Download Computer Networks (CSC 345)

Computer Networks
An Introduction
Prepared by S.Perry (February 2010)
Unit Objectives
To present a comprehensive view of the principles
and fundamental concepts in Computer Networks
 To learn about the basics in design and
implementation of network protocols
 To provide an understanding of the components of
a network and how they are connected.
 To acquire some hands-on experience
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Presentation Outline
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Introduction
◦ Fundamental concepts
◦ Basic definitions
◦ Network architecture
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Communication Basics
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Media and signals
Asynchronous and synchronous communication
Relationship among bandwidth, throughput, and noise
Frequency-division and time-division multiplexing
Presentation Outline (continued)
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Networking and network technologies
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Packing switching
Framing, parity, and error detection
Local and wide area technologies
Network addressing
Connection, wiring and extension (repeaters, bridges, hubs,
switches)
◦ Forwarding and measuring of delay and throughput
Presentation Outline (continued)
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Internets and Internetworking
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Motivation and concept
Internet Protocol (IP) datagram format and addressing
Internet routers and routing
Transmission Control Protocol (TCP)
Presentation Outline (continued)
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Network Applications
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Domain Name System (DNS)
File Transfer Protocol (FTP)
Remote Login Protocol (TELNET)
Email Transfer (SMTP)
Web technologies and protocol (HTTP)
Putting all pieces together
What is a Computer Network?
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A collection of computers (PCs, workstations) and other
devices (e.g. printers, credit card readers) are all
interconnected (each is called a “node”)
Components:
◦ Hosts (computers)
◦ Links (coaxial cable, twisted pair, optical fiber, radio, satellite)
◦ Switches/routers (intermediate systems)
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Goal: provide ubiquitous access to resources (e.g.,
database servers, Web), allow remote users to
communicate (e.g., email)
User runs applications
What is a Computer Network?
Application
Application
Frame
Networks connect applications
on different stations
What is a Computer Network?
Frame
Client
Station
Mobile Client
Station
Stations are computers
and other devices
such as cellphones and PDAs
Server
Station
Server
Station
What is a Computer Network?
Frame
Client
Station
Mobile Client
Station
Stations communicate by
sending messages called
Frames
Server
Station
Server
Station
What is a Computer Network?
Frame
Switch
Switch
Switch
Switch
Frames may pass
through multiple switches;
Each switch reads the frame
And passes it on
What is a Computer Network?
Trunk
Link
Trunk
Link
Trunk links
connect switches
Higher capacity
than access links
Often optical
fiber
Trunk Link
Trunk
Link
Trunk
Link
What is a Computer Network?

In summary, a network is a system of hardware,
software and transmission components that
collectively allow two application programs on
two different stations connected to the network
to communicate well
What is a Computer Network?

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Point-to-Point Communication
Multiple Access Communication
What is a Computer Network?

Switched Networks
◦ Circuit - switched network:
public telephone network
(dedicated circuit per call)
◦ Packet switched network:
Internet (collection of
networks where data is sent
in chunks)
Circuit-Switching
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Set up a connection path (circuit) between the
source and the destination (permanent for the
lifetime of the connection)
All bytes follow the same dedicated path
Used in telephony
Advantages: dedicated resources
Disadvantages: not very efficient (lower utilization,
e.g., a person talks < 35% of the time during a call)
While A talks to C, B cannot talk to D on the same
line.
Packets
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Packets
◦ Data are chopped up into small blocks called packets
(e.g., ~ 4500 bytes)
◦ Each packet carries extra information to allow it to
reach its destination
Packet-Switching

