Introduction There are two fundamentally different types of data: Digital -Computer produced signals that are binary, either on or off. Analog - Electrical signals which are shaped like the sound waves they transfer.

Introduction Data can be transmitted through a circuit in the same form they are produced. Data can also be converted from one form into the other for transmission over network circuits. Likewise, it is possible to translate analog voice data into digital form for transmission over digital computer circuits using a device called a codec.

Introduction Digital transmission offers five key benefits over analog transmission. Digital transmission produces fewer errors than analog transmission. Digital transmission is more efficient. Digital transmission permits higher maximum transmission rates. Digital transmission is more secure because it is easier to encrypt. Finally, and most importantly, integrating voice, video and data on the same circuit is far simpler with digital transmission.

Digital Transmission of Digital Data All computers produce binary data. For this data to be understood by both the sender and receiver, both must agree on a standard system for representing the letters, numbers, and symbols that comprise the messages.

Coding A character is a symbol that has a common, constant meaning. Characters in data communications, as in computer systems, are represented by groups of bits [1’s and 0’s]. The group of bits representing the set of characters in the “alphabet” of any given system are called a coding scheme, or simply a code.

Coding A byte is a group of consecutive bits that is treated as a unit or character. There are two predominant coding schemes in use today: United States of America Standard Code for Information Interchange (USASCII or ASCII) Extended Binary Coded Decimal Interchange Code (EBCDIC)

Transmission Modes Parallel Mode is the way the internal transfer of binary data takes place inside a computer. Serial Mode is the predominant method of transferring information in data communications.

Transmission Modes Parallel Mode Serial Mode

Baseband Transmission Digital transmission is the transmission of electrical pulses. Digital information is binary in nature in that it has only two possible states 1 or 0. Digital signals are commonly referred to as baseband signals. In order to successfully send and receive a message, both the sender and receiver have to agree how often the sender can transmit data (data rate).

Baseband Transmission With unipolar signaling technique, the voltage is always positive or negative (like a dc current). In bipolar signaling, the 1’s and 0’s vary from a plus voltage to a minus voltage (like an ac current). In general bipolar signaling experiences fewer errors than unipolar signaling because the signals are more distinct.

Baseband Transmission

Baseband Transmission Manchester encoding is a special type of unipolar signaling in which the signal is changed from a high to low or low to high in the middle of the signal. Manchester encoding is less susceptible to having errors go undetected, because if there is no transition, the receiver knows that an error must have occurred. Manchester encoding is commonly used in local area networks (ethernet, token ring).

Analog Transmission of Digital Data Telephone networks were originally built for human speech rather than data. Analog Transmission occurs when the signal sent over the transmission media continuously varies from one state to another in a wave-like pattern.

The North American Telephone System The telephone system (commonly called POTS for plain old telephone service) enables voice communication between any two telephones within its network. Houses and offices are connected to a telephone company end office (central office class 5) by a set of two twisted pair wires (called the local loop).

The North American Telephone System The end office is connected to a central office class 4 by a trunk line. The central offices are arranged in a hierarchy; a class 4 is connected to a class 3 office which is connected to a class 2 etc. The telephone system was originally designed as an analog system, but today, most trunk lines are digital.

Bandwidth on a Voice Circuit Every sound wave has two parts, half above the zero point (positive), and half below (negative) and three important characteristics. The height of the wave is called amplitude. The length of the sound wave is expressed as the number of waves per second or frequency, expressed in Hertz (Hz). The phase is the direction in which the wave begins. Bandwidth refers to a range of frequencies.

Bandwidth on a Voice Circuit Frequency: 1 Period/Sec = 1 Hertz

Bandwidth on a Voice Circuit Human hearing ranges from about 20 Hz to about 14,000 Hz (some up to 20,000 Hz). Human voice ranges from 20 Hz to about 14,000 Hz. The bandwidth of a voice grade telephone circuit is 0 to 4000 Hz or 4000 Hz (4 KHz). Guardbands prevent data transmissions from interfering with other transmission when these circuits are multiplexed using FDM.

Bandwidth on a Voice Circuit

Bandwidth on a Voice Circuit It is important to note that the limit on bandwidth is imposed by the equipment used in the telephone network. The actual capacity of bandwidth of the wires in the local loop depends on what exact type of wires were installed, and the number of miles in the local loop. Actual bandwidth in North America varies from 300 KHz to 1 MHz depending on distance.

