Networking and the Internet 10

1
Networking and the Internet (10)

• Week 8
• Data transmission (Willis, chapter 12)
• Modulation Media used (Coope, chapter 13)
• Network types (Willis, chapter 13, Coope chapter
  14)
• Relieving Bottlenecks
• Communication protocols (Willis, chapter 14
  Coope, ch.15)
• Final Workshop for May 7th Assignment
• Week 9 Self-study understanding network
  concepts
• Week 10 Focus
• More communication protocols (Willis, chapter 14)
  
• Application protocols, includingData formats for
  transmission

2
Over the May Holiday Did You Work out?

• Why Ethernet collisions run away when the load
  grows too large
• Would you expect to be OK at higher utilizations
  if there were just one client using the server?
• Find out what an Ethernet switch is and how it
  can help
• What is Wired Equivalent Privacy on a wireless
  LAN?
• Why is WPA better?
• Did you learn anything about IEEE 802.11n?
• Why does a 50KB jpeg picture take longer to
  transmit through a modem than a 50KB Word
  document?

3
Communication Protocols

• A vital part of communication that is not the
  message
• Attracting attention
• Handshaking
• Turn-taking
• Body language
• Acknowledgement (e.g. nod)
• Failure to acknowledge (blank look)
• Interruptions
• Request to repeat
• Humans are very adaptiveComputers need more
  formal protocols

4
Formal Protocols

• Error Control (assuming we can recognize errors)
• ACKs and NAKs depending on results of error
  checks
• Sequence Control
• Putting packets back together into a message
• Flow Control
• Request to sender to slow down (or speed up)
• Time-out Control
• How to deal with absence of responses
• Direction Control
• Turning round a half-duplex link (over)
• Start-up Control
• Establishing a connection

5
Error Detection and Correction

• In any transmission, its possible to lose bits
• Error rate usually measured as 1 bit in 106 or
  similar
• Errors may be random or clustered
• Parity checks let us detect (not correct)
  single-bit errors
• But we could always ask to have the data sent
  again
• Parity is just one form of redundancy
• If we sent everything three times and took a
  vote,wed be pretty sure the data had arrived
  OK!
• In the real world, we go for smallest effective
  redundancy
• Cyclic Redundancy Checks let us validate an
  entire block
• ITU algorithms let us correct all single and
  double errors, all errors with odd number of
  bits, burst errors up to 16 bits
• 99.998 of 18-bit and longer bursts

6
Protocols for the layers

• Protocols effectively define how layers talk to
  their peers

7
Notes on use of Layers

• In reality, every flow between two programs (or a
  program and a terminal) goes along some physical
  medium, which is controlled by the bottom layer.
• But since the interface to the next layer down is
  consistent, and the other end of the flow will
  ascend through the same layers, its much simpler
  to ignore the lower layers, and think of a
  logical connection between higher layers.
• So file-transfer between two E-mail systems might
  need the application to code the file up in some
  way, packaging it with a header to indicate where
  it is to go, and what is to be done when it
  arrives (maybe the sender needs acknowledgement
  when the receiver opens the file).
• The session layer then needs to establish where
  the recipient is, and set up a session to that
  remote system. The file might be capable of
  tying up a circuit for hours on end, so it makes
  sense to chop it up into packets to be sent on
  the session.
• Network layer routes each packet down a
  particular link (or down one of a group of links)
  that is known to be on the way to the
  destination. At the other end of an intermediate
  link, the network layer looks at the destination
  and picks a suitable link for onward
  transmission.
• Back on the starting system, when the network
  layer hands a packet to the data-link, it is
  coded in a way appropriate to the link chosen.
  For example, there might be a risk of the ends
  getting out of sync if you send more than 7
  concurrent ones or zeros down a link, in which
  case you need a way to encode data that would
  cause that to happen. You may need to code the
  data as tone changes, so that it can be sent down
  an analogue circuit. Finally, there has to be a
  standard for voltage, bandwidth etc on the
  physical link.

