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COMP210 Physical Layer Dr Ahmad Al-Zubi The Physical Layer • The Physical Layer performs bit by bit transmission of the frames given to it by the Data Link Layer. • The specifications of the Physical Layer include: • Mechanical and electrical interfaces • Sockets and wires used to connect the host to the network • Voltage levels uses (e.g. -5V and +5V) • Encoding techniques (e.g. Manchester encoding) • Modulation techniques used (e.g. square wave) • The bit rate and the baud rate. Signal Transmission • Electronic energy to send signals that communicate from one node to another • Two methods of transmitting data ¯ Digital signaling ¯ Analog signaling Comparison of Digital and Analog Bandwidth-Limited Signals Bandwidth-Limited Signals Digital Signaling • Digital signal represents discrete state (on or off) • Practically instantaneous change Digital Signaling Current State Encoding • Data is encoding by the presence or absence of a signal • A positive voltage might represent a binary zero or binary one or visa versa • The current state indicates the value of the data Digital Signaling Current State Digital Signaling State Transition Analog Signaling • Signals represented by an electromagnetic wave • Signal is continuos and represents values in a range • Uses one or more of the characteristics of an analog wave to represent ones and zeros Characteristics of an Analog Signal Parts of a Wave The maximum intensity of a wave is called the amplitude. The distance between two crests is the wavelength. Wavelen gth () or period Phase: Amplitud e (a) Relative state of one wave to another in regards to timing Phase differenc e () The number of complete wave cycles every second is the frequency. The phase difference measures (as an angle) how far ahead one wave is when compared to another wave. ELECTROMAGNETIC SPECTRUM Lambda = c / f = (3 * 10**8)[m / sec] / f f = 10 MHz -> lambda = 30 m f = 10 GHz -> lambda = 3 cm f = 10**15 Hz -> lambda = 3 * 10**-4 mm Three equations describing data transmissions 1. lambda = c / frequency lambda is the wave length c is the speed of light (3 * 10**8 m/sec) 2. Max_data_rate = 2 * freq * log(2) #of_levels 3. delta(freq) = c * delta(lambda) / lambda ** 2 Wire Propagation Effects •Propagation Effects ¯ Signal changes as it travels ¯ Receiver may not be able to recognise it Original Signal Final Signal Distance Propagation Effects: Attenuation • Attenuation: signal gets weaker as it propagates ¯ Attenuation becomes greater with distance ¯ May become too weak to recognise Signal Strength Distance Propagation Effects: Distortion • Distortion: signal changes shape as it propagates ¯ Adjacent bits may overlap ¯ May make recognition impossible for receiver Distance Propagation Effects: Noise • Noise: thermal energy in wire adds to signal ¯ Noise floor is average noise energy ¯ Random signal, so spikes sometimes occur Signal Strength Signal Noise Spike Noise Floor Time Propagation Effects: SNR • Want a high Signal-to-Noise Ratio (SNR) ¯ Signal strength divided by average noise strength ¯ As SNR falls, errors increase Signal Strength Signal SNR Noise Floor Distance Propagation Effects: Interference • Interference: energy from outside the wire ¯ Adds to signal, like noise ¯ Often intermittent, so hard to diagnose Signal Strength Signal Interference Time Propagation Effects: Termination • Interference can occur at cable terminator (connector, plug) ¯ ¯ ¯ ¯ Often, multiple wires in a bundle Each radiates some of its signal Causes interference in nearby wires Especially bad at termination, where wires are unwound and parallel Termination Bandwidth • Capacity of a media to carry information • Total capacity may be divided into channels • A channel is a portion of the total bandwidth used for a specific purpose Bandwidth • Baseband ¯ The total capacity of the media is used for one channel ¯ Most LANs use baseband • Broadband ¯ Divides the total bandwidth into many channels ¯ Each channel can carry a different signal ¯ Broadband carries many simultaneous transmissions Analog versus Digital • Digital ¯ Is less error prone ¯ Distortion of the signal between the source and destination is eliminated • Analog ¯ Little control over the signal distortion ¯ Old technology Analog versus Digital In digital communication, it is often possible to reconstruct the original signal even after it has been effected by noise voltag e voltag e tim e voltag e analogue signal tim e voltag e digital signal tim e analogue signal + noise When noise effects an analogue signal, it is hard to deduce