SEMINARIO WIND 2 GIORNI - Politonetgroup.polito.it/images/Didattica/CN/Physical_layer_Access... · Pag. 9 COMPUTER NETWORKS – Transmission media and physical layer ... Puntatori

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

[email protected]

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:

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or send a letter to Creative Commons, 559 Nathan Abbott

Way, Stanford, California 94305, USA.

Copyright Gruppo Reti – Politecnico di Torino

Pag. 2



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



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


Pag. 3



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)


10-1

10-2

10-3

10-4

Prx

Ptx

50 100 150 200 meters

attenuation

crosstalk


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


Pag. 4



RJ45 connector (Ethernet)


(TX) (RX) (RX) (TX)


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


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



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


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


CORE PRIMARY COVER

PROTECTION CLADDING

Pag. 6



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



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



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


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


Pag. 8



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


Transport network

Gruppo Reti TLC

[email protected]


Pag. 9



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



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



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


.

.

CH1

CH2

CH23

CH24

MUX

x x x x x x x x

MSB LSB

frame

Sample

CH1 CH2 CH23 CH24 CH3 CH22 . . . .


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



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


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



SONET/SDH hierarchy


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


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



SONET: STS-1 frame

• 51.84 Mbit/s


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


Access network

Gruppo Reti TLC

[email protected]


Pag. 14



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)



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)


Pag. 15



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



POTS: the modem

• DTE = user terminal

• DCE = modem (netwprk device)


MODEM MODEM Analog public

Telephone network

DTE DTE

DCE DCE

RS232 or USB interface

Pag. 16



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



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


Pag. 17



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)



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



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



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)


Pag. 19



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.



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


Pag. 20



TE2 TE1

TA

NT2 NT1

R

U T S S

S-bus

ISDN: reference points

and functional architecture


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



ADSL: scenario


ADSL at user premises

• Splitter filter

– Separates voice signal

from data

• Modem

– Modulates/demodulates

the signals to the proper

frequency band


Voice Data

Pag. 22



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



HFC access network

• CATV (cable TC) are

also named Hybrid

Fiber Coax (HFC)

• Designed originally for

unidirectional

transmission


headend remote

node fiber coax

amplifier

tap

Pag. 23



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


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


Pag. 24



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)


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



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



Globalstar

• 48 active satellites

and 4 backup

satellites”

• Constellation with

crossing multiplanes

• CDMA transmission


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SEMINARIO WIND 2 GIORNI - Politonetgroup.polito.it/images/Didattica/CN/Physical_layer_Access... · Pag. 9 COMPUTER NETWORKS – Transmission media and physical layer ... Puntatori