Modulation Modulation is the technique that modifies the form of a digital electrical signal so the signal can carry information on a communications media. There are three fundamental methods of analog modulation of an analog signal: Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation(PM)

Modulation There are also three fundamental methods of analog modulation of a digital signal: Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK)

Amplitude Modulation and ASK

Frequency Modulation and FSK

Phase Modulation and PSK

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

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.

Sending Multiple Bits Simultaneously Trellis coded modulation (TCM) is an enhancement of QAM that combines phase modulation and amplitude modulation. The problem with high speed modulation techniques such as TCM is that they are more sensitive to imperfections in the communications circuit.

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.

Capacity of a Voice Circuit The capacity of a voice circuit (the maximum data rate) is the fastest rate at which you can send your data over the circuit. The maximum symbol rate in any circuit depends upon the bandwidth available and the signal to noise ratio. Voice grade lines provide a bandwidth of 3000 Hz.

Modems Modem is an acronym for Modulator/ Demodulator, and takes digital electrical pulses from a computer, terminal, or microcomputer and converts them into a continuous analog signal, for transmission over an analog voice grade circuit. It then re-converts the analog signal to its original digital format. Most modems accept commands from a microcomputer keyboard.

Modem Standards There are many different types of modems available today. Most modems support several standards so that they can communicate with a variety of different modems. Better modems can change data rates during transmission to reduce the rate in case of noisy transmission (fast retrain).

Modem Standards

Modem Standards V.22 V.32 and V.32bis V.34 and V.34bis 1200-2400 baud/bps, FSK V.32 and V.32bis full duplex at 9600 bps (2400 baud at QAM) bis uses TCM to achieve 14,400 bps. V.34 and V.34bis for phone networks using digital transmission beyond the local loop.uses rates above the 2400 baud. Up to 28,800 bps (TCM) bis up to 36,600 with TCM

Modem Standards V.42bis data compression modems, accomplished by run length encoding, code book compression, Huffman encoding and adaptive Huffman encoding MNP5 - uses Huffman encoding to attain 1.3:1 to 2:1 compression. bis uses Lempel-Ziv encoding and attains 3.5:1 to 4:1. V.42bis compression can be added to almost any modem standard effectively tripling the data rate.

Modem Standards There are two drawbacks to the use of data compression: Compressing already compressed data provides little gain. Data rates over 100 Kbps place considerable pressure on the traditional microcomputer serial port controller that controls the communications between the serial port and the modem.

Digital Transmission of Analog Data Analog voice data can be sent over digital networks using a pair of special devices called CODECs (Coder/Decoder). Operation is very similar to how modems function.

Pulse Amplitude Modulation Analog voice data must be translated into a series of binary digits before they can be transmitted. With Pulse Amplitude Modulation, the amplitude of the sound wave is sampled at regular intervals and translated into a binary number. The difference between the original analog signal and the translated digital signal is called quantizing error.

Pulse Amplitude Modulation

Pulse Amplitude Modulation

Pulse Amplitude Modulation

Pulse Amplitude Modulation For standard voice grade circuits, the sampling of 3300 Hz at an average of 2 samples/second would result in a sample rate of 6600 times per second. There are two ways to reduce quantizing error and improve the quality of the PAM signal. Increase the number of amplitude levels Sample more frequently (oversampling).

Pulse Code Modulation Pulse Code Modulation is the most commonly used technique in the PAM family and uses a sampling rate of 8000 samples per second. Each sample is an 8 bit sample resulting in a digital rate of 64,000 bps (8 x 8000).

Analog/Digital Modems (56k Modems) The V.34+ modem is probably the fastest analog modem that will be developed. The basic idea behind 56K modems (V.pcm) is simple. 56K modems take the basic concepts of PCM and turn them backwards. They are designed to recognize an 8-bit digital signal 8000 times per second. Subtract the one bit in the PCM symbol used for control, and the maximum data rate becomes 56K.

Analog/Digital Modems (56k Modems) Noise is a critical issue. Recent tests found 56K modems to connect at less than 40 Kbps 18% of the time, 40-50 Kbps 80% of the time, and 50+ Kbps only 2 % of the time. It is easier to control noise in the channel transmitting from the server to the client than in the opposite direction. Because the current 56K technology is based on the PCM standard, it cannot be used on services that do not use this standard.