1. The University of Adelaide, School of Computer Science
Computer Networks: A Systems Approach, 5e
Larry L. Peterson and Bruce S. Davie
The University of Adelaide, School of Computer Science
20 April 2017
Chapter 1
Foundation
Copyright © 2010, Elsevier Inc. All rights Reserved
Chapter 2 — Instructions: Language of the Computer

2. The University of Adelaide, School of Computer Science
20 April 2017
Problems
What is a computer network?
How to build a scalable network that will support different applications?
How is a computer network different from other types of networks?
What is a computer network architecture?
Chapter 2 — Instructions: Language of the Computer

3. The University of Adelaide, School of Computer Science
20 April 2017
Chapter Outline
Applications
Requirements
Network Architecture
Implementing Network Software
Performance
Chapter 2 — Instructions: Language of the Computer

4. The University of Adelaide, School of Computer Science
20 April 2017
Chapter Goal
Exploring the requirements that different applications and different communities place on the computer network
Introducing the idea of network architecture
Introducing some key elements in implementing Network Software
Define key metrics that will be used to evaluate the performance of computer network
Chapter 2 — Instructions: Language of the Computer

5. The University of Adelaide, School of Computer Science
20 April 2017
Applications
Most people know about the Internet (a computer network) through applications
World Wide Web
Just a single application of the Internet
Online Social Network
Streaming Audio Video
File Sharing
Instant Messaging …
Chapter 2 — Instructions: Language of the Computer

6. Application Protocol (WWW)
The University of Adelaide, School of Computer Science
20 April 2017
Application Protocol (WWW)
URL
Uniform resource locater
is the name of the machine that serves the page
IP:
nslookup
whois
/~misaleh/Teaching/cs342spring2011/mainpage.html uniquely identifies the required page
Chapter 2 — Instructions: Language of the Computer

7. Application Protocol (Cont.)
HTTP
Hyper Text Transfer Protocol
TCP
Transmission Control Protocol
17 messages for one URL request
6 to find the IP (Internet Protocol) address
3 for connection establishment of TCP
4 for HTTP (GET) request and acknowledgement (Response)
Request: I got your request and I will send the data
Reply: Here is the data you requested; I got the data
4 messages for tearing down TCP connection

8. Example of an application
A multimedia application including video-conferencing

9. Expectations!! HTML requests: Emails: Video conferencing:
A couple of seconds delay is acceptable
Pages are complete
Images can be delivered out of order
Etc… s:
No missing parts (i.e., the is identical to what you sent)
A minute or two delay is acceptable
Video conferencing:
Video and sound MUST be continuous and on a timely matter
Some missing parts are acceptable

10. The University of Adelaide, School of Computer Science
20 April 2017
Requirements
Application Programmer
List the services that his application needs: delay bounded delivery of data
Network Designer
Design a cost-effective network with sharable resources
Network Provider (usually called Service Provider, for the Inernet, it ISP)
List the characteristics of a system that is easy to administer and manage.
Add new clients and remove/isolate faulty nodes
Chapter 2 — Instructions: Language of the Computer

11. Requirements Connectivity Cost-Effective Resource Sharing
Support for Common Services
Reliability
Manageability

12. Connectivity Need to understand the following terminologies
Scale: A system that is designed to support growth to an arbitrarily large size
Link: Physical medium that connects nodes
Node: a device (could be a computer, switch, etc) on the network
Point-to-point: direct link between two nodes
Multiple access: multiple nodes share the same link
Point-to-point
Multiple access

13. Connectivity
Switched Network: a network that uses switches for forwarding
Circuit Switched
Telephone companies
Dedicated/reserved when connection is established
Less utilization of the resources
Packet Switched
Overwhelming majority of computer networks
Messages are divided into pieces (discrete blocks) called packets
No dedication is required
More sharing, thus more utilization
Store-and-forward: a switch stores the incoming traffic (packets) in its own buffers then forwards them.

