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VoIP

VoIP. Contents. What is VoIP Background & Benefit VoIP Concepts What is H.323 Another VoIP Protocol SIP Considerations. PBX. PBX. Router. IP Network. 1. What is VoIP?. PSTN. Voice is transported over Internet with compression of Audio data. ①. Router. ②. 1/20.

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VoIP

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  1. VoIP Contents • What is VoIP • Background & Benefit • VoIP Concepts • What is H.323 • Another VoIP Protocol SIP • Considerations

  2. PBX PBX Router IP Network 1. What is VoIP? PSTN Voice is transported over Internet with compression of Audio data. ① Router ② 1/20

  3. 2. Background & Benefit • Offices are located everywhere, cost grows heavier. Voice망 Voice Voice PBX PBX Data망 Data System Data System • Business IP LAN/WAN grown rapidly, and Ethernet network is everywhere. Voice & Data Data System Data System Voice Voice PBX PBX 2/20

  4. 3. VoIP Concepts OSI-7 layer • International Standards Organization’s Open System Interconnection 7-layer Reference Model. • Defines functional layers needed for communication between 2 or more network end-points. • Message delivery requires each function layer. 7 Application Interactive Voice (VoIP) 6 Presentation Codec 5 Session RTP, RTCP 4 Transport TCP & UDP 3 Network IP, Diffserv 2 Data Link Ethernet 1 Physical UTP Cat 5 3/20

  5. General 3. VoIP Concepts • VoIP standards define protocol messages and packet structure/content for each layer. • “Protocol Stack” sum of protocol layers in OSI model • Several VoIP standards: • IEEE developed H.323 • IETF developed SIP • MGCP used by DOCSIS (Cable TV), disassociated Gateway • IEEE/IETF Megaco/H.248 IETF/IEEE disassociated Gateway Message Layer 7 Layer 7 Layer 6 Layer 6 Layer 5 Layer 5 Sender Receiver Layer 4 Layer 4 Layer 3 Layer 3 Layer 2 Layer 2 Layer 1 Layer 1 4/20

  6. Layer 2 Data Link 3. VoIP Concepts • Defines functions for sharing physical medium and media access. • Ethernet, 10/100 Base T LAN IEEE 802. • Uses Layer 1 as UTP Cat 5 cable. • MAC (Media Access Control) 6 byte address unique for each device. • Sometimes called MAC layer 5/20

  7. 3. VoIP Concepts Layer 3 Network • IP (Internet Protocol) actually lower layer neutral. • IP Packet Network, IP defines packet content w/Sender, Receiver address and IPv4 header. • IPv4 Packet Header 20 bytes (five 32-bit words) include: • Type of Service byte (Diffserv code point) • Header & Packet size • Sequence • Fragmentation flag, Offset • Type of transport (TCP/UDP) • Check sums and DLL CRC • Router protocols exist at this layer. 6/20

  8. 3. VoIP Concepts Layer 4 Transport • Layer 4 defines controls reliability and other route characteristics. • For VoIP TCP (Transport Control Protocol) & UDP (Uniform Datagram Protocol) employed. • TCP “reliable” includes retransmit, used for VoIP network call signaling. • UDP relies on application for transport reliability, Real-time nature of voice suggests use of unreliable transport, UDP for voice packets. 7/20

  9. 3. VoIP Concepts Layer 5 Session • Each session (call or request) carries a unique ID. • RTP (real-time protocol) & RTCP (real-time control protocol), session statistics. • Receiver requests, application processes, e.g. for excessive delay, app could request change codec. • Call (session) control signaling messages: • request • alerting • connect • release • ReSerVation Protocol (RSVP), Quality of Service protocol. 8/20

  10. 3. VoIP Concepts Layer 6 Presentation • To conserve LAN/WAN bandwidth, codec applies compression at encoding, common Codecs: • G.711, PCM codec, 64 kbps • G.726, ADPCM, 16 to 32 kbps • G.723.1, MP-MLQ/ACELP 5.3/6.4 kbps • G.729A, CS-ACELP, 8 kbps • Codec is selected from “Capability” message, may be predefined. • Silence suppression using VAD (Voice Activity Detection) and CNG (Comfort Noise Generation). 9/20

  11. 3. VoIP Concepts Layer 7 Application • Inter-active Voice communications: • IP Phone call • Office to Office FAX • IP Conference Calling • Voice Mail • Unified Messaging, TTS “reads” e-mails 10/20

