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100 Chapter 2: QoS Tools and Architectures

Table 2-6

Comparison of Link-Efficiency Tools

 

 

 

 

 

 

 

 

Types of Packets to Which Tool

 

Tool

Data Links Supported

Can Be Applied

 

 

 

 

 

Payload compression

All; recommended on serial

All IP packets

 

 

links (T/1, E/1, and slower)

 

 

 

 

 

 

RTP header compression

All; recommended on serial

All packets with IP/UDP/RTP

 

(cRTP)

links (T/1, E/1, and slower)

headers

 

 

 

 

 

TCP header compression

All; recommended on serial

All IP packets with TCP headers

 

 

links (T/1, E/1, and slower)

 

 

 

 

 

 

Multilink PPP fragmentation

Multilink PPP

All packets larger than a configured

 

and interleaving (MLPPP LFI)

 

length

 

 

 

 

 

Frame Relay fragmentation

Frame Relay

All packets larger than a configured

 

(FRF*)

 

length (FRF.12) or all non-VoFR

 

 

 

frames (FRF.11c)

 

 

 

 

 

Link fragmentation and

Frame Relay and ATM

All IP packets

 

interleaving for Frame Relay

 

 

 

and ATM VCs

 

 

 

 

 

 

All functions listed are based on 12.2 mainline IOS code levels.

*The Frame Relay Forum is often referred to as FRF; their document names tend to begin with the letters FRF as well. The QoS feature called Frame Relay fragmentation is also referred to as FRF. In this book, FRF refers to fragmentation, and not the Frame Relay Forum, unless otherwise stated.

Call Admission Control and RSVP

Call admission control (CAC) protects network bandwidth by preventing more concurrent voice and video flows than the network can support. By doing so, it not only protects data traffic, but it also protects the quality of voice and video calls that are already set up. If a network engineer designs a network to support 3 concurrent G.729 calls, for instance, which take roughly 85 kbps, depending on the data links used, but if 10 concurrent calls occur, taking roughly 285 kbps, many bad things happen. Data applications may not get enough bandwidth. Also all the voice calls tend to degrade quickly—not just the “extra” calls.

Cisco defines three general categories of CAC tools: local, measurement based, and resource based. When a device makes a decision about whether to allow a new call to occur, a CAC decision has been made. When a single device makes a CAC decision without asking other devices, or without any perception of network health beyond itself, the CAC tool used is considered to be a local CAC tool—in other words, the decision was made locally. Measurement-based CAC tools measure the network performance and load by sending probes that measure delay and packet loss through the network. This “measurement” of the current health of the network determines whether a new call is allowed. The third category, resource-based CAC, uses information about usage of resources to decide whether a new call occurs. Resources include things such as

Introduction to IOS QoS Tools 101

voice digital signal processors (DSPs), available DS0 channels on trunks, and bandwidth inside the IP network. Table 2-7 lists a general definition of each type of CAC.

Table 2-7 Definitions of Types of CAC Tools

Type of CAC Tool

Definition

 

 

Local

When making the decision to allow or disallow a new call, a single device

 

considers information about itself, but does not consider information about

 

the current network behavior, and does not ask about resource utilization in

 

other devices in the network.

 

 

Measurement based

When making the decision to allow or disallow a new call, a single device

 

considers the results of probes that measure delay and loss in the network.

 

 

Resource based

When making the decision to allow or disallow a new call, a single device

 

considers information about resource utilization, as perceived by itself and

 

by other devices in the network. Resources include DSPs, DS0 channels on

 

trunks, and IP bandwidth.

 

 

The Resource Reservation Protocol (RSVP) enables devices to reserve bandwidth inside the IP network. RSVP is one of the resource-based VoIP CAC methods, because IP bandwidth is a resource in the network. However, RSVP can be used as a more general tool, not just for VoIP CAC. Chapter 8, “Call Admission Control and QoS Signaling,” happens to be a good place to cover RSVP in some depth, because some RSVP discussion is already required for the CAC coverage in that chapter.

CAC Tools

Table 2-8 lists each of the specific CAC tools. The table categorizes each tool based on whether it is a local CAC, resource-based CAC, or measurement-based CAC mechanism. Chapter 8 covers each tool to varying levels of detail.

Table 2-8

Comparison of CAC Tools

 

 

 

 

 

 

 

Tool

CAC Type

CAC Decision Is Based on Whether . . .

 

 

 

 

 

Physical DS0 limitation

Local

A DS0 channel is available on a trunk

 

 

 

 

 

Max-connections

Local

A configured number of maximum connections on the dial

 

 

 

peer used for the call has been exceeded

 

 

 

 

 

Voice-bandwidth for

Local

VoFR PVC CIR has been exceeded; VoFR only (not VoIP)

 

Frame Relay

 

 

 

 

 

 

 

Trunk conditioning

Local

Keepalives sent to other end of network keep working or

 

 

 

not; used for “connection trunk” calls only*

 

 

 

 

continues