the original signal. tim e digital signal + noise The original digital signal can be deduced despite the noise. Benefits of Digital Transmission • Reliability ¯ Can regenerate slightly damaged signals ¯ There are only two states. Change to closest ¯ E.g., if two states are voltages +10v (1) and -10v (0) and the signal is +8v, the signal is a 1 Original ReceivedRegenerated Benefits of Digital Transmission • rror detection and E correction ¯ Can correct errors in transmission - Add a few bytes of errorchecking information - Can ask for retransmission if an error is detected Benefits of Digital Transmission • Encryption ¯ Encrypt (scramble) messages so that someone intercepting them cannot read them Benefits of Digital Transmission • Compression ¯ Compress message before transmission ¯ Decompress at other end ¯ Compressed message places lighter load on transmission line, so less expensive to send ¯ Not always used 10101001 1010 Original Signal Compressed Signal Modulation • Because attenuation is frequency dependent, modems use a sine wave carrier of a particular frequency, and then modulate that frequency. Various modulations include: Amplitude modulation: Two different amplitudes of sine wave are used to represent 1's and 0's. Frequency modulation: Two (or more) different frequencies, close to the carrier frequency, are used. Phase modulation: The phase of the sine wave is changed by some fixed amount. Binary Signal Nyquist’s Limit Suppose we know the bandwidth (H) of a channel and the number of signal levels (V) being used. What is the maximum number of bit we could transmit? Nyquist’s Limit says: Max_bps = 2*H*log(base 2)(V) bits/sec For example, if the bandwidth is 3100Hz and we are using 16 level modulation then the maximum number of bits per second is: max_bps = 2 3100 log2(16) = 24800 bps Analog Signaling Amplitude Modulation (ASK) Analog Signaling Frequency Modulation (FSK) Analog Signaling Phase Shift Keying (FSK) Constellation Diagrams We can represent the different combinations of phase shifts and amplitudes using a constellation diagram. 101 0110 001 45 0000 15 110 010 000 100 1000 011 111 Each dot represents a combination of phase shift (the angle from the positive x axis) and amplitude (the distance from the origin). Baud Rate / Bit Rate The maximum number of times a signal can change in a second is called the baud rate. The number of bits (1s and 0s) transmitted in a second is called the bit rate. In the examples we have seen so far, the bit rate and the baud rate are the same. This is not always the case. The bit rate can be higher than the baud rate if we use more than two signal levels (more later on…). Modems • Modulation demodulation • Used to connect a digital computer to an analog phone system • Can be installed internally a card inserted into the motherboard • Can be connected to the serial port (external modem) Modems • Transfer speeds ¯ Bit rate BPS ¯ Baud ¯ Bandwidth • Compression ¯ appears to increases speed by decreasing the number of bits sent (usually some data does not compress well) ¯ sending and receiving modem must use same compression standard Modems • Error detection and correction ¯ asynchronous modems use parity check ¯ checksum counting the number of data words sent Digital Modem • Miss named • Used to connect to a digital telephone • ISDN (integrated Services Digital Network) is an example • Again do not connect digital modems to an analog phone • Higher quality lower errors Transmission Modes Parallel Mode Serial Mode Channel Types A channel is any conduit for sending information between devices. There are three basic types of channel: simplex, half-duplex and full-duplex. A simplex channel is unidirectional, which means data can only be sent in one direction. For example, a TV channel only carries data from the transmitter to your TV set. Your TV set cannot send information back. Channel Types A half-duplex channel allows information to flow in either direction (but not simultaneously). Devices at either end of the channel must take it in turns to transmit information whilst the other listens. For example, a walky-talky either transmits or receives but not both at the same time. Channel Types A full-duplex channel allows data to be sent in both directions simultaneously. A full-duplex channel can be formed from two simplex channels carrying data in opposite directions. This may make it more expensive than a half-duplex channel. There is no waiting for turns or for the devices swap roles, as is the case with a half-duplex channel. This means fullduplex can be faster and more efficient. Network