14. Connectivity Cloud: used to represent any kind of network technology
Point-to-point, multiple-access, or switched network
Hosts: nodes outside the cloud (usually computer or end users devices and use the network)
Switches: nodes inside the cloud and implement the network (store and forward packets)
Internetwork (or internet with small “i”): a set of independent interconnected networks (clouds)
Internet (with big “I”): is the globally known network

15. Connectivity
Router/gateway: connects two or more networks (plays much the same role as a switch—stores and forwards)
Host-to-host connectivity: hosts can talk to hosts
Address: the way to find nodes. It is a byte string that identifies a node.
Routing: the process of determining systematically how to forward messages toward the destination node based on its address
Unicast: send to single destination
Broadcast: send to all nodes on the network
Multicast: send to subset of nodes

16. Connectivity
(a)
(b)
A switched network
Interconnection of networks

17. Cost-Effective Resource Sharing
Resource: links and nodes
How to share a link?
Multiplexing
Analogy to a timesharing computer system
De-multiplexing
Synchronous Time-division Multiplexing (STDM)
Equal-sized quanta
Round-robin fashion
Time slots/data transmitted in predetermined slots
Multiplexing multiple logical flows over a single physical link

18. Cost-Effective Resource Sharing
FDM: Frequency Division Multiplexing
Diff. TV stations with diff. frequencies.
Both STDM and FDM waste resources and hard to accommodate changes (fixed time slots and frequencies)
Statistical Multiplexing
Like STDM: sharing over time but data is transmitted based on demand rather than during a predetermined time slot

19. Statistical Multiplexing
Packets vs. Messages
Fair decision: FIFO, Round-Robin, Priorities (Quality-of-Service (QoS) allocate bandwidth for a particular flows)
A switch might run out of buffers because it receives data much faster than it can send on the shared link
Congestion
Start to drop packets
A switch multiplexing packets from multiple sources onto one shared link

20. LAN, MAN, WAN, and SAN
Characterize networks according to their size (they usually use diff. technologies):
LAN: Local Area Network, typically less than 1km
WAN: Wide Area Network, worldwide
MAN: Metropolitan Area Network, tens of kilometers
SAN: Systems/Storage Area Network, single room that has high-performance components (like leading-edge storage devices) connected together

21. Support for Common Services
Overly simplistic to think about network as simple delivering packets among a collection of computers
But rather a means for application processes to communicate

22. Support for Common Services
One option would be for the app designers to build all that complicated functionality into each application.
However, since many applications need common services, it is much more logical to build those common services once and let the app designers to use the services.

23. Support for Common Services
Process communicating over an abstract/logical channel

24. Support for Common Services
The challenge then is recognize what functionality the channel should provide to application programs:
Reliable delivery?!
Same order?!
Eavesdropping parties?!
In general, a network provides a variety of different types of channels
An applications can choose the best that meets its needs

25. Common Communication Patterns
The University of Adelaide, School of Computer Science
20 April 2017
Common Communication Patterns
Designing abstract channels involve
Understanding representative collection of apps
Extract the common needs
Client/Server
FTP (file transfer protocol), NFS (network file system), digital libraries like ACM
Video conferencing
Two-way traffic (or one-way with video on demand)
No need to guarantee the delivery of ALL messages.
Same order is required
Might need multicast
Chapter 2 — Instructions: Language of the Computer

26. Common Communication Patterns
Two types of common communication channels
Request/Reply Channels
Message Stream Channels
It is dangerous to have too much abstractions
Every application has to choose between the provided channels
If you have a hammer then you might see everything looks like a nail, 
Independent of exactly what functionality a given channel should provide is where this functionality should be implemented.
On switches => dump hosts, like the telephone handsets
On hosts => keep switches as simple as possible

27. The University of Adelaide, School of Computer Science
20 April 2017
Reliability
Network should hide the errors
Bits are lost
Bit errors (1 to a 0, and vice versa)
Burst errors – several consecutive errors
Outside forces like: lightning strikes, microwave ovens, etc.
Packets are lost (Congestion)
Links and Node failures (crashes and misconfig.)
Messages are delayed
Messages are delivered out-of-order
Third parties eavesdrop
Chapter 2 — Instructions: Language of the Computer

