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Pag. 1
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 1 Copyright Gruppo Reti – Politecnico di Torino
Physical layer
Transport and access networks
Gruppo Reti TLC
http://www.telematica.polito.it/
COMPUTER NETWORKS – Physical layer - 2
Copyright
• Quest’opera è protetta dalla licenza Creative Commons
NoDerivs-NonCommercial. Per vedere una copia di questa
licenza, consultare
http://creativecommons.org/licenses/nd-nc/1.0/
oppure inviare una lettera a: Creative Commons, 559
Nathan Abbott Way, Stanford, California 94305, USA.
• This work is licensed under the Creative Commons
NoDerivs-NonCommercial License. To view a copy of this
license, visit:
http://creativecommons.org/licenses/nd-nc/1.0/
or send a letter to Creative Commons, 559 Nathan Abbott
Way, Stanford, California 94305, USA.
Copyright Gruppo Reti – Politecnico di Torino
Pag. 2
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 3
Transmission media
• Electrical
– Unshielded/shielded twisted pair
– Coaxial cable
• Optical
– Optical fiber
– Laser
• Radio
– Radio link (antennas) for point-to-point communication
– Satellite
– Cellular network
– Wi-Fi
Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 4
Electrical media characteristics
• Optimal media characterized by
– Resistance, capacities and impedance
– Traction resistance
– Flexibility
• Electrical media characteristics depend on
– Geometry
– Number of cables and their distance
– Material used for isolation
– Shielding type
Copyright Gruppo Reti – Politecnico di Torino
Pag. 3
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 5
Parameters for electrical media
• Impedance (as a function of frequency)
• Signal propagation speed in the media (0.5c-0.7c for cables,
0.6c for fiber opticcs)
• For a given frequency, we report below
– Attenuation (linearly increasing, in dB, with distance and with the
square root of frequency)
– Cross-Talk (noise introduced by adjacent cables – increases with the
distance and then saturates)
Copyright Gruppo Reti – Politecnico di Torino
10-1
10-2
10-3
10-4
Prx
Ptx
50 100 150 200 meters
attenuation
crosstalk
COMPUTER NETWORKS – Physical layer - 6
Twisted pair
• Also simply named pair
• Used in the access segment of telephone
networks
• Two copper conductors twisted to reduce
electromagnetic interference using
differential transmission techniques
• Low costs and ease of cabling
Copyright Gruppo Reti – Politecnico di Torino
Pag. 4
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 7
RJ45 connector (Ethernet)
Copyright Gruppo Reti – Politecnico di Torino
(TX) (RX) (RX) (TX)
COMPUTER NETWORKS – Physical layer - 8
UTP
• The Unshielded Twisted Pair is used in both
telephone and data networks
• Seven categories of increasing transmission quality
• Category 7 with shielded twisted pair
Copyright Gruppo Reti – Politecnico di Torino
1 Analog telephony
2 ISDN telephony
3 LANs up to 10 Mbit/s
4 LANs up to 16 Mbit/s
5 LANa up to 100 Mbit/s
5e LANs up to1 Gbit/s
6 LAN up to 1 Gbit/s (better quality than 5e)
Pag. 5
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 9
Coaxial cable
• One central connector plus one or more
covers to protect against electromagnetic
interference
• Reduced interference thanks to its schiedling
properties (Faraday cage)
• Higher cost, complex cabling
• Transmission speed ~ hundreds of Mbit/s
• Two dominant types
– Oscilloscope cable (RG-58)
– TV cable (RG-59) Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 10
Fiber optic
• Very thin and flexible glass-based conductor
composed by two parts (core and cladding) with
different refraction index
• According to Snell law, the luminous ray (generated
by a LED or by a laser) inserted in the fiber is
restricted to propagate in the core if the incident