Media Types • Types of Media ¯ Cable (conducted media) - Coaxial Twisted pair (UTP) Shielded twisted pair (STP) Fiber optic ¯ Radiated - Infrared Microwave Radio Satellite Media Selection Criteria • Cost ¯ For actual media and connecting devices such as NICs hubs etc • Installation ¯ Difficulty to work with media ¯ Special tools, training Media Selection Criteria • Capacity ¯ The amount of information that can be transmitted in a giving period of time ¯ Measured as - Bits per second bps (preferred) - Baud (discrete signals per second) - Bandwidth (range of frequencies) Media Selection Criteria • Node Capacity ¯ Number of network devices that can be connected to the media • Attenuation ¯ Weakening of the signal over distance Media Selection Criteria • Electromagnetic Interference (EMI) ¯ Distortion of signal caused by outside electromagnetic fields ¯ Caused by large motors, proximity to power sources • Other noise sources ¯ White (Gaussian) noise ¯ Impulse noise ¯ Crosstalk ¯ Echo Cable Media • Unshielded Twisted Pair UTP • Shielded Twisted Pair STP • Coaxial • Fiber optic Unshielded Twisted Pair (UTP) • One or more pairs of twisted copper wires insulated and contained in a plastic sheath • Twisted to reduce crosstalk Unshielded Twisted Pair (UTP) • Categories ¯ categories 1 and 2 - voice grade - low data rates up to 4 Mbps ¯ category 3 - suitable for most LANs - up to 16 Mbps ¯ category 4 - up to 20 Mbps Unshielded Twisted Pair (UTP) • Categories ¯ category 5 - supports fast ethernet - more twists per foot - more stringent standards on connectors • Data grade UTP cable usually consists of either 4 or 8 wires, two or four pair • Uses RJ-45 telephone connector Unshielded Twisted Pair (UTP) Shielded Twisted Pair (STP) • Same as UTP but with a aluminum/polyester shield • Connectors are more awkward to work with • Usually comes in pre made lengths • Different standards for IBM and Apple Shielded Twisted Pair (STP) Coaxial Cable Coax • Two conductors sharing the same axis • A solid center wire surrounded insulation and a second conductor Coaxial Cable Coaxial Cable Coax • Size of Coax ¯ RG-8, RG-11 - 50 ohm Thick Ethernet ¯ RG-58 - 50 ohm Thin Ethernet ¯ RG-59 - 75 ohm Cable T.V. ¯ RG-62 - 93 OHM ARCnet Fiber Optic Cable • Thin strand(s) of glass or plastic protected by a plastic sheath and strength wires or gel • Transmits laser (single mode) or LED (multi mode) • Single mode more expensive but can handle longer distances Fiber Optics • Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. • Light trapped by total internal reflection. Optical Fibers The main advantage of optical fiber is the great bandwidth it can carry. There are three main bands of wavelength used. Attenuat dB/km 0.80 band 1.30 band 1.55 band 2. 0 1. 5 1. 0 0. 5 0 0. 0. 1. 1. 1. 1. 1. 1. 1. 1. 1. 8 9 0 1 2 3 4 5 6 7 8 Wavelen (microns) Optical Fibers One problem with optical fibers is that the light pulses become distorted over distance due to dispersion. Dispersion occurs when photons from the same light pulse take slight different paths along the optical fiber. Because some paths will be longer or shorter than other paths the photons will arrive at different times thus smearing the shape of the pulse. Over long distances, one pulse may merge with another pulse. When this happens, the receiving device will not be able to distinguish between pulses. Overcoming Dispersion Normal fiber optic cable is called multimode because photons can take different paths along it. The more expensive monomode fibre optic overcomes dispersion by having a core so thin that the light can only take one path along it. Fiber Optic Networks A fiber optic ring with active repeaters. Fiber Optic Networks Star connection Fiber Cables A comparison of semiconductor diodes and LEDs as light sources. Characteristics of Cable Media Wireless Media • Uses the earth’s atmosphere as a conducting media • Main types ¯ radio wave ¯ microwave (including satellite) ¯ infrared The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication. Radio Transmission • (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. • (b) In the HF band, they bounce off the ionosphere. Radio Wave • Most radio frequencies are regulated • Must obtain a license from a regulatory board (CRTC, FCC) • A range of radio frequencies are unregulated Radio Wave • Low power single frequency ¯ uses one frequency ¯ limited range 20 to 30 meters ¯ usually limited to short open environments Radio Wave • High power single frequency ¯ long distance may use repeaters to increase distance ¯ line of sight or bounced of the earth’s