• IP - Internet Protocol, which was originally designed to support data

• Current version 4 is the most common in use (RFC 791)

• Other protocols used with the IP :
  -- TCP - Transmission Control Protocol (reliable)
  -- UDP - User Datagram Protocol (non-reliable)
  -- RTP - Real Time Protocol (RFC 1889)
  -- RSVP - Resource reSerVation Protocol)

• Most common telephone system today - analogue

• A very simple implementation

• Keeps the end-to-end delay of voice transmission

• Inexpensive (relatively) when users are not too far apart

BUT

• Other protocols used with the IP :

• Requires one pair of wires per active conversation

• Analogue switches require lot of electromechanical gear which is expensive to buy and maintain

• Parasitic noise adds up at all of the stages of transmission

• For all of the previous reasons - many countries use digital telephone networks.

• In most cases - subscriber line remains analogue. Usually first local exchange converts to digital data stream (64kbps).

• Many voice channels can be multiplexed by TDM.

• The switch copies the content of one time slot in the incoming transmission to another time slot in the outgoing transmission.

• Conclusion - switching can by done by computers!

• Usually, speak session takes less than half of the time during the conversation (35 percent). 

• Data can be sent only when Voice Activity is detected, avoiding transmission of silence. Data is delivers as 'chunks' of data - Packets.

• Advantage - gain of bandwidth.

• Disadvantage - if line transmission is full, need to wait until there is spare capacity.

•  The Network delay is called Jitter and must be corrected by the receiving side.

MARKET

• International VOIP calls could cost 1/5th of normal rates T Big share of $18B US to foreign calls. $15B within Europe.

• 500,000 IP telephony users at the end of 1995. 

• 15% of all voice calls on IP/Internet by 2000 ??10M users and $500M in VOIP product sales in 1999 [IDC] 

• US VOIP service will grow from $30M in 1998 to $2B in 2004 [Forester Research], $2B in 2001 and $16B by 2004 [Frost & Sullivan]

Applications

• Any voice communication where PC is already used:

• Helpdesk access

• On-line order placement

• International callbacks (many operators use voice over frame relay). 

• Intranet telephony

• Internet fax

1. Applications - PC to PC

• Need a PC with sound card

• IP Telephony software: Cuseeme, Internet Phone, ...Intranet telephony

• Video optional

2.Applications – PC to Phone

• Need more gateways that connect IP network to phone networks

• The IP network could be dedicated intra-net or the Internet.

• The phone networks could be intra-company PBXs or the carrier switches

• “Problem” - A sequence of packets must be delivered to the recipient in the same sequence as was transmitted by the source

• Protocol is failed if any packets are lost or damaged or any packet is duplicated or packets are received in a different order

• “Basic Solution” - Packet is transmitted and Recipient acknowledges its receipt

• Packet has a checksum so recipient can detect if packet arrives Okay.

• Once transmitter received the acknowledge - goes on with the next packet

• Defined in RFC 1889

• Provides an End-to-End delivery service for Real-Time traffic applications (Voice, Video)

• The service include :
  -- Payload Type identification
  -- Sequence numbering
  -- Time-Stamping
  -- Delivery monitoring

• RTCP - Real-Time Transport Control Protocol which is used to provide feedback on the quality of the received data on the connection (among other things)

• Defined in draft RFC 2205

• Reserve network bandwidth for a flow of information to provide QoS to information flow

• Uses a “flow descriptor” to request a particular QoS from the network

• Provides the necessary resources in the network to guarantee resources

• 64kbps (serial) data stream is converted into data packets by a codec

• ITU (International Telecommunication Union) is the international organization that specifies codecs

• ITU evaluates the codec’s performance: