VoIP Technologies by Sorin M. SCHWARTZ VOICE OVER CIRCUIT SWITCHING www.sorin-schwartz.com NETWORKS PHASE TWO - ANALOG TELEPHONE, DIGITAL EXCHANGE - Signals - User to exchange: analog voice - Exchange to exchange: digitized voice - Technology- Virtual circuit switching and multiplexing Advantages: - Switching is faster (electronic) - Circuits are “logical” not “physical”. Multiple logical (virtual) circuits may be multiplexed over one single high speed trunk 24 x 64 kbps ⇒ 1.544 Mbps (T1) 32 x 64 kbps ⇒ 2.048 Mbps (E1) - As most of the circuit carries digital signals, noises have less influence ⇒ better voice quality But: - Two phones still require two separate physical lines! - No multiplexing in the local loop. - Digital signals (generated by PCs, smoke detectors, TV cameras, etc.) have to be downgraded to analog signals just to be digitized again in the exchange, to be converted to analog at the remote site, to be converted back in digital format by the final device! - Lines are noisy (analog signals, digital and a lot of A/D processing) EXCHANGE EXCHANGE analog analog X Y A/D EXCHANGE EXCHANGE EXCHANGE A A/D B EXCHANGE EXCHANGE VoIP 03 1
VoIP Technologies by Sorin M. SCHWARTZ VoIP CALL FLOW www.sorin-schwartz.com VoIP DEVICE to VoIP DEVICE zone A zone B Router A Router B R R R R R Terminal T1 Terminal T3 System System Controller Controller R R (Gatekeeper) (Gatekeeper) Terminal T2 Terminal T4 Internet / intranet (GK A) (GK B) Multipoint Multipoint Media Media Control Control Gateway Gateway Unit Unit Long distance Local Local (GW A1) (GW B1) (MCU A) (MCU B) PSTN-A PSTN-B PSTN GW PABX A PABX B GW A2 B2 A D B C In different zones Phone number IP address - T1 to T3 Phone number IP address GK A Table - GK B finds “tel-(B)T3” in tel-(A)T1 IP-T1 GK B Table its table and sends to GK tel-(B)T3 IP-T3 tel-(A)T2 IP-T2 A, “IP-T3” tel-(B)T4 IP-T4 - GK A sends to T1, tel-(972)…(e.g.B) IP-GW A1 “IP-T3” tel-(972)…(e.g.B) Zone A; IP-GK A tel-(A)…(e.g.A) IP-GW A2 - T1 generates VoIP tel-(B)…(e.g.D) IP-GW B2 packets with tel-(86)…(e.g.C) Zone B; IP-GK B IP DA = IP-T3 tel-(86)…(e.g.C) IP-GW B1 tel-(B)…(e.g.D,T3,4) Zone B; IP-GK B tel-(A)…(e.g.A,T1,2) Zone A; IP-GK A VoIP 03 2
VoIP Technologies by Sorin M. SCHWARTZ VoIP SYSTEM ARCHITECTURE www.sorin-schwartz.com ITU-T H.323 Summary ITU-T H.323 Architecture terminal terminal C C Router A SG Router B SG IP net Router A Router B IP net MGC MGC MG MG S S Syst. Syst. Contr. Contr. GK A GK B B A MC MCU MC GW A1 GW B1 MCU SG A1 SG B1 MP MP C C PSTN PSTN MGC MGC A1 B1 GW A2 S S C MG MG SG A1 B1 MGC MG S H.323 basic call flow 1.- End points register with the zone GK 2.- Terminal / GW query GK for IP address to be used for controls 3.- GK may indicate as the entity to send controls to: - itself (GK routed call), or - the remote end (direct routed call) - If GK has no relevant information, it may contact other servers (local or in the Internet) 4.- Terminal / GW query the entity indicated by GK in step 3, for the IP address to be used to send media VoIP 03 3
