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Chapter 4 Circuit-Switching Networks Multiplexing SONET Transport Networks Circuit Switches The Telephone Network Signaling Traffic and Overload Control in Telephone Networks Cellular Telephone Networks Circuit Switching Networks z End-to-end dedicated circuits between clients z Client can be a person or equipment (router or switch) z Circuit can take different forms z Dedicated path for the transfer of electrical current z Dedicated time slots for transfer of voice samples z Dedicated frames for transfer of Nx51.84 Mbps signals z Dedicated wavelengths for transfer of optical signals z Circuit switching networks require: z Multiplexing & switching of circuits z Signaling & control for establishing circuits z These are the subjects covered in this chapter How a network grows (a) A switch provides the network to a cluster of users, e.g. a telephone switch connects a local community Network Access network (b) A multiplexer connects two access networks, e.g. a high speed line connects two switches A Network Keeps Growing 1* a b 2 a b 4 (a) Metropolitan network A 3 A viewed as Network A of A Access Subnetworks c d c d Network of Metropolitan (b) National network viewed Access as Network of Regional Subnetworks Subnetworks (including A) A zVery high- speed lines α Network of Regional National & Subnetworks International Chapter 4 Circuit-Switching Networks Multiplexing Multiplexing z Multiplexing involves the sharing of a transmission channel (resource) by several connections or information flows z Channel = 1 wire, 1 optical fiber, or 1 frequency band z Significant economies of scale can be achieved by combining many signals into one z Fewer wires/pole; fiber replaces thousands of cables z Implicit or explicit information is required to demultiplex the information flows. (a) (b) Shared A A A Channel A B B B MUX MUX B C C C C Frequency-Division Multiplexing z Channel divided into frequency slots A f 0 Wu (a) Individual signals occupy B z Guard bands f required W Hz 0 u Wu z AM or FM radio stations C f 0 W z TV stations in u air or cable (b) Combined z Analog signal fits into telephone channel A B C systems f bandwidth 0 W Time-Division Multiplexing z High-speed digital channel divided into time slots … A1 A2 t 0T 3T 6T z Framing B … required (a) Each signal 1 B2 t transmits 1 unit 0T 3T 6T z Telephone every 3T digital seconds … transmission C1 C2 t z Digital 0T 3T 6T transmission in (b) Combined backbone signal transmits A B C … network A1 B1 C1 2 2 2 t 1 unit every T 0T 1T 2T 3T 4T 5T 6T seconds T-Carrier System z Digital telephone system uses TDM. z PCM voice channel is basic unit for TDM z 1 channel = 8 bits/sample x 8000 samples/sec. = 64 kbps z T-1 carrier carries Digital Signal 1 (DS-1) that combines 24 voice channels into a digital stream: 1 1 2 MUX MUX 2 22 23 24 b . b 1 2 . 24 . 24 Frame 24 Framing bit Bit Rate = 8000 frames/sec. x (1 + 8 x 24) bits/frame = 1.544 Mbps North American Digital Multiplexing Hierarchy 1 . DS1 signal, 1.544Mbps . Mux 24 1 DS2 signal, 6.312Mbps 24 DS0 . 4 DS1 . Mux 4 1 . DS3 signal, 44.736Mpbs 7 DS2 . Mux 7 1 . z DS0, 64 Kbps channel 6 DS3 . Mux 6 z DS1, 1.544 Mbps channel z DS2, 6.312 Mbps channel DS4 signal z DS3, 44.736 Mbps channel 274.176Mbps z DS4, 274.176 Mbps channel CCITT Digital Hierarchy z CCITT digital hierarchy based on 30 PCM channels 1 . 2.048 Mbps . Mux 30 1 8.448 Mbps 64 Kbps . Mux 4 1 . 34.368 Mpbs . Mux 139.264 Mbps 1 z E1, 2.048 Mbps channel . Mux z E2, 8.448 Mbps channel 4 z E3, 34.368 Mbps channel z E4, 139.264 Mbps channel Clock Synch & Bit Slips z Digital streams cannot be kept perfectly synchronized z Bit slips can occur in multiplexers Slow clock results in late bit arrival and bit slip MUX t 514 3 2 1 5 4 3 2 Pulse Stuffing z Pulse Stuffing: synchronization to avoid data loss due to slips z Output rate > R1+R2 z i.e. DS2, 6.312Mbps=4x1.544Mbps + 136 Kbps z Pulse stuffing format z Fixed-length master frames with each channel allowed to stuff or not to stuff a single bit in the master frame. z Redundant stuffing specifications z signaling or specification bits (other than data bits) are distributed across a master frame. Muxing of equal-rate signals Pulse stuffing requires perfect synch Wavelength-Division Multiplexing z Optical fiber link carries several wavelengths z From few (4-8) to many (64-160) wavelengths per fiber z Imagine prism combining different colors into single beam z Each wavelength carries a high-speed stream z Each wavelength can carry different format signal z e.g. 1 Gbps, 2.5 Gbps, or 10 Gbps Optical Optical λ1 MUX deMUX λ1 λ2 λ λ 1 λ2. λm 2 Optical fiber λm λm Example: WDM with 16 wavelengths 30 dB 1540 nm 1550 nm 1560 nm Typical U.S. Optical Long-Haul Network Chapter 4 Circuit-Switching Networks SONET SONET: Overview z Synchronous Optical