Chapter 2

Electrical Interface

2

Introduction

• 2 진 data 를 전송선을 통해 전송하기 위하여 전기신호로 변환– 전송된 전기신호는 전송 매체에 의해 attenuated &

distorted

– Figure 2.1

• attenuation 과 distortion 에 영향을 주는 요인– 전송매체의 종류– data 의 bit rate

– 통신거리

3

전송매체

• Transmission line– a pair of conductors or wires

– electromagnetic wave

– glass fiber

• 전송매체의 종류– bps 에 따라 결정

4

5

Two-wire open lines

• Simplest transmission medium– 50m 이내– 19.2kbps 이하

• The complete set of wires – multicore cable

– flat ribbon cable

6

Two-wire open lines (cont.)

• Crosstalk– cross-coupling of electrical signals between adj

acent wires in the same cable– 유도 , 전자파 방사

7

Twisted Pair lines

• 잡음에 대한 면역성을 높이기 위해 twisted pair

사용• Any interface signal is picked up by both wires

reducing its effect on the difference signal

• 1Mbps 이하의 bit rate

• 100m 이하

8

Twisted Pair lines (cont.)

• 종류– UTP(Unshielded Twisted Pairs)

• 전화선으로 널리 이용– STP(Shielded Twisted Pairs)

• 외부 간섭 신호의 효과 감소

9

Coaxial cable

• Skin effect– Twisted-pair 의 주된 제한 요인– increases bit rate the current flowing tends to flow on

the outer surface of the wire less of the available cross-section increases electrical resistance higher attenuation

• 수백 m 거리• 10Mbps 이하• point-to-point & multipoint topologies 에 적합

10

Optical fiber

• Optical fiber– in form of a fluctuating beam of light– a much wider bandwidth– hundreds of Mbps– immune to electromagnetic interference and cro

sstalk– useful in electronically noisy environments

11

Optical fiber

• Transmitter– uses LED or LD

• Receivers– uses photodiode or

photo transistor

12

• Transmission mode– depending on the type and width of core materi

al used– multimode stepped index fiber– multimode graded index fiber– monomode fiber

Optical fiber (cont.)

13

Satellites

• Transmitted– using electromagnetic (radio) waves through

free space in satellite system

14

Terrestrial microwave

• Communication links– when it is impractical or too expensive to install

physical transmission media– for example, across a river or perhaps a swamp

or desert

15

Radio

• It is used in place of fixed-wire links over modest distances using ground-based transmitters and receivers

16

Attenuation and distortion sources

• The various attenuation and distortion effects that can degrade a signal during transmission

17

Attenuation

• Signal attenuation– As a signal propagates along a transmission

medium its amplitude decreases

• Amplifiers (Repeaters)– to restore the received signal to its original level

• Equalizers– to equalize the attenuation across a defined band

of frequencies

18

Delay distortion

• Delay distortion

– the rate of propagation of a sinusoidal signal along a tra

nsmission line varies with the frequency of the signal

• Intersymbol interference

– as the bit rate increases, so some of the frequency comp

onents associated with each bit transition are delayed a

nd start to interfere with the frequency components ass

ociated with a later bit.

19

Noise

• Line noise level– there are random perturbations on the line even

when no signal is being transmitted

• Signal-to-noise ration (SNR, ratio S/N)– the ration of the average power in a received

signal, S, to the power in the noise level , N

20

Signal types

• Modem– When we use telephony service

• to convert the electrical signals output by the source DTE into a form analogous to the signals used to convey spoken messages

• on reception, to convert signals back into a form suitable for use by the destination DTE

21

Coaxial cable signals

• Two ways– Baseband mode

• all the available bandwidth is used to derive a signal high bit rate transmission path

– Broadband mode• the available bandwidth is divided to derive a numb

er of lower bandwidth subchannels on one cable

22

Baseband mode (cont.)

23

TDM

• Time-division multiplexing– to share the available capacity of a based transmission

channel

– two types• Synchronous

– Each user has access to the channel at precisely defined time intervals

• Asynchronous– Each user has random access to the channel and, on acquiring

access, is the sole user of the channel for the duration of transmission

24

Broadband

• FDM(Frequency-division multiplexing)– using the broadband mode, multiple

transmission channels are derived from a single distribution cable

– requires a device, a radio frequency(RF) modem

25

Optical fiber signals

• A variety of forms of optical signal encoding

• based on a bipolar encoding scheme– produces a three level optical output– makes it suitable for operating the cable from D

C up to 50 Mbps

• is used mainly in a point-to-point mode

26

Optical fiber signals (cont.)

27

Satellite and radio

• Access control methods– Random access

• all stations compete for a transmission channel in a random way

– Fixed assignment• Both the channel frequency and the channel time slo

t are preassigned to each ground or cordless station

28

Satellite and radio (cont.)

– Demand assignment• when a station wishes to transmit data, it first

requests channel capacity from a central site which assigns the required capacity to the requesting station

29

Aloha

• Definition– the oldest type of

access control method

– be first used to control access to a single satellite channel

30

Aloha (cont.)

• Pure Aloha– operation

• when a station has a message to transmit, it simply transmits it. If a second station starts to transmit while the first station is transmitting its message, both transmissions are corrupted and a collision occurs

– less than 20% of the available capacity

31

Aloha (cont.)

• Slotted Aloha– establish a synchronous time clot structure– constrain all transmissions to be carried out in

these time slots– a transmission can corrupt another transmission

only in the same time slot– utilization in excess of 30% are possible

32

Aloha (cont.)

• Preassigned FDMA– with fixed assignment, both the channel frequency and/

or channel time slot are preassigned to each station in advance

– be easier than time slot assignment

• demand-assigned TDMA– much better channel utilization

– using a demand assignment access control method

33

Signal propagation delay

• Transmission propagation delay– time delay for a signal to propagate from one end of a

transmission medium to the other

• Round-trip delay– the time delay between the first bit of a block being

transmitted by the sender and the last bit of its associated acknowledgement being received

34

Public carrier circuits

• When we wish to transmit data between two DTEs in different establishments– The solution is widely used

• switched circuits

• leased circuits

35

Analog PSTN circuits

• When we wish to transmit data using existing analog PSTN transmission lines– we must convert the electrical signals output by the

source DTE into a form that is acceptable to the PSTN

36

Analog PSTN circuits (cont.)

• Modulator / Demodulator– we must convert the binary data into a form compatible

with a speech signal at the sending end of the line and reconvert this signal back into its binary form at the receiver

37

Digital leased circuits

• Digital leased circuits– be used not only to provide a direct connection

between two DTEs but also as the basis of most private data network

– normally, be used when a single organization or enterprise has a very high level of intersite traffic

• ISDN(Integrated services digital network)– without modem

38

Voice digitization

• Nyquist sampling theorem– to convert signals into digital

form

– amplitude must be sampled at a minimum rate of greater than twice the highest frequency component

39

Voice digitization (cont.)

• Pulse amplitude modulated(PAM) signal– the sampled signal is converted into a pulse stre

am

• Quantizing– PAM signal is converted into an all-digital form– Pulse code modulated)PCM) signal

40

Multiplexing

• TDM– circuits that link exchanges carry multiple calls

concurrently in a digital form

• Aggregate link bit– North America and Japan

• 24 voice channel

• 1.544Mbps

– ITU-T recommendation• 30 channel

• 2.048Mbps

41

Multiplexing

42

Physical layer interface standards

• Transmission media and the associated electrical signals– be used to transmit a binary data stream betwee

n two DTEs– physical layer interface standards

43

EIA-232D

44

Null modem

45

X.21

46

ISND interface

47

Modem summary