Networking and the Internet 7

1
Networking and the Internet (7)

• Last Time
• Checkpoint review of the module so far
• Data integrity in a networked environment
• Workshop for assignment due Thursday May 7th
• Week 7 Focus
• Message/Queuing model for distributing function
• Data Transfer inside and outside the computer
• Local- and Wide-Area Networking
• Data Transmission on a network
• Assignment workshop
• Useful book White, Curt M (2006) Data
  Communications Computer Networks, A Business
  Users Approach, Thomson

2
Distributed Systems

• Goal is to put user-interface close to
  user data-interface close to shared data
• Improves data integrity only one system touches
  data
• Reduces network traffic graphics dont have to
  flow
• Exploits low cost of workstation processing
• Can be synchronous
• Remote procedure call client and server work in
  step
• Function shipping all data returned to client
• Transaction routing typical of form processing
  on www
• or asynchronous main examples are
• Interactions on web never sure if theyve
  worked (until application confirms)
• MQ Series messaging and queuing guaranteed
  delivery

3
Distributed Programming Models

• Function Shipping, as in CICS or Network File
  System
• Application issues data request to environment
• Environment generates messages to another system
  to request action on the data
• Transaction Routing, used by CICS
• Client system invokes a transaction thats not
  local
• CICS routes the request to another system
  (usually server) where associated program is run
  in its entirety
• Remote Procedure Call
• Logic on workstation invokes procedure on the
  server
• Procedure runs, and returns control (and results)
  to caller
• Environment handles RPC messages
• Permits complex structures of networked logic

or vice versa
4
Insurance Example

• Customer asks agent for a quotation
• Agent takes details on local PC
• Quotes using premium tables downloaded from HQ
• Offers a deal to the customer
• Customer accepts it
• Agent sends data to HQ as firm proposal
• HQ updates database and accepts proposal (or not)

5
An Asynchronous Model
Proposal queue
Insurance Company HQ
Agents PC
Process proposal
QuoteQueue proposal
Queue printout
Agents print server

• Alternative approach to distributed systems is
  Queuing
• Infrastructure guarantees message delivery
• But not how quickly itll arrive
• Agent can get on with other work as soon as
  message has been sent
• Application needs to consider risk that message
  will not produce desired effect on arrival

6
Advantages of MQ Approach

• Simple
• Allows easy connection of heterogeneous systems
• Asynchronous operation is built into the model
• Coping with failure is inherent
• You never know when the message will arrive,so
  you have to design around non-instant delivery
• Network failure is simply an extended case of
  this
• Depends on integrity of infrastructure
• Since many customers and systems use same
  vehicle, problems will get ironed out quickly
• BUT
• Transmission is often very fast
• Designers may wrongly depend on this

7
Data Transfer Inside the Computer

• How the controller chipsets drive peripherals

8
Data Transfer Inside the Computer

• Every device needs an interface to exchange data
  with the computer
• Generally, these interfaces share a bus to
  communicate with the CPU or with memory

Front side bus
Memory
CPU
Register
I/O bus
cable
9
I/O and the CPU

• I/O through the CPU
• In very early days of computing, programmed with
  CPU looping while waiting for data transfer
• Later programmed via interrupts, grabbing CPU
  only when a chunk of data arrives
• Inefficient, because every transfer stops other
  processing
• Better to transfer data autonomously
• Only use the CPU to initiate I/O
• Then have separate processor to move data into
  memory
• Basis of PCs DMA (direct memory access), IBM
  channels and ICLs Autonomous transfer units
• Only impacts CPU throughput when theres a clash
  for memory access (cycle stealing)

10
Parallel and Serial Connectors

• Parallel devices (like printers and most IDE
  disks)have one wire per bit, plus control wires
• Keep cables short enough to avoid spread of bits
  in time
• Usually bi-directional (to provide
  acknowledgement data)
• Serial devices (like networks) send bits one
  after another
• Examples
• RS232-C interface COM1 etc on PCs (with UART
  chip)
• USB (Universal Serial Bus) on iMacs and PCs from
  W98
• IEEE1394 (Firewire Apple Computers, Sony
  i-link), USB2
• Need to keep both ends in step to keep bytes
  separate
• Asynchronously, as in most modems
  start/send-a-bit/stop
• or synchronously synchronize clocks in both
  ends, then send a whole block (SNAs SDLC does
  this)
• Surprisingly, serial is usually faster than
  parallel ?

