VoIPVoice 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