Physical Layer 9/17/2018 Dr Kamesh Duvvuri

Communication Satellites Geostationary Satellites Medium-Earth Orbit Satellites Low-Earth Orbit Satellites Satellites versus Fiber

Structure of Telephone System Major components: 1. Local loops (analog twisted pairs going into houses and businesses). 2. Trunks (digital fiber optics connecting the switching offices). 3. Switching offices (where calls are moved from one trunk to another) 9/17/2018 Dr Kamesh Duvvuri

Modems Computer is digital Telephone line is analog Need translation device called a modem Analog Signal Digital Signal Modem 9/17/2018 Dr Kamesh Duvvuri

MODEM 9/17/2018 Dr Kamesh Duvvuri

Modulation A Modem is a Modulator and Demodulator Modulation is converting outgoing digital device signals into analog transmission line signals Demodulation is converting incoming analog transmission line signals into digital device signals Analog Signal Digital Signal Modem 9/17/2018 Dr Kamesh Duvvuri

Problems in Transmission Lines Attenuation is the loss of energy as the signal propagates outward. Delay distortion is caused by different Fourier components propagating at different speeds in the wire. Noise is unwanted energy from sources other than the transmitter. Thermal noise is caused by the random motion of the electrons in a wire and is unavoidable. Sometimes when talking on the telephone, you can hear another conversation in the background. That is crosstalk. DC Signaling is subject to strong attenuation and delay distortion. Hence on telephone lines, AC signaling is used. 9/17/2018 Dr Kamesh Duvvuri

Why modulation is necessary ? Signals are transmitted between a transmitter over some form of transmission medium But normally signals are not in the form that is suitable for transmission and need to be transformed Bandwidth requirement Signals multiplexing Complexity of transmission system Preventing noise, interference, attenuation Modulation is a process of impressing (applying) a low frequency information signals to onto a relatively high frequency carrier signal 9/17/2018 Dr Kamesh Duvvuri

Modulation techniques 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. 9/17/2018 Dr Kamesh Duvvuri

9/17/2018 Dr Kamesh Duvvuri

Bandwidth of a medium is the range of frequencies that pass through it with minimum attenuation. It is a physical property of the medium and measured in Hz. The baud rate is the number of samples/sec made. Each sample sends one piece of information, that is, one symbol. The baud rate and symbol rate are thus the same. All advanced modems use a combination of modulation techniques to transmit multiple bits per baud. Often multiple amplitudes and multiple phase shifts are combined to transmit several bits/symbol.

What is DSL? Digital Subscriber Line-New modem technology. Data transmission is based on digital encoding (digital). Use digital coding techniques to provide more capacity. Allows high-speed Internet access over existing twisted-pair and ordinary copper telephone wires. Provides "always-on" connection. To transport high-bandwidth data. A special hardware attached to both the user and switch ends of line. 9/17/2018 Dr Kamesh Duvvuri

9/17/2018 Dr Kamesh Duvvuri

Advantages of DSL High-speed. Secure connection. No dial-up, waiting or dropped connections. It's always on connection. Saves both money and time. Provides large file transfers. Multiple users on a network can connect to a single DSL. 9/17/2018 Dr Kamesh Duvvuri

What is ADSL? Asymmetric Digital Subscriber Line. Is a form of DSL. A high-speed Internet access service. Speed depends on the length and the diameter of the cable and the type of the mode Requires a special ADSL modem and an Internet service provider (ISP) . 9/17/2018 Dr Kamesh Duvvuri

9/17/2018 Dr Kamesh Duvvuri

What is ADSL? It is asymmetric since the data coming to your computer from the Internet (download) is faster than the data traveling from your computer to the Internet (upload). Uses standard telephone lines. Telephone can be used normally, even when surfing in the Web with ADSL service. An "always on" service. Not available to everyone. 9/17/2018 Dr Kamesh Duvvuri

ADSL(Asymmetric Digital Subscriber Line) Operation of ADSL using discrete multitone modulation. 9/17/2018 Dr Kamesh Duvvuri