NETwork z North American TDM physical layer standard for optical fiber communications z 8000 frames/sec. (Tframe = 125 μsec) z compatible with North American digital hierarchy z SDH (Synchronous Digital Hierarchy) elsewhere z Needs to carry E1 and E3 signals z Compatible with SONET at higher speeds z Greatly simplifies multiplexing in network backbone z OA&M support to facilitate network management z Protection & restoration SONET simplifies multiplexing Pre-SONET multiplexing: Pulse stuffing required demultiplexing all channels MUX DEMUX MUX DEMUX Remove Insert tributary tributary SONET Add-Drop Multiplexing: Allows taking individual channels in and out without full demultiplexing MUX ADM DEMUX Remove Insert tributary tributary SONET Specifications z Defines electrical & optical signal interfaces z Electrical z Multiplexing, Regeneration performed in electrical domain z STS – Synchronous Transport Signals defined z Very short range (e.g., within a switch) z Optical z Transmission carried out in optical domain z Optical transmitter & receiver z OC – Optical Carrier SONET & SDH Hierarchy SONET Electrical Optical Signal Bit Rate (Mbps) SDH Signal Electrical Signal STS-1 OC-1 51.84 N/A STS-3 OC-3 155.52 STM-1 STS-9 OC-9 466.56 STM-3 STS-12 OC-12 622.08 STM-4 STS-18 OC-18 933.12 STM-6 STS-24 OC-24 1244.16 STM-8 STS-36 OC-36 1866.24 STM-12 STS-48 OC-48 2488.32 STM-16 STS-192 OC-192 9953.28 STM-64 STS: Synchronous OC: Optical Channel STM: Synchronous Transport Signal Transfer Module SONET Multiplexing DS1 Low-speed DS2 mapping E1 function STS-1 51.84 Mbps Medium DS3 speed STS-1 44.736 mapping OC-n function STS-n . E/O . STS-3c Scrambler MUX E4 High- STS-1 speed STS-1 mapping STS-1 139.264 function STS-3c STS-1 High- STS-1 ATM or speed STS-1 POS mapping function SONET Equipment z By Functionality z ADMs: dropping & inserting tributaries z Regenerators: digital signal regeneration z Cross-Connects: interconnecting SONET streams z By Signaling between elements z Section Terminating Equipment (STE): span of fiber between adjacent devices, e.g. regenerators z Line Terminating Equipment (LTE): span between adjacent multiplexers, encompasses multiple sections z Path Terminating Equipment (PTE): span between SONET terminals at end of network, encompasses multiple lines Section, Line, & Path in SONET PTE PTE LTE LTE STE STE STE SONET SONET terminal terminal MUX Reg Reg Reg MUX Section Section Section Section STS Line STS-1 Path STE = Section Terminating Equipment, e.g., a repeater/regenerator LTE = Line Terminating Equipment, e.g., a STS-1 to STS-3 multiplexer PTE = Path Terminating Equipment, e.g., an STS-1 multiplexer z Often, PTE and LTE equipment are the same z Difference is based on function and location z PTE is at the ends, e.g., STS-1 multiplexer. z LTE in the middle, e.g., STS-3 to STS-1 multiplexer. Section, Line, & Path Layers in SONET Path Path Line Line Line Line Section Section Section Section Section Section Section Optical Optical Optical Optical Optical Optical Optical z SONET has four layers z Optical, section, line, path z Each layer is concerned with the integrity of its own signals z Each layer has its own protocols z SONET provides signaling channels for elements within a layer SONET STS Frame z SONET streams carry two types of overhead z Path overhead (POH): z inserted & removed at the ends z Synchronous Payload Envelope (SPE) consisting of Data + POH traverses network as a single unit z Transport Overhead (TOH): z processed at every SONET node z TOH occupies a portion of each SONET frame z TOH carries management & link integrity information STS-1 Frame z810x64kbps=51.84 Mbps 810 Octets per frame @ 8000 frames/sec 90 columns A1 A2 J0 J1 B1 E1 F1 B3 1 D1 D2 D3 C2 Order of 2 transmission H1 H2 H3 G1 9 rows B2 K1 K2 F2 D4 D5 D6 H4 D7 D8 D9 Z3 Special OH octets: D10 D11 D12 Z4 A1, A2 Frame Synch S1 M0/1 E2 N1 B1 Parity on Previous Frame (BER monitoring) 3 Columns of Synchronous Payload Envelope (SPE) J0 Section trace Transport OH 1 column of Path OH + 8 data columns (Connection Alive?) H1, H2, H3 Pointer Action Section Overhead Path Overhead K1, K2 Automatic Protection Line Overhead Data Switching SPE Can Span Consecutive Frames Pointer First octet Frame 87 Columns k Synchronous 9 Rows payload envelope Pointer Last octet Frame k+1 First column is path overhead z Pointer indicates where SPE begins within a frame z Pointer enables add/drop capability Stuffing in SONET z Consider system with different clocks (faster out than in) z Use buffer (e.g., 8 bit FIFO) to manage difference z Buffer empties eventually z One solution: send “stuff” z Problem: z Need to signal “stuff” to receiver FIFO 1,000,000 bps 1,000,001 bps Negative & Positive Stuff Frame Frame k Pointer k Pointer First octet First octet of SPE of
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