Transcript Networks

Networks
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Computer Networks
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“An interconnected collection of autonomous end-systems”
End Systems: hosts, PCs, dedicated computers, network
components
Connection can be via various means: copper wire, fiber optic
cables, microwaves, satellites, ham radio.
Distributed Systems:
– Existence of multiple autonomous computers is transparent
(not visible), machines managed automatically
Network:
– Users explicitly log onto a machine, machines managed
personally
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Network Hardware
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Broadly speaking : two types of transmission technology
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Broadcast Networks
Point-to-Point Networks
Broadcast (multiaccess)
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Point-to-point
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Broadcast Networks
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Single communication channel that is shared by all the
machines on the network
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Share wire, frequency etc.
Packets (small messages) sent by one mechanism is received
by the others.
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Each machine checks the address field in the provided
message. If the address is the same as receiving machine’s
address, packet is processed, otherwise it is ignored.
Some arbitration mechanism is needed when more than one
machine want to send message at the same time.
Multicasting: transmission to a subset of machines.
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Point-to-Point Networks
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Consists of many connections between individual pair of
machines.
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To go from source machine to destination machine, packet may
have to visit intermediate machines.
• Various routes (multiple paths) possible  Routing algorithms
are required.
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Connecting Networks
- Repeaters •
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Copies bits from one network to another
Does not look at (interpret) bits
Allows the extension of a network beyond physical length
limitations
REPEATER
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Connecting Networks
- Bridges •
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Copies frames from one network to another
Can operate selectively – does not copy all forms
Extends the network beyond physical length limitations
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Connecting Networks
- Routers •
Switching Elements, packet switching nodes, intermediate
systems, data switching exchanges
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Copies packets from one network to another
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Subnets in which intermediate routers can store a packet and
forward it is called point-to-point or store-and-forward, packet
switched.
Makes decisions about what route (path) a packet should take
If two routers not connected by direct link want to
communicate, they do so by using intermediate routers
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Subnet
Router
LAN
LAN
host
SUBNET
LAN
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LAN
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Connecting Networks
- Gateways •
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Operates as a router
A gateway is a host that is connected to two or more physical
networks simultaneously and is configured to switch packets
between them
• Can perform
– Data conversions
– Translation: connect different application protocols
– encryption
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Hardware vs. Software
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Repeaters are typically hardware devices
Bridges can be implemented in hardware or software
Routers and gateways are typically implemented in software so
that they can be extended to handle new protocols
• Many workstations can operate as routers or gateways
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Local Area Networks (LANs)
• LANs connect computers that are physically close together
• LANs are restricted in size
• Worst case transmisson time is bounded and known in advance
• High speed
• Transmission technology: multi-access (broadcast)
• Speeds
– Ethernet: 10 Mbps
– Token Ring: 16 Mbps
– FDDI: 100 Mbps
– Fast Ethernet: 100 Mbps
– Gigabit Ethernet: 1 Gbps
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Classification of Networks
• Smaller, geographically localized networks tend to use broadcasting. Larger
networks are usually point-to-point.
Interprocessor Distance
Processors located in the same: Example
1m
system
10m
Room
100m
Building
1km
Campus
10km
City
multicomputer
LAN
Metropolitan Area
Network (MAN)
100km
Country
Wide Area
1000km
Continent
Network (WAN)
10000km
Planet
The Internet
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Transmission Media
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Can be classified as:
– Guided : waves are guided along a solid medium such as
copper wire, optical fiber etc.
– Unguided : wireless transmission, atmosphere, outer space,
transmit electromagnetic signals, radio, laser etc.
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Design issues in transmission systems:
– Data rate, distance, bandwidth, transmission impairments,
interference, number of receivers etc.
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Twisted Pair (guided trans. media)
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Oldest, least expensive, most widely used
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Can transmit both analog and digital signals:
Telephones connected to telcos by twisted pair (subscriber
loops)
– For analog: amplifiers needed every 5 to 6 km
– For digital: repeaters required every 2 to 3 km
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Limited distance, bandwidth compared to others
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Twisted Pair (guided trans media)
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2 kinds:
1) Unshielded (UTP, 10 Base T) is ordinary telephone wire,
least expensive transmission media for LANs
2) Shielded: shield twisted pair with a metalic braid/sheating
to reduce interference, more expensive
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UTP kinds:
– Category 3: 3 or 4 twists per foot, (data rates: 16 Mbps)
– Category 5: 3 to 4 twists per inch (data rates: 100 Mbps)
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Coaxial Cable (guided trans. media)
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Can be used over longer distances and supports more stations
on a shared line than twisted pair.
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Bandwidth possible depends on cable length. For 1 km cables
1-2 Gbps feasible.
