Networking and the Internet 4

1
Networking and the Internet (4)

• Last Week
• Online Computing (Interactive versus
  Transactional)
• e-commerce issues what do we need for B2C?
• HTML introduction
• HTML Practical
• Week 4 Focus
• Network Architecture
• Computer Memory Virtual and Real
• CPU design and Performance (Coope chapter
  6 Willis, chapters 6, 7 and 8)
• HTML Practical
• HTML tables

2
Network Architecture

• In earlier weeks we looked at networks in various
  waysPhysical, logical, data, meaning of data
• Will now consider Packet Switching

3
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 taken over (even in IBM
  shops)

Telephone companies
4
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

5
Packet-Switching

• All modern networks rely on chopping
  transmissions into packets, and identifying them
  with control information
• Think of the address and return-address on a
  letter
• Packets usually have other controls, such as
• error-correction (for fix or detect transmission
  errors)
• Sequence control (to get the packets back into
  order)
• Examples of packet-switching
• Local Area Networks (like Ethernet)
• Cellular phone networks
• The phone system (apart from the local loop
  the copper wire to your house)
• Internet

6
A general network

• A has direct links to B, C H
• Can manage links as sessions
• Flows from A-E could go
• A-C-E
• A-H-G-E
• 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
• If packet for E reaches H, H works out which link
  to send it out on
• Sender and receiver dont care about route taken
• or about how each link works
  for example

7
A general network

• 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
• Key issue is to avoid routing round in circles

Slow
Fast
Fast
wireless
Fast
LAN
LAN
8
Seven Layer Model

• Protocols effectively define how layers talk to
  their peers

9
TCP/IP has superseded SNA

• The World of e-Commerce is built on LANs and
  TCP/IP
• Well cover Ethernet and other LAN technology
  later they provide the links to carry Internet
  Protocol data
• Every location on the Internet has an IP address
• To send data to a location, system applies its
  address to every packet to be transmitted
• Also attaches a return address for replies
• Sender works out which link to send the packet
  down
• Routes it when theres no direct link
• for example, at home, any non-local Internet
  address is down the Pipex connection from my home
  network
• Other end of link works out onward routing

10
Memory Concepts

• Virtual and Real Memorywell start with Real

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Random Access Memory

• CPU accesses memory through two registers
• Memory Address Register (where)
• Memory Buffer Register (what)
• To write to memory (e.g. STORE 1234)
• CPU puts address (1234) into MAR, and
• data (contents of general register or
  accumulator) in MBR
• Matches John Von Neumanns model of a computer
• Memory is linear array of cells
• No physical separation between instructions and
  data
• Only one cell can be accessed at any one time
• Well nearly multi-cell groupings are accessed
  today
• 8-byte double-words in the case of a
  Pentium(Coope uses Pentium III addressing on
  p.51)

12
Register Sizes on a Pentium

• MAR needs to handle the full 4GB address-range
• 4GB is 2³² so we need 32 bits
• Though the last 3 wont be used by the memory
  hardware, as 8 bytes flow over the bus even if
  only one is to be used
• MBR needs room for all 8 bytes 64 bits
• How long will 4GB be sufficient?
• Even I can afford 2½GB today
• Software grows to fill the space available (e.g.
  Windows Vista)
• The architecture limit will become a practical
  issue
• Now were moving to 64-bit systems 64-bit MAR
  able to address How much?

13
Associative Memory

• Memory is read by content, not by address
• Essentially you get locations matching a given
  key
• Place key in interrogate register
• Filter matches with a mask
• Parallel memory electronicsflag all locations
  that match
• Expensive Little used for ordinary data
• Valuable for paging systems
• As we shall see
• Gordon Scarrotts CAFS was never a commercial
  success

Content Addressable File Store
14
Memory Contention

• CPU is much faster than memory access
• So it helps if we can pre-fetch instructions
  and data
• Cant do this if MAR/MBR are a bottleneck
• We could split memory into banks, each with an
  MAR
• Send request to one bank, and while its
  working...
• Send request to another one
• If we store odd locations in one bank and even in
  another we have 2-way interleaving
• Within limits, more interleaving increases
  speed(Willis has four-way example on p.88)
• Depends on having a fast bus to feed CPU requests
  to MAR(s) this is why a 800MHz front side bus
  is advertised

