Transcript Physical Layer

Chapter 6 - Physical Layer
It is about the characteristics of Physical Layer, the
lowest layer in the ISO/OSI reference model.
Most of the network problems such as noise
interference, incompatible cable connection and line
disconnection come from this layer.
Objective
• Understand various definition of
standards related to this layer
• Identify the characteristics and role of
physical layer
• Understand the PC communications using
8250 UART communication chip
Physical Layer
• The lowest layer in ISO/OSI reference model and is
about the transmission of data from one point to
another over the communications media
• This layer is dealt with the physical data
transmission and is related to:
– Type of transmission medium (Copper wire, air, optical
fiber etc.)
– The transmission media such as telephone cable are
sometimes regarded as Layer 0.
– Modulation and demodulation scheme (FSK, PSK, digital
data).
– Message switching techniques
Four Specification
• Mechanical specification such as socket layout, pin
size etc.
• Electrical specification such as the electronic
signals for certain pins.
• Functional specification such as timing or clocking
signals
• Procedural specification such as the sequence of
events required to effect the data transfer across a
pair of modems.
EIA-232-D
• The EIA standard RS-232C (RS refers to
Recommended Standard) was developed in
1969 is still widely used.
• The latest version is called RS-232D.
– Electrical, ITU-T V.28
– Functional, ITU-T V.28
– Mechanical, ISO 2110
– Procedural, CCITT V.24
Characteristics
• This is a 25-pin connector with data, control, timing and
ground signal pins.
• Not all of them are used for data transmission
DTE-DTE (Computer to Computer port)
• When terminals are connected directly to a computer port, it
is necessary to cross over some of the wires in order to
communicate properly.
• When two computer ports are connected running at
synchronous protocol, either one should provide the correct
timing or a null modem with proper timing signals is required.
PC to PC
• Three Pins, Transmit, Receive and Signal
Return
Null
Modem
Transmit
Receive
Signal Ground
Transmit
DTE-DCE (Synchronous)
• This is a normal configuration of the V24
interface between the computer and
modem.
• It therefore uses the straight through
cable. That is to say there is no cable
cross.
Serial communications
• It takes place by transmitting and receiving
data in a stream of consecutive electrical
pulses that represents bits.
• The EIA (Electronic Industries Association)
has recommended several standards for
serial communication such as:
– RS-232-D (Similar to ITU-T V.24)
– RS-422
– RS-423
– RS-449
Wiring Diagram
• Apart from the conventional wiring
diagrams between computer & modem
and computer & computer.
Serial communication controller
• The fundamental element of the serial port is an IC
called 8250, universal asynchronous receiver and
transmitter (UART).
Modem Commands
• The modem supports the standard and extended Hayes* AT
command set.
• The AT prefix (also known as the Attention Code), signals the
modem that one or more commands are to follow.
• These commands are industry standard language used to
communicate with the modem.
• The modem is always either in the command mode, or the online mode.
• Commands are only accepted by the modem when it is in
command mode.
• Commands may be entered from the terminal mode of most
communications software packages.
Basic Commands
• AT, This prefix begins all but two commands you issue to the
modem locally,, and tells the modem "ATtention! commands
to follow".
• D, Dial. Use the D command to dial a telephone number from
the command line. The format of the command is ATD and
the string parameter can contain up to 45 characters
• +++, Escape Character Sequence. After the users have
connected to another modem
• A/, Repeat. This command does not use the AT prefix nor
does it require a carriage return to enter. Typing this
command causes the modem to repeat the last command line
entered
• Zn, Reset Modem. This command (in which n=0 or 1) resets
the modem to the configuration profile stored in non-volatile
memory location 0 or 1.
An example: ATD 27888639
• Below shows the connection to CityU's Link Plus to access the
Internet service.
• The AT commands include ATZ, AT10, ATTD2xxxxxx etc.
X.20 and X.21 Recommendation
• For digital networks such as X.25 packet switching network,
• X.20 and X.21 are another interface standards.
• X.21 is a 15-pin connector. However, only eight lines as shown
below are required.
EIA RS-449/RS-422/RS-423
• RS449 corresponding to CCITT (ITU-T)
V.35 is a 37-pin connector and supports
transmission rates up to 2Mbps with
cable length to 60 meters.
• It defines both mechanical (pin and plug
configuration) and procedural (signal
descriptions) aspects similar to RS232.
• The electrical specifications (balanced or
unbalanced circuit) are defined by RS422
and RS423 respectively.
Current Loop
• An obsolete technology to extend the
physical distance up to 300 meters.
• It uses the current signal, presence of
current signal or absence (no current), to
represent binary 0 or 1 (mark or space)
instead of voltage level in RS232D.
• The most common current values are 20 mA
and 60 mA.
• This transmission method could support half
or full duplex, which requires 2 or 4 wire
connection