Transmission Modes

• Serial Transmission One bit is transmitted on a circuit at a time Usually there is one transmit circuit and one

receive circuit

transmit

receive

receive

transmit

Used with modem ports, early mouse ports, some printer ports

Transmission Modes

• Parallel Transmission Multiple bits transmitted simultaneously Multiple circuits (lines) must be used

receivetransmit

One entire byte could be transmitted in one interval

Printers have often been connected to PCs using this mode

Transmission Techniques

• How do we organize the bits for transmission?

• How do we keep bits synchronized?• If we transmit bytes, what distinguishes

the start of each byte?• How is the data rate determined?• We must define the rules, the standards,

in order for different equipment to properly communicate

Transmission Techniques

• Asynchronous Transmission Also called Start-Stop Each character is framed by start and stop

bits Hence, each character is individually

synchronized Spacing between characters is undefined

• May be short• May be long

Asynchronous Transmission

• Consider the character ‘Z’ ASCII code• In binary, this can be represented as

1 0 1 1 0 1 0

• Often, an additional bit is added for parity 0 1 0 1 1 0 1 0

• This would be called even parity, the number of one bits is even

• Some times, the parity bit might not be used

Asynchronous Transmission

• To the ASCII character, additional bits are added• By convention

Start bit = positive value Stop bit = negative value

• We can now represent this character as follows

Asynchronous Transmission

• The width of the pulse determines the speed of transmission

• Width of a pulse must be set at both ends, that is, both end must agree on this ahead of time

• Note that bits are sent one at a time, not a character at a time

• This is a serial transmission.

Interface Specification

• Now that we know how we might encode data we need to define an interface

• How many circuits? What will each do?• If voltages, what are the levels, what are

the tolerances?• What types of connectors will we agree

on?• Typically, interfaces have four major areas

to define

Interface Specification• Mechanical

Physical connection Connector specifications

• Electrical/Optical Voltage levels Meaning of each circuit Rate at which voltages change Determines data rates, distances

• Functional Defines the meaning of each circuit Generally are broad categories of functional circuits

• Control• Ground• Data• Timing

• Procedural Defines sequences of events for establishing connections and data

exchange

RS232c (EIA)V.24 (ITU)

• The is the most common interface

• In the past, was used for everything, like USB interfaces are used now.

• This is a serial (asynchronous) interface

• Common past interface between modems and PCs, or Data Terminal Equipment (DTE) Data Communications Equipment (DCE)

RS232c

• Mechanical Various connectors have been defined

• DB25• DB15• DB9• RJ45

• Electrical Digital signal (2 states – voltage) Voltage measured with respect to a common ground Voltage: -15v to -3v (1) +15v to +3v (0)

RS232cConnectors

RS232c

Functional Assignment of functions to specific circuits

• Ground FG 1 Frame ground SG 7 Signal ground

• Data transmission TD 2 Transmit data

DTE DCE RD 3 Receive data

DTE DCE

RS232c

• Control DTR 20 Data Terminal Ready

DSR 6 Data Set Ready

RTS 4 Request to Send

CTS 5 Clear to Send

DCD 8 Data Carrier Detect

DTE DCE

DTE DCE

DTE DCE

DTE DCE

DTE DCE

RS232c

• Timing

• TC 15 Transmit Clock

• RC 17 Receive Clock

DTE DCE

DTE DCE

Used in synchronous communications

Modem Connection

PC ModemDTE DCE

DTRDSRDCD

RTSCTS

TD RDRD TD

SG SG

Modem PCDCE DTE

DTRDSRDCD

RTSCTS

RD TDTD RD

SG SG

Null ModemPC PC

TD TDRD RD

RTS RTSCTS CTS

DSR DSRDCD DCDDTR DTR

SG SG

Synchronous Transmission• In asynchronous transmission, each

character was ‘framed’ with start-stop bits

• Synchronous transmission involves collecting characters into blocks and ‘framing’ each block.

• Does not involve extra start-stop bits

• But, may involve special characters or special strings to delimit each block

• Hence, transmission are synchronized on blocks rather than characters

Synchronous Transmission

• Since synchronous transmission involves long string of bits, it is possible clocks at receivers to ‘slip’

• After some time the receiver might be sampling too close to the edge of a bit time rather than in the middle

• For this reason, synchronous modems usually provide timing circuits to indicate when the transmit a bit and when to sample one

• Synchronous communications can also use an RS232 interface

Limitations of RS232c

• Limited distances (originally limited to 50 ft.)• Limited speeds (originally limited to 20 Kbps,

maximum 115 Kbps)• Communications is point-to-point• Some lack of functions

Loopback• Does not lend itself to support modern devices

such as scanners, cameras, etc• Other interfaces have been developed to

address these issues (RS449)• RS232 has still remained the most commonly

used interface of all time

Serial Interfaces

Universal Serial Bus (USB) Developed to overcome most RS232 limitations Has become the commodity serial interface Allows connection of up to 126 devices on single port

(multipoint) using hubs Plug and Play support incorporated into Operating

Systems Two versions

• USB 1.1 (1996) up to 12 Mbps• USB 2.0 (2000) up to 480 Mbps

USB Connectors

Type A Type B

IEEE-1394

• High speed multipoint serial interface like USB

• Speeds up to 800 Mbps

• Complements USB

• Developed by Texas Instruments and implemented widely by Apple Computer

• Also uses two types of connectors

IEEE-1394 Connector