© 2005 Cisco Systems, Inc. All rights reserved. IPTX v2.01-1 Introducing Cisco CallManager Express Describing the Cisco CallManager Express Voice Packet. - презентация

1 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Introducing Cisco CallManager Express Describing the Cisco CallManager Express Voice Packet Handling Methods

2 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v On-Net Calls SCCP Signaling RTP SCCP is sent between IP Phones and Cisco CallManager Express. The voice connection is carried in IP packets between two IP Phones and has voice samples in an RTP segment. There is no per-call CPU loading on the Cisco CallManager Express router except for call setup and teardown. RTP : :18355 Cisco CallManager Express listens for SCCP messages on TCP port

3 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v PSTN Calls with Cisco CallManager Express As the PSTN Gateway Signaling TCP 2000 Voice over IP UDP 16,384– 32,768 SCCP signaling is used between the IP Phone and Cisco CallManager Express. Appropriate signaling is used between Cisco CallManager Express and the PSTN. RTP is used to carry traffic between the IP Phone and the Cisco CallManager Express router. Cisco CallManager Express acts as an MTP. Voice is sent to the PSTN. Analog or Digital Trunk(s) Voice Cisco CallManager Express PSTN

4 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v PSTN Calls with a Separate Voice Gateway SCCP Signaling RTP SCCP signaling is used between the IP Phone and Cisco CallManager Express. H.323 is used between Cisco CallManager Express and the PSTN gateway. RTP is used to carry traffic between the IP Phone and the voice gateway. The voice gateway acts as an MTP. Voice is sent to the PSTN from the voice gateway. Analog or Digital Trunk(s) Voice H.323 PSTN Gateway Cisco CallManager Express PSTN

5 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Intersite Calls SCCP SCCP signaling is used between the IP Phone and Cisco CallManager Express. H.323 or SIP signaling is used between the Cisco CallManager Express routers. RTP is used to carry traffic between the IP Phones. If Voice over IP is used on the WAN, the RTP header will be preserved. RTP PSTN IP WAN H.323 or SIP

6 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Cisco CallManager Express Local QoS These markings are used to give voice traffic priority over most other types of data on the network. The Cisco CallManager Express system requires all IP Phones under its control to be local on the same LAN network. Layer 2 CoS Marking of 5 Layer 3 IP Precedence Marking of 5 A call has QoS markings on the Layer 2 header and in the IP packet header q Trunk

7 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v RTP Stream Information RTP RTP headers carry voice across an IP-based network. The RTP header is carried inside a UDP segment. The UDP segment is carried inside IP packets. UDP ports are randomly selected from 16,384 through 32,768. If the whole path is Voice over IP, the RTP header will be preserved. RTPVoice PayloadUDPIP

8 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v The Need for Call Admission Control Is there enough bandwidth on the WAN for three simultaneous calls? If allowed, the third call will cause quality problems not only for the third call, but also for all three calls. The third call should be prevented. X IP WAN CAC is useful for the WAN environment, where bandwidth is often limited.

9 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Call Admission Control Locally CAC is not needed for traffic to IP Phones because Cisco CallManager Express assumes that the media is Ethernet LAN and therefore that the bandwidth is effectively unlimited.

10 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Call Admission Control Across WANs CAC should be used for WAN links that could be even temporarily saturated. CAC is implemented through an H.323 mechanism called a gatekeeper. The voice gateway asks the gatekeeper if there is enough bandwidth to set up the call with a specific codec. The gatekeeper answers the question with either an affirmative or a negative response. If the answer is negative, the dial plan of the voice gateway must either connect the call using a secondary path, like the PSTN, or give a fast busy signal to the caller.

11 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v DTMF Relay over the WAN DTMF tones are normally carried in-band with voice. Low-bandwidth codecs such as G.729 are designed for human voice, not for DTMF tones, and they can distort DTMF tones carried in-band. Symptoms of this problem are DTMF tones that are interpreted as another digit or not detected at all. The solution is to send DTMF tones out-of-band in packets. Various types of DTMF relay mechanisms exist.

12 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Summary Local calls are set up and torn down by Cisco CallManager Express, but the RTP goes between the two IP Phones. SCCP is used between the IP Phones and Cisco CallManager Express. Calls to the PSTN can use the Cisco CallManager Express router as the gateway or as a separate router. The PSTN gateway must act as an MTP and convert the RTP stream to and from the format of the connection to the PSTN. Intersite calls that use an IP WAN link between sites preserve the RTP headers. Voice packets originating from the voice on the IP Phones have QoS markings at Layers 2 and 3. CAC should be used when going across low-bandwidth WAN links. DTMF relay should be used when low-bandwidth codecs are used across WAN links.

13