Analog Transmission of Digital Data

The Telephone Network Originally designed for analog communications only. Today, standard analog telephone service is called POTS (Plain Old Telephone Service). Modem communications use the telephone network to send digital data that has been converted into an analog format.

Carrier Waves Modems use carrier waves to send information (Figure 3-13). Each wave has three fundamental characteristics: –Amplitude, meaning the height (intensity) of the wave –Frequency, which is the number of waves that pass in a single second and is measured in Hertz (cycles/second) (wavelength, the length of the wave from crest to crest, is related to frequency.). –Phase is a third characteristic that describes the point in the wave’s cycle at which a wave begins and is measured in degrees. (From example, changing a wave’s cycle from crest to trough corresponds to a 180 degree phase shift).

Figure 3-13 A Carrier Wave

Modulation Modulation is the modification of a carrier wave’s fundamental characteristics in order to encode information. There are three basic ways to modulate a carrier wave: –Amplitude Modulation –Frequency Modulation –Phase Modulation

Amplitude Modulation Amplitude Modulation (AM), also called Amplitude Shift Keying (ASK), means changing the height of the wave to encode data. The AM dial on the radio uses amplitude modulation to encode analog information. Figure 3-14 shows a simple case of amplitude modulation in which one bit is encoded for each carrier wave change. –A high amplitude means a bit value of 1 –Zero amplitude means a bit value of 0

Figure 3-14 Amplitude Modulation

Frequency Modulation Frequency Modulation (FM), also called Frequency Shift Keying (FSK), means changing the frequency of the carrier wave to encode data. The FM dial on the radio uses frequency modulation to encode analog information. Figure 3-15 shows a simple case of frequency modulation in which one bit is encoded for each carrier wave change. –Changing the carrier wave to a higher frequency encodes a bit value of 1 –No change in the carrier wave frequency means a bit value of 0

Figure 3-15 Frequency Modulation

Phase Modulation Phase refers to the point in each wave cycle at which the wave begins. Phase Modulation (PM), also called Phase Shift Keying (PSK) means changing the phase of the carrier wave to encode data. Figure 3-16 shows a simple case of phase modulation in which one bit is encoded for each carrier wave change. –Changing the carrier wave’s phase by 180 o corresponds to a bit value of 1 –No change in the carrier wave’s phase means a bit value of 0

Figure 3-16 Phase Modulation

Sending Multiple Bits Simultaneously Each of the three modulation techniques can be refined to send more than one bit at a time. It is possible to send two bits on one wave by defining four different amplitudes. This technique could be further refined to send three bits at the same time by defining 8 different amplitude levels or four bits by defining 16, etc. The same approach can be used for frequency and phase modulation.

Sending Multiple Bits Simultaneously

In practice, the maximum number of bits that can be sent with any one of these techniques is about five bits. The solution is to combine modulation techniques. One popular technique is quadrature amplitude modulation (QAM) involves splitting the signal into eight different phases, and two different amplitude for a total of 16 different possible values.

Simplified Example Combination of Modulation Techniques

Bits Rate Versus Baud Rate Versus Symbol Rate The terms bit rate (the number of bits per second) and baud rate are used incorrectly much of the time. They are not the same. A bit is a unit of information, a baud is a unit of signaling speed, the number of times a signal on a communications circuit changes. ITU-T now recommends the term baud rate be replaced by the term symbol rate.

Bits Rate Versus Baud Rate Versus Symbol Rate The bit rate and the symbol rate (or baud rate) are the same only when one bit is sent on each symbol. If we use QAM or TCM, the bit rate would be four to eight times the baud rate.

Digital Transmission of Analog Data

Pulse Amplitude Modulation (PAM) An analog voice signal can be converted into digital form using a device called a codec (coder/decoder) which also converts it back to analog data at the receiving end. The codecs used by the phone system use Pulse Amplitude Modulation. PAM involves 3 steps (see Figures 3-19a-c): –Measuring the signal –Taking samples of the signal –Encoding the signal as a binary data sample

Quantizing

5ms

Quantizing

5ms

Quantizing

sps

Multiplexing

Multiplexing means breaking up a higher speed circuit into several slower circuits. The main advantage of multiplexing is cost; multiplexing is cheaper because fewer network circuits are needed. There are four categories of multiplexing: –Frequency division multiplexing (FDM) –Time division multiplexing (TDM) –Statistical time division multiplexing (STDM) –Wavelength division multiplexing (WDM)

