Kommunikationssysteme [KS]

Transkript

1 Kommunikationssysteme [KS] Dr.-Ing. Falko Dressler Computer Networks and Communication Systems Department of Computer Sciences University of Erlangen-Nürnberg [KS], WS 2005/

2 Course Overview Network Infrastructure Basics of communication networks, packet / circuit switched networks, telephone networks, ATM Quality of Service and Multimedia Policing, scheduling, AQM, audio/video streaming, conferencing and VoIP, IntServ, DiffServ, ATM, MPLS Simulation Tools, accomplishment, measures Protocol Engineering FSM, SDL, MSC, UML Wireless / Mobile Communications GSM, UMTS, mobility issues, WLAN, WPAN, WSN Network Security Threats and security goals, basic mechanisms [KS], WS 2005/

3 Circuit Switching Networks [KS], WS 2005/

4 Circuit Switching Networks End-to-end dedicated circuits between clients Client can be a person or equipment (router or switch) Circuit can take different forms Dedicated path for the transfer of electrical current Dedicated time slots for transfer of voice samples Dedicated frames for transfer of Nx5.84 Mbps signals Dedicated wavelengths for transfer of optical signals Circuit switching networks require: Multiplexing & switching of circuits Signaling & control for establishing circuits [KS], WS 2005/

5 Multiplexing Multiplexing involves the sharing of a transmission channel (resource) by several connections or information flows Channel = wire, optical fiber, or frequency band Significant economies of scale can be achieved by combining many signals into one Fewer wires/pole; fiber replaces thousands of cables Implicit or explicit information is required to demultiplex the information flows. (a) A B A B (b) A B MUX Shared Channel MUX A B C C C C [KS], WS 2005/

6 Frequency-Division Multiplexing Channel divided into frequency slots 0 A W u f (a) Individual signals occupy W u Hz (b) Combined signal fits into channel bandwidth B 0 C W u 0 W u A B 0 C W f f f Guard bands required AM or FM radio stations TV stations in air or cable Analog telephone systems [KS], WS 2005/

7 Time-Division Multiplexing High-speed digital channel divided into time slots (a) Each signal transmits unit every 3T seconds 0T 0T 0T A A 2 3T B B 2 3T C C 2 3T 6T 6T 6T t t t Framing required Telephone digital transmission Digital transmission in backbone network (b) Combined signal transmits unit every T seconds A B C A 2 B 2 C 2 0T T 2T 3T 4T 5T 6T t [KS], WS 2005/

8 T-Carrier System Digital telephone system uses TDM. PCM voice channel is basic unit for TDM channel = 8 bits/sample x 8000 samples/sec. = 64 kbps T- carrier carries Digital Signal (DS-) that combines 24 voice channels into a digital stream: 2 MUX MUX b b Frame Framing bit 24 Bit Rate = 8000 frames/sec x ( + 8 x 24) bits/frame =.544 Mbps [KS], WS 2005/

9 North American Digital Multiplexing Hierarchy 24. Mux DS signal,.544mbps 24 DS0 4 DS 4. Mux DS2 signal, 6.32Mbps 7 DS2 7. Mux DS3 signal, Mpbs DS0, 64 Kbps channel DS,.544 Mbps channel DS2, 6.32 Mbps channel DS3, Mbps channel DS4, Mbps channel 6 DS3 6. Mux DS4 signal Mbps [KS], WS 2005/

10 CCITT Digital Hierarchy CCITT digital hierarchy based on 30 PCM channels 30.. Mux Mbps 64 Kbps 4.. Mux Mbps.. Mux Mpbs Mbps E, Mbps channel E2, Mbps channel E3, Mbps channel E4, Mbps channel 4.. Mux [KS], WS 2005/

11 Clock Sync & Bit Slips Digital streams cannot be kept perfectly synchronized Bit slips can occur in multiplexers Slow clock results in late bit arrival and bit slip MUX t [KS], WS 2005/

12 Pulse Stuffing Pulse Stuffing: synchronization to avoid data loss due to slips Output rate > R+R2 i.e. DS2, 6.32Mbps=4x.544Mbps + 36 Kbps Pulse stuffing format Fixed-length master frames with each channel allowed to stuff or not to stuff a single bit in the master frame. Redundant stuffing specifications signaling or specification bits (other than data bits) are distributed across a master frame. Muxing of equal-rate signals requires perfect sync Pulse stuffing [KS], WS 2005/

