Networking and the Internet 4 l Last Week

Networking and the Internet (4) l Last Week: » » » l Task Management revision Online Computing (Interactive versus Transactional) e-commerce issues – what do we need for B 2 C? 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 BS 2911 Week 4 1

Network Architecture Last week we looked at networks in various ways: Physical, logical, data, meaning of data Will now consider “Packet Switching” BS 2911 Week 4 2

Early Computer Networks l Initially had two distinct purposes: » to connect terminals to mainframe computers » to link computers together l And two ranges: » Local – low errors, wiring under enterprise control » Wide-area – wiring regulated by PTTs (was error-prone) l And two topologies: l Network architectures developed to share connections, based on packet-switching concept: Telephone companies » Point-to point » Concentrated (multiple devices sharing a connection) » Systems Network Architecture led through 70 s and 80 s » Internet Architecture now taken over (even in IBM shops) BS 2911 Week 4 3

Networking Requirements l 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 or SMS text l 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 l 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 BS 2911 Week 4 4

Packet-Switching l 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) l 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 BS 2911 Week 4 5

A general network l l A has direct links to B, C & H and 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 Node A Node B Node C If we break flows into packets, and address them to ultimate destination, Node D Node E we can mix flows on the links l If packet for E reaches H, H works out which link to send it out on Node G l Sender and receiver don’t care about Node F route taken… l … or about how each link works Node H for example 6 BS 2911 Week 4 l

A general network l l l Sender and receiver don’t 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 Node A Slow Node B Node C Fast l Each node functions as a router Key issue is to avoid routing round in circles BS 2911 Week 4 Node D Node E wireless Fast Node F Node G LAN l LAN Node H 7

Seven Layer Model l Protocols effectively define how layers talk to their peers Purpose/Examples Application File transfer Application Presentation show screen image Presentation Session manage sign-on Session Transport manage connections Transport Network route over several links Network Data Link V. 34 , HDLC Data Link Physical LAN, digital-, analoguecircuit Physical BS 2911 Week 4 SNA/OSI terms Application Transport (TCP) Internet (IP) Network Interface Internet terms 8

TCP/IP has superseded SNA l l l The World of e-Commerce is built on LANs and TCP/IP We’ll 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 l Sender works out which link to send the packet down » Routes it when there’s no direct link – » for example, at home, any non-local Internet address is down the Talk connection from my home network » Other end of link works out onward routing BS 2911 Week 4 9

Memory Concepts Virtual and Real Memory: we’ll start with Real BS 2911 Week 4 10

Random Access Memory l CPU accesses memory through two registers: » Memory Address Register (where) » Memory Buffer Register (what) l To write to memory (e. g. STORE 1234): » CPU puts address (1234) into MAR, and » data (contents of general register or accumulator) in MBR l Matches John Von Neumann’s 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 l 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) BS 2911 Week 4 11

Register Sizes on a Pentium l MAR needs to handle the full 4 GB address-range » 4 GB is 2³² so we need 32 bits » Though the last 3 won’t be used by the memory hardware, as 8 bytes flow over the bus even if only one is to be used l MBR needs room for all 8 bytes – 64 bits l How long will 4 GB 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 we’re moving to “ 64 -bit systems” – 64 -bit MAR able to address… How much? BS 2911 Week 4 12

Associative Memory l l l l Memory is read by content, not by address Essentially you get locations matching a given key Place key in interrogate register Interrogate Register Filter matches with a mask Mask Register Parallel memory electronics flag all locations that match Associative Registers Expensive – Little used for ordinary data Valuable for paging systems » As we shall see l Gordon Scarrott’s CAFS* was never a commercial success BS 2911 Week 4 * Content Addressable File Store 13

Memory Contention l CPU is faster than memory access » So it helps if we can “pre-fetch” instructions and data » Can’t do this if MAR/MBR are a bottleneck l We could split memory into banks, each with an MAR » Send request to one bank, and while it’s 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 “ 800 MHz front side bus” is advertised BS 2911 Week 4 14

Cache Memory l l 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 l l 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 cache) l How do we know if a location is in cache? » An opportunity for associative memory (Coope p. 51) BS 2911 Week 4 15

Virtual Memory l 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 l 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 memory using “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 BS 2911 Week 4 16

Organizing Virtual Memory l Early virtual systems (like OS/VS 1 and DOS/VS) used a single 16 MB 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 16 MB » - Left risk that programs would “tread on” each other l Better and safer to have separate address spaces » Each process runs in a separate chunk of virtual memory (initially 16 MB to be compatible with 24 -bit programs) » No risk of writing to other process’s memory – can’t see it » CP/67, VM/370, MVS, most Unix, Windows (NT and later) BS 2911 Week 4 17

