1. 0907422 Computer Networks Dr. Walid Abu-Sufah
Computer Engineering Department
University of Jordan

2. Course Information Instructor: Teaching Assistant:
Dr. Walid Abu-Sufah
Office: Room 10 - Computer Engineering
Office Hours:  Su, T, Th 11-12; Wed: 1-2 and by appointment.
Teaching Assistant:
To be announced
Class Website:

3. Goals for Today’s Class: course overview
Goals of the course
Learning the material
Course grading
Academic policies

4. What You Learn in This Course
Knowledge: how the Internet works
IP protocol suite
Internet architecture
Applications (Web, , VoIP, …)
Insight: key concepts in networking
Protocols
Layering
Resource allocation
Naming
Network programming skills NOT here but in
Socket programming
Designing and implementing protocols

5. Prerequisite
Communications (1)
Probability and Statistics

6. Textbook
Computer Networks: A Systems Approach, by Peterson & Davie, 4th Edition

7. Grading Policy Homework 4% Quizzes 16% Mid-term Exam 30%
5 homework assignments
Quizzes %
5 quizzes, based on homeworks
Mid-term Exam 30%
Sunday, March 30 ; 5 - 6:30
Final Exam 50%
Thursday, June 5 ; 5 - 7

8. Policies: The Course Web Site
Posted on the course website you will find lecture notes; homeworks and their solutions; solutions of quizzes, old exams, and the midterm; classroom seat assignments (for enforcing attendance regulations); and grades. Class announcements, such as homework/quizzes dates and changes in assignments, will also be posted  on the Web. You are therefore responsible for checking the course web site. If you make an error because you did not check the Web, I will hold you fully responsible.

9. Policies: Homework and Quizzes
Five sets of homework problems will be assigned. Unless otherwise announced in class, the solutions of the homework are due at the beginning of the first lecture in the weeks indicated in the course calendar on the website. No late homework will be accepted.
One quiz will be given at the beginning of the same lecture when each of the five homeworks is collected. The quiz will be based on the homework. For each student, the lowest of the 5 grades of quizzes will be dropped. As a result, there will be no make-up quizzes for any reason. A similar grading approach will be used for the homeworks.

10. Policies: Makeup Midterm
There will be no make-up for the midterm. In case of medical/ or other DISABLING emergencies, the instructor should be notified BEFORE  the midterm and his approval for missing the midterm should be obtained before the midterm. If for any reason the instructor could not be reached, the department secretary should be notified before the midterm. The phone number is Extension

11. Policies: Grading Corrections
Ask the instructor for any grading correction requests within a week of returning the exam/quiz papers. After that, your grade will not be adjusted. If you find any mistake in grading, please let the instructor know. Your grade will not be lowered.

12. Policies: Class Attendance
Class attendance will be taken. University regulations regarding attendance will be strictly enforced. If you miss class, you must obtain the covered material from a willing classmate and/or the course web site. The instructor will not be available (during office hours or other times) to repeat material covered.

13. Policies: Academic Honesty
Your work in this class must be your own.
Penalties for excessive collaboration and cheating are severe.
Sharing homework answers is forbidden

14. Course Objectives At the end of the semester, you should be able to:
Identify the problems that arise in computer networks
Explain the advantages and disadvantages of existing solutions to these problems in the context of different networking regimes
Understand the implications of a given solution for performance in various networking regimes
Evaluate novel approaches to these problems

15. Okay, so let’s get started

16. What is a Network? Something that connects two or more computers
Tools that help computers communicate
.. and people communicate

17. Network Overview
What functionality must a network provide? What are the requirements? (Sections 1.2 & 1.5)
Connectivity
Cost-effective Resource Sharing
Support for common services needed by application processes
Performance
What is network architecture? How are networks designed and built? (Section 1.3)
Layering
Protocols
Standards

18. Connectivity … Building Blocks Links:
Links: coax cable, optical fiber, …
Nodes: workstations, routers, …
Links:
Point-to-point
Multiple access …

19. Indirect Connectivity
Switched Networks
Two types of nodes: hosts & switches
Internetworks
Networks (clouds) interconnected to form an internetwork
A router or gateway: a node that is connected to two or more networks
The cloud in Figure 1.3 distinguishes between the nodes on the inside that implement the network (they are commonly called switches, and their primary function is to store and forward packets) and the nodes on the outside of the cloud that use the network (they are commonly called hosts, and they support users and run application programs). Also note that the cloud in Figure 1.3 is one of the most important icons of computer networking. In general, we use a cloud to denote any type of network, whether it is a single point-to-point link, a multiple-access link, or a switched network. Thus, whenever you see a cloud used in a figure, you can think of it as a placeholder for any of the networking technologies covered in this book.

