2. www.s ain t jo e.ed u / ~ b r ian c/ OSCON
b r ian c@s ain t jo e.ed u
3. Bas ic Plan
It 's all ab ou t s p eech
●
An alog t elep h on y s t ill m at t er s
●
Digit iz at ion of s ou n d
●
Cir cu it ver s u s p acket - s wit ch ed com m u n icat ion s
●
En cod in g d igit iz ed s p eech
●
In t er n et s ign alin g p r ot o cols
●
As t er is k Fu n d am en t als
●
– Brief h istory
– Overview
– Basic Op eration al Con cep t s
As t er is k in Act ion
●
– Th e CLI
– Th e Man ager In terface
4. Me chanical As pe cts o f Spe e ch
Air flow is m o d u lat ed t o m ake “s p eech ”
●
– Vocal cord s p rovid e “voice”
– Sh ap e of cavity m akes “vowels”
– Im p ed im en ts to airflow m ake “con son an ts”
E.g. lip s, tongue, teeth, glottis
●
Par t icles of s p eech ar e pho ne m e s
●
– Rem em ber th is com e cod ec tim e
5. The Te le pho ne Te rm inal
Fo u r fu n d am en t al p ar t s :
●
– Sign alin g elem en ts
Hook switch
●
Ringer
●
– Au d io (bearer) elem en ts
Microp hone
●
Sp eaker
●
Ph on e lin es s u p p ly t h eir own p ower !
●
6. Te rm inals co nne ct to o ffice s
Two- en d ed p r o t o col
●
– Sim ilar to clien t/ server, DCE/ DTE, etc.
Ser vice p r ovid ed on t wo wir es (“p air ”)
●
– Both A/ C an d D/ C Elem en ts
– Tip an d Rin g
Swit ch es p r ovid e s er vices t o t er m in als
●
– In bou n d an d ou tbou n d calls
– Tru n ks to rem ote switch es
– Featu res
Th e PSTN is n et wor k of s wit ch es
●
7. Fo re ign Exchange (FX)
Th e office (FXO) is p r ovid er of s er vices
●
– Origin ation : “Som eon e calls you ”
– Term in ation : “You call som eon e”
– Featu res: CallerID, Call Wait in g, etc.
Th e s t at ion (FXS) m akes r eq u es t s
●
Eas y t o d is cer n wit h POTS:
●
– Station s h ave p lu gs
– Offices h ave sockets
Th is p oin t is m ajo r s ou r ce of con fu s ion
●
– How cou ld th at be???
8. Co m pute r- POTS Inte rface s
Nam ed aft er wh ich s id e con n ect s t o t h em
●
– So t h eir beh avior is o ppo s ite of th eir n am es!
FXO p orts behave like stations
●
FXS p orts behave like offices
●
Both use hard ware “sockets”
●
Lead in g cau s e o f b egin n er in s an it y
●
Th er e is a co s t t o t h e con fu s ion
●
– Rin g voltage can d efin itely be felt
– Rin g voltages can kill POTS p orts
Exp er ien ce is t h e b es t t each er
●
9. 1 9 7 0 s : The Digital Age Be gins
Fir s t d igit al cir cu it s wer e in Ch icago
●
Ad van t ages of d igit al
●
– Lower circu it cost
– Im m u n it y to n oise
– Im m u n it y to crosstalk
– Ease of sign al p rocessin g
– Sep aration of sign al an d bearer fu n ction s
So m e d is ad van t ages , t oo
●
– Less efficien t u se of ban d wid th
– More com p lex to u n d erstan d an d im p lem en t
10. Analo g to Digital Co nv e rs io n
Co n ver t an alo g s ign al t o puls e train
●
– Each p u lse is an in stan tan eou s voltage
– Each su ch volt age read in g is a sam p le
Th e Nyq u is t Fr eq u en cy s et s s o m e lim it s
●
– PSTN sam p les at 8Kh z
Th e gr an u lar it y of s am p les is im p or t an t
●
– More gran u larity= h igh er fid elity
– PTSN u ses 8 bits/ sam p le
12. Quantiz atio n No is e
A/ D con ver s ion is a lw a y s ap p r oxim at e
●
Er r or s in t h is d om ain called quantiz atio n
●
So ft er s ou n d s (lower valu es ) s u ffer m or e
●
Co m p an d in g: n o n - u n ifor m q u an t iz at ion
●
– More resolu tion at softer levels th an lou d er
– Stu ffs 12 bits of resolu tion in to 8 bits
Tr an s lat io n p r oces s co d in g- de cod in g
●
– Co de cs are tran slators
u - law an d a- law ar e u s ed in PSTN
●
– Stan d ard iz ed as ITU G.711
13. POTS Jargo n co nfus e s IP ty pe s
Or igin at ion ver s u s Ter m in at ion
●
– An oth er m ajor cau se of con fu sion
– Both are n etwork su bscriber services
– Term in ate: You call som eon e
– Origin ate: Som eon e calls you
Seem s s im p le en o u gh on t h e s u r face
●
Wh y d is cer n ? In VoIP, t h ey can b e sep a ra ted !
