8 bits/byte = 12,000 bits
Line rate: 56,000
bps
Result: 12,000 bits/56,000bps = .214 sec or 214 ms
Another method of efficiency is by eliminating overhead bits.
For example, RTP headers have a 40-byte header. With a payload
of as little as 20 bytes, the overhead can be twice that of the
payload in some cases. RTP header compression (also known as
Compressed Real-Time Protocol header) reduces the header to a
more manageable size. Traffic Shaping and Policing
Shaping is used to create a traffic flow that limits the
full bandwidth potential of the flow(s). This is used many
times to prevent the overflow problem mentioned in the
introduction. For instance, many network topologies use Frame
Relay in a hub-and-spoke design. In this case, the central site
normally has a high-bandwidth link (say, T1), while remote
sites have a low-bandwidth link in comparison (say, 384 Kbps).
In this case, it is possible for traffic from the central site
to overflow the low bandwidth link at the other end. Shaping is
a perfect way to pace traffic closer to 384 Kbps to avoid the
overflow of the remote link. Traffic above the configured rate
is buffered for transmission later to maintain the rate
configured. Policing is similar to shaping, but it
differs in one very important way: Traffic that exceeds the
configured rate is not buffered (and normally is discarded).
Note
Cisco's implementation of policing (committed
access rate [CAR]) allows a number of actions besides discard
to be performed. However, policing normally refers to the
discard of traffic above a configured rate. In summary, QoS is
the ability of the network to provide better or “special”
services to selected users and applications, but with a cost to
other users and applications. In any bandwidth-limited network,
QoS reduces jitter, delay, and packet loss for time-sensitive
and mission-critical applications.
Content
3.2 Implementing Cisco IOS QoS 3.2.2
Congestion-Management Tools One way network elements
handle an overflow of arriving traffic is to use a queuing
algorithm to sort the traffic and then determine some method of
prioritizing it onto an output link. Cisco IOS software
includes the following queuing tools: - First-in,
first-out (FIFO) queuing
- Priority queuing (PQ)
- Custom queuing (CQ)
- Flow-based weighted fair
queuing (WFQ)
- Class-based weighted fair queuing
(CBWFQ)
Each queuing algorithm was designed to solve
a specific network traffic problem and has a particular effect
on network performance, as described in the following
sections. Note
Queuing algorithms take effect when
congestion is experienced. By definition, if the link is not
congested, then there is no need to queue packets. In the
absence of congestion, all packets are delivered directly to
the interface. FIFO: Basic Store-and-Forward Capability
In its simplest form, FIFO queuing involves storing
packets when the network is congested and forwarding them in
order of arrival when the network is no longer congested. FIFO
is the default queuing algorithm in some instances, thus
requiring no configuration, but it has several shortcomings.
Most important, FIFO queuing makes no decision about packet
priority; the order of arrival determines bandwidth,
promptness, and buffer allocation. Nor does it provide
protection against ill-behaved applications (sources). Bursty
sources can cause long delays in delivering time-sensitive
application traffic, and potentially to network control and
signaling messages. FIFO queuing was a necessary first step in
controlling network traffic, but today's intelligent networks
need more sophisticated algorithms. In addition, a full queue
causes tail drops. This is undesirable because the dropped
packet could be a high-priority packet. The router can’t
prevent this packet from being dropped because there is no room
in the queue for it (in addition to the fact that FIFO cannot
tell a high-priority packet from a low-priority packet). Cisco
IOS software implements queuing algorithms that avoid the
shortcomings of FIFO queuing. PQ: Prioritizing Traffic
PQ ensures that important traffic gets the fastest
handling at each point where it is used. It was designed to
give strict priority to important traffic. Priority queuing can
flexibly prioritize according to network protocol (for example
IP, IPX, or AppleTalk), incoming interface, packet size,
source/destination address, and so on. In PQ, each packet is
placed in one of four queues—high, medium, normal, or low—based
on an assigned priority. Packets that are not classified by
this priority list mechanism fall into the normal queue. During
transmission, the algorithm gives higher-priority queues
absolute preferential treatment over low-priority queues. As
shown in Figure , PQ puts data into four levels of queues:
High, Medium, Normal, and Low. PQ is useful for making sure
that mission-critical traffic traversing various WAN links gets
priority treatment. For example, Cisco uses PQ to ensure that
important Oracle-based sales reporting data gets to its
destination ahead of other, less-critical traffic. PQ currently
uses static configuration and thus does not automatically adapt
to changing network requirements. CQ: Guaranteeing
Bandwidth
CQ allows various applications or
organizations to share the network among applications with
specific minimum bandwidth or latency requirements. In these
environments, bandwidth must be shared proportionally between
applications and users. You can use the Cisco CQ feature to
provide guaranteed bandwidth at a potential congestion point,
ensuring the specified traffic a fixed portion of available
bandwidth and leaving the remaining bandwidth to other traffic.
Custom queuing handles traffic by assigning a specified amount
of queue space to each class of packets and then servicing the
queues in a round-robin fashion). As shown in Figure , CQ
handles traffic by assigning a specified amount of queue space
to each class of packet and then servicing up to 16 queues in a
round-robin fashion. As an example, encapsulated Systems
Network Architecture (SNA) requires a guaranteed minimum level
of service. You could reserve half of available bandwidth for
SNA data and allow the remaining half to be used by other
protocols such as IP and Internetwork Packet Exchange (IPX).
The queuing algorithm places the messages in one of 17 queues
(queue 0 holds system messages such as keepalives, signaling,
and so on) and is emptied with weighted priority. The router
services queues 1 through 16 in round-robin order, dequeuing a
configured byte count from each queue in each cycle. This
feature ensures that no application (or specified group of
applications) achieves more than a predetermined proportion of
overall capacity when the line is under stress. Like PQ, CQ is
statically configured and does not automatically adapt to
changing network conditions. Flow-Based WFQ: Creating
Fairness Among Flows
For situations in which it is
desirable to provide consistent response time to heavy and
light network users alike without adding excessive bandwidth,
the solution is flow-based WFQ (commonly referred to as just
WFQ). WFQ is one of Cisco's premier queuing techniques. It is a
flow-based queuing algorithm that creates bit-wise fairness by
allowing each queue to be serviced fairly in terms of byte
count. For example, if queue 1 has 100-byte packets and queue 2
has 50-byte packets, the WFQ algorithm will take two packets
from queue 2 for every one packet from queue 1. This makes
service fair for each queue: 100 bytes each time the queue is
serviced. WFQ ensures that queues do not starve for bandwidth
and that traffic gets predictable service. Low-volume traffic
streams that comprise the majority of traffic, receive
increased service, transmitting the same number of bytes as
high-volume streams. This behavior results in what appears to
be preferential treatment for low-volume traffic, when in
actuality it is creating fairness. As shown in Figure , if
high-volume conversations are active, WFQ makes their transfer