angle is below a given threshold
Copyright Gruppo Reti – Politecnico di Torino
CORE PRIMARY COVER
PROTECTION CLADDING
Pag. 6
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 11
Fiber optics
• Advantages
– Immune to electromagneti interference
– High available bit rate (tens ofTerabit/s)
– Low attenuation (~0.1dB/km) see next slide
– Relatively low cost and reduced size
• Disadvantages
– Can be used only for point to point connections
– Difficult to connect
– Difficult to align transceivers
– Not easy to lay
– Suffers vibration
Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 12 Copyright Gruppo Reti – Politecnico di Torino
Fiber attenuation
Wavelength (nm) 800 1000 1200 1400 1600
Att
en
ua
tio
n (
dB
/km
)
0.01
0.1
1.0
10
Infrared
absorption
1800
Rayleigh
scattering
UV
absorption
Oxygen
First
Window
850 nm
a=1.2 dB/Km
Second
window
1300 nm
a=0.4 dB/Km
Third
window
1550 nm
a=0.2 dB/Km
Pag. 7
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 13 Copyright Gruppo Reti – Politecnico di Torino
Submarine cables
• Amplifiers needed
every 30/50 Km
• Each amplifier is
backed up
• Need to transport also
power supply over the
cable
• Costly and time
consuming
maintenance
COMPUTER NETWORKS – Physical layer - 14
Radio (Ether)
• Environment may affect signal propagation
– Interference for multiple paths created by reflected signal
• Fading (quick signal amplitude variation due to the in phase
combination of “copies” of the same signal
– Natural obstacles
• Shadowing (slow signal amplitude variation)
• Co-channel interference
• May suffer for atmospheric phenomena (fog, rain,
clouds)
• Signal attenuation is a function of the squared
distance
Copyright Gruppo Reti – Politecnico di Torino
Pag. 8
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 15 Copyright Gruppo Reti – Politecnico di Torino
Transport and access networks
• Access network is the portion of the network
including
– Devices and transmission media that connect
the user to the access node of the service
provider network
• Transport network comprise
– Devices and transmission media managed by
one or more service providers connecting
networks nodes
– Metro and core segments
COMPUTER NETWORKS – Physical layer - 16 Copyright Gruppo Reti – Politecnico di Torino
Transport network
Gruppo Reti TLC
http://www.telematica.polito.it/
Pag. 9
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 17
Plesiochronous Digital Hierarchy
• Plesiouchonous Digital Hierarchy (PDH) was
the standard for digital transmission in
telephone networks
• Time Division Multiplexing scheme
– Defined to transfer 64kbit/s voice channels
– Avoids Store-and-Forward
• Strict synchronization bewteen TX and RX is needed
• Almost synchronous behaviour (plesio-synchronous)
– Different standard in USA/Europe/Japan
• Creates interface and interoperability complexity
Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 18 Copyright Gruppo Reti – Politecnico di Torino
T and E hierarchies
Layer America
(T-)
Europe
(E-)
Japan
0 0.064 Mb/s 0.064 Mb/s 0.064 Mb/s
1 1.544 Mb/s 2.048 Mb/s 1.544 Mb/s
2 6.312 Mb/s 8.488 Mb/s 6.312 Mb/s
3 44.736 Mb/s 34.368 Mb/s 32.064 Mb/s
4 274.176 Mb/s 139.264 Mb/s 97.928 Mb/s
Pag. 10
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 19
T-1 carrier system
• 24 voice channels coded in a TDM PCM frame
• One bit per frame signaling channel
• T-1 carries speed is (24*8+1)*8000=1.544Mbit/s
• Frame last 125sec
• One sample per channel every 125sec
• Frame multiplexing to increase transmission speed
Copyright Gruppo Reti – Politecnico di Torino
.
.
CH1
CH2
CH23
CH24
MUX
x x x x x x x x
MSB LSB
frame
Sample
CH1 CH2 CH23 CH24 CH3 CH22 . . . .
COMPUTER NETWORKS – Physical layer - 20 Copyright Gruppo Reti – Politecnico di Torino
T- and DS- hierarchy
CH1 CH2 CH23 CH24 CH3 CH22 . . .
