BS2911 Week 3 1

Networking and the Internet (3) Last Week:

» Computer Architecture

» Operating System concepts and examples

Week 3 Focus» Views of a network (Coope, chapters 12 & 14)

– Electrical; transmission management; addressing; contents– Data formats (the codes that flow) (Coope, chapter 2)

» More on systems for Online Computing

» e-commerce issues – what do we need for B2C?

» Introduction to Writing HTML

BS2911 Week 3 2

Revision of Task Dispatching

Remember that every action involves an interrupt that returns control to the

Operating System

The queues are OS control blocks

BS2911 Week 3 3

Dispatching Tasks Let’s assume a Ready Queue of tasks waiting to run

» Add tasks to queue according to a priority pattern (cheapest is FIFO, but we may want to improve on that)

» Dispatch the task at the head of the queue» When it gives up control, dispatch new head of queue» May want to maintain queue of blocked tasks too

What if task doesn’t give up control?» Need to interrupt it at end of time slice to let others run

(Windows 3.x didn’t do this except for DOS tasks)» Can return it directly to Ready Queue

(with risk that it’ll consume too much CPU time)» Maybe we should favour short interactions over long

BS2911 Week 3 4

Life Cycle of an Interaction

Task Created

User hits ENTER

Terminated

Executable Task

FinishesRunningOn Ready Queue

Blocked

Dispatched

Requeued on Event

Time expires

Enters WAIT

But how do we know when it’s finished?

BS2911 Week 3 5

Working On-line On a PC, you start a program

» Feed it with data e.g. edit your document» Produce some results saved or printed

document» Quit the program

While you’re editing, you expect the computer..» To keep the program running, even if you walk away» and Maintain all the information you’ve entered» This involves keeping program and data in memory

Very hard for an Internet Server to match this» How long should it hold stuff in memory for you?» What if there are 5,000 users like you?» Chosen solution is to treat interactions as “atomic”

BS2911 Week 3 6

Keeping the Process Alive

Task Created

User hits ENTER

TerminatedFinishesRunningOn Ready

Queue

Blocked

Dispatched

Requeued on Event

Time expires

Enters WAIT

User hits ENTER

Maintain page tables to access process’s memory, though it’ll get paged out

Nothing to do

Executable Task

On idle queue

Can free process’s memory

BS2911 Week 3 7

Transactional & Interactive Computing

Lynn Brooks (IBM Poughkeepsie) recognized and described two distinct kinds of on-line computing:

Transactional» Composed of separate, “atomic” actions» Take an order, handle a billing query» Very little “state” to carry between transactions» Each transaction may use a different program

Interactive» Usually lots of interactions with a single program,

as in editing a document» Copious state information – process’s whole memory» Typically heavy on memory, light on processing

BS2911 Week 3 8

The Internet is Transactional In general:

» Every time you interact, browser sends message to server» Server responds by sending you something» Then it forgets you

Presents a problem if you want a “session”» For example, you sign on to authenticate yourself» Server then treats you as a privileged user,

for a whole series of interactions Server usually maintains minimum status information

» Table of who’s signed on, with IP address and privilege» On each request, you send a “token” for server to look up» How long must it keep your entry in the table?

BS2911 Week 3 9

Impact on B2C e-commerce Successful Business to Consumer e-commerce needs

friendly service to the end-user» Server should “recognize” the user» Authenticate when needed, but do it only once per session» Not grind to a halt because server’s run out of memory

Example: amazon.co.uk» Stores “cookie” on your disk when you sign up » Retrieves it each time you connect in future» Asks you to confirm that it’s really you» Requires user name and password for secure area (anything

to do with account status or payment)» Times out if you don’t sign out of the secure area

BS2911 Week 3 10

Other e-commerce Considerations Server performance

» Users need to be able to rely on availability» With adequate response time

Not usually a problem for delivering HTML pages» “http daemons” in server are tuned to be efficient

Much more serious for transactional work» Server may have to start a new process for each message» Heavy-duty activity involving address-space creation

unless it can exploit a pool of processes» Database activity may involve multiple accesses» User may interact multiple times per business activity» Unless we can send a Java applet for the user to run

BS2911 Week 3 11

Networking and Communications

Resolving confusion with concept of “layers”

BS2911 Week 3 12

Networking and Communications An area made confusing by range of different activities that

fit under this heading:» Electrical (or equivalent) signals actually transmitted

(down twisted-pair, coaxial cable, or by radio)» How the transmission medium is managed

(modem, network card, hub)» How we address data to make sure it gets to destination» How we code the information so receiver understands it» How we package information to send it to other application

(email, screen-shots, file transfer…) Can resolve confusion by the concept of “layers” Convention is to show electrical signals at bottom

BS2911 Week 3 13

Communication Layers Seven layer model of SNA and OSI aims to let us think

about one element of the network at a time: » Application layer Services for applications» Presentation layer Maps data to what is displayed» Session layerTies multiple flows into logical “call”» Transport layer Hides network details from above» Network layer Routing and sequencing of packets» Data Link layer Flow control and error handling» Physical layer Point-to-point flows of bits

