This document defines VoIP and its protocols. VoIP allows routing of phone calls over IP networks using packet switching instead of traditional circuit switching. Key protocols include H.323 for call setup, control and transport, and RTP for real-time media transport over UDP. H.323 defines codecs for digitizing and compressing voice, and uses signaling protocols like H.225 and H.245 as well as gatekeepers for call routing and quality of service control. RTCP monitors RTP transport quality. H.323 gateways enable interworking between IP and circuit-switched networks.

1. VoIP
IP Telephony
Internet Telephony
Digital Phone

2. Defined
 The Routing of telephonic conversations on
the Internet or any other IP-based network.
 General packet switching as opposed to
traditional circuit switching.
 VoIP Protocols are defined to address
communication.

3. VoIP Protocols
 H.323 (ITU-T) vs IETF
 H.323 is a suite of protocols recommended
by ITU-T.
 Layered architecture.
 Codecs – Voice digitization, compression
 Signaling – Call establishment, control
 Transport – End-to-end transport of voice.

4. H.323

5. Codecs
Analog to digital conversion (& Compression)
 Waveform Codecs
 Preserve the actual waveform of the speech
signal.
 PCM, G.711, 64 kbps.
 DPCM, 32kbps.
 ADPCM, G.721,722,726, 64-16 kbps.

6. Codecs
 Vocoders
 Encode the parameters of speech signal
separately to achieve low bit rates.
 Code Excited Linear Predictive
 G.729, 8 kbps
 G.723.1, 5.3/6.3 kbps
 AMR

7. Transport
 Real Time Transport Protocol (RTP) RFC
3550.
 Designed for real-time multimedia
communication over IP networks.
 It runs over UDP (TCP is also supported).
 RTP packets carry encoded voice frames in
the case of VoIP.

8. RTP/RTCP
 Support for multicast
 Data transport is monitored by a sister
protocol RTCP.
 RTP aids in
 Payload type Identification
 Sequencing of the out-of-order packets.
 Timely play out of the media data using
timestamps.

9. RTP/RTCP
 RTCP
 Monitors the quality of service.
 Conveys information about the participants in on-
going session.
 Report packets are exchanged between
senders and receivers of multimedia
information.
 Three Reports are defined: RTCP-SR, RTCP-
RR, RTCP-SDES.

10. RTCP
 RTCP-SR/RTCP-RR contain information on
statistics such as number of packets lost,
round-trip delay and jitter etc.
 Used to modify sender transmission rates
and for diagnostic purposes.

11. H.323 Control Protocols
 Call Setup
 H.225 {Resource Access Service (RAS), Q.931}
 Gatekeeper
 Call Control
 H.245

12. Call Setup
H.323
gatekeeper
Initiating end system
(terminal/computer)
Called end system
(terminal/computer)
ARQ(1)
ACF/ARJ(2)
Setup (3)
Call Proceeding (4)
ARQ(5)
ACF/ARJ (6)
Alerting (7)
Connect (8)
Resource Access Service
(RAS) Messages/UDP
Q.931/TCP
signaling messages
ARQ = Access Request
ACF = Access Confirm
ARJ = Access Reject

13. H.323 Gatekeeper
 To limit the number of concurrent VoIP calls
on a LAN.
 The objective is to provide guaranteed QoS.
 Optional

14. Call Control – H.245/TCP
 Exchange of system Control messages
between end-points.
 Opening of Logical channels for audio, video
and user data streams.
 Negotiation of capabilities.

15. Interworking
 H.323 also addresses Interworking between
end-points that are attached to circuit
switched networks.
 This is achieved through H.323 Gateway.

16. The Role of H.323 Gateway
 To provide translation between different procedures and control
messages associated with each network type.
 To perform transcoding if communicating end systems are
using different coding standards.
 Address translation – in case where communicating end points
are connected to different networks because each uses a
different naming convention.
 Example – All the systems on a LAN are assigned an alias
PSTN number and all the systems external to LAN are
assigned an alias IP address.

17. H.323 Gateway
The
Internet
The
Internet
H.323
Gateway
GW
GW
GWGW
ISDNISDN
PSTNPSTN
GUARANTEE
D QoS LAN
GUARANTEE
D QoS LAN
B-ISDNB-ISDN
NON-
GUARANTEED
QoS LAN
NON-
GUARANTEED
QoS LAN

18. H.323 Sources
 Open Source Projects:
 WinRTP {www.vovida.org}.
 OpenPhone, ohphone {www.openh323.org}.
 Multimedia Communications, {Fred Halsall}

19. Thank you!