Voice over IP. Presentation Outline. Objectives

Transcription

1 Voice over IP Professor Richard Harris Presentation Outline Brief overview of VoIP and applications Challenges of VoIP IP Support for Voice Protocols used for VoIP (current views) RTP RTCP RSVP H.323 Semester Advanced Telecommunications Slide 2 Objectives To understand the current state of the art in Internet Telephony. To be able to describe the key protocols involved in VoIP and their roles in this new technology. To be able to discuss the limitations of current VoIP technology. Identify key factors influencing the growth of Internet telephony services for global communications. Semester Advanced Telecommunications Slide 3 1

2 References Internet RFCs RFC RTP RFC RSVP ITU-T H Packet-based multimedia communications systems Voice over IP - Technology Guide Series (techguide.com) IEEE Network May/June 1999 Features Internet Telephony Issues Semester Advanced Telecommunications Slide 4 Introduction Many companies have seen advantages in minimising costs by transporting voice over IP networks. This has set the stage for standards development and the design of terminals and gateways and the rolling out of services on a global scale. Adding voice to packet networks generates many challenges: interoperability packet loss delay scalability reliability quality Semester Advanced Telecommunications Slide 5 Examples of Possible VoIP Applications PSTN Gateways PC based telephone accessing a public network by calling a gateway at a point close to the destination to minimise long distance charges. Internet aware telephones Enhancement of ordinary telephones to serve as an Internet access device as well as ordinary telephony. Directory services could be accomplished via the Internet. Tie line replacement Intranet links could replace tie lines between company PBXs Remote access from branch or home Small office could gain access to corporate voice, data and fax. Voice calls from a mobile PC via the Internet Internet call centres Semester Advanced Telecommunications Slide 6 2

3 HEWLETT Vectra PACKARD Ofice More on Challenges Voice quality has to be comparable to PSTN Underlying network must meet strict performance criteria including: minimising call rejections network latency packet loss disconnects Call control (signalling) must make the telephone calling process transparent so that the callers need not know the technology involved. PSTN/VoIP service interworking. System management and security and accounting and consolidated with PSTN OSSs Semester Advanced Telecommunications Slide 7 End to End Delay for VoIP PC Phone Call Delay (End to End) Round trip PSTN IP PSTN Telephone Client PSTN Gateway Access PC Client Above chart from IEEE Network May/June 1999 Author Bill Goodman Internet Telephony and Modem Delay Semester Advanced Telecommunications Slide 8 IP Network Support for Voice - 1 We need a network that is capable of supporting real time telephone and fax. Three techniques for improving network QoS: Controlled networking environment Capacity pre-planned and adequate performance Management tools to configure network nodes, monitor performance, and manage capacity and flow on a dynamic basis. Control protocols and mechanisms Protocols such as RSVP (Resources Reservation Protocol) and RTP (Real Time Protocol) Semester Advanced Telecommunications Slide 9 3

4 IP Network Support for Voice - 2 Real time voice traffic can be carried over IP networks in 3 ways: Voice trunks Voice packets are transferred between pre-defined IP addresses Eliminate the need for phone number conversions. Fall back to the PSTN as an option. PC to PC Voice Multimedia PCs can utilise this technique without connecting to the PSTN. System emulates an Internet Chat group and can be combined with multimedia whiteboards. Telephony Appears like normal phone but employs various forms of voice over packet networks. Gateway functionality is required if the PSTN needs to be accessed. Semester Advanced Telecommunications Slide 10 IP Network Support for Voice - 3 IP Network Protocols currently being used to implement VoIP: H.323 RTP, RSVP, RTCP UDP/TCP Network Layer (IPv4 and IPv6) Data Link Layer Physical Layer We shall look at RTP, RTCP and RSVP to see their functions. This will be followed by a brief overview of H.323. Semester Advanced Telecommunications Slide 11 RTP - Real-time Transport Protocol - 1 Reference: RFC 1889 A real time end to end protocol Utilises existing transport layers for data that has real time properties. Used by H.323 Takes the bitstream generated by the media encoder breaking it into packets, sending the packets over the network and recovering the bitstream at the receiver. Plays a key role in Internet telephony since it is the component that moves the actual voice among the participants. Signalling protocols provide the parameters for RTP transport. Semester Advanced Telecommunications Slide 12 4