VoIP Technologies by Sorin M. SCHWARTZ VoIP SYSTEM ARCHITECTURE www.sorin-schwartz.com SIP (Session Initiation Protocol) SIP Architecture SIP REDIRECT server UAC UAC C C Router A Router A SG Router B SG SG Router B SG IP net IP net MGC MGC MGC MGC UAS SIP SIP UAS MG MG MG S S MG Syst. Syst. PROXY PROXY Contr. Contr. UA server server UA B A (A) (B) MC MCU MC MCU SG A1 SG B1 MP UAC UAC C C MP SG SG PSTN PSTN MGC MGC MGC MGC A1 B1 UAS UAS MG MG S S UAC C MG MG SG SG UA UA A1 B1 MGC MGC UAS MG S MG SIP REGISTRAR UA server LOCATION server SIP basic call flow 1.- User Agents register with the Registrar server (Registrar server maintains user’s whereabouts in a Location Server 2.- User Agent Client query SIP server for IP address to be used for controls 3.a.- Proxy server indicates itself as entity to send controls to. It forwards client’s requests to the called party or to a better informed server (it acts on client’s behalf). It executes call signaling on behalf of the party it serves (retaining billing information). 3.b.- Redirect server responds to client’s request by providing to it the coordinates of the called party or those of another server, better informed about called party location. Client has to contact directly the new server. 3.c.- Last server has to indicate the coordinates of the called party (for media) VoIP 03 4
VoIP Technologies by Sorin M. SCHWARTZ VoIP SYSTEM ARCHITECTURE www.sorin-schwartz.com Gateway Decomposition “GW Decomposition” Architecture C C C Router A Router A SG Router B SG Router B SG IP net IP net MGC MGC MGC Syst. MG MG MG S S S Syst. Syst. Syst. Contr. Contr. Contr. Contr. H.323 or SIP B B A behaves as one H.323 or SIP MC C MCU MG Control SG A1 SG B1 MGC SG B1 MP C C C PSTN PSTN SG end point S MGC MGC MGC MG Control MG Control MG A1 B1 B1 S S S C MG MG SG MG SG SG A1 B1 B1 MGC MG MG MG S - The GW is decomposed into its basic elements: - MG is simple (cheap) - one Media Gateway Controller (MGC)(a.k.a - Future services are transparent to MG. Changes affect Agent, Call Agent) only MGC, which will generate new sets of commands - multiple Media Gateways (MG) to MG, in order to provide the new services - MG reports to MGC every event (incoming call, - For the external world, MGC together with all its MGs off hook, hang up, etc.). looks like a node with multiple connections - In response, MGC instructs MG what has to be - MGs are unaware that the call is established by MGC done (ring the phone, bring dialed numbers, etc.). using H.323 or SIP. - Only MGC understands both MGCP and SIP or H.323 VoIP 03 5
VoIP Technologies by Sorin M. SCHWARTZ VoIP STANDARDS www.sorin-schwartz.com MAIN STANDARDS REDIRECT Router A IP net server SIP Router Router terminal terminal UAC UAC Router Router A IP net B Syst. A B Contr. IP net H.323 or SIP UAS SIP SIP UAS PROXY PROXY GK GK server server UA UA behaves as one H.323 or SIP (B) (A) A B C MCU MGC MG Control UAC PSTN SG end point S GW A1 GW B1 location MG Control UAC server PSTN UAC MG Control MG PSTN GW A2 UAS SG UAS SG UAS UA MG MG UA UA SIP REGISTRAR server H.323 SIP GW Decomposition ITU-T SG16 IETF Telcordia ETSI Field (H.323 suite) (SIP suite) (Bellcore) (TIPHON) Registration SIP RAS (-) Hand shaking processes Restrictions on H.323 usage: Call signaling H.225.0 (Q 931) (-) SIP DTS 2001 H.245 SIP (SDP) (-) Capabilities exchange - PC to phone DTS 2002 - Internal to GW - MEGACO (defined with - phone to PC Media GW Controller (MGC) - H.248 (defined with IETF ITU-T SG16 H.248) MGCP DTS 2003 to Media GW (MG) MEGACO) - MGCP - phone to phone - MGCP DTS 2004 System controller - PC to PC not defined not defined (-) to servers VoIP 03 6
Recommend
More recommend