●
– Term in ation services m ore com m on th an origin ation
14. Vo ice as ano the r ty pe o f data
Ad van t ages
●
– Dram atically lower cost s
– Leveraged in frast ru ct u re
– Con vergen ce wit h com p u ter ap p lication s
Dis ad van t ages
●
– Packet switch in g versu s circu it switch in g
– Ch aot ic vs. Cen traliz ed m an agem en t
– Dyn am ic rou tin g cau ses su rp rises
– Secu rity con cern s abou n d
15. The Big Thre e
Lat en cy
●
– Alm ost always slower th an PSTN
e.g. d om estic PSTN latency ~ 20m s m ax
●
VoIP latency can reach 1000 m s!
●
Packet Los s
●
– Not every p acket get s d elivered
– Ou t of ord er d elivery p ossible, too
Jit t er
●
– Defin ed as variation in laten cy
– Can cau se very seriou s qu ality p roblem s
16. Packe tiz ing Digital Audio
Fir s t q u es t ion : h o w m u ch p er p acket ?
●
– Too little: overh ead will kill you
– Too m u ch : Packet loss or d elay will get you
– 20- 30m s are m ost com m on valu es
i.e. 33/ 50 p ackets p er second
●
VoIP term : fram es
●
Man y ways t o en co d e t h e au d io
●
– Clever sch em es d ram atically red u ce b/ w
One could d o a whole tutorial on cod ecs
●
Som e are p rop rietary, introd ucing m ore confusion
●
Som e require a lot of p rocessor resources
●
Processing itself introd uces d elays
●
17. Co de cs carry v o ice
Th is is t h e be are r p ar t of t h e call
●
Two b as ic t yp es
●
– Waveform
– Vocod er (AKA sou rce)
– Hybrid s are com m on resu lt
Bas ic id ea: h igh q u alit y, lo w b an d wid t h
●
Th eo r y is r an k m at h em at ical t er r it or y
●
Qu alit y m et r ic is Mean Op in ion Sco r e
●
– MOS, valu es 1- 5
Man y cod ecs go b y t h eir ITU n am es
●
Lot s of var iat ion in m an y ar eas
●
18. So m e ty pical e xam ple s
g.7 1 1 (var ian t s u law an d u law)
●
– Waveform ,64kbs, MOS 4.3
Ad ap t ive PCM (g.7 2 1 , g.7 2 6 )
●
– Waveform , variou s bitrates an d MOS
g.7 2 9
●
– Hybrid , proprietary, 8kbs, 4.0 MOS
iLBC (In t er n et Low Ban d wid t h Cod ec)
●
– Hybrid , ~ 13- 15kbs, > 4.0 MOS
– Freely- licen sed version of p rop rietary
– Design ed to with stan d p acket loss
19. Mo re co m m o n co de cs
gs m
●
– Derived from classic cellp h on e tech n ology
– Hybrid
– Variou s low (5- 8kbs) rates
– Variou s (3.5- 4.0) MOS valu es
Sp eex
●
– Tru ly Op en Sou rce cod ec
– Hybrid
– Dyn am ically ad ap tive bitrates
– High - m ed iu m - low MOS varian ts
Th is s cr at ch es t h e s u r face; go go oglin g
●
20. Signaling Pro to co ls
No t e con t r as t t o b ear er fu n ct ion
●
Th er e ar e m an y, an d it 's an act ive ar ea of r es ear ch
●
Most of t h em u s e RTP for b ear er p ar t
●
– So cod ecs en cod e for RTP, n ot sign alin g
– Cod ecs are n egotiated by sign alin g p rotocols
Her e we'll m en t ion t h r ee, p lay wit h t wo:
●
– SIP, th e u n d isp u ted lead er, IETF- based
– IAX, d evelop ed by an d for Asterisk
– H.323, old classic, d evelop ed by ITU
TDM h as it s o wn s et , t oo
●
21. SIP
Ses s ion In it iat ion Pr ot ocol
●
Develop ed b y IETF WGs
●
St ar t ed ou t s im p le
●
– Cu rren tly over 1K p ages of RFCs
– Th is lead s to in terop erability p roblem s
Th e am az in g layer b ou n d ar y flaw
●
– SIP en cod es IP in form ation in layer 4
– Cau se of fam ou s NAT d ifficu lties
– Mot h er of session bord er con troller m arket
Us es RTP for b ear er t r an s p or t
●
22. IAX
In t er - As t er is k Exch an ge Pr ot ocol
●
Develop ed b y Mar k Sp en cer an d As t er is k co m m u n it y
●
– In form ation al RFC in IETF
Co m b in ed s ign al an d b ear er p r ot ocol
●
– Th is m ean s NAT- h ard in ess