• Widespread use: television distribution, long distance telephone
transmission, short-run computer system, local area networks
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Varieties: thin, thick coaxial
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Fiber Optics (guided trans. media)
•Physiscs: light ray is refracted (bent) when passing from one
medium to another)
•Above a critical angle of incidence, light is refracted back into
silica (hence trapped inside fiber) and can travel for many
kilometers
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Fiber Optics (guided trans. media)
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Lower attenuation
Fibers thinner and lighter weight (considerably)
Data Rates: 1 Gbs, 100 Gbps, 1 Tbps
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Fiber Optics (guided trans. media)
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Electromagnetic isolation (i.e. not affected by external
electromagnetic fields) no interference
• Difficult to tap, therefore better security
• Enables greater spacing between repeaters
• Multimode fiber: allows many different rays incident at
different angles to travel
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Singlemode fiber: employs fiber core which has small distance
(of the order of a wavelength) and allows only a single ray to
pass. Singlemode fiber is more expensive but can be used for
longer distances.
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Global Wireless Network Standards
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IEEE 802.15 (Bluetooth) for the Personal Area Network (PAN)
IEEE 802.11 (Wi-Fi) for the Local Area Network (LAN)
IEEE 802.16 (WiMax) for the Metropolitan Area Network
(MAN)
• IEEE 802.20 standard for the Mobile Broadband Wireless
Access (MBWA)
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Bluetooth
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Can link up to 8 devices in 10-m area
Can transmit up to 722 Kbps in the 2.4 GhZ band
Low power requirements
Radio based communication
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Bluetooth
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Wi-Fi
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Three standards:
– 802.11a: up to 54 Mbps (10 – 30 meters)
– 802.11b : up to 11 Mbps (30 – 100 meters)
– 802.11g : up to 54 Mbps (30 – 100 meters)
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Infrastructure mode: Devices use access point to communicate
with wired network
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Ad-hoc mode (peer-to-peer): Wireless devices communicate
directly with each other
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Wireless LAN
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The Telephone System
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Telephone was patented by Alexander Graham Bell, 1876.
Evolution of the structure of telephone system:
(a) Fully-interconnected network.
(b) Centralized switch.
(c) Two-level hierarchy.
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The Telephone System
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Basic Bell system model remained essentially intact up to now.
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Transmission media used:
Presently, we have highly redundant multilevel hierarchy.
Hierarchy: Home, End Office, Toll Office, Primary Office,
Sectional Office, Regional Office
– Local loops consist of twisted pair
– Between switching offices: coaxial cables, microwaves,
fiber optics
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Transmission:
– Local loops: analog signaling
– Trunks: mostly digital
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Structure of Telephone System
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Local Loop
• Between home and the end office
• Modems can be used to connect computer at home
• Baud Rate: Signalling speed – the number of times per second that signal
changes its value (i.e. its voltage)
– b-baud line does not necessarily transmit b bits/sec
– each signal may convey several bits
– Example: if voltages 0,1,2,3,4,5,6,7 are used, each signal can convey 3
bits. So bit rate: 3 X baud rate
• DSL (Digital subscriber lines) : Use existing telephone lines to transmit
signals. 500Kbps-1.5Mbps possible.
– ADSL is asymmetric DSL : download and upload speeds are different.
Upload speed is slower than download speed.
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Trunks and Multiplexing
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Cost of installing and maintaining both high-bandwidth and
low bandwidth trunk the same (essentially)
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Therefore, choose high-bandwidth trunk installation
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Multiplexing schemes:
To efficiently use high bandwidth, multiplex many
conversations over a single trunk
– Frequency Division Multiplexing
– Wavelength Division Multiplexing
– Time Division Multiplexing
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Example of Time Division Multiplexing
• T1 Carrier (1.544 Mbps):
– 24 voice channels multiplexed together
– Analog signals are sampled on a round robin basis
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Multiplexing T1 streams into higher carriers
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Layered Network Software Architecture Model
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Protocol Hierarchies
•Design issues for the layers: addressing, error control,
flow control, multiplexing, routing
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OSI Reference Model
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Network architecture model based on a proposal developed by
International Standards Organization (ISO)
• OSI: Open Systems Interconnection
• Standardized protocols for network layers
• OSI has 7 layers
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OSI Layers
Layer
Responsibility
1) Physical Layer
Transmission of raw bits over a communication
channel
2) Data Link Layer
Data Link Control
- Provide an error free communication link. Issues:
framing, addressing
MAC Sublayer
- Medium Access Control needed by multiaccess
communications. MAC provides DLC with virtual
wires on multiaccess networks
3) Network Layer
-Path selection between end-systems (routing)
-Subnet flow control
- Translation between different network types
4) Transport layer
-Provides virtual end-to-end links between peer
processes. Fragmentation and reassembly. End-to-end
flow control
5) Session Layer
- Establishes, manages and terminates sessions
between applications. Service location lookup.
6) Presentation Layer
- Data encryption, data compression, data conversion
7) Application Layer
- Anything not provided by other layers
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TCP/IP Reference Model
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First used in ARPANET (Advanced Research Projects Agency)
sponsored by US Department of Defense
• Now used in worldwide Internet
• Goal: ability to connect multiple networks in a transparent way
• Requirements: Connections should remain intact as long as
source and destination machines are functioning even if some
of the intermediate machines, links are out of operation
• Internetwork Layer (Internet) : Packet switching network based
on a connectionless internetwork layer.
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TCP/IP Reference Model
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Protocols and networks in the TCP/IP model
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Why OSI Model/Protocols were not implemented
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Bad timing
Bad technology
Bad implementations
Bad politics
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