Physical layer Transport and access networks Gruppo Reti TLC nome.cognome@polito.it http://www.telematica.polito.it/ COMPUTER NETWORK DESIGN Physical layer review - 1 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. COMPUTER NETWORK DESIGN Physical layer review - 2 Transport networks Used to connect network nodes Access networks connect users to the network Two TDM-based scheme Plesiouchronous Digital Hierarchy (PDH) Synchronous Digital Hierarchy (SDH) Both completely avoid Store-and-Forward operation No delay Derived from the telephone network PDH Strict synchronization between TX and RX is needed Almost synchronous behaviour (plesio-synchronous) The SDH network is fully synchronous COMPUTER NETWORK DESIGN Physical layer review - 3 Pag. 1
Transport networks Both define a limited set of available transmission speeds Multiple of a voice channel @64kbit/s PDH is simpler but limited in bit rate 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 SDH Provides more functionalities 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. COMPUTER NETWORK DESIGN Physical layer review - 4 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 COMPUTER NETWORK DESIGN Physical layer review - 5 Access network Gruppo Reti TLC nome.cognome@polito.it http://www.telematica.polito.it/ COMPUTER NETWORK DESIGN Physical layer review - 6 Pag. 2
Access networks Used to connect users to the network (last mile) 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) LANS (also wireless such as Wi-Fi, see later) COMPUTER NETWORK DESIGN Physical layer review - 7 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 for mobility (handover) COMPUTER NETWORK DESIGN Physical layer review - 8 POTS: modem Transmission media is the telephone twisted pair Analog MODEM: MOdulator / DEModulator 56 kb/s in reception and 33.6 kb/s in transmission 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) COMPUTER NETWORK DESIGN Physical layer review - 9 Pag. 3
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 based scheme) Packet and circuit services over a circuit switched network Telephone, fax, data transmission COMPUTER NETWORK DESIGN Physical layer review - 10 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 In principle any speed such as nb + md (with arbitrary n and m) In practice BRI - Basic Rate Interface 2B + D (128kb/s) PRI - Primary Rate Interface 30B + D (EU) 23B + D (USA) Channels multiplexed in time (TDM) No resource sharing COMPUTER NETWORK DESIGN Physical layer review - 11 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 No sharing COMPUTER NETWORK DESIGN Physical layer review - 12 Pag. 4
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 Voice Data COMPUTER NETWORK DESIGN Physical layer review - 13 ADSL: scenario COMPUTER NETWORK DESIGN Physical layer review - 14 HFC access network CATV (cable TC) are also named Hybrid Fiber Coax (HFC) tap headend fiber remote node Designed originally for unidirectional transmission coax amplifier COMPUTER NETWORK DESIGN Physical layer review - 15 Pag. 5
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 NETWORK DESIGN Physical layer review - 16 ADSL vs HFC 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 COMPUTER NETWORK DESIGN Physical layer review - 17 Radio-Mobile Access 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 COMPUTER NETWORK DESIGN Physical layer review - 18 Pag. 6