64 × 24 = 1.544 Mb/s
T1 Frame trasmessi in
un canale DS1
DS1 DS1 DS1 DS1
DS2 DS2 DS2 DS2 DS2 DS2 DS2 4 DS1 = 1 DS2
4 × 1.544 = 6.312 Mb/s
DS3 7 DS2 = 1 DS3
7 × 6.312 = 44.736 Mb/s DS3 DS3 DS3 DS3 DS3
DS4 6 DS3 = 1 DS4
6 × 44.736 = 274.176 Mb/s
Difficult to identify a
single channel in a
frame: need to
demultiplex all levels to
extract /insert other
channels
Pag. 11
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 21 Copyright Gruppo Reti – Politecnico di Torino
PDH: synchronization
• Every node has its own clock
• No global (network wide) synchronization
• Only link by link synchronization
• Local clock drift
– Synchronization errors
– Bit are stuffed to compensate for clock drifts
COMPUTER NETWORKS – Physical layer - 22
SONET/SDH
• New technology
– SONET - Synchronous Optical NETwork (optical
signals mutiple of the base transmission speed
di 51.84 Mbit/s)
– SDH - Synchronous Digital Hierarchy
(international name of SONET)
– STS - Synchronous Transport Signal (standard
defining electrical signals)
• Ring topologies for fault resilience
– A bidirectional ring degenerates to a
unidirectional ring in case of single link failure Copyright Gruppo Reti – Politecnico di Torino
Pag. 12
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 23
SONET/SDH hierarchy
Copyright Gruppo Reti – Politecnico di Torino
OC level STS level SDH level Mbit /s
OC-1
OC-3
OC-12
OC-24
OC-48
OC-192
OC-768
OC-3072
STS-1
STS-3
STS-12
STS-24
STS-48
STS-192
STS-768
STS-3072
STM-1
STM-4
STM-8
STM-16
STM-64
STM-256
STM-1024
51.84
155.52
622.08
1244.16
2488.32
9953.28
39813.12
159252.48
COMPUTER NETWORKS – Physical layer - 24
SONET framing
• Synchronous transmission
– Bit continuously sent
– Only possible on a point to point link
• Flow multiplexing obtained via a TDM
scheme
– Designed to ease VLSI implementation
• Every frame includes a physical layer PCI
– Synchronisation infos
– Service voice channell
– Error/fault management Copyright Gruppo Reti – Politecnico di Torino
Pag. 13
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 25
SONET: STS-1 frame
• 51.84 Mbit/s
Copyright Gruppo Reti – Politecnico di Torino
90 byte
Puntatori
Framing
3 byte 87 byte
125s
0 s
Section overhead Path overhead
Payload Payload
Line overhead
A1 C1 A2
D2 D1 D3
H1 H3 H2
K2
D6
Z5
Z4
Z3
H4
F2
G1
C2
B1 F1 E1 B3
J1
D9
D12
E2
K1
D5
D8
D11
Z2 Z1
D10
D7
D4
B2
COMPUTER NETWORKS – Physical layer - 26 Copyright Gruppo Reti – Politecnico di Torino
Access network
Gruppo Reti TLC
http://www.telematica.polito.it/
Pag. 14
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 27
Access networks
• Used to reach the users (last mile)
• Also named local loop
• Main technologies:
– Plain Old Telephone Service (POTS)
– Integrated Services Digital Network (ISDN)
– Asymmetric Digital Subscriber Loop (ADSL)
– cable-modem over Cable-TV infrastructures (CATV)
– wireless: Local Multipoint Distribution Service (LMDS),
Wi-MAX
– Cellular networks (GPRS, UMTS)
– PONs (Passive Optical Networks)
Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 28
Radio access networks
• Wireless network
– Access to the network is obtained through a terminal
connected via a wireless link
• An access point can be identified
– No support for mobility
• Cellular network
– A large geographical area is covered via adjacent
(sometimes superimposed) cells
• Small areas under the control of an antenna.