20. Recursive definition of a network
Two or more nodes connected by a physical link OR
Two or more networks connected by one or more nodes
The cloud in Figure 1.3 distinguishes between the nodes on the inside that implement the network (they are commonly called switches, and their primary function is to store and forward packets) and the nodes on the outside of the cloud that use the network (they are commonly called hosts, and they support users and run application programs). Also note that the cloud in Figure 1.3 is one of the most important icons of computer networking. In general, we use a cloud to denote any type of network, whether it is a single point-to-point link, a multiple-access link, or a switched network. Thus, whenever you see a cloud used in a figure, you can think of it as a placeholder for any of the networking technologies covered in this book.

21. Effects of Indirect Connectivity
Switching/Routing nodes receive data on one link and forward it onto another  switching network
Circuit Switching
Telephone Stream-based (dedicated circuit)
Links reserved for use by communication channel
Send/receive bit stream at constant rate
Packet Switching
Internet Message-based (store-and-forward)
Links used dynamically
Admission policies and other traffic determine bandwidth

22. Addressing Addressing Types of Addresses Routing
Unique byte-string used to indicate which node is the target of communication
Types of Addresses
Unicast: node-specific
Broadcast: all nodes on the network
Multicast: subset of nodes on the network
Routing
The process of determining how to forward messages toward the destination node based on its address

23. Network Overview (continued I)
What functionality must a network provide? What are the requirements? (Sections 1.2 & 1.5)
Connectivity
Cost-effective Resource Sharing
Support for common services needed by application processes
Performance

24. Cost-Effective Sharing of Resources
Physical links and switches must be shared among many users
Common multiplexing strategies
(Synchronous) Time-Division Multiplexing (TDM)
Frequency-Division Multiplexing (FDM)

25. Sharing Time-Division Multiplexing Frequency-division multiplexing
Synchronous
Frequency-division multiplexing
frequency
time

26. Statistical Multiplexing
Statistical Multiplexing (SM)
On-demand time-division multiplexing
Defines an upper bound on the size of the block of data that each flow is permitted to transmit at a given time, a packet
ensures that all flows eventually get their turn to transmit
Scheduled on a per-packet basis
Packets from different sources are interleaved
Statistical multiplexing is really quite simple, with two key ideas. First, it is like STDM in that the physical link is shared over time—first data from one flow is transmitted over the physical link, then data from another flow is transmitted, and so on. Unlike STDM, however, data is transmitted from each flow on demand rather than during a predetermined time slot. Thus, if only one flow has data to send, it gets to transmit that data without waiting for its quantum to come around and thus without having to watch the quanta assigned to the other flows go by unused. It is this avoidance of idle time that gives packet switching its efficiency.

27. Statistical Multiplexing in a Switch
Packets buffered in switch until forwarded
Selection of next packet depends on policy
How do we make these decisions in a fair manner? Round Robin? FIFO?
How should the switch handle congestion?

28. Network Overview (continued II)
What functionality must a network provide? What are the requirements? (Sections 1.2 & 1.5)
Connectivity
Cost-effective Resource Sharing
Support for common services needed by application processes
Performance

29. Support for common services needed by application processes
Goal:
Meaningful communication between application programs running on hosts connected to the network
Idea
Common services simplify the role of applications
Hide the complexity of the network without overly constraining the application designer
Semantics and interface depend on applications
Request/reply: FTP, HTTP, Digital Library
Message stream: video-on-demand, video conferencing
The request/reply channel would be used by the file transfer and digital library applications.
Guarantee that every message sent by one side is received by the other side
Only one copy of each message is delivered
Protect the privacy and integrity of the
The message stream channel could be used by both the video-on-demand and videoconferencing applications, provided it is parameterized to support both one-way and two-way traffic and to support different delay properties.
Might not need to guarantee that all messages are delivered
Ensure that those messages that are delivered arrive in the same order in which they were sent
Ensure the privacy and integrity of the video data
Support multicast, so that multiple parties can participate in the teleconference or view the video.