28. Manageability Last requirement that is usually neglected
Making changes as the network grows to carry more traffic or reach more users
Troubleshooting when something goes wrong or performance is not as desired
Related to scalability
Configuring new devices/routers is always problematic
Needs more automation
Great if it becomes plug-and-play (like home routers)

29. Requirements Summary
A computer network must provide general, cost-effective, fair and robust connectivity among large number of computers.
The network must be manageable by number of varying levels of skills

30. Network Architecture
Example of a layered network system

31. Network Architecture
Layered system with alternative abstractions available at a given layer

32. Abstraction and Layering
The University of Adelaide, School of Computer Science
20 April 2017
Abstraction and Layering
Abstraction
The hiding of details behind a well-defined interface
Define a model
Capture important aspects of the system
Abstractions naturally leads to layering
The general idea
Start from services offered by the underlying hardware
Add a sequence of layers, each providing a higher (i.e., more abstract) level of service.
Manageability and Mudularity
Chapter 2 — Instructions: Language of the Computer

33. Protocoals
The abstract objects the make up the layers of a network system are called protocols
Building blocks of a network architecture
Each protocol object has two different interfaces
service interface: operations on this protocol
peer-to-peer interface: messages exchanged with peer (indirect communication, except for the hardware)

34. Interfaces
Service and Peer Interfaces

35. The University of Adelaide, School of Computer Science
20 April 2017
Protocols
Protocol Specification:
Written description (prose)
pseudo-code
state transition diagram
Packet format
RFCs: Request For Comments
IETF: Internet Engineering Task Force
Standardization body
Ex. RFC 2616 for HTTP protocol
Interoperable: when two or more protocols that implement the specification accurately
Chapter 2 — Instructions: Language of the Computer

36. Protocol Graph Example of a protocol graph
nodes are the protocols and links the “depends-on” relation

37. High-level messages are encapsulated inside of low-level messages
Encapsulation
High-level messages are encapsulated inside of low-level messages

38. Packets General Format
Header
Payload
Trailer
Peers talk to each other through Headers and Trailers

39. OSI – Open Systems Interconnection
OSI Architecture
The OSI 7-layer Model
OSI – Open Systems Interconnection

40. The University of Adelaide, School of Computer Science
20 April 2017
Description of Layers
Physical Layer
Handles the transmission of raw bits over a communication link
Data Link Layer
Collects a stream of bits into a larger aggregate called a frame
Network adaptor along with device driver in OS implement the protocol in this layer
Frames are actually delivered to hosts
Network Layer
Handles routing among nodes within a packet-switched network
Unit of data exchanged between nodes in this layer is called a packet
The lower three layers are implemented on all network nodes
Chapter 2 — Instructions: Language of the Computer

41. Description of Layers Transport Layer
Implements a process-to-process channel
Unit of data exchanges in this layer is called a message
There is a disagreement on the top three
Session, Presentation, and Application
Mainly because the are not always present

42. The University of Adelaide, School of Computer Science
20 April 2017
Description of Layers
Session Layer
Provides a name space that is used to tie together the potentially different transport streams that are part of a single application
Ex., tie the audio and video together in videoconference
Presentation Layer
Concerned about the format of data exchanged between peers
Ex., integer formats, audio/video format, most/least significant
Application Layer
Include things like the Hypertext Transfer Protocol
Basis the world wide web
Used by web browsers
The transport layer and the higher layers typically run only on end-hosts and not on the intermediate switches and routers
Chapter 2 — Instructions: Language of the Computer

43. Internet Architecture
Sometimes called TCP/IP
Evolved from an earlier packet-switched network called ARPANET
Internet and ARPANET were funded by ARPA (Advanced Research Projects Agency)
Both existed before the OSI architecture
Both affected the OSI model

44. Internet Architecture
Alternative view of the Internet architecture. The “Network” layer shown here is sometimes referred to as the “sub-network” or “link” layer.
Internet Protocol Graph

