Data Transmission

Data transmission is essentially the same thing as digital com- munications, and implies physical transmission of a message as a digital bit stream, represented as an electro-magnetic signal, over a physical point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibers, wireless communication channels, and storage media.

Data transmission is a subset of the field of data communications, which also includes computer networking or computer communica- tion applications and networking protocols, for example, routing and switching.

Applications and History

The first data transmission applications in modern time were telegraphy (1809) and teletypewriters (1906). The fundamental meo- retical work in data transmission and information theory by Harry Nyquist, Ralph Hartley, Claude Shannon and omers during the early 20th century, was done with these applications in mind.

Data transmission is utilized in computers in computer buses and for communication with peripheral equipment via parallel ports and serial ports such us RS-232 (1969), Firewire (1995) and USB (1996). The principles of data transmission are also utilized in storage media for error detection and correction since 1951.

Data transmission is utilized in computer networking equipment such as modems (1940), local area networks (LAN) adapters (1964), repeaters, hubs, microwave links, wireless network access points (1997), etc.

In telephone networks, digital communication is utilized for trans- ferring many phone calls over the same copper cable or fiber cable by

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means of Pulse Code Modulation (PCM), i.e. sampling and digitiza- tion, in combination with Time Division Multiplexing (TDM) (1962). Telephone exchanges have become digital and software controlled, facilitating many value added services. For example the first AXE telephone exchange was presented in 1976. Since late 1980th, digital communication to the end user has been possible using Integrated Services Digital Network (ISDN) services. Since me end of 1990th, broadband access techniques such as ADSL, Cable modems, fiber- to-the-building (FTTB) and fiber-to-the-home (FTTH) have become wide spread to small offices and homes. The current tendency is to replace traditional telecommunication services by packet mode com- munication such as IP telephony and IPTV.

Protocols and Handshaking

Protocol

A protocol is an agreed-upon format for transmitting data between two devices, e.g.: computer and printer. All communications between devices require that the devices agree on the format of the data. The set of rules defining a format is called a protocol.

The protocol determines the following:

the type of error checking to be used if any, e.g.: check digit (and what type/ what formula to be used);

data compression method, if any, e.g.: zipped files if the file is large, like transfer across me Internet, LANs and WANs.

how the sending device will indicate that it has finished sending a message, e.g.: in a Communications port a spare wire would be used, for serial (USB) transfer start and stop digits maybe used.

how the receiving device will indicate that it has received a mes- sage

rate of transmission (in baud or bit rate)

whether transmission is to be synchronous or asynchronous.

In addition, protocols can include sophisticated techniques for detecting and recovering from transmission errors and for encoding and decoding data.

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Handshaking

Handshaking is the process by which two-devices initiate com- munications, e.g.: a certain ASCII character or an interrupt signal/ request bus signal to the processor along the Control Bus. Handshaking begins when one device sends a message to another device indicating that it wants to establish a communications channel. The two devices then send several messages back and forth that enable them to agree on a communications protocol. Handshaking must occur before data transmission as it allows the protocol to be agreed.

ASYNCHRONOUS AND SYNCHRONOUS DATA TRANSMISSION

Asynchronous and synchronous communication refers to methods by which signals are transferred in computing technology. These signals allow computers to transfer data between components within the computer or between the computer and an external network. Most actions and operations that take place in computers are carefully controlled and occur at specific times and intervals. Actions that are measured against a time reference, or a clock signal, are referred to as synchronous actions. Actions that are prompted as a response to another signal, typically not governed by a clock signal, are referred to as asynchronous signals.

Typical examples of synchronous signals include the transfer and retrieval of address information within a computer via the use of an address bus. For example, when a processor places an address on the address bus, it will hold it there for a specific period of time. Within this interval, a particular device inside the computer will identify itself as the one being addressed and acknowledge the commencement of an operation related to that address.

