© 2006 Cisco Systems, Inc. All rights reserved.ONT v Describe Cisco VoIP Implementations Encapsulating Voice Packets for Transport

© 2006 Cisco Systems, Inc. All rights reserved.ONT v End-to-End Delivery of Voice Packets

© 2006 Cisco Systems, Inc. All rights reserved.ONT v Voice Transport in Circuit-Based Networks Analog phones connect to CO switches. CO switches convert between analog and digital. After call is set up, PSTN provides: –End-to-end dedicated circuit for this call (DS-0) –Synchronous transmission with fixed bandwidth and very low, constant delay

© 2006 Cisco Systems, Inc. All rights reserved.ONT v Voice Transport in IP Networks Analog phones connect to voice gateways. Voice gateways convert between analog and digital. After call is set up, IP network provides: –Packet-by-packet delivery through the network –Shared bandwidth, higher and variable delays

© 2006 Cisco Systems, Inc. All rights reserved.ONT v Explaining Protocols Used in Voice Encapsulation

© 2006 Cisco Systems, Inc. All rights reserved.ONT v Which Protocols to Use for VoIP? Feature Voice Needs TCPUDPRTP ReliabilityNoYesNo ReorderingYes NoYes Time- stamping YesNo Yes MultiplexingYes No

© 2006 Cisco Systems, Inc. All rights reserved.ONT v Voice Encapsulation Examples Digitized voice is encapsulated into RTP, UDP, and IP. By default, 20 ms of voice is packetized into a single IP packet.

© 2006 Cisco Systems, Inc. All rights reserved.ONT v Reducing Header Overhead

© 2006 Cisco Systems, Inc. All rights reserved.ONT v Voice Encapsulation Overhead Voice is sent in small packets at high packet rates. IP, UDP, and RTP header overheads are enormous: –For G.729, the headers are twice the size of the payload. –For G.711, the headers are one-fourth the size of the payload. Bandwidth is 24 kbps for G.729 and 80 kbps for G.711, ignoring Layer 2 overhead.

© 2006 Cisco Systems, Inc. All rights reserved.ONT v RTP Header Compression Compresses the IP, UDP, and RTP headers Is configured on a link-by-link basis Reduces the size of the headers substantially (from 40 bytes to 2 or 4 bytes): –4 bytes if the UDP checksum is preserved –2 bytes if the UDP checksum is not sent Saves a considerable amount of bandwidth

© 2006 Cisco Systems, Inc. All rights reserved.ONT v When to Use RTP Header Compression Use cRTP: –Only on slow links (less than 2 Mbps) –If bandwidth needs to be conserved Consider the disadvantages of cRTP: –Adds to processing overhead –Introduces additional delays Tune cRTPset the number of sessions to be compressed (default is 16)

© 2006 Cisco Systems, Inc. All rights reserved.ONT v Summary In packet telephony, digitized voice is carried in IP packets, which are routed one by one across the IP network. Voice is encapsulated using RTP, UDP, and IP protocol headers. IP, UDP, and RTP headers can be compressed using cRTP to substantially reduce the encapsulation overhead.

© 2006 Cisco Systems, Inc. All rights reserved.ONT v