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COMP 421 /CMPET 401. COMMUNICATIONS and NETWORKING Chapter 3 (Continued) Data Transmission. History. Although electricity has been known to exist for centuries, experiments into its friendly use did not begin until the 1700s when scientists such as Volta, Ampere, and Watt explored
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COMP 421 /CMPET 401 COMMUNICATIONS and NETWORKING Chapter 3 (Continued) Data Transmission
History Although electricity has been known to exist for centuries, experiments into its friendly use did not begin until the 1700s when scientists such as Volta, Ampere, and Watt explored ways to harness its potential. In the 19th century, other scientists used electricity to invent the telegraph, telephone, and radio. With the advent of Marconi’s transoceanic wireless and the inauguration of radiotelegraph service, the 20th century saw dramatic breakthroughs in communications technology. The world of telecommunications has since exploded to include such developments as television, communications satellites, lasers, and fiber optics. Each new invention, like all of its predecessors, relies on parts of the electromagnetic spectrum to carry information from origin to destination.
Bandwidth • Width of the spectrum of frequencies that can be transmitted • if spectrum=300 to 3400Hz, bandwidth=3100Hz • Greater bandwidth leads to greater costs • Limited bandwidth leads to distortion • Analog measured in Hertz • Digital measured in baud or Bps
Bandwidth The most common means for measuring path/circuit/channel size is describing the "range" (bandwidth) of radio frequency (RF) spectrum necessary to carry the information assigned to a particular path, channel, circuit, etc. The wider the path (the larger the bandwidth), the greater its capacity. Bandwidth is expressed in hertz (Hz) or cycles per second (CPS) of radio frequency. One Hz equals one CPS. The capacity of paths that carry digital data is usually expressed in kilobits or megabits per second (kbps/Mbps) as a more meaningful measure of data throughput capacity.
Analog and Digital Data Transmission • Data • Entities that convey meaning • Signals • Electric or electromagnetic representations of data • Transmission • Communication of data by propagation and processing of signals
Bit rate and Baud rate • Bit rate number of bits that are transmitted in a second • Baud rate number of line signal changes (variations) per second If a modem transmits 1 bit for every signal change bit rate = baud rate If a signal change represents 2 or more or n bits bit rate = baud rate *n
Switching Switching is the means by which traffic is routed through a communications or system network. Switches may be manual (operator assisted) or automatic; they may serve local (in a city or on a military base) subscribers or perform area network (tandem [many switches connected to one another]) functions. An electrical path established between terminals, switches, and/or transmission systems is commonly referred to as a line, circuit, or channel. The "size" of the path is important in determining the full capability or capacity.
Switching Systems Electronic Switching Systems: A switch using solid-state switching devices and computer software that provides preprogrammed instructions to accomplish the switching of calls. Digital Switches: An electronic switching system that processes all signals to be switched into a digital mode. The circuit switch can also be used to route record and data traffic from a terminal to the nearest message switch for further processing. This dial-up, or hybrid, switching method uses the data adapter/RS-232 port feature of a digital telephone to accommodate a teletypewriter/data facsimile terminal connection to the circuit switch, which then routes the traffic forward as if it were a telephone call.
Switching Types There are three broad types of switches: circuit, message, and packet: Circuit switching - is the process of interconnecting a specific circuit to provide a direct connection between calling and called stations. For example, a local civilian telephone company interconnects telephone calls through its central office computerized circuit switch. Message switch = accepts a group of characters called a message, reads the message’s attached routing information, and stores it in computer memory. When a circuit path becomes available, the message is forwarded either to its destination or to another message switch closer to its destination. Message switches are called “store and forward” because they receive and store an entire message before sending it on its way. Packet switching - is a specialized technique of dividing messages into many standardized transmission blocks (packets), whereby the switching center does not store the packets, but routes them through a network independent of each other. At the destination, the packets are reassembled into the original message. Packet switching is an efficient and relatively inexpensive method to transfer data between local area networks.