Trunks & Multiplexing Trunks (digital fiber optics connecting the switching offices). How to collect multiple calls together and send them out over the same fiber. This subject is called multiplexing. Telephone companies have developed elaborate schemes for multiplexing many conversations over a single physical trunk. 9/17/2018 Dr Kamesh Duvvuri

Multiplexing Multiplexing schemes can be divided into two basic categories: FDM (Frequency Division Multiplexing) TDM (Time Division Multiplexing). In FDM, the frequency spectrum is divided into frequency bands, with each user having exclusive possession of some band. In TDM, the users take turns (in a round-robin fashion), each one periodically getting the entire bandwidth for a little burst of time. Advanced FDM applied to fiber optics called WDM (wavelength division multiplexing). Advanced TDM system used for fiber optics (SONET)Synchronous Optical Network. 9/17/2018 Dr Kamesh Duvvuri

Multiplexing Multiplexor (MUX) Demultiplexor (DEMUX) Sometimes just called a MUX 9/17/2018 Dr Kamesh Duvvuri

Multiplexing Two or more simultaneous transmissions on a single circuit. Transparent to end user. Multiplexing costs less. 9/17/2018 Dr Kamesh Duvvuri

Frequency Division Multiplexing Assignment of non-overlapping frequency ranges to each “user” or signal on a medium. Thus, all signals are transmitted at the same time, each using different frequencies. A multiplexor accepts inputs and assigns frequencies to each device. The multiplexor is attached to a high-speed communications line. A corresponding multiplexor, or DE multiplexor, is on the end of the high-speed line and separates the multiplexed signals. 9/17/2018 Dr Kamesh Duvvuri

9/17/2018 Dr Kamesh Duvvuri

Frequency Division Multiplexing Analog signaling is used to transmits the signals. Broadcast radio and television, cable television and the cellular phone systems use frequency division multiplexing. This technique is the oldest multiplexing technique. Since it involves analog signaling, it is more susceptible to noise.

Time Division Multiplexing Sharing of the signal is accomplished by dividing available transmission time on a medium among users. Digital signaling is used exclusively. Time division multiplexing comes in two basic forms: 1. Synchronous time division multiplexing, and 2. Statistical, or asynchronous time division multiplexing.

Wavelength Division Multiplexing Prisms form the basis of optical multiplexing and demultiplexing a multiplexor accepts beams of light of various wavelengths and uses a prism to combine them into a single beam a de multiplexor uses a prism to separate the wavelengths.

9/17/2018 Dr Kamesh Duvvuri

Switching Types of Switching Circuit switching Packet switching The phone system is divided into two principal parts: Outside plant (the local loops and trunks, since they are physically outside the switching offices) Inside plant (the switches), which are inside the switching offices. Types of Switching Circuit switching Packet switching 9/17/2018 Dr Kamesh Duvvuri

Circuit & packet Switching 9/17/2018 Dr Kamesh Duvvuri

Circuit Switching When you or your computer places a telephone call, the switching equipment within the telephone system seeks out a physical path all the way from your telephone to the receiver's telephone. This technique is called circuit switching. An important property of circuit switching is the need to set up an end-to-end path before any data can be sent. 9/17/2018 Dr Kamesh Duvvuri

Message Switching An alternative switching strategy is message switching. No physical path is established in advance between sender and receiver. Instead, when the sender has a block of data to be sent, it is stored in the first switching office (i.e., router) and then forwarded later, one hop at a time. Each block is received in its entirety, inspected for errors, and then retransmitted. A network using this technique is called a store-and-forward network

Packet Switching With message switching, there is no limit at all on block size, which means that routers (in a modern system) must have disks to buffer long blocks. Packet-switching networks place a tight upper limit on block size, allowing packets to be buffered in router main memory instead of on disk. In packet Switching individual packets are sent as need be, with no dedicated path being set up in advance. It is up to each packet to find its way to the destination on its own.

Timing of events in (a) circuit switching, (b) message switching, (c) packet switching 9/17/2018 Dr Kamesh Duvvuri