8
Layer 1 Physical

• Weve already covered I/O Bus handshaking
• Ready line says theres data to be transmitted
• Accept line turned on when data has been received
• Serial Protocol (RS-232C) visible on modem lights
• DTR (Data terminal ready) makes modem ready to
  answer
• When called, modem raises Ring Indicator
• When RI and DTR, modem goes off-hook (answers)
• Computer raises Request to Send, modem sends
  1850Hz, raises Clear to Send
• Remote modem raises Carrier Detect, sends 1180Hz
  tone
• Local modem raises CD
• Transmission can take place
• When a carrier drops, connection is unpicked

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Layer 2 Data Link

• Main task is Error Control
• Early systems ECHOed every character (errors
  obvious)
• Now rely on more efficient error checking
  examples?
• with Automatic Repeat requests
• Principle is to acknowledge every packet (or not)
• Potentially attach ACK/NAK to reverse packet
  flows why?
• Various timings possible
• Sender needs ACK for packet n before sending n1
• If NAK (negative acknowledgement), resends packet
  n
• Simple, but wastes potential transmission time
• Sender will send up to m packets without Ack
  (continuous RQ)
• Waits for Ack (n) before sending packet nm
• In event of Nak, may request just the bad
  packetor throw away all later packets and go
  round again

10
More Data Link Protocols

• Sequence Control
• Reassemble packets in correct order
• Usually counted modulo some number not less than
  the number of outstanding packets in Error
  control
• Flow Control
• Window mechanism is inherent in continuous RQ
• May also wish to stop and start the sender
• e.g., if it functions best in streaming mode,
  like a tape
• To reduce number of interrupts at sender
• Collections of the protocols have been
  standardized
• Asynchronous Start/Stop byte at a time
• Binary Synchronous block at a time (now
  obsolete)
• SDLC and HDLC continuous RQ block transmission

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Continuous RQ block transmission

• TCP/IP implementation is based on Sliding Window
• Also called ARQ Automatic ReQuest
• Works by acknowledging which block is expected
  next
• See animation number 6
• Principle is that transmission can continue until
  a fixed number of packets are outstanding 7 in
  this animation
• This is the minimum number of buffers needed at
  each end
• Cant send more than this without getting an ACK
• Sequence numbering must go at least one more
  (e.g. 0-7)
• Benefits
• Fewer line turn-rounds (vital with satellite
  transmission)
• ACK can be attached to data going in other
  direction

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During ARQ transmission on Data-Link

• Sender has transmitted 3 packets
• Up to 7 can be outstanding 8 unique numbers

13
Wireless LANs

• The hot technology in 2005 now you can connect
  over most of the campus, and at many other places
• Its how most houses with a shared ADSL link
  access it
• Access point may be built into router or ADSL
  modem
• Range is 100m can be more for eavesdropping
• Increasingly accepted in business locations
• Protocol used is a variant of Ethernet (IEEE
  802.11)
• 802.11b runs at 11Mbps
• Now largely superseded by 802.11g running at
  54Mbps (or even 108Mbps)
• New 802.11n variant will be secure by default,
  and faster
• 802.11 is combination of Physical and Datalink
  protocols

14
Wireless LAN Security References

• 802.11b and g are only secure if you
• turn on WEP (Wireless Equivalent Protection) or
  better, WPA (Wi-Fi Protected Access), as used on
  campus
• Turn OFF broadcasting of your network identity
  (the Access point name is broadcast unless you
  stop it)
• Wired Equivalent Protection (WEP) Security
  isnt activated by default need to turn it on
  with 128 bit keyhttp//www.wi-fiplanet.com/tutori
  als/article.php/2106281
• Can restrict MAC addresses, but these can be
  spoofed
• IEEE 802.11n coming soon better security
• Security activated by default
• Part of key changes with time
• http//www.embedded.com/showArticle.jhtml?articleI
  D34400002

