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Voice Quality

Users can take advantage of the extensive voice and fax capabilities of the Cisco AS5850 Universal Gateway to build a reliable, high-

quality VoIP network. Voice-quality tests confirm that the Cisco AS5850 delivers end-to-end voice-quality performance that meets the

high standards established for toll-quality voice services in the PSTN. Comprehensive voice-quality testing is a critical component in the

Cisco AS5850 development process. Cisco conducts subjective voice-quality tests to determine mean opinion scores using a methodology

derived from ITU-T Recommendations P.830 and P.831. Objective voice-quality tests are also conducted using the Perceptual Analysis

Measurement System.

The high-performance design of the Cisco AS5850 minimizes delay and packet loss during the voice encoding and packetization processes.

The Cisco AS5850 introduces minimal delay as voice data is received from the PSTN and transmitted to the IP network for G.711 calls.

Cisco quality-of-service (QoS) features, including IP Precedence, Resource Reservation Protocol (RSVP), and Weighted Random Early

Detection (WRED), implemented on both the universal gateway and backbone routing infrastructure, can provide a low-latency, high-

reliability path for sensitive voice traffic through today’s networks.

Echo control is essential for packet-switched networks to carry voice traffic successfully. The Cisco AS5850 supports ITU-T

Recommendation G.168 for echo cancellation with a tail length up to 128 milliseconds (ms). Fixed and adaptive jitter buffering and

comfort-noise generation further enhance voice quality.

Voice Codecs

The Cisco AS5850 Universal Gateway offers multiple codecs to meet interoperability, compression, and latency requirements for

various phone-to-phone and PC-to-phone applications: G.711, G.723.1 (5.3K and 6.3K), G.726, G.729ab, G.Clear, and GSM-FR. The

same number of calls is supported across all codec types to simplify network engineering. Enabling voice activity detection (VAD) reduces

packet traffic through the network. With VAD enabled, the Cisco AS5850 detects silence and stops transmitting packets when callers stop

speaking. Variable frame sizing provides further control over speech packetization.

CAC for Voice Services

For VoIP to be a realistic replacement for standard PSTN telephony services, customers need to receive the same consistently high-quality

voice transmissions they receive with basic telephone services. For real-time delay-sensitive traffic such as voice, it is better to deny

network access under congestion conditions than to allow traffic onto the network to be dropped and delayed, causing intermittent impaired

QoS and resulting in customer dissatisfaction.

A variety of QoS mechanisms exist in Cisco IOS Software to allow service providers to design and configure packet networks that provide

the necessary low latency and guaranteed delivery required for voice traffic. These mechanisms include tools such as queuing, policing,

traffic shaping, packet marking, fragmentation, and interleaving.

CAC extends the QoS tool suite to protect voice traffic from being negatively affected by other voice traffic, keeping excess voice traffic

off the network. CAC allows the Cisco AS5850 Universal Gateway to make deterministic and informed decisions before a voice call is

established based on whether the required network resources are available to provide suitable QoS for the new call. Some CAC

mechanisms include:

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Local CAC mechanisms—Call-admission decisions are based on individual gateway-resource conditions.

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Measurement-based CAC mechanisms—Look ahead into the packet network to evaluate the state of network resources.

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Resource-based CAC mechanisms—Calculate resources needed or available, and reserve resources.

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