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)
Bandwidth vs Throughput
• Bandwidth and throughput are different concepts!
– Bandwidth: frequency band in which a signal can be fully represented.
• – Throughput:
Signal impairments
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
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