Physical Sciences in Medicine
IT Networks - Lecture 1
Mark Gleeson
gleesoma@cs.tcd.ie
(01) 896 2666
5th May 2009
1
Objectives
• Understand some network terminologyenough to be able to read further on the
topic.
• Understand some issues of network layout.
• Emphasis on practical aspects
• Recommended Text
–
Computer Networks; Andrew S. Tannenbaum; 4th edition;
Prentice Hall International 2003; ISBN 0-13-066102-3,
•
TCD Library shelf mark 500.17 N691*3
2
Section 1 – Introduction - Network Basics
• Initially computers were highly centralized,
usually within a single room. Computers
were physically large.
• The development and advances made in the
computer industry are huge.
• Now – lots of small independent computers
communicating to do a job. These are called
Computer Networks
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What is a Computer Network? (1/2)
• An interconnected collection of computers
which are:
– Co-operative
• Co-operative action is required between the
components
– Autonomous
• All components are capable of independent
action
• Any resource is capable of refusing requests
– Mutually Suspicious
• Components verify requests
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What is a Computer Network? (2/2)
• Any computer connected to a network is
known as a host.
– Local host
• Your own computer
– Remote host
• The computer elsewhere you are in contact with
• There are hardware and software aspects to
computer networks
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Section 2 - Network characteristics
• What Are Networks
• Network Types and Topologies
• Communication concepts
• Basic Message Types
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What are Networks?
• Tanenbaum’s definition:
"A network is an interconnected collection
of autonomous computers"
???
OSPF
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Types of Networks
• Bus-based networks
– Original Ethernet (802.3)
• Star-based networks
– Switched (Modern) Ethernet (802.3ab)
• Ring-based networks
– FDDI
– Token Ring (802.5)
• Wireless networks
– WiFi (802.11a/b/g/n), Bluetooth, IrDA,
WiMax, GSM, EDGE, 3G
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LAN Topologies
Bus architecture (Ethernet)
Ring architecture (Token Ring)
FDDI Ring
Star architecture (switched Ethernet)
Double ring architecture (FDDI)
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Types of Networks
• Classification based on diameter:
1 m System
10 m Room
Multi-processor
PAN (Personal Area Networks)
100 m Building
1 km Campus
10 km City
LAN (Local Area Networks)
MAN (Metropolitan Area Networks)
100 km Country
WAN (Wide Area Networks)
1,000 km Continent
10,000 km Planet
The Internet
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Local-Area Networks (LANs)
* Figure is courtesy of B. Forouzan
11
Metropolitan Area Networks (MANs)
Network Cloud
* Figure is courtesy of B. Forouzan
12
Wide-Area Networks (WANs)
• Frequently used to join companies offices
worldwide together
• Latency
• Administration/Jurisdiction
* Figure is courtesy of B. Forouzan
13
Simplex
* Figure is courtesy of B. Forouzan
14
Duplex
Half-Duplex
Full-Duplex
* Figure is courtesy of B. Forouzan
15
Point-to-Point & Multipoint
* Figure is courtesy of B. Forouzan
16
Basic Message Types
• Three basic message types
– 1. Unicast - one sender to one receiver
Sender
Receiver
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Basic Message Types
• Three basic message types
– 1. Unicast - one sender and one receiver
– 2. Broadcast - one sender, everybody
receives
Sender
• Broadcast addresses:
– network ID +
– all bits of host ID set
– e.g. 134.226.255.255
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Basic Message Types
• Three basic message types
– Unicast - one sender and one receive
– Broadcast - one sender, everybody receives
– Multicast - one sender and a group of
Sender
receivers
Receivers
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The Physical Layer
• The Physical Layer is the lowest layer and is concerned
with wiring and electrical standards. The design issues
have to do with making sure that when a sender sends a 1
bit that the receiver receives a 1 bit and not a 0 bit.
• Example issues to be agreed when building this layer
– How many volts to represent a 1
– How many volts to represent a 0
– How many microseconds a bit lasts.
– Does transmission proceed simultaneously in both
directions
– How are connections established and torn down
– How many pins are on connectors and what each pin
does.
– What kind of transmission medium, wired, fiber optic
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Communication between End-Systems
* Figure is courtesy of B. Forouzan
21
Data Link Layer
* Figure is courtesy of B. Forouzan
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Duties of the Data Link Layer
The data link layer is responsible for transmitting frames from one
node to the next on the same network.
* Figure is courtesy of B. Forouzan
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Packetizing & Addressing
• Packetizing: Encapsulating data in frame or
cell i.e. adding header and trailer
• Addressing: Determining the address of the
next hop (LANs) or the virtual circuit
address (WANs)
* Figure is courtesy of B. Forouzan
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LAN Technologies - Ethernet
• Developed by Metcalfe 1972/3 while at Xerox PARC
• Standards in 1978, 1995, 1998
• Types of Ethernet
– Original Ethernet
– Switched Ethernet
– Fast Ethernet
– Gigabit Ethernet
Metcalfe’s Ethernet sketch
• Medium Access Control
– CSMA/CD
• IEEE 802.2: Logical Link Control
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Ethernet Addresses – The ‘MAC’ Address
• A unique 48 bit long number
– Eg 00:A0:4A:21:19:13
vendor-specific
• Types of Addresses:
– Unicast – delivered to one station
– Multicast – delivered to a set of stations
• 01-80-C2-00-00-00
Spanning tree (for bridges)
– Broadcast – delivered to all stations
• FF-FF-FF-FF-FF-FF
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Switched Ethernet
• Switch delivers packets to individual
machines
– Without affecting communication with other
machines
• Collisions only occur on individual links
* Figure is courtesy of B. Forouzan
27
Full-duplex Switched Ethernet
• No collisions
– One line to send
– One line to transmit
* Figure is courtesy of B. Forouzan
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Switches in Comms Rooms
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Wireless (1/2)
• IEEE 802.11 standard of 1997 started the revolution
with 2Mbps top speed
–
–
–
–
Now on 802.11g with 54Mbps
802.11n to promise 150+Mbps
Referred by some as Wireless Ethernet
Shares significant similarities with original bus style
Ethernet
• Reliability and Performance much less than wired
network
– Current max speed 54Mbps shared by all on same
access point
– Prone to interference and poor reception
– Speed drops under poor conditions to reduce errors
– Range 100m+ in open much less in office situation
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Wireless (2/2)
• Star like network
– Your laptop talks to a ‘access point’ which connects
to your wired network
– Laptop will move between access points to keep the
strongest signal
• Uses the Industrial, Medical and Scientific Band
– No licence needed
– Healthcare staff should be aware of this shared use
and verify before installation that there won’t be a
conflict
• Advantages
– No need to install ethernet cabling everywhere
– Network access everywhere in range
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The Network Layer
• The Network Layer is concerned with
controlling the operation of the subnet. A key
design issue is determining how packets are
routed from source to destination. They can be
static, dynamic.
• Example issues to be agreed when building this
layer
–
–
–
–
–
Routing mechanisms
How is subnet congestion to be dealt with
How are costings included- national boundaries
Addressing mechanisms.
In broadcast networks the network layer may be
very thin or non-existent.
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Position of the Network Layer
• Sends frames through data link layer
• Accepts data from transport layer
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Duties of Network Layer
• Problems the Network Layer needs to address:
– Transfer over networks of various architectures
– Addressing on a “global” scale
– Adjusting to maximum transmission units
• Hop-to-hop delivery provided by data link layer
• Transfer of packets between end systems
provided by network layer
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