Im p lem en t ed in s om e fir m war es
●
Allows exch an ge of d ialp lan in for m at ion
●
23. H.3 2 3
Develop ed b y t h e ITU
●
Clas s ic p r ot o col, n ow in wan in g u s e
●
Co m p lex, m an y s u b - p ar t s
●
Us es RTP for b ear er t r an s p or t
●
Mas t er - s lave o r ien t at ion
●
– Masters are called gatekeep ers
Bas is o f m os t ear ly VoIP s ys t em s
●
24. His to ry / Ov e rv ie w
Th e Ad t r an Tr ad es h o w St or y
●
Lin u x Su p p or t Ser vices
●
L.S.S. b eco m es Digiu m
●
Th e Zap at a Pr oject
●
Ot h er Op en Sou r ce PBX- t yp e p r oject s
●
– VOVIDA
– Bayon n e
– SER
25. Functio nal Ov e rv ie w
PBX: Swit ch in g fu n ct ion alit y
●
– Both TDM an d IP
Tr an s co d er
●
Co n fer en ce Br id ge
●
Call Cen t er Fu n ct ion s
●
Ro u t in g En gin e
●
Feat u r es Ser ver
●
– Voicem ail
– Directory
– CDR
26. Ins tallatio n
So u r ce Bu ild s fair ly t r ivial
●
– Requ ires kern el h ead ers at m in im u m
Packages , e.g. RPM, .ip k
●
Fu ll d is t r ib u t ion s , e.g. As t lin u x
●
ABE
●
In t egr at or s
●
St ealt h Ver s ion s (3 Co m ?)
●
Nu m b er ed ver s ion s , p lu s SVN t r u n k
●
– 1.2 series m ost com m on in p rod u ction , d ep recated
– 1.4 cu rren t version
– SVN- tru n k will becom e 1.6
27. Fundam e ntal Ide as
Ever yt h in g h ap p en s o n ch an n els
●
– A sign alin g sch em e on an in terface
TDM Ch an n els
●
– Each in terface t yp e h as ch an n el typ e
e.g. T1, ISDN, POTS, etc.
●
IP Ch an n els
●
– Man y IP ch an n els sh are an in terface
Usually Ethernet
●
– e.g. SIP, IAX, H.323, SCCP, etc.
Typ e vs . in s t an ce vs . call- in s t an ce
●
28. Co nfiguring the s e rv e r
Gen er ally d o n e wit h t ext .con f files
●
– / etc/ asterisk is u su al h om e d irectory
On e file p er ch an n el t ech n o lo gy
●
– e.g. sip .con f, iax.con f. h 323.con f
Co n figu r at ion for feat u r es p r o ces s or s
●
– voicem ail.con f
– m eetm e.con f for con feren cin g
– agen ts.con f for call agen ts
Th e d ialp lan is t h e h ear t o f t h e s ys t em
●
– exten sion s.con f
29. Dialplan Bas ics
Glo b al p ar am et er s an d d efin it ion s
●
Bas ic Hier ar ch ical Or ien t at ion
●
– Dialp lan is a set of con texts
– Each con text is a set of exten sion s
– Each exten sion is a set of ap p lication s in p riority ord er
Logic for m acr os , var iab les , flow con t r ol
●
Co n t ext s ar e ver y im p or t an t
●
– Lin kage between d ialp lan an d ch an n els
– Determ in es allowable exten sion s
– Con text s can in clu d e ot h er con text s
30. Exte ns io ns
Mo s t ext en s ion s ar e n u m er ic
●
– Th ey d on 't h ave to be, “brian c” is valid
– Sm all set of bu ilt- in sp ecial exten sion s
e.g. s is the start extension for POTS calls
●
Each is a s et , in p riority o r d er
●
– Priorities can be n u m eric 1,2,3, etc.
– Th ey can also h ave labels
– Flow con trol p ossible to oth er p riorities
Always as s o ciat ed wit h at leas t on e ch an n el
●
31. Applicatio ns pro v ide be hav io r
So m e ar e b u ilt in t o s er ver
●
So m e ar e load ab le m od u les
●
A few com m on in t u it ive exam p les
●
– Dial
– Voicem ail
– Meetm e (Con feren ce en gin e)
– Mu sicon h old
– Han gu p
– Record
32. Bas ic Call Flo w
Mo s t act ion s t ar t s wit h ch an n el even t
●
– Asterisk calls it a “rin g”
Ch an n el con figu r at ion file is con s u lt ed
●
– Au th en ticates
– Negotiates cod ec ch oices
– Miscellan eou s op tion s (CallerID, DTMF)
– Sp ecifies d ialp lan con text
Dialp lan con t ext is s ear ch ed for ext en s ion m at ch
●
Ap p licat io n s ar e execu t ed b y p r io r it y
●
