VoIP Voice Transmission Over Data Network

What is VoIP?  A method for Taking analog audio signals Turning audio signals into digital data Digital data can be transmitted over the Internet

More Definition  A technology used to transmit voice conversations over a data network using IP

Brief Explanation of Voice Transmission  Audio is digitizing by using sampling circuits  Digital audio is compressed  Therefore, voice transmission with less bandwith

Types of Voice Transmission  Through data network without IP TDM, FR or ATM Directly transmission VoDSL, VoTDM, VoFR or VoATM Network dependency

Types of Voice Transmission  IP Packets VoIP Independent of data network

Types of VoIP  IP to IP (Computer to computer) Typically by using server-client based programs Connect to same server

Types of VoIP  IP to PSTN (Computer to Phone) Need media gateway Compatible with circuit switching and packet switching

Types of VoIP  PSTN to PSTN (Computer to Phone) Through internet For long distance voice transmission Decrease bandwidth cost

Circuit Switching  The path is decided before data transmission  Route is dedicated and exclusive

Packet Switching  IP breaks data into packet  Data load and IP address of source and destination  Reassemble packets  No predetermined path, decide own path  Find its way using the info

Comparision  PSTN Old and expensive Circuit dedication More reliable (Dedicated for only one session)  VoIP More modern No circuit dedication Using the same path with other people at the same time Cost is shared Delay and packet loss

Codecs  Stands for coder-decoder  Coder converts audio signal into a compressed digital form for transmission  Decoder returns back into an uncompressed audio signal for replay

Codecs  Sample  Sort  Compress  Packetize audio data

Soft Switches  Translate phone numbers to IP addresses  Find the current IP address of requested number  Called mapping process handled by central call processor running a softswitch

Soft Switches  A specialized database/mapping program  Connects endpoints  Include Where the endpoint is on the network What phone number is associated with that endpoint The current IP address assigned to that endpoint

Protocols  To communicate efficiently between completely different pieces of software and hardware

H.323  Comprehensive and complex protocol  Originally designed for video conferencing  Provide specifications for  Real time applications  Interactive videoconferencing  Data sharing  Audio applications such as VoIP  Not specific to VoIP

SIP(Session Initiation Protocol)  Smaller, flexible and more efficient  Specifically developed for initiating, building and terminating multimedia applications  More suitable for VoIP

Factors that Affect Call Quality in VoIP  Packet Loss For real time applications, requesting packet again is impossible. Limited packet loss is omitted.

Factors that Affect Call Quality in VoIP  Latency Duration is related to connection type between source and destination To minimize, priority for VoIP packets. Coders also results in latency.

Factors that Affect Call Quality in VoIP  Jitter Variation in delay times Connection state and density vary. Packet delay time change To prevent, use RTP protocol

RTP( Real-time Transport Protocol)  Calculate average delay time of packet  Postpone packets that comes early  Try to equalize delay time

VoIP Packet Headers  For real time applications, no tolerance to error check and retransmission of loss packet mechanism UDP  UDP header info < TCP header info  However, no packet order info on UDP  Again, RTP

RTP  RTP add its own header to packets  RTP includes the order number of packets

VoIP Packet  Coded audio (data) 20 to 150 byte  IP Header (20 bytes)  UDP Header (8 bytes)  RTP Header (12 bytes)  UDP+RTP<TCP

Advantages of VoIP  Low cost for operators  Low or no price for customers  Flexibility  Efficiency  Less bandwidth

Disadvantages of VoIP  Reliability  Latency, jitter, packet loss  Dependent on wall power  No integration to other systems like digital video recorders, digital tv home security systems  Attacked by worms and viruses, hacking  No universal and specific VoIP standards