45. Internet Architecture
The University of Adelaide, School of Computer Science
20 April 2017
Internet Architecture
Defined by IETF
Three main features
Does not imply strict layering. The application is free to bypass the defined transport layers and to directly use IP or other underlying networks
An hour-glass shape – wide at the top, narrow in the middle and wide at the bottom. IP serves as the focal point for the architecture (host-to-host connectivity is separate from all channel types)
In order for a new protocol to be officially included in the architecture, there needs to be both a protocol specification and at least one (and preferably two) representative implementations of the specification
Chapter 2 — Instructions: Language of the Computer

46. Internet Architecture
The University of Adelaide, School of Computer Science
20 April 2017
Internet Architecture
NET1, NET2, …
Could be Ethernet, Wireless, etc.
Encapsulate both hardware and data link layers from OSI model
IP (Internet Protocol)
Supports the interconnection of multiple networking technologies into a single logical internetwork
Analogy to the network layer in the OSI
The routing protocol
Chapter 2 — Instructions: Language of the Computer

47. Internet Architecture
The University of Adelaide, School of Computer Science
20 April 2017
Internet Architecture
TCP provides reliable, byte-stream channel (connection oriented protocol)
UDP provides unreliable, datagram (message) delivery channel.
TCP and UDP are called (besides Transport) end-to-end protocols
Chapter 2 — Instructions: Language of the Computer

48. Internet Architecture
The University of Adelaide, School of Computer Science
20 April 2017
Internet Architecture
Application Protocols
HTTP, FTP, Telnet (remote login), Simple Mail Transfer Protocol (SMTP), and much more
Enables the interoperation of popular applications
Many different web browsers interoperate with web servers because they all conform/use the HTTP protocol
Chapter 2 — Instructions: Language of the Computer

49. The Success of the Internet
1.8 billion Internet users
Much of its functionality is provided by software running in general-purpose computers
Small matter of programming
Massive increase in computing power

50. Application Programming Interface
The University of Adelaide, School of Computer Science
20 April 2017
Application Programming Interface
Interface exported by the network
Since most network protocols are implemented (those in the high protocol stack) in software and nearly all computer systems implement their network protocols as part of the operating system, when we refer to the interface “exported by the network”, we are generally referring to the interface that the OS provides to its networking subsystem
The interface is called the network Application Programming Interface (API)
Chapter 2 — Instructions: Language of the Computer

51. Application Programming Interface (Sockets)
The University of Adelaide, School of Computer Science
20 April 2017
Application Programming Interface (Sockets)
Socket Interface was originally provided by the Berkeley distribution of Unix
- Now supported in virtually all operating systems
Each protocol provides a certain set of services, and the API provides a syntax by which those services can be invoked in this particular OS
Chapter 2 — Instructions: Language of the Computer

52. The University of Adelaide, School of Computer Science
20 April 2017
Socket
What is a socket?
The point where a local application process attaches to the network
An interface between an application and the network
An application creates the socket
The interface defines operations for
Creating a socket
Attaching a socket to the network
Sending and receiving messages through the socket
Closing the socket
Chapter 2 — Instructions: Language of the Computer

53. The University of Adelaide, School of Computer Science
20 April 2017
Socket
int sockfd = socket(protocol_family, type, protocol);
Protocol Family
PF_INET denotes the Internet family
PF_PACKET denotes direct access to the network interface (i.e., it bypasses the TCP/IP protocol stack)
The socket number returned is the socket descriptor for the newly created socket
Chapter 2 — Instructions: Language of the Computer

54. The University of Adelaide, School of Computer Science
20 April 2017
Creating a Socket
int sockfd = socket(protocol_family, type, protocol);
Socket Type
SOCK_STREAM is used to denote a byte stream (TCP)
SOCK_DGRAM is an alternative that denotes a message oriented service, such as that provided by UDP
int sockfd = socket (PF_INET, SOCK_STREAM, 0);
int sockfd = socket (PF_INET, SOCK_DGRAM, 0);
The combination of PF_INET and SOCK_STREAM implies TCP
Chapter 2 — Instructions: Language of the Computer