11
Significance in Networking

• The idea of autonomous transfer can be applied
  widely
• Network card moves data from memory to LAN cable
• Router on LAN cable sorts out data to leave LAN
• ASDL modem takes Router output and sends over
  line
• Much of this work involves buffering
• Accepting a packet of data and writing it into
  local storage
• Then sending it on, perhaps using a different
  technology
• Effectively all data is being stored and
  forwarded, though we only use this term when
  entire message is stored
• Have you noticed FreeView is delayed wrt analogue
  TV?
• Data in buffer is independent of how it arrived
• So we can change transmission formats, for
  example PC bus to LAN to ADSL to ATM to LAN to
  mainframe bus

12
Links to Real Life

• Theres very little technology unique to data
  networks, so think about the products you use
• How does a Discman handle shocks?
• Technical aspects Buffering, error detection,
  retransmission
• What are quality issues with a mobile phone?
• Multiplexing, signal/noise ratio, compression
• Why does AM radio sound so grotty?
• Noise, attenuation, bandwidth
• How does FreeView put multiple programmes on a
  channel why is movement sometimes jerky?
• Multiplexing, lossy compression, buffering
• Well cover all these technologies and see how
  they apply to data transmission

13
Packet-Switched Networks

• Including The Internet

14
Early Computer Networks

• Initially had two distinct purposes
• to connect terminals to mainframe computers
• to link computers together
• And two ranges
• Local low errors, wiring under enterprise
  control
• Wide-area wiring regulated by PTTs (was
  error-prone)
• And two topologies
• Point-to point
• Concentrated (multiple devices sharing a
  connection)
• Network architectures developed to share
  connections, based on packet-switching concept
• Systems Network Architecture led through 70s and
  80s
• Internet Architecture now taking over (even in
  IBM shops)

Telephone companies
15
Networking Requirements

• Two fundamental forms of communication
• Session-based, where you set up a call, exchange
  data, then hang up like a telephone call
• Message-based, where you create a message, put an
  address on it, and send it like a postcard
• Sessions are often used synchronously, with a
  conversation between their endpoints
• Example is terminal emulator to host
• but they can support bursty flows, including
  messages
• Message flows can be mixed together, as in the
  mail
• If the packaging/unpacking is fast enough, you
  can get the impression of synchronous
  conversation
• This is the basis of packet-switched networking

16
A general network

• Flows from A-E could go
• A-C-E or A-H-G-E or A-B-D-F-H-G-E
• If we break flows into packets, and address them
  to ultimate destination, we can mix flows on the
  links
• Sender and receiver dont care about route taken
• or about how each link works
• In this case, a packet would get there faster via
  H
• Fast wide-area connection to H
• Local connection to G
• Would even be faster to E
• Each node functions as a router
• Need to avoid routing round in circles

Slow
Fast
Fast
wireless
Fast
LAN
LAN
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Some problems of this model

• Every packet has to be routed onwards by the
  node(s)
• Creates overhead on each intermediate node
• Would not be acceptable to have (say) server to
  printer traffic passing through a users PC
• Packets may arrive out of sequence
• Hard to provide end-to-end integrity
• But we cant afford links direct to every PC
• Solved by specialized networking hardware
• Mainframe network controllers
• Routers and bridges
• Buses and hubs

18
Wide-area Data Transmission

• In the 70s, phone lines were slow and unreliable
• They could only handle sounds in pitch range of
  voice, so data had to be modulated into tones and
  demodulated at the other end by a box called a
  modem
• Phone companies monopolized the supply of modems
• Voice lines had 4KHz bandwidth, handled 2400
  tone-changes a second 2400 baud
• Modem technology improved used several tones at
  once, thus getting more bits into each
  tone-change 4 tones lets you run 9600
  bits/second (but its still a 2400 baud line)
• In 1980, even leased lines were flat out at 9600
  bps, now a 53Kbps dial up modem costs under 30
• Phone network is now digital, apart from local
  loop