Circuit Switches Circuit switches principally route voice telephone traffic, while message switches route the electrical form of hard copy messages. Message switches are further categorized as either store and forward or packet. A store and forward switch receives and electrically stores an entire message, retrieves it, determines where it should go, and routes it to its destination either directly or through another switch. Packet switches, which are especially adept at handling data, receive message segments (packets)
Circuit Switching Every time a telephone is used, a circuit switch routes the call. The switching center serves as the focal point for the interconnection of subscribers, via trunk circuits, to subscribers at other locations. In a circuit switched network, the calling party is connected to an end office (private branch exchange, or PBX) via a "subscriber loop." When the caller lifts the handset, a signal sent to the end office indicates a request for service. The end office switch places a dial tone on the loop, which alerts the caller that the switch is prepared to accept his calling instructions. The caller issues instructions to the switch by dialing the digits of the subscriber being called. These digits appear as dial pulses in the case of a rotary dial or as multiple frequency tones (touch-tones) that represent discrete digits
Circuit Switching The switch interprets these digits as an indicator of the destination of the call, and, through preprogrammed instructions, logically and sequentially executes the actions to complete the call. Call processing is never a random process: it adheres to strict procedural rules established in the preprogrammed instructions. The following are typical circuit switches that may be used in a network. Electromechanical Switches: A long proved application whereby program control is executed by preset electrical/mechanical relays. Stored Program Control Switches: Sometimes a hybrid, where the switching is completed by electrical/mechanical relays under the direction of a computer-like stored program.
Message Switching A message switch is a central routing mechanism for teletypewriter and low-speed data information. The majority of switching networks in service employ the store and forward message switch technique. A switch simply receives and stores a message, retrieves and determines where it is addressed, and routes it to the next appropriate node. This process is particularly valuable in handling multi-addressed messages for near simultaneous delivery without the need for retransmissions. In military message switching networks, a precedence system provides expedited service for critical messages having a higher priority or urgency than the other messages. Message precedences from lowest to highest are routine, priority, immediate, and flash.
Packet Switching Packet Switching The packet switched network is used to route digital data traffic, including electronic mail (E-mail). In packet switched networks, the subscriber transmits the message to the switching center as a total message. The message is next divided into discrete packets and routed over any available transmission path to the next node. Each node of the packet switched network contains an internal processor that constantly surveys the traffic loads and conditions throughout the network. Upon receipt of the packets at the destination node, the switch reassembles the packets in sequence for delivery to the addressee.
Packet Switching Packet switching requires highly structured protocols to maintain network status and control of the packets. The preamble to each packet must contain identification of the message. A normal schedule is to limit the message to approximately eight packets of 1000 bits per packet (125-250 characters). The nodal points within a network do not store the messages except for the very brief time it takes to “packetize” the message and forward it through the network. Therefore, if an incomplete message is received by the addressee, the originator must retransmit the message. The packet switched network is designed to handle computer-to-computer exchanges, interactive queries to a computer, and batch processing, as well as processing narrative traffic, such as E-mail. However, since this technology does not switch whole messages, it is an uneconomical method to use for multi-addressed traffic.
Frequency Division Multiplexing The oldest form of multiplexing, FDM, divides a circuit into several smaller channels by frequency for simultaneous transmission. FDM, which is analog only, allows a user to cluster many terminals at a given location and share the same transmission path. Because the bandwidth can be divided into just so many parts, the number of terminals supported is limited. The speed or transmission rate of each channel is reduced due to narrower channel bandwidth. Thus, digital signals, such as data, must first be converted to analog.
FDM For example, when 12 individual 4-kHz analog voice channels are fed to an FDM multiplexer (channel bank), each channel is assigned a frequency slot until all 12 channels are allocated. The channel bank output, a composite 48-kHz analog signal, is sent to the receiver by some transmission path where the reverse process (demultiplexing) occurs, restoring the original 12 channels. FDM can be used for voice, teletypewriter, analog data, and facsimile.
Time Division Multiplexing In TDM, a digital multiplexing scheme, each individual channel, called a subchannel, is allocated the entire transmission bandwidth for specific regular intervals or time slots. A time slot is allocated to each subchannel whether or not information is being transmitted. TDM is more flexible than FDM and allows the user to vary the number or duration of the time slots, depending on network requirements. In slightly more technical terms.