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Layer 3 Network

• Although the Internet isnt strictly aligned with
  OSI model, IP (Internet Protocol) maps closely to
  layer 3
• Provides connectionless service to deliver
  packets over multiple links
• receives datagrams (packets with destination
  address)
• selects a link to route each datagram
  over(potentially, this selection might be
  cached)
• Routers perform switching at level 3
• Ignore Williss view that IP is likely to be
  replaced by OSI transport protocol the trend is
  in the opposite direction
• SNA networks are being converted to TCP/IP
• Window of opportunity for OSI closed long ago

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Layers 4 5 Transport Session

• The TCP part of TCP/IP is Transmission Control
  Protocol
• Can establish a connection end-to-end
• Session layer issues a connect request
• Transport layer sends indication to partner at
  other end
• When indication acknowledged, confirms connection
  to session layer
• Session layer then sends/receives data requests
• Each data element sent to partner transport layer
• Until
• Session layer asks to terminate the connection
• Transport layers then disconnect it at both
  ends often the application does this (without
  session or presentation layer)

17
Application Protocols

• Too numerous to cover in detail any application
  can define its own, and many do
• Main thing is that Application should be
  insensitive to lower layers
• Some common examples
• HTTP (handle request to transmit a file for
  display)
• FTP (handle request to transmit a file)
• SMTP (Simple Mail Transmission Protocol)
• Remote Procedure Call parcel up a request to
  get a program run at the far end, then deliver
  its results
• CICS Function-ship ask remote application for a
  record, and deliver the record as if the request
  were local

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Two approaches to standards

• Self-defining data formats
• Everybody works internally in an efficient format
• Describes format in any transmitted unit
• Translation needed at interface with
  heterogeneous system (and nowhere else)
• For example, most CICS systems use EBCDIC
  characters, but CICS for AIX uses ASCII character
  formatSo convert only at connection to CICS for
  AIX
• Working through a reference format
• All transmitted data converted into a standard
  form
• Converted to local form at other side of
  interface
• Avoids need to be aware of variety of formats
• BUT double translation is often a futile overhead

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Self-defining format (1)
English Application
Presentation
Session below
(English) I should like a beer
Network flow
20
Self-defining format (2)
English Application
Presentation
Translation
Session below
(English) I should like a beer
Network flow
21
Using a standard intermediary
English Application
I should like a beer
I want beer
Presentation
cervisiam volo
cervisiam volo
Session below
cervisiam volo
Network flow
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Some Conversions

• ASCII to EBCDIC
• Needed at Distributed/Mainframe interface
• Code Pages
• ASCII isnt a single standard the same code
  points are used for different things in different
  languages
• e.g in UK English, in US
• Japanese EBCDIC has Upper-case Roman, plus Kana
• Unicode an attempt to unify all the character
  sets
• At the expense of needing 2 bytes per character
• Big-endian and Little-endian integers
• (Called after Lilliputian political parties)
• Intel has low-order byte before high-order (four
  and twenty)
• Everyone else is the other way round

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Self-defining format
French Appl
English Appl
English Application
Presentation
Session below
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Goal of Exam

• To test the learning outcomes not measured by the
  practical assignment. You should
• be familiar with a range of computer system
  concepts
• understand the theory underpinning network
  technology
• have studied current network configurations
  applicable to business
• understand Internet technologies
• have accumulated sufficient practical experience
  of Internet services to understand how to apply
  them to business (tested by the HTML assignment)
• be able to review innovations in network
  technology in a critical manner, without
  succumbing to fashion

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Preparation for Exam

• You may bring to the examination
• One reference book or textbook of your choice
• the notes handed out during the course
• any notes you have made
• Well finish the theory next week, and revise to
• Ensure you understand the topics that may come up
• Answer your questions about technical side of the
  module
• Books
• Module handouts are intended to cover the
  content, but
• If you have Coope or Willis, so much the better
• John OGorman Operating Systems (Macmillan)
  covers File I/O and memory well