Simplex, Half Duplex & Duplex Serial & Parallel transmission Synchronous & Asynchronous Bit & Baud Rate

Various modes of communication/transmission:  Simplex, Half Duplex & Full Duplex  Serial & Parallel transmission  Synchronous & Asynchronous

 Simplex: Transmit in one direction. Example: a radio station broadcasting and data logging system  Half-duplex: Two interconnected devices exchanges information (data) alternately Example: a “walkie-talkie”  Duplex: To interconnected devices transmit and receive simultaneously. Example: A telephony system

 Serial – the process of sending data one bit at a time, sequentially, over a communication channel Example: Modem  Parallel – the process of sending data multi bits at a time. Example: Printer

 Synchronous data transmission is a data transfer method in which a continuous stream of data signals is accompanied by timing signals (generated by an electronic clock) to ensure that the transmitter and the receiver are in step (synchronized) with one another.  The data is sent in blocks (called frames or packets) spaced by fixed time intervals.  In brief, synchronous communication is direct communication where the communicators are time synchronized  E.g. a chat room event and instant messaging

 The transmitter sends out one or more unique characters called SYNC characters are inserted at the beginning of the bit stream by the sender to mark the start of transmission  These characters are used to synchronize a block of information  When data is not being sent through a synchronous data link the line is held in a marking condition.  Two ways of organizing a synchronous data link : Character (or byte) oriented and bit oriented.

character-oriented: Also known as byte-oriented protocols Interpret a transmission frame as a succession of characters bit-oriented: Interpret a transmission frame as a succession of individual bits Control information in a bit-oriented protocol can be one or multiple bits depending on the information embodied in the pattern

The information is received and translated by agreed upon patterns Data are generated at random intervals It is necessary for the receiver to resynchronize at the beginning of each character. This is done by adding additional bits to the beginning and end ( start and stop bits respectively) to encapsulate the original data bits Examples are e- mail messages, discussion boards, blogging, and text messaging over cell phones

Data is what we need to send. Signal is what we can send. So, signal is the carrier which carriers data. Smallest entity of the data that can represent a piece of information is called data element. Shortest meaningful unit of a signal is called signal element

Consider a train. Each carriage is a signal element, each passenger inside the train is a data element. The train as whole is a signal and all passengers together represent a data.

Data rate – number of data elements transmitted per second Signal rate – number of signal elements transmitted per second Unit of data rate is bit rate. Unit of signal rate is baud rate.

The bit rate is the number of bits sent in 1s, expressed in bits per second (bps)

Assume we need to download text documents at the rate of 100 pages per minute. What is the required bit rate of the channel? –Note: A page is an average of 24 lines with 80 character in each line. One character requires average 8 bits. Bit rate = 100 x 24 x 80 x 8 = 1,636,00 bps = Mbps

HDTV uses digital signal to broadcast high quality video signals. The HDTV Screen is normally a ratio of 16:9 (in contrast to 4:3 for regular TV), which means the screen is wider. There are 1920 by 1080 pixels per screen, and the screen is renewed 30 times per second. Twenty four bits represent one color pixel. Bit rate = 1920 x 1080 x 30 x 24 = 1,492,992,000 bps = 1.5 Gbps

The baud rate refers to the number of signal elements or symbol per second. A symbol may have 2 or more bits represented by it. In the analog transmission of digital data, the baud rate is less than or equal to the bit rate.

Baud Rate is calculated using the below formula: S = baud rate N = data rate (bps) r (rand)= the number of data elements carried in one signal element The value of r in analog transmission is r = log2 L, where L is the type of signal element, not the level.

An analog signal carries 4 bits per signal element. If 1000 signal elements are sent per second, find the bit rate. R = 4, S = 1000 N = S x r = 1000 x 4 = 4000 bps

An analog signal has a bit rate of 8000 bps and a baud rate of 1000 baud. How many data elements are carried by each signal element? How many signal elements do we need? In this example, S = 1000, N =8000, and rand L are unknown. We find first the value of rand then the value of L. r = N / S = 8000 / 1000 = 8 r= log 2 L L = 2 r = 2 8 = 256

QUESTIONS?