Transcript Transmission Characteristics

Transmission Characteristics
1. Introduction (Information Interchange
codes)
2. Asynchronous and Synchronous
Transmissions
3. Error detection (bit errors)
4. Data compression
Introduction
DataTransmission
Asy nchronous
receiv er
clock
*4, *8,*16
receiv er
clock ratio
Low bit
rate
High bit
rate
Asynchronous
Sy nchronous receiv er
clock
Synchronous
Clock Encoding
RZ,biphase, NRZ,
Start/stop
bits
per
AMI
dif f erential
Manchester
character
SY N
character
sy nchronizatio
Single
parity bit
n
STX/ETXfram ing
blockchecks um
Character
Oriented
BinaryData
Bit
Oriented
Flagframing
CRC frame check
sequence
Blocks ofCharacter
Data compression
DLE-STX,DLE-ETXfram ing
and characters tuffing
CRC fram e checks equence
Com m unication Control Circuits
Com m unication control devices
blockm ode devices
Data LinkProtocols
STX/ETX framing
block sum check
Com puter
Networks
1. Introduction (Information Interchange
codes)
Input encodes 7 (128 different element) or 8 bits a
codeword, output decodes.
Codes

EBCDIC (Extended Binary Coded Decimal Interchange code)
 8 bit, proprietary (equipment manufactured by IBM)

ASCII (American Standard Committee for Information Interchange)
 7 bit, ISO 645


Printable characters
Control characters (Non-printable characters),
 Format control: BS, CR, SP, DEL, ESC
 Information Separator: FS (file), RS (record)
 Transmission Characters:
 SOH (start-of-heading), STX (Start of Text), ETX, ACK,NACK, SYN

Word = several bytes.
1. Introduction (characteristics)
Parallel transfer mode, bit-serial mode
Communication modes (simplex,half duplex, full duplex)
Transmission modes

Receiver DTE must detect
 Start of each bit cell period (bit or clock sync)
 Start or end of each element (character or byte) char or byte sync
 Start or end of each message block (frame) frame sync.

Asynchronous transmission
(for a long time in idle state, or low data rate)
 (resynchronize at the start of each character received)
 Uses start bit, stop bit (or 2 stop bits)

Synchronous transmission (at high data rate)
Bit stream suitable encoded
 Frames preceded by (one or more) reserved bytes, character synchronization
 Frame content encapsulated between a pair of reserved characters

1. Introduction (cont.)
Error Control


Asynchronous Tx: Parity bit for each transmitted character.
Synchronous Tx: Possible errors on the complete frame
 Based on frame content transmitted, sequence of error check digits.
 Receiver detects error
 Need a scheme to get correct frame.
Flow control


Avoid congestion in network communication devices
Control of flow between two DTE
Data Link Protocols




Protocol set of convention rules between two communicating entities
Allows Error control, Flow control
Can also define the format of data being exchanged and type of encoding
scheme
The type and order of messages that are to be exchanged (Sender DTE
may set up a connection to Receiver DTE)
1. Parallel Transmission and Serial Transmission
Bits in a group are sent
simultaneously, each using a
separate link
n wires are used to send n bits
at one time
Advantage: speed
Disadvantage: cost; limited to
short distances
Transmission of data one bit at
a time using only one single link
Advantage: reduced cost
Disadvantage: requires
conversion devices
Methods:
 Asynchronous
 Synchronous
2. Asynchronous and Synchronous
Transmissions
Transfer of data with start and stop bits
and a variable time interval between data
units
Timing is unimportant
Start bit alerts receiver that new group of
data is arriving
Stop bit alerts receiver that byte is
finished
Synchronization achieved through
start/stop bits with each byte received
Requires additional overhead (start/stop
bits)
Slower but Cheap and effective
Ideal for low-speed communication when
gaps may occur during transmission (ex:
keyboard)
Requires constant timing
relationship
Bit stream is combined into longer
frames, possibly containing multiple
bytes
Any gaps between bursts are filled
in with a special sequence of 0s and
1s indicating idle
Advantage: speed, no gaps or extra
bits
Byte synchronization accomplished
by data link layer
2.1 Asynchronous Transmission
Transmitting and receiving clocks assumed out of sync
Within each DTE:




Parallel-to-serial conversion of each character (byte) in preparation for its transmission.
Serial-to-parallel conversion of each received character in preparation of its processing
and storage
Receiver must have a mean to achieve bit, character, and frame synchronization
A generation of suitable error checks for error detection
Bit synchronization

Receiving clock samples N times faster (ex. N=6)
Character Synchronization


After received start bit, simple count to detect the character
Use a buffer register.
Frame Synchronization


Use STX and ETX
Use DLE-STX, DLE-ETX, DLE-DLE (byte stuffing) for pure binary data (ex. Compiled
program)
2.2 Synchronous Transmission
Synchronous receiver clock
Clock encoding:

RZ, biphase, NRZ, AMI, differential manchester.
Synchronous control scheme:


Character-oriented
Bit-oriented

Difference in Frame synchronization
Character-oriented control
Bit-oriented
Uses a synchronous idle (SYN)
Helps in bit synchronization
And frame synchronization
Uses DLE-STX, DLE-ETX, DLE-DLE
for pure binary data.



Preamble for bit synchronization
Bit encoded violation for a start-offrame delimiter JK0JK000 (start
delimiter, JK1JK111 (end of frame)
Character-oriented inefficient for binary
data because of


Additional DLE characters
Control characters varies for different
character sets.