Communication Networks

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Communication Networks

• Its really not magic!!

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A Network consists of

• Links
• bit pipe
• Transfer systems
• routers or switches
• Hosts /end systems
• servers and computers

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Links

• Implementation of a bit pipe
• A link has
• rate R
• delay D
• bit error rate BER
• Transmission time for a packet of P bits
• TDP/R
• arrives correctly with a probability (1-BER)P

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Routers and Switches

• A box with input and output links (ports)
• when a packet arrives, an output link is chosen

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Servers and Computers

• Servers - computer systems at the end of a
  network that store files and provide other
  services
• Computers - computer systems that are end points
  for users of the data

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Example Ethernet

• each computer has a name, network address (IP),
  and Ethernet address (48bits)
• computers are attached to a hub
• hub broadcasts packets to other ports

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Switches routers

• Switch
• Selects a path for sending data to next
  destination
• output ports define destination addresses
• switch uses a list of addresses and outports
• Router
• Connects networks
• uses network addresses (IP)
• keeps ARPs and RARPs for transformations
• router changes packet as it goes through

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ARPs and RARPs(Reverse) Address Resolution
Protocol

• ARP
• each computer has a table
• network address (IP) local (ethernet) address
• RARP
• computers do not keep own table
• list of addresses is kept on server
• computer asks server for its own network address

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But what happens to a packet?

• Incoming packet arrives at router looking for
  local host with IPX
• Router checks ARP for that IP
• Finds it, inserts the local address and sends it

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Router Example
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ARPs and RARPsAddress resolution Protocol

• Network address is used to refer to destination
• independent of Ethernet address (48bits)
• packet needs both!!
• ARP - each computer keeps a list of matching
  network-ethernet address. Pairs
• RARP - list is kept on the network server

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ATM Asynchronous Transfer Mode

• Complements Ethernet and Internet
• moves data in 53 byte cells on virtual circuits
• identifies whole path before sending
• uses routing tables to define that path

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Network Architectures

• ISOs OSI model
• 1970s
• International Organization for Standards
• Open Systems Interconnection reference Model
• 7 layer architecture

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ISO - OSI Model

• Application layer
• presentation layer
• session layer
• transport layer
• network layer
• data link layer
• physical layer

• Ftp, telnet, etc
• data compression, format
• set up connections
• end-to-end trans of packets
• guide packets along links
• send packet between nodes
• deliver bits between nodes

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ISO OSI model
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INTERNET MODEL

• 4 layers
• Application layer
• communication services (ftp, telnet, email)
• transport layer
• transmission of messages end-to-end
• network layer
• transmission of messages sequence of links
• link layer
• transmission of packet across one link

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Internet layers
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Application Layer

• FTP
• HTTP
• SMTP
• telnet
• etc

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Transport layer TCP and UDP

• TCP
• transmission control protocol
• full duplex byte stream
• virtual path (connected)
• error free
• uses acknowledgements
• 16 bit address of ports

• UDP
• user datagram protocol
• connectionless
• no acknowledgements
• no flow control
• no resending of erroroneous packets
• some error detection
• 16 bit port addresses

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TCP and IP
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Network Layer IP

• Delivers packets up to 64kbytes, 1 at a time
• Each packet has a header
• sending host and intended host network addresses
• 32 bit addresses
• IP layer (like UDP)
• unreliable
• connectionless

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Link Layer links

• Connect computer to Internet
• SLIP
• serial line IP (asynchronous, 1 char at a time)
• move IP packets on common link (phone line)
• PPP
• point-to-point protocol
• also synchronous transfer for packets

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Data encapsulation using TCP on Ethernet
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Sum Up

• Networks take care of local connections
• Routers take care of inter-network connections
• Tables are kept to manage different addresses
• All packets need at least source and destination
  IP addresses