Media VoIP

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Lesson Objectives

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By the end of this lesson you will:

• Become familiar with transport technologies and protocols used to transport media over IP networks

• RTP/RTCP• Codecs• Fax• DTMF

RTP / RTCP

Codecs

DTMF

Summary

SIGNALIG AND MEDIA PROTOCOLS

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• VOIP signaling protocols define how to set up and tear down calls, how to carry information required to locate users and negotiate capabilities

• VOIP media protocols define how to transport audio and video as packets over the IP network

• The transport technologies and protocols are fairly independent of the signaling protocols.

CMSNetworkINVITE

TRYING

RINGING

OK

ACK

2 Way media path

Signaling Protocol SIP

Media Protocol RTP

RTP

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• In a VOIP enabled network, the voice signal is digitized, compressed and converted to IP packets and then transmitted over the IP network.

• The mechanism that provides end-to-end delivery services for real-time data such as audio and video or simulation data is defined by Real-Time Transport protocol (RTP).

• What the RTP does:• payload type identification, • sequence numbering, • time stamping• delivery monitoring

• What the RTP doesn't do:• ensure timely delivery• provide quality-of-service

RTCP

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• A new set of rules were introduced to address the quality of service for real-time data

• The Real Time Transport Control Protocol (RTP control protocol or RTCP) is based on the periodic transmission of control packets to all participants in the session, using the same distribution mechanism as the data packets.

• What RTCP does:• RTCP provides feedback on the quality of the data distribution. This

feedback function is performed by the RTCP sender and receiver reports.

• RTCP carries a persistent transport-level identifier for an RTP source called the canonical name or CNAME. The CNAME is been used to keep track of each participant in case of multicast environment.

• Allows large number of participants in a conference. By using CNAME each party can observe the number of participants and this number is used to calculate the rate at which RTP packets are sent.

• What RTCP doesn't do:• ensure timely delivery

How It works - RTP

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• Using a signaling protocol (SIP) 2 or more end-points negotiate the addresses and a pair of ports. One port is used for audio data (RTP), and the other is used for control (RTCP) packets.

• Each end-point sends audio data in small chunks of, say, 20 ms duration.

• Each chunk of audio data is preceded by an RTP header; RTP header and data are in turn contained in a UDP packet.

How It Works - RTP

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IP

Header

UDP

Header

RTP

Header

RTP

PayloadData

• The RTP header indicates the type of audio encoding (such as PCM, ADPCM or LPC), timing information and a sequence number that allow the receivers to reconstruct the timing produced by the source, so that in this example, chunks of audio are contiguously played out the speaker every 20 ms.

• The RTP Payload represent the data transported by RTP in a packet, for example audio samples or compressed video data.

How It works

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• Payload types codec and RFC reference

PT Name Type Clock Rate (Hz) RFC0 PCMU Audio 8000 RFC 35512 G721 Audio 8000 RFC 35513 GSM Audio 8000 RFC 35514 G723 Audio 80005 DVI4 Audio 8000 RFC 35516 DVI4 Audio 16000 RFC 35517 LPC Audio 8000 RFC 35518 PCMA Audio 8000 RFC 3551

18 G729 Audio 800019 reserved Audio 800026 JPEG Video 9000 RFC 243531 H261 Video 9000 RFC 203234 H263 Video 9000 RFC 2190

Dynamic MPEG Video 9000 RFC 2250

How It works - RTCP

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• To monitor the quality of service each end-point periodically multicasts a reception report plus the name of its user on the RTCP (control) port. The reception report indicates how well the current speaker is being received and may be used to control adaptive encodings.

How It works - RTCP

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IP

Header

UDP

Header

RTCP

Header

RTCP

Payload Data

RTCP header contains information how to interpret the payload.

The RTCP payload may contain

SR: Sender report, for transmission and reception statistics from participants that are active senders

RR: Receiver report, for reception statistics from participants that are not active senders

SDES: Source description items, including CNAME.

Not all end-points are equal

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• So far, we have assumed that all sites want to receive media data in the same format. However, this may not always be appropriate. For users having connections of different bandwidth or those working behind a firewall which won't allow IP packets to pass will need some extra processing.

• The mechanism that defines how to connect two or more transport-level “clouds“ is called RTP translator/mixer.

• Typically, each cloud is defined by a common network and transport protocol (e.g., IP/UDP) plus a multicast address and transport level destination port or a pair of unicast addresses and ports.

Mixer in RTP

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• Not all participants in a conference have a connection with the same bandwidth. So how do they take part simultaneously? .

• A mixer may be placed near the low-bandwidth area. This mixer resynchronizes incoming audio packets to reconstruct the constant 20 ms spacing generated by the sender, mixes these reconstructed audio streams into a single stream, translates the audio encoding to a lower-bandwidth one and forwards the lower-bandwidth packet stream across the low-speed link .

Mixer in RTP

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SSRC1

SSRC2

SSRC3

Mixer Receiver

High-bandwidth Low-bandwidth

The mechanism that receives streams of RTP data packets from one or more sources, possibly changes the data format, combines the streams in some manner and then forwards the combined stream is called Mixer

Translator in RTP

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• A problem occurs if one or more participants of a conference are behind a firewall which won't allow an IP packet containing the RTP message to pass. For this situation translators are used .