5 RTP - Real-time Transport Protocol - 2 Specific functions provided by RTP are: Sequencing Each RTP pack has a sequence number used for loss detection and compensation for reordering. Intramedia synchronisation Packets within the same stream can suffer different delays (jitter). Applications use playout buffers to compensate for this jitter. RTP provides timestamps to assist in this. Payload Identification Since network conditions may vary during a call, it may be necessary to change encoding dynamically. RTP contains a payload type identifier in each packet. Frame Indication Video and audio are sent in logical units called frames. It is used to mark B of Frame and E of Frame for upper layers. Semester Advanced Telecommunications Slide 13 RTP - Real-time Transport Protocol - 3 Source Identification In a multicast session, many users are participating and so there has to be a mechanism for a packet to say which participant actually sent it. A special identifier called a SSRC - Synchronisation Source is included in the protocol. The RTP protocol has a companion protocol called the RTCP. RTCP = Real Time Control Protocol Semester Advanced Telecommunications Slide 14 RTCP - Real Time Control Protocol Facilities provided by the RTCP: QoS feedback Receivers in a session report back the quality of their reception from each sender. Feedback includes Lost packets, jitter, round trip delays Intermedia Synchronisation Audio and video are often carried as separate streams but need to be synchronised at the receiver (eg lip sync ) Identification RTCP packets contain full details of , phone number and name of participant - this is available to other participants. Session Control Allows you to send small notes or say goodbye!! Semester Advanced Telecommunications Slide 15 5

6 RSVP Protocol Reference: RFC General purpose signalling protocol that allows network resources to be reserved for a connectionless data stream, based on receiver controlled requests. Reserve Reserve Reserve Semester Advanced Telecommunications Slide 16 Signalling There are currently 3 major protocols that support signalling in the IP network for VoIP applications, viz: H.323 MGCP SIP H.323 is described in the next few slides. MGCP - Media Gateway Protocol A control protocol allowing for the monitoring of events in IP phones and gateways and to instruct them to send media to specific addresses. SIP - Session Initiation Protocol Protocol developed by IETF for lightweight call control and capabilities negotiation. Semester Advanced Telecommunications Slide 17 H323 Protocol - 1 Reference: ITU-T H.323 Title: Packet-based Multimedia Communications Systems Conceived originally for multimedia conferencing on a LAN, but now extended to cover Internet Telephony. (Revised in 1998) Provides: Call control Conferencing functions Call management Capability negotiation Supplementary services Semester Advanced Telecommunications Slide 18 6

7 H.323 Protocol - 2 This Recommendation describes terminals and other entities that provide multimedia communications services over Packet Based Networks (PBN) which may not provide a guaranteed Quality of Service. H.323 entities may provide real-time audio, video and/or data communications. Support for audio is mandatory, while data and video are optional, but if supported, the ability to use a specified common mode of operation is required, so that all terminals supporting that media type can inter-work. The packet based network over which H.323 entities communicate may be a point-to-point connection, a single network segment, or an internetwork having multiple segments with complex topologies. Semester Advanced Telecommunications Slide 19 Voice Gateway/Terminal Functions The following picture shows the functional components of terminals that use the H.323 standards: Speech Signalling Voice Processing Call Processing Network Management Packet Processing SNMP Messages IP packets Semester Advanced Telecommunications Slide 20 Other Associated Protocols Among the many protocols relevant to implementations of VoIP are: TCP, UDP IPv4 and IPv6 ATM and Frame Relay SNMP LDAP WWW etc Semester Advanced Telecommunications Slide 21 7

8 Software Support for VoIP - 1 The software functionality required for voice to packet conversion in a VoIP gateway or terminal device is: A Voice Processing module Preparation of voice samples for transmission over the packet network A Call Processing module Signalling gateway that allows calls to be established across packet networks A Packet Processing module Processes voice and signalling packets by adding the appropriate transport headers prior to submitting the packets to the IP network. Signalling information is converted from telephony protocols to the packet signalling protocol. Semester Advanced Telecommunications Slide 22 Software Support for VoIP - 2 A Network Management module Management agent functionality Remote fault Accounting Configuration management Security? Dialling directories Remote access support. Semester Advanced Telecommunications Slide 23 Conclusions This technology is in its infancy! Many problems to overcome. Hot topics: Quality of service Echo cancellation Manageability Many of the protocols that currently support VoIP are not adequate for the task and will need modification before they can be really useful. Need to determine tariffing structures also. Semester Advanced Telecommunications Slide 24 8