How Mlp-Based Lfi Works - Cisco 10000 Series Configuration Manual

Multilink PPP-Based Link Fragmentation and Interleaving
The router supports MLP-based fragmentation and interleaving on serial, Frame Relay, and ATM links.
For information on how MLP works and MLP-based LFI, see the following sections:
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How MLP-Based LFI Works

To understand how MLP-based LFI works, it helps to understand the problem it addresses. The complete
end-to-end delay target for real-time packets, especially voice packets, is 150 to 200 milliseconds (ms).
The IP-based datagram transmission techniques for audio transmission do not adequately address the
problems posed by limited bandwidth and the very stringent telephony delay bound of 150 ms.
Unacceptable queuing delays for small real-time packets exist regardless of the use of QoS features such
as weighted fair queuing (WFQ), and the use of voice compression algorithms such as code excited
linear prediction (CELP) compression, which reduces the inherent bit rate from 64 kbps to as low as
8 kbps. Despite these measures, real-time delay continues to exist because per-packet header overhead
is too large and large maximum transmission units (MTUs) are needed to produce acceptable bulk
transmission efficiency.
A large MTU of 1500 bytes takes 215 ms to traverse a 56-kbps line, which exceeds the delay target.
Therefore, to limit the delay of real-time packets on relatively slow bandwidth links—links such as
56-kbps Frame Relay or 64-kbps ISDN B channels—a method for fragmenting larger packets and
queuing smaller packets between fragments of the large packet is needed. MLP helps to solve this
problem through LFI.
MLP provides a method of splitting, recombining, and sequencing datagrams across multiple logical
data links. The LFI scheme is relatively simple: large datagrams are multilink encapsulated and
fragmented to packets of a size small enough to satisfy the delay requirements of the delay-sensitive
traffic; small delay-sensitive packets are not multilink encapsulated, but are interleaved between
fragments of the large datagram.
MLP allows the fragmented packets to be sent at the same time over multiple point-to-point links to the
same remote address. The multiple links come up in response to a dialer load threshold that you define.
The load can be calculated on inbound traffic, outbound traffic, or on either, as needed for the traffic
between the specific sites. MLP provides bandwidth on demand and reduces transmission latency across
WAN links.
To ensure correct order of transmission and reassembly, LFI adds multilink headers to the datagram
fragments after the packets are dequeued and ready to be sent.
Cisco 10000 Series Router Quality of Service Configuration Guide
16-12
How MLP-Based LFI Works, page 16-12
MLP Over Serial-Based LFI, page 16-13
Single-VC MLP Over ATM-Based LFI, page 16-14
Multi-VC MLP Over ATM-Based LFI, page 16-19
MLP Over Frame Relay-Based LFI, page 16-21
Chapter 16 Fragmenting and Interleaving Real-Time and Nonreal-Time Packets
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