Before we discuss various techniques of transmission on physical links, let us quickly review some characteristics and the types of physical media. A circuit could be just the point-to-point type connecting only two network elements, or could be a multi- point type where many network elements are connected through the same circuit. The fl ow of the transmission on these circuits could be simplex, half duplex, or full duplex indicated that the transmission could be in one direction only, in both direc- tions but one direction at a time, or both directions simultaneously, respectively. The physical media can be grouped into two categories: guided media where transmis- sion fl ows along a physical guide, and unguided media where there is no physical boundaries (transmission take place in the air). Guided media includes twisted pair wires, coaxial cables, and optical fi ber cables, where as wireless media (a.k.a., radi- ated media) includes radio (microwave, satellite) and infrared communications. Note that the electricity is used in twisted pair wires and coaxial cables, whereas the light created by lasers is used to transmit signals in optical fi ber cables. In the unguided category, transmission of electrical (electromagnetic) waves over the air is used in wireless media, whereas “invisible” light waves (frequency below the red light) is used in infrared communications.
Analog versus Digital Transmission
The types of data transmitting on these circuits could be analog or digital. Analog data can take on any value at any time in a wide range of possibilities. (A sound wave and corresponding electrical, continuous, analog waves produced by telephones are good examples of analog data.) Digital data are produced by computers in a binary form as a series of ones and zeros. Similarly, a transmission could be analog or digital. In analog transmissions, the data are transmitted in an analog form (i.e., in a continu- ous wave format). In digital transmissions, the data are transmitted as square waves (i.e., pulses) with a clear beginning and ending. Typically, telephone circuits at the local loop use analog transmission whereas LAN circuits use digital transmission.
Data need to be converted between analog and digital formats to be transmitted appropriately. To do these, conversion-specifi c devices are used: a modem (modulator/ demodulator) used to convert digital data into analog form that is suitable for sending it over the circuits using analog transmission, whereas a codec (coder/decoder) used to convert analog data into the digital format that is suitable for sending it over the circuits using digital transmission.
Techniques for Transmitting Digital Data over Digital Circuits
At the physical layer, signaling (a.k.a., encoding) refers to the representation of bits (0 and 1) in electrical voltages. (Note that the term signaling is also used in telecom- munications to refer to an overlay network and related procedures and protocols to setup and disconnect calls.) In digital transmission, signals are sent as a series of “square waves” of either positive or negative voltage corresponding to 0 or 1. Typical values of voltage levels vary between +3/–3 and +24/–24 depending on the circuit. The signaling (encoding) then defi nes what voltage levels correspond to a bit value of 0 or 1. There are a number of different encoding techniques. We mention only
three of them to illustrate the concept: unipolar, bipolar, and Manchester. The uni- polar approach uses voltage levels that vary between 0 V and a positive value or between 0 V and some negative value. The bipolar techniques use both positive and negative voltages. A bit transmitted via a bipolar method has a better chance of being interpreted correctly at the destination since the signals are more distinct (more diffi cult for interference to change the polarity of the current). Figure 3.16 illustrates these techniques.
The Manchester encoding used by Ethernet defi nes a bit by a mid-bit transition of voltage value. As shown in Figure 3.16, a high- to low-voltage transition represents a 0 and a low- to high-mid-bit transition defi nes a 1. The advantage of the Manchester scheme is that it is less susceptible to having errors go undetected because simply no transition indicates that an error took place. The duration of an interval in which a transition takes place depends on the specifi cations of the protocol. This time period (i.e., the interval), in turns, defi nes the data rate of the circuit. For example, if the time period is 1/64,000 of a second, then, the data rate will be 64 Kbps (sending a bit every 1/64,000 of a second).
Techniques for Transmitting Digital Data over Analog Circuits
In the case of analog circuits, digital data (0s and 1s) must be modulated into analog format. A well-known example for this case is the use of phone lines to connect PCs to the Internet. A modem is used for this purpose between the computer and the phone line. Modems simply convert the digital data into the analog format. Another modem must be used at the receiver to regenerate the digital data. There are a num- ber of modulation techniques used by the modems: Amplitude Modulation (AM), Frequency Modulation (FM) and Phase Modulation (PM). They are all based on the idea of using an agreed upon so-called carrier wave signal between the sender and the receiver. Each bit is, then, sent, in an interval, by changing one of the charac- teristics (i.e., amplitude, frequency, or phase) of the carrier. These basic modulation
1 0 1 1 0 0 1 0 0 0 1 Unipolar Bipolar Manchester +5V 0V +5V 0V +2V –2V –5V 0V
schemes typically send one bit (of information) at a time; 1 bit encoded for each symbol (carrier wave change Æ 1 bit per symbol). It is also possible to send multiple bits simultaneously. Multiple bits per symbol might be encoded using amplitude, frequency, and phase modulation (or combination of them). A widely used modula- tion scheme, Quadrature Amplitude Modulation (QAM), combines Amplitude and Phase Modulation (a common form: 16-QAM uses eight different phase shifts and two different amplitude levels). This implies that there are 16 possible symbols and therefore we can transmit 4 bits/symbol. Various modem standards defi ne these modulation schemes and other interface details. For example, on coaxial cables modems use 64-QAM (or 256-QAM) on downstream modulation, whereas ADSL modems on the existing local telephone lines use a QAM technique with up to 15 bits/symbol.
Techniques for Transmitting Analog Data over Digital Circuits
In the case of the analog data (such as sound, image, movies) sent over a digital network, a pair of special devices called codec are used. This is a device that con- verts the analog signal into a digital format. During this conversion, the analog data is translated into a series of bits before transmission of a digital circuit. This is performed by a technique called Pulse Amplitude Modulation (PAM) involving three steps: measuring the signal by using an agreed upon set of amplitude levels, encoding the signal as a binary data sample, and taking samples of the signal. This creates a rough (digitized) approximation of original signal. The more number of amplitude levels you have and the more samples you take, the better sound quality you get. The wire line telephone network uses 128 levels of amplitudes (resulting in a 7-bit representation) and a music codec may use more than 65,536 levels of amplitudes resulting in a 16-bit representation. The telephone network uses 80,000 samples per second whereas a music player may use about 64,000 and more samples per second.