DATA COMMUNICATIONS AND COMPUTER
NETWORKS
ASSIGNMENT
1.
Propagation Delay
2.
Transmission Delay,
3.
Queuing Delay
4.
Processing Delay.
Discuss each of these concepts with respect to DELAY and
highlight the contribution of each towards overall delay during
data communication.
Compiled by JUDE MARK KAGANDA
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PROPAGATION DELAY
A network suffers some delay because a signal requires some amount of time to travel across a
transmission medium i.e from node A to node B. A packet is initiated at node A, after the packet
is transmitted to the transmission medium, it has to go through the medium to reach node B.
Hence, the time taken by the last bit of the packet to reach node B is called propagation delay.
Propagation delay has a direct proportion to distance spanned by the packet. A typical LAN
used within a single building has a propagation delay under a millisecond despite the use of
long cable runs. Such delays seem negligible/irrelevant to a human but a modern computer can
execute over one hundred thousand instructions in a millisecond. Thus, a millisecond delay is
significant when a set of computers need to coordinate. A network that uses a geo-satellite has
much higher delay — even at the speed of light, it takes hundreds of milliseconds for a bit to
travel to the satellite and back to earth owing to the distance from the satellite to the earth.
Let Dprop be the Propagation delay.
Let d be the distance from Node A to Node B
Let s be the velocity/speed of the signal
Thus;
Dprop = d/s
Factors affecting propagation delay:
Distance – The longer the distance of propagation, the more time reaching the destination.
Velocity – If the velocity(speed) of the signal is higher, the packet will be received faster.
Velocity =3 X 108 m/s (for air)
Velocity= 2.1 X 108 m/s (for optical fibre)
For data communication using metal wire cables, such as ethernet cables, propagation delay is
the time needed for an electrical signal to be propagated from one host to another. An Ethernet
card transmits bits by varying the electric potentials at its pins, and all other Ethernet cards
attached to it by a wire will detect this change in electric potential and correctly interpret the bits
(Ethernet protocol, IEEE 802.3 standard).
In a fiber-optic cable, the time needed to propagate a light signal from end to end is the
propagation delay.
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TRANSMISSION DELAY
Transmission delay is the rate at which an interface(transmission link) pushes all bits in a
packet into the transmission medium.
It can also be known as store-and-forward delay or packetization delay.
This mimics behaviour at an airport. When goods are shipped in from abroad, they are stored at
the airports so as to verify clearance of all taxes on them and other necessary checks made and
then later they are let out to be received after the paperwork checks out hence, store and
forward delay.
Let Dtrans be the transmission delay
Let L be the length of the packet in bits
Let t be the time in seconds
Thus;
Dtrans = L/t
If bandwidth is 1 bps (every second 1 bit can be transmitted on to the transmission medium)
and data size is 70 bits then what is the transmission delay?
Solution
In 1s ----------> 1 bit is transferred
Then ;
70 bits will be transferred in (70 x 1)seconds
Thus;
Dtrans = 1bps
This delay depends upon the following factors:
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If there are multiple active sessions, delay will become significant.
Transmission delay is related to transmission rate of an interface which is bandwidth of a
link. Increasing bandwidth decreases transmission delay.
MAC protocol largely influences the delay if a link is shared among multiple devices.
Sending and receiving a packet involves context switch in the operating system, which takes
finite time.
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QUEUING DELAY
When packets arrive at a router, they have to be processed and transmitted. A router can only
process one packet at a time. If packets arrive faster than the router can process them (such as
in a burst transmission) the router puts them into a buffer (queue) until it can get around to
transmitting them. So, the amount of time a packet takes to be transmitted from the queue at the
link to the recipient is Queuing Delay.
It varies from packet to packet depending on time of arrival at the link. In an instance when 20
packets arrive at a previously empty link, the first packet to arrive suffers no queuing delay while
the next packet will suffer negligible delay and the delay size increases as the packets are
queued up which makes the 20th packet have the largest queuing delay. Since queueing delay
is not the same for all the packets we cannot have a definite declaration and so calculations like
average queuing delay, variance of queuing delay, probability of queuing delay are used to
estimate the queuing delay at a link.
This delay depends upon the following factors:
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If the size of the queue is large, the queuing delay will be huge. If the queue is empty there
will be less or no delay.
Less number of servers/links increases the queuing delay.
If more packets are arriving in a short or no time interval, queuing delay will be large. In
particular, if traffic intensity is close to zero, then packets are pushed out at a rate much
higher than the packet arrival rate; therefore, the average queuing delay will be close to zero.
On the other hand, when the traffic intensity is close to 1, there will be intervals of time when
the arrival rate exceeds the transmission capacity (due to the burstiness of arrivals), and a
queue will form thus the average queue length gets larger and larger. The qualitative
dependence of average queuing delay on the traffic intensity is shown below.
In reality a queue preceding a link has finite capacity, although the queuing capacity greatly
depends on the switch design and cost. Because the queue capacity is finite, packet delays do
not really approach infinity as the traffic intensity approaches one. Instead, a packet can arrive
to find a full queue and with no place to store such a packet, a router will drop that packet i.e
the packet will be lost.
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PROCESSING DELAY
Processing delay is the CPU cycles needed to look at the packet headers and decide what to
do with the packet, and do it – basically the time needed to process the packet.
Nodal processing involves checking bit-level errors in the packet that occurred in transmitting
the packet, performing header checksum calculations like looking up the link to the next node,
based on the destination address.
In the past, the processing delay has been ignored as insignificant compared to the other forms
of network delay. However, in some systems, the processing delay can be quite large especially
where routers are performing complex encryption algorithms and examining or modifying packet
content. Deep packet inspection done by some networks examine packet content for security,
legal, or other reasons, which can cause very large delay and thus is only done at selected
inspection points. Routers performing network address translation also have higher than normal
processing delay because those routers need to examine and modify both incoming and
outgoing packets.
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REFERENCES
1. Queuing Delay. https://www.its.bldrdoc.gov/fs-1037/dir-029/_4318.htm retrieved 3/11/2020
2. Delays in Computer Network. https://www.geeksforgeeks.org/delays-in-computer-network/
retrieved 3/11/2020
3. Network Delays and Losses. https://www.d.umn.edu/~gshute/net/delays-losses.xhtml
retrieved 3/11/2020
4. Notes on different types of delays.
http://cs.newpaltz.edu/~easwaran/CN/Module2/delays.html retrieved 3/11/2020
5. Latency or Delay. https://www.cs.purdue.edu/homes/comer/cnai/page-473.pdf retrieved
4/11/2020
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