55. Client-Serve Model with TCP
The University of Adelaide, School of Computer Science
20 April 2017
Client-Serve Model with TCP
Server
Passive open
Prepares to accept connection, does not actually establish a connection
Server invokes
int bind (int socket, struct sockaddr *address,
int addr_len)
int listen (int socket, int backlog)
int accept (int socket, struct sockaddr *address,
int *addr_len)
Chapter 2 — Instructions: Language of the Computer

56. Client-Serve Model with TCP
The University of Adelaide, School of Computer Science
20 April 2017
Client-Serve Model with TCP
Bind
Binds the newly created socket to the specified address i.e. the network address of the local participant (the server)
Address is a data structure which combines IP and port
Listen
Defines how many connections can be pending on the specified socket
Chapter 2 — Instructions: Language of the Computer

57. Client-Serve Model with TCP
The University of Adelaide, School of Computer Science
20 April 2017
Client-Serve Model with TCP
Accept
Carries out the passive open
Blocking operation
Does not return until a remote participant has established a connection
When it does, it returns a new socket that corresponds to the new established connection and the address argument contains the remote participant’s address
Chapter 2 — Instructions: Language of the Computer

58. Client-Serve Model with TCP
The University of Adelaide, School of Computer Science
20 April 2017
Client-Serve Model with TCP
Client
Application performs active open
It says who it wants to communicate with
Client invokes
int connect (int socket, struct sockaddr *address,
int addr_len)
Connect
Does not return until TCP has successfully established a connection at which application is free to begin sending data
Address contains remote machine’s address
Chapter 2 — Instructions: Language of the Computer

59. Client-Serve Model with TCP
The University of Adelaide, School of Computer Science
20 April 2017
Client-Serve Model with TCP
In practice
The client usually specifies only remote participant’s address and let’s the system fill in the local information
Whereas a server usually listens for messages on a well-known port
A client does not care which port it uses for itself, the OS simply selects an unused one
Chapter 2 — Instructions: Language of the Computer

60. Client-Serve Model with TCP
The University of Adelaide, School of Computer Science
20 April 2017
Client-Serve Model with TCP
Once a connection is established, the application process invokes two operation
int send (int socket, char *msg, int msg_len,
int flags)
int recv (int socket, char *buff, int buff_len,
Chapter 2 — Instructions: Language of the Computer

61. Example Application: Client
The University of Adelaide, School of Computer Science
20 April 2017
Example Application: Client
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define SERVER_PORT 5432
#define MAX_LINE 256
int main(int argc, char * argv[]) {
FILE *fp;
struct hostent *hp;
struct sockaddr_in sin;
char *host;
char buf[MAX_LINE];
int s;
int len;
if (argc==2) {
host = argv[1]; }
else {
fprintf(stderr, "usage: simplex-talk host\n");
exit(1);
Chapter 2 — Instructions: Language of the Computer

62. Example Application: Client
The University of Adelaide, School of Computer Science
20 April 2017
Example Application: Client
/* translate host name into peer’s IP address */
hp = gethostbyname(host);
if (!hp) {
fprintf(stderr, "simplex-talk: unknown host: %s\n", host);
exit(1); }
/* build address data structure */
bzero((char *)&sin, sizeof(sin));
sin.sin_family = AF_INET;
bcopy(hp->h_addr, (char *)&sin.sin_addr, hp->h_length);
sin.sin_port = htons(SERVER_PORT);
/* active open */
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
perror("simplex-talk: socket");
if (connect(s, (struct sockaddr *)&sin, sizeof(sin)) < 0) {
perror("simplex-talk: connect");
close(s);
/* main loop: get and send lines of text */
while (fgets(buf, sizeof(buf), stdin)) {
buf[MAX_LINE-1] = ’\0’;
len = strlen(buf) + 1;
send(s, buf, len, 0);
Chapter 2 — Instructions: Language of the Computer

