SEMINARIO WIND 2 GIORNI - Politecnico di Torino

COMPUTER NETWORK DESIGN – Physical layer review
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Physical layer
Transport and access networks
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Gruppo Reti TLC
[email protected]
http://www.telematica.polito.it/
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Copyright Gruppo Reti – Politecnico di Torino
COMPUTER NETWORK DESIGN – Physical layer review - 1
Transport networks
Transport networks
• Used to connect network nodes
• Both define a limited set of available transmission
speeds
– Access networks connect users to the network
– Multiple of a voice channel @64kbit/s
• Two TDM-based scheme
• PDH is simpler but limited in bit rate
– Plesiouchronous Digital Hierarchy (PDH)
– Synchronous Digital Hierarchy (SDH)
– Both completely avoid Store-and-Forward operation
– Europe: 2 Mbit/s, 34,3 Mbit/s, 139 Mbit/s
– USA: 1,5 Mbit/s, 44 Mbit/s, 274 Mbit/s
– It offers a signalling channel
• No delay
– Derived from the telephone network
– PDH
• SDH
– Provides more functionalities
• Strict synchronization between TX and RX is needed
• Almost synchronous behaviour (plesio-synchronous)
• E.g., Automatic protection among faults (ring topology)
– Has a real physical layer PCI
– 55Mbit/s, 155Mbit/s, 622Mbit/s, 1,2 Gbit/s, 10Gbit/s …….
– The SDH network is fully synchronous
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Transport networks
• In some cases (mainly POPs or MANs) Gigabit
Ethernet is starting to be used
• Much simpler, much cheaper, automatic
reconfiguration against single fault slower and
under study
Access network
Gruppo Reti TLC
[email protected]
http://www.telematica.polito.it/
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COMPUTER NETWORK DESIGN – Physical layer review
Access networks
Radio access networks
• Used to connect users to the network (last mile)
• Main technologies:
• Wireless network
– Access to the network is obtained through a terminal
connected via a wireless link
–
–
–
–
–
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)
– LANS (also wireless such as Wi-Fi, see later)
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• 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 for mobility (handover)
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COMPUTER NETWORK DESIGN – Physical layer review - 7
ISDN: digital access
to telephone network
POTS: modem
• Transmission media is the telephone twisted pair
• Analog MODEM: MOdulator / DEModulator
• ISDN: Integrated Services Digital Network
• Integrated network (almost )
– 56 kb/s in reception and 33.6 kb/s in transmission
– Voice and data transport over the same telephone infrastracture
• 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
• Connection oriented (charge by time)
• Bit rate dedicated to a single user (no sharing)
• Digital access
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Copyright Gruppo Reti – Politecnico di Torino
– From the user terminal
– Classical telephones need A/D converters
• Connection oriented
– Time based pricing
• Exploits plesiochronous transmission (TDM based scheme)
• Packet and circuit services over a circuit switched network
– Telephone, fax, data transmission
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ISDN: transmission interface
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DSL access
• DSL (Digital Subscriber Line) is a family of technologies
(also named xDSL)
• Two types of channels:
– B channel - Bearer - 64 kb/s
• Voice, data, fax, low resolution video
– Data transfer in the access segment ad high speed
– D channel - Data - 16 kb/s (o 64 kb/s)
• Most widely deployed ADSL (Asymmetric DSL)
• Signalling, Data, telecontrol
– Higher bit rate in downstream, lower in upstream
• In principle any speed such as
• Designed for client-server applications, web browsing
– nB + mD (with arbitrary n and m)
– In practice
• Maximum ADSL bit rate
• BRI - Basic Rate Interface –
– Highly dependent on the distance between the user and the first
access node
– From few Mbit/s to tens of Mbit/s
– 2B + D (128kb/s)
• PRI - Primary Rate Interface –
– 30B + D (EU)
– 23B + D (USA)
• Dedicated bit rate from the user to the first access node
• Channels multiplexed in time (TDM)
• No resource sharing
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COMPUTER NETWORK DESIGN – Physical layer review - 8
– No sharing
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COMPUTER NETWORK DESIGN – Physical layer review
ADSL: scenario
ADSL at user premises
• Frequency separation
among voice and data
• Splitter filter
– Separates voice signal
from data
• ADSL Modem
– Modulates/demodulates
the signals to the proper
frequency band
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Voice
Data
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COMPUTER NETWORK DESIGN – Physical layer review - 13
HFC
HFC access network
• Exploit the cable TV transmission medium (fiber
in the network and coax in the last mile)
• Tree topology
• Bandwidth multiplied among all users
• CATV (cable TC) are
also named Hybrid
Fiber Coax (HFC)
– Shared bandwidth
tap
headend
fiber
remote
node
• Data and TV signals exploits separate bandwidth
(filter used at the receiving end in user premises)
amplifier
– 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)
coax
• Designed originally for
unidirectional
transmission
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COMPUTER NETWORK DESIGN – Physical layer review - 14
• Cable modem used by users to decode data
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ADSL vs HFC
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Radio-Mobile Access
• HFC bandwidth is shared amon all users in a
given area, ADSL bandwidth is dedicated
• 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
• Well established technologies
– Data access through cellular access: GPRS, UMTS, HSDPA
• Up to 170 kbit/s for GPRS, 470 kbit/s for EDGE, 384 kbit/s for UMTS,
7.2 Mbit/s for HSDPA
– Hot Spot coverage: IEEE 802.11 (Wi-Fi)
• See later. Mbit/s speed
• More recent technologies
– IEEE802.16 (Wi-Max)
• All these technologies are based on the resource sharing
approach
– Multiple access
– FDMA/TDMA, access protocols
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