• Two translators are installed, one on either side of the firewall, with the outside one funneling all multicast packets received through a secure connection to the translator inside the firewall. The translator inside the firewall sends them again as multicast packets to a multicast group restricted to the site's internal network.

Translator in RTP

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Source FireWallTranslator ReceiverTranslator

The mechanism that receives streams of RTP data packets from one or more sources and forwards RTP packets with their SSRC identifier intact is called Mixer.

RTP / RTCP

Codecs

DTMF

Summary

What is a Codec

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• The word codec is a blending of two words, 'compressor-decompressor' or, more accurately, 'coder-decoder'.

• A codec is an algorithm (a program), most of the time installed as a software on a server or embedded within a piece of hardware (ATA, IP Phone, etc’), that is used to convert Voice/Video signals into digital data to be transmitted over the Internet or any network during a VoIP call.

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Mechanisms

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• PCM (Pulse Code Modulation) is a simple technique of sampling the sound signal at a fixed rate (8000 times/second) and generate a number corresponding to each sample. It assumes no specific property of the signal. So it works reasonably well with all types of sounds.

• LPC (Liner Predictive Coding) assumes specific properties of human voice and uses a more complex algorithm to digitize and compress voice data. It works well for sending human utterances offering a low data rate but is not suitable for transmitting music or fax.

• SBC (Sub Band Coder) uses a different approach of representing sounds in terms of frequencies rather than sampling at regular intervals.

Audio Codecs

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• G.7xx, including G.711, G.721, G.722, G.726, G.727, G.728, G.729, is an suite of ITU-T standards for audio compression and de-compression.

• In telephony, there are 2 main algorithms defined in the standard, mu-law algorithm (used in America) and a-law algorithm (used in Europe and the rest of the world). Both are logarithmic, but the later a-law was specifically designed to be simpler for a computer to process.

Video Codecs

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• H.261 is video coding standard defined by ITU. It was designed for data rates which are multiples of 64Kbit/s, and is sometimes called p x 64Kbit/s (p is in the range 1-30).

• The coding algorithm is a hybrid of inter-picture prediction, transform coding, and motion compensation.

• The H.263, defined by ITU, supports video compression (coding) for video-conferencing and video-telephony applications.

• The coding algorithm of H.263 is similar to that used by H.261, however with some improvements and changes to improve performance and error recovery.

• The H.264 and the MPEG-4 Part 10, also named Advanced Video Coding (AVC), is jointly developed by ITU and ISO. H.264/MPEG-4 supports video compression (coding) for video-conferencing and video-telephony applications.

RTP / RTCP

Codecs

DTMF

Summary

DTMF

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• Dual-tone multi-frequency (DTMF) signaling is used for telephone signaling over the line in the voice-frequency band to instructing a telephone switching system of the telephone number to be dialed, or to issue commands to switching systems or related telephony equipment.

DTMF keypad frequencies DTMF event frequencies

DTMF over VoIP

• In-band• In-band means that DTMF tones are transmitted as audio

packets along with voice data. Therefore only uncompressed codecs like g711 alaw or ulaw can carry in-band DTMF reliably.

• RFC2833: DTMFs are detected at the Gateway level and transmitted as special RTP packets (with different payload ID) along with voice data

• Out-of-band• Out-of-band means that DTMF is transmitted outside of the

audio phone conversation usually with signaling protocol.

H.323 SIP

In-band In-Band In-Band

RFC 2833Out-of-Band H.245 SIP INFO

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DTMF Schemes with VoIP Protocols

DTMF Out-of-Band SIP INFO

• The SIP INFO method can be used by SIP network elements to transmit DTMF tones out-of-band as telephone-events in a reliable manner independent of the media stream.

• In the DTMF relay method the body of the SIP message consists of signaling information and uses the content-type application/dtmf-relay

INFO sip:7007471000@example.com SIP/2.0 Via: SIP/2.0/UDP alice.uk.example.com:5060 From: <sip:7007471234@alice.uk.example.com>;tag=d3f423d To: <sip:7007471000@example.com>;tag=8942 Call-ID: 312352@myphone CSeq: 5 INFO Content-Length: 24 Content-Type: application/dtmf-relay

Signal=5 Duration=160

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DTMF Out-of-Band SIP INFO

• Another. less common, variation of sending DTMF with INFO method is the DTMF trigger mechanism which uses the application/dtmf mime-type. The body of the message consists only of the DTMF digit.

INFO sip:7007471000@example.com SIP/2.0 Via: SIP/2.0/UDP alice.uk.example.com:5060 From: <sip:7007471234@alice.uk.example.com>;tag=d3f423d To: <sip:7007471000@example.com>;tag=8942 Call-ID: 312352@myphone CSeq: 5 INFO Content-Length: 1 Content-Type: application/dtmf

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Summary

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For a given call Signaling protocols defines how to setup the call and RTP/RTCP protocols defines how to transport voice/video between end-points

Codecs represent the mathematical algorithms that define how to encode/decode audio/video data.

Fax can be transmitted over IP in real-time or not in real-time.

DTMF are sent either in-band or out-of-band

Summary