atmosphere ¯ uses a single frequency Radio Wave Spread Spectrum Maintains security of the radio transmission by: 1. Spreading the carrier signal frequency 2. Modulating the carrier frequency by a Pseudo Random signal Radio Wave Spread Spectrum Two methods: 1. Frequency hopping – carrier frequency keeps on changing according to the Pseudo Random code 2. Direct Sequence Spread SpectrumPseudo Random digital signal modulates the carrier directly Transmitter and receiver must use the same Pseudo Random code Radio Wave Spread Spectrum Direct Sequence Spread Spectrum (DSSS) is used as the Physical Layer for the Mobile networking (IEEE 802.11) Radio Wave Microwave • Terrestrial ¯ line of sight ¯ use relay towers ¯ uses license frequencies Communication Satellites Communication Satellites One disadvantage of satellite transmission is the delay that occurs because the signal has to travel out into space and back to Earth (propagation delay). One problem associated with some types of satellite transmission is raindrop attenuation (some waves at the high end of the spectrum are so short they can be absorbed by raindrops). Communication Satellites The principal satellite bands. Communication Satellites VSATs using a hub. Globalstar Infrared • Uses same technology as remotes for T.V. • signals can not penetrate objects • Can be point to point or broadcast • Point to point requires precise alignment of devices • Point less immune to eavesdropping Multiplexing • several lines (one for each device) enter a multiplexer (mux) at the host side • the host side mux combines all incoming signals • combined signals are transmitted to a mux on the receiving side Types of Multiplexers • Frequency Division Multiplexing (FDM) • Time Division Multiplexing (TDM) • Statistical Time Division Multiplexing (STDM) Frequency Division Multiplexing • The frequency spectrum is divided up among the logical channels - each user hangs on to a particular frequency. The radio spectrum (and a radio) are examples of the media and the mechanism for extracting information from the medium. Frequency division multiplexed circuit Frequency Division Multiplexing One problem with FDM is that it cannot utilise the full capacity of the cable. It is important that the frequency bands do not overlap. Indeed, there must be a considerable gap between the frequency bands in order to ensure that signals from one band do not affect signals in another band. FDM is usually used to carry analogue signals although modulated digital signals can also be sent using this technique. Wavelength Division Multiplexing • The same as FDM, but applied to fibers. There's great potential for fibers since the bandwidth is so huge (25,000 GHz). • Fibers with different energy bands are passed through a diffraction grating prism • Combined on the long distance link • Split at the destination • High reliability, very high capacity Time Division Multiplexing (TDM) Like FDM, time division multiplexing (TDM) saves money by allowing more than one telephone call to use a cable at the same time. Multiplexer Demultiplexer Device#1 Terminal#1 Terminal#2 Terminal#3 #3 #2 #1 #3 #2 #1 Device#2 Device#3 Instead of dividing the cable into frequency bands, TDM splits cable usage into time slots. Each channel is given a regular time slot in which to send a PCM signal. Time Division Multiplexing • In TDM, the users take turns, each one having exclusive use of the medium in a round robin fashion. TDM can be all digital. Time Division Multiplexing Statistical Time Division Multiplexing (STDM) • allows connection of more nodes to the circuit than the capacity of the circuit • works on the premise that not all nodes will transmit at full capacity at all times • must transmit a terminal identification • may require storage Digital Technology in Telephone Networks Over the past 30 years, much of the traditional analogue telephone network has been replaced by digital technology. A device called a codec (coder/decoder) is used to convert analogue voice signals into digital information that can be handled by the digital technology. WAN Transmission Media • Public Switched Telephone Network • High speed, High bandwidth dedicated leased circuits • High speed fiber optic cable • Microwave transmission links • Satellite links Services provided by PSTN • Voice – Plain Old Telephone Service (POTS) Based on the Voice Channel (3600 Hz) • Data transmission services ¯ consist of services such as : - switched 56 X.25 T1, T3 circuits Frame relay ISDN ATM Switching • Switching send data across different routes • Three types of switching ¯ Circuit