Multiplexing

Frequency Division Multiplexing (FDM) FDM works by making a number of smaller channels from a larger frequency band (Figure 3-21). FDM is sometimes referred to as dividing the circuit “horizontally”. In order to prevent interference between channels, unused frequency bands called guardbands are used to separate the channels. Because of the guardbands, there is some wasted capacity on an FDM circuit. CATV uses FDM. FDM was also commonly used to multiplex telephone signals before digital transmission became common and is still used on some older transmission lines.

Figure 3-21 Frequency Division Multiplexing

Time Division Multiplexing (TDM) TDM allows multiple channels to be used by allowing the channels to send data by taking turns. TDM is sometimes referred to as dividing the circuit “vertically” Figure 3-22 shows an example of 4 terminals sharing a circuit, with each terminal sending one character at a time. With TDM, time on the circuit is shared equally with each channel getting a specified time slot, whether or not it has any data to send. TDM is more efficient than FDM, since TDM doesn’t use guardbands, so the entire capacity can be divided up between the data channels.

Figure 3-22 Time Division Multiplexing

Multiplexing - TDM Buffer Terminal 9600 bps Buffer TDM Mux Buffers 28.8kbps trunk

Statistical Time Division Multiplexing (STDM) STDM is designed to make use of the idle time created when terminals are not using the multiplexed circuit. Like regular TDM, STDM uses time slots, but the time slots are not fixed. Instead, they are used as needed by the different terminals on the multiplexed circuit. Since the source of a data sample is not identified by the time slot it occupies, additional addressing information must be added to each sample. If all terminals try to use the multiplexed circuit intensively, response time delays can occur. The multiplexer also needs to contain memory to store data in case more data samples come in than its outgoing circuit capacity can handle.

Statistical TDM Idle time No unused time slots Time slots on demand Total of input greater than multiplexed line Uses lower data rate or more devices on same data rate Requires additional overhead for addressing info Buffer Vs. Delay –Peak periods require larger buffer –Larger buffer means additional delay

Statistical TDM A B C D To Remote System Users/Data Source t0t0 t1t1 t2t2 t3t3 t4t4 A1A1 B1B1 C1C1 D1D1 A2A2 B2B2 C2C2 D2D2 First cycle Second cycle A1A1 B1B1 B2B2 C2C2 Extra bandwidth available First cycleSecond cycle Data Address Wasted Bandwidth

Wavelength Division Multiplexing (WDM) Optical fiber uses lasers or LEDs which previously transmitted at only a single frequency, with a typical transmission rate being around 622 Mbps. With WDM, data is transmitted at several different frequencies over the same fiber. The data transmission capacity of optical continues to increase dramatically. A new version of WDM, Dense WDM or DWDM promises data rates in the terabits, with over a hundred channels per fiber, each transmitting at a rate of 10 Gbps, making aggregate data rates in the low terabit range possible. Future versions of DWDM will make petabit aggregate transmission rates possible as per channel data rates and the total number of channels both continue to rise.

Inverse Multiplexing (Figure 3-24) Instead of using a single line, an inverse multiplexer (IMUX) shares the load by sending multiplexed data over two or more lines. For example, two T-1 lines can be used to send data, creating a combined multiplexed capacity of 2 x = Mbps. A recent IMUX standard, the Bandwidth ON Demand Network Interoperability Group (BONDING) standard, is often used for videoconferencing applications. With the BONDING standard, six 64 kbps lines can be combined to create an aggregate line of 384 kbps for transmitting video.

Figure Inverse Multiplexing

How DSL Transmits Data Digital Subscriber Line is becoming popular as a way to increase data rates in the local loop. Instead of using the KHz voice channel, DSL uses the physical capacity of the copper phone lines of up to 1 MHz. The 1 MHz capacity is split into: 1) a 4 KHz voice channel, 2) an upstream channel and 3) a downstream channel. There are several versions of DSL, with the main differences being how much of the bandwidth is allocated between the upstream and downstream channels. One form of DSL, G.Lite provides a 4 Khz voice channel, 384 kbps upstream and 1.5 Mbps downstream (provided line conditions are optimal).