EECS 122, Lecture 24 Introduction to the Telephone Network. Kevin Fall Jean Walrand

Transcription

1 EECS 122, Lecture 24 Itroductio to the Telephoe Network Kevi Fall Jea Walrad

2 Outlie Overview Cocepts Recet History Structure Hierarchical Addressig Ed Systems Trasmissio Switchig Sigalig Routig

3 Overview Telephoe Call Telephoe Network Sigalig Mutiplexig Routig

4 Overview Telephoe Network 5ESS (Lucet) DMS100 (Nortel)

5 Overview Telephoe Network

6 Overview Telephoe Network Logic Diagram:

7 Overview Sigalig S1 Early days: I-Bad; Coupled to Hdw Later: Out-of-bad; Decoupled rig B A A Off-Hook S1 Listes A dials S1 Registers B Off-Hook ---- B O-Hook Release

8 Overview Multiplexig: Circuits C Circuits share commo fiber: TDM B A D Voice sigals use the reserved circuits

9 Overview Routig B A Issues: Myopic of log-term objectives? Decetralized or Cetralized?

10 Telephoe Net Cocepts > 1 billio telephoes, > 200 millio calls a day just o oe carrier (AT&T) Circuit switchig two-party, small ed-to-ed delay ad jitter, reserved resources oce call admitted full duplex coectios Itelligece placed withi the etwork, ot i ed-systems (telephoe sets) [cotrast with Iteret]

11 History The importat 1980 s: deploymet of large digital switches adaptatio of computer-cotrolled switches to provide switchig multiple data types deploymet of fiber optic trasmissio media The breakup (Ja 1, 1984) AT&T -> 7 RBOCs plus AT&T ad others log-distace carriers (IXCs) ope local area (LATA) carriers (LECs) regulated

12 History (cotiued) The 1996 Telecom Act removes umerous restrictios o LECs LECs ca provide log-distace ad IXCs ca provide local callig, if certai restrictios are met [like equal access to IXC, space sharig] players: w ILECs (icumbet LECs; ow COs ad loops) w CLECs (competitive LECs) w your competitor is your ladlord

13 Telephoe Net Structure Ed systems (phoes, faxes, etc) Cetral offices (COs) local aggregatio poits for phoe lies wire pair (local loop) to each telephoe most are aalog, provide A/D coversio Exchages switches coectig ed systems coect to back-boe (core) switches

14 Telephoe Net Structure Backboe switches (ear) fully-coected set of switches < 500 (vs exchages) Simplified architecture picture: Ed Systems Backboe Cetral Offices

15 Hierarchical Addressig Example To route: must pass through backboe hadlig 512 area code ad 224 exchage may also pass through others Special area codes (700,800,888,900) umber used as idex ito table table cotais actual telephoe umber table may be modified by time of day, etc

16 Ed Systems Traditioally a telephoe (POTS): soud-to-electric trasducer electric-to-soud trasducer dialer, riger, switch hook Echo issues: with oly 2 wires, side-toe (hearig yourself talk) must be limited but preset received soud may be echoed back (ok for small local delay, actively cacelled with circuitry at backboe switches [costly])

17 Ed-Systems - Newer Digital local loops: ISDN (BRI) xdsl ISDN (BRI): DSL: 2x64kb/s circuit chaels 1x16kb/s packet chael up to 1Mb/s, possibly asymmetric, FDM with respect to POTS service

18 Trasmissio familiar characteristics: badwidth, delay, atteuatio atteuatio addressed with regeerators: with optical fiber, every 5000km o-electric optical amplificatio possible digital multiplexig: 8000 samples/sec at 256 levels=64kb/s mu-law ecodig i US, Japa

19 Trasmissio: TDM Operatio TDM muxig of digital voice streams Commo service is T1(lie), DS1(std): 1.544Mb/s, 8000f/s at 193 bits/frame 192/8 = 24 bytes(tdm d calls)/frame + 1 bit Digital Sigalig (DS) Hierarchy: DS0 (64kb/s), DS1(1.544Mb/s), DS3(44.736Mb/s) ot exact multiples due to framig overheads

20 Trasmissio: TDM Operatio Plesiochroous Operatio Almost sychroous: compoets geerate data at omially the same bit rate, but are allowed to vary by a bouded amout [used for DS2,3] Requires a good, but ot perfect, clock Muxig uses bit iterleavig; differeces i clock rates are accommodated by justificatio or bit stuffig

21 Trasmissio: TDM Operatio Plesiochroous Operatio Justificatio or (Pulse/Bit) Stuffig: output chael rate higher tha sum of iput rates additioal bits iserted to pad iput rate allocatios of iput rates at output are at the miimum rate (o uderflow), so slightly-fast iputs use up stuff bits eed to read a whole frame to properly extract the idividual iputs

22 Trasmissio: TDM Operatio Plesiochroous Operatio Problems with Plesiochroy: Each part of the world has its ow (ot directly iter-operable) format Justificatio spreads data from tributaries all across frame, makig it difficult to add/drop data from a particular stream Hard to build switches that switch budles of voice calls istead of idividual oes [all must demux dow to DS0 to fid idividual calls]

23 Trasmissio: TDM Operatio Sychroous Operatio (SONET) If etwork was completely sychroous, o eed for justificatio (i theory ) SONET defies a multiplexig hierarchy with exact multiples of data rates: OC-3(155.52Mb/s), OC-12(622.08Mb/s), OC- 24( Gb/s), OC-48( Gb/s) Assumes sychroized clock Uses byte iterleavig across lesser-speed sigals (tributaries)