– The mobile terminal can move from one cell to another
cell without any communication interruption
– Support fo mobility (handover)
Copyright Gruppo Reti – Politecnico di Torino
Pag. 15
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 29
POTS: modem
• MODEM: MOdulator / DEModulator
• Used for connection over public telephone
networks
• Transmission: adapt the digital signal to the
analog signal suited to be sent over the
twisted pair
• Reception: analog to digital conversion
• Make the digital signal suitable for analog
transmission on the voice band
Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 30
POTS: the modem
• DTE = user terminal
• DCE = modem (netwprk device)
Copyright Gruppo Reti – Politecnico di Torino
MODEM MODEM Analog public
Telephone network
DTE DTE
DCE DCE
RS232 or USB interface
Pag. 16
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 31
Modem standard
• Analog modulation (Bell and CCITT)
– V.21 300 b/s
– V.22 1200 b/s (Bell 212A)
– V.22 bis 2400, 1200 b/s
– V.23 75/1200 b/s usato per Videotel
– V.32 9600, 4800 b/s
– V.32 bis 14400, 12000, 9600, 7200, 4800 b/s
– V.34 33600, 31200, 28800, 26400, 24000,
21600, 19200, 16800, 14400, …. b/s
• Last modem generation reached (standard V.90)
56 kb/s in reception and 33.6 kb/s in transmission
Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 32
ISDN: digital access
to telephone network • ISDN: Integrated Services Digital Network
• Integrated network (almost )
– Voice and data transport over the same telephone infrastracture
• Digital access
– From the user terminal
– Classical telephones need A/D converters
• Connection oriented
– Time based pricing
• Exploits plesiochronous transmission (TDM frame based
scheme)
• Packet and circuit services
– Telephone, fax, data transmission
Copyright Gruppo Reti – Politecnico di Torino
Pag. 17
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 33
ISDN: motivations and
standardization • ISDN main goals were
– Extend TLC services beyond telephone
– Uniform and standardized access
– Unified digital interface for all services
• ISDN standardization process
– From 1980 to 1988 within CCITT (ITU-T)
– Standardized starting in the last '70 s up to early
'90s
– Commercially available in the ‘80s (starting in
the USA)
Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 34 Copyright Gruppo Reti – Politecnico di Torino
ISDN architecture
node
node node
node node
Interface toward private networks
ISDN access
Interface toward other networks
Advanced ISDN services
PABX
ISDN
ISDN
terminal
voice data
video appliances
ISDN
terminal
voice data
video appliances
ISDN
terminal
voice data
video appliances
ISDN
terminal
voice data video appliances
node
Pag. 18
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 35
ISDN: transmission interface
• Two types of channels:
– B channel - Bearer - 64 kb/s
• Voice, data, fax, low resolution video
– D channel - Data - 16 kb/s (o 64 kb/s)
• Signalling, Data, telecontrol
• An ISDN access can be obtained by freely
combining the two channels
– nB + mD (with arbitrary n and m)
• In practice, only few combinations of m and n
are available
Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 36
ISDN: transmission interface
• Few standard interfaces where defined
– BRI - Basic Rate Interface –
• 2B + D (128kb/s)
– PRI - Primary Rate Interface –
• 30B + D (EU)
• 23B + D (USA)
• Channels are separated in time (TDM)
Copyright Gruppo Reti – Politecnico di Torino
Pag. 19
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 37
ISDN: Basic Rate Interface
• Used for domestic access or in small offices
• Adaptors are used to keep compatibility with
existing devices
• The digital signal is distributed among
devices in user premises through the S-bus.
Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 38
ISDN: Primary Rate Interface
• Used for business access
• Groups several B channels in a single H
channel:
– H0 - 6B - 384 kb/s
– H11 - 24B - 1536 kb/s - equivalent to DS1
– H12 - 30B - 1920 kb/s - equivalent to E1
Copyright Gruppo Reti – Politecnico di Torino
Pag. 20
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 39 Copyright Gruppo Reti – Politecnico di Torino
TE2 TE1
TA
NT2 NT1
R
U T S S
S-bus
ISDN: reference points
and functional architecture
COMPUTER NETWORKS – Physical layer - 40 Copyright Gruppo Reti – Politecnico di Torino
DSL access
• DSL (Digital Subscriber Line) is a family of technologies
(also named xDSL)
– Data transfer in the access segment ad high speed
• Most widely deployed ADSL (Asymmetric DSL)
– Higher bit rate in downstream, lower in upstream
• Designed for client-server applications, web browsing
• Maximum ADSL bit rate
– Highly dependent on the distance between the user and the first
access node
– From few Mbit/s to tens of Mbit/s
• Dedicated bit rate from the user to the first access node
Pag. 21
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 41 Copyright Gruppo Reti – Politecnico di Torino
ADSL: scenario
COMPUTER NETWORKS – Physical layer - 42
ADSL at user premises
• Splitter filter
– Separates voice signal
from data
• Modem
– Modulates/demodulates
the signals to the proper
frequency band
Copyright Gruppo Reti – Politecnico di Torino
Voice Data
Pag. 22
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 43
ADSL in the network
• Filter/modem POTS
– separates voice and
data flows
• DSLAM (DSL Access
Multiplexer)
– Receives several data
flows from users and
multiplex them on a
single channel
Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 44
HFC access network
• CATV (cable TC) are
also named Hybrid
Fiber Coax (HFC)
• Designed originally for
unidirectional
transmission
Copyright Gruppo Reti – Politecnico di Torino
headend remote
node fiber coax
amplifier
tap
Pag. 23
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 45 Copyright Gruppo Reti – Politecnico di Torino
HFC • Exploit the cable TV transmission medium (fiber
in the network and coax in the last mile)
• Tree topology
• Bandwidth multiplied among all users
– Shared bandwidth
• Data and TV signals exploits separate bandwidth (filter used at the receiving end in user premises) – 50-450 Mhz for TV, 6Mhz per channel
– 450-750 Mhz for downstream data
– 5-50 Mhz for upstream data (often not usable due to mono directional amplifiers, may rely on the telephone network)
• Cable modem used by users to decode data
COMPUTER NETWORKS – Physical layer - 46
ADSL vs HFC
• HFC bandwidth is shared amon all users in a
given area, ADSL bandwidth is dedicated (in
the access link)
• HFC have security issues (shared medium)
• DSL exploits telephone twisted pairs, HFC
requires Cable TV or laying ad hoc cables
• ADSL bit rate decreases with the distance,
HFC bit rate is almost distance independent
Copyright Gruppo Reti – Politecnico di Torino
Pag. 24
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 47 Copyright Gruppo Reti – Politecnico di Torino
Accesso Radio Mobile
• Well established technologies
– Data access through cellular access: GPRS,
UMTS
– Hot Spot coverage: IEEE 802.11 (Wi-Fi)
• More recent technologies
– IEEE802.16 (Wi-Max)
COMPUTER NETWORKS – Physical layer - 48 Copyright Gruppo Reti – Politecnico di Torino
Mobile radio access
• Satellite networks
– GEO (35000 km, 270ms, 3 satellites for global coverage), used for broadcast tranmission and data services
– MEO (15000 km, 50ms, >10 satellies), used for GPS not for telecom applications
– LEO (<1000 km, 5ms, >50 satellites), satellite telephony with worldwide coverage. Low delays
• Iridium, Globalstar
• Stratospherich platform (under study)
Pag. 25
COMPUTER NETWORKS – Transmission media and physical layer
COMPUTER NETWORKS – Physical layer - 49
Iridium
• 66 active satellites and 11
backup satellites
• Constellation of six polar
planes
• Each plane has 11
satellites acting as
switching nodes
• One satellite available on
each earth region
• Original value $5 billion,
sold at $25 milions
Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORKS – Physical layer - 50
Globalstar
• 48 active satellites
and 4 backup
satellites”
• Constellation with
crossing multiplanes
• CDMA transmission
Copyright Gruppo Reti – Politecnico di Torino