Packets from different sources are interleaved
Efficient use of resources (since they are used
on a demand): statistical multiplexing. Nobody
reserves a lane on a freeway
 Can accommodate bursty traffic (as opposed to
circuit-switching where transmission is at
constant rate).
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Features of a Packet-Switching
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Store and forward: intermediate nodes (e.g.,
routers) store (buffer) incoming packets, process
them and forward them to the appropriate outgoing
link.
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Allows for flexibility and robustness. Packets can
travel through alternate paths (adaptive routing).
Undesired situations such congestion, long delays
may occur.
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Packet Switched Networks: Example
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Packets can travel on different networks/links that may
have different line speeds
Packet-Switched Networks: Topologies
What is the Internet?
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In the 60’s and 70’s the Internet (ARPANET) was a small
network connecting universities, research labs and
government agencies. Main application: email, FTP.
Motivation: share & research
Today it is a global, non-regulated communications
network with millions of hosts and users. Main
applications: Web, multimedia (audio/video), email.
Motivation: commercialization
A large number of different network technologies and
standards exist: LANs, WANs, B-ISDN, Optical Nets,
Wireless, Satellite.
The Internet Today-- Complicated
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A huge and arbitrary collection of heterogeneous
nets. A network of networks!
◦ More than 240 million hosts
◦ Growing exponentially– doubling every 18 months
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Hierarchically structured
◦ LANs (e.g., Ethernet)
◦ CANs (e.g., FDDI)
◦ National/global (e.g., ATM or optical backbone)
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Fully distributed operation (i.e., no centralized
system or computer)
An Internet
Multiple Networks
Connected by Routers
Path of a Packet is its Route
Single Network
Routers
Packet
Single Network
Route
A Network Example
Issues
Resource sharing (i.e., accommodate many users
over the same link or through the same router)
 Addressing and routing (i.e., how does an email
message finds its way to the receiver)
 Reliability and recovery: guarantee end-to-end
delivery
 Traffic management: monitoring and policing the
network! Regulate traffic
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Network Performance
There is a number of measures that characterize and
capture the performance of a network
 It is not enough that networks work
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◦ They must work well
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Quality of service (QoS) defines quantitative
measures of service quality
◦ Speed
◦ Delay (Latency)
◦ Reliability
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Security (not a QoS measure but crucial)
Network Performance
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Speed
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Bits per second (bps)
Multiples of 1,000 (not 1,024)
Kilobits per second (kbps)  Note the lower case “k”
Megabits per second (Mbps)
Gigabits per second (Gbps)
Terabits per second (Tbps)
Related to link bandwidth
Network Performance
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Congestion and Latency
◦ Congestion because traffic chronically or momentarily
exceeds capacity
◦ Latency delay measured in milliseconds (ms),
microseconds ( s ).
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Especially bad for some services such as voice
communication or highly interactive applications
Network Performance
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Delay:
◦ Transmission time: time it takes to transmit a packet
(depends on the link speed) = packet size/ speed
◦ Propagation delay: time for a bit to travel across a link
(depends on the distance, physical medium)
◦ Queuing delay: waiting time inside a buffer
◦ Processing delay: time to process a packet
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RTT (round-trip time): time for a bit to travel to the
destination and come back
Reliability and Recovery
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Reliability
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Examples:
◦ Availability – percentage of time the network is available to
users for transmission and reception
◦ Error rate – percentage of lost or damaged messages or
bits.
◦ Bit errors (bits are flipped, e.g., due to electrical signal
interference.)
◦ Packet loss (packets may be dropped due to insufficient
buffer space.)
◦ Packet delays (e.g., due to large queue size)
◦ Nodes or links can fail (go down)
◦ Malicious users
Reliability and Recovery
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As a consequence:
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Packets delivered to the wrong destination
Long delays on packets
Packets delivered out-of-order
Duplicate packets
Recovery:
◦ Implement error-control mechanism
 Hop by hop (I.e., between nodes)
 End-to-end (source-to-destination).
◦ Retransmissions
◦ End-to-end security (e.g., encryption, authentication)
Overload and Congestion
Overload: Too many packets occur in a
subnetwork in the same time, which prevent
each other and in such a way the throughput
decreases
 Congestion: the queues in the routers are too
long, the buffers are full.
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◦ As a consequence some packages are dropped if
the buffers of the routers are overloaded
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In extreme case: grid-lock, lock-up (often
used in a DNS (Denial of Service attack)
User Applications
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Users run application programs (web, email, ftp) at the
hosts interconnected through a network
Hosts need to communicate in a meaningful way. User
should not be concerned with the underlying network
Network supports process-to-process (uni- or bidirectional) communication among the hosts
Applications need to take into consideration limitations
imposed by the networks physical characteristics
The Need for a Protocol Architecture
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Procedures to exchange data between devices
can be complex
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High degree of cooperation required between
communicating systems
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destination addressing, path
readiness to receive
file formats, structure of data
how commands are sent/received and acknowledged
What is a Protocol?
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Set of rules that specify the format and meaning of
messages exchanged between computers across a
network
◦ Format is sometimes called syntax
◦ Meaning is sometimes called semantics
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Example from everyday life: traffic laws!
Internet Protocol Architecture
Currently, Internet is mostly based on the TCP/IP
protocol suite (designed in late 70’s)
 TCP/IP became popular as it was bundled with the
UNIX/C environment
 ISO is still influential in designing networks
 Other architectures: ATM. Frame Relay
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Key Features of a Protocol
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Set of rules or conventions to exchange blocks
of formatted data
Syntax: data format
Semantics: control information (coordination,
error handling)
Timing: speed matching, sequencing
Actions: what happens when an event occurs
Network Tools
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Repeater: connects network segments logically to
one network
Hub: multiport repeater
Bridge: datalink level connection of two networks
Switch: multiport bridge
Router: connects networks that are compatible in
transport level
◦ subnetworks are connected to the interfaces of the
repeater
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Gateway (proxy server): router between two
individual network. The “Way Out”
Characteristics of High-Speed LANs
Data Rate
Transmission Mode
Access Method
Supporting
Standard
Fast Ethernet
Gigabit Ethernet
Fibre Channel
Wireless LAN
100 Mbps
1 Gbps, 10 Gbps
100 Mbps – 3.2
Gbps
1 Mbps – 2 Gbps
UTP,STP, Optical
Fiber
UTP, shielded
cable, optical fiber
Optical fiber,
coaxial cable, STP
2.4 GHz, 5 GHz
Microwave
CSMA/CD
CSMA/CD
Switched
CSMA/CA Polling
IEEE 802.3
IEEE 802.3
Fibre Channel
Association
IEEE 802.11
Performance Considerations
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The following affect performance metrics
◦ Overhead: CPU time to put packet on wire
◦ Throughput: Maximum number of bytes per second
 Depends on “wire speed”, but also limited by slowest router (routing delay) or
by congestion at routers
◦ Latency: time until first bit of packet arrives at receiver
 Raw transfer time + overhead at each routing hop
Router
LW1
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LR1
Router
LW2
LR2
Contributions to Latency
◦ Wire latency: depends on speed of light on wire
 about 1–1.5 ns/foot
◦ Router latency: depends on internals of router
 Could be < 1 ms (for a good router)
Lw3