Cisco Systems OL-14619-01 manual Packetization SCCP Integrations Only, 6-17

Chapter 6 Integrating Cisco Unity with the Phone System

Integrating with Cisco Unified Communications Manager Express (by Using SCCP or SIP)

Packetization (SCCP Integrations Only)

The Real-Time Transport Protocol (RTP) is used to send and receive audio packets over the IP network. Each discrete packet has a fixed-size header, but the packets themselves can vary in size, depending on the size of the audio stream to be transported (which varies by codec) and the packetization setting. This variable size function helps utilize network bandwidth more efficiently. Reducing the number of packets that are created per call sends fewer total bytes over the network.

Packetization is set in the Cisco Unified CM Service Parameters, in the Preferred G711 Millisecond PacketSize and Preferred G729 Millisecond PacketSize parameters. Cisco Unity supports any packet size up to 30ms for G.711 audio, and any packet size up to 60 ms for G.729a audio. The default setting is 20ms for both; there may be latency issues with lower settings.

DSCP is a priority setting on each packet. DSCP helps intermediary routers manage network congestion and lets them know which packets to prioritize ahead of others. Following Cisco AVVID standards, the Cisco Unity-CM TSP marks the SCCP packets (call control) with a default DSCP value of 26 (the TOS octet is 0x68), and the RTP packets (audio traffic) with a default DSCP value of 46 (the TOS octet is 0xB8). Thus, the RTP audio packets can be assigned priority over other packets by using the router settings. Note that even though Cisco Unified CM allows you set different DSCP values, when integrated with Cisco Unity, the DSCP values set by the Cisco Unity-CM TSP always take precedence.

With each new audio stream (once per call), Cisco Unified CM tells Cisco Unity which packet size to use, and the Cisco Unity-CM TSP sets the DSCP priority for the stream. The entire stream (call) stays at the specified packet size and priority. For example, an audio stream could be broken up into packets of 30ms each. A 30ms G.729a audio stream would be 30 bytes plus the header per packet, and a 30ms G.711 stream would be 240 bytes plus the header per packet. For information on setting Cisco Unified CM Service Parameters, see the Cisco Unified CM documentation at http://www.cisco.com/en/US/products/sw/voicesw/ps556/tsd_products_support_series_home.html.

Integrating with Cisco Unified Communications Manager Express (by Using SCCP or SIP)

Cisco Unity supports Cisco Unified Communications Manager (CM) Express (formerly known as Cisco Unified CallManager Express) integrations through both SCCP and SIP interfaces. Figure 6-8shows the connections.

Figure 6-8

PSTN

Cisco Unity SCCP and SIP Connections to Cisco Unified Communications Manager Express Over a LAN

Cisco

Unity server

Cisco Unified

Communications

Manager Express

LAN 191850

See Table 6-5for information on the differences in these integration methods.

	Design Guide for Cisco Unity Release 5.x
OL-14619-01		6-17