Dynamic Address Translation l l Hardware translates a virtual address into a Real address For every (virtual) address space, we have a “page table, ” giving addresses of every frame that contains a page » If a page is present, its entry will contain a frame number » For pages not present, entries will be flagged invalid l How it works (example with 4 KB pages in 16 MB space) » treat address as page||offset-in-page – 12 + 12 bits » So page-table will need 4 K 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 l Assumes page table is in real memory » Move to two-level (segment table) when that’s a problem BS 2911 Week 4 18

Getting to a byte in real memory 8 F 4 A 10 Page number Offset in page Virtual address A 10 Relevant page frame Page table 8 F 4 Addressed byte contains real address Real memory BS 2911 Week 4 19

Machine code & CPU structure l Some of Coope chapter 5 (Willis 7 -8) will be familiar to you » » l Instruction sets for Machine code Optional detail Register instructions (you’ll not be examined on anything Stacks beyond simple Virtual Memory) 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 (<32 -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 * inside the instruction BS 2911 Week 4 20

World Wide Web Publishing Writing HTML Some recommended references: http: //www. web-nation. com/lessons/html-pri. htm http: //www. w 3 schools. com/ BS 2911 Week 4 21

World Wide Web Publishing l How confident are you about: » » Basic structure of an HTML document Headings, Paragraphs and Lists Including Images Making Hyperlinks ? l Did you do the exercise last week? See document and zip file of samples on Learning Network l Main structures still to study are: » Tables (this week) » Frames BS 2911 Week 4 22

Reminder: Creating Hyperlinks l l Any hyperlink reference needs to say what it is linking to In HTML, we use an Anchor tag with the HREF attribute » Quick Univ » Local reference l l 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 Important in Assignment » Makes it much easier to move the site » Can also use relative references, e. g. to a sub-folder relative reference BS 2911 Week 4 23

Internal Bookmarks l You can also name a target inside a resource » Like a bookmark in Word l Also coded on Anchor tag, this time as NAME attribute » target of the link » Click here to get to target » or if the link may be from a different document. . . Click l 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 BS 2911 Week 4 24

Tables l As in Word, a table is an array of cells, arranged in: » Rows, usually containing related data » Columns l l Each cell can contain multiple lines of data In HTML, we need to : » Indicate where a table starts and finishes

can appear anywhere in table, but browser will always show caption text at the top BS 2911 Week 4 26

More Complex Tables l l 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 cell’s tag –

caption text

l Some bells and whistles: » Heading rows contain special table headers » And cells may contain nulls BS 2911 Week 4	25 HTML Tables l tag starts structure & provides attributes to lay it out » border attribute frames each cell as whole table l l l Heading item identified with Each cell is “Table Data” identified with can have a dramatic impact!) l Most tags automatically terminate previous tag » so end-tags can be omitted (not Rows introduced with » align attribute places text at right, left or center of cell » valign attribute places text at top, bottom or center of cell l No explicit statement of number of columns » It’s inferred from maximum number of cells in a row of the table (so missing a of course) l
: » COLSPAN=c says how many columns are spanned » Also used to give width in horizontal units, without spanning » ROWSPAN=r says how many lines are spanned by the cell l Empty Cells » Usually represented as if the cell were not there (“raised surface” in most browsers) » Put in the cell to create a true cell containing a blank l Graphics » Yes, the tag works fine in tables » It’s often the easiest way to control where pictures appear BS 2911 Week 4 28 HTML Table Exercise l l l l Starting with the zip file of samples from Week 4 of the BS 2911 area of the Learning Network Make a copy of the source in My Documents, preferably with a new name (samplesXXXtable. 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 (don’t forget to Refresh/Reload) BS 2911 Week 4 30 How tolerant is your browser? l Try messing around with the HTML to see what you can get away with – is it safe to rely on quirks of your browser? l Col one heading Col two heading Column three row 1 item 1 row 1 item 2 row 2 item 1 row 2 item 3 Crazy table BS 2911 Week 4 l l Next arrange that there isn’t 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? 31 Challenging Table Exercise l Make a table to display your Semester 2 timetables l BM single hons Year 2 timetable is quite complex » Should follow Sch. BM. html model in htmlsam 2. zip file » Please find and correct the bug in Sch. BM. html – it is only obvious with Firefox » Timetables are posted on the BA Student Info area of LN » 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? l If you do something good, I’d welcome it to go on the LN BS 2911 Week 4 32 HTML Covered So Far l Overall Document Structure » , , tags l Headings and other text tags » , . . tags; , , , l Lists and the items within them » , with l Anchors and Hyperlinks » Text labelled (if any) » Explanatory text » . / means “same folder” and. . / means “folder above” l In-line graphics » Extra HTML in Week 4 l Tables » » » l framed between and tags rows start with
except for header rows, where it’s	Include the break tag if the cell is empty Label table with