CS475 – Networks
Lecture 3
Chapter 1: Foundation
Assignments
• Reading for Lecture 4: Sections 2.1 – 2.3
• Homework 1: 1.13, 1.18, 1.33, 1.34. Start each of 1.33 and 1.34 with code after last in-class exercise. (I.e., don't do both
problems together.) Tar/zip each set of server/client programs together and email to instructor. Due: Thursday, Sept. 8.
• Reminder: Wireshark Project 1 due on Tuesday.
1.5 Performance
The primary network performance measures are bandwidth
(BW) and _________________.
BW (or throughput) is the number of bits that can be sent over
the network in a period of time (usually bits-per-second or bps).
The link BW is the maximum physical rate at which data can be
sent. The measured performance (often called throughput) is the
max rate at which we can send data. The throughput is a
Fig. 1.16: The bit width in a slower BW channel (a) is wider
than the bit width in a faster channel (b).
function of the link BW and other factors (congestion, server
load, etc)
Latency
Latency (or _________________) is the time it takes a message to travel across the network.
Latency = PropagationTime + TransmitTime + QueueTime
where
PropagationTime = Distance/SpeedOfPropagation
TransmitTime = MessageSize/Bandwidth
The PropagationTime is the time it takes for a single pulse to be sent across the network. The SpeedOfPropagation depends on the
_______________________ (2x108 m/s for fiber, 2.3x108 m/s for cable and 3.0x108 m/s for wireless).
The TransmitTime is the time it takes to push the entire message onto the medium. The TransmitTime for a 1 MB file on a 10
Mbps link is (1 MB x 8 b/B)/10 Mbps = 0.8 s.
The QueueTime is the sum of the queuing delays encountered at the switches and routers by the message. The QueueTime is not
included when calculating the latency over a single link.
In some problems the round-trip-time or RTT is of interest. The RTT includes the time it takes for a signal to be sent plus the time
it takes for an ____________________________________ to be received.
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For small messages the latency is dominated by the propagation
time, for large messages the ___________________ time
determines the latency.
Fig. 1.17: Latency vs RTT for different
message sizes and link speeds.
Delay – Bandwidth Product
The Delay-BW product represents the number of bits in transit
(on the network).
Alternatively, it can be thought of as the number of bits
transmitted before the first bit reaches the _________________.
High-Speed Networks
On a high-speed network, the RTT for an acknowledgment can
Using 100 ms for a cross-country RTT, the TransferTime for a 1
have a significant impact on _______________________:
MB file on a 1 Gbps link is 108 ms for Throughput of only 74.1
Throughput = MessageSize/TransferTime
where
Mbps.
TransferTime = RTT + MessageSize/BW
Application Requirements
Applications do not necessarily need “all the bandwidth they can of 30 frames per second would require a throughput of 75 Mbps.
get”. Streaming video with a resolution of 352 x 240 pixels and Much greater __________________ would be of no interest to
24 bit color would have a frame size of 247.5 KB. A frame rate this application.
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Jitter
The __________________ in latency or jitter can be important
know the maximum jitter in order to allocate a buffer of the
in streaming applications. If a frame arrives late quality will be
appropriate size.
affected.
The client can account for jitter via buffering. The client must
1.6 Summary
Chapter 1 lays the foundation for understanding how to build a
•
fully functional computer network from the ground up.
•
Identified the requirements
•
Defined a layered architecture
Discussed programmer interface between the network
protocols and applications
•
Identified performance metrics
In-Class Exercises
1.
Problem 1.5 on pages 61-62
2.
Problem 1.19 on page 64
Turn these problem in at the end of class
Work on Wireshark Project 1 and the programming problems of Homework 1.
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