13 Wavelength-Division Multiplexing Optical fiber link carries several wavelengths From few (4-8) to many (64-60) wavelengths per fiber Imagine prism combining different colors into single beam Each wavelength carries a high-speed stream Each wavelength can carry different format signal e.g. Gbps, 2.5 Gbps, or 0 Gbps λ Optical Optical MUX demux λ λ 2 λ λ 2. λ m λ 2 λ m Optical fiber λ m [KS], WS 2005/

14 Example: WDM with 6 wavelengths 30 db 560 nm 550 nm 540 nm [KS], WS 2005/

15 Optical Switching Optical fiber switch MUX Output Input DeMUX WDM WDM Wavelength cross-connect WDM WDM Dropped wavelengths Added wavelengths [KS], WS 2005/

16 Circuit Switches [KS], WS 2005/

17 Network: Links & Switches Circuit consists of dedicated resources in sequence of links & switches across network Circuit switch connects input links to output links Network Switch Control Link User Switch User n User n 2 3 Connection of inputs to outputs 2 3 N N [KS], WS 2005/

18 Circuit Switch Types Space-Division switches Provide separate physical connection between inputs and outputs Crossbar switches Multistage switches Time-Division switches Time-slot interchange technique Time-space-time switches Hybrids combine Time & Space switching [KS], WS 2005/

19 Crossbar Space Switch N x N array of crosspoints Connect an input to an output by closing a crosspoint 2 Nonblocking: Any input can connect to idle output Complexity: N 2 crosspoints N 2 N N [KS], WS 2005/

20 Multistage Space Switch Large switch built from multiple stages of small switches The n inputs to a first-stage switch share k paths through intermediate crossbar switches Larger k (more intermediate switches) means more paths to output In 950s, Clos asked, How many intermediate switches required to make switch nonblocking? 2 (N/n) nk + k (N/n) 2 crosspoints N inputs n k n k 2 n k N/n N/n N/n N/n 2 k n 3 3 k n 2 k n N outputs n k N/n N/n N/n k k n N/n [KS], WS 2005/

21 Clos Non-Blocking Condition: k=2n- Request connection from last input to input switch j to last output in output switch m Worst Case: All other inputs have seized top n- middle switches AND all other outputs have seized next n- middle switches If k=2n-, there is another path left to connect desired input to desired output nxk N/n x N/n kxn Desired input nxk j n- busy N/n x N/n n- N/n x N/n n+ N/n x N/n 2n-2 n- busy kxn m Desired output # internal links = 2x # external links nxk N/n Free path N/n x N/n 2n- Free path kxn N/n [KS], WS 2005/

22 Minimum Complexity Clos Switch C(n) = number of crosspoints in Clos switch N = 2Nk + k( ) 2 N = 2N(2n )+(2n )( ) n n 2 Differentiate with respect to n: 0 = δc = 4N 2N 2 + 2N 2 4N 2N 2 ==> n δn n 2 n 3 n 2 N 2 The minimized number of crosspoints is then: N 2 C* = (2N + )(2( ) /2 ) 4N 2N = 4 2N.5 N/2 N 2 This is lower than N 2 for large N [KS], WS 2005/

23 Example: Clos Switch Design Circa 2002, Mindspeed offered a Crossbar chip with the following specs: 44 inputs x 44 outputs, 3.25 Gbps/line Aggregate Crossbar chip throughput: 450 Gbps Clos Nonblocking Design for 52x52 switch N=52, n=8, k=6 N/n=44 8x6 switches in first stage 6 44x44 in centre stage 44 6x8 in third stage Aggregate Throughput: 3.6 Tbps! Note: the 44x44 crossbar can be partitioned into multiple smaller switches 52 inputs 8x6 8x6 2 44x44 2 8x x x44 6 6x8 6x8 6x8 6x8 N/n 52 outputs [KS], WS 2005/

24 Time-Slot Interchange (TSI) Switching Write bytes from arriving TDM stream into memory Read bytes in permuted order into outgoing TDM stream Max # slots = slot rate / (2 x memory cycle time) Example: slot rate = 8k/s; memory cycle time = 20ns max # slots = 500 a Read slots 2 b according to 3 connection d c b a permutation b a d c Write 22 slots in Incoming order of 23 c Outgoing TDM stream arrival TDM stream 24 d Time-slot interchange [KS], WS 2005/