switching ¯ Message switching ¯ Packet switching Switching • Circuit Switching: A connection (electrical, optical, radio) is established from the caller phone to the callee phone. This happens BEFORE any data is sent. • Message Switching: The connection is determined only when there is actual data (a message) ready to be sent. The whole message is re-collected at each switch and then forwarded on to the next switch. This method is called store-and-forward. This method may tie up routers for long periods of time - not good for interactive traffic. • Packet Switching: Divides the message up into blocks (packets). Therefore packets use the transmission lines for only a short time period - allows for interactive traffic. Circuit Switching • Connects the sender and receiver by a single physical path for the duration of the session • PSTN uses circuit switching • Before transmission a dedicated circuit must be established Circuit Switching • Advantages ¯ guaranteed data rate ¯ once connected no channel access delay • Disadvantages ¯ inefficient use of the transmission media (idle time) ¯ long connection delays (first time) Message Switching • Each message is treated as an independent unit ¯ has its own source and destination address • Each is transmitted from device to device • Each intermediate device stores the message until the next device is ready ¯ store and forward Message Switching • Route messages along varying paths for more efficiency • Switching devices are often PCs with special software Message Switching • Advantages ¯ efficient traffic management ¯ reduces network congestion ¯ efficient use of network media ¯ messages can be sent when receiver down • Disadvantages ¯ delay of storing and forwarding ¯ costly intermediate storage Packet Switching • Packet switching breaks messages into packets • Packets travel different routes (independent routing) • Each packet has its own header information • Packets small enough to be stored in RAM thus quicker than message switching Packet Switching • Advantages ¯ improves the use of bandwidth over circuit switching ¯ can adjust routes to reflect network conditions ¯ shorter transmission delays than message switching (stored in RAM) ¯ less disk space ¯ smaller packets to retransmit Packet Switching • Disadvantages ¯ More RAM ¯ More complex protocols ¯ more processing power for switching device ¯ Greater number of packets greater chance for packet loss or damage Packet Switching • Two methods of packet switching ¯ Datagram packet switching ¯ Virtual circuit packet switching Datagram Packet Switching • Message divided into a stream of packets • Each packet has it’s own control instructions • Switching devices route each packet independently Datagram Packet Switching • Switching devices can route packets around busy network links • Require sequence numbers to reassemble • Small packet size facilitates retransmission due to errors Virtual Circuit Packet Switching • Similar to circuit switching • Before transmission of the sending and receiving device agree on: ¯ maximum message size ¯ network path ¯ establish a logical connection (virtual circuit) • All packets travel on the same virtual path CIRCUIT SWITCHED versus PACKET SWITCHED NETWORKS (a) circuit switching (b) message switching (c) packet switching NARROWBAND ISDN - WHAT IS IT? Integrated Services Digital Network: A completely digit, circuitswitched phone system. Integrates voice and nonvoice services. ISDN allows integration of computers and voice. It means that caller ID can be used to look up your account on the computer so that by the time a human answers the phone, a screen has your information already available. Integrated Services Digital Network ISDN SYSTEM ARCHITECTURE: • ISDN uses TDM to handle multiple channels. For home use, the NT1 (Network Terminator) connects the twisted pair going to the phone company with the house wiring. Various ISDN devices can be connected to this NT1. • Businesses may have more channels active than the home configuration internal bus can handle. So a PBX ( Private Branch eXchange ) is used to provide the internal bus containing more switching capacity. This in turn is connected to NT1. Integrated Services Digital Network THE ISDN INTERFACE: • Typically a number of channels are combined together. In the USA, Primary Rate ISDN contains 23 channels (each 64 kbps carrying voice or data) + 1 channel for signaling and control (16 kbps digital channel.) In Europe, instead of 23 channels, 30 are used. • The primary Rate is designed to connect to a business with a PBX. As it turns out, most companies now