24 Trasmissio: TDM Operatio Sychroous Operatio (SONET) Beefits of SONET Creates a stadard muxig format for ay umber of 51.84Mb/s sigals Creates a optical stadard for itercoectig multiple-vedor equip. Creates stadard operatio, admiistratio, ad maiteace (OAM) Defies sychroous muxig format for lower-speed (DS1, 2, etc) sigals

25 Trasmissio: TDM Operatio Sychroous Operatio (SONET) Complicatios of SONET SONET ca hadle aggregatios of lowerspeed plesiochroous sigals, so still eed a form of justificatio SONET has overhead: about 27/810 bytes (about 3%) SONET is complicated: recall 2-D frame format & poiter offset SONET requires good clock (+/-1 i 10^11)

26 Switchig Telephoe switch is actually two parts: switchig hardware (moves data) switch cotroller (hadles set up/clearig) (we covered most of the issues already) Cotrollers kow as overlay etwork Messagig betwee cotrollers form sigalig etwork, with its ow protocol

27 Sigalig Basic Versio Toes or pulses from ed system iterpreted at switch cotroller If itra-exchage call, rigs bell o receiver, sets up billig record If iter-exchage, seds set-up message to switch cotroller o earest backboe Cotroller ot directly ivolved i the forwardig of voice samples (cotrol versus data plae)

28 Sigalig Simplified Cotroller FSM Idle Clearig Off-Hook Talkig Dialig Rigig Set-up wait Simplified actual #5 ESS switch cotrol software is 15 millio lies of C code (much of which is this stuff)

29 Sigalig Commo Chael Iter-office Sigalig (CCIS) Older phoe etwork used i-chael & i-bad sigalig betwee cotrollers usig toes Curret system uses out-of-bad sigalig more secure ad flexible uses packet switchig messages use SS7 protocol

30 Sigalig Sigalig System 7 Covers call establishmet, routig ad ehaced services (coferece calls, etc) Etire protocol stack SCCP (aalogous to TCP) MTP-3 (aalogous to IP) MTP-{2,1} (datalik, physical) predates ISO; hard to exted Q.931 stadard defies ISDN-UP sematics (call cotrol, admissio, etc)

31 Routig Structure calls are routed as closely as possible (withi exchages, betwee exchages i same area, or through backboe if ecessary) (ear) fully-coected backboe makes routig decisio fairly straightforward hierarchical area/prefix/umber format provides global uiqueess ad scalig

32 Routig Telephoe Network Routig COs or tadem switches coect to [multiple] core switches (toll switches). Multiple cores from various IXCs. Dese coectivity withi core provides for reasoably simple routig: if src/dst i same CO, coect them if src/dst i same LEC, use 1-hop path betwee COs otherwise, call to (oe of) the core(s)

33 Routig Iteret vs Telco Routig Phoe call traffic relatively easy to predict (both load ad time), so ca pre-select paths Telephoe switches/liks very reliable Cetralized cotrol over core Highly coected with multiple equalcost paths QoS for each path (but same for all)

34 Routig Dyamic No-Hierarchical Routig (DNHR) 10 time periods each day each toll switch assiged a primary (1-hop) path to aother toll switch ad a list of alterate (2-hop) paths [by time] try 1-hop path first, the try others i order (called crakback) crakback useful whe routig supports QoS but wats small coectio rejectio rate

35 Routig The Erlag Map used to compute blockig probability o a truk group give load, capacity DNHR assigs alterate paths to toll switches to miimize blockig probability So, path depeds o expected load which depeds o the path selectio! How to deal with this: system of equatios (Erlag Map) uique fixed-poit solutio (Erlag fixed poit)

36 Routig The Erlag Map: Erlag Formula B(k)=blockig probability o lik k B(k)=E(L(k),C(k)), E() is Erlag formula L(k)=load o lik k, C(k) is capacity lik k r = route = set of liks, v(r)=load o route r Approximate L(k) by: L(k) = Σ v(r) Π [1 B(j)] r: r k j: r j j k r k meas route r goes through lik k

37 Routig The Erlag Map: Erlag Formula Ituitio v(r) is the itrisic load o router r each (1-B(j)) is a thiig of the load, so load o truk k is just the thied sum of the loads o all routes that share truk k B(k) depeds o some other B(j) s through the B()=E() equatio ad eq. So, each B(k) is implicitly defied by the others, formig the Erlag map

38 Routig Metastability i DNHR Solutio of Erlag map is the log-term mea blockig probability This mea is usually the mea of two high low values b ad b Two values more represetative of operatig states of the etwork usig DNHR. Why?

39 Routig Metastability i DNHR B C D A E A->B calls saturate 1-hop lik

40 Routig Metastability i DNHR B C D A E New calls routed thru {ACB} & {ADB}

41 Routig Metastability i DNHR B C D A E A->D call wo t fit across {A,D} lik

42 Routig Metastability i DNHR System ca reach a stable poit i which most or all traffic is takig 2-hop paths whe it could all be takig 1-hop paths udesirable because ca lead to high blockig probability with moderate load ca be preveted by reservig part of each lik badwidth for 1-hop calls (called truk reservatio). May icrease idividual blockig probability, but leads to overall smaller blockig probability

43 Routig Other Techiques TSMR (Truk Status Map Routig) i DNRH, alterative paths updated about oce a week based o traffic studies each switch measures load, tells cetral computer periodic alterate path recomputatio by cetral computer

44 Routig Other Techiques RTNR (Real-Time Network Routig) curret-geeratio routig algorithm (replaced DNHR i ATT switches i 1991) distributed cotrol each switch measure load o all outgoig truks to make decisio, origiator asks destiatio for its truk loadig list, takes logical AND of the two lists ad chooses path (which is symmetric) about 1 or 2 blocked i core (of 260 millio)