25 Time-Space-Time Hybrid Switch Use TSI in first & third stage; Use crossbar in middle Replace n input x k output space switch by TSI switch that takes n-slot input frame and switches it to k-slot output frame nxk N/n x N/n kxn N inputs nxk 2 nxk 3 Input TDM frame with n slots 2 Output TDM frame with k slots nxk N/n n 2 k 2 n Time-slot interchange [KS], WS 2005/

26 Pure Optical Switching Pure Optical switching: light-in, light-out, without optical-to-electronic conversion Space switching theory can be used to design optical switches Multistage designs using small optical switches Typically 2x2 or 4x4 MEMs and Electro-optic switching devices Wavelength switches Very interesting designs when space switching is combined with wavelength conversion devices [KS], WS 2005/

27 Telephone Network [KS], WS 2005/

28 Telephone Call User requests connection Network signaling establishes connection Speakers converse User(s) hang up Network releases connection resources Source Signal Signal Go ahead Message Release Destination Signal [KS], WS 2005/

29 Call Routing (a) C D 5 Local calls routed through local network (In U.S. Local Access & Transport Area) A B (b) Net Net 2 Long distance calls routed to long distance service provider LATA LATA 2 [KS], WS 2005/

30 Telephone Local Loop Pedestal Serving area interface Distribution cable Serving area interface Feeder cable Local Loop: Last Mile Copper pair from telephone to CO Pedestal to SAI to Main Distribution Frame (MDF) 2700 cable pairs in a feeder cable MDF connects voice signal to telephone switch DSL signal to routers Local telephone office (CO) [KS], WS 2005/ Distribution frame Switch For interesting pictures of switches & MDF, see web.mit.edu/is/is/delivery/5ess/photos.html

31 Fiber-to-the-Home or Fiber-to-the-Curve? Data rates of 24-gauge twisted pair Standard Data Rate Distance T-.544 Mbps 8,000 feet, 5.5 km DS Mbps 2,000 feet, 3.7 km /4 STS Mbps 4500 feet,.4 km /2 STS Mbps 3000 feet, 0.9 km Fiber connection to the home provides huge amount of bandwidth, but cost of optical modems still high Fiber to the curve (pedestal) with shorter distance from pedestal to home can provide high speeds over copper pairs STS Mbps 000 feet, 300 m [KS], WS 2005/

32 Two- & Four-wire connections From telephone to CO, two wires carry signals in both directions Inside network, wire pair per direction Conversion from 2-wire to 4-wire occurs at hybrid transformer (CO) Signal reflections can occur causing speech echo Echo cancellers used to subtract the echo from the voice signals Original signal Transmit pair Received signal Four Wires Echoed signal Hybrid transformer Receive pair Two Wires [KS], WS 2005/

33 Integrated Services Digital Network (ISDN) First effort to provide end-to-end digital connections B channel = 64 kbps, D channel = 6 kbps ISDN defined interface to network Network consisted of separate networks for voice, data, signaling BRI PRI Circuitswitched network Private channelswitched network Packetswitched networks Signaling network Basic rate interface (BRI): 2B+D BRI PRI Primary rate interface (PRI): 23B+D [KS], WS 2005/

34 Setting Up Connections Manually Automatically Human Intervention Telephone Voice commands & switchboard operators Transport Networks Order forms & dispatching of craftpersons Management Interface Operator at console sets up connections at various switches Automatic signaling Request for connection generates signaling messages that control connection setup in switches [KS], WS 2005/

35 Stored-Program Control Switches SPC switches (960s) Crossbar switches with crossbars built from relays that open/close mechanically through electrical control Computer program controls set up opening/closing of crosspoints to establish connections between switch inputs and outputs Signaling required to coordinate path set up across network SPC Control Signaling Message [KS], WS 2005/

36 Message Signaling Processors that control switches exchange signaling messages Protocols defining messages & actions defined Modems developed to communicate digitally over converted voice trunks Office A Trunks Office B Switch Switch Processor Modem Signaling Modem Processor [KS], WS 2005/

37 Signaling Network Common Channel Signaling (CCS) #7 deployed in 970s to control call setup Protocol stack developed to support signaling Signaling network based on highly reliable packet switching network Processors & databases attached to signaling network enabled many new services: caller id, call forwarding, call waiting, user mobility Access Signaling Dial tone STP Internodal Signaling Signaling System 7 STP SCP SSP STP STP Signaling Network SSP Transport Network SSP = service switching point (signal to message) STP = signal transfer point (packet switch) SCP = service control point (processing) [KS], WS 2005/