CSC 335
Data Communications
and
Networking
Lecture 4b: Communication and
Multiplexing
Dr. Cheer-Sun Yang
Standards for Communication
A standard for communication defines among
others the timing of signals and the electrical
details of voltage and current. If two venders
follow a given standard, their equipment will
interoperate.
Standards for Communication
There are several organizations that are
contributing to ensure that communication
hardware built be different venders will
interoperate. The defines the specifications for
various communication hardware.
• International Telecommunications Union (ITU)
• the Electronic Industries Association (EIA)
• Institute for Electrical and Electronic Engineers
(IEEE)
Interfacing
• Data processing devices (or data terminal
equipment, DTE) do not (usually) include data
transmission facilities
• Need an interface called data circuit terminating
equipment (DCE)
– e.g. modem, NIC
• DCE transmits bits on medium
• DCE communicates data and control info with
DTE
– Done over interchange circuits
– Clear interface standards required
Interfaces between DTE and DCE
Characteristics of Interface
• Mechanical
– Connection plugs
• Electrical
– Voltage, timing, encoding
• Functional
– Data, control, timing, grounding
• Procedural
– Sequence of events
V.24/EIA-232-F
• ITU-T v.24
• Only specifies functional and procedural
– References other standards for electrical and
mechanical
• EIA-232-F (USA)
– RS-232
– Mechanical ISO 2110
– Electrical v.28
– Functional v.24
– Procedural v.24
Electrical Specification
• Digital signals
• Values interpreted as data or control,
depending on circuit
• More than -3v is binary 1, more than +3v is
binary 0 (NRZ-L)
• Signal rate < 20kbps
• Distance <15m
• For control, more than-3v is off, +3v is on
Functional Specification
Pins – nine most frequently used pins:
– Data Terminal Ready(pin 20) – as
terminal or computer is powered up
– Data Set Ready (pin 6) – as modem
is powered up
– Carrier Detect (pin 8) – as modem
detects a carrier on the telephone line
Functional Specification(cont’d)
– Request to Send (pin 4) – terminal
wants to send data
– Clear to Send (pin 5) – modem ready
to accept data
– Transmit (pin 2) – data transmitted
– Receive (pin 3) – data received
Mechanical Specification
Procedural Specification
• E.g. Asynchronous private line modem
• When turned on and ready, modem (DCE) asserts
DCE ready
• When DTE ready to send data, it asserts Request
to Send
– Also inhibits receive mode in half duplex
• Modem responds when ready by asserting Clear to
send
• DTE sends data
• When data arrives, local modem asserts Receive
Line Signal Detector and delivers data
Dial Up Operation (1)
Dial Up Operation (2)
Dial Up Operation (3)
DTE and DCE Interfacing
Data Transmission Using RS-232
Transmission Using RS-232
This figure shows that RS-232 uses negative 15
volts to represent a 1 bit and positive 15 volts to
represent a 0 bit.
Null Modem
• Sometimes we may need to allow two
devices such as PC to communicate directly,
that is, with no network or DCEs between
them.
• Your first reaction may be connecting two
RS232 interface together. However, they
both try to send a signal to request to send,
or receive data from the same pin.
Null Modem
Summary of RS-232 Main Features
RS-232 is a popular standard used for
asynchronous serial communication over short
distance between a computer (DTE) and a modem
or ASCII terminal(DCE). RS-232 precedes each
character with a start bit, follows each character
with an idle period at least one bit long (stop bit),
and send each bit in exactly the same length of
time.