19
Notes on Wide-area Networking

• Theres nothing new about digital transmission
  its how the 19th century telegraph worked. The
  teleprinter used a similar approach, sending a
  signal to convey each bit of the Baudot code used
  to represent characters on 5-hole paper tape.
  Wireless telegraphy introduced the concept of
  modulation, with a tone for a 1-bit (or hole) and
  a silence for a 0. The speed of transmission was
  stated in bauds, and teleprinter lines often ran
  at 50 baud (10 char/second). Once
  data-transmission became electronic, faster
  speeds were needed, and better lines were
  modulated. A typical voice line is good for a
  frequency range of about 100Hz to 4kHz, so it was
  easy to squeeze in 1200 tone-changes a second.
  Improved modulation techniques used multiple
  tones, so the bit rate is now a multiple of the
  baud-rate.
• Modern phone lines are digital, with each voice
  call being turned into a 56Kbps digital
  data-stream. Between exchanges, this runs on
  64Kbps channels (kilostream), combined into 2Mbps
  circuits (E1 or megastream). The US equivalent
  of E1 is slightly slower 1.5Mbps, called T1.
  That speed was picked because its fastest you
  can run down an old copper cable by putting
  repeaters at the standard distance between
  manhole covers. Except first when demonstrating
  video-on-demand, BT do not run digital data
  directly down old copper, so were stuck with
  analogue lines to our houses. However, using
  ADSL modems, we can still get gt1Mbps down the
  line. Cable TV operators were slow to compete,
  but now offer broadband services too.
• The fastest current standard for a voice-grade
  line is V.92 (53Kbps) though you cant beat V.34
  (33 Kbps) on the voice channel of a line set up
  for ADSL.
• You can improve effective data-rates with
  compression, irrespective of the link technology.

20
Wide Area Networks

• Traditionally slower and more error-prone than
  LANs
• Links run over telephone company circuits
• Leased lines usually digital these days
• Kilostream (64Kbps) and Megastream (2Mbps) aka E1
  
• US mainly T1 (1.55Mbps) with some 56Kbps around
• Unless you pay for ISDN, Dial-up is still
  largely analogue (modems needed to send bits over
  analogue circuits)
• Can use local copper with ADSL for fast Internet
  access
• IP networks usually link Routers rather than
  computers
• Multi-protocol networks now becoming available
• Vendor provides line termination that looks like
  direct links to multiple nodes. For example,
  frame relay
• Or you create a virtual circuit on the Internet

Dont
21
Campus Networking

• No need to modulate data on phone-lines if you
  can run a wire from point-to-point over your own
  land
• If you send bits down a wire, they start off as
  square waves
• Losses knock the corners off and your signal
  gets weaker
• Happens less if you screen the signal using
  coaxial cable...
• or use twisted pair and clean up signal every so
  often
• Terminals used to be wired to controllers using
  coax
• Local Area Networks dominant since 80s
• Concept of LAN is that devices share the medium
• add destination addresses to data packets they
  send
• ignore incoming packets unless addressed to them
• Main implementations are Ethernet and Token Ring

22
Local Area Networks

• Network card in PC creates addressed packets
• Sends down wire as soon as it can
• In Token ring LAN, this is when it gets the token
• In Ethernet, its when nobody else is sending
• Listens for incoming packets addressed to the PC
• In principle, LANs are buses the wire is shared
  among all users, who effectively broadcast on it
• In practice, most physical LANs (apart from
  Ethernet on coax) are stars, with direct links
  from hub to PC
• Switching hubs only send PCs the data thats
  addressed to them effectively theyre smart
  nodes

23
Ethernet

• Logically a bus
• each device throws data on the bus when its
  quiet,..
• and hopes that nobody else does so at the same
  time
• Each device listens, in case theres been a
  collision
• if so, both back off for a random time, then try
  again
• the busier the bus, the greater risk of collision
• One collision increases risk of another
• Retransmission raises traffic on the LAN
• LAN may be busy when station finishes its delay
• Other station may finish delay during this
  wait,in which case theyll collide again
• Limits effective speed to below half of nominal
  speed

24
Ethernet Hardware

• Ethernet originally ran 10Mbps over coax, but...
• Single break in the bus can cut off many users
• Coax is fairly expensive to buy and install
• Hub allows radial wire to individual stations
• Can clean signal, so uses cheap and flexible
  twisted-pair with cheap RS45 connector
• Smart hub can filter out information not meant
  for station, and even work full-duplex down the
  cable
• Most hubs run at 100Mbps, newer ones at 1Gbps
• Popular cable is CAT5e inexpensive and good to
  1Gbps
• Can also use fibre-optics over longer range

25
Token Ring largely superseded

• Avoids collisions by sending tokens round the LAN
  to each station in turn
• When your token arrives, you can add data to it
• Any other token you just forward to the next
  station
• Can approach nominal speed (4 or 16 Mbps)
• Risk that one rogue station can bring down whole
  ring is solved by building logical ring over star
  wiring
• MAU sends stream of tokens down two wires of the
  cable, station returns them down two others
• MAU shorts out any station that doesnt respond
  
• Hardware typically over twice Ethernet price
• Didnt sell well enough to be extended beyond
  16Mbps
• Obsolescent in face of 100Mbps and Gigabit
  Ethernet

