chapter 2 electrical interface. 2 introduction 2 진 data 를 전송선을 통해 전송하기...
TRANSCRIPT
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