Chap 4 Circuit-Switching Networks
• Provide dedicated circuits between users •Example: 1. telephone network: provides 64Kbps circuits for voice signals 64Kbps=8 k samples/sec * 8 bits/sample 2. transport network: high bandwidth circuit. • Backbone interconnects telephone switches • Backbone interconnects large routers (internet) – Provide the physical layer that transfer bits • Circuit switching networks require: – Multiplexing & switching of circuits – Signaling & control for establishing circuits
4.1 Multiplexing • Sharing of transmission systems by several connections • Desirable when the bandwidth of individual connections is much smaller than that of the transmission system. – E.g. FM radio 25MHz (total) a standard FM radio signal 150Khz • Cost can be reduced by combining many signals into one – Fewer wires/pole; fiber replaces thousands of cables
(a) (b) A A A A
B B B MUX MUX B
C C C C
1 4.1.1 Frequency-Division Multiplexing (FDM) • Frequency slots: each connection uses different frequency slot • Demultiplexer: recovers the signals •Example: – Broadcast radio AM, FM, Television AM: 10 KHz, FM: 200KHz, Television: 6MHz – Cellular telephony: e.g. AMPS, 30KHz – Guard bands: voice signal 3.4KHz, assigned 4KHz to provide guard bands
Frequency-Division Multiplexing • Channel divided into frequency slots (Fig 4.2)
A f 0 Wu (a) Individual signals occupy B f W Hz 0 u Wu
C f 0 Wu
(b) Combined signal fits into channel A B C f bandwidth 0 W
2 4.1.2 Time-Division Multiplexing (TDM) • Share a high-speed digital transmission line using temporal interleaving (Fig 4.3)
… A1 A2 t 0T 3T 6T
B … (a) Each signal 1 B2 t transmits 1 unit 0T 3T 6T every 3T seconds … C1 C2 t 0T 6T (b) Combined 3T signal transmits 1 unit every T A B C A B C … seconds 1 1 1 2 2 2 t 0T 1T 2T 3T 4T 5T 6T
Digital Multiplexing Hierarchy • TDM used in the telephone network from early 1960s • Digital voice: 64 Kbps = 8K samples/sec *8 bit/sample PCM sample (pulse code Modulation) • T-1 carrier: 24 digital telephone connections • T-1 frame: 24 slots (8 bits/slot) + 1 “framing bit”
1 1
2 MUX MUX 2 22 23 24 24 b . . b 1 2 . . . . . 24 Frame 24 Framing bit Bit Rate = 8000 frames/sec. x (1 + 8 x 24) bits/frame = 1.544 Mbps
3 Digital Multiplexing Hierarchy • T-1 or DS1 (Digital signal 1) North America and Japan basic building block of digital multiplexing hierarchy •DS2: 4DS1 + 136Kbits synchronization information (per sec) = 6.312 Mbps •DS3: 7DS2 + 552Kbits sync info = 44.736 Mbps • In Europe: CEPT-1 (E-1) = 32 64Kbps channels (30 for voices and 2 for signaling, frame alignment etc)
Primary M12 M23 DS2 6.312 Mbps DS3 44.736 Mbps multiplex DS1 1.544 Mbps Multiplex Multiplex e.g., digital . . switch x4 x7 24 channel PCM M13 Multiplex DS3 44.736 Mbps
. . x28 North American digital hierarchy (Fig 4.5)
Clock Synch & Bit Slips • Digital streams cannot be kept perfectly synchronized • If the clock of input stream is slower than that of the multiplexer, bit slips can occur (Fig 4.6) • If faster, bits will accumulate at the multiplexer and get dropped
Slow clock results in late bit arrival and bit slip
MUX t
514 3 2 1 5 4 3 2
4 Clock Synch & Bit Slips
Solution: – Multiplexer operates at a speed slightly higher than the combined speed of inputs – Indicate to the receiving multiplexer when a slip occurs – To extract an individual input stream, need to de-multiplex the entire combined signal. 64 Kbps → DS1 → DS2→ DS3→ DS2→ DS1→ voice
4.1.3 Wavelength-Division Multiplexing (WDM) • Optical domain version of FDM (λf=c) • Electrical signals → optical signals ⇒ optical → electrical
limited at tens of Gbps. fiber: tens of terahertz (THz)
1 THz = 1000 GHz – e.g. 160 wavelengths * 10Gbps/wavelength = 1.6 Tbps • Huge increase in available bandwidth without investment on additional optical fiber
5 4.2 SONET
• Synchronous optical network (SONET) North America • Synchronous Digital Hierarchy (SDH) Europe • S: synchronous (tightly synchronized to network based clocks, atomic clocks)
4.2.1 SONET Multiplexing
• STS-1 (Synchronous-transport signal level-1) – 51.84 Mbps is the basic building block • STS-n: obtained by interleaving of bytes from n STS-1 – no additional synchronization information needed – e.g. STS-3: 3 x 51.84 = 155.52 Mbps • OC-n (optical carrier level-n) – Modulation STS-n electrical signal to optical signal • In SDH, STM-n (Synchronous transfer module-n) –STM-1 ⇔ STS-3 • STS-48/STM-16 is widely deployed in the backbone of modern communication networks
6 4.2.1 SONET Multiplexing • SONET uses the term “tributary” to refer to component streams that are multiplexed (Fig 4.10) –Flexible • Several STS-1 frames can be concatenated to accommodate signals with bit rates that cannot be handled by a single STS-1, e.g. STS-3C
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 . . . . STS-3c MUX Scrambler E/O 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
4.2.2 SONET Frame Structure
• Four layers: optical, section, line, path – Section: the span of fiber between two adjacent devices e.g. two regenerators • regenerator – Line: the span between two adjacent multiplexers – Path: the span between two SONET-terminals • e.g. large routers, can be SONET terminals
7 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
Fig 4.11
SONET Frame Structure • STS-1 frame structure (Fig 4.12) –9 rows x 90 columns per frame (810 bytes) – Frame rate: 8000 frames/sec – The bits are physically transmitted row by row and from left to right – Overall bit rate: 8 x 9 x 90 x 8000 = 51.84 Mbps – First three columns: section and line overhead • Section overhead: 3 rows (framing, error monitoring) • Line overhead: 6 rows (synchronization, multiplexing etc)
•H1, H2, H3, in line overhead; very important – Remaining 87 columns “information payload” • 8 x 9 x 87 x 8000 = 52.122 Mbps
8 SONET Frame Structure
90 bytes
Section A1 A2 87B
overhead B1 3rows H H H 1 2 3 9 rows B K K Line 2 1 2 overhead 6rows
125 μs Transport overhead Information payload
SONET Frame Structure • SPE (synchronous payload envelope) Fig 4.13 – First column: path overhead – SPE is not necessarily aligned to the information payload of an STS-1 frame
–H1, H2 (first two bytes of line overhead) are used as a pointer to indicate where the SPE begins – When n STS-1 signals are multiplexed, they are first synchronized to the clock of the multiplexer – The STS-n frame is produced by interleaving the bytes of the n synchronized STS-1 frames ⇒ 9 rows x 90n columns (3n section, line overhead, 87n payload information)
9 Pointer First octet Frame 87 Columns k
Synchronous 9 Rows payload envelope
Pointer Last octet Frame k+1 First column is path overhead
10