In such an instance, all devices involved in ensuing bus cycles must obey the time constraints applied to their actions — this is known as a synchronous operation. In contrast, asynchronous signals refer to the operations that are prompted by an exchange of signals with one another, and are not measured against a reference time base. Devices that cooperate asynchronously usually include modems and many net- work technologies, bom of which use a collection of control signals to notify intent in an information exchange. Asynchronous signals, or extra control signals, are sometimes referred to as handshaking signals

because of the way they mimic two people approaching one another and shaking hands before conversing or negotiating.

Within a computer, both asynchronous and synchronous protocols are used. Synchronous protocols usually offer the ability to transfer information faster per unit time than asynchronous protocols. This happens because synchronous signals do not require any extra negotia- tion as a prerequisite to data exchange. Instead, data or information is moved from one place to another at instants in time that are measured against the clock signal being used. This signal is usually comprised of one or more high frequency rectangular shaped waveforms, gener- ated by special purpose clock circuitry. These pulsed waveforms are connected to all the devices that operate synchronously, allowing them to start and stop operations with respect to the clock waveform.

In contrast, asynchronous protocols are generally more flexible, since all the devices that need to exchange information can do so at their own natural rate — be these fast or slow. A clock signal is no longer necessary; instead the devices that behave asynchronously wait for the handshaking signals to change state, indicating that some transaction is about to commence. The handshaking signals are gener- ated by the devices themselves and can occur as needed, and do not require an outside supervisory controller such as a clock circuit that dictates the occurrence of data transfer.

Asynchronous and synchronous transmission of information oc- curs both externally and internally in computers. One of the most popu- lar protocols for communication between computers and peripheral devices, such as modems and printers, is the asynchronous RS-232 protocol. Designated as the RS-232C by the Electronic Industries As- sociation (EIA), this protocol has been so successful at adapting to the needs of managing communication between computers and supporting devices, that it has been pushed into service in ways that were not intended as part of its original design. The RS-232C protocol uses an asynchronous scheme that permits flexible communication between computers and devices using byte-sized data blocks each framed with start, stop, and optional parity bits on the data line. Other conductors carry the handshaking signals and possess names that indicate their purpose — these include data terminal ready, request to send, clear to send, data set ready, etc.

Another advantage of asynchronous schemes is that they do not demand complexity in the receiver hardware. As each byte of data has

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its own start and stop bits, a small amount of drift or imprecision at the receiving end does not necessarily spell disaster since the device only has to keep pace with the data stream for a modest number of bits. So, if an interruption occurs, the receiving device can re-establish its operation with the beginning of the arrival of the next byte. This ability allows for the use of inexpensive hardware devices.

Although asynchronous data transfer schemes like RS-232 work well when relatively small amounts of data need to be transferred on an intermittent basis, they tend to be sub-optimal during large information transfers. This is so because the extra bits that frame incoming data tend to account for a significant part of the overall inter-machine traf- fic, hence consuming a portion of the communication bandwidth.

An alternative is to dispense with the extra handshaking signals and overhead, instead synchronizing the transmitter and receiver with a clock signal or synchronization information contained within the transmitted code before transmitting large amounts of informa- tion. This arrangement allows for collection and dispatch of large batches of bytes of data, with a few bytes at the front-end that can be used for the synchronization and control. These leading bytes are variously called synchronization bytes, flags, and preambles. If the actual communication channel is not a great distance, the clocking signal can also be sent as a separate stream of pulses. This ensures that the transmitter and receiver are both operating on the same time base, and the receiver can be prepared for data collection prior to the arrival of the data.

An example of a synchronous transmission scheme is known as the High-level Data Link Control, or HDLC. This protocol arose from an initial design proposed by the ШМ Corporation. HDLC has been used at the data link level in public networks and has been adapted and modified in several different ways since.

A more advanced communication protocol is the Asynchronous Transfer Mode (ATM), which is an open, international standard for the transmission of voice, video, and data signals. Some advantages of ATM include a format that consists of short, fixed cells (53 bytes) which reduce overhead in maintenance of variable-sized data traffic. The versatility of this mode also allows it to simulate and integrate well with legacy technologies, as well as offering the ability to guar- antee certain service levels, generally referred to as quality of service (QoS) parameters.

Exercises