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Chapter 6

ECE495 Chapter Notes - Chapter 6: Propagation Delay, Packet Loss, Tail Drop


Department
Electrical and Computer Engineering
Course Code
ECE495
Professor
mint
Chapter
6

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Introduction to QoS v1.21 – Aaron Balchunas
* * *
All original material copyright © 2010 by
Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
1
- Introduction to QoS -
Obstacles to Network Communication
Modern networks support traffic beyond the traditional data types, such as
email, file sharing, or web traffic. Increasingly, data networks share a
common medium with more sensitive forms of traffic, like voice and video.
These sensitive traffic types often require guaranteed or regulated service,
as such traffic is more susceptible to the various obstacles of network
communication, including:
Lack of Bandwidth – Describes the simple lack of sufficient throughput,
which can severely impact sensitive traffic. Increasing bandwidth is
generally considered the best method of improving network communication,
though often expensive and time-consuming.
Bandwidth is generally measured in bits-per-second (bps), and can be
offered at a fixed-rate (as Ethernet usually is), or at a variable-rate (as
Frame-Relay often is). Various mechanisms, such as compression, can be
used to pseudo-increase the capacity of a link.
Delay – Defines the latency that occurs when traffic is sent end-to-end
across a network. Delay will occur at various points on a network, and will
be discussed in greater detail shortly.
Jitter – Describes the fragmentation that occurs when traffic arrives at
irregular times or in the wrong order. Jitter is thus a varying amount of
delay. Voice communication is especially susceptible to jitter. Jitter can be
somewhat mitigated using a de-jitter buffer.
Data Loss – Defines the packet loss that occurs due to link congestion. A
full queue will drop newly-arriving packets - an effect known as tail drop.
All of above factors adversely affect network communication. Voice over IP
(VoIP) traffic, for example, begins to degrade when delay is higher than 150
ms, and when data loss is greater than 1%.
Quality of Service (QoS) tools have been developed as an alternative to
merely increasing bandwidth. These QoS mechanisms are designed to
provide specific applications with guaranteed or consistent service in the
absence of optimal bandwidth conditions.
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Introduction to QoS v1.21 – Aaron Balchunas
* * *
All original material copyright © 2010 by
Aaron Balchunas (aaron@routeralley.com),
unless otherwise noted. All other material copyright © of their respective owners.
This material may be copied and used freely, but may not be altered or sold without the expressed written
consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
2
Types of Delay
Delay can occur at many points on a network. Collectively, this is known as
end-to-end delay. The various types of delay include:
Serialization Delay – refers to the time necessary for an interface to
encode bits of data onto a physical medium. Calculating serialization
delay can be accomplished using a simple formula:
________# of bits________
bits per second (bps)
Thus, the serialization delay to encode 128,000 bits on a 64,000 bps
link would be 2 seconds.
Propagation Delay – refers to the time necessary for a single bit to
travel end-to-end on a physical wire. For the incredibly anal geeks, the
rough formula to estimate propagation delay on a copper wire:
____Length of the Physical Wire (in meters)___
2.1 x 10
8
meters/second
Forwarding (or Processing) Delay – refers to the time necessary for
a router or switch to move a packet between an ingress (input) queue
and an egress (output) queue. Forwarding delay is affected by a
variety of factors, such as the routing or switching method used, the
speed of the device’s CPU, or the size of the routing table.
Queuing Delay – refers to the time spent in an egress queue, waiting
for previously-queued packets to be serialized onto the wire. Queues
that are too small can become congested, and start dropping newly
arriving packets (tail drop). This forces a higher-layer protocol (such
as TCP) to resend data. Queues that are too large can actually queue
too many packets, causing long queuing delays.
Network (Provider) Delay – refers to the time spent in a WAN
provider’s cloud. Network delay can be very difficult to quantify, as it
is often impossible to determine the structure of the cloud.
Shaping Delay – refers to the delay initiated by shaping mechanisms
intended to slow down traffic to prevent dropped packet due to
congestion.
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