TDM TDM allows simultaneous transmission of two or more signals by sampling each approximately 8000 times a second. Each channel sample is trans-formed into a pulse that is further coded to represent the incoming signal. The pulses from the channel sample are multiplexed in time. Each pulse is sequenced in a serial time slot of the output of the channel bank. TDM can accept various numbers of low-, medium-, or high-speed data channels directly and sequence them into a higher capacity data stream. TDM multiplexing is also referred to as pulse code modulation. Most FDM tactical multiplex equipment is rapidly being replaced by newer digital TDM equipment
Terminals Terminals are the most recognizable communications components. A telephone, radio, facsimile, computer, television, and teletypewriter are all examples of terminals used to transmit (send) and receive information. Information, often called traffic, can take the form of voice, data, message, video, or other means. Traffic may be secure (encrypted/covered) or nonsecure (clear). Radios and telephones are the common terminals associated with voice communications. Facsimiles transmit and receive maps, photographs, sketches, and printed or handwritten text. Teletypewriters or printers attached to computers are used for messages and cables. This is often referred to as "record traffic" because the printer produces a "hard copy" record of what is received. Data terminals and computers transmit and receive binary data, while video terminals communicate imagery and sound.
Inter-Relationship of Terminals, Switches and Switching Facilities
Data Rate The speed of transmission of digital data is reflected as a "data rate." As data rate increases, so does the bandwidth of the path, channel, or circuit carrying the data stream.. A single 4-kHz-wide VF channel can also be subdivided by bandwidth to accommodate modes other than voice. For example, one VF channel can be multiplexed to commonly provide up to 16 teletypewriter (TTY) channels of about 200 Hz each.
Voice Circuits A typical voice conversation transmitted over a standard telephone line or circuit requires a radio frequency of sufficient bandwidth to handle the range of voice variations (called modulations) needed to convey information. This voice frequency (VF) range is approximately 3-4 kHz wide (1000 hertz = kHz) and is the standard for defining a single channel or "narrowband" circuit. Circuits established between switches are called trunks. Trunks ride transmission systems that are normally equipped with a multi-channel capability and employ modems. They use various protocols (rules and codes) required for processing traffic in a switched network.
Duplex Circuits Circuits may be established to provide a one-way, one-way reversible, or simultaneous two-way traffic capability, depending upon user need and the availability of assets. The one-way reversible circuit is commonly referred to as "half duplex," meaning that traffic can be passed in either direction but in only one direction at a time. A simultaneous two-way path is referred to as "full duplex"--traffic can be passed in either or both directions at the same time. The simple one-way-only path is often referred to as a "receive-only" or "transmit-only" circuit.
Modem A modem (a contraction of "modulator" and "demodulator") is a device that converts incoming and outgoing electronic signals from one form to another. For example, the digital output of a home computer may be fed to a modem that converts the digital bit stream (digital signal) into a series of audio tones (analog signals) transmitted over an analog-capable standard-grade public telephone circuit. The home computer modem may also convert an inbound analog signal from a telephone line to a digital signal that can be used by the computer..
Analog and Digital Transmission Analog signals are represented as continuous wave-like (sine wave) signals, such as those electrical signals generated by a telephone keyed to a human voice. The analog signal varies by frequency in cycles per second, or hertz, to represent different voice sounds. Analog voice signals are transmitted over a channel in one of two ways: at their original frequency as a baseband signal or modulated onto another (carrier) signal and transmitted at the carrier frequency. Voice is transmitted in the audio frequency range of about 300 to 3400 Hz, or a voice signal bandwidth of 3100 Hz. Invariably, long-distance calls will be modulated to a higher frequency for transmission to achieve greater efficiency and lower cost.
Digital Signals By contrast, digital signals are discrete and consist of two possible states: the presence or absence of an electrical signal (on or off), or two different electrical signal levels. Communications between computers use a digital form, where information is conveyed as binary digits or bits (1s or 0s). For digital communications, the bandwidth describes the amount of data that can be transmitted on a channel over time in bits per second (bps). High-speed data networks might offer bandwidths of 10 to 50 Mbps, while telephone systems used for digital transmission offer a bandwidth of 1.544 Mbps, the standard commercial T-1 carrier rates.