Interfaces between layers are well-defined protocols

BS2911 Week 3 14

Seven Layer Model

Simplify programming by encapsulating lower levels

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Transport

(TCP)Internet

(IP)

Network Interface

Application

Presentation

Session

Transport

Network

Data Link

Physical

Purpose/Examples

File transfer

show screen image

manage sign-on

manage connections

route over several links

V.34 , HDLC

LAN, digital-, analogue-circuit

Internet termsOSI terms

BS2911 Week 3 15

Data Formats Computers work on numbers,

but human communications involve more than numbers:» Need letters, signals, pictures, sounds

Computer must be able to handle these formats Popular character format is ASCII

» 7-bit code representing characters (now 8-bit) » Each letter or number or punctuation mark is coded» to represent a character, need to know the right code

Network needs to perform “conversational” actions » Acknowledgement, slow down, “what was that?”» Extra ASCII codes were added to handle these

Also needs to check integrity of data

BS2911 Week 3 16

ASCII (examples)

32 <space> 48 0 64 @

33 ! 49 1 65 A

34 “ 50 2 66 B

35 # 51 3 67 C

36 $ 52 4 68 D

37 % 53 5 69 E

38 & 54 6 70 F

BS2911 Week 3 17

Other forms of coding Can store integers in two or four bytes

» 2 (half-word) can go from -215 to +215-1 (-32,768 to 32,767)» Full-word from -2,147,483,648 to 2,147,483,647

How about floating point? Can also code pure numeric decimal data with

half a byte per decimal digit» Main implementation is “Packed decimal,” for example:

+597 would pack as ‘59 7C’X; -12345 as ‘12 34 5D’X zSeries (System/390) still uses EBCDIC for characters

» Conversion overhead on Mainframe/PC transmissions» Different collating sequences can cause problems

Double-byte character sets such as Unicode

BS2911 Week 3 18

Double-byte character sets

Some orthographies use more than 256 characters, usually ideograms» Examples: Japanese Kanji, Korean Hangul, Chinese» May also need room for syllabic characters (e.g. Kana)» and for a range of alphabets

Problem solved by coding each character in 2-bytes» 64K values are enough» Fortunately, you need fewer characters per message

DBCS can be pure or “SISO”» Pure DBCS uses two bytes even for Latin letters» An example is Unicode» Alternatively you could Switch In and Switch Out of DBCS

BS2911 Week 3 19

Other SBCS Encodings Even Single-byte coding isn’t trivial

» Currency symbols» Latin alphabet extensions

– National characters (æ ð þ ø …)– diacritical marks (Turkish beats even Czech)

» Greek» Cyrillic» Thai

Some run Right to Left (with L->R insertions!)» Hebrew» Arabic

BS2911 Week 3 20

Non-conforming Data What if the data we’re sending isn’t just characters?

» Pictures and sounds are a stream of bytes that can be close to a random selection of binary values

» Binary data might well contain bytes that match the ASCII code to terminate the transmission

One way is to take seven bytes in and write eight out» That way you can avoid the values that give trouble» done in MIME encoding, and in 3270 emulator downloads» This is why e-mail attachments are sometimes gibberish

We may also be sending data coded in a different way» Such as EBCDIC, used on mainframes

BS2911 Week 3 21

World Wide Web Publishing

How to write HTML

(see references for more detail)

BS2911 Week 3 22

Publishing to a Diverse Audience Some users have 14” VGA screens Others have 21” high-resolution displays Users run many different browsers

» Multiple versions of Internet Explorer, Firefox, Chrome, Safari» or something you’ve never heard of!

Some users have character-based terminals, or don’t have bandwidth to spare for graphics

Others have high bandwidth via LAN to server How can we make content attractive and yet still

comprehensible to all these users?

The trick is to describe the content, not the intended layout

BS2911 Week 3 23

Resources on the Internet Internet is a network of networks, sharing a common

4-byte addressing scheme (e.g. 194.66.202.100) Computer resources are collected into Domains, which are

usually named, e.g. www.winchester.ac.uk Domain Name Servers map domain to 4-byte address Uniform Resource locator (URL) made up of three parts:

» Service ID (such as http, ftp, mailto, gopher) terminated with :» Domain name[:port], terminated with /» Resource within domain

Most URLs written service://domain/resource(mailto: does not need the //)

BS2911 Week 3 24

The http Service

HyperText Transfer Protocol is the keystone of the World Wide Web

Browser transmits request to target domain, where http daemon – the server – responds by sending back the requested resource (usually a file)

Browser then reads file and formats it for display While doing so, browser will find mark-up requesting other

resources, and forward them to the appropriate server – usual example is for embedded graphics

If browser can’t handle the resource alone, it can:» Use a “helper application” such as Excel or media player» Offer to save the file to disk for future use

BS2911 Week 3 25

Overall Document Structure

<HTML><HEAD><TITLE>Simple HTML document</TITLE></HEAD><BODY text="#000060"> (here’s hexadecimal again)<H1>Chapter 1</H1><P>There isn’t much in this document.

