Data Communication Topics to be discussed:  Data Communication Terminology.  Data Transmission Signals.  Data Transmission Circuits.  Serial & Parallel Data Transmission.  Data Transmission Protocols.  Types of Data circuits.

Data Communication Terminology.  The transfer of data from one machine to another machine such that the sender and receiver both interpret the data correctly.

Data Communications Terminology  Channel  Baud Rate  Bandwidth

Channel  Portion of the communication medium allocated to the sender & receiver for conveying information between them.

Baud Rate  The signaling rate of a line, which is the number of transitions (voltage or frequency changes) that are made per second.

Bandwidth  The amount of data that can be transmitted in a fixed amount of time.  For digital devices, the bandwidth is usually expressed in bits per second(bps) or bytes per second.  For analog devices, the bandwidth is expressed in cycles per second, or Hertz (Hz).

Human Communication v. Data Communication 1.Human communication is richer, less predictable 2.Words vary in meaning with context 3.Many factors influence meaning and perception of message 4.Data communication is more precise 5.Exact replication of information 6.Computers do not interpret, they simply relay

Analogue Signals

 Analogue signals are what we encounter every day of our life. Speech is an analogue signal, and varies in amplitude (volume)  and frequency (pitch).  Analog Signaling are represented by sine waves  The main characteristics of analogue signals are,

Basic Analog Terms n Wave frequency: Number of times a cycle occurs in given time period n Wave amplitude: Height of a wave cycle n Hertz: The number of times a wave cycle occurs in one second (commonly used measure of frequency)

Digital Data n Represented as a sequence of discrete symbols from a finite “alphabet” of text and/or digits n Rate and capacity of a digital channel measured in bits per second (bps) n Digital data is binary: uses 1s and 0s to represent everything n Binary digits can be represented as voltage pulses

Digital Signals

 Digital signals are the language of modern day computers. Digital signals normally comprise only two states. These are  expressed as ON or OFF, 1 or 0 respectively.

Basic Digital Terms  Bit: digit in a binary number 1 is a 1-bit number (=1 in base 10) 10 is a 2-bit number (=2 in base 10) is an 8-bit number (=153 in base 10)  Byte: eight bits

Data Transmission Circuits  Simplex  Half Duplex  Full Duplex

Simplex

 only transmit in one direction  rarely used in data communications  e.g., receiving signals from the radio station or CATV  the sending station has only one transmitter the receiving station has only one receiver

Half Duplex

 data may travel in both directions, but only in one direction at a time  provides non-simultaneous two-way communication  computers use control signals to negotiate when to send and when to receive  the time it takes to switch between sending and receiving is called turnaround time

Full Duplex

 complete two-way simultaneous transmission  faster than half-duplex communication because no turnaround time is needed

Serial Data Transmission

 Each bit is sent over a single wire, one after the after. The organisation looks like,

Parallel Data Transmission

 Each bit uses a separate wire. The organisation looks likes,

Data transmission protocols  Synchronous Serial Transmission  Asynchronous Serial Transmission

Synchronous Serial Transmission  The transmission of data in which both stations are synchronized.  Codes are sent from the transmitting station to the receiving station to establish the synchronization.  A communication protocol that controls a synchronous transmission eg. SDLC & HDLC.

Synchronous Serial Transmission

Asynchronous Serial Transmission  The transmission of data in which each character is a self contained unit with its own start & stop bits.  Intervals between characters may be uneven.  A communication protocol that controls a asynchronous transmission eg. ASCII

Asynchronous Serial Transmission

Types of Data Circuits Available  Switched Dial-Up Lines.  Datel (data over dial-up telephone circuit).  Leased Lines.  Packet Switched Circuits.  ISDN.

Switched Dial-Up Lines  Here subscribers send routing information (ie. The dialed number) to the network which connects them to the receiver, then follow this with the information.

Datel  DATEL (data over dial-up telephone circuit).  Public communication carrier provides a dial-up line & modem for the user  The line may be used for speech or data, but not at the same time.  The circuit is non-permanent and is switched.

Leased Lines.  Permanent non-switched end to end connection.  There is no need to send routing information along with the data.  Suited for high volume, high speed data requirements.

Packet Switched Circuits  This is a special network which connects users which send data grouped in packets.  Special hardware & software is required to packetize the data before transmission, & depacketize the data on arrival.  Packet switched circuits exists for the duration of the call.

ISDN.  Abbreviation of integrated services digital network, an international communications standard for sending voice, video, and data over digital telephone lines or normal telephone wires.  ISDN supports data transfer rates of 64 Kbps (64,000 bits per second).