Multiple Access Unit like a hub
26
Application view of network

• Target is to deliver data without having to be
  aware of the network, for example
• Send data-stream to paint characters on 3280
  screen
• Deliver WWW page to browser that requested it
• Transmit file to remote system
• This requires abstraction to hide intermediate
  steps
• Envelop data to say where it is to go
• Break large package into transmittable chunks if
  needed
• Route each chunk down the appropriate link
• Correct any transmission errors that arise
• Perform onward routing to destination
• Reassemble chunks at destination (in sequence)
• Open envelope and pass to target application

27
Seven Layer Model

• Simplify programming by encapsulating lower levels

Examples File transfer show screen image manage
sign-on manage connections route over several
links V.34 , HDLC LAN, digital-,
analogue-circuit (Colours show TCP/IP layers)
Application Presentation Session Transport Network
Data Link Physical
28
SNA Networking

• Largely confined to the IBM mainframe and
  mid-range market (a huge market)
• Has concept of Networks usually one or a
  small number per enterprise
• Each networks contain subareas that can be
  separately managed
• Each subarea contains a number of
  individually-addressed Physical and Logical
  Units
• Wide-area circuits usually run SDLC protocol
  (Synchronous data-link control layer 2)
• Local connections via LAN or direct wiring
• Now being superseded by TCP/IP, even in IBM shops

29
Data Transmission

• Concepts and Technologies

30
Data Transmission

• Features of a waveform
• Frequency usually measured in Hertz (Hz
  1/time)
• Amplitude measured in volts, bars, etc
• Phase usually measured in degrees
• Wavelength speed divided by frequency
• Timbre can be represented by sine wave
  overtonesSquare wave is made up of F 3F 5F
  7F
• Information Capacity
• Information content is number of bits needed to
  discriminate among possible symbols (128 symbols
  gt 7 bits)
• Capacity of channel is in symbols or bits per
  second
• Bandwidth (Hz) is half of capacity in bits/sec
  (on good line)
• But if line is noisy, capacity drops relative to
  bandwidth

31
Modulation

• Base-band transmission is sending digital data
  as is
• effectively sending square waves down the wire
• Signal degrades with distance and
  bit-rate(remember all those odd harmonics?)
• So we need to modulate over longer distances
• Modulation involves using the signal to do
  something to a carrier signal (see pictures on
  Coope p.224)
• Amplitude modulation changes carriers amplitude
• Problem is that noise and attenuation look like
  signalThink of the low quality of AM radio
  broadcasts
• Frequency modulation changes carriers frequency
• Less susceptible to interference and noise
• Phase modulation shifts the phase to indicate bits

32
Baud versus Bits per Second

• Baud refers to signalling rate the number of
  transitions per second
• If each transition carries 2 bits, a 2400 baud
  line has a capacity of 4800 bps
• Various ways to carry multiple bits per
  transition
• Multiple voltage levels you need 4 levels to
  transmit 2 bits
• By more than one modulation technique
• Modulate different carrier frequencies
• Use phase frequency modulation
• or by combining these three techniques

33
Multiplexing

• Early technique for sharing lines
• Frequency-division multiplexing works on analogue
  lines
• e.g. 48kHz group was split into 12 voice-grade
  circuits by shifting each circuit to a higher
  frequency
• This was the main reason for limiting bandwidth
  of voice lines to below 4kHz (the local copper
  can carry far more)
• Now obsolete in telephone network (lines are
  digital)
• Time-division multiplexing sends a chunk of one
  circuit, then a chunk of the next, and so on
• If circuit whose time has come has nothing to
  send,the time-slice is unused
• Works well for digital and analogue transmission
• Intelligent multiplexing exploits the silences

34
Transmission media

• Wires
• Cheap and robust, capacity limited by distance
• Suffer from losses and stray radiation (noise
  getting in, secrets getting out)
• Hard to intercept without getting caught (its
  usually illegal)
• Microwave
• Fairly expensive, but high capacity over long
  distance
• Very easy to intercept not always illegal
  hard to detect
• Some sensitivity to weather
• Optical fibres
• Fairly cheap, vast capacity
• Very difficult to intercept without detection
• Termination equipment cheap and still falling in
  price