63. Example Application: Server
The University of Adelaide, School of Computer Science
20 April 2017
Example Application: Server
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define SERVER_PORT 5432
#define MAX_PENDING 5
#define MAX_LINE 256
int main() {
struct sockaddr_in sin;
char buf[MAX_LINE];
int len;
int s, new_s;
/* build address data structure */
bzero((char *)&sin, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = INADDR_ANY;
sin.sin_port = htons(SERVER_PORT);
/* setup passive open */
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
perror("simplex-talk: socket");
exit(1); }
Chapter 2 — Instructions: Language of the Computer

64. Example Application: Server
The University of Adelaide, School of Computer Science
20 April 2017
Example Application: Server
if ((bind(s, (struct sockaddr *)&sin, sizeof(sin))) < 0) {
perror("simplex-talk: bind");
exit(1); }
listen(s, MAX_PENDING);
/* wait for connection, then receive and print text */
while(1) {
if ((new_s = accept(s, (struct sockaddr *)&sin, &len)) < 0) {
perror("simplex-talk: accept");
while (len = recv(new_s, buf, sizeof(buf), 0))
fputs(buf, stdout);
close(new_s);
Chapter 2 — Instructions: Language of the Computer

65. The University of Adelaide, School of Computer Science
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Performance
Old programming adage: “first get it right and then make it fast”.
In networking: necessary to “design for performance”
Two fundamental ways to measure the performance
Bandwidth (sometimes called throughput)
Latency (also called delay)
Chapter 2 — Instructions: Language of the Computer

66. Bandwidth Frequency band (measured in Hertz): we don’t mean that
Number of bits per second that can be transmitted over a communication link
Throughput vs. bandwidth (from the most confusing terms in computer networks.
Bandwidth: the maximum data rate (bits per second)
Throughput: number of bits per second that we actually transmit over the link in practice
1 Mbps: 1 x 106 bits/second = 1x220 bits/sec
1 x 10-6 seconds to transmit each bit or imagine that a timeline, now each bit occupies 1 micro second space.
On a 2 Mbps link the width is 0.5 micro second.
Smaller the width more will be transmission per unit time.

67. Bandwidth
Bits transmitted at a particular bandwidth can be regarded as having some width:
(a) bits transmitted at 1Mbps (each bit 1 μs wide);
(b) bits transmitted at 2Mbps (each bit 0.5 μs wide).

68. Latency
How long it takes a message to travel from one end of a network to the other.
Measured in time
Transcontinental might have a latency of 24 ms
Round-Trip Time (RRT): send a message from one to the other and back

69. Latency Three components Speed-of-light propagation delay
Different media at different speeds
3.0 × 108 m/s in a vacuum
2.3 × 108 m/s in copper cable
2.0 × 108 m/s in optical fiber
Amount of time to transmit a unit of data
Queuing delays (switches store packets)

70. The University of Adelaide, School of Computer Science
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Performance
Latency = Propagation + transmit + queue
Propagation = distance/speed of light
Distance: the length of the wire
Transmit = size/bandwidth
Size: the size of the packet
One bit transmission => propagation is important
Large bytes transmission => bandwidth is important
Chapter 2 — Instructions: Language of the Computer

71. Performance (ex1)
A client sends1-byte message to a server and receives a 1-byte message
Suppose RTT = 100ms
Case 1: bandwidth 1 Mbps
=> Transmit = 8 bits / 1Mbps = 8 micro seconds
Case 2: bandwidth 100 Mbps
=> Transmit = 8 bits / 100Mbps = 0.08 micro seconds
Insignificant difference between Case 1 and 2

72. Performance (ex2) A client wants to download a 25MB image
Bandwidth 10Mbps
=>Transmit = 25 × 106 × 8 bits / 10 × 106 bits/sec = 20 seconds
Case 1: RTT = 100ms
Latency = 20.1 sec
Case 2: RTT = 1ms
Latency = sec
Propagation delay is insignificant in this case

73. Copyright © 2012, Elsevier Inc. All rights Reserved
FIGURE 1.17 Perceived latency (response time) versus round-trip time for various object sizes and link speeds.
Chapter 1
Copyright © 2012, Elsevier Inc. All rights Reserved