need far more capacity than 64 kbps for the many uses beyond voice. So this is less than adequate. • N-ISDN may have a life as a connection to homes for people wanting to download images etc. But it's not useful for serious business applications. B-ISDN (Broadband Integrated Services Digital Network) • Broadband transmission A type of data transmission in which a single medium (wire) can carry several channels at once (ex. Cable TV). • Baseband transmission one signal at a time (most communication between computers). • B-ISDN will offer video on demand, live television from many sources, full motion multimedia email, CD-quality music, LAN interconnection, high-speed data transport for science and industry and many more, ALL over the telephone line. • A digital virtual circuit capable of 155 Mbps • Underlying technology that makes B-ISDN possible ATM (Asynchronous Transfer Mode) Comparing Virtual Circuits and Circuit Switching A Virtual Circuit A kind of circuit-switched path implemented with packet switching The service offered is connection oriented (from the customer's point of view) but is implemented with packet switching. Services offered include: • Permanent virtual circuits that remain in place for long periods of time. • Switched virtual circuits that are set up and torn down with each request. • The method for establishing these circuits is shown in the Figure. The circuit is really entries in a series of switches, each mapping a circuit number onto a forwarding line. Why all the interest in ATM? • These days it is more common for companies to want to interconnect networks. There is one international standard for ATM networks and so interconnection is easier. • ATM can be used to provide both LAN and WAN networks. • ATM can be made to behave like other standard networks and so you do not have to throw away all your old equipment. • ATM provides a high speed network. ATM Technology • ATM is based on Cell Relay technology. This uses virtual circuits to carry small packets (just 53 bytes long) over a predetermined path through the network. • Section of the path can be shared by other virtual circuits, thus ensuring that the network is used more efficiently than in the case of circuit switching. • Little error checking is performed by the network (this keeps overheads down). Instead, the transmitting and receiving hosts are responsible for error checking. Establishing a Connection • When information needs to be communicated, the sender NEGOTIATES a "requested path" with the network for a connection to the destination. • When setting up this connection, the sender specifies the type, speed and other attributes of the call, which determine the end-to-end quality of service. • The network then determines a path through the network, and sets up a virtual circuit along this path. • An analogy for this is sending mail. One can choose to send 1st class, overnight, 2 day delivery, etc. and can ask for certified mail. ATM Cells The format of the ATM cell looks like this: Bits: 1 2 VP I 16 3 1 8 VCI T P H CR C Payload 48 bytes VPI Virtual Path ID VCI T Virtual Channel ID Payload Type P Priority H Header CRC Checksum 5 bytes (40 bits) are used to carry header data and the remaining 48 bytes carry data. The Virtual Channel ID is used identify which virtual circuit the cell is to be routed along. ATM Cell Transmission – Unlike in synchronous communication, there is no requirement that cell input rigidly alternate among sources – Continuous cell stream is not required – ATM cells can be transmitted on many types of transmission technologies – ATM current common speed is 155Mbps (OC3c), 622Mbps (OC12) and 45Mbps (T3) also getting common. – Fiber media preferred. Short runs may be coax or twisted pair. ATM Speed • ATM can deliver data at rates of either 155.52 Mbps or 622.08 Mbps and higher data rates are likely to follow. • ATM can operate at these high speed because specialised switching mechanisms have been developed that can switch the short 53 byte cells very quickly through the network. • ATM can work over a variety of media. Coaxial cable and optical fibre are the most commonly used. • ATM technology is currently being used to develop the next generation of high-bandwidth telephone systems. ATM Switches Operation – Cells arrive at 360000 cells/sec per 155Mbps connection • Cycle time of 2.7 ms – Many ports per switching fabric • Fixed size cells facilitate fast switching – Switch cells • Keep discard rate very low • Never reorder cells on virtual circuit – Problem: Two cells destined for same output – Problem: Head-of-line blocking