Introduction to Data Communication The need to communicate is parts of mans inherent being. Since the beginning of time man has communicated using different techniques and methods. Circumstances and available technology have dictated the method and means of communications.
Data communications concerns itself with the transmission(sending and receiving) of information between two parties. In modern times, this means sending information between machines , which are connected together by physical wires or radio links.
There are FOUR basic elements involved in communication.
The TRANSMITTER initiates the communication.
The CHANNEL is the mechanism by which the communication is conveyed to the receiver.
The RECEIVER receives the communication.
The MESSAGE is the information content that is transferred between the sender and receiver via the medium.
In real world terms, this can be illustrated by a simple telephone conversation between two people. The person who initiates the call by lifting the telephone handset and dialing a number is the TRANSMITTER, whilst the person who answers ringing telephone is the RECEIVER. The CHANNEL is the Public Telephone Switched Network, and the MESSAGE is the topic of conversation [speech] that was the reason for the call being made.
The message source is the transmitter, and the destination is the receiver. A channel whose direction of transmission is unchanging is referred to as a simplex channel. For example, a radio station is a simplex channel because it always transmits the signal to its listeners and never allows them to transmit back. A half-duplex channel is a single physical channel in which the direction may be reversed. Message may flow in two directions, but never at the same time, in a half-duplex system. In a telephone call, one party speaks while the other listens. After a pause, the other party speaks and the first party listens. Speaking simultaneously results in garbled sound that cannot be understood. A full-duplex channels allows simultaneous message exchange in both directions. It really consists of two simplex channels, a forward channel and a reverse channel, linking the same points.
Consider Morse Code, a two-state data communication system that functions very similarly to today’s computerized data communication systems. Developed by Samuel E.B. Morse in the 19th century, Morse code uses electrical current to transmit a series of dashes and dots that represent letters of the alphabet, numbers, a comma and a period. A basic Morse Code transaction works as follows: A “message” is given to an operator who translates that message into dots and dashes (TRANSMITTER),then the transmitting operator uses the telegraph key to send an electrical signal to the receiving operator at the desired location to indicate that a message is about to come through. The receiving operator (RECEIVER) sends back an acknowledgment that he is ready, and the transmitting operator then sends the message which the receiving operator takes down. when the message is completely transmitted, the transmitting operator signals to the receiving operator that he is done, and the transmission line is closed, the receiving operator then translates the code back into the original message, and delivers it to the designated recipient.
Clearly, in a system of this type, accuracy is extremely important. As only two characters-dot or dash-are used to create a code for an entire language system, the transmitting and receiving operators must be extremely accurate. This system can only work if both sides of the data communication system know the code and can encrypt and decode messages. It is also essential that the transmitter not send faster than the receiver can take down the information. Even using expert operators, static on the line could obscure the signals making a dash sound like a dot and thereby corrupting the message. Thus, it becomes obvious that the most important aspect of designing a data communication system is ensuring not only that RECEIVER can receive and understand the data transmitted by TRANSMITTER, but also that the data remain uncorrupted during transmission.
These are the very same concerns faced by computerized data communication system designers. Indeed Morse code is often though of as the forerunner of the computer’s binary communication system. The binary system uses the numbers 0 and 1 as the symbols for transmission of data. Using position notation, any value is represented by a weighted series of 1s and 0s. thus the decimal number “33”would be represented by “100001” (1*25+0*24+0*23+0*22+0*21+1*20).
The above is a simple example of binary coding, but the same system is used by computers to transmit complex text messages, complex graphics, and streaming video. In order to accurately transmit many different types of data, numerous interfaces and protocols have been created. In such a system, the distance over which data moves within a computer many vary from a few thousandths of an inch, as is the case within a single chip, to as much as several feet along the backplane of the main circuit board. Frequently, however, data must be sent beyond the local circuitry that constitutes a computer. In many cases, the distances involved may be enormous. Unfortunately, as the distance between the source of a message and its destination increases, accurate transmission becomes increasingly difficult. This results from the electrical distortion of signals traveling through long distance, and from noise added to the signal as it propagates through a transmission medium. It is the relationship of the true signal to the noise signal, known as the signal to noise ratio, which is the most interest to the communication engineer.
Data may be transmitted between two points in two different ways. Lets consider sending 8 bits of digital data(1 byte). These bits may be sent all at once (in parallel), or one after the other (serial).
To transfer data on parallel link, a parallel transmission may convey eight bits at a time through eight parallel channels. Although transfer rate is eight times faster than in bit-serial transmission, eight channels are needed, and the cost may be as much as eight times higher to transmit the message. When distances are short, it may nonetheless be both feasible and economic to use parallel channels in return for high data rates. The popular Centronics printer interface is a case where parallel transmission is used. As another example, it is common practice to use a 16-bit-wide data bus to transfer data between a microprocessor and memory chips; this provides the equivalent of 16 parallel channels. On the other hand, when communicating with a timesharing system over a modem, only a single channel is available, and serial transmission is required.