35
Other media

• Satellite microwave repeated at geo-stationary
  satellite
• Good capacity
• Portable (the obvious answer at a new oil well)
• Very long transmission time (0.25 second)so you
  have to buffer between acknowledgements
• Easy to intercept without detection (must
  encrypt)
• A finite resource (there arent that many slots
  in the sky)
• Infra-red and radio
• IR is similar technology to TV remote control
  used to swap data with cell-phone, or for
  printing from a laptop
• Radio can provide a wireless LAN by broadcasting
  within a location, using IEEE802.11 standards
• IEEE802.11b (Wi-Fi) at 11Mbps 802.11g at 54Mbps
• IEEE802.11n draft now implemented 270Mbps
• IR and Bluetooth operate mainly for local
  point-to-point

36
Cellular Radio

• The technology used by mobile phones
• Depends on computer-controlled network of
  transmitters, each with a cell (as in a
  honey-comb)
• Phone handshakes to get allocated to a cell
• Therefore networks computer knows where you
  are(using Data Protection Act RIPA so does
  Government!)
• GSM phones use intelligent Time Division
  Multiplexing
• Call set-up provides you with frequent time-slots
• Tiny periods of transmission are digitised and
  compressed
• Each packet of data is transmitted in your time
  slot
• For data, maximum rate supported is 9600 bps
• New generation mobile phones are always
  connected
• This concept successful in Japan for several years

37
Always Connected Technologies

• Most commercial data networks are always active
  (think of LANs and Internet connections in
  Winchester)
• But telephone calls are set up when you dial
• True for analogue and ISDN digital services
• At home and on the road, we mostly use dial-up
  Why?
• The copper to the exchange is dedicated to us
• Our mobiles have to handshake regularly with the
  cell
• Digital Subscriber Lines (ADSL and SDSL) always
  on
• Permanent connection to the Internet
• e-mail arrives immediately you can offer a web
  server
• Gives you some security concerns get a firewall
  router
• New generation mobile phones stay online

Dont touch with a bargepole
38
The Internet

• Grew out of ARPAnet, connecting DoD and
  universities
• Now extended to most large organizations
• Publicly accessible through Internet Service
  Providers
• Consists of a backbone, to which ISPs connect
• Companies and organizations lease connections to
  ISP
• Services offered include
• WWW Worldwide Web of pages with Hypertext links
• e-mail Files routed between mail servers
• FTP File Transfer Protocol
• Main protocol stack is called TCP/IP
• Transmission Control Protocol
• Internet Protocol

Well cover protocols next time
39
The Layers and your browser

• When you request a URL, your browser application
  needs to communicate with appropriate server
• Browser builds request, including destination
  address, passes it to socket (interface to
  networking)
• Socket effectively manages session to Internet
  gateway, builds TCP/IP header, sends request to
  network card
• Networking card handles bottom layers
• Data link including error correction
• Physical layer (twisted-pair, coaxial cable..)
• Router passes packets to Internet gateway
• Gateway strips header addressing it, determines
  next link on route to destination, calls Data
  link layer
• and so on...

40
Networking Summary

• All modern networking is Packet-switched
• Application sends messages to network software
• Messages wrapped in headers indicating
  destination
• Each message becomes one or more packets
• Packets passed to software layer or link as
  needed(may require adding more wrapping to the
  data)
• Significant LAN technology is Ethernet
• Throw data on the bus, detect collisions,
  retransmit
• Usually 100Mbps (formerly 10Mbps, 1Gbps now
  common)
• LANs based on hubs, switches or routers
• Wide-area connections usually via the Internet,
  accessed on leased lines, ADSL or dial-up

41
HTML and Assignment

• Due Thursday 7 May 09Develop on disk, hand in
  on USB stick Ill give you later

42
Assignment Workshop

• Chance to check understanding of
• Document structure lthtmlgt ltheadgt lt/headgt ltbodygt
• Basic HTML lthngt ltpgt ltulgt ltligt etc
• Hyperlinking lta hrefgt lta namegt
• Tables lttablegt lttrgt lttdgt
• Handling graphics ltimg srcgt
• Frames ltframesetgt ltframegt
• Attributes used inside a tag
• width, height never both in an ltimggt tag!)
• href dont forget http// in remote links
• border1
• Try to avoid direct reference to appearance on
  screen
• Font, bold, italic

43
Marking Scheme

• Two thirds for quality of site, including
• Success in achieving what you said you set out to
  do
• Usability, including ease of navigation between
  pages
• Accuracy minimal impact from errors
• One third for evidence of understanding and
  logical development
• Clear statement of scope what is and isnt to
  be included
• Review of what you discovered in research
  development
• Reflection on how you developed your site
• Both aspects will be affected by how ambitious
  youve been, but you can usually do better with a
  simpler site you get right than aiming for the
  sky and falling short