15
Cache Memory

• Memory access is seldom truly random in real life
• Groups of locations get used much more than
  others
• So if we can make a local copy of them, we save
  time
• Cache is a place we can look without waiting for
  the data to come from memory (Coope, pp.53-57)
• Good chance that active working variables will
  fit in cache
• Can also be used to pre-fetch instructions
• Usually implemented as small, high-speed memory
• Very fast, but very expensive if actually on CPU
  chip
• Cheaper and fairly fast if on separate silicon
  (level 2)
• How do we know if a location is in cache?
• An opportunity for associative memory (Coope p.51)

16
Virtual Memory

• Introduced with Ferranti Atlas (1965)
• Commercial debut with IBM System/360 model 67
• Achieved success with IBM System/370 in 1971
• Now everybody does it, even PCs
• Basic principles
• Virtual Memory has more addresses than real
  memory
• Divide address range of virtual memory into pages
• Put pages of virtual memory into frames in real
  memory
• Map virtual address to address of frame in real
  memoryusing dynamic address translation
• OK until every frame is full, then we have to
  page out
• find a page we can sacrifice write it to disk
• allocate the frame to another (virtual) page

17
Organizing Virtual Memory

• Early virtual systems (like OS/VS1 and DOS/VS)
  used a single 16MB virtual address range
• chopped it up between processes just as real
  operating systems had done previously
• Minimized change needed to OS and 24-bit
  applications
• - Limited total concurrent programs to 16MB
• - Left risk that programs would tread on each
  other
• Better and safer to have separate address spaces
• Each process runs in a separate chunk of virtual
  memory (initially 16MB to be compatible with
  24-bit programs)
• No risk of writing to other processs memory
  cant see it
• CP/67, VM/370, MVS, most Unix, Windows (NT and
  later)

18
Dynamic Address Translation

• Hardware translates a virtual address into a Real
  address
• For every (virtual) address space, we have a
  page table, giving addresses of frames that
  contains each page
• If a page is present, its entry will contain a
  frame number
• For pages not present, entries will be flagged
  invalid
• How it works (example with 4KB pages in 16MB
  space)
• treat address as pageoffset-in-page 12 12
  bits
• So page-table will need 4K entries (212)
• Use 12 high-order bits to look in the page table
  for address of frame containing the page
• Add (by OR) the offset (low-order bits) get that
  address
• Assumes page table is in real memory
• Move to two-level (segment table) when thats a
  problem

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Getting to a byte in real memory
8F4 Page number
A10 Offset in page
A10
Virtual address
Relevant page frame
Page table
Addressed byte
8F4
contains real address
Real memory
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Machine code CPU structure

• Some of Coope chapter 5 (Willis 7-8) will be
  familiar to you
• Instruction sets for Machine code
• Register instructions
• Stacks
• Components of the CPU
• Base Displacement addressing may not be
• IBM System/360 instructions use 16-bit
  address,but memory needs a 24-bit address
  (31-bit from S/370)
• So have a register to hold a base address
  (lt32-bits)
• add displacement to decide which location to
  access
• Displacement can be literal or held in a
  register
• Can also add contents of another (Index) register
  great for stepping along an array

Optional detail (youll not be examined on
anything beyond simple Virtual Memory)
within instruction
21
World Wide Web Publishing

• Writing HTML
• Some recommended references
• http//www.web-nation.com/lessons/html-pri.htm
• http//www.w3schools.com/

22
World Wide Web Publishing

• How confident are you about
• Basic structure of an HTML document
• Headings, Paragraphs and Lists
• Including Images
• Making Hyperlinks
• Did you do the exercise last week? See document
  and zip file of samples on Learning Network
• Main structures still to study are
• Tables (this week)
• Frames

?
23
Reminder Creating Hyperlinks

• Any hyperlink reference needs to say what it is
  linking to
• In HTML, we use an Anchor tag with the HREF
  attribute
• ltA href"http//www.pdq.edu/freds.htm"gtQuick
  Univlt/Agt
• ltA href"freds.htm"gtLocal referencelt/Agt
• You can reference any resource on the Internet
  Just give its URL
• Any lack of specificity means look locally, so
  missing out the domain means it is in the local
  file system
• Always leave out as much as you can
• Makes it much easier to move the site
• Can also use relative references, e.g. to a
  sub-folder ltA href./subdir/fred2.htm"gtrelative
  referencelt/Agt