74. The University of Adelaide, School of Computer Science
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Delay X Bandwidth
We think the channel between a pair of processes as a hollow pipe
Latency (delay) length of the pipe and bandwidth the width of the pipe
Delay of 50 ms and bandwidth of 45 Mbps
50 x 10-3 seconds x 45 x 106 bits/second
2.25 x 106 bits ≈ 280 KB data.
Network as a pipe
Chapter 2 — Instructions: Language of the Computer

75. The University of Adelaide, School of Computer Science
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Delay X Bandwidth
Relative importance of bandwidth and latency depends on application
For large file transfer, bandwidth is critical
For small messages (HTTP, NFS, etc.), latency is critical
Chapter 2 — Instructions: Language of the Computer

76. The University of Adelaide, School of Computer Science
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Delay X Bandwidth
The max number of bits that could be in transit through the pipe at any given instant
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
If the sender does not fill the pipe (i.e., send a whole delay × bandwidth product’s worth of data before it stops to wait for a signal), then the sender will not fully utilize the network
Chapter 2 — Instructions: Language of the Computer

77. Delay X Bandwidth
One-way latency of 50ms and a bandwidth of 45 Mbps is able to hold
50 × 10-3s × 45 × 106 bits/s = 2.25 × 106 bits
This number is doubled if we consider the RTT time instead of the one-way
Until the send hears the first bit from the receiver to stop sending or something
Most of the time, we will be interested in the RTT × Bandwidth

78. The University of Adelaide, School of Computer Science
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High-Speed Networks
Bandwidths are increasing dramatically
What is the impact network design of having infinite bandwidth available??!!
RTT dominates
What does not change as bandwidth increases: the speed of light
You can not change the laws of physics
Its all relative
1-MB file to 1-Gbps link looks like a 1-KB packet to 1-Mbps link
Chapter 2 — Instructions: Language of the Computer

79. Relationship between bandwidth and latency
A 1-MB file would fill the 1-Mbps link 80 times,
but only fill the 1-Gbps link 1/12 of one time

80. High-Speed Networks Delay is 100 x 10-3 sec File is 1MB
Bandwidth1 = 1Mbps
Bandwidth2 = 1Gbps
Delay x Bandwidth1 = 100 x 10-3 sec x 106 bits/sec = 105 bits
1MB / 105bits = 8 x 106 / 105 = 80
=> the file is 80 times larger than the channel
Delay x Bandwidth2 = 100 x 10-3 sec x 109 bits/sec = 108 bits
1MB / 108 bits = 8 x 106 / 108 = 1 / 12.5
=> the channel is 12.5 times larger than the file

81. Throughput and TransferTime
The effective end-to-end throughput
Throughput = TransferSize / TransferTime
TransferTime = RTT + TransferSize / Bandwidth
Insert the two-way, RTT to the formula
Ex., a user wants to fetch 1MB file across 1Gbps network with a round-trip time of 100ms.
Transmit time = 1MB / 1Gbps = 8ms
TransferTime = Latency = RTT + transmit = 108ms
Effective throughput = 1MB/108ms = 74.1 Mbps
Much less than the available bandwidth
Transferring a larger amount of data will help improve the effective throughput
Infinite large transfer size => throughput approaches the bandwidth

82. Units Confusion MB, Mbps, KB, Kbps
Mbps is always 106. Kbps is always 103
MB you can use 220 or 106
KB you can use 210 or 103
5% error
Bits and bytes
b (small letter) for bits
B (capital letter) for bytes (each byte is 8 bits, you can use 10 sometimes, but with 20% errors)

83. The University of Adelaide, School of Computer Science
20 April 2017
Summary
We have identified what we expect from a computer network
We have defined a layered architecture for computer network that will serve as a blueprint for our design
We have discussed the socket interface which will be used by applications for invoking the services of the network subsystem
We have discussed two performance metrics using which we can analyze the performance of computer networks
Chapter 2 — Instructions: Language of the Computer