CSC 335 Data Communications

and Networking

Lecture 4b: Communication and Multiplexing

Dr. Cheer-Sun Yang

Standards for CommunicationA 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 CommunicationThere 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 FeaturesRS-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.