Most digital messages are vastly longer than just a few bits. Because it is neither practical nor economic to transfer all bits of a long message simultaneously, the message is broken into smaller parts and transmitted sequentially. Bit-serial transmission conveys a message one bit at a time through a channel. Each bit represents a part of the message. The individual bits are then reassembled at the
destination to compose the message. In general, one channel will pass only one bit at a time. Thus, bit-serial transmission is necessary in data communications if only a single channel is available. Bit-serial transmission is normally just called serial transmission and is the chosen communications method in many computer peripherals.
Serialized data is not generally sent at a uniform rate through a channel. Instead, there is usually a burst of regularly spaced binary data bits followed by a pause, after which the data flow resumes. Packets of binary data are sent in this manner, possibly with variable-length pauses between packets, until the message has been fully transmitted. In order for the receiving end to know the proper moment to read individual binary bits from the channel, it must know exactly when a packet begins and how much time elapses between bits. When this timing information is known, the receiver is said to be synchronized with the transmitter, and accurate data transfer becomes possible. Failure to remain synchronized throughout a transmission will cause data to be corrupted or lost.
Two basic techniques are employed to ensure correct synchronization. In synchronous systems, separate channels are used to transmit data and timing information. The timing channel transmits clock pulses to the receiver. Upon receipt of a clock pulse, the receiver reads the data channel and latches the bit value found on the channel at that moment. The data channel is not read again until the next clock pulse arrives. Because the transmitter originates both the data and the timing pulses, the receiver will read the data channel only when told to do so by the transmitter(via the clock pulse), and synchronization is guaranteed.
Techniques exist to merge the timing signal with the data so that only a single channel is required. This is especially useful when synchronous transmissions are to be sent through a modern. Two methods in which a data signal is self-timed are nonreturn-to-zero and biphase Manchester coding. These both refer to methods for encoding a data stream into an electrical waveform for transmission.
The synchronous systems, a separate timing channel is not used. The transmitter and receiver must be preset in advance to an agreed-upon baud rate. A very accurate local oscillator within the receiver will then generate an internal clock signal that is equal to the transmitter’s within a fraction of a percent. For the most common serial protocol, data is sent in small packets of 10 or 11 bits, eight of which continuous message information. When the channel is idle, the signal voltage corresponds to a continuous logic 1. A data packet always begins with a logic 0(the start bit) to signal the receiver that a transmission is starting. The start bit triggers an internal timer in the receiver that generates the needed clock pulses. Following the start bit, eight bits of message data are sent bit by bit at the agreed upon baud rate. The packet is concluded with a parity bit and stop bit.
The packet length is short in asynchronous systems to minimize the risk that the local oscillators in the receiver and transmitter will drift apart. When high-qualify crystal oscillators are used, synchronization can be guaranteed over an 11-bit period. Every time a new packet is sent, the start bit resets the synchronization, so the pause between packets can be arbitrarily long. Note that the EIA232 standard defines electrical, timing, and mechanical characteristics of a serial interface.
In circuit switching, resources remain allocated during the full length of a communication, after a circuit is established and until the circuit is terminated and the allocated resources are freed. Resources remain allocates even if no data is flowing on a circuit, hereby wasting link capacity when a circuit does not carry as much traffic as the allocation permits. This is a major issue since frequencies (in FDM) or time slots
(in TDM) are available in finite quantity on each link, and establishing a circuit consumes one of these frequencies or slots on each link of the circuit. As a result, establishing circuits for communications that carry less traffic than allocation permits can lead to resource exhaustion and network saturation, preventing further connections from being established. If no circuit can be established between a sender and a receiver because of a lack of resources, the connection is blocked.
A communications protocol is an agreed-upon convention that defines the order and meaning of bits in a serial transmission. It may also specify a procedure for exchanging messages. A protocol will define how many data bits compose a message unit, the framing and formatting bits, any error-detecting bits that may be added, and other information that governs control of the communications hardware. Channel efficiency is determined by the protocol design rather than by digital hardware considerations. Note that there is a tradeoff between channel efficiency and reliability protocols that provide greater immunity to noise by adding error-detecting and correcting codes must necessarily become less efficient.
Protocol layering
In modern protocol design, protocols are “layered” .layering is a design principle which divides the protocol design into a number of smaller parts, each of which accomplishes a particular sub-task, and interacts with the other parts of the protocol only in a small number of well-defined ways.
For example, one layer might describe how to code text (with ASCII, say), while anther describes how to inquire for messages (with the Internet’s simple mail transfer protocol, for example), while another may detect and retry errors (with the Internet’s transmission control protocol), another handles addressing (say with IP, the Internet Protocol), another handles the encapsulation of that data into a stream of bits (for example, with the point-to-point protocol), and another handles the electrical encoding of the bits (with a V.42 modem, for example)
The standard model for networking protocols and distributed applications is the International Standard Organization’s Open System Interconnect (ISO/OSI) model. It defines seven network layers.
