VoIP and QoS
Real-time applications, such as voice applications, have different characteristics and
requirements from those of traditional data applications. Because they are real-time
based, voice applications tolerate minimal variation in the amount of delay affecting
delivery of their voice packets. Voice traffic is also intolerant of packet loss and jitter,
both of which unacceptably degrade the quality of the voice transmission delivered to
the recipient end user. To effectively transport voice traffic over IP, mechanisms are
required that ensure reliable delivery of voice packets. Cisco IOS QoS features collectively
embody these techniques, offering the means to provide priority service that meets the
stringent requirements of voice packet delivery.
The QoS components for Cisco Unified Communications are provided through the
IP traffic management, queuing, and shaping capabilities of a Cisco IP network
infrastructure.
Following are a few of the Cisco IOS features that address the requirements of end-toend
QoS and service differentiation for voice packet delivery:
■ Header Compression: Used in conjunction with Real-time Transport Protocol (RTP)
and Transmission Control Protocol (TCP), it compresses the extensive RTP or TCP
header, resulting in decreased consumption of available bandwidth for voice traffic.
A corresponding reduction in delay is realized.
■ Frame Relay Traffic Shaping (FRTS): Delays excess traffic using a buffer or queuing
mechanism to hold packets and shape the flow when the data rate of the source is
higher than expected.
■ FRF.12 (and Higher): Ensures predictability for voice traffic, aiming to provide better
throughput on low-speed Frame Relay links by interleaving delay-sensitive voice
traffic on one virtual circuit (VC) with fragments of a long frame on another VC utilizing
the same interface.
■ Public Switched Telephone Network (PSTN) Fallback: Provides a mechanism to
monitor congestion in the IP network and either redirect calls to the PSTN or reject
calls based on the network congestion.
■ IP RTP Priority and Frame Relay IP RTP Priority: Provides a strict priority queuing
scheme that allows delay-sensitive data, such as voice, to be dequeued and sent
before packets when other queues are dequeued. These features are especially useful
on slow-speed WAN links, including Frame Relay, Multilink PPP [MLP], and T1
ATM links. It works with weighted fair queuing (WFQ) and Class-Based WFQ
(CBWFQ).
■ IP to ATM Class of Service (CoS): Includes a feature suite that maps QoS characteristics
between IP and ATM. Offers differential service classes across the entire WAN,
not just the routed portion. Gives mission-critical applications exceptional service
during periods of high network usage and congestion.
■ Low Latency Queuing (LLQ): Provides strict priority queuing on ATM VCs and
serial interfaces. This feature enables you to configure the priority status for a class
within CBWFQ and is not limited to User Datagram Protocol (UDP) port numbers,
as is IP RTP Priority.
■ MLP: Allows large packets to be multilink encapsulated and fragmented so they are
small enough to satisfy the delay requirements of real-time traffic. MLP also provides
a special transmit queue for smaller, delay-sensitive packets, enabling them to
be sent earlier than other flows.
■ Resource Reservation Protocol (RSVP): Supports the reservation of resources
across an IP network, allowing end systems to request QoS guarantees from the network.
For networks supporting VoIP, RSVP (in conjunction with features that provide
queuing, traffic shaping, and voice call signaling) can provide call admission
control (CAC) for voice traffic. Cisco also provides RSVP support for LLQ and
Frame Relay.