Important in Assignment
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Internal Bookmarks

• You can also name a target inside a resource
• Like a bookmark in Word
• Also coded on Anchor tag, this time as NAME
  attribute
• ltA name"phred"gttarget of the linklt/Agt
• ltA href"phred"gtClick here to get to targetlt/Agt
• or if the link may be from a different
  document...ltA href"http//www.pdq.edu/freds.htm
  phred"gtClicklt/Agt
• So the full form of the HREF attribute is a URL
  followed by and a named anchor inside the URL
• Missing out the resource means its in this
  document
• If the name is omitted, it refers to the top of a
  resource

25
Tables

• As in Word, a table is an array of cells,
  arranged in
• Rows, usually containing related data
• Columns
• Each cell can contain multiple lines of data
• In HTML, we need to
• Indicate where a table starts and
  finishes lttablegt
• Show the start of each row lttrgt
• Provide table data for each cell lttdgt
• Some bells and whistles
• Heading rows contain special table headers ltthgt
• And cells may contain nulls ltbr/gt

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HTML Tables

• ltTABLEgt tag starts structure provides
  attributes to lay it out
• border attribute frames each cell as well as
  whole table
• Heading item identified with ltTHgt
• Rows introduced with ltTRgt
• Each cell is Table Data identified with ltTDgt
• align attribute places text at right, left or
  center of cell
• valign attribute places text at top, bottom or
  center of cell
• No explicit statement of number of columns
• inferred from maximum number of cells in a row of
  the table (so missing a lttrgt can have a dramatic
  impact!)
• Most tags automatically terminate previous tag
• so end-tags can be omitted (not lt/TABLEgt of
  course)
• ltcaptiongt can appear anywhere in table, but
  browser will always show caption text at the top

27
More about tables

• Tables are always the most complicated structures
  in any text-processing system, and their precise
  syntax often changes as the mark-up language
  develops. The table tags of mainframe GML were
  initially unpublished, then modified in the
  commercial GML called BookMaster, then modified
  again before being published in starter set.
• Similarly, in Microsoft Word, table commands have
  changed in every release, with graphical creation
  of complex tables being a key selling point of
  Word 97. Word XP added yet more bells and
  whistles.
• HTML took a fairly simple view that you pour
  cells (table data) into a table, creating columns
  as you pour new cells into a row. Rows are ended
  explicitly, or implicitly by beginning a new row.
  A side-effect of this is that omitting a ltTRgt
  tag will create a double-length row, thus
  doubling the number of columns in the table.
• Check out the table in table.htm in the zip file
  on the Learning Network or athttp//www2.winches
  ter.ac.uk/bm/courses/BS2911/htmlsam2.zip
• This cuts corners in terms of the tags actually
  used, but stays within the rules of syntax for
  HTML tables, so appears correctly.
• In the following exercise, well see how
  Microsoft Internet Explorer can tolerate sloppy
  HTML. Even so, its safer to be pedantic!

28
More Complex Tables

• Tables are not always simple C R arrays
  sometimes we want to combine cells horizontally
  and/or vertically
• This is done with attributes on the cells tag
• COLSPANc says how many columns are spanned
• Also used to give width in horizontal units,
  without spanning
• ROWSPANr says how many lines are spanned by the
  cell
• Empty Cells
• Usually represented as if the cell were not there
  (raised surface in most browsers)
• Put ltBR /gt in the cell to create a true cell
  containing a blank
• Graphics
• Yes, the ltIMGgt tag works fine in tables
• Its often the easiest way to control where
  pictures appear

29
Table Exercise

• Columns are automatically scaled so that the
  largest possible cell fits on a line (if you
  shrink the window so that the largest cell
  word-wraps, the column scaling will change so the
  widest remaining cell is just wide enough for its
  contents).
• The file table3.htm uses the ROWSPAN attribute to
  produce a table displaying the main table tags.
• table4.htm adds the COLSPAN attribute to split
  one of the cells in what is visually the RH
  column. This has to be done by treating all the
  other cells in that column as if they span two
  columns the best way to think of it is as a
  grid on to which the cells are placed the grid
  has to be fine enough to contain the smallest
  cell required.

