transmission modes: either parallel or serial -...

Transmission Mode 8 Copyright 1998, Professor John T. Gorgone 1

Transmission Modes:

either Parallel

or Serial


1000000 1

Transmission Mode: Serial

Serial transmission of one 8-Bit ASCII Character

Bits of a character are transmitted one at a time.Only a single channel is required.

Transmitter Receiver


0 1




ReceiverTransmitter

1 0 0 0 0 0





Transmitter0 0 1

Receiver1 0 0 0 0





Transmitter0 0 0 1

Receiver

1 0 0 0





Transmitter0 0 0 0 1

Receiver

1 0 0





Transmitter0 0 0 0 0 1

Receiver

1 0





Transmitter0 0 0 0 0 0 1

Receiver

1





Transmitter1 0 0 0 0 0 0 1

Receiver


10000001#1

Transmission Mode: Parallel

Parallel transmission of four 8-bit ASCII character

All bits of a character are transmitted simultaneously.Each bit has a separate channel dedicated to it.







10001010#2

10000001#1






10011000#3

10001010#2

10000001#1






10100001#4

10011000#3

10001010#2

10000001#1


There are two common forms of serial transmission:- Asynchronous- Synchronous

Bit and character Synchronization: The receiver must correctly interpret the bit pattern generated by the transmitter if there is to be a meaningful exchange of information.

There is a need to periodically Resynchronize.


Two ways to perform this - - Asynchronous Transmission where the transmitter and receiver are resynchronized at the beginning of each character, or

Synchronous Transmission where timing signals are set throughout the network, permitting the receiver to derive a clock that is precisely in step with the transmitter clock.


Asynchronous Transmission

The "1" state or "Mark" condition is when current is flowing,andThe "0" state or "Space" condition is when current is not flowing.

The receiver mechanism is started by changing the date communication line from the idle state "1" to the "0" state for the length of time it takes to transmit one bit.

This is called the Start Bit.

The line is then changed, or conditioned, to a "1" bit or "0" bit to represent each bit in the character being sent.


Asynchronous Transmission

Following the character bits, one or more bits times of 1 state (idle) are sent to allow the receiver to coast back down to a known position in time for the start bit of the next character.

The first 1 bit following the character bits is called the Stop Bit.


Asynchronous Transmission of Two 8-Bit Characters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

1 1 1 1 0 1 0 0 1 0 1 1 0 1 1 1 1 1 0 1 1 1 1 0 0 1 1 1 1 1 1 1 1

Idle Line one 8-bit character

Idle Line Idle Lineone 8-bit character

10 bits required to transmit one 8-bit EBCDIC character.

startbit

stopbit

startbit

stopbit


Asynchronous Transmission is also called

Start/Stop Transmission

There is no fixed time interval between characters except for the requirement that the line be idle for at least the stop bit interval. The stop bit is necessary to guarantee that each character will begin with a 1 or 0 transition to clearly define the start of each character.


Asynchronous

n The start of each character, or block of characters is arbitrary; once started the time of occurrence of each signal representing a bit within the character, or block has the same relationship to significant instants of a fixed time frame.

n All bits of a character are synchronized and occur at a set interval. However, different characters within a message are not synchronized with each other.


Asynchronous Mode Characteristics

n Data may have a period of inactivity between characters of a message.

n Start and stop bit(s) frame each character.

n Bits within a character are sent at prescribed time intervals synchronized with the start bit.


Asynchronous Mode Characteristics (Continued)

n Allows timing to be established independently in transmitting and receiving station.

n Transmission time is increased by the time required to transmit start and stop bits.

Character size = Data bits+ Start bits+ Stop bits


P bit

Stop

1

1.5

2

StartBit

Idle stateof line

Odd/Evenor Unused

Remain idleor next start bit

5 to 8 data bits

1 1 1 1 000 1 00 1 0 11 00 1 0 0 0 1 0 1 0

(b) 8-bit asynchronous bit stream Idle line

~~ ~~

Start bit 1st Char

Stop bit

Start bit

Random time interval between characters

2nd Char

Stop bit

3rd Char

(a) Data character format


n Synchronous Transmission: is used for high-speed transmission of a block of characters.

Start/Stop bits are not required for each character. Instead, synchronization is established by sending synchronization characters prior to each block of message bits.

This is accomplished by either a separate clock lead from the transmission point to the reception point, in addition to the data lead, or a modem that includes the clock information in the modulation process the encodes the data.

Synchronous transmission permits more information to be transmitted per unit of time than asynchronous transmission because fewer total bits are required to send a long message.


Synchronous Transmission of One Block of Data

Transmitter ReceiverData Message SynChar

SynChar

SynChar


Synchronous Mode

n Synchronous data transmission mode in which the time of occurrence of each signal representing a bit is related to a fixed time frame.

n All bits of character are synchronized and all characters within a message are synchronized with each other.


Synchronous Transmission:

n Once bit synchronization is achieved, character synchronization must be accomplished.

n Character synchronization is simply a matter of knowing how many bits are required to form one character.


Synchronous Mode Characteristics

n The bits of one character are followed immediately by those of the next and are transmitted at the maximum line speed.

n Bit synchronization, which ensures that the receiving station knows at what instant a bit starts and ends, is provided by control timings.


Synchronous Mode Characteristics (Continued)

n Character synchronization which ensures that the receiving station knows which bit is in which character is achieved by transmitting two or more unique synchronization codes (SYN) at the beginning of each message.

n If there are data character gaps, it will be filled by SYN characters to maintain continuity of transmission.


0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 0 1 1 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 0 0 0 1 0

Sync Sync Sync A B

Synchronous Mode


Syn Syn Syn

One or more SYN characters

Control characters Data characters Control characters

~~ ~~~~ ~~

F F

Control fields Control fieldsData field

8-bitflag

8-bitflag

(a) Character - oriented frame

(b) Bit - oriented frames

SYNCHRONOUS TRANSMISSION

transmission modes: either parallel or serial -...

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