Transcript Introduction to Networks

Computer Networks
N Amanquah, PhD
What is a Network?
• A connection between two nodes for the
purpose of electronic data exchange
• Same medium, independent hosts -Tanenbaum
Usage
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Communication
Information Access/dissemination
Entertainment & games
E-commerce
Education
Social networking
Resource sharing
Illicit- eg hacking, snooping, gambling etc
Types of networks
• By Size and Scale
– PAN, LAN, WAN, MAN, the internet
• Media type
– copper, fibre, wireless
• Topology
– Star, bus, ring, mesh
A basic network
Wireless: BT, IrAD, Wifi etc
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serial, parallel, twisted pair
coax
Media: 3 types: copper, fibre, wireless
Media
• Copper
– UTP
– Coax
• Fibre
• Wireless
• **Connectors
Not so basic
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topologies
Topologies
Ultimate network
• The internet
• *pix – cabinet, satellite, dish, microwave
links, etc
Network nodes
• 3 or more nodes:
– Hub, Switch
• Integrate networks
– Routers, bridges, repeaters, gateways
• Transmission lines
From
here
B1 & back
From
here
to to
Google
& back
Components of network
• Transmission lines
– Transmission mechanisms
• Store and Forward (excludes satellites)
• Circuits & Switching elements
– Packet switched vs circuit switched
– Subnets
– routers
Count down to transmission
….….Between two very different hosts
• 7. Need a program –an application
– Eg FTP, email, Telnet, http(web)
• 6. syntax and semantics, presentation
– Unix, Mac, Windows are different
– Big/little endian
IBM: Extended binary coded decimal interchange code
– ASCII and EBCDIC, Unicode
Count down …II
• 5. May want to log the session for future review,
or to resume the session.
• 4 End-to-end connection management
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Connection setup
Must be able to stream,
distinguish traffic sessions
Guarantee QoS,
Fragmentation,
Error free end to end in-order delivery
• Also broadcasting, non guaranteed
• 3. Route to the destination (next hop),
– addressing
– Control/avoid congestion,
– communication with other networks
(heterogeneity)
– Datagram segmentation
Count down ..III
• 2a. Further segmentation and reassembly
– Error detection
• Need to be able to able to detect errors that may
occur on the imperfect transmission line
– Flow control
• Avoid swamping a slow receiver
– Framing
• Determine where one frame/chunk of data starts
and ends (be able to distinguish 000 from a
transmission line gone dead)
Count down …IV
• 2b Medium access control
– With more than two hosts, need some
arbitration for transmission.
• 1. Electronic Transmission
– Convert the data (in 01010) to electrical
format and transmit
– Modulation etc : electrical, mechanical, timing
Reference Model
Have enough layers with distinct
functionality
Well defined functionality
Minimize information flow across
layers
Internet Model
Network Architecture
• “Set of layers and protocols”
• Services
– Operations a layer should perform
• Protocols
– rules/agreements on format of packets
• Protocol Stack
– Set of protocols used by one system
Layers
• Communication takes place between peer layers
– Virtual comms: eg bt layer 3 and layer 3
– Actual comms: down/up the stack
• An interface is between two layers
• Layers perform clearly defined task
– Are replaceable
– Each layer provides a service to the one above
– Abstracts layers below
Design issues
• Broadcast networks
– single communication channel shared by all
– May use broadcast address  every to host
– Multicasting to subset
• Point to Point network (unicasting)
Design issues
• Duplexing:
– Simplex
• One way only
– (broadcasting is simplex!)
– Half duplex
• one way at a time
– Full duplex
• simultaneously
Design issues
• Error control:
– Error correcting codes
– Acknowledgements
• *Packet sequencing
• *Flow control
• Multiplexing & de-multiplexing
– Same connection for multiple communications
• Connection oriented service
– Establish connection, transmit, tear down
connection.
– Sequencing is usually preserved.
• Connectionless
– Each packet carries destination information
and is routed independently.
Reliability & QoS
• Acknowledgement
– overhead & delay, vs reliability
• Negotiation for QoS parameters
• **Un/Reliable datagram service
Wireless networks
• Infrastructure
– APs
• Infrastructure-less
– Peer to peer,
– Ad hoc
Network standards
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IEEE 802.3 – Ethernet
IEEE 802.4 –token bus
IEEE 802.5 –token ring -IBM
IEEE 802.11 –wireless LAN
IEEE 802.15 –wireless PAN
IEEE 802.16-wirleess WAN
Example networks
• The Internet is a network of networks
• ARPANET- Advanced Research Projects
Agency Network (DoD)
– Precursor to Internet. A resilient network to
survive a knock out of network hosts
• NSFNET – university research
Example Networks
• Connection oriented
– X.25
– Frame relay,
• For interconnecting LANS
• in-order delivery, no flow control
– ATM
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Used by telcos
Move data eg IP over virtual circuits @ high speed
53byte cell, with 5 byte header.
Inorder delivery
Small overhead. NO error control
Example networks
• Ethernet
– Developed from ALOHANET
– Uses coax, or UTP
• Wifi
Readings
• Andrew Tanenbaum
– Sections 1.0 to 1.9