long distances, fiber is not without problems. When light travels through fiber, some of the light energy is lost. • The most important factor is scattering. – The scattering of light in a fiber is caused by microscopic non-uniformity (distortions) in the fiber that reflects and scatters some of the light energy. 07-Apr-09 26 51 Signals and noise in optical fibers • Absorption is another cause of light energy loss. • Another factor that causes attenuation of the light signal is manufacturing irregularities or roughness in the core-to-cladding boundary. • Dispersion of a light flash also limits transmission distances on a fiber. – Dispersion is the technical term for the spreading of pulses of light as they travel down the fiber 52 Installation, care, and testing of optical fiber 07-Apr-09 27 53 Wireless WAN Standards • IEEE is the prime issuer of standards for wireless networks. • The standards have been created within the framework of the regulations created by the Federal Communications Commission (FCC). • 802.11 standard is Direct Sequence Spread Spectrum (DSSS). – DSSS applies to wireless devices operating within a 1 to 2 Mbps range. • 802.11b may also be called Wi-Fi™ or high-speed wireless and refers to DSSS systems that operate at 1, 2, 5.5 and 11 Mbps. – The majority of 802.11b devices still fail to match the 11 Mbps throughput and generally function in the 2 to 4 Mbps range. • 802.11a covers WLAN devices operating in the 5 GHZ transmission band. – 802.11a is capable of supplying data throughput of 54 Mbps and with proprietary technology known as "rate doubling" has achieved 108 Mbps. – In production networks, a more standard rating is 20-26 Mbps. • 802.11g provides the same throughout as 802.11a but with backwards compatibility for 802.11b devices. 54 Wireless devices and topologies 07-Apr-09 28 55 Wireless devices and topologies • Access point (AP) is commonly installed to act as a central hub for the WLAN "infrastructure mode". • The AP is hard wired to the cabled LAN to provide Internet access and connectivity to the wired network. APs are equipped with antennae and provide wireless connectivity over a specified area referred to as a cell. • Depending on the structural composition of the location in which the AP is installed and the size and gain of the antennae, the size of the cell could greatly vary. • Most commonly, the range will be from 91.44 to 152.4 meters (300 to 500 feet). 56 Wireless devices and topologies • Overlap, on multiple AP networks, is critical to allow for movement of devices within the WLAN. • Although not addressed in the IEEE standards, a 20-30% overlap is desirable. • This rate of overlap will permit roaming between cells, allowing for the disconnect and reconnect activity to occur seamlessly without service interruption. 07-Apr-09 29 57 The radio wave and microwave spectrums • Computers send data signals electronically. • Radio transmitters convert these electrical signals to radio waves. • Changing electric currents in the antenna of a transmitter generates the radio waves. • However, radio waves attenuate as they move out from the transmitting antenna. 58 Modulation • The process of altering the carrier signal that will enter the antenna of the transmitter is called modulation. • There are three basic ways in which a radio carrier signal can be modulated. • For example, Amplitude Modulated (AM) radio stations modulate the height (amplitude) of the carrier signal. • Frequency Modulated (FM) radio stations modulate the frequency of the carrier signal as determined by the electrical signal from the microphone. • In WLANs, a third type of modulation called phase modulation is used to superimpose the data signal onto the carrier signal that is broadcast by the transmitter. 07-Apr-09 30 Actual Networks • Apendix 3