COMPUTER NETWORKS Transmission media and physical layer

Transcription

1 Physical layer Transport and access networks Gruppo Reti TLC COMPUTER NETWORKS Physical layer - 1 Copyright Quest opera è protetta dalla licenza Creative Commons NoDerivs-NonCommercial. Per vedere una copia di questa licenza, consultare 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: or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. COMPUTER NETWORKS Physical layer - 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 COMPUTER NETWORKS Physical layer - 3 Pag. 1

2 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 COMPUTER NETWORKS Physical layer - 4 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) meters P rx P tx crosstalk attenuation COMPUTER NETWORKS Physical layer - 5 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 COMPUTER NETWORKS Physical layer - 6 Pag. 2

3 RJ45 connector (Ethernet) (TX) (RX) (RX) (TX) COMPUTER NETWORKS Physical layer - 7 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) COMPUTER NETWORKS Physical layer - 8 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) COMPUTER NETWORKS Physical layer - 9 Pag. 3

4 Attenuation (db/km) COMPUTER NETWORKS Transmission media and physical layer 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 CLADDING PROTECTION CORE PRIMARY COVER COMPUTER NETWORKS Physical layer - 10 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 COMPUTER NETWORKS Physical layer - 11 Fiber attenuation 10 Oxygen Rayleigh scattering UV absorption Infrared absorption First Second Third Wavelength (nm) Window window window 850 nm 1300 nm 1550 nm a=1.2 db/km a=0.4 db/km a=0.2 db/km COMPUTER NETWORKS Physical layer - 12 Pag. 4

5 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 Submarine cables COMPUTER NETWORKS Physical layer - 13 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 COMPUTER NETWORKS Physical layer - 14 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 - 15 Pag. 5

6 Transport network Gruppo Reti TLC COMPUTER NETWORKS Physical layer - 16 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 COMPUTER NETWORKS Physical layer - 17 T and E hierarchies Layer America (T-) Europe (E-) Japan Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s Mb/s COMPUTER NETWORKS Physical layer - 18 Pag. 6

7 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 125 sec One sample per channel every 125 sec Frame multiplexing to increase transmission speed CH1 CH2. MUX CH23 frame CH24 Sample CH1 CH2 CH3... CH22 CH23 CH24 x x x x x x x x MSB LSB COMPUTER NETWORKS Physical layer - 19 T- and DS- hierarchy CH1 CH2 CH3... DS1 DS1 DS1 DS1 CH22 CH23 CH = Mb/s T1 Frame trasmessi in un canale DS1 Difficult to identify a single channel in a frame: need to demultiplex all levels to extract /insert other channels DS2 DS2 DS2 DS2 DS2 DS2 DS2 4 DS1 = 1 DS = Mb/s DS3 DS3 DS3 DS3 DS3 DS3 7 DS2 = 1 DS = Mb/s DS4 6 DS3 = 1 DS = Mb/s COMPUTER NETWORKS Physical layer - 20 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 - 21 Pag. 7

8 SONET/SDH New technology SONET - Synchronous Optical NETwork (optical signals mutiple of the base transmission speed di 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 COMPUTER NETWORKS Physical layer - 22 SONET/SDH hierarchy OC level STS level SDH level Mbit /s OC-1 STS-1 OC-3 STS-3 OC-12 STS-12 OC-24 STS-24 OC-48 STS-48 OC-192 STS-192 OC-768 STS-768 OC-3072 STS STM STM STM STM STM STM STM COMPUTER NETWORKS Physical layer - 23 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 COMPUTER NETWORKS Physical layer - 24 Pag. 8

9 51.84 Mbit/s SONET: STS-1 frame 0 s 90 byte 3 byte 87 byte A1 Framing A2 C1 B1 E1 F1 D1 D2 D3 H1 Puntatori H2 H3 B2 K1 K2 D4 D5 D6 D7 D8 D9 D10 D11 D12 Z1 Z2 E2 Section overhead Payload J1 B3 C2 G1 F2 H4 Z3 Z4 Z5 Line overhead Payload Path overhead 125 s COMPUTER NETWORKS Physical layer - 25 Access network Gruppo Reti TLC nome.cognome@polito.it COMPUTER NETWORKS Physical layer - 26 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) COMPUTER NETWORKS Physical layer - 27 Pag. 9

10 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) COMPUTER NETWORKS Physical layer - 28 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 COMPUTER NETWORKS Physical layer - 29 POTS: the modem DTE = user terminal DCE = modem (netwprk device) DTE DTE RS232 or USB interface MODEM DCE Analog public Telephone network MODEM DCE COMPUTER NETWORKS Physical layer - 30 Pag. 10

11 Modem standard Analog modulation (Bell and CCITT) V b/s V b/s (Bell 212A) V.22 bis 2400, 1200 b/s V.23 75/1200 b/s usato per Videotel V , 4800 b/s V.32 bis 14400, 12000, 9600, 7200, 4800 b/s V , 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 COMPUTER NETWORKS Physical layer - 31 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 COMPUTER NETWORKS Physical layer - 32 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) COMPUTER NETWORKS Physical layer - 33 Pag. 11

12 ISDN architecture voice data video appliances ISDN terminal ISDN terminal voice data video appliances Advanced ISDN services voice data video appliances ISDN terminal node node voice data video appliances ISDN terminal PABX ISDN node node ISDN access node node Interface toward private networks Interface toward other networks COMPUTER NETWORKS Physical layer - 34 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 COMPUTER NETWORKS Physical layer - 35 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) COMPUTER NETWORKS Physical layer - 36 Pag. 12

13 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. COMPUTER NETWORKS Physical layer - 37 ISDN: Primary Rate Interface Used for business access Groups several B channels in a single H channel: H0-6B kb/s H11-24B kb/s - equivalent to DS1 H12-30B kb/s - equivalent to E1 COMPUTER NETWORKS Physical layer - 38 ISDN: reference points and functional architecture TE2 TE1 R TA S S-bus S NT2 T NT1 U COMPUTER NETWORKS Physical layer - 39 Pag. 13

14 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 COMPUTER NETWORKS Physical layer - 40 ADSL: scenario COMPUTER NETWORKS Physical layer - 41 ADSL at user premises Splitter filter Separates voice signal from data Modem Modulates/demodulates the signals to the proper frequency band Voice Data COMPUTER NETWORKS Physical layer - 42 Pag. 14

15 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 COMPUTER NETWORKS Physical layer - 43 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 NETWORKS Physical layer - 44 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) Mhz for TV, 6Mhz per channel 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 - 45 Pag. 15

16 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 COMPUTER NETWORKS Physical layer - 46 Accesso Radio Mobile Well established technologies Data access through cellular access: GPRS, UMTS Hot Spot coverage: IEEE (Wi-Fi) More recent technologies IEEE (Wi-Max) COMPUTER NETWORKS Physical layer - 47 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) COMPUTER NETWORKS Physical layer - 48 Pag. 16

17 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 Iridium COMPUTER NETWORKS Physical layer active satellites and 4 backup satellites Constellation with crossing multiplanes CDMA transmission Globalstar COMPUTER NETWORKS Physical layer - 50 Pag. 17