Layer 1-Physical
Physical layer defines the cable or physical medium itself, e.g., thinnet, thicknet, Unshielded Twisted Pairs (UTP). All media are functionally equivalent. The main difference is in convenience and cost of installation and maintenance. Converters from one media to another operate at this level.
Layer 2-Data Link
Data Link layer defines the format of data on the network. A network data frame, data packet, includes checksum, source and destination address, and data. The data link layer handles the physical and logical connections to the packet’s destination, using a network interface. A host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world, and a loopback interface to send packets to itself.
Ethernet addresses a host using a unique, 48-bit address called its Ethernet address or Media Access Control (MAC) address. MAC addresses are usually represented as six colon-separated pairs of hex digits, e.g., 8:0:20:11:64:85. this number is unique and is associated with a particular Ethernet device. The data link layer’s protocol-specific header specifies the MAC address of the pack et’s source and destination. When a packet is sent to all hosts (broadcast), a special MAC address is used.
Layer 3-Network
Network uses Internetwork Protocol (IP) as its network layer interface. IP is responsible for routing, directing datagrams from one network to another. The network layer may have to break large datagrams into smaller packets and host receiving the packet will have to reassemble the fragmented datagram. The Internetwork Protocol identifies each host with a 32-bit IP address. IP addresses are written as four dot-separated decimal numbers between 0 and 255, e.g., 129.79.16.40. the network portion of the IP is assigned by InterNIC Registration Services, and the local network administrators assign the host portion portion of the IP.
Even though IP packets are addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. The Address Resolution Protocol (ARP) is used to map the IP address to it hardware address.
Layer 4-Transport
Two transport protocols, Transmission Control Protocol (TCP) AND User Datagram Protocol (UDP), sits at the transport layer. Reliability and speed are the primary difference between these two protocols. TCP establishes connections between two hosts on the network through sockets which are determined by the IP address and port number. UDP on the other hand provides a low overhead transmission service, but with less error checking.
Layer 5-Session
The session protocol defines the format of the data sent over the connectio n. The NFS uses the Remote Procedure Call (RPC) for its session protocol. RPC may be built on other TCP or UDP. Login sessions uses TCP whereas NFS and broadcast use UDP.
Layer 6-Presentation
External Data Representation (XDR) sits at the presentation level. It converts local representation of data to its canonical form and vice versa. The canonical uses a standard byte ordering and structure packing convention, independent of the host.
Layer 7-Application
Provides network services to the end-users. mail, ftp, telnet, DNS, NIS,NFS are examples of network applications.
The oldest and hitherto largest telecommunications network in existence is the Public Switched Telephone Network (PSIN) which has in excess of 700million subscribers.
The Public Switched Telephone Network (PSIN) is the concatenation of the world’s public circuit-switched telephone networks, in much the same way the Internet is the concatenation of the world’s public IP-based packet-switched networks. It is an interconnection of switching centers and connections to subscribers, which offers voice dial-up between any two subscribers connected to the PSIN. In addition, overseas connections to other countries are possible.
Originally a network of fixed-line analog telephone systems, the PSIN is now almost entirely digital, and now includes mobile as fixed telephones. The PSIN is largely governed by technical standards created by the ITU-T, and uses E.163/E.164addresses (known more commonly as telephone numbers) for addressing.
The PSIN started as human-operated analogue circuit switching systems (plug boards), electromechanical switches. By now this has almost completely been made digital, except for the final connection to the subscriber (the “last mile”): The signal coming out of the phone set is analogue. It is usually transmitted over a twisted pair cable still as an analogue signal. At the telecommunication office this analogue signal
is usually digitized, using 8000 samples per second and 8 bits per sample, yielding a 64 kbit/s data stream. So the basic digital circuit in the PSIN is a 64-kilobit-per-second channel, originally designed by Bell Labs, called a “DS0” or Digital Signal 0.The DS0 is the basic granularity at which switching takes place in a telephone exchange. DS0 is also known as timeslots because they are multiplexed together in a time-division fashion. Multiple DS0s are multiplexed together on higher capacity circuits, such that 24DS0 make a DS1 signal, which when carried on copper is the well-known, T-carrier system, T1 (the European equivalent is an E1, containing 32 64 kbit/s channels). In modern networks, this multiplexing is moved as close to the end user as possible, usually into cabinets at the roadside in residential areas, or large business premises. 资料来源360毕业设计网https://www.doczj.com/doc/207046090.html,
PSIN and the Internet world interwork to add hybrid PSIN/Internet capabilities. Incumbent carriers will move quickly to offer new hybrid services that combine functionality available in the PSIN as well as the Internet and other packet networks. Concurrently, the incumbent carriers will develop parallel packet networks and begin to examine how to interwork circuit-switched and packet networks. Initially, carriers many offer V oice over Packet (V oP) services, but will move to fully incorporate the characteristics of Next Generation Network (NGN). These characteristics include a comprehensive and integrated architecture with a new distributed processing approach for control, management, and signaling capabilities to enable advanced multimedia services.