Chapter 7: Voice over IP

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The simplest IP telephone system uses two basic components: - IP telephone: end device allowing humans to place and receive calls. - Media Gateway Controller: providing overall control and coordination between IP phones; allowing a caller to locate a callee (e.g. call forwarding)

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Nội dung Text: Chapter 7: Voice over IP

Chapter 7: Voice over IP Lecturer: M.Eng. P.T.A. Quang
Outline  An overview of the concept  Data transmission vs Conventional digital telephony  Drawbacks and Challenges for transmitting voice on data packets  VoIP, introductory technical description  Media gateway controller and its protocol
An overview of the concept  Digitize the voice  Break up into packets  Add header  Send packets over data packet network  Voice over IP (VoIP), Voice over ATM,Voice over MPLS (VoMPLS)
Data transmission vs conventional digital telephony  Conventional digital telephony:  Full-duplex mode  Analog voice  PCM format (1 sample = 8bits)  Circuit (set up by signaling routine)  Telephone address = 7 to 12 digits  send just once  How VoIP works  Continuously sample audio  Convert each sample to digital form  Send digitized stream across Internet in packets  Convert the stream back to analog for playback 4
Drawbacks and challenges for transmitting voice on data packets  Mouth-to-ear delay  Impact of error frames (packets)  Lost frames (packets)  Variation of packet arrival time, jitter buffering  Prioritizing VoIP traffic over regular Internet and data services  Talker echo  Distortion  Sufficient bit rate capacity on interconnecting transmission media  Voice coding algorithm standardization  Optimized standard packet payload size  Packet overhead  Silence suppression
Data packets
A Basic IP Telephone System • The simplest IP telephone system uses two basic components: - IP telephone: end device allowing humans to place and receive calls. - Media Gateway Controller: providing overall control and coordination between IP phones; allowing a caller to locate a callee (e.g. call forwarding) 7
Interconnection with Others (1)  IP telephone system needs to interoperate with PSTN or another IP telephone system.  Two additional components needed for such interconnection:  Media Gateway  Signaling Gateway 8
Interconnection with Others (2)  Media gateway: translates audio between IP network and PSTN.  Signaling Gateway: translates signaling operations. 9
VoIP, introductory technical description  Introductory  VoIP gateway  An IP Packet as Used for VoIP  The delay tradeoff  Lost packet rate  Concealment of lost packets  Echo and echo control
VoIP, introductory technical description  Gateway: between voice codec and the digital data transport circuit  Signal is converted to digital  Binary output is applied to a conversion device (load 1s and 0s into IP load – 20 to 40 octets)  At the far end, IP packets are input to converter, strips off IP header, stores the payload, release in a constant bit stream to a code  Codec convert to digital bit stream back to an analog signal
VoIP Gateway  Gateway (server) – media gateway  On the edge of the network, have switching function of a local, tandem, or toll-connecting PSTN
 Media gateway functions:  A/D conversion  Convert DS0/E0 to binary signal compatible with IP  Support types of access network  Availability (99.999%)  Handle several voice and data interface protocols  Interoperability  Provide interface between NGC and MG: SIP, H.323, MGCP, and Megaco  Handle switching and media processing based on standard network PCM, ATM, and traditional IP  Transport of voice. 4 tranmissions categories  Standard PCM  ATM over AAL1/AAL2  IP-based RTP/RTCP  Frame relay
VoIP Gateway  Most powerful gateway supports PSTN  high reliability device to meet PSTN availability requirement  Network management probability: SNMP  Integrated access device (IAD): small and medium-sized business can handle data and video products  Residential gateway is deployed in at least five settings:  POTS  Set-top-box provides telephony as well  PC/modem  XDSL termination  Broadband last mile connectivity PSTN vs POTS???
Media gateway
VoIP Gateway • A gateway senses the presence of data by the presence of a 2100-Hz tone in the time-slot •DTMF signaling tones •Human speech
An IP packet as used for VoIP  G.711 or G.726 packets  The packet consists: header and payload
An IP packet as used for VoIP  G.711 (PSTN PCM): transmission rate 100 packets/s, 80B in payload/packet = 8000 samples per second or 50 packets/second, 160B /packet  Total raw bytes= 40B (L3 and L4 overhead) + 8B (L2 overhead) +80B/160B (payload) = 128B/208B