as the software queue. FRTS supports FRF.12 (Frame
Relay fragmentation), while class-based shaping does not
support FRF.12 fragmentation for Frame Relay. Traffic-shaping
mechanisms can interact with a Frame Relay network, adapting to
indications of Layer 2 congestion in the WAN. For example, if
the backward-explicit congestion notification (BECN) bit is
received, the router can lower the rate limit to help reduce
congestion in the Frame Relay network. And if the
forward-explicit congestion notification (FECN) bit is
received, the router can generate a test frame with the BECN
bit set. This enables the sender to notice congestion even if
there is no data traffic flowing back from the receiver to the
sender.
Content 4.7 Introducing Traffic
Policing and Shaping 4.7.7 Applying Traffic
Policing Figure shows how traffic policing is often
implemented in an enterprise network. Typically, the access or
distribution layer employs traffic policing to limit certain
traffic classes before that traffic exits the campus onto the
WAN. Traffic shaping is often implemented at the WAN edge when
there are speed mismatches or oversubscription. In a typical
service provider network, traffic policing is often implemented
inbound at the Provider Edge (PE) router to rate-limit incoming
traffic from the Customer Edge (CE) router to ensure that the
customer traffic rate is not exceeding the contractual rate.
Traffic shaping is often implemented outbound at the Provider
Edge and at the Customer Edge to limit the traffic rate between
the Provider Edge and Customer Edge and to allow for FRF.12
fragmentation on Frame Relay connections between the Customer
Edge and Provider Edge.
Content 4.8
Understanding WAN Link Efficiency Mechanisms 4.8.1
Link Efficiency Mechanisms Although there are many
quality of service (QoS) mechanisms for optimizing throughput
and reducing delay in network traffic, QoS mechanisms do not
create bandwidth. QoS mechanisms optimize existing resources
and enable differentiation of traffic according to a policy.
WAN links can use bandwidth optimizing link efficiency QoS
mechanisms such as payload compression, header compression, and
link fragmentation and interleaving (LFI). Figure lists these
three mechanisms, all available on Cisco IOS software. These
features are applicable to low-speed WAN interfaces and are
emerging for use on high-speed Ethernet interfaces. -
Payload compression: Payload compression does create
additional bandwidth, because it squeezes packet payloads, and
therefore increases the amount of data that can be sent through
a transmission resource in a given time period. Payload
compression is mostly performed on Layer 2 frames and, as a
result, compresses the entire Layer 3 packet.
-
Header compression: The basis of header compression,
like other compression methods, is the elimination of
redundancy. This applies especially to often-repeated protocol
headers. The header information of packets in the same flow
does not change much over the lifetime of that flow. Header
compression reads header information only at the beginning of
the flow and then stores it in a dictionary for later reference
through a short dictionary index.
- LFI: LFI is
a Layer 2 technique in which large frames are broken into
small, equal-sized fragments and transmitted over the link in
an interleaved fashion. Using LFI, smaller frames are
prioritized, and a mixture of fragments is sent over the link.
LFI reduces the queuing delay of small frames because the small
frames are sent almost immediately. LFI, therefore, reduces
delay and jitter by expediting the transfer of smaller frames
through the hardware transmit (Tx) queue. The three primary LFI
mechanisms supported by Cisco are as follows:
-
Multilink PPP (MLP): Used on PPP links
-
FRF.12: Used on Voice over IP over Frame Relay
(VoIPovFR) links
- FRF.11 Annex C: Used on Voice
over Frame Relay (VoFR) links
Content 4.8 Understanding WAN Link Efficiency
Mechanisms 4.8.2 Compression Overview
Figure provides an overview of the concept of compression. Data
compression works by identifying patterns in streams of data.
Data compression chooses a more efficient method to represent
the same information. Essentially, a compression algorithm
removes as much redundancy as possible. The compression ratio
measures the efficiency and effectiveness of a compression
scheme—the ratio of the size of uncompressed data to compressed
data. A compression ratio of 2:1 (which is relatively common)
means that the compressed data is half the size of the original
data. Several compression algorithms exist. Some algorithms
take advantage of a specific medium and the redundancies found
in it. However, they do a poor job when applied to other
sources of data. For example, the MPEG standard takes advantage
of the relatively small difference between one frame and
another in video data. It does an excellent job in compression
of motion pictures, but does not compress text well. For text
compression, the Huffmann compression algorithm is better. One
of the most important concepts in compression theory is that
there is a theoretical limit, known as Shannon's limit, which
describes how far a given source of data can be compressed.
Modern compression algorithms coupled with the fast processors
available today allow compression to approach Shannon’s limit.
Hardware compression and software compression refer to the site
in the router to which the compression algorithm is applied. In
software compression, compression is implemented in the main
CPU as a software process. In hardware compression, the
compression computations are off-loaded to a secondary hardware
module. This frees the central CPU from the computationally
intensive task of calculating compression. If you assume that
the router has the clock cycles available to perform the
compression calculations—for example, CPU utilization remains
at a reasonable level—then there is no difference between the
efficiency of hardware compression and software compression.
The achieved compression ratio is a function of the compression
algorithm selected and the amount of redundancy in the data to
be compressed, not where the compression calculations take
place. Figure illustrates the concepts of payload and header
compression. Payload compression squeezes payloads, either the
Layer 2 payload or the Layer 3 payload. With Layer 2 payload
compression, the Layer 2 header remains intact, but its payload
(Layer 3 and above) is compressed. With Layer 3 payload
compression, Layer 2 and 3 headers remain intact. Payload
compression increases the throughput and decreases the latency
in transmission, because smaller packets (with compressed
payloads) take less time to transmit than the larger,
uncompressed packets. Layer 2 payload compression is performed
on a link-by-link basis, whereas Layer 3 payload compression is
generally used on a session-by-session basis. Header
compression methods work by not transmitting repeated
information in packet headers throughout a session. The two
peers on a PPP Layer 2 connection (a dialup link) agree on
session indices that index a dictionary of packet headers. The
dictionary is built at the start of every session and is used
for all subsequent (noninitial) packets. Only changing, or
nonconstant, parameters in the headers are actually sent along
with the session index. Header compression cannot be performed
across multiple routers because routers need full Layer 3
header information to be able to route packets to the next
hop.
Compression increases the amount of data sent through
a transmission resource. Most payload compression schemes work
on Layer 2 frames. This results in compressing the entire Layer
3 packet. The Layer 2 payload compression methods include
these: - Stacker
- Predictor
-
Microsoft Point-to-Point Compression (MPPC)
These
algorithms differ vastly in their compression efficiency and in