This sentence will flow after the previous one.<P>But this will be a new paragraph.</BODY></HTML>

BS2911 Week 3 26

Other Common Tags

<HR /> Produces a horizontal rule on the column<BR /> “Break” – a empty text element that is at

the start of a line (you can misuse it to be like a paragraph, but more compact )

<UL> Starts an unordered list (with bullets)<OL> Starts an ordered list (usually numbered)<LI> List item in either sort of list(Lists can be nested, so it’s important to close them in reverse sequence)

<EM> Emphasised text (usually italic)<STRONG> Strong text (usually bold)

BS2911 Week 3 27

Advanced Text-descriptor Tags <DIR> Starts a Directory list (multi-column) <MENU> Starts an unordered list without bullets <DL> Starts a Definition (or Glossary) List

This contains two kinds of list item:» <DT> The Term being defined» <DD> The Description defining that term

<PRE> Pre-formatted text (as in an example) <TABLE> We’ll build tables later

Exercise:Experiment with the various list types. Try nesting them.

BS2911 Week 3 28

Creating Hyperlinks Any hyperlink reference needs to say what it is linking to In HTML, we use an Anchor tag with the HREF attribute

» <A href="http://www.pdq.edu/freds.htm">Quick Univ</A>» <A href=“./freds.htm">Local reference</A>

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

<A href=“./subdir/fred2.htm">relative reference</A>

BS2911 Week 3 29

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

» <A name="phred">target of the link</A>» <A href="#phred">Click here to get to target</A>» or if the link may be from a different document...

<A href="http://www.pdq.edu/freds.htm#phred">Click</A> 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 it’s in this document» If the name is omitted, it refers to the top of a resource

BS2911 Week 3 30

Graphical Images Easy to imbed with the IMG tag

» Need to define the source of the image» Example <img src="fred.jpeg" alt="[fred]“/> » Text of “alt” attribute appears on non-graphical displays

You can also use attributes to modify the image» <img src="/mh.jpg" width="80" alt="Picture of Mike">» Image will be 80 dots wide, in original aspect ratio

(don’t set width and height, it can distort the image)» Use align attribute to place it right left or center

(HTML uses US spelling – even though it was originally written by an Englishman working in Switzerland!)

Use images in JPEG or GIF format (never BMP!)» GIF better for line drawings, JPEG better for pictures

BS2911 Week 3 31

Stick to Relative Addressing At some point, we’ll want to move pages and images on to a

real Web Server Can test most* things on local disks, providing we address

directories relative to the current page, for example:» <a href="next.html”>forward to next page</a>

– meaning switch to next.html in the current directory » <img src="../graphics/freddo.jpeg" alt="[fred]">

– meaning go to next directory up, then look in its graphics subdirectory

* CGI scripts and old-style Image maps do require a server, so can’t usually be tested purely from disk

BS2911 Week 3 32

HTML Covered So Far Overall Document Structure

» <HTML>, <HEAD>, <BODY> tags Headings and other text tags

» <H1>, <H2>..<H6> tags; <P>, <BR>, <EM>, <STRONG> Lists and the items within them

» <OL>, <UL> with <LI>» <DL> with <DT> and <DD>

Anchors and Hyperlinks» <A name="target"> Text labelled (if any)</A>» <A href="where_to_link_to"> Explanatory text</A>

In-line graphics» <IMG src="path to image" alt="don’t forget non-graphic user">

BS2911 Week 3 33

HTML Reference Materials Library has some O’Reilly pocket reference books about

HTML and the Web (see module handbook for references)

Or you can use one of the primers on the WWW… » Tom Drewry’s Guide to HTML tags at

http://www.cems.uwe.ac.uk/~tdrewry/html/index.htm» Marc Andreessen’s at

http://www.web-nation.com/lessons/html-pri.htm » http://www.mcli.dist.maricopa.edu/tut/index.html» http://www.telacommunications.com/nutshell/primer.htm » Web design guidance http://

www.winchester.ac.uk/designproject/ » http://www2.winchester.ac.uk/bm/courses/bs2911/htmlcard.htm

l local copy (archive of NCSA primer)

BS2911 Week 3 34

Writing your own HTML See separate document for simple exercises

(Week 3 on the Learning Network)

Exercise 1 gets you in there looking at the HTML source» Pretty easy to do, providing you think about what each element of

the document is, rather than what it should look like on the screen

» Normal process is to have a Notepad window for sourceand a browser window to view the results

» Update source in Notepad and Save» Flip to browser and Reload to see the effect

Exercise 2 has you writing your own HTML» You can re-use stuff from the other files if you wish