Computer Networks Chapter 4: Transmission systems and the telephone network

B B C C A A B C A B C A MUX (a) (b) Trunk group Multiplexing

A C B f C f B f A f W W W (a) Individual signals occupy W Hz (b) Combined signal fits into channel bandwidth FDM ● Original multiplexing systems used it for voice comm. ● Group=12*4kHz (60-108kHz), Supergroup=5 groups ● Mastergroups=10 supergroups ● Currently used in radio and TV comms: ● AM channels(10kHz), FM (200kHz), TV(6MHz)

(b) Combined signal transmits 1 unit every T seconds t A1A1 A2A2 t B1B1 B2B2 t C1C1 C2C2 3T3T 0T0T 6T6T 3T3T 0T0T 6T6T 3T3T 0T0T 6T6T t B1B1 C1C1 A2A2 C2C2 B2B2 A1A1 0T 1T 2T 3T 4T 5T 6T (a) Each signal transmits 1 signal every 3T TDM ● Obviously the outgoing line has n times the rate of an incoming line ● Practically the outgoing rate is slightly higher: make up sync. defficiencies ● Most existing multiplexing hierarchies based on TDM ● Ex: T1= (1+24*8)bits/frame*8000frames/s=1.544Mbps

Primary Multiplex Eg. Digital Switch 30 chan PCM 4th order Multiplex x4 2nd order Multiplex x4 3rd order Multiplex x Mbps Mbps Mbps Mbps CEPT 1 CEPT 4 North American Digital Hierarchy European Digital Hierarchy 28 M13 Multiplex M23 Multiplex x7 Primary Multiplex Eg. Digital Switch 24 chan PCM M12 Multiplex x4 1 DS Mbps DS Mbps DS MbpsDS Mbps Existing digital multiplexing hierarchies

Low-Speed Mapping Function Medium Speed Mapping Function High- Speed Mapping Function DS DS1 DS2 CEPT-1 CEPT ATM 150 Mbps STS-1 STS-3c OC-n Scrambler E/O Mbps High- Speed Mapping Function Mux STS-n SONET multiplexing ● Interconnection of cities/networks

MUX DEMUX MUX DEMUX MUX DEMUX (a) pre-SONET multiplexing remove tributary insert tributary ADM remove tributary insert tributary (b) SONET Add-Drop multiplexing Add/drop in multiplexing

a b c logical fully-connected net (a) a b c 3 ADMs connected in physical ring topology OC-3n (b) Ring topology in SONET

Inter-Office Rings Metro Ring Regional Ring Ring structures: Local Metropolitan, regional networks

BBB 87B Information Payload 9 Rows 125  s Transport overhead 90 bytes Section Overhead 3 rows 6 rows Line Overhead Sonet ST-1 frame structure ● Data bit rate 8x9x90x8000=51.84Mbps ● Information bit rate 8x9x87x8000=50.122Mbps ● Efficiency =96.7%

1 2 m Optical MUX 1 2 m Optical deMUX 1 2. m Optical fiber Wavelength-division multiplexing: The present and future (a) WDM chain network a b c d (b) WDM ring network a b c 3 ADMs

User 1 Switch Link User n User n-1 (a) Network (b) Switch Control N N Connection of inputs to outputs Circuit switching ● Switchs: non-blocking or blocking, complexity is limiting factor

N N 2  N-1 nxknxk nxknxk nxknxk nxknxk N/n x N/n kxnkxn 1 2 N/n N inputs N/n N outputs 1 2 k 2(N/n)nk + k (N/n) 2 crosspoints kxnkxn kxnkxn kxnkxn Space-division and multistage switch Crossbar: Non-blocking Multistage: If k<n can be blocking

From TDM DeMUX To TDM MUX Read slots in permuted order

nxknxk nxknxk nxknxk nxknxk N/n x N/n kxnkxn 1 2 N/n N inputs n input TDM frame with n slots output TDM frame with k slots

nxknxk nxknxk nxknxk nxknxk N/n x N/n Time-Shared Space Switch kxnkxn 1 2 N/n N inputs N/n N outputs TDM n slots kxnkxn kxnkxn kxnkxn TDM k slots TDM k slots TSI Stage Space Stage ● We will skip the study of the telephone system, and its signaling schemes

N(t) t all trunks busy trunk #

# trunks Blocking Probability offered load

Tandem Switch 2 Tandem Switch 1 B C A (b) Trunk group 10 Erlangs between each pair 90 Erlangs E F D B C A (a) E F D Routing control 1%blocking prob., 10Erl=>18trunks 18*9=162trunks 106 trunks

High Usage Route B-E Tandem Switch 1 Alternate Routes for B-E, C-F High Usage Route C-F Switch A Switch B Switch C Switch E Switch D Switch F Tandem Switch 2

High Usage Route Alternate Routes Tandem Switch 3 Tandem Switch 1 Tandem Switch 2 Switch A Switch B

Carried Load Offered Load Network Capacity We will skip cellular telephone networks and satellite cellular networks