ECEN4533 Data Communications
Lecture #24
4 March 2013
Dr. George Scheets
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
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Read 4.6
Problems Web 16 - 18
Corrected Exams
 Due 6 March (Live)
 One week after return (DL)
Design #2
 Due 15 March (Live)
 Due 22 March (DL)
Quiz #2 on 27 March
ECEN4533 Data Communications
Lecture #25
6 March 2013
Dr. George Scheets
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

Corrected Exams
 Due Today (Live)
 One week after return (DL)
Design #2
 Due 15 March (Live)
 Due 22 March (DL)
Quiz #2 on 27 March
ECEN4533 Data Communications
Lecture #26
8 March 2013
Dr. George Scheets
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Read 8.1 – 8.3
Problems 8.1, 8.2, Web 19
Corrected Exams
 One week after return (DL)
Design #2
 Due 15 March (Live)
 Due 22 March (DL)
Quiz #2 on 27 March
Normalized Propagation Delay
 NPD
= End-to-End Propagation Delay
Average time to inject a Packet
 NPD > 1
1 or more packets can simultaneously be in transit
 NPD < 1
Packet front end hits far side before back end
transmitted
Transmitter
NPD > 1
NPD < 1
Receiver
Layer 4 Acknowledgement

Stop & Wait
Transmit one packet. Wait until ACK.
 Throughput
Horrible as NPD increases
High Speed
 Long Distances
 Shorter Packets

 Used

on WiFi (wireless- layer 2)
Window Flow Control
Allow N unACKnowledged packets enroute
 Used
on Wired systems
 On Wireless, TCP uses windows at Layer 4 &
802.11 uses form of Stop & Wait at Layer 2
Layer 4 ARQ w/ Window Flow Control
How do you retransmit when a packet is
mangled or lost?
 Go Back N

 Retransmit

packet in error & all that follow
Selective Repeat
 Retransmit
packet in error
Layer 4 Flow Control Efficiencies
 Stop
& Wait
 Error
Free
1/(K + 2*NPD)
K = 1 + Tack/E[Ts]
 Errors
(1 - Pe)/(K + 2*NPD)
K = 1 + (Tack/E[Ts]) (1 - Pe)
Layer 4 Flow Control Efficiencies

Windows
 error free
 1 if W (in segments) > K + 2*NPD
 W/(K + 2*NPD); otherwise
K = 1 + Tack/E[Ts]
 with
errors using Selective Repeat
 [1-Pe] if W (in segments) > K + 2*NPD
 W(1 - Pe)/(K + 2*NPD); otherwise
Layer 4 Flow Control Efficiencies

Windows
 error free
 1 if W (in segments) > K + 2*NPD
 W/(K + 2*NPD); otherwise
K = 1 + Tack/E[Ts]
 with
errors using Go Back N
 ≈ [1-Pe]/[1 + Pe*2*NPD]
if W (in segments) > K + 2*NPD
 ≈ W(1 - Pe)/[(1 - Pe + WPe)(K + 2*NPD)];
otherwise
TCP Flow Control

Sliding Window
 Allows

multiple packets to be enroute
Positive Acknowledgements
 Packet
Dumped (Congestion, Bit Error(s))?
ACK never sent from RCV to TRANS
Transmitter needs a Retrans Timer
 Adaptive Timers are best

 Can
respond to network congestion
 Requires Estimate of Round-Trip Time
ISO OSI Seven Layer Model
Layer 7
 Layer 6
 Layer 5
 Layer 4
 Layer 3
 Layer 2
 Layer 1
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Application
Presentation
Session
Transport
Network
Data Link
Physical
Software
Windows API
TCP, Windows
TCP, Windows
IP, Windows
PC NIC
PC NIC
Typical Packet
20 B
Layer 2
Header

20 B
TCP
IP
Header Header
Layer 6 & 7 Layer 2
Traffic
Trailer
Exception:
UDP used for Interactive Voice or Video
TCP Header
Source Port
Destination Port
Sequence Number
ACK Number
Window
Checksum
Wireshark Capture showing
Opening of TCP Logical Link
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