Transcript phys-layer-interface..

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
transmit
receive
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
101 1010
• Often, an additional bit is added for parity
0101 1010
• 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)
RS232c
Connectors
RS232c
Functional
 Assignment of functions to specific circuits
• Ground
 FG
 SG
1
7
Frame ground
Signal ground
• Data transmission
 TD
2
DTE
 RD
DTE
Transmit data
DCE
3
Receive data
DCE
RS232c
• Control
 DTR
20
DTE
 DSR
DCE
6
DTE
 RTS
4
DTE
Request to Send
DCE
5
DTE
 DCD
Data Set Ready
DCE
DTE
 CTS
Data Terminal Ready
Clear to Send
DCE
8
Data Carrier Detect
DCE
RS232c
• Timing
• TC
15
DTE
• RC
DCE
17
DTE
Transmit Clock
Receive Clock
DCE
Used in synchronous
communications
Modem Connection
PC
DTE
Modem
DCE
Modem
DCE
PC
DTE
DTR
DTR
DSR
DCD
DSR
DCD
RTS
RTS
CTS
CTS
TD
RD
RD
TD
RD
TD
TD
RD
SG
SG
SG
SG
Null Modem
PC
PC
TD
RD
TD
RD
RTS
CTS
RTS
CTS
DSR
DCD
DTR
DSR
DCD
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