• table4a.htm was based upon Fig 10.3 of the 10
  Minute Guide to HTML by Tim Evans (QUE
  publishing, 1995), where the author decided to
  work on a much finer horizontal grid. This works
  fine until one actually tries to use the COLSPAN
  attribute to split cells, at which point it gave
  odd results on both the browsers I tried. In
  neither case were the two cells occupying the
  right-hand column shown in the ratio prescribed
  by the COLSPAN values.
• Try making one of the cells wider than the others
  in its column. And narrower. What happens in
  Microsoft Internet Explorer? And in Firefox?
• Finally, make a personal copy of one of the
  tables, and insert an image into it.

30
HTML Table Exercise

• Starting with the zip file of samples from Week 4
  of the BS2911 area of the Learning Network
• Make a copy of the source in My Documents,
  preferably with a new name (samples\XXXtable.htm)
• Start Windows Explorer and get your file visible
  on it
• Open it with the NOTEPAD Accessory (right-click)
• Open it with Internet Explorer (double-click)
• Open Firefox as well if you have it
• You can drag the file from Explorer into each
  browser
• Has anything gone wrong?
• Make changes in NOTEPAD, Save, and look at the
  effect in both the browsers (dont forget to
  Refresh/Reload)

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How tolerant is your browser?

• Try messing around with the HTML to see what you
  can get away with is it safe to rely on quirks
  of your browser?
• lttable bordergtltthgtCol one headingltthgtCol two
  headingltthgtColumn threelttrgtlttdgtrow 1 item
  1lt/tdgtlttdgtrow 1 item 2lttrgtlttdgtrow 2 item
  1lttdgtlttdgtrow 2 item 3ltcaptiongtCrazy
  tablelt/tablegt

• Next arrange that there isnt quite enough room
  for every cell to be displayed on a single line
  (you can add text, increase the number of
  columns, or shrink the browser window, or a
  combination of these). Note how the text in a
  cell gets split across lines, and the cell
  boundaries stay lined up.
• Add the attribute align"xxx" to various data and
  heading cells, where xxx takes the values right
  left and center. Try other forms, such as L,
  centre etc. What can you tell about the defaults
  in IE and Firefox?

32
Challenging Table Exercise

• Make a table to display your Semester 2
  timetables
• Should follow SchBM.html model in htmlsam2.zip
  file
• Please find and correct the bug in SchBM.html
  it is only obvious with Firefox
• Timetables are posted on the BA Student Info area
  of LN
• BM single hons Year 2 timetable is quite complex
• Timing varies from day to day
• Have to consider optional modules
• My approach was to slice the day into hours and
  then join cells to represent the sessions
• You may be able to think of a better way what
  if hours ran across with days going down the
  page?
• If you do something good, Id welcome it to go on
  the LN

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HTML Covered So Far

• Overall Document Structure
• ltHTMLgt, ltHEADgt, ltBODYgt tags
• Headings and other text tags
• ltH1gt, ltH2gt..ltH6gt tags ltPgt, ltBR/gt, ltEMgt,
  ltSTRONGgt
• Lists and the items within them
• ltOLgt, ltULgt with ltLIgt
• Anchors and Hyperlinks
• ltA name"target"gt Text labelled (if any)lt/Agt
• ltA href"where_to_link_to"gt Explanatory textlt/Agt
• ./ means same folder and ../ means folder
  above
• In-line graphics
• ltIMG src"path to image" alt"dont forget
  non-graphic user"gt

34
Extra HTML in Week 4

• Tables
• framed between lttablegt and lt/tablegt tags
• rows start with lttrgt tag (lt/trgt is optional)
• normal table data starts with lttdgt
• except for header rows, where its ltthgt
• Include the break tag ltbr/gt if the cell is empty
• Label table with ltcaptiongtcaption textlt/captiongt
• Table tags can have attributes
• lttable frame"border"gt
• lttd colspan2gt
• lttd align"center"gt (HTML was designed in
  Switzerland by an Englishman, so it's in
  American!)
• Still to do Frames