introduction to communications data communications and networks mr. greg vogl uganda martyrs...

Introduction to Communications

Data Communications and NetworksMr. Greg VoglUganda Martyrs UniversityLecture 1, 19 March 2003

19 March 2003 Data Communications and Networking: Introduction to Communications

2

Overview Communication principles Asynchronous and synchronous transmission Packet switching, routing, errors, flow control Simplex, half-duplex, duplex operation Serial vs. parallel transmission Modems incl. modulation techniques Multiplexing Guided transmission media Other data communications


3

Communication Principles Data is sent by sender Data is received by receiver Check if data was received correctly Receiver informs sender of receipt If not correctly received, attempt error

recovery (either repair or resend data) Various media join sender and receiver


4

Communication Principles Standards of communication are used

RS232C, X.21 Protocols specify who says what when Digital data sent using analog media

like voltages (0=low, 1=high) Data rate in bps = inverse of signal

duration


5

Types of Connections Synchronous

Sender and receiver use same data rate Best for large amounts of data (nearly

100% efficient, % overhead is small) Asynchronous

Requires start, stop, and parity information

Parity bit checks if data was corrupted 0=even number of 1’s, 1=odd


6

Reliable Transmission Analog signals deteriorate or

attenuate with distance Larger amounts of data increase

probability of error and need to resend (e.g. 10MB Internet download)


7

Packets So a large data message is divided

into packets which are sent individually

How big should the packets be? Tradeoff: overhead vs. chances of error

Receiver reassembles packets in order Each packet has an ID


8

Routing Across Networks Connection-based (virtual circuit)

Virtual connection is open, all data sent along same path, then connection closed

Guaranteed correct sequence Connectionless (datagram)

Each packet routed independently Packets can arrive out of sequence Receiver must put in correct sequence


9

Error Detection/Correction Detection

Parity bit (only works for one-bit errors) Checksum or cyclical redundancy check

Uses polynomial to generate remainder

Correction/Recovery If receiver sends ACK, no need to resend If receiver sends NAK, repeat data If no reply after a timeout period, resend data If timeout a few times, give up


10

Flow Control Sender can send multiple packets Receiver’s ACK tells which it

received Receiver may be busy processing Receiver can send RNR

Received but not ready for more yet Data lines can be used for flow

control


11

Types of Connections Simplex

One direction only Half-duplex

One direction at a time Full-duplex

Both directions at the same time May need two sets of wires


12

Types of Connections Serial

Sequentially, one bit at a time (one wire) Slow (few characters/sec.) Uses: Modem, mouse, keyboard

Parallel Several bits at a time (several wires) Wire signals can skew so cables are

short Uses: Printer


13

Modem Modulator-Demodulator

Sender modem modulates (digitalanalog) Old telephone system uses analog signals Receiver demodulates (analogdigital)

Carrier signal is modulated Change wave: amplitude, frequency, phase Changing two can make bits/sec > baud

Standards for bit rate, baud rate, mod. V series (V.21, V.90, etc.)


14

DTE/DCE Computer->modem->phone lines-

>modem->computer DTE->DCE->analog signal->DCE-

>DTE Computer=data terminating

equipment Modem=data circuit-terminating

equipment


15

Multiplexing Several terminals communicate w/

host Each has its own process/socket Multiple data inputs into one

channel Data rate divided by no. of

channels


16

Time Division Multiplexing each terminal polled during its time

slot need start and stop bits if asynchronous wastes time when no data to send

any terminal can send at any time include sender’s ID use input storage buffer to hold >1

input


17

Frequency Division Multiplexing Uses analog line; best for

asynchronous Each terminal given frequency

channel Data rate divided by no. of channels Guard bands waste some of

bandwidth Same as tuning to a radio station


18

Cable Transmission Limitations Signal can be damaged

attenuation over long distance; regenerate

interference, line noise, etc. Maximum cable length depends on:

Signalling methods (analog/digital) Access method Data rates/signal speed Type of cable and its physical properties


19

Guided Transmission Media Twisted Pair Wire

Unshielded (UTP): 100 m Shielded (STP)

Co-axial cable Thinnet: 200 m Thicknet: 500 m

Fiber Optic Cable: 2 km


20

Twisted pair cable categories Category 1: RS232 telephone cable Category 2: 4 Mbit/s: token ring Category 3: 16 Mbit/s: older

Ethernet Category 4: 20 Mbit/s: token ring Category 5: 100 Mbit/s: LANs, ATM

most common for Ethernet LANs


21

Connectors D-Connectors

25-pin RS232C, <15m BNC connectors

T and round used with coax cable by LANs and cable TV

RJ-45 connectors 8-wire plug four pairs used with UTP by different types of LANs


22

EIA/TIA 568B specification Wire/pin layout for RJ-45 network plug

white/orange orange/white white/green blue/white white/blue green/white white/brown brown/white


23

Other Data Communications Microwave, wireless, satellite The public switched telephone

network The mobile telephone system Cable television

introduction to communications data communications and networks mr. greg vogl uganda martyrs...

Documents

updata communications

iddata communications

receiverdata communications

odddata communications

data ratebest

whendigital data

large amounts of data

large data message