30. Channels
Channel
The abstraction for application-level communication
Idea
Turn host-to-host connectivity into process-to-process communication
Host
APP
Channel

31. Inter-process Communication
Problems typically masked by communication channel abstractions
Bit errors (electrical interference)
Packet errors (congestion)
Link/node failures
Message delays
Out-of-order delivery
Eavesdropping
Goal
Fill the gap between what applications expect and what the underlying technology provides

32. Network Overview (continued III)
What functionality must a network provide? What are the requirements? (Sections 1.2 & 1.5)
Connectivity
Cost-effective Resource Sharing
Support for common services needed by application processes
Performance

33. Bandwidth/throughput Performance Metric
Data transmitted per unit time
Example: 10 Mbps
Link bandwidth vs. end-to-end throughput
Notation
KB = 210 bytes
Mbps = 106 bits per second

34. Latency/delay Performance Metric
Time from A to B
Latency (delay): how long it takes to perform a particular function such as delivering a message or moving an object
Example: 30 msec (milliseconds)
Many applications depend on round-trip time (RTT)

35. Latency Components Propagation delay over each link Transmission time
the specific amount of time it takes a signal to propagate from one end of a link to another
Transmission time
Queuing delays
Software processing overheads

36. Performance Notes Speed of Light Comments Key Point
3.0 x 108 meters/second in a vacuum
2.3 x 108 meters/second in a cable
2.0 x 108 meters/second in a fiber
Comments
No queueing delays in a direct link
Bandwidth is not relevant if size = 1bit
Software overhead can dominate when distance is small
Key Point
Latency dominates small transmissions
Bandwidth dominates large transmissions

37. Perceived latency (response time) vs. RTT

38. Delay x Bandwidth Product Metric
Relative importance of bandwidth and latency depends on application
Large file transfer is bandwidth critical
Small messages (HTTP, NFS, etc.) are latency critical
Variance in latency (jitter) can also affect some applications (e.g., audio/video conferencing)
Delay x bandwidth product
Amount of data in “pipe”
Half of data that must be buffered before sender responds to slowdown request

39. Delay x Bandwidth Product
channel between two processes = pipe
delay = length
bandwidth = area of a cross section
bandwidth x delay product = volume
Delay
Bandwidth

40. Delay x Bandwidth Product
Bandwidth x delay product
How many bits the sender must transmit before the first bit arrives at the receiver if the sender keeps the pipe full
Takes another one-way latency to receive a response from the receiver 4 5 6 7 8 9 10 11 1 2 3 A B

41. Delay x Bandwidth Product
Example: Transcontinental Channel
BW = 45 Mbps
delay = 50ms
bandwidth x delay product
= (50 x 10–3 sec) x (45 x 106 bits/sec)
= 2.25 x 106 bits

42. Bandwidth vs. Latency Relative importance 1-byte: Latency bound
1ms vs 100ms latency dominates 1Mbps vs 100Mbps BW
25MB: Bandwidth bound
1Mbps vs 100Mbps BW dominates 1ms vs 100ms latency
100 Mbps
1 Mbps
25MB
100Mbps 1B
1Mbps
100ms
1ms

43. Bandwidth vs. Latency (continued)
Infinite bandwidth
RTT dominates
Throughput = TransferSize / TransferTime
TransferTime = RTT + (1/Bandwidth) x TransferSize
What is “Infinite or High” bandwidth?
Its all relative
1 MB file to 1 Gbps link looks like a 1KB packet to 1 Mbps link
Note that most of the time we are interested in the RTT scenario, which we simply refer to as the delay × bandwidth product, without explicitly saying that this product is multiplied by two. Again, whether the “delay” in “delay × bandwidth” means one-way latency or RTT is made clear by the context .