Channel Capacity Voice lines used in conjunction with modems to transmit data typically offer speeds of 2.4 and 19.2 kbps, and wide data compression yielding asynchronous throughputs up to 76.8 kbps. To conserve channel capacity, the multiplexing of many signals into one channel can be accomplished normally using FDM or TDM techniques. The FDM method requires that digital signals be converted into analog. This conversion of analog-to-digital (A/D) and digital-to-analog (D/A) is usually accomplished by a modem, which is often used to transmit digital data over a telephone (analog) network. A/D and D/A conversion; and it is incompatible with existing analog equipment.
Digitized Analog There is a bandwidth penalty for digitizing analog signals. For example, in a voice transmission, the bandwidth requirement increases 16-fold, in comparison to music that is 20:1 digital versus analog ratio. By converting signals from a digital telephone to analog prior to transmission, however, the output can be sent over a standard 4 kHz voice frequency circuit versus a wideband path.
Transmission Rate Vs. Information Rate In general, the greater the information rate (or resolution), the larger the transmission bandwidth. Required transmission bandwidth varies directly with desired information speed. A single voice conversation or teletype channel can easily be accommodated by a nominal 3- kHz-wide voice frequency channel. However, beyond voice, required bandwidth increases precipitously. Digital facsimile requires 7.5 to 50 kHz, high-speed data more than 100 kHz, and full-motion video 4 MHz.
Information Rate In theory, information rate can equal but never exceed the transmission rate. In practice, the actual information rate measured in wpm or bps is almost always slower than the transmission rate, and is frequently significantly so. There are several reasons for this. Many times, information rate is dictated by the speed of the user’s terminal device, which may be much slower than the transmission “pipe.” This will be the case with slow-speed teletypewriters and with the human voice.
Bits and Bytes A Bit is actually a contraction for the term "binary digit" and is the smallest unit of information used in data communications. It is represented by either a zero or a one. A Byte is a grouping of bits that may or may not be translatable to the user. Although the number of bits in a byte depends upon the type of communications equipment in use, eight is the most common. Bit rate is a measure of transmission and speed and is usually expressed in bits per second (bps); bit rates greater than 1000 are given in kbps, greater than one million in Mbps, etc. Baud (or sometimes Baud Rate) is the number of signal transitions per period of time on the phone line. Bit and baud rates are typically the same at 300 bps, but baud rates are limited to 2400 (the approximate bandwidth of a phone line). It is possible to transmit about 2400 bps by coding/increasing the number of bits per baud (e.g., 4800 bps can be transmitted at 2400 baud by encoding two bits per baud).
Transmission rates Many factors determine transmission and information rates. The transmission rate of a network or link includes the trunk speed in bits per second of the trunk and switching facilities or both. The information rate (traffic throughput), on the other hand, takes into consideration the entire path from one user terminal (A) to another (B), and reflects the speed at which the receive end can accept information from the transmit end as dictated by the slowest terminal in the loop. For example, if terminal A transmits data at 19.2 kbps to terminal B, which can handle traffic input at only 4.8 kbps, the resultant information rate is 4.8 kbps. The difference is stored/buffered by the system to accommodate the lower rate of the receive terminal.
Channel Capacity The capacity of a channel may be described as the maximum rate at which information can be sent over the channel without error. Bandwidth is the frequency range of a given electrical path or circuit. For data transmission purposes, channel capacity is measured in bps. The rate at which data may be transmitted is proportional to the channel bandwidth. Overall, data channel capacity is a function of volume, rate or speed, and the quality of the transmission path. These factors are important to the planner because they dictate how well any given transmission link can pass intelligence data from one point to another.
Transmission Speeds Different terminal types have various transmission speed requirements. Standard 4-kHz narrowband telephone channels can handle data transmission speeds up to 9.6 kbps. Data sent faster than 9.6 kbps over these channels becomes unintelligible. Consequently, high-speed modems operating at speeds from over 9.6 kbps to 56 kbps or higher will not work using this type of transmission channel.
Transmission Speeds The maximum data transmission speeds achieved over a given bandwidth are considerably lower than the theoretical maximum. These speeds fall into the shaded area on the above graphic. The dotted lines are theoretical maximum speeds in the presence of noise, and are calculated for an error rate of one error bit in every 10,000 bits transmitted. One way to increase the capacity of a channel is to raise the signal-to-noise ratio. While some types of noise can partially be controlled, the level of random noise is determined by natural phenomena, which are uncontrollable. There is a level below which noise cannot be suppressed.