  12. Feature Protocol Call Signaling H.225 (Q.931) Media Control H.245 Audio Codecs G.711, G.723, G.729 Video Codecs H.261, H.263 FAX T.38 Media Transport Security Supplementary SVC RTP/RTCP H.235 H.450 4. What is H.323? General • Multimedia communication(audio, data & video) over an unreliable network, aimed at IP and UDP. • Application Layer control protocol. • An umbrella standard employs many other protocols by reference and Annex. 11/20

  13. Internet PSTN PSTN 4. What is H.323? Elements • Four “entities”: • Gatekeeper Zone controller, RAS channel (register, status, admit) • Gateway Provides access between disparate networks • Terminal (IP Phone) End-point, user interface device • MCU (Multi-point Control Unit) Conference controller & audio processor Gatekeeper W/MCU Zone Gateways Terminals 12/20

  14. G.711 G.729 G.723.1 H.261 H.263 RTP 4. What is H.323? Protocol Architecture H.323 Stack Audio Apps Video Apps Terminal control and management H.225.0 RAS T.120 Data H.225.0 Call signaling H.245 Control signaling RTCP UDP TCP IP Link Layer 802.3 13/20

  15. 4. What is H.323? Call Flow • Endpoint gets GK permission (ARQ/ACF) • ARQ request for Fast start to GK with destination & media. • Setup request between endpoints, includes media and codec. • Other messages (alerting, connect) then open audio channel. • Close audio channel (either endpoint) • Disconnect, return to idle advise to GK. ARQ ACF setup ARQ H.323 endpoint H.323 endpoint ACF alerting Gatekeeper connect Audio end session end session Release complete DRQ DRQ DCF DCF RAS = black H.225 = blue H.245 = green 14/20

  16. 5. Another VoIP Protocol - SIP General SIP (Session Initiation Protocol) • SIP: Establish multi-media communications sessions IETF’s RFC-2543. • Text based protocol, similar to HTTP, SIP URLs SIP:me@myco.com. • Low layer neutral, Reliable (TCP) or Unreliable (UDP), packet or byte network. • Client/Server Application layer control protocol. • SIP Entities include User Agents and proxy, registrar, location servers, redirect server. • Session has unique Call ID (call leg id) with CSeq (command sequence) for each transaction controlled by a Call Agent. 15/20

  17. G.711 G.729 G.723.1 H.261 H.263 RTP/RTCP 5. Another VoIP Protocol - SIP Protocol Architecture SIP Stack Audio Apps Video Apps Terminal control and management SDP SAP/Q.931 SIP UDP TCP IP Link Layer, 802.3 16/20

  18. 5. Another VoIP Protocol - SIP Call Flow INVITE+SDP • Requests from the User Client include: • Invite • Ack • Cancel • Bye • Option • Register Proxy 200+SDP ACK User Agent A Use r Agent B • Response message has 3-digit “Status Code”: • 1xx Information • 2xx Success • 3xx Redirect • 4xx Client error • 5xx Server error • 6xx Global failure RTP/RTCP BYE 200 17/20

  19. 6. Considerations QoS QoS (Quality of Service) • IP a “best effort” network, packet may be delayed or discarded at router/switch. • Codec, highly compressed voice requires significant processing power and time, DSPs common. • 723.1 delay 37.5 msec (30 msec frame + 7.5 msec look-ahead) • 729A delay 15 msec (10 msec frame + 5 msec look ahead) • Bandwidth Required = (Packet header + payload). • IP/UDP/RTP packet headers = 40 bytes • Adding header & payload, 723.1 needs 17Kbps • DiffServ prioritizes packets at layer 3 with TOS byte. • 802.1 p/Q, priority & queuing at layer 2, with TOS bit in Ethernet packet. 18/20

  20. Internet 6. Considerations NAT NAT (Network Address Translation) • NAT server controls use of scarce Public IP addresses. • NAT generally not compatible with standard VoIP protocols. • H.323 • NAT assigns public IP address and port for outgoing request changing address & port in original packet • Far end sends H.245 request to open a logical second port on same IP address. NAT does not recognize the request on a different port and discards packet as illegal. IP = yyy.yyy.yyy.yy1 IP = yyy.yyy.yyy.yy2 IP = yyy.yyy.yyy.yyn IP = xxx.xxx.xxx.xxx LAN Router running NAT Private IP Address 19/20

  21. Local Network Internet Firewall 6. Considerations Firewalls • Firewalls • Prevent unauthorized incoming access from WAN to LAN. • Prevent unauthorized outgoing access to WAN from LAN. • Typically use Allow/Deny Table of address/port. • IP address can be allowed use of defined ports. • IP address can be allowed full I/O access. • Certain applications, ftp, telnet, etc. use fixed ports. • Most Firewalls support H.323/SIP traffic 20/20

  22. Thank you

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