Internet dan Jaringan Komputer

Layer Fisik

Singgih Jatmiko

Analog vs digital signals

takes values from a continuous range

takes values from a discrete range (discrete levels).

How to carry information on a wire?

• Voltage, Current: varying over distance and time

• both analog signals

• But bits are digital (discrete) representations! What to do?

– Represent digits (1’s and 0’s) as analog signal patterns (symbols)

Digital signal

Bandwidth vs Throughput

• Bandwidth and throughput are different concepts!

– Bandwidth: frequency band in which a signal can be fully represented.

• – Throughput:

Signal impairments

Signal impairment

Transmission media

Multiplexing

• WDM (optical network) vs FDM (radio network)

• TDM

• CDM

Wavelength Division Multiplexing

Frequency Division Multiplexing

Time Division Multiplexing

TDM/FDM network -example-

Comparing different multiplexers

• TDM: 802.11 CSMA/CA

• TDM/FDM: GSM

• CDM: UMTS

Analog Transmission: Modulation / Demodulation

• Can use channels that have finite bandwidth (band-pass)

– Easier to use / deploy.

Modulation / Demodulation (cont’d)

Internet @ Home

• Dial-up

• ADSL

• Cable modem

• Fiber-to-the-Home

ADSL Asymmetric Digital Subscriber Line

• DM/FDM

• Asymmetric – higher bit-rate for downloading

• Adapts data rate based on the line conditions

ADSL (cnt’d)

Cable modem

• Theoretical downstream data rate 30 Mbps, upstream - 12 Mbps

Cable modem (cnt’d)

Conclusions

• Physical layer is the basis of all networks

– limitations: attenuation and bandwidth

• Transmission media: guided & unguided

• Multiplexing techniques introduced to efficiently utilize bandwidth

– TDM; FDM; WDM; CDM

– Broadband access: ADSL; Cable modem; FTTH