Cisco 10000 Series Configuration Manual page 506
Quality of service configuration guide
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Link Fragmentation and Interleaving
An important part of delay is the time it takes to actually place the bits onto an interface, referred to as
serialization delay. We recommend that serialization delay not exceed 20 ms. Serialization delay is
calculated using the following formula:
Serialization Delay = Frame Size (bits) / Link Bandwidth (bps)
As shown in
a 56-kbps link. While waiting for the large data packet to transmit, the router queues real-time packets.
However, real-time traffic cannot tolerate delay. For example, good voice quality requires delay to be
less than 150 ms. By fragmenting the nonreal-time large data packet into smaller frames and interleaving
real-time packets between the fragments, both real-time packets and data frames can be carried together
on low-speed links, without causing excessive delay to the real-time traffic.
Figure 16-1
Voice
Data
The Cisco 10000 series router supports the following types of link fragmentation and interleaving (LFI):
MLP over Serial-based LFI—Uses the fragmentation and interleaving capability of MLP to
•
integrate real-time packets (such as voice packets) and nonreal-time packets (such as data transfers)
on the same link while reducing real-time packet latency. MLP defines the mechanisms to fragment,
reassemble, and sequence large datagrams across multiple logical data links. MLP over serial-based
LFI supports up to 10 member links per MLP bundle, one of which is LFI-enabled. You can
terminate the serial links on multiple line cards in the router chassis if all of the links are the same
type, such as T1 or E1. The router supports subrate T1 interfaces as member links. The link speeds
must be the same for all of the links in the bundle.
Single-VC MLP over ATM-based LFI—Uses the fragmentation and interleaving capability of MLP
•
to integrate real-time and nonreal-time packets together on the same link. MLP defines the
mechanisms to fragment, reassemble, and sequence large datagrams across multiple logical data
links. MLP uses the fragmentation and packet sequencing specifications defined in RFC 1990 to
implement link fragmentation and interleaving at the bundle level. Single-VC MLP over ATM-based
LFI supports only one member link per MLP bundle and the link is LFI-enabled.
Multi-VC MLP over ATM-based LFI—Uses the fragmentation and interleaving capability of MLP
•
to integrate real-time packets and nonreal-time packets on the same link while reducing real-time
packet latency. MLP implements link fragmentation and interleaving at the bundle level. Multi-VC
MLP over ATM-based LFI supports up to 10 member links, one of which is LFI-enabled.
Cisco 10000 Series Router Quality of Service Configuration Guide
16-2
Chapter 16
Figure
16-1, a nonreal-time data packet of 1500 bytes takes 214 ms to leave the router over
Integrating Voice and Data Packets on Low-Speed Links
Before
for 1500-byte frame at 56 kbps
After
Data
Voice
Fragmenting and Interleaving Real-Time and Nonreal-Time